Short Wave - An icy mystery: What are lake stars?

Episode Date: March 13, 2026

When producer Berly McCoy was out on her local frozen lake, she saw something she'd never seen before. There were dark spidery, star-shaped patterns in the ice and they freaked her out. So, we called an expert to find out more about them. In today’s episode, geophysicist Victor Tsai tells us about lake stars and how he became the first person to scientifically prove how they form. Plus, he explains how knowing more about lake stars can potentially give us clues about the presence of water on Europa, one of Jupiter’s icy moons.Read Victor Tsai’s full paper on lake stars here.Have a question about something in the environment? Email us at [e-mail] to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave. See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. Hey, Sherwaiver is Regina Barber here. Recently, our producer Burley McCoy was out on a frozen lake near her home. It's been a weird winter, so I was already kind of worried about the ice safety. Don't worry, Burley knows about being safe on the ice. But then she saw something that freaked her out. There were these star-looking kind of spidery patterns in the ice. They were black and branching, and they kind of just,

made me nervous because I spent a lot of time on the ice and I hadn't seen them before and I didn't know what they were. Lucky for Burley, she works on a science podcast. So we called up an expert to ask about this icy phenomenon. They're actually quite common, but you need a particular set of circumstances to happen. Victor Sci is a geophysicist at Brown University. He says the thing Burley was seen is called a lake star. He studied them as a visiting graduate student at Woods Hole Oceanographic Institution. There, his advisor gave him a task. Figure out the science behind Lake Stars. Victor did by creating them in the lab.

And here's the cool thing. The research eventually led Victor somewhere unexpected. A water world in space. Today on the show, Lake Stars, how they're formed, and why studying them may help us understand where the water is on Europa, one of Jupiter's largest moons. We'll also tell you, and Burley, what Lake Stars might say about the safety of the ice.

You're listening to Shortwave, the science podcast from NPR. Let's start with a little Lake Star Science 101 with geophysicist Victor Sci. The basic idea is just that there's a thin layer of ice with some snow on top of it. The snow pushes down on the ice. At the same time, if a small hole forms on the underside of that ice... That allows warm lake water to seep up through that hole. and then melt through this slushy snow layer and eventually form these star patterns. When all these things happen, you get a lake star.

But Victor wanted to actually figure out the math and physics behind this process. In our conversation, Victor told me field work wasn't possible. It was summer when he was working at Woods Hole. So what we decided to do was to go into a cold lab and create our own miniature lake stars by forming slushy ice on top of a cold surface, and then pour relatively warm, just barely above freezing water through that slushy ice. So how did you make this slush?

So the lab didn't have a way of producing this slushy, snowy layer. And so I purchased a blender, and one of the challenges is trying to make a slushy snow that's similar to what you can find in nature. But I found that playing around with different blender settings and things like that, I could make a slush that was similar to what we observe. Wow, that is really, really cool. So you had, like, different kinds of slushies. Did you actually make, like, drink slushies and, like, drink them and stuff?

I did. Although not at the same time. Because, you know, it's very cold in the cold lab. Yeah, you don't want it in there. But it was the summer, you know. It was. So you like basically have this like flat surface and you put this fun slushy stuff from your blender on top. And then you said you drip water.

How fast do you have to drip it? Like how much water is going in? And then what do you see after that happens? Yeah, we have to drip the water relatively slowly so that it's a sort of constant flow that doesn't like a leaky faucet. Yeah, like a leaky faucet so that it doesn't catastrophically break up this slushy layer. And then basically what you observe is that first you start with a little drip, and so you have a sort of circular melt pattern. But then as it spreads out, you start to get these star-like patterns forming with these arms where you are melted. Let's say you wanted to do this at home.

What could people do to try to make their own lake stars? Yeah, you can certainly make your own. You can do exactly what I did. Take a kitchen blender, form some slush, place. it on a flat surface and then try to pour some slightly warm water through that slush pattern. Slightly warm, like room temperature. Actually, it's better if it's even below room temperature so that it happens more slowly. Otherwise, it can be too catastrophic.

Okay. And then we've got to do this kind of quick, right? Assuming people don't have a cold room, they're just in like room temperature. How fast do they have to drip that water? You do have to drip the water somewhat slowly and carefully to give you a sort of constant flow rate. Otherwise, if you just like poured the whole bucket of warm water, you would sort of destroy the slush. Right. Yeah.

So you're dripping it like a slow drip of like a faucet, like what you're supposed to do. So your pipes don't freeze in the winter. What are they going to see in those few minutes? Yeah. Within a few minutes, you should see these star-pruders. patterns forming. So you'll see some branching of the of the melt patterns. So originally, you might start with something kind of circular. And then after the drops are. Yeah, exactly. Exactly where you dropped it. You'll see a little circular melt pattern. And then as it expands,

so as more of the ice starts to get melted, you should see these branches form. And the branches form because the water likes to channelize. So if there were no diffusion of heat, It would basically just channelize into one single channel. And you would have all of the water flowing through this bigger channel. Yeah, it wouldn't be a star. It'd be a snake. Exactly. It would be a single snake. Yeah. I want to talk about how all of this is related to one of my favorite subjects, Jupiter's icy moon Europa.

When I was a kid, I was completely obsessed with Europa. And you recently published a paper with a group of scientists that related these lake stars to a formation on Europa. First of all, can you describe that formation? What did it look like? Sure. So there's a formation on Europa that I think was previously called a spider pattern, but it's basically very similar to what we've been talking about,

a star-like pattern. So cool. And as you mentioned already, the surface of Europa is icy. And so the question was, what caused this star-like or spider-like pattern to form in this icy, on the surface of this icy body? How big is this spider, like, on Europa? Like, if you could compare it to, like, any sort of size on Earth? One to two kilometers.

That's like a mile. Yeah, yeah, it could be, yeah, a mile wide. Wow. That's a very big lake star. It is a very big lake star. On Earth, we basically never get such big lake stars. The biggest ones that I've seen are maybe 30 feet across. So what did you learn from this work of like kind of studying this spider, mile-long lake star on Europa?

Yeah. Well, my co-authors and I found that this same theory that we could use to explain lake stars on Earth could approximately make a feature that looks like what's observed on Europa. So I wouldn't say that we know for sure that it's because of the same physics, but at least it's a good possibility. Yeah. Did you simulate something in the lab to try to get something that look like this spider on Europa? Yeah. One of my co-authors did try to do this in the lab. And it's actually a very similar experiment to what I had done for the lake stars. More slushies. Yeah, exactly. Yeah. So how can comparing these features on Earth and Europa, how can that tell us more about the moon, Europa, and its ice?

Yeah. Well, one of the key questions for Europa is where there might be water, liquid water, and how close to the surface that could be. And so some of the models have that liquid water being more than 40 kilometers from the surface. and some of the models have it being much closer to the surface. Right. And so if this star pattern really was formed by melted liquid water, then it really suggests that at least at some time in Europa's history, you had to have water close to the surface. Yeah.

So it doesn't exactly tell you that today it has to be close to the surface because we don't know exactly when this star pattern or spider-fews. future formed. For like somebody who doesn't know anything about planets, like, why does this matter? One reason that it's very important to know is in terms of any future mission that we send to Europa, in terms of how likely would it be successful in getting to that water layer, right? Oh, to drill? Yeah. So if we really want, if we really want to have a better chance of knowing whether or not there is life in that water down there, the easiest way to do it is to really drill there and sample it and see what's there. If it's 40 kilometers thick, that ain't happening.

No, yeah. But yeah, if it's right near the surface, then yeah, maybe we can get in there. Right. We can put probes. Dang, that's so cool. Okay. Okay, coming back to like Lake Stars on Earth, Let's, let's, we were in space for so long. Let's come back to Earth. I know a lot of people actually live around frozen lakes, like one of our producers, Burley. Does the presence of lake stars, like you see them, you maybe could walk on top of them, does it indicate that it might not be safe to, to like walk over those lake stars? Yeah. Or in the ice in the area.

Right. So if a lake star had recently formed, then it would be very nice. safe. Because that means that the ice was very thin and then there's mostly snow and slush on top of that and it probably would not be able to support your weight. So if it had just formed, then you can still see the kind of snow on top. But in many cases, you get very thick ice forming afterwards that preserves the lake stars. And so actually most of the cases where people have observed them are in cases where there's thicker ice

and sometimes people have been walking on that thicker ice and then see the lake stars there. So basically if you see one, wait a couple days, then don't go over there until it's been a couple days. Okay, all right. Victor, thank you so much for talking with me today about lake stars. I've never seen one. Now I'm going to be looking.

Great. Yeah, sure, no problem. If you like this episode, we have a request. Please share it with a friend. It really helps our show out. Also, check out our series about Lake Ice Loss and what it means for community, cultures, ecosystems, and safety. This episode was produced by Burley McCoy, edited by Viet Le and fact-checked by Tyler Jones. The audio engineer was Jimmy Keely. I'm Regina Barber.

Thank you for listening to Shorewave from NPR.