After Cassini, A Mission To Icy Planets Full Of Diamond Rain?

16:57 minutes

cut out of planet neptune with a shower of diamonds
A recent experiment observed the formation of diamonds from hydrocarbons under conditions like those found in the interiors of Uranus and Neptune. Credit: Greg Stewart/Stanford Linear Accelerator Center National Accelerator Laboratory

Now that Cassini’s 13-year mission to Saturn has finally wrapped up, astronomers are already starting to think about where to send the next robotic orbiter.  

Among the proposals for a future mission are the “ice giants,” Uranus and Neptune. The two planets are good candidates because their far distance from Earth makes them especially hard to study. But, scientists are curious about one particularly interesting feature: the weather.

[Do edible dormice hold secrets to the fountain of youth?]

Astronomers believe that inside the atmospheres of Neptune and Uranus it’s literally raining diamonds. Tim Dowling, a professor of planetary physics at the University of Louisville, joins Ira to describe what we could learn from the wacky weather of these icy planets if we were able to get a closer look at them.  

two images of Uranus with a teal blue tint
Photos from the Keck II telescope in Hawaii reveal weather on Uranus. Credit: Lawrence Sromovsky, Pat Fry, Heidi Hammel, Imke de Pater/University of Wisconsin
jupiter's blue south pole
This image shows Jupiter’s south pole, as seen by NASA’s Juno spacecraft from an altitude of 32,000 miles (52,000 kilometers). The oval features are cyclones, up to 600 miles (1,000 kilometers) in diameter. Multiple images taken with the JunoCam instrument on three separate orbits were combined to show all areas in daylight, enhanced color, and stereographic projection. Credits: NASA/JPL-Caltech/SwRI/MSSS/Betsy Asher Hall/Gervasio Robles

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Segment Guests

Tim Dowling

Tim Dowling is a Professor of Planetary Physics at the University of Louisville in Louisville, Kentucky.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow, coming to you from the Brown Theatre in Louisville, Kentucky.


As you know, our theme tonight is On the Rocks, and I know what you’re thinking. In Louisville, that usually refers to the ice you put in your glass of bourbon. Yeah. But tonight, on Science Friday, it also means ice in space. And I’m talking about Uranus and Neptune, of course. They’re called the ice giants of our solar system.

And now that Cassini’s mission to Saturn has been wrapped up, people are already thinking about, where should we send the next orbiter? The ice giants of Uranus and Neptune are good candidates for this type of mission, not only because their distance from Earth makes them especially hard to study, but also because there’s some interesting weather happening there that scientists would like to take a closer look at.

For example, astronomers believe that inside the atmosphere of Neptune and Uranus it’s literally raining diamonds. Whoa, I want to go. I know it’s hard to believe, so here with me to describe what we could learn from the wacky weather of these icy planets, if we were to get a closer look at them, is my guest, Tim Dowling, Professor of Planetary Physics at the University of Louisville. Welcome to Science Friday.

TIM DOWLING: Thanks for having me.


IRA FLATOW: So diamond rain? How do we know it’s raining diamonds on Uranus and Neptune?

TIM DOWLING: Well, we don’t. But that idea has been around for 30 years. It’s basically the methane, which is very, very common, and it gets crushed into the pressure that makes diamonds. Now, this idea actually was confirmed this summer, in Stanford. They had a couple of lasers hit polystyrene, which is just eight C’s and eight H’s, and that’s very much like methane. And darned if all the carbon didn’t turn into diamond.

IRA FLATOW: While in the laboratory?


IRA FLATOW: They tried it. So if you went there you actually would see diamonds, rain–

TIM DOWLING: Well, you’d have to go about halfway in. So you have to think like Jules Verne, right? So I think, instead of the Nautilus we’d have to call it Meshach and Shadrach or something, because it’s very hot. But yeah, to imagine a probe going down there is a challenge, but–

IRA FLATOW: So if we sent a probe we might be able to confirm that?

TIM DOWLING: Yes, well, you know the temperature is 5,000 de– it’s the same temperature as the surface of the sun. So not only are you talking real bling here, it’s bathed in a golden light, just like the sun. So it would be amazing to be able to see it.

IRA FLATOW: Wow, wow. And before Cassini there was the Juno mission, which sent back data about Jupiter. And we learned something from that data about Jupiter and Neptune. They’re similar, right?

TIM DOWLING: Right. So the big surprise is– and Juno’s an ongoing mission– it’s our one and only big mission right now in the outer solar system. It gets very close. It has what’s called proximity orbits. It gets almost in the peach fuzz. It’s so close and then it swings out every 53 days. Well they’re getting the gravity field from that, and from the details they can actually figure out the interior structure. The math is very similar to an MRI at a doctor’s office.

So they can actually reconstruct the interior structure without ever going inside. It’s the same idea, and it just takes a little longer. And so what they found to their surprise is that the core is not concentrated. It’s very diffuse, in fact, almost out to halfway out. And then they started looking more closely at it, and it turns out you can think of Neptune and Uranus as being the core of Jupiter. That’s the new paradigm.

IRA FLATOW: Wow, so there’s a sort of a rocky core with a gas bag surrounding it?

TIM DOWLING: Maybe not so rocky. It’s mostly this ice we’re talking about. Now we say ice– it’s very hot, so it’s melted– so but it’s methane, ammonia, and water. And that’s what makes an ice giant an ice giant. It was just after the Voyager encounters which were in 1986, for Uranus, 1989, for Neptune.

So in the early 90s it was realized that these two planets, even though we call them gas giants, were fundamentally different. And what’s different about them is their mantles. Between the core and the outer atmosphere it is not hydrogen and helium. In fact, it’s water, methane, and ammonia. So that’s the ice– that’s where the name comes from.

IRA FLATOW: So should we stop calling Jupiter then a gas giant?

TIM DOWLING: Jupiter is– so Jupiter has– its mantle is actually hydrogen. And the funny thing about hydrogen, it sits over there on the left of the periodic table above lithium, it’s in the lithium group. It’s in the alkali metal group. But you don’t think of it as a metal. But at those pressures it becomes a metal.

And so it’s funny, but that’s extremely exotic. But you think about it, most of the planetary mass in the solar system is metallic hydrogen. So that’s what makes Jupiter and Saturn different. They’re big enough for that to happen. And then the ice giants are the smaller class of planets.

IRA FLATOW: You know the one time we took a close look at Neptune was in 1989, when Voyager 2 passed by. I remember covering that as a journalist. And we got a view of something called the Great Dark Spot. Wow.

TIM DOWLING: Yes. During the far encounter, as Voyager was coming closer and closer, all of a sudden the spot appeared. And so we said, oh, my gosh, it’s just like the Great Red Spot on Jupiter, except that it gallops. The other difference is it was drifting. It’s a high pressure system and it was drifting towards the equator.

You know our hurricanes on Earth actually are low pressure and they drift away from the equator, and for the same reason. So the physics is very similar. But this is anticyclone. It’s a high pressure and it drifts. And darned if the thing didn’t die about four months after it was discovered. It was pure luck that we were there.

IRA FLATOW: Is that right?

TIM DOWLING: Yeah. We’ve had several appear since then. And all of them drift towards the equator, and they all basically kamikaze into the equator.

IRA FLATOW: So what makes Neptune’s Great Dark Spot different from Jupiter’s Great Red Spot? We’re all know about that.

TIM DOWLING: I have much better flow visualization on Jupiter. So I get almost all of my information from the incredible clouds that you can track on Jupiter. On Neptune, we have it by analogy, but you know we couldn’t even see the temperature anomaly for the Great Dark Spot, so it was a real frustrating– because the flyby happened so quickly. And basically we were in discovery mode.

The neat thing about what– the way NASA does its lap around the solar system is the first time it’s guaranteed to always get great science, right? You put a camera in a new place and you always get great science, but that’s the discovery mode. And we’ve pretty much lapped the planet– the solar system. So now the next step is the hypothesis mode that most– like NSF itself– has to do all their science that way. You have to have a hypothesis. And so that’s the mode we’re in now. That’s why you follow up with these orbiters.

IRA FLATOW: So you know, we’re looking now at a picture of Neptune from Voyager, I think, probably.

TIM DOWLING: They knew this was going to happen. You see every feature on Neptune is on the same shot. Well, there’s an enormous shear. The biggest shear in the solar system between these different– and so they actually predicted that the thing about these planets as you predict the weather with a ruler and a stopwatch. And so this was known weeks before it was going to happen that there were going to get this family portrait, and so that becomes the poster child for the whole– for the whole mission.

IRA FLATOW: If you have a question you’d like to ask, we have the microphone set up on the aisles here. And also there’s a microphone in the balcony. It’s so gorgeous blue. I mean does the blue and the dark– little dark spot there– where’s the blue come from? There are no oceans like we have.

TIM DOWLING: Right. It’s not blue for the same reason. But don’t you agree with me, though, that a blue planet with white clouds is a very comforting– especially to have traveled that far out and to see those. It’s because the methane gas is absorbing the red light. So it’s not the same– we have what’s called Rayleigh scattering. So when we look up– but you know when you look down on the Earth, you’re not seeing blue because of the atmosphere. We don’t really have a lot of methane. You see it because of the ocean.


TIM DOWLING: But when you look up from inside, you see the blue. And here it’s all basically the methane gas.

IRA FLATOW: We have a question from the audience. Let’s go right to this side. Yes, step up to the mic.

AUDIENCE: Hi, Dr. Dowling. Thank you. I know you’d like to see more missions, orbiters as you said, to some ice giants, to Neptune or Uranus. Two questions in regards to that. The easy one first. Do you think the hypothesized Planet Nine also would count as an ice giant, is it– what’s likely there?

And the harder one, what of the current political environment, both admin– the whole administration, does their antipathy for aliens extend to potential massive discoveries? Or does–


And, our local reps, Kentucky and Indiana. Would they be likely to support?

TIM DOWLING: The NASA budget is actually doing pretty well with this current administration. So I won’t go too much more into that. We only have the two ice giants here. Most of the other planets, or minor or dwarf planets, are going to be ice, literally rock ice. That’s interesting, though, you know we have over 3,500 confirmed exoplanets, planets out of our solar systems. That’s a large number and they’re starting to get good statistics.

And it turns out 85%, to our great surprise, are planet types that we do not have in this solar system. So there’s a gap between the size of Venus and Earth– there’s a gap between that size– and the ice giant size. And so they’re called mini-Neptunes and super-Earths. Those are actually two planet classes. There’s a gap between them. And so why does our solar system not have the most common planet type? We’re trying to put our solar system in context, and we find out that it’s unusual.

IRA FLATOW: Why does a weather person care about what’s going on in those planets? I mean– how would that affect what we know about ours?

TIM DOWLING: We take it– it turns out that the weather on these planets is easier to predict and easier to unders– it’s cleaner. I think back to the Nobel Prize in medicine in 2000. It was the California sea hare. Did you cover that? Do you remember that? It was the big, thick neuron. And they could do the pulse and get the timing with a sea slug.

And it was not– it was to learn about humans, and to make progress with physiology, but you don’t do it with humans. It’s the same thing here. So we very much need to learn about Earth. But Earth doesn’t come with a user manual, so we have to write our own, right? So you want to go to the other planets for that, and Jupiter is our sea slug.

IRA FLATOW: Let’s go to the audience. Yes sir, here on this side. Step up to the mic, please.

AUDIENCE: Yeah, hey Ira. I love what you do, man. I have one quick question. The James Webb Space Telescope, I know you’d love to send orbiters to these planets, but is it feasible to do a great amount of science with that possible mission? Thank you.

TIM DOWLING: Yes. The James Webb Space Telescope is the successor to the Hubble Space Telescope, and it’s just about to go up. It’s all ready, and it’s very exciting. So we have good news is that the planetary community is getting a fair bit of time for James Webb. It’s very heavily subscribed, of course, by all the different galactic and extragalactic astronomy.

But it’s going to have some new wavelengths. And we’re going to be able to see deeper and more often. And it’s turning out that even the ground-based telescopes, with what’s called adaptive optics where they can actually warp the telescope, and actually account for the turbulence in the atmosphere, is giving us some incredibly crystal clear images. But it’s really tantalizing because there’s nothing like being there.

IRA FLATOW: Yeah, so let me– let’s say this audience is your board at NASA, or someplace. And you want to talk this audience into sending a probe to one of these planets. Give me your pitch, your elevator pitch, for where you’d like to go and what kind of probe we should be sending.

TIM DOWLING: Well, we’ve discovered that 85% of the planets in the universe are mini-Neptunes. And we have a Neptune and Uranus waiting for us. And the whole story of the formation of the solar system is how Jupiter moved around, and how Uranus and Neptune moved around.

And we need to go there with a probe to get the noble gases. That’s the only way you can do it, you can’t do it remotely. You have to send a probe in situ to get the noble gases. And you can’t change the noble gases with regular chemistry. That’s the point. So they tell you like a fingerprint– they tell you the history of the formation of the solar system. That’s where we came from.

IRA FLATOW: OK. Let’s– where I’m going to is this young lady here and the mic. Go ahead.

AUDIENCE: Hi. I was wondering if the diamonds on different planets are the same, in like shape, or like how much pressure is needed to be made, than on Earth?

IRA FLATOW: That’s a great question.

TIM DOWLING: That’s a great question. Yeah. So obviously Earth is making diamonds– we have diamond envy in this country. We have 50 states and there’s only one state where you can find diamonds. It’s Arkansas. It’s actually next door to Kentucky here. And the–

IRA FLATOW: What does that mean, do you think?

TIM DOWLING: I don’t know. I don’t know. But the diamond itself is going to be the same everywhere it’s formed. You know if you put a little aluminum in these atmospheres you can get rubies and sapphires as well. But the pressure–

IRA FLATOW: Is Elon Musk listening to this as you talked about [INAUDIBLE].

TIM DOWLING: –the pressure is one and a half megabars That means there’s one bar in this room. You know we’re in the middle of the country, but we’re at 300 feet above sea level here because we’re right– seven blocks from the Ohio River. So exactly the pressure that you’re breathing right now, multiply that by a million and a half, that’s the pressure. The temperature is about between 5,000 and 6,000 degrees, which is the same as the temperature of the surface of the sun.

IRA FLATOW: And you thought it was bad outside tonight. Let me go right here. Yes, sir.

AUDIENCE: How much would it cost to bring an orbiter to an ice planet, and how long would it take?

TIM DOWLING: It takes about 10 years. It’s a big commitment just to get there, and then you would want the orbiter to last about that long again. But Cassini was a wonderful model for that. It lasted 13 years. It cost somewhere above a billion dollars.

So NASA has these, yes– NASA has these different levels of mission. And so there’s a mission called the New Horizons– or New Frontiers Mission, which is capped at a billion dollars. The one above that is called a flagship.

And so I think what Cassini has taught us– those cost about $3 or $4 billion– it’s a very expensive– so expensive that NASA actually almost canceled the Cassini mission, because they thought that was too much. But I think what Cassini showed us is that if you do put the kitchen sink in and you go all in, that you have incredible flexibility, and you have a 13 year mission that was originally a five year mission. It was just fantastic. They made it– they did everything with Cassini. That’s what we need to send to Uranus or Neptune.

IRA FLATOW: This is Science Friday from PRI. Let’s talk about the atmosphere of Uranus. We study these– Uranus and these planets also– I think we have a slide of this from ground based telescopes, right?


IRA FLATOW: What have we been able to learn about the weather on Uranus? Is it very similar to the weather on Neptune?

TIM DOWLING: This is just incredible. I want you to remember that Uranus is the size of a quarter at a miles’ distance in the sky. And this is done with a ground-base under the atmosphere telescope at Keck. And this is that adaptive optics. And that’s some beating swords into plowshares. That’s a military application that was turned into an astronomical technique. The surface weather is what the model that we’ve been developing here in Louisville for 20 years, the EPIC model, this is what we do. So this data is just really exciting. And all the white clouds are actually methane ice.

IRA FLATOW: Wow. So you get excited by it.

TIM DOWLING: Oh, yeah. Oh, yeah.


TIM DOWLING: Don’t you–

IRA FLATOW: Well, yeah. What would make your day about– tell me the best thing that, really, that you could discover that would say, you know–

TIM DOWLING: I think what gives me the greatest kick for doing this is bringing results back to Earth. And so, it was– especially Jupiter, but all four of these gas giants, they are so clean. You know the weather is going east and west. There’s just not a lot of turbulence. The prediction horizon is in the months here, not weeks.

You know, for Earth, it’s two weeks. And it’s always going to be that. So we can learn more by studying this. But I’m basically saying I love being a veterinary scientist. And love how it helps us learn about the human body. It’s the same thing. I love being a planetary scientist to learn about the most important planet of all.

IRA FLATOW: I see that you hate your job, Tim. Thank you. Tim Dowling, Professor of Planetary Physics, University of Louisville. Thank you very much.


That’s about all the time we have for this evening. Wow. But we have a lot of people we want to thank. We want to give our heartfelt thanks to John Grants, Michael Skoler, Kirsten Pfalzgraf, Aaron Palmer, and all the wonderful folks at Louisville Public Media. Thank you all for–


Also we want to thank everybody here at the Brown Theatre for making this such a wonderful evening to everybody. We also want to thank the National Speleological Society. Yes, the largest organization on Earth dedicated to the exploration, study, and conservation of caves. Visit them on www.caves.org.

And most of all, I want to thank all of you for coming out tonight. You’ve been a great crowd. Thank you, coming out in the weather. Also, let’s give one last round of applause to our amazing musical guests, Bridge 19.

[MUSIC – BRIDGE 19, “CHAIN”] What would I do, if I lost you–

IRA FLATOW: Thank you all for coming. Drive home safely, everybody. In Louisville, Kentucky, I’m Ira Flatow.

[MUSIC – BRIDGE 19, “CHAIN”] I’m tied to a chain I want you, I hate to, I need a way to break through. I’m tied to a chain, a chain. A chain, a chain. A chain, a chain. A chain, a chain. A chain, a chain.

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