An icy mystery: What are lake stars?

Summary

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Podcast Summary: Short Wave — "An Icy Mystery: What Are Lake Stars?"

Podcast: Short Wave (NPR)
Episode Title: An Icy Mystery: What Are Lake Stars?
Air Date: March 13, 2026
Hosts: Regina Barber
Guest Expert: Victor Tsai, Geophysicist, Brown University

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Overview

In this enlightening and playfully-scientific episode, host Regina Barber explores the mysterious patterns known as "lake stars" alongside geophysicist Victor Tsai. What starts as NPR producer Burley McCoy’s fascination (and concern) with odd star-shaped ice patterns on her local frozen lake spirals into an investigation of the physics behind lake stars, their DIY recreation, and their uncanny resemblance to massive formations on Europa, Jupiter’s icy moon. The episode also addresses important questions about ice safety, all in under 15 minutes.

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Key Discussion Points & Insights

1. The Mystery on the Lake (00:20–00:50)

• Burley McCoy discovers star-shaped, spidery patterns in the ice on her local frozen lake.
  • "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." — Burley McCoy (00:37)

2. Introducing the Expert: What Are Lake Stars? (00:57–02:26)

• Lake Stars Defined:
  • Victor Tsai explains: “They're actually quite common, but you need a particular set of circumstances to happen.” (00:57)
  • Lake stars emerge when snow covers a thin ice sheet over a lake, and a hole in the ice allows relatively warm lake water to seep up, melting slushy paths in a star-shaped pattern.
  • “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.” — Victor Tsai (02:43)

3. Lab Science: Recreating Lake Stars (03:08–06:06)

• Laboratory Lake Stars:

  • Without winter ice for field studies, Tsai devised an experiment with a kitchen blender to make slush in a cold lab.
  • “I purchased a blender... I found that playing around with different blender settings, I could make a slush that was similar to what we observe.” — Victor Tsai (03:31)
  • Experiment involved slowly dripping just-above-freezing water onto the slush to mimic lake conditions.
  • Key procedural insight: It’s all about slowly dripping water—"like a leaky faucet"—to see branching, star-like patterns instead of just a melted puddle or a 'snake' channel. (04:39, 06:52)

• DIY Lake Stars at Home:

  • Listeners encouraged to try the experiment at home using a blender to make slush and drip cold (but not too warm) water over it.
    • “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.” — Victor Tsai (05:08)
  • Patience is key—too much or too warm water destroys the pattern.

4. Earth to Space: Lake Stars and Europa (06:58–10:11)

• Jupiter's Moon Europa:

  • There is a "spider" or "star-like" formation on Europa—a mile-wide feature resembling lake stars.
  • Europa's icy surface and hidden ocean make these features tantalizing.
  • Tsai’s group found that the same physics explaining Earth’s lake stars might explain Europa’s spider:
    • “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.” — Victor Tsai (08:22)
  • Physical experiments: Similar slushy setups used in the lab to model these extraterrestrial patterns. (08:55)

• Significance for Space Exploration:

  • If Europa’s spider patterns are made by water closer to the surface, that could shape future missions.
  • "If it’s 40km thick, that ain’t happening. But yeah, if it’s right near the surface, then yeah, maybe we can get in there." — Regina Barber (10:48)
  • "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." — Victor Tsai (10:26)

5. Practical Takeaway: Are Lake Stars Safe? (10:53–12:14)

• Ice Safety:
  • Recent lake stars = thin, risky ice—stay off!
  • “If a lake star had recently formed, then it would be very not safe. ...But in many cases you get very thick ice forming afterwards that preserves the lake stars. ...Most of the cases where people have observed them are in cases where there's thicker ice.” — Victor Tsai (11:23–12:06)
  • General advice: If you see a lake star, wait a few days—the ice may become safer after thickening.

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Notable Quotes & Memorable Moments

• “It wouldn't be a star, it'd be a snake.” — Regina Barber (06:52)
• “I purchased a blender... playing around with different blender settings and things like that, I could make a slush that was similar to what we observe.” — Victor Tsai (03:31)
• “That's a very big lake star.” — Regina Barber, on Europa’s kilometer-sized feature (08:01)
• “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.” — Victor Tsai (10:26)

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Important Timestamps

• 00:37: Discovery of mysterious ice patterns
• 02:32: Explanation of how lake stars form
• 03:31: DIY approach using a blender for slush
• 05:08: How to try making lake stars at home
• 06:52: Explanation of the "snake" vs "star" formation
• 07:20: Linking lake stars to Europa’s spider features
• 09:17: Why this matters for understanding Europa
• 11:23: Advice on ice safety around lake stars

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Final Thoughts

This episode offers a delightful blend of curiosity-driven storytelling, practical DIY science, and a cosmic perspective—showing how a seemingly small icy mystery from Earth might hold clues for distant moons. It’s engaging, accessible, and leaves listeners with both knowledge and wonder, along with real-world safety advice for winter lake exploration.

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Transcript

[A]

This message comes from Capella University. That spark you feel, that's your drive. For more. Capella University's flexpath learning format lets you earn your degree at your pace without putting life on pause. Learn [e-mail]. you're listening to Shortwave from NPR.

[B]

Hey, Shortwavers, Regina Barber here. Recently our producer Burley McCoy was out on a frozen lake near her home.

[A]

It's been a weird winter, so I was already kind of worried about the ice safety.

[B]

Don't worry. Burley knows about being safe on the ice. But then she saw something that freaked her out.

[A]

There were these like 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.

[B]

Lucky for Burley, she works on a science podcast. So we called up an expert to ask about this icy phenomenon.

[C]

They're actually quite common, but you need a particular set of circumstances to happen.

[B]

Victor Tsai is a geophysicist at Brown University. He says the thing Burleigh was seeing is called a lake star. He studied them as a visiting graduate student at Woods Hole Oceanographic Institution. There, his advisor gave him a 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 Burleigh what lake stars might say about the safety of the ice. You're listening to Shortwave, the science podcast from npr.

[D]

Support for this podcast and the following message come from E Trade from Morgan Stanley. Discover a wide range of investing choices and banking solutions all in one place. Plus get up to $1,500 when you open a brokerage account with a qualifying deposit today. Learn [e-mail] offer banking products and services are provided by Morgan Stanley Private Bank national association member FDIC Terms and other fees apply. Investing involves risks. Morgan Stanley Smith Bar Member SIPIC let's

[B]

start with a little Lake Star Science 101 with geophysicist Victor Tsai.

[C]

The basic idea is just that there's a thin layer of ice with some snow on top of it.

[B]

The snow pushes down on the ice at the same time. If a small hole forms on the

[C]

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.

[B]

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 fieldwork wasn't possible. It was summer when he was working at Woods Hole.

[C]

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.

[B]

So how did you make this slush?

[C]

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.

[B]

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?

[C]

I did, although not at the same time, because, you know, it's very cold in the cold lab.

[B]

So I. Yeah, you don't want it in there. You want it different. 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?

[C]

Yeah, we have to drip the water relatively slowly so that it's a sort of constant flow that doesn't.

[B]

Like a leaky faucet.

[C]

Catastrophic. 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.

[B]

Let's say you wanted to do this at home. What could people do to try to make their own lake stars?

[C]

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.

[B]

Slightly warm? Like room temperature?

[C]

Actually, it's better if it's even below room temperature so that it happens more slowly, otherwise it can be too catastrophic.

[B]

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 you do they have to drip that water?

[C]

You do have to drip the water somewhat slowly and carefully to give you a sort of constant flow rate. Otherwise.

[B]

Okay.

[C]

Otherwise if you just like poured the whole bucket of warm water, you would sort of destroy the slush, right? Yeah.

[B]

So you're, 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.

[C]

Exactly.

[B]

In the winter. What are they going to see in those few minutes?

[C]

Yeah, within a few minutes you should see these star 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 where 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.

[B]

It wouldn't be a star, it'd be a snake.

[C]

Exactly. It would be a single snake. Yeah.

[B]

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?

[C]

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.

[B]

So cool.

[C]

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,

[B]

how big is this spider like on Europa? Like if you could compare it to like any sort of size on Earth,

[C]

one to two kilometers.

[B]

That's like a mile.

[C]

Yeah. Yeah, it could be. Yeah, a mile wide. Wow.

[B]

That's, that's a very big lake star.

[C]

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 30ft across.

[B]

So what, what did you learn from this, this work of like kind of studying this spider mile long lake star on Europa?

[C]

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.

[B]

Yeah. Did you simulate something in the lab to try to get something that looked like this spider on Europa?

[C]

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.

[B]

More slushies.

[C]

Yep, exactly. Yeah.

[B]

So how can comparing these features on Earth and Europa, how can that tell us more about the moon Europa and its ice?

[C]

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.

[B]

Yeah.

[C]

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 feature formed.

[B]

For like somebody who doesn't know anything about planets, like why does this matter?

[C]

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.

[B]

To drill.

[C]

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, if it's 40km thick, that ain't happening.

[B]

But yeah, if it's right near the surface, then yeah, maybe we can get in there.

[C]

Right.

[B]

We can put probes. Dang, that's so cool. Okay. Okay, Coming back to like lake stars on Earth. 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, them you maybe could walk on top of them. Does it indicate that it might not be safe to walk over those lake stars?

[C]

Yeah.

[B]

Or in the ice in the area.

[C]

Right. So if a lake star had recently formed, then it would be very not safe. Okay. 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.

[B]

So basically, if you see one, wait a couple days, then. Then don't go over there until it's been a couple days. Okay.

[C]

All right.

[B]

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.

[C]

Yeah, sure, no problem.

[B]

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 sandals safety. This episode was Produced by Burleigh McCoy, edited by Viet Le and fact checked by Tyler Jones. The audio engineer was Jimmy Keeley. I'm Regina Barber. Thank you for listening to Short Wave from npr.

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