Astrum Space - This Is the Most Extreme Planet in the Solar System

Episode Date: September 25, 2025

Ready to explore the most extreme planet in our solar system? This Astrum video dives into the incredible secrets of Mercury, a world of scorching heat and freezing cold. Learn why a day on this tiny planet lasts longer than a year, and the other surprising discoveries uncovered on this secretive world. ▀▀▀▀▀▀Astrum's newsletter has launched! Want to know what's happening in space? Sign up here: ⁠https://astrumspace.kit.com⁠A huge thanks to our Patreons who help make these videos possible. Sign-up here: ⁠https://bit.ly/4aiJZNF

Having the sun as your neighbor is hard, searing hot temperatures, deadly radiation, and the onslaught of meteor collisions don't exactly make it the most welcoming part of the solar system. As you can imagine, these challenges make Mercury a particularly tricky place to get to. For this reason, even though it's much closer to us than Jupiter or Saturn, Mercury remains relatively unexplored. Still, scientists managed to send two different missions out to study Mercury. Mariner 10 flew by in the 1970s, followed by NASA's Messenger in 2015, which was the first probe to ever successfully orbit the planet.

And now, as I speak, a third Mercury mission is hurtling through the constellation of Pisces over 177 million kilometers away from Earth. Beppe Colombo, a labor of love between the European Space Agency and the Japanese Aerospace Exploration Agency, is set to arrive on December 5, 2025. It will carry out a comprehensive study of the planet's magnetic field, surface, and internal structure. But, as they say, the journey is the destination.

and Bepi Colombo's journey is a long one. I'm Alex McColgan and you're watching Astrum. Join me today as we reveal what Bepi Colombo has revealed of Mercury so far, why it's travelling farther than the distance of Pluto to get there, and what we're hoping to learn about the closest planet to the sun. On October 20th, 2018, Beppe Colombo took off from the French Guiana Space Center, beginning its seven-year journey to Mercury.

Its trajectory would take it around the sun 18 times, including nine planet flybys. The first was an extremely close flyby of Earth on April 10th 2020. Bepicolombo came within just 12,700 kilometers of our planet's surface, about the distance from Copenhagen to Santiago, Chile. Its journey will then take it on two flybys of Venus and Sun. 6 of Mercury, the last three of which are due to happen in September, December and January of next year. In total, the mighty spacecraft will cover a distance of 8.5 billion kilometres. Taking such a long route was the only way Bepi Colombo could slow down enough before approaching

Mercury's orbit. But why is slowing down so important? Well, if the spacecraft arrived with too much speed, it would swing right past the planet and get sucked into the sun's powerful gravitational field instead. Bepi Colombo was designed to complement and deepen the findings of its predecessor, the Messenger probe. While Bepi Colombo carries many of the same instruments as Messenger did, it is equipped with higher caliber tools able to obtain larger ranges of data. Additionally, Messenger's orbit of Mercury was rather one-sided. as periapsis was over the south pole, meaning we have great detail and mapping of the northern hemisphere and almost non-existent data on the southern hemisphere.

Beppe Colombo plans to rectify this by examining Mercury from an orbital angle more in line with the planet's equator. Beppe Colombo is a very unique spacecraft. It's made up of three component parts, two probes and a propulsion module, which were launched and travelled together in a united configuration known as the Mercury Cruise System. Run by Issa, the Mercury Planetary Orbiter, or MPO, will study the planet's surface and interior, while Jax's Mercury Magnetosphere Orbiter, affectionately known as Mio, will study the planet's suspiciously low magnetic field that has baffled scientists for decades.

The name of Mio was actually chosen by the Japanese general public, It means a waterway, and according to Jaxa, it symbolizes the scientific milestones reached so far, and wishes for safe travels to come. Mio and the MPO sit either side of an interplanetary propulsion unit called the Messenger Transfer Module. Stacking the different parts of the craft together like this means most, but not all of the scientific instruments on board will be inoperative until Beppie Clombo settles into its orbit around Mercury. At that point, the MPO and Mio will be jettisoned from the spacecraft and go their separate ways

to study Mercury from different vantage points. But don't worry, there's still enough data being sent back to keep us entertained. Why don't we take a look back at how far this mission has come, what it's already taught us, and explore some of the amazing things it's seen along the way? Bepi Colombo's first encounter with Mercury came in October 2021. Passing just 200 kilometers above the planet's surface, it sent back some stunning images. As it drifted past, Bepi Colombo snapped Mercury's northern hemisphere. To the average person, it might look like our rocky inert moon.

But the trained eye sees the Calvino crater, which could hold the key to deciphering Mercury's crust composition. The Lamontov crater hosts traces of volcanic activity, and mysterious pockets where an unidentified compound in the planet's crust is being lost to space, and scientists have no idea how or why. These images remind us just how enigmatic Mercury is, and how much we have left to learn about it. Another camera mounted on the spacecraft caught Mercury from another perspective, one that left the planet's volcanic history on full display. Previously, lava flooded and over 250 kilometers wide, the Hayden crater comes into view, along with the Pampu Facula, one of Mercury's many bright spots, probably formed by volcanic

eruptions from billions of years ago. Even though Messenger had already shown as Mercury's volcanic past, it's still pretty amazing that this activity left a mark visible after such an inconceivable amount of time. And Beppe Colombo didn't just show us how the flyby looked. It captured how the flyby felt. You know how your phone has an accelerometer in it that helps it know when and how its orientation changes? Scientists mounted a similar accelerometer on Beppy Colombo's MPO. It measured the movements and vibrations of the spacecraft as it flew past Mick. Mercury, creating a sort of sound map of the flyby.

Scientists then plotted that data to frequencies us humans can actually hear, so you can actually hear the flyby. I'll play it for you now. Pay attention to the two distinct sounds towards the beginning. They represent the changes in solar radiation pressure when the spacecraft enters and then exits Mercury Shadow. The big clunk towards the end is the sound of the sound. of the Phoebus instrument assuming its parking position.

Here's the clip. Pretty cool, right? I've got another one, and it's even better. Pepe Colombo captured magnetic field data during its flyby that was also later deciphered and sonified by scientists on the ground. This one is a little more abstract than the other, but we'll give it a go. You're going to hear two synthesizers at the same time. The first synthesizer changes pitch with the changing magnetism.

of the background magnetic field. A lower pitch means more intensity in the magnetic field. The second synthesizer changes pitch with the variations of the magnetic field magnitude. A more turbulent magnetic field is represented by a faster change in pitch. The dotted vertical line shows where the closest approach was. After the closest approach, you'll hear something that sounds like a DJ scratching a disc. Lots of fast changes in pitch. That right there marks when the spacecraft crosses the magneto sheath, where the magnetic field is highly turbulent.

Let's have a listen. On June 23, 2022, just over halfway in its planned journey, Bebe Colombo crossed paths with Mercury once again. This time, the spacecraft turned its gaze towards Mercury's largest impact crater, Caleris Basin. At a whopping 1,550 kilometers wide, it's actually one of the widest known craters in the solar system. Calarice Basin was likely created by the collision of a comet, asteroid, or other celestial object some 3.9 billion years ago.

The highly reflective lava craters on its floor make it easy to see as the bright semi-circular area of Mercury's disk below the magnetometer boom. The second flyby gave us beautiful images of Mercury's craters like we'd never seen before. Like this one of Hieni, a crater embedded in smooth volcanic plains, potentially near an ancient volcano which Beppe Colombo plans to examine when it finally settles into orbit. Almost exactly a year later, on June 19th, 20203, Beppe Colombo re-approached Mercury from a height of just 263 kilometers. The flyby images captured the 600 km long Beagle rupees, one of the many lobate scarves

on Mercury that was first imaged by Messenger in 2008. Other than our Moon, Mercury is the only planetary body in our solar system known to exhibit this tectonic signature. When the interior of the planetary body cools, it leads to a global contraction and surface expression of a thrust fault, aka a lobate scarb. Beagle Rupee's has an especially pronounced curvature, more so than other scarps on Mercury, offering geologists a rare insight into the planet's tectonic history. As a result of the third Bepi Colombo flyby, a large 218 km-wide peak ring impact crater received the name Manley,

after 20th century visual artist Edna Manley. The original basin floor was flooded by lava, given the crater its visibly smooth interior. texture. Bebi Colombo will explore Manly Crater further from orbit, seeking to measure its carbon composition and detect other minerals that may be present in order to learn more about Mercury's geological history. Gliding past Mercury on its most recent flyby, Bebe Colombo snapped a series of Farewell Mercury images.

At 236 kilometers from the surface, Bebe Colombo appears to hug the planet goodbye. back home on Earth will have to wait another 16 months for the next set of images. Ambition comes in all shapes and sizes. At First Citizens Bank, we roll with your goals because we're built for what you're

building. Fit for your ambition for Citizens Bank. Now these flybys are more than an indulgence for our senses. They also revealed important nuances of the planet's particle environment and magnetic boundaries. One of the instruments aboard the MPO

probe is Phoebus. Probing of Hermion exosphere by ultraviolet spectroscopy. Phoebus uses UV emission from Mercury's exosphere, the planet's outermost and only atmospheric layer to determine its composition, structure and interactions with the surface. It does this by detecting specific types of light emissions in the ultraviolet spectrum, ranging from 55 to 350 nanometers. By capturing these light emissions across the whole spectrum, it helps us figure out what the exosphere is made of and how its particles are spread out. As Bepi Colombo emerged from Mercury's shadow, Fubas detected the presence of hydrogen and calcium, confirming earlier findings from Messenger that established the presence of calcium in the planet's exosphere, particularly around the

equator and on its night side. Interestingly, this is the opposite of the distribution observed for sodium and potassium, which more readily concentrate around the poles. Once in orbit around Mercury, Phoebus will be able to send back much more detailed analysis of Mercury's exosphere, composition, and dynamics, including how it changes with location and time. Speaking of Messenger, back in its day, it discovered ice on Mercury's North Pole. Beppe Colombo will continue this legacy by continuing the search for ice in permanently shadowed regions of high-latitude craters. As you might know, Mercury is the only planet

in our solar system to have a magnetic field similar to Earth's. Thanks to its liquid iron core, Mercury exhibits a dynamo effect, which creates an innate magnetic field around the planet. However, Mercury's magnetic field is just 1.1% that of Earth's magnetic field, leaving scientists fascinated with how it's generated. One of the big goals of the Beppe Colombo mission is understanding this magnetic field better. So far, the only magnetic activity of the Northern Hemisphere has been mapped by NASA's messenger mission. Scientists hope that Bepicolomba will be able to shed light on the dynamics of the planet's

magnetism in the Southern Hemisphere. This magnetic field is of great importance. By understanding it, scientists aim to shed light on Mercury's origins, its evolutionary journey, and the current state of the planet's interior. MPO and MEO will travel through different areas of Mercury's magnetosphere on different timescales, simultaneously measuring how the magnetic field changes over time and space, and its relationship to the powerful solar wind. All this new data from the BepiClombo mission will eventually be compared with global

magnetic field models from the Messenger mission to build the most accurate picture yet of Mercury's magnetic field, which could unlock telling insights into the planet's. elusive past. As we await Beppe Colombo's next flyby in September 2024, the journey to understanding Mercury continues. The spacecraft is gearing up for its next major maneuver. A prolonged solar electric propulsion thruster arc planned to begin in early August, and it should last until mid-September. Thruster arcs, along with strategic flybys, help Epi-Colombo break against the enormous gravitational pole of the sun.

This is the only way to successfully insert into orbit around Mercury when the time comes. Once in orbit in December 2025, the real exploration begins. Mercury's heavily created surface records a 4.6 billion year history of asteroid and comet bombardment, which, together with unique tectonic and volcanic curiosities, will help scientists unlock the secrets off the planet's place in solar system evolution. Baby Colombo is set to remain in orbit around Mercury for one year, and if all goes well, might get the green light for an extra year on the mission. Scientists hope to study the planet like never before, from its composition, to its geophysics,

its atmosphere and magnetosphere, both its history and the history of other inner planets like it, including Earth. Until then, we've got a few more flybys to look forward to, and the haunting pictures they're bound to capture for us earthlings back home. Imagine yourself standing on the surface of Mercury. But not the Mercury you usually picture, not the scorched, sun-blasted daytime inferno. Instead, you're here during the planet's incredibly long night. The sun set weeks ago, and it won't rise for weeks more.

Above you, the stars are stark and brilliant against a perfect black, untouched by atmosphere haze. The ground beneath your heavily insulated boots is rigid, radiating away the day's heat into the vacuum of space. It's a silent, desolate landscape. But tonight you've brought special equipment, not just for survival but for observation. You point a sensitive detector, tuned to a specific wavelength of light towards the anti-solar point in the sky, the direction directly opposite where the sun lurks below the horizon, and you see it. Something faint, ethereal, almost unbelievable. Stretching away from the planet, reaching impossibly far into the blackness, is a ghostly

elongated glow. It's a whisper of light, invisible to the unaided human eye, a delicate structure millions of kilometers long. trailing behind this tiny battered world like the train of a spectral gown. Mercury, the seemingly inert little rock closest to the sun, is doing something deeply strange in the dark. It's venting a tail into space. A tail made not of gas or dust in the way we usually think of comet tails, but of something far more specific and intriguing. Sodium atoms.

I'm Alex McCulligan and you're watching Astrum. Join me today as we delve into one of the solar systems more subtle and surprising phenomena, the vast, invisible sodium tale of Mercury. We'll explore how this bizarre feature was discovered, investigate the complex processes that create and shape it, and understand what this ghostly appendage tells us about the innermost planet and its relationship with the sun. When we think of planets with distinct features, Mercury rarely comes to mind.

It's the runt of our planetary family, a tiny, created world baking in the sun. But in 2001, everything changed. Scientists have been using specialized sodium filters to observe Mercury since the mid-1980s, when it was found in the planet's exosphere. That's exactly what the team at McDonald Observatory in Texas were doing when they spotted something strange, a faint glow extending far beyond Mercury's tiny disc. Initially, they didn't believe their own eyes. They thought it must be the result of an equipment malfunction or data processing errors. But after thoroughly checking and double checking, the astonishing

truth emerged. Mercury was trailing an enormous structure through space. A tail of sodium stretching almost 2.5 million kilometers behind it. For perspective, that's 700 times the width of Mercury itself. The discovery was humbling. Mercury isn't some distant, obscure object, it's one of our closest cosmic neighbors. We've been observing it since ancient times and even visited it in the 1970s and the 2000s thanks to NASA's Mariner 10 and Messenger spacecraft. And yet, this colossal feature remained completely undetected until the 21st century. The discovery was a big deal because it challenged our classification of what can have a tail in our solar system.

Suddenly, Mercury wasn't just a bland, created world, but a planet with a dynamic feature rivaling the most spectacular comets. So, how is Mercury's tale different from comet tales? And why would this planet have a sodium tail specifically? The answer is the fascinating result of Mercury's unique position, composition, and the sun's fierce influence. Comets are kind of like snowballs hurtling through space. They are mainly made of ice, which sublomates as the comet approaches the sun, leaving

behind a trail of ionized gas and dust particles. In contrast, Mercury is a rocky planet with minimal, if any, ice. Scientists think there may be ice hidden in permanently shadowed craters at its poles, but these frozen deposits remain stable precisely because they never receive direct sunlight. So Mercury's tail isn't formed through the sublimation of ice into gas. made of sodium. But why? To understand that, we need to take a look at its proximity to the Sun. At Perihelian, Mercury comes as close as 47 million kilometers from the Sun, where it's exposed to solar radiation up to 10 times more intense than what we receive

on Earth. The solar wind is absolutely torrential. Imagine a constant stream of charged particles flying at 1.6 million kilometers per hour from the sun straight at you. When those particles strike, they physically knock out other atoms on Mercury's surface in a process called sputtering. Picture a bunch of fridge magnets sitting on your fridge. They don't fall on their own because the fridge holds them in place, but then say you throw a bouncy ball at the fridge, when it hits, the energy spreads out and shakes some of the magnets. If enough energy makes it to the weakest magnet, more energy than

is what's keeping it stuck, it will pop off and fall down. That's exactly what's happening in a sputtering collision cascade. Ions from the solar wind shower mercury, creating high energy impacts that dislodge the weakest link atoms from the surface of the planet, just like the bouncy ball dislodge the weakest fridge magnet. It just so happens one of the the most weakly bound elements on Mercury's surface is, you guessed it, sodium. Messenger's data show that during periods of high solar wind activity, these solar wind particles rain down on Mercury's surface and result in a 50% rise in sodium group ions in the planet's atmosphere.

This indicates that stronger solar wind activity enhances surface particle interactions, leading to a greater release of sodium into Mercury's atmosphere and hail. But there is more to the story. In addition to solar wind, Mercury is also tormented by micrometeoroids, and without an atmosphere to slow them down, even dust-sized particles strike the planet with tremendous force. These tiny space rocks vaporize surface materials on impact, releasing even more sodium atoms. We're essentially watching a process of space erosion, a planet that is slowly disintegrating

over billions of years right in our cosmic front yard. Some researchers have noticed enhancements in the tail that seem to correlate with times when Mercury is predicted to pass through denser streams of interplanetary dust, like debris from comet anchor. This would boost the contribution from micrometeoroid impact vaporization, temporarily increasing the amount of sodium injected into the exosphere and subsequently fed into the tail. But the shaping of the sodium tail isn't primarily due to the solar wind blowing it back, as is largely the case for a comet's ion tail.

Instead, the dominant force sculpting Mercury-Sodium tail is solar radiation pressure. This is a subtle but powerful effect. Photons, despite having no mass, carry momentum. When a sodium atom in Mercury's exosphere absorbs a photon of light, it gets a tiny push in the direction the photon was traveling, away from the sun. It then quickly reemits a photon in a random direction, causing negligible recoil on average. However, it's immediately ready to absorb another photon coming from the sun. This continuous process of absorbing solar photons and receiving tiny directed pushes,

effectively acts like a gentle but constant wind, pushing the sodium atoms away from the sun. Imagine countless tiny solar sails, each one a sodium atom being persistently nudged by sunlight. This relentless pressure accelerates the atoms anti-sunward, overcoming Mercury's weak gravity and stretching them out into the vast, elongated tail structure observed from Earth. Because radiation pressure acts directly away from the sun, the tail reliably points in the anti-solar direction. But as I mentioned, despite being so big, it's not actually visible to the naked eye. So how did scientists spot it?

When sodium atoms absorb energy, they become excited, or their outermost electron jumps to a higher energy level. When this electron falls back to its ground state, it emits photons at a very specific wavelength, 589 nanometers. Light at this wavelength has a distinct color you've definitely seen before. It's the color of old street lights. This process is happening constantly in Mercury's tail. However, the tail is hidden from the human eye and from standard telescopes.

To reveal it, astronomers use specially designed filters. called sodium D-line filters that isolate light at precisely 589 nanometers. These filters block all other wavelengths, allowing the faint glow of sodium to stand out against the black background of space. By deleting all other wavelengths of light, scientists were able to see the sodium tail. Mercury's sodium tail varies dramatically throughout its 80-day orbit around the sun, which is highly elliptical. When Mercury reaches perihelion, the increased solar radiation dramatically intensifies the rate

of sodium ejection from its surface. You might expect the tail to reach peak visibility at perihelion, since that's where the solar wind and radiation are strongest. However, it actually reaches peak 16 days later. Let me explain. Since Mercury's orbit is highly elliptical, its orbital speed also varies a lot, from 39 kilometers per second when it's farther from the sun to 59 kilometers per second when it's closer. Through repeated observations, scientists noticed that Mercury reaches its maximal radial velocity

after slingshotting past the sun, exactly 16 days after perihelion. In other words, that's when it's traveling away from the sun at its top speed, creating optimal conditions for the sodium tail to extend and become more observable. But why would higher speed lead to a brighter tail? Two words, Doppler shift. When we look at the spectrum of light from the sun, we see dark lines called Fraunhofer absorption lines, where certain wavelengths of light are being absorbed by elements in the sun's atmosphere before the sunlight is emitted.

The sun's atmosphere contains sodium, just like Mercury's tail, which absorbs all the sodium-specific light before the light travels to Mercury. But the faster Mercury travels away from the sun, the more the sunlight is Doppler shifted or stretched. This means the wavelength gets longer, or redder. The entire sunlight spectrum is shifted, including the Frownhofer lines. So the sodium-specific light is no longer blocked, and is available for the tail to absorb and scatter.

The result? The tail gets brighter, and we can see it from Earth. So if astronomers wanted to spot Mercury's tail, they just need to look up on day 16 past Perihelian, right? Well, it's not that simple. There are three other main factors at play to consider. Firstly, Mercury must be at a sufficient elongation from the Sun in Earth's sky so that it isn't caught in the glare.

Secondly, it can't be behind the Sun from Earth's perspective. And finally, it must be at the perfect angle for Earth's sky. based telescopes to view his anti-sunward direction. This astronomical alignment only occurs during very specific periods roughly every few years. And to think, the scientists at McDonald's Observatory caught it basically by coincidence, talk about perfect timing. Some of the most spectacular images of Mercury's sodium tail have come from observatories utilizing coronagraphs, instruments that block the sun's direct light, allowing the faint glow of the tail to become visible even when Mercury is relatively close to the Sun in our sky.

Each successful observation represents a triumph of astronomical planning and precision. These rare glimpses show a dynamic that changes dramatically with Mercury's position relative to the Sun, stretching, shortening, brightening, and dimming in a cosmic dance governed by orbital mechanics and atomic physics. But this story stretches beyond Mercury itself. It has provided an unexpected window into key processes occurring throughout our solar system and beyond. Firstly, it gives us a glimpse into planetary evolution.

While the current rate of surface material loss is too small to significantly alter Mercury in human timescales, over billions of years, this process could potentially affect its surface composition, gradually depleting sodium and other easily liberated elements. We could use this knowledge to infer what might have happened to older planets in other solar systems and understand their life cycle better. Secondly, and perhaps most excitingly, the discovery has profound implications for exoplanet research. Many star systems contain hot Jupiter's, massive planets orbiting extremely close to their stars.

Mercury can generate a 2.5 million kilometer tail, these giants likely sport even more dramatic features, potentially shedding significant mass through similar mechanisms. The detection methods developed to study Mercury's tail are now being applied in the search for similar phenomena around distant worlds. By looking for the distinctive spectral signatures of elements being stripped from exoplanets, astronomers may gain new insights into their composition and environment. In fact, we have already detected similar sodium tales around our own moon, suggesting this phenomenon may be common around airless bodies exposed to solar wind.

Mercury's sodium tail represents a perfect opportunity for studying planet-star interactions throughout the universe. By better understanding this phenomenon, we gain crucial insights into how stars shape and a row of their planetary systems over billions of years. ESA's Beppe Colombo mission has already completed six rapid flybys of Mercury. It's set to enter into orbit in December 2025, where it will spend at least one Earth year studying Mercury's internal composition and its impressive exosphere. This will provide data on the sodium tail in unprecedented detail, promising new discoveries

about this fascinating phenomenon. The story's elusive tale reminds us that even after centuries of astronomical observation, fundamental discoveries about our closest cosmic neighbors are still being made. What other secrets might our cosmic neighborhoods still be hiding? The universe never stops surprising us. All we need to do is keep looking. Everyone knows the name of the first of the planets in our solar system. Mercury was seen in ancient times and carries the name of a Roman god.

We've known of its existence for thousands of years. It's not hidden in darkness, but thanks to its position next to the sun is bathed in brilliant light. And yet, it is the least explored terrestrial planet, having only been visited by two probes in all of our space-faring history. Why is that? Is it boring? What do we truly know about Mercury?

What are its characteristics and history? And how have we learned about it? I'm Alex McColgan and you're watching Astrom. And in watching this Supercut, you're about to find out. Join with me as I teach you everything you might want to know about the first planet in our solar system, Mercury. When you think about the physical characteristics of Mercury, I'm sure you imagine it being the closest planet to the sun,

but also that it's this giant rock floating in space. You wouldn't be too far wrong with that, but it is much more interesting than what you may first think. For example, when I look at Mercury, I do think of our moon, but Mercury actually is visually more appealing than our moon. Look at it in its true colour. The first thing that I notice is that it actually does have a colour. It's not just different shades of grey.

And what else? Well, did you know, for example, that Mercury consists of approximately 70% of a colour metallic and 30% silicate materials. So it's actually more metallic than rocky. Because of this, Mercury's density is the second highest in the solar system at 5.427 grams per centimetre cubed, only slightly less than the planet with the greatest density, that of Earth, at 5.515 grams per centimeter cubed. If Mercury happened to be the same size as Earth, that would mean it would pretty much have

the same gravitational pull at its surface. But being the size that it is, its surface gravity is only 3.7 meters per second squared. If you were to compare its gravity to Earth, it would look something like this. This means the surface gravity of Mercury is only slightly less than what it is on Mars, and considering that Mars is a much bigger planet, that just says something about the density of Mercury. But before we leave the subject of Mercury's size, I want to show you one last comparison, that of Ganymede and Titan against Mercury. Now, Ganymede is the solar system's biggest moon and also the biggest moon of Jupiter, while Titan is Saturn's biggest moon and the second biggest moon in the solar system.

These two giant moons are bigger than Mercury, as you can see here, but their masses are far less. If you look closely at Mercury's surface, you'll see its appearance is similar to that of our moon. It shows extensive, mare-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. But it obviously was geologically active at one point, because one of the distinctive features of Mercury's surface is the presence of many narrow ridges, extending up to several hundred kilometers in length. We'll talk more about these later. One of the most distinctive things you'll notice about Mercury is this

this huge crater on its surface called Caleris Basin, with a diameter of 1,550 kilometers. The impact that created Caleris Basin was so powerful, it caused lava eruptions and left a concentric ring over two kilometers tall surrounding the impact crater. At the antipode of Caleris Basin is a large region of unusually hilly terrain, known as the weird terrain. If you compare this region to the rest of Mercury, you can see why. it would have this name. So, what's it like on the surface of Mercury? Well, to start with, the surface temperature is hugely different all over.

It can range from minus 173 degrees Celsius to over 400 degrees Celsius. It never rises above minus 93 degrees at the poles, though, because there's no atmosphere retaining the heat. This means that there's quite a big difference between the equator and the poles, but this This variation is also due to its orbit and rotation, which we will get back to later. The sub-solar point reaches about 400 degrees, while on the dark side of the planet, the temperatures are, on average, minus 163 degrees Celsius. Because Mercury is too small and hot for its gravity to retain any significant atmosphere

over long periods of time, it's not able to retain any of the heat it gets from being so close to the sun, which is why the dark side of the heat of the heat of the sun. of the planet is so much colder than the side facing the sun. Mercury, however, does have an exosphere, which is like an extremely thin, atmospheric-like volume surrounding the planet. Molecules in an exosphere are gravitationally bound to a planet, but the density is so low that it can't behave like a gas because the molecules don't collide with each other. In this picture, you can see the messenger probe's view of Mercury's exosphere. When solar wind, The wind hits the planet, it rips off certain atoms out of the exosphere, and what's left

is this trail of atoms going into space. We call this the planet's tail, and every planet has this to a certain extent. Earth even does have an exosphere, but it starts at 600 kilometers above the surface. It's really the point where space and the atmosphere meet. Now, in the case of Mercury, this exosphere is not at all stable. are continuously lost and replenished from a variety of sources, which we'll discuss in more detail later. NASA has been able to confirm that craters at the North Pole of Mercury contain water ice.

Mercury also has something which Mars lacks, an actual magnetosphere, or a magnetic field all around the planet. It is only about 1.1% as strong as Earth's, but it's still strong enough to deflect a lot of the solar wind around the planet. Mercury has the most eccentric orbit of all the planets, with a distance from the Sun ranging from 46 million kilometres to 70 million kilometers. Now this is something a bit hard to imagine, but bear with me. Mercury takes about 88 Earth days to complete an orbit around the Sun.

It also has a 3-2 spin-orbit resonance of the planet's rotation around its axis. This means it spins three times around its axis for every two times that it orbits around the Sun. It takes about 59 Earth days for Mercury to rotate on its axis once, which is what we call a sidereal day. By pure coincidence, this is almost exactly half its synodic period in respect to Earth, which is 116 days. So between conjunctions of Earth and Mercury, Mercury rotate on Earth.

its axis exactly twice. Historically, there was a big problem with that. Because of this coincidence, we believed that Mercury was tidily locked to the Sun for the longest time. You see, Mercury orbits closely around the Sun, meaning it was always tricky for astronomers to get a good look at it for most of its year. When it finally got in a good viewing angle from our perspective, we'd have a look at the face of the planet.

118 days later, we'd have another look during this prime observable. observation alignment, and see the same face again. So to them, it showed that Mercury was tidily locked to the Sun. What astronomers didn't realize is that Mercury had rotated exactly twice on its axis during this time. It wasn't until radar observations of the planet that we found out that it does rotate slightly faster than it orbits. This three-two orbital resonance means that if you were actually standing on Mercury, it

would appear that one day, from sunrise to sunrise, or what is called a solar day, is two Mercurian years. Standing on Mercury, that would look something like this. You would see the sun rise relatively fast, and then as it approaches midday, it slows down and even starts going backwards before continuing on again to sunset. As you can see, that took a whole year, which means a night time on Mercury also takes a year. The sun starts going backwards in the sky because approximately four Earth days before perihelion, the speed in which Mercury travels along its orbit equals the speed in which

it's rotating. At this point, the sun's apparent motion stays stationary. A perihelion itself, Mercury's orbital speed exceeds its rotational speed. So to a person actually standing on Mercury, the sun appears to move backwards. Four days after perihelion, the sun's normal motion resumes. You can see this even clearer from a top-down perspective of Mercury. Twice a day on one of its poles, the sun seems to pause and then continue on again. Something else to notice about Mercury's orbit is that it's inclined by 7 degrees to the

plane of Earth's orbit. As a result of this, we can only see Mercury transit in front of the sun when it's directly between us on Earth and the Sun itself, and because its orbit is inclined by 7 degrees, this only happens about once every 7 Earth years. The last thing we'll discuss about the rotation of Mercury is that its axle tilt is almost zero, with the best measured value as low as 0.027 degrees. This is even smaller than that of Jupiter, which has been measured at 3.1 degrees. And finally, do you want to see Earth from Mercury?

Well, here we are, just a couple of pixels across. This photo was taken from the Messenger probe several years ago, and, barring the newborns, every single one of us was in this picture. But what was Messenger, and why was it important? Well, let's start with a little context. When mankind first started sending spacecraft out to explore the solar system, the first planet To be visited was Venus, our closest neighbor, in 1962. Next was Mars in 1965, and then Jupiter in 1973.

Only then came Mercury in 1974, and already this order might seem a little odd. The closest distance between Earth and Mercury is 77 million kilometers. In fact, it is the closest planet to us on average. The closest distance between Earth and Jupiter is 58 million kilometers, almost 8 times that. And Jupiter was visited again in 1974, twice in 1979, in 1992, in 1992, in 2009, in 1992, in 2007. Multiple missions were launched to Saturn, Uranus, Neptune, to comets and asteroids, While Mercury got nothing for 30 years.

Is this because it was deemed uninteresting? Did we discover everything there was to discover about it with that single first mission? No. The first mission was a flyby and only mapped about 40 to 45% of Mercury's surface. Actually, the real reason is that Mercury is one of the most challenging planets to visit in our entire solar system. Why? Well, as previously mentioned, Mercury exists in a furnace.

Due to its proximity to the sun, its surface temperature reaches highs of 430 degrees Celsius, so any probe visiting it would need to be highly heat resistant. But that same proximity to the sun means that any probe launched towards it will accelerate faster and faster due to the immense gravitational pull from our star. Using rocket fuel against that would be like swimming up white water rapids. Combating the Sun's gravity required too much fuel for a Discovery-class spacecraft to carry. Slowing down the spacecraft enough to be caught up in Mercury's orbit seemed impossible. It was a question of weight.

Weight is a challenging limitation when it comes to spacecraft. The heavier a craft, the larger a rocket needed to get it out of Earth's orbit, and the more expensive everything becomes. Scientists try to keep everything as light as possible to reduce this cost. As fuel takes up precious weight allocations that could go towards scientific instruments, scientists tried to only take what is necessary to help them complete their journey. However, for about 30 years, scientists could think of no way to put enough fuel on a probe to get it to slow down enough to enter Mercury's orbit, especially if they wanted scientific

equipment on board too. So, after the success of Mariner Tens fly-by missions of Mercury in 1974 to 1975, Mercury exploration was put on hold. But in 1985, an orbital mechanics expert named Chen Wan-Yen realized that there was a way of getting a probe into orbit around Mercury that didn't need new technology. Instead, she had worked out a particular route an orbiter could take around the solar system that would slow it down enough to enter Mercury's orbit with only a few course corrections. Rather than going straight to Mercury, the orbiter would need to go a longer way.

How long? Under Chen Wan Yan's model, a craft would orbit the Sun about 15 times, flying past the Earth once, Venus twice, and Mercury three times before finally slowing down enough to enter its orbit on the fourth pass. All these planetary flybys would be essential. By skimming the planet's atmospheres, vital speed could be shaved off from atmospheric drag and due to the gravity of the planets. The entire route would cover a mammoth 7.9 billion kilometers, and would take 6.5

years. Chen Wan Yan's findings were not immediately picked up, but in 1998, NASA began to take an interest in the idea, and after seeing the feasibility of the route, they launched the messenger probe in 2004. Messenger, or the Mercury's surface, space environment, geochemistry, and ranging probe, was only about 1.8 meters long and 1.3 meters wide, and weighed 1,100 kilograms. This is small and light for a typical NASA mission. Just a comparison, Juno is 20 meters long.

Messenger, a ceramic heat shield to protect it from the sun, two solar panels, and a whole suite of scientific equipment for imaging and measuring data from Mercury. Scientists hoped to take advantage of this opportunity to learn as much as they could about the chemical composition of Mercury's surface, its geological history, its magnetic field, and its core, among other things. Messenger spent its first year in space making one orbit around the sun before meeting back up again with Earth. This gave scientists a chance to test its equipment on a known and a new space.

astronomical body to make sure there weren't any errors and to make any adjustments as needed. Messenger took some photos of Earth and the Moon, and also tested its other equipment to take readings of our atmosphere and magnetosphere. Fortunately, everything was working perfectly. As it began to head further Sunward, Messenger employed a clever technique to help reduce its acceleration towards the Sun. It used its solar panels to catch solar radiation, like sails on a ship might catch the much wind.

Solar radiation hitting an object actually pushes it very slightly, while this force is very tiny. Because Messenger's journey was so long, it really added up. Making the most of this phenomenon was one of the ways Messenger saved propellant and decelerated naturally. The next notable landmark in Messenger's journey came in 2006, when it did its first flyby of Venus. Sadly, for scientists, this moment came a moment.

the time when Venus was exactly on the opposite side of the Sun from Earth, which meant Messenger was not in radio contact. It did take some photos of the planet, which it later sent, but otherwise it performed no science. However, in 2007, it passed Venus again. At that time, another spacecraft was orbiting Venus, Issa's Venus Express. Messenger and the Venus Express took the opportunity to work together, performing the first-ever simultaneous measurements of particle and field characteristics of the planet.

But then it was on to the main event, Mercury. Messenger made its first flyby of Mercury on the 14th of January 2008, with everything going smoothly. The same was true of the second flyby. But during the third flyby in 2009, something went wrong. Messenger went into safe mode, which was designed to protect systems on the craft in the event of an error. How disappointing to have come so far, only for the mission to potentially fail during one

of the final stages. Messenger remained in safe mode for what must have been seven hours of stress for all the scientists involved. You see, Messenger had to pass through Mercury's shadow during this flyby, meaning it had to rely on its batteries for 18 minutes. Something wasn't configured right in the power management part of the software. Fortunately, Messenger's computer reset once power from the panels charged the battery, and it was able to continue with its mission, swinging around the Sun one more time before

finally entering orbit around Mercury on the 11th of March 2011. Messenger took up an elliptical orbit around Mercury, alternating between as close as 200 kilometers and as far away as 15,000 kilometers. This is because Mercury acts sort of like a very. a giant sun mirror, radiating heat back into space. Remaining too close to Mercury was too hot for Messenger, even with its heat shield, which was more designed to protect it from the sun, seven times brighter by Mercury than it is on Earth.

So moving further away every 12 hours gave it a chance to cool off. Messenger spent the next four years in Mercury's orbit, far exceeding scientists' hopes and expectations for the mission, as they had originally planned for it to only be able to only be last one year. Before launch, scientists had hoped that Messenger would take at least 1,000 photos over the course of its lifetime. However, Messenger took over 200,000 photographs, giving us a complete map of Mercury's surface in high resolution and colour, as well as photographing nearby comets and other planets. On the 25th of December 2014, Messenger's propellant, so carefully saved up until that point, was finally about to run out.

By this point, Messenger was orbiting a mere 25 kilometres from the surface of the planet. Scientists gave the thrusters one last burst to extend its orbit for as long as possible, but on the 30th of April 2015, Messenger crashed into the surface of Mercury. After a journey that had lasted over a decade and had covered literally billions of kilometres, Messenger's journey had come to an end. Messenger gave us a wealth of insights into Mercury before it died. Onboard Messenger were a host of scientific instruments, including a magnetometer to map out Mercury's magnetic field, which is thought to be generated by a dynamo effect in its molten core.

Our fast rotation and tidal stretching from our moon keeps our core molten. But Mercury doesn't have a moon or a fast rotation. What it does have, however, is an eccentric orbit. more so than any other planet. Gravitational strength increases and decreases as it gets closer and further away from the Sun, so the tidal forces pull and squeeze on the planet, the friction of which keeps Mercury's core hot and the dynamo going. Unlike Earth's, it is offset from the centre by about 20% of the planet's radius, and we don't really know why. Its magnetic field is only about 1% as strong as Earth's, but this still has an impact on the planet's, but this still has an impact on

deflecting a lot of the solar wind around the planet. However, due to it being closer to the sun, the solar wind pressure is a lot greater here than it is around Earth. Add a weak magnetic field to the mix, and the magnetosphere around Mercury is compressed closely to the planet's surface. Earth's, on the other hand, extends many times the diameter of the planet away from the surface. Interestingly, these factors make the magnetosphere of Mercury highly dynamic. What does that entail? Well, for one, reconnection events are 100 times more common around Mercury than around Earth.

Reconnection events occur when magnetic field lines snap together as the charged solar wind pushes against the planet's magnetosphere. When this happens, it allows a few of these charged particles to break into the planet's magnetosphere, entering a region of plasma in the planet's magnetotail. The flows you see in this simulation in the plasma region are from reconnection events. Another feature of the magnetosphere that Messenger detected was energetic bursts of electrons, producing hundreds of thousands of electron volts of energy. As Messenger orbited Mercury, it picked up thousands of these events, and mysteriously,

they were mainly localized in the northern hemisphere, and were compressed towards the planet along the sun-facing side. This is still an ongoing field of study, however, scientists believe these electrons have been accelerated through breakdowns in the magnetotail, and they follow the direction of the magnetic field around from the south pole to the north. Messenger also hosted a wide array of spectrometers. Spectrometers are important for detecting the composition of mineral deposits on the surface without actually having to take a sample.

Spectrometers can also be used to detect the particles in the atmosphere. Now, Mercury doesn't have an atmosphere per se, but as previously mentioned, it has an exosphere, or an extremely tenuous atmosphere. It is so thin that the particles within it don't interact with each other. But what messenger found out about this exosphere's relationship to the surface really surprised scientists? Mercury is covered with volatile substances. It isn't just a fried, rocky planet.

It seems to be covered in potassium, magnesium, sulphur, sodium and chlorine, at a higher level than any other terrestrial planet, and much higher than on our moon. The fact that its volatile ratios have more in common with Mars than with Earth and Venus have completely disproved a lot of solar system formation theories that existed before messenger arrived at Mercury. These volatiles are blasted by radiation from the Sun, more so at the equator than near the poles, which may explain why on the surface, some substances like potassium are more abundant in the northern hemisphere than around the equator.

It is much hotter on Mercury around the equator than the poles, so the potassium there would have been heated enough that much of it has been lost from the surface to the exosphere. Now, the exosphere contains a lot of the particles you would also find on the surface, like sodium, potassium, and the others I mentioned. This exosphere is not at all stable. Solar wind picks up and carries away a lot of charged particles, and solar light pressure also pushes a lot of the neutral particles away. Where it not for the processes that replenish the exosphere, Mercury would lose it all to space over a relatively short time frame.

While most substances certainly do come from the planet's surface, it also contains other elements like hydrogen and helium, which cannot be found there. So where did they come from? Well, as you may know, the sun is made predominantly of hydrogen and helium, and interestingly, the solar wind carries these particles to Mercury. Some of the solar wind actually gets caught up in the exosphere and stays for a while. As far as we know, this is the only major source of hydrogen and helium in the exosphere. In these images, we see calcium, an unknown process of which means it's much more prevalent

in the exosphere during the planet's dawn than dusk. and magnesium streaming away from the night side of the planet. In fact, Mercury's tail has been known about for a while. In these images, sodium ions are lit up as they stream away from the planet, making Mercury look like a comet. Incredibly, if you were to look up into the night sky on Mercury, you would actually see a faint yellow glow, reminiscent of city lights on Earth. This tail is seasonal.

The eccentric orbit of Mercury means that its distance to the Sun very very much. varies throughout its year, and as it orbits, its orbital speed also changes. So the time of greatest sodium emission is actually when Mercury is at its middle distance from the Sun. There was one other curious substance found in Mercury's exosphere that scientists really weren't expecting. Water vapour. This could come from cometry tales as they passed by, or it could come from the ice deposits

messenger detected around the planet's poles. Surprisingly, water ice can exist on this scorched planet, but only at the bottom of permanently shadowed craters, forever protected from directly interacting with the sun's light rays. The Earth-based Ariseboe telescope had already detected highly reflective regions around the poles, and as images from Messenger came in, these regions matched up with regions of permanent shadow at the bottom of large craters. Estimates put the amount of water ice found on Mercury at a quadrillion kilograms. This isn't huge by Earth standards, but it would be a significant boost to any future colony

there to have that much water accessible. There were some other surprising features found on Mercury's surface too. Hollows were found, dispersed all over. This is a unique feature to Mercury. While we aren't completely sure what causes them, they may be with. volatile substances sublimating, and they are unique to Mercury simply due to the proximity of Mercury to the Sun. They seem to be an active geological process, apparently some of the youngest features on the planet, and they are certainly not the result of meteor impacts.

There is much more going on on Mercury's surface. Ancient dried-up lava flows, evidence of volcanic activity, craters from massive asteroid strikes that warmed its surface. And surprisingly, the thin scarps that were evidence of its gradual cooling. These scarps show that Mercury is contracting, and from Messenger's data, Mercury has contracted by over 14 kilometres in diameter since its formation, a lot more than was expected. All these findings have thrilled scientists. Yet even though we would barely know anything about Mercury were it not for Messenger, Somehow this mission isn't that well known among the general public.

Perhaps Issa's Beppe Colombo mission, already on its way to Mercury right now, will better capture the public's imagination when it arrives in 2025. Let's go back to when the planet was warmer. So warm in fact that it becomes necessary to ask an important question. What happens when a planet melts? For Mercury, this is no idle question. At the risk of it being understated, Mercury is a very hot planet. With daytime temperatures reaching an incredible 430 degrees Celsius, the temperature of some wood-burning

fires, the rocks and dust on Mercury's surface bake beneath a blistering heat that pushes them towards their limits. It's not the hottest planet in the solar system, that honour goes to Venus, thanks to its thick atmosphere, but it's certainly up there. The mercury we know today has actually cooled considerably over the years. There is ice at its polar caps, and we discuss the signs on its surface that show it has contracted over time as its interior became colder. So what was it like back then?

When rock is sufficiently heated, its solid structure breaks down and it turns into the gloomy, viscous liquid known as magma, with viscosity or runnyness, 10,000 or 100,000 or 100 100,000 more viscous than water. For a point of reference, this is similar viscosity to tomato ketchup, although I would not recommend putting this on your food. Depending on the rock type, magma forms at temperatures of at least 600 degrees Celsius, but potentially as high as 1,300 degrees Celsius. So, for mercury to have begun to melt, we know that it must have reached at least these temperatures. In spite of being much less runny than water, The lava can still travel for great distances before stopping.

This is because once the surface of lava hardens, it forms an insulating layer that keeps the rest of the lava within protected so it can flow freely. How do we know this happened on Mercury? The clues can be found in craters like Raditladi. Scientists estimate that Raditladi is a relatively young crater, likely under a billion years old, with well-preserved walls and a floor relatively clear of other later impacts. It's large, over 25 kilometers in diameter. Notice how rough the hills are around the crater, and yet inside is a smooth plane.

This is no coincidence. Originally, the terrain inside Raditladi was likely about as rugged as the hills around it. So why is it so smooth now? The answer is lava. When lava is left on its own, it will try to form the flattest surface possible, just like water does if you put it in a bowl, as it is dragged down under the effects of gravity. The same happened here. An asteroid crashed into the planet's surface, and the crater quickly filled with lava.

Once the lava cooled, it formed the smooth plane you see here. But where did this lava come from? There are two theories. The first is that the impact of the meteor triggered a creeping volcanic eruption. as magma from beneath the surface rose up through the cracks to fill the basin. The second explanation is that the surface within the crater got so hot due to the impact of the meteor that it pushed the already hot rock crust over the tipping point into melting.

This kind of lava is known as impact melt. The true explanation is likely a combination of both. Now that we know that smoothness is a sign of lava flow, we suddenly realize that there are numerous other craters on Mercury that similarly must have been filled with lava. Just look at Rustaveli, where crags of mountain can be seen poking up through the smooth lava layer. Or Copland, Polygnottes, or Rachmaninov. Rachmaninov is particularly interesting, as here you can see the strong indicators of

lava bubbling up through from beneath the surface to the center of the crater. Take a look at the strange, crinkled cracks forming a rough circle inside the central crater. Such craters are a signal that a slower outpouring of magma pushed up from beneath the surface, breaking the plane, then pushing up, and then cooling again under the effects of Mercury's fluctuating temperature. Here, and in many of these impact craters, the collisions from space triggered deep volcanic activity from within Mercury's shell. Lava didn't just flow within the craters. Let's look at the valley known as Ancorvalis. Here you can see clear signs of smooth lava flow, but this time moving like a river.

The lava travelled from high to low ground, until it eventually poured into the basin next to it. Flows like these ended up filling massive seas, taking a vast swaths of the planet, and turning them the more orangey colour we see today. Scientists have begun to recognize this telltale orange color as a sure sign of volcanic activity, and from it a more detailed picture has begun to emerge of conditions on early mercury, that make it even less hospitable. Areas like this one, to the northeast of Rachmaninov, are likely formed by volcanic activity.

When Messenger flew over this area in 2015, it took detailed photos of it, and found the surface to be covered in a fine dust. In review, it was obvious what this dust was, volcanic ash, that must have fired out events and covered the terrain around it. NASA scientists likened it to snow, fiery, hot, angry snow. So it wasn't just lava flowing beneath your feet that you'd have to contend with on Mercury, but burning ash falling from the sky. And that was just the calmer volcanoes.

The final indicator of volcanic activity on Mercury hints at eruptions so much. destructive that whole chunks were scooped out of the planet. Take a look at this crater Navoy. This is no impact crater. When a crater is formed onto a hard surface, one that's not sufficiently hot to melt into lava, a central peak is usually formed. This is because when the crater wall suddenly find themselves exposed, gravity suddenly exerts itself on all that loose particulate, which rushes down the walls of the new

scooped out basin towards the center. Once there, having built up momentum, it comes crashing into all the rocks and landslide that is sliding down from the other side of the crater. The two sides meet, and all that momentum and energy forces them to keep moving in the only direction they can, up. You see the same effect more clearly when you throw a large rock into water. The water of the newly formed basin rushes in to fill the gap, but then crashes into water from the other side and all of it shoots upward in a powerful secondary splash.

But unlike water, the sand and loose rock of a crater does not level out, but forms a central peak. Depending on what angle the meteor impacted, this peak is either perfectly rounded or possibly teardrop shaped. However, the ray's central formation of Navoy is neither of these things. As scientists looked at this, they came to the conclusion left, that this crater was not formed by an impact at all. Instead, it had been carved out through the force of an erupting volcano. At 66 kilometers in diameter, the amount of force exploding upwards that would have been necessary to carve out this crater and scatter its remnants for kilometers all around must have been truly massive. So there you have it. Meteors raining from the sky,

tipping the rocks they landed on over the melting point. Volcanoes bursting forth, either filling the landscape slowly with bubbling magma in lakes and fiery rivers, or choking the air with burning ash. Not that there was any air to begin with, beyond the thick toxic gases emitted with the eruptions. And even the ground you could stand on might at any moment explode under your feet. This is what it was like when a planet was melting. Mercury is quiet now.

As near as we can tell, there are not. no longer any active volcanoes on the planet. Although the sun still bakes down on it, the unbridled fury that raged beneath its surface is now calm and soothed. Yet for all those who know how to look, the evidence of what once was is still there, locked in the geological record. It's the scars that tell the story of a violent past. When something is as incredibly difficult to get to as Mercury, it is extremely tricky

to study. which is one of the reasons why in all of human history there have only ever been two missions to Mercury, with just one more on the way. Mariner 10 in 1974, Messenger in 2011, and Bepi Colombo, due to arrive in 2025. Of these prior two missions, only Messenger went into orbit around Mercury, and so it is the only mission to ever give us close-up shots of Mercury's surface. And crazily enough, some of the formations we've seen on the surface are still unsolved mysteries even more than a decade on, while other formations give us hints at the raw primal power of the

early solar system. So let's finish by taking a look at some of those mysteries and formations and see what answers we can find. When you look at the surface of Mercury, there are a few features that immediately jump out at you. First, it's color. Mercury's color is not actually monochrome and is smatted with speckled grays, creams and beages with lighter sections and lines. These darker sections are believed to indicate high levels of graphite, the same material used in pencils, and the lighter sections, well, we'll get onto them later. Beyond that, you most likely noticed the craters.

Much like the moon, Mercury is covered with craters, as ancient pieces of space debris crash down on the unprotected planet with a roughly even distribution. These offer fascinating insights into the planet's violent history. You can get a sense for how old crater likely is by how sharp its crater rims are. Sharp and crisp rims are likely a lot more recent than the older, rougher rims that have had more time to erode down due to the natural processes happening on the planet. Sometimes asteroids strike within the same place as older collisions, creating overlapping craters of differing ages such as the craters here.

But it is in the difference between these older and younger craters that we get our first fascinating clue about the surface of Mercury. It is an active, flowing place. Although there is no real atmosphere to produce the weathering we would imagine, evidently things on the planet's surface do not remain static on an astronomical time scale. As previously mentioned, Mercury is cooling, and as it does so, its surface bunches together in kilometre long scarps. But Mercury's surface is not just crumpling, it is also smoothing out.

In this crater there is evidence of slumping having taken place. While about 90 degrees of the crater wall has retained its shape, the remaining 270 degrees has slipped further into the crater bottom, breaking away from the rest of the rim under the force of its own weight. Scientists are not entirely sure why this happened to only some of the crater and not all of it. Is the soil particularly hard in the bottom right corner? Was it something to do with the angle the impact is struck at?

We don't really know. And that's one of the things that is so intriguing about Mercury. There is still so much more to discover. Here's another interesting phenomenon. In my videos about the moon, I mentioned crater rays or ray systems. These spidery lines that radiate out from certain craters are a prominent path of Mercury surface as well, with some stretching over 400 kilometers across. A lighter colour is a sign that the material kicked up from under Mercury's surface is less

graphite-rich, or at least is a different chemical composition to the sun-exposed surface. But did you know that for a while, scientists could not account for how these spidery limes were formed? When they tested different weights, consistencies of terrain, and speeds of impact, they were unable to recreate these patterns in lab conditions. Whatever they tried, the material they kicked up would always return back down in a consistent circle, not thin spider-web lines. Scientists racked their brains for years, but now it seems that this mystery has been solved.

In 2018, a scientist called Tapan Saboala was scouring the internet, and discovered that a group of students had managed to recreate the spidery line pattern of crater rays. Sabuwala was excited, but also confused. Why were these students able to manage what other scientists had not? Interestingly, he realized that this was an example of scientists being too neat. Before performing their tests in lab conditions, Saboala and other researchers had always prepped the experiment by smoothing out the sand their test asteroid was impacting into.

The students had not done this step, leaving the test surface rough. This made sense in hindsight, as it more closely mimics the rough terrain on the surface of an alien planet. And as it turns out, this was the entire key to know how these rays were formed. Crater rays do not care about the speed of the impact, the angle, or the composition of the crust. They only care about the surface shape and how rough it is. Knowing that this is how these lines are formed, it really opens your eyes to the scope of some of the impacts that have struck Mercury in its past.

Remember I mentioned how some of these ray systems stretch over 400 kilometers? That was just the smaller ones. Look at the ray system originating from the crater known as Hokusai. These rays must have been created from an incredible impact, as their lines stretch almost entirely around Mercury's surface, which, by the way, has a circumference of 15,000 kilometers. And although not quite as large, the ray system of the crater Debussy covered over 1,000 kilometers. While the moon also has ray systems, they are usually smaller than these. In fact, one of the main visual distinguishing aspects of Mercury are these giant ray systems.

These show us that one of the formative processes that explain Mercury's unique surface is incredibly powerful bombardments. Seeing as the sun is so nearby, objects caught in this intense gravitational pull would crash into Mercury with far more force than Mercury could produce with its own gravity. Mercury's gravitational pull is so weak compared to the Sun that Mercury cannot normally capture objects as moons. They get pulled past instead. This is one of the reasons why visiting Mercury is so difficult for spacecraft. But that's not to say that Mercury can't stop such an object reaching the Sun, it just has to body block it. Yet, this explanation cannot explain this

last formation. Take a look at one of the most fascinating craters on Mercury, Apollodorus and its surrounding pantheon fossae. At first glance, you might think there is nothing unusual about the crater Apollodorus. Yes, those fractures running out from the centre are a little unusual. There are a few features that are odd here. To begin with, the fractures and the surrounding radial fractures bear strange resemblance to fracture glass. Glass fractures in this way due to its hard but brittle qualities. As you have seen, most other craters we have seen on Mercury do not follow this pattern. The crust of Mercury does not tend to fracture, but instead sprays in ray systems, or

just leaves perfectly round craters. This is in keeping with a loose material makeup. Sand does not fracture when hit. We do not see this fracturing anywhere else on the planet either, so something unusual is clearly happening here. Was the surface of Mercury particularly cold and hard when this impact occurred, thus making it more brittle? Mercury's nights can get as cold as minus 180 degrees Celsius.

But if that was so, why has this not happened in other places? About half the impact should be hitting Mercury's night side, at least. When we take a closer look at Apollodorus, the crater you might assume is the cause of the fractures, We noticed something even stranger. Apollodorus is not quite the epicenter. While it's pretty close, it doesn't actually line up. This might suggest that Apollodorus and the spidery fractures of the pantheon fosci are actually unrelated.

Whatever cause this phenomenon may have happened only to be later hit by an asteroid near to but not on its epicenter. But if that's so, what cause pantheon fosci? The intriguing thing is that we don't know. Evidently, something created this fractured glass shape, but left no crater. Might that imply that this is the result of not something hitting it from above, but pushing its way up from beneath? Perhaps this is the result of immense volcanic activity, suddenly pressing up and cracking

the crust. That's just my guess. There are still many mysteries to be found out on Mercury's surface, and many other fascinating insights to be gleaned as science advances. When Issa's Bepi Colombo arrives at Mercury in 2025, it will begin another extensive study of the planet, and perhaps then we will have the answers. Bepi Colombo will uncover the characteristics of Mercury's magnetosphere and exosphere, and will take a clearer look at its geology and composition.

But until then, scientists will continue to pour over the data we have. For now, Mercury and Jaws, baked in its solar furnace. It has survived there for millions of years, and will likely survive for millions of years, in spite of all that the Sun and the Solar System have to throw at it. The hellish conditions of its environment make it challenging to get to, but there is no denying its resilience, shown in its charred, crater beauty. Perhaps one day we will know all there is to know about the first of the Solar System's planets, but that day is not yet. In spite of it being the most illuminated planet in the

solar system, thanks to its location, there is still plenty of light to shed on Mercury. Mercury The solar system's closest planet to the Sun. Everything I'll show you today will be an actual picture or video image of Mercury from the messenger probe. We'll discuss Mercury's orbit and rotation, its physical characteristics, its surface conditions, and the magnetic field and magnetosphere of the planet. I'm Alex McColgan, and you're watching Astrum. Stick with me on this video, and you will learn almost everything you could want to know

about this tiny, yet fascinating planet. Now, when you think about the physical characteristics of Mercury, I'm sure you imagine it being the closest planet to the sun, but also that it's this giant rock floating in space. You wouldn't be too far wrong with that, but it is much more interesting than what you may first think. For example, when I look at Mercury, I do think of our moon, but Mercury actually is visually more appealing than our moon.

Look at it in its true colour. The first thing that I notice is that it actually does have a colour. It's not just different shades of grey. And what else? Well, did you know, for example, that Mercury consists of approximate colour? 70% metallic and 30% silicate materials. So it's actually more metallic than rocky. Because of this, Mercury's density is the second highest in the solar system at 5.427 grams

per centimetre cubed, only slightly less than the planet with the greatest density, that of Earth, at 5.515 grams per centimeter cubed. If Mercury happened to be the same size as Earth, that would mean it would pretty much have the same gravitational pull at its surface. But being the size that it is, its surface gravity is only 3.7 meters per second squared. If you were to compare its gravity to Earth, it would look something like this. This means the surface gravity of Mercury is only slightly less than what it is on Mars, and considering that Mars is a much bigger planet, that just says something about the density of Mercury. But before we leave the subject of Mercury's size,

I want to show you one last comparison, that of Ganymede and Titan against Mercury. Now, Ganymede is the solar system's biggest moon and also the biggest moon of Jupiter, while Titan is Saturn's biggest moon and the second biggest moon in the solar system. These two giant moons are bigger than Mercury, as you can see here, but their masses are far less. If you look closely at Mercury's surface, you'll see its appearance is similar to that of our moon. It shows extensive, mare-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. But it obviously was geologically active at one point,

because one of the distinctive features of Mercury's surface is the presence of many narrow ridges, extending up to several hundred kilometers in length. It's believed that these were formed as Mercury's core and mantle cooled and contracted over time, when the crust had already solidified. And one of the most distinctive things you'll notice about Mercury is this huge crater on its surface called Caleris Basin, with a diameter of 1,550 kilometers. The impact that created Caleris Basin was so powerful, it caused lava eruptions and left a concentric ring over two kilometers tall surrounding the impact crater. At the antipode of Caleris Basin is a large region of unusually hilly terrain, known as the weird terrain. If you compare this region to the rest

of Mercury, you can see why it would have this name. So, what's it like on the surface of Mercury? Well, to start with, the surface temperature is hugely different all over. It can range from minus 173 degrees Celsius to over 400 degrees Celsius. It never rises above minus 93 degrees degrees at the poles, though, because there's no atmosphere retaining the heat. This means that there's quite a big difference between the equator and the poles, but this variation is also due to its orbit and rotation, which we will get back to later. The sub-solar point reaches about 400 degrees, while on the dark side of the planet, the temperatures are, on average, minus 163 degrees Celsius. Because Mercury is too small and hot for its gravity to retain any sense,

significant atmosphere over long periods of time, it's not able to retain any of the heat it gets from being so close to the sun, which is why the dark side of the planet is so much colder than the side facing the sun. Mercury, however, does have an exosphere, which is like an extremely thin, atmospheric-like volume surrounding the planet. Molecules in an exosphere are gravitationally bound to a planet, but the density is so low that it can't behave like a gas, because the the molecules don't collide with each other. In this picture, you can see the messenger probe's view of Mercury's exosphere. When solar wind hits the planet, it rips off certain atoms out of the exosphere, and

what's left is this trail of atoms going into space. We call this the planet's tail, and every planet has this to a certain extent. Earth even does have an exosphere, but it starts at 600 kilometres above the surface. It's really the point where space and the atmosphere meet. Now, in the case of Mercury, this exosphere is not at all stable. Atoms are continuously lost and replenished from a variety of sources. NASA has been able to confirm that craters at the North Pole of Mercury contain water ice.

Mercury also has something which Mars lacks, an actual magnetosphere, or a magnetic field all around the planet. It is only about 1.1% as strong as Earth's, but it's still strong enough to deflect a lot of the solar wind around the planet. Now we're going to get to one of the things I find the most interesting about Mercury, its orbit and its rotation. Mercury has the most eccentric orbit of all the planets, with a distance from the Sun ranging from 46 million kilometers to 70 million kilometers.

Now, this is something a bit hard to imagine, but bear with me. Mercury takes about 88 Earth days to complete an orbit around the Sun. It also has a 3-2 spin-orbit resonance of the planet's rotation around its axis. This means it spins three times around its axis for every two times that it orbits around the Sun. So although it takes about 59 Earth days for Mercury to rotate on its axis once, which is what we call sidereal day, this 3-2 orbital resonance means that if you were actually standing on Mercury, it would appear that one day, from sunrise to sunrise, or what is called a solar

day, is two Mercurian years. Standing on Mercury, that would look something like this. You would see the sun rise relatively fast, and then as it approaches midday, it slows down and even starts going backwards before continuing on again to sunset. As you can see, that took a whole year, which means a night time on Mercury also takes a year. The sun starts going backwards in the sky because approximately four Earth days before perihelion, the speed in which Mercury travels along its orbit

equals the speed in which it's rotating. At this point, the sun's apparent motion stays stationary. A perihelion itself, Mercury's orbital speed exceeds its rotational speed. So to a person actually standing on Mercury, the Sun appears to move backwards. Four days after Perihelian, the Sun's normal motion resumes. You can see this even clearer from a top-down perspective of Mercury. Twice a day on one of its poles, the Sun seems to pause and then continue on again. Something else to notice about Mercury's orbit is that it's inclined by 7 degrees to the

plane of Earth's orbit. As a result of this, we can only see Mercury transit in front of the Sun when it's directly between us on Earth and the Sun itself, and because its orbit is inclined by 7 degrees, this only happens about once every 7 Earth years. The last thing we'll discuss about the rotation of Mercury is that its axle tilt is almost zero, with the best measured value as low as zero. 0.027 degrees. This is even smaller than that of Jupiter, which has been measured at 3.1 degrees. And finally, do you want to see Earth from Mercury? Well, here we are. Just a couple of pixels

across. This photo was taken from the messenger probe several years ago, and, barring the newborns, every single one of us was in this picture. Mercury's orbit and rotation produces a bizarre phenomenon. This time lapse shows Mercury orbiting the Sun, as it does so it spins like you would expect. But look carefully and you'll notice that every so often the spinning seems to stop before carrying on again. From a ground perspective, it appears like this. What's going on here? Well, sunrise to sunrise on Mercury, or what is called a solar day, last longer than a Mercurian year. In fact, there are two Maccurian years for every one solar day. Simply put, It rotates incredibly slowly.

Mercury's orbit is also pretty elliptical, and it travels faster at this point in its orbit compared to this part. During this fastest point, Mercury's angular orbital velocity increases so much it actually overcomes its rotational velocity, making the sun appear to pause and even go backwards a bit in the sky before carrying on like normal. Amazing, huh? Check the pin comment for more. Messenger was a triumph of a mission.

It was designed to last one year in orbit around Mercury, but ended up lasting four, from 2011 to 2015. During those four years, it took over 200,000 images of the planet, it scanned the surface for various minerals, and investigated its bizarre magnetic field. Its findings have been critical to understanding Mercury, because the only other spacecraft to even get close to the planet was Marina 10 back in 1975. Without Messenger, we wouldn't even know what the entirety of the planet looks like. I'm Alex McColgan and you're watching Astrum.

Join me today as we investigate the science collected by the Messenger probe, the things that surprise NASA scientists, and look at some of the most impressive features on Mercury's surface. One of the fascinating things about Mercury's orbit and rotation is that while it isn't tidily locked to the Sun, it is locked in a rare three-three, three. to spin orbit resonance, meaning it rotates three times for every two orbits around the Sun. This means that its sidereal day, all the time it takes to do a full rotation on its axis, takes 59 days. By pure coincidence, this is almost exactly half its synodic period in respect

to Earth, which is 116 days. So between conjunctions of Earth and Mercury, Mercury rotates on its axis exactly twice. Historically, there was a big problem with that. Because of this coincidence, we believed that Mercury was tidily locked to the Sun for the longest time. You see, Mercury orbits closely around the Sun, meaning it was always tricky for astronomers to get a good look at it for most of its year. When it finally got in a good viewing angle from our perspective, we'd have a look at the

face of the planet. 118 days later, we'd have another look during this prime observation alignment, and see the same face again. So to them, it showed that Mercury was tidily locked to the Sun. What astronomers didn't realize is that Mercury had rotated exactly twice on its axis during this time. It wasn't until radar observations of the planet that we found out that it does rotate slightly faster than it orbits. Marina 10 had a similar problem. While it did give us great images of the planet, its orbital period was almost exactly twice that of Mercury's, meaning that even though So Marina 10 did three flybys of Mercury, the same side of the planet was always sunlit

every time it passed by. In order to map the full surface in detail, we needed an orbiter, something that could follow Mercury as it went through its day-night cycle while it travelled around the Sun. As my previous video showed, this was harder than it sounds. However, in 2011, Messenger successfully overcame those issues and entered Mercury's orbit. On board messenger were a host of scientific instruments, including a magnetometer to map out Mercury's magnetic field. Unlike Venus and Mars, Mercury has a significant magnetic field originating from its core. Like Earth's, it is likely generated by a dynamo effect in its molten core.

Our fast rotation and tidal stretching from our moon keeps our core molten. But Mercury doesn't have a moon or a fast rotation. What it does have, however, is an eccentric orbit, more so than any other planet. Gravitational strength increases and decreases as it gets closer and further away from the sun, so the tidal forces pull and squeeze on the planet, the friction of which keeps Mercury's core hot and the dynamo going. Unlike Earth's, it is offset from the center by about 20% of the planet's radius, and we don't really know why.

This magnetic field is only about 1% as strong as Earth's, but this still has an impact on deflecting a lot of the solar wind around the planet. However, due to it being closer to the Sun, the solar wind pressure is a lot greater here than it is around Earth. Add a weak magnetic field to the mix, and the magnetosphere around Mercury is compressed closely to the planet's surface. Earth's, on the other hand, extends many times the diameter of the planet away from the surface. Interestingly, these factors make the magnetosphere of Mercury highly dynamic.

What does that entail? Well, for one, reconnection events are 100 times more common around Mercury than around Earth. Reconnection events occur when magnetic field lines snap together as the charged solar wind pushes against the planet's magnetosphere. When this happens, it allows a few of these charged particles to break into the planet's magnetosphere, entering a region of plasma in the planet's magnetotosphere. The flows you see in this simulation in the plasma region are from reconnection events. Another feature of the magnetosphere that Messenger detected was energetic bursts of electrons,

producing hundreds of thousands of electron volts of energy. As Messenger orbited Mercury, it picked up thousands of these events, and mysteriously, they were mainly localized in the northern hemisphere, and were compressed towards the planet along the sun-facing side. This is still an ongoing field of study, however, scientists believe these electrons have been accelerated through breakdowns in the magneto tail, and they follow the direction of the magnetic field around from the south pole to the north. Messenger also hosted a wide array of spectrometers.

Spectrometers are important for detecting the composition of mineral deposits on the surface without actually having to take a sample. Spectrometers can also be used to detect the particles in the atmosphere. Now, Mercury doesn't have an atmosphere per se, rather an exosphere, or an extremely tenuous atmosphere. It is so thin that the particles within it don't interact with each other. But what messenger found out about this exosphere's relationship to the surface really surprised scientists?

Mercury is covered with volatile substances. It isn't just a fried, rocky planet. It seems to be covered in potassium, magnesium, sulfur, sodium and chloro. Thorine, at a higher level than any other terrestrial planet, and much higher than on our moon. The fact that its volatile ratios have more in common with Mars than with Earth and Venus have completely disproved a lot of solar system formation theories that existed before messenger arrived at Mercury.

These volatiles are blasted by radiation from the Sun, more so at the equator than near the poles, which may explain why on the surface, some substances like potassium, are more abundant in the northern hemisphere than around the equator. It is much hotter on Mercury around the equator than the poles, so the potassium there would have been heated enough that much of it has been lost from the surface to the exosphere. Now, the exosphere contains a lot of the particles you would also find on the surface, like sodium, potassium, and the others I mentioned. This exosphere is not at all stable. Solar wind picks up and carries away a lot of charged particles, and solar light pressure also pushes

a lot of the neutral particles away. Were it not for the processes that replenish the exosphere, Mercury would lose it all to space over a relatively short time frame. While most substances certainly do come from the planet's surface, it also contains other elements like hydrogen and helium, which cannot be found there. So where did they come from? Well, as you may know, the Sun is made predominantly of hydrogen and helium, and interestingly, the solar wind carries these particles to Mercury.

Some of the solar wind actually gets caught up in the exosphere and stays for a while. As far as we know, this is the only major source of hydrogen and helium in the exosphere. In these images, we see calcium, an unknown process of which means it's much more prevalent in the exosphere during the planet's dawn than dusk, and magnesium streaming away from the night side of the planet. In fact, Mercury's tale has been known about for a while. In these images, sodium ions are lit up as they stream away from the planet, making Mercury look like a comet.

Incredibly, if you were to look up into the night sky on Mercury, you would actually see a faint yellow glow, reminiscent of city lights on Earth. This tail is seasonal. The eccentric orbit of Mercury means that its distance to the Sun varies throughout its year, and as it orbits, its orbital speed also changes. So, the time of greatest sodium image. mission is actually when Mercury is at its middle distance from the Sun. There was one other curious substance found in Mercury's exosphere that scientists really

weren't expecting. Water vapour. This could come from cometry tales as they pass by, or it could come from the ice deposits messenger detected around the planet's poles. Surprisingly, water ice can exist on this scorched planet, but only at the bottom of permanently shadowed craters, forever protected from directly interacting with the sun's light rays. The Earth-based Aricebo Radio Telescope had already detected highly reflective regions around

the poles, and as images from Messenger came in, these regions matched up with regions of permanent shadow at the bottom of large craters. Estimates put the amount of water ice found on Mercury at a quadrillion kilograms. This isn't huge by Earth standards, but it would be a substantial. significant boost to any future colony there to have that much water accessible. There were some other surprising features found on Mercury's surface too. Hollows were found, dispersed all over. This is a unique feature to Mercury.

While we aren't completely sure what causes them, they may be volatile substances sublimating, and they are unique to Mercury simply due to the proximity of Mercury to the sun. They seem to be an active geological process, apparently, some of the same. of the youngest features on the planet, and they are certainly not the result of meteor impacts. Other young features include evidence of volcanic deposits. Look how this crater appears to be completely filled in by this volcanic flow. Mercury certainly isn't volcanically active today, but billions of years ago it may have hosted numerous volcanoes across the surface. We see evidence of shield and compound volcanoes that were active in the past, with at least nine vents spotted in

Mercury's most famous surface feature, Caleris Basin. Caleris Basin is a huge impact crater, one of the largest in the solar system at 1,600 kilometers in diameter. A 100 km-wide impactor likely caused this crater, creating a global event that would have changed the very nature of Mercury at the time. The scars of it remain. There is a 2 km tall mountain range surrounding the rim, and radial troughs coming away from the centre.

Further away from the centre, these troughs turn into concentric rings. The reason for this is not known. At the antipode of the impact, or the opposite side of the planet, is found a region of weird terrain, likely formed when shockwaves from the collision converged. Here the terrain is hilly and lineated, unlike much of the Mercurian surface. The last thing I'll discuss today are these long scarves, evidence that Mercury is cooling. That may be a surprise to you, considering it's getting blasted by the heat of the sun. However, overall, Mercury still loses more heat than it gains from the sun.

These scarps show that Mercury is contracting, and from Messenger's data, Mercury has contracted by over 14 kilometres in diameter since its formation, a lot more than was expected. All these findings have thrilled scientists, yet even though we would barely know anything about Mercury, were it not for Messenger, somehow this mission isn't that well known among the general public. Perhaps Issa's Beppe Colombo mission, already on its way to Mercury right now, will better capture the public's imagination when it arrives in 2025. In any case, personally, I really loved the Messenger mission, simply because it brought Mercury to life in my eyes.

Thanks for watching. I was honestly blown away by the numbers of you that signed up to the Patreon. Like I said in the replies to your DMs on Patreon, everyone here at the Astrum team is so grateful to have such an amazing community. If you haven't joined the Patreon party yet, we're still on our long-term thousand patron member drive, so you can go to the link in the pin comment to become a part of that effort. When you join, you'll be able to watch the whole video ad-free, see your name in the credits, and submit questions to our team.

Meanwhile, click the link to this playlist for more Astrum content. I'll see you next time. Thank you.