Gravity Assist Podcast: Uranus and Neptune, with Amy Simon, Part 1

The Gravity Assist Podcast is hosted by NASA’s Director of Planetary Science, Jim Green, who each week talks to some of the greatest planetary scientists on the planet, giving a guided tour through the Solar System and beyond in the process. This week, he’s joined by Amy Simon of the Goddard Space Flight Center to talk about not just one, but two planets of the Solar System: the ice giants Uranus and Neptune, as well as Neptune’s moon Triton.

Here is a short teaser for this week’s podcast:

You can listen to the full podcast here, or read the transcript below.

Uranus, seen here by Voyager 2, is a deceptively bland planet. Image credit: NASA/JPL–Caltech.

Jim Green: Uranus and Neptune are probably the least known of all our planets, and the reason, of course, is only one spacecraft has visited them, and that’s Voyager 2 – which flew by Uranus in 1986 and Neptune in 1989. We call these planets the ice giants, but Amy, what do we mean by ice giants?

Amy Simon: Uranus and Neptune are really unique in our Solar System. They’re very different planets [than the other worlds in our Solar System]. Part of the reason we call them ice giants is because they actually [contain] a lot of water-ice. So, while [Jupiter and Saturn are] gas giant planets with mostly hydrogen and helium, the ice giants are predominately water and other ices.

Jim Green: How were they able to acquire that much ice?

Amy Simon: They formed much further out in the Solar System where there was a lot of ice available. They didn’t quite form as big as Jupiter or Saturn, so they couldn’t pull in quite as much gas. That’s part of why we believe they’re so different.

Jim Green: Some of the simulations of how our planets form seem to indicate that they formed closer to the Sun, and then through gravitational interactions, were pushed out. That includes Uranus and Neptune. Could they have acquired a lot of the Kuiper Belt objects as they were doing that?

Planetary scientist Amy Simon of NASA’s Goddard Space Flight Center. Image credit: NASA.

Amy Simon: Absolutely. As a matter of fact, we think that a lot of Neptune’s moons are captured Kuiper Belt objects.

Jim Green: Yeah, that kind of gives it away a little bit, I think. Of course, that one moon that we love so much at Neptune, called Triton, is such an unusual body and it was quite a shock when Voyager 2 saw it for the first time. Why is Triton such a different moon?

Amy Simon: For one thing, we think Triton has geysers on it, but these aren’t geysers like we’re used to on Earth where we have hot water and steam. It’s so cold [on Triton] that there is actually nitrogen-ice spewing out of the surface. So, that’s really weird to start with.

But, if you move away from the south pole of Triton, then you get into this weird terrain that we call “cantaloupe terrain,” because it looks like the skin of a cantaloupe. It’s all wrinkly.  We have no idea what’s forming that. And we’ve never even seen the other side of Triton, so who knows what’s there?

Jim Green: Are there any analogies between that cantaloupe terrain on Triton and some of the terrain we see on Pluto?

Amy Simon: There is some, and a lot of it has to do with the fact that they’re so cold. Even though they’re cold, they still have some sort of activity that’s moving the ice around. We think that Triton and Pluto actually have quite a lot in common, and that’s something we’d like to go back and learn a lot more about.

Jim Green: Triton’s such a spectacular moon. It’s larger than Pluto, and as we talked about, it may actually be a Kuiper Belt object. It also has a funny orbit around Neptune.

Amy Simon: Right. All the planets in our Solar System move in the same direction, and they all pretty much rotate in the same direction. All their moons go around them in the same direction. But, Triton doesn’t. It’s retrograde, [meaning] it’s going backwards. This is partly because we think it was captured, so it got too close to Neptune and got stuck there.

The Hubble Space Telescope observes Uranus every year. This view, in near infrared light, highlights not only the planet’s rings but also atmospheric belts and clouds. Image credit: NASA/JPL/STScI.

Jim Green: How hard is it to see Uranus and Neptune from the Earth?

Amy Simon: They are so far away, they’re just really faint. The ancient astronomers that originally found the other planets didn’t even see Uranus and Neptune. It took telescopes to find them. So, if you were to go out and look, you’d have to know exactly where to look, and you’d still need a telescope to be able to find them.

Jim Green: When were they discovered?

Amy Simon: Uranus was first seen by [William] Hershel in 1781. Neptune wasn’t seen for almost 50 years later, in 1846.

Jim Green: The discovery of Neptune is really kind of fascinating in the sense that observing Uranus really gave away the fact that there’s something else out there. How did that happen?

Amy Simon: It is interesting. How they inferred all these outer planets was they were looking at the orbits of planets closer in and kept seeing them being tweaked a little bit. They kept inferring there had to be something else out there or something with a lot of mass [gravity] pulling them around. So, that’s kind of how we got an idea there was a Uranus and a Neptune. But, even after that, we still thought there was more mass out there, which led to the hunt for Pluto. [Editor’s note: Ironically, Pluto turned out not to be massive enough to perturb Uranus or Neptune, and discrepancies in the orbits of the ice giants were reconciled by more accurate measurements of their masses.]

Jim Green: Uranus seems so featureless. Why does it look like that?

Amy Simon: I think poor Uranus is misunderstood, actually. Uranus is very bland in appearance most of the time. It’s kind of a pale blue planet. It’s the real pale blue dot. Part [of the reason for that] is it is so cold, and it doesn’t have a lot of internal heat. All of our outer giant planets give off more heat than they receive from the Sun except for Uranus. We think that is slowing down convection inside the planet. You don’t get the equivalent of thunderstorms, so you don’t see the bright clouds on Uranus that you see on the other planets.

Jim Green: Another really fascinating aspect about Uranus is its rotational axis. It’s so different than all the other planets. Why is that?

Miranda is one of the most intriguing of Uranus’ moons, with a surface that looks like it has been smashed apart, melted and put back together again. Image credit: NASA/JPL–Caltech.

Amy Simon: That’s another big puzzle. Uranus is tilted over on its side. [Imagine] you were looking straight up in the Solar System, that would be zero degrees. It’s tilted over by 98 degrees. So, it is pretty much rolling around on its side. We have no [theoretical] way of making it do that. The best guess we have at the moment is that, while it was forming, it collided with something even bigger or as big, and it got knocked over. So that’s a real puzzle when we try to explain how the Solar System formed.

Jim Green: Are all Uranus’ moons in the same plane in the equatorial region?

Amy Simon: They are. It’s a little different than what we can see on the other planets because it is tilted on its side. We get a different view than we do when we fly by other planets.

Jim Green: In addition to the fabulous moons that Uranus has, doesn’t it have rings?

Amy Simon: That’s correct. All of the outer planets have rings around them. And Uranus’ are very narrow. It has about nine rings. They’re hard to see because they are so narrow.  We were able to see them with Voyager 2, and that’s how we discovered them. [Editor’s note: The rings were actually discovered in 1977 by James Elliot, Edward Dunham and Jessica Mink during a stellar occultation, where a background star winked on and off as it moved behind the rings from our point of view.]

But, rings are great because they’re one way that we actually can do kind of the equivalent of seismology on the planets. We can look at how the rings oscillate and how their shapes change and learn a little bit about the inside of the planets.

Jim Green: So, the planet must be shaking and moving the rings back and forth. That’s pretty astounding.

Amy Simon: We’ve learned this while looking at the other planets, at Saturn especially, because it has such extensive rings. The fact that we have rings at all around the outer planets tells us they’re pretty common. But, they’re also very different from each planet. It tells us that we don’t actually know what forms a ring and keeps a ring.

Jim Green: Does Uranus have a magnetic field?

Amy Simon: It does have a magnetic field, and it’s a lot different than what we have here on Earth, where we have a north magnetic pole and a south magnetic pole. For both Uranus and Neptune, [the magnetic field is] offset from the center. So, it’s not directly in the center of the planet, and it’s also not just a north and south [orientation]. It’s actually kind of a “multi” pole. If you can think about two magnets crossed with each other, it’s almost like that. It’s really strange.

Part 2 of the transcript is available here.