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Gravity Assist: Ice Giants (Uranus & Neptune) with Amy Simon (1)
Jan. 17, 2018
Left: Arriving at Uranus in 1986, Voyager 2 observed a bluish orb with subtle features. A haze layer hid most of the planet's cloud features from view. Right: This image of Neptune was produced from Voyager 2 and shows the Great Dark Spot and its companion bright smudge. Credits: Left: NASA/JPL-Caltech - Right: NASABeyond Saturn are two of the most misunderstood and bizarre planets in our solar system—the “ice giants” Uranus and Neptune. Did you know that Uranus has rings and appears to spin on its side? And that windy and intensely blue Neptune once had an Earth-sized Great Dark Spot? In this episode of Gravity Assist, NASA’s Jim Green and Amy Simon discuss Uranus, Neptune, and Neptune’s intriguing moon – Triton -- and what we still have to learn about these mysterious bodies.
Transcript
Jim Green: Our solar system is a wondrous place with a single star, our Sun, and everything that orbits around it - planets, moons, asteroids and comets - what do we know about this beautiful solar system we call home? It's part of an even larger cosmos with billions of other solar systems.
Hi, I'm Jim Green, Director of Planetary Science at NASA, and this is Gravity Assist.
With me today is Dr. Amy Simon. She's a planetary scientist from NASA's Goddard Space Flight Center. And we're talking about the ice giants. These are two enormous planets in our outer solar system: Uranus and Neptune.
Now, Uranus and Neptune are probably the least known of all our planets, and the reason, of course, is only one spacecraft had visited them, and that's Voyager 2 – which flew by Uranus in '86 and Neptune in '89.
So, Amy, what do we mean by ice giants, and what are these objects all about?
Amy Simon: So, Uranus and Neptune are really unique in our solar system. They're very different planets than the other ones we think of. And part of the reason we call them ice giants is because they actually have a lot of water ice. So, while some of the other gas giant planets are mostly hydrogen and helium, they're predominately water and other ices.
Jim Green: That's kind of amazing when we think about that. How were they able to acquire that?
Amy Simon: So, these planets formed much further out in the solar system where there was a lot of ices available. And they didn't quite form as big as, say, Jupiter or Saturn. So, they couldn't pull in quite as much gas. And so, that's kind of part of why we believe they're so different.
Jim Green: You know, some of the simulations on how our planets form seem to indicate that they formed closer to the Sun, and then through gravitational interactions, were pushed out. And that includes Uranus and Neptune. Could they have acquired a lot of the Kuiper Belt objects as they were doing that?
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. And of course, that one moon that we love so much at Neptune called Triton, that's such an unusual body and was quite a shock when Voyager 2 went by. Why is that such a different moon?
Amy Simon: So, Triton really is just such a bizarre world. For one thing, we think it 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 that we actually have nitrogen ice spewing out of the surface. And 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. And we have no idea what's forming that. And we've never even seen the other side of Triton, so who knows what's on the other side?
Amy Simon: So, Triton really is just such a bizarre world. For one thing, we think it 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 that we actually have nitrogen ice spewing out of the surface. And 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. And we have no idea what's forming that. And we've never even seen the other side of Triton, so who knows what's on the other side?Global color mosaic of Triton, taken in 1989 by Voyager 2 during its flyby of the Neptune system. The greenish areas includes what is called the cantaloupe terrain, whose origin is unknown, and a set of "cryovolcanic" landscapes apparently produced by icy-cold liquids (now frozen) erupted from Triton's interior. Credits: NASA/JPL/USGSJim Green: You know, are there any analogies between that cantaloupe terrain and some of the terrain we see on Pluto?
Amy Simon: There is some, and a lot of this has to do with the fact that they're so cold. And even though they're cold, they still have some sort of activity that's moving the ice around. And so, 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: You know, Triton's such a spectacular moon. It's larger than the body Pluto, and as we talked about, may actually be a Kuiper Belt object. But, it also has a funny orbit around Neptune.
Amy Simon: Right. So, all the planets in our solar system move in the same direction. 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. So, it's going backwards. And this is partly because we think it was captured, so it got too close to Neptune and got stuck there.
Jim Green: How hard is it to see Uranus and Neptune from the Earth?
Amy Simon: So, Uranus and Neptune are so far away, they're just really faint. And so, the ancient astronomers that originally found the other planets didn't even see Uranus and Neptune. It took telescopes to find them in the first place. 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 it.
Jim Green: Since it required telescopes to see Uranus and Neptune, when were they discovered?
Amy Simon: So, Uranus was first seen by Hershel in 1781. Neptune wasn't seen for almost 50 years later, in 1846.
Jim Green: You know, 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 go?
Amy Simon: Yeah, you know, that's kind of interesting. That's actually 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. And they kept inferring there had to be something else out there or something with a lot of mass pulling them around. And 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.
Jim Green: You know, I think it took a long time to really find Pluto because of Uranus' perturbations. It had to go around the Sun at least once during that time period to be able to understand its full perturbations. But, it was that that really discovered Neptune.
Amy Simon: Correct. So, it was almost 80 years to the date when they found Pluto, and it's partly because they knew where to look. So, you're looking at regions of the sky with photographic plates and trying to find something moving in the right place in the sky to be able to find it.
Jim Green: Uranus is one of those planets that I think is 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. And part of it is just that it is so cold, and it doesn't have a lot of internal heat. All of our outer planets or giant planets give off more heat than they receive from the Sun except for Uranus. And 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 others. Why is that?
Amy Simon: Yeah, that's another big puzzle. So, Uranus is tilted over on its side. So, if you were looking straight up in the solar system, that would be zero degrees. It's tilted over 98 degrees. So, it is pretty much rolling around on its side. And we have no way of making it do that. And so, 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. And so, that's a real puzzle when we try to explain how the solar system formed.
Jim Green: Are all the Uranus moons in the same plane in the equatorial region?
Amy Simon: They are. And so, 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: You know, in addition to the fabulous moons that Uranus has, doesn't it have rings?
Amy Simon: That's correct, actually. All of the outer planets have rings around them. And Uranus' are very narrow. It has about nine rings. It's--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.
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: Yeah, it really is unique. And we've learned this while look at the other planets, at Saturn especially, because it has such extensive rings. But, 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, and that's just, you know, interesting. It tells us that we don't actually know what forms a ring and keeps a ring.
Jim Green: Now, 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. So, here on Earth, we have a north magnetic pole and a south magnetic pole. For both Uranus and Neptune, actually, that 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. It's actually kind of a “multi” pole.
So, if you could think about two magnets crossed with each other, it's almost like that. It's really strange.
Jim Green: So, we've really got to go back to these planets and visit them. There's so much for us to learn.
Amy Simon: Absolutely.
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