An icy mystery: What are lake stars?

When producer Berly McCoy https://www.npr.org/people/985775371/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 https://vivo.brown.edu/display/vtsai2 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 https://doi.org/10.1103/PhysRevE.75.066105. Have a question about something in the environment? Email us at [email protected] mailto:[email protected]. Listen 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 http://plus.npr.org/shortwave. See pcm.adswizz.com https://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 https://www.npr.org/about-npr/179878450/privacy-policy

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• 0:00Speaker 00

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• 0:20Regina Barber?

  Hey, Short Waver, it's Regina Barber here. Recently, our producer Berly McCoy was out on a frozen lake near her home.

• 0:28Speaker 02

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

• 0:32Regina Barber?

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

• 0:38Speaker 02

  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.

• 0:50Regina Barber?

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

• 0:57Speaker 03

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

• 1:02Regina Barber?

  Victor Tsai is a geophysicist at Brown University. He says the thing Burley 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 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.

• 1:30Regina Barber?

  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 Short Wave, the science podcast from NPR.

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• 3:18Regina Barber?

  Let's start with a little Lake Star Science 101 with geophysicist Victor Sae.

• 3:23Speaker 03

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

• 3:30Regina Barber?

  The snow pushes down on the ice. At the same time, if a small hole forms on the underside of that ice...

• 3:36Speaker 03

  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.

• 3:46Regina Barber?

  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.

• 4:00Speaker 03

  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.

• 4:19Regina Barber?

  So how did you make this slush?

• 4:22Speaker 03

  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 observed.

• 4:48Regina Barber?

  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?

• 4:56Speaker 03

  I did, although not at the same time. Because you know, it's very cold in the cold lab, so I wouldn't be.

• 5:02Regina Barber?

  Yeah, you don't want it in there. You want it different, but it was the summer, you know?

• 5:06Speaker 03

  It was.

• 5:06Regina Barber?

  So you like basically have this like flat surface and then 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?

• 5:23Speaker 03

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

• 5:30Regina Barber?

  Like a leaky faucet.

• 5:31Speaker 03

  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 then 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.

• 5:53Regina Barber?

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

• 6:00Speaker 03

  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.

• 6:15Regina Barber?

  Slightly warm, like room temperature?

• 6:17Speaker 03

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

• 6:26Regina Barber?

  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?

• 6:35Speaker 03

  You do have to drip the water somewhat slowly and carefully.

• 6:39Regina Barber?

  Okay.

• 6:39Speaker 03

  To give you a sort of constant flow rate. Otherwise.

• 6:43Regina Barber?

  Okay.

• 6:43Speaker 03

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

• 6:49Regina Barber?

  Right.

• 6:50Speaker 03

  Yeah.

• 6:51Regina Barber?

  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.

• 6:57Speaker 03

  Yeah, exactly.

• 6:58Regina Barber?

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

• 7:01Speaker 03

  Yeah, within a few minutes, you should see these star patterns forming. So you'll see some branching of the melt patterns. So originally, you might start with something kind of circular, and then after.

• 7:15Regina Barber?

  Where the drops are.

• 7:16Speaker 03

  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.

• 7:44Regina Barber?

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

• 7:46Speaker 03

  Exactly, it would be a single snake.

• 7:49Speaker 03

  Yeah.

• 7:49Regina Barber?

  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?

• 8:11Speaker 03

  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.

• 8:23Regina Barber?

  So cool.

• 8:24Speaker 03

  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?

• 8:42Regina Barber?

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

• 8:47Speaker 03

  One to two kilometers.

• 8:49Regina Barber?

  That's like a mile.

• 8:50Speaker 03

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

• 8:52Regina Barber?

  Wow, that's a very big lake star.

• 8:55Speaker 03

  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.

• 9:06Regina Barber?

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

• 9:14Speaker 03

  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.

• 9:39Regina Barber?

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

• 9:46Speaker 03

  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.

• 9:56Regina Barber?

  More slushies.

• 9:57Speaker 03

  Yep, exactly, yeah.

• 9:59Regina Barber?

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

• 10:09Speaker 03

  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.

• 10:31Regina Barber?

  Right.

• 10:31Speaker 03

  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.

• 10:47Regina Barber?

  Yeah.

• 10:48Speaker 03

  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.

• 10:57Regina Barber?

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

• 11:03Speaker 03

  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?

• 11:16Regina Barber?

  Oh, to drill?

• 11:17Speaker 03

  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.

• 11:38Regina Barber?

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

• 11:44Speaker 03

  Right.

• 11:45Regina Barber?

  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, those lake stars?

• 12:14Speaker 03

  Yeah, so if-

• 12:15Regina Barber?

  Or in the ice in the area.

• 12:17Speaker 03

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

• 12:23Regina Barber?

  Okay.

• 12:24Speaker 03

  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. Uh, so if it had just formed, then you can still see the kind of snow on top. But, um, in many cases, you get very thick ice forming afterwards that preserves the lake stars. Um, 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.

• 12:58Regina Barber?

  So basically, if you see one, wait a couple of days, then, then, like, don't go over there.

• 13:04Speaker 03

  Yeah, exactly.

• 13:05Regina Barber?

  Until it's been a couple of days. Okay, all right. Um, Victor, thank you so much for talking with me today about lake stars. I've never seen one. Now I'm gonna, now I'm gonna be looking.

• 13:15Speaker 03

  Great. Yeah, sure. No problem.

• 13:19Regina Barber?

  If you liked 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 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 Shore Wave from NPR.

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• 14:27Speaker 04

  This message comes from Capella University. You know that feeling when there's a spark building inside you? That you were meant for more? That's your own drive, pushing you towards what's next. Capella University gets that. With their FlexPath learning format, you can set the pace and earn your degree without putting life on pause. You've built experience and know what you're capable of. Now, this is your time to turn that momentum into more. The only real question is, what can't you do? Learn more at capella.edu.