Gravity Assist Podcast: Uranus and Neptune, with Amy Simon, Part 2

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’s a short teaser of this week’s podcast:

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

The deep blue atmosphere of Neptune is more active than Uranus’ atmosphere. Image credit: NASA/JPL.

Jim Green: Let’s move our discussion on to Neptune. What did Voyager 2 discover about Neptune during its fly-by that really surprised us?

Amy Simon: Neptune and Uranus are not at all like each other in a lot of ways. Besides being much more Earth-like in its tilt – Neptune’s tilted about 28 degrees [Earth is tilted by 23,5 degrees] – it’s not the same color as Uranus. It’s a deeper blue, and when we got there, we were shocked to see it had a Great Dark Spot, which was a big storm that was raging in the atmosphere of Neptune.

Jim Green: A dark spot? You mean sort of like the Great Red Spot on Jupiter?

Amy Simon: Exactly the same type of thing. It’s a gigantic anti-cyclone, so it’s a high-pressure storm. It was raging throughout the entire Voyager fly-by. But, when we looked again with Hubble when we could first look at Neptune, it was gone.

Jim Green: On Jupiter and Saturn, we’re seeing lightning. Has there ever been lightning detected on Uranus and Neptune?

Amy Simon: We haven’t seen lightning, but that’s partly because we expect lightning to form in the water-ice clouds, and on these cold planets the water-ice is deep down. We’re actually seeing methane ice clouds when we see clouds on Neptune, and we haven’t seen lightning yet.

Jim Green: Does Neptune have a ring, also? What do we know about it?

Amy Simon: Neptune does have rings, as well, but these are not as well formed as what we see around the other planets. They’re kind of clumps. We see arcs – partial rings – at various points around the planet.

Jim Green: In addition to Voyager 2 discovering Neptune’s rings, there’s also other techniques that we’ve used to discover and look at the rings of Uranus and Neptune. What’s the most important technique?

Amy Simon: The best technique we have, especially since we’re not up close to them, is to use stars. When a star passes behind the planet, it gets dimmed out. Well, it turns out, when it passes behind the rings, the same thing happens. So, we can watch a star twinkling in and out as it goes behind Neptune and its rings.

The surface of Neptune’s moon Triton, as seen by Voyager 2. The greenish areas are called “cantaloupe terrain” while the pinkish regions is methane ice in the south polar cap. Image credit: NASA/JPL/USGS.

Jim Green: Are there any other moons besides Triton that are notable?

Amy Simon: Both Neptune and Uranus actually have quite a few satellites around them. In the case of Uranus, they’re all fairly small, although we think some of them do have interesting ice on their surface, as well. For Neptune, there’s that one big moon, which is Triton, and then the rest are much smaller. So, it’s more similar to what we see around Saturn.

Jim Green: You know, it’d be fantastic to go back to either one of these and take a look at the moons more carefully.  I’d be willing to bet we could find some captured comets, too.

Amy Simon: Oh, I think we’d find all sorts of interesting things. We have no idea how many of these moons might be active and actually helping to form the ring systems, for example. That’s what we found at Saturn with Enceladus. And the fact that we haven’t seen the other side of any of these moons, we have no idea what’s out there.

Jim Green: I think we’re gonna learn an enormous amount when we have an opportunity to return and really spend some time at Uranus and Neptune. What are some of the mission ideas that we’ve been talking about?

Amy Simon:  We’ve studied quite a lot of different ways to get out to Uranus and Neptune. I think the biggest problem is they’re just so far away, so you can’t get out there very fast.

So we’ve looked into whether we could do a fly-by mission, which is similar to what Voyager 2 did, and if you did that, what would you add? I think the primary thing we’d add to any mission is an atmospheric probe, because we want to understand what the layers are in the atmosphere, what the temperatures are. We have also looked at orbiters, and these are nice because it gives you a chance to explore that whole system – the rings, the moons, Triton in particular – to see what’s going on on all sides of those different moons.

Neptune’s Great Dark Spot. This was the spot seen by Voyager. It had disappeared from the planet’s southern hemisphere by the time the Hubble Space Telescope looked for it in 1994, only to be replaced later by a dark spot in the northern hemisphere. Image credit: NASA/JPL.

Jim Green: When we look at other planets around other stars and we try to figure out what the most populous type of exoplanet is, it turns out that the most common are mini- neptunes. So, this tells us that Uranus and Neptune are objects that we really need to study further. How are we studying them today?

Amy Simon: We’re actually looking every year now at both of these planets with the Hubble Space Telescope. We’ve also been using the Kepler Astrophysical Telescope to look at the light curves [i.e. a graph of how their brightness changes with time] and study how their clouds are changing. Even if we can’t see the clouds, we can see the change in their light curves. So, we’re using as many different ground-based and space-based observatories as we can to look at both Uranus and Neptune.

Jim Green: What kind of things are we finding out from the Hubble and Kepler observations?

Amy Simon: At the moment we’re seeing a lot more clouds on Neptune, and we’re finding that they vary on really short time scales, and that’s partly because Neptune has winds that blow at hundreds of miles an hour. So, those clouds change really fast. In the last few years we’ve also seen another Great Dark Spot. We discovered it with Hubble, and we’ve been able to watch it get smaller and eventually disappear. And we think [the dark spots] move around a little bit, too, which is interesting.

Uranus, on the other hand, has been really, really quiet. When it passed its northern hemisphere equinox in 2007, kind of its springtime, we suddenly saw an outbreak of clouds all over the place, but we haven’t seen a whole lot since then. We just see occasional ones. So they really are quite different planets from each other.

Jim Green: One of the things that the Cassini spacecraft found at Saturn was that the rings were really shading the planet during certain seasons and causing all kinds of changes in the atmosphere. How long does it take for seasons on Uranus and Neptune?

Amy Simon: A year on Uranus is about 84 [Earth] years. Each of its seasons is 21 [Earth] years. Because Uranus is tilted over on its side, that means that, for example, the south pole wouldn’t see sunlight for about 40 years. It’s got really extreme seasons, which help to drive the weather.

Neptune takes 164 years to go around the Sun, so almost twice as long, but it doesn’t have that extreme tilt. We haven’t, at least by this point, been able to observe any seasonal changes because we haven’t been observing long enough.

Neptune’s rings are faint and dark. They are seen clearly here only with Neptune blotted out of the picture. Image credit: NASA/JPL.

Jim Green: I ask all my guests how they got into this business, what was their ‘gravity assist’ that propelled them forward and made them the scientist they are today. Amy, what’s yours?

Amy Simon: I think I had a two-body gravity assist, actually, the first one being when the shuttle program came around and Sally Ride – I really wanted to be an astronaut, to be the first woman on Mars. But, honestly, the second one was Voyager 2 and when it flew by Uranus and Neptune and you saw all these exotic worlds and pink ice and all sorts of blue colors, I was so just so enthralled by the planets, I really wanted to be a planetary scientist.

Jim Green: I can resonate with that. Those are just tremendous events that have happened in our space program, and like many others, they have indeed inspired another generation. Amy, you’ve been working for Goddard now for several years. How did you become an employee for NASA?

Amy Simon: My start actually was working on the Galileo mission to Jupiter, and I was doing that as a student and got invited to do a post-doctoral position working on Galileo, at which point I got asked if I would like to also help out with Cassini, which was on its way to Saturn. And the instrument I was working at was based at NASA Goddard. So, eventually, I moved down to NASA Goddard.

Jim Green: As a civil servant, you provide opportunities during the summer for students to come and work. How many students have been involved in your organization?

Amy Simon: It depends on the summer, but some summers we have hundreds and hundreds of students at the center doing all sorts of projects, from engineering to science and through all of the different science fields that we have.

Jim Green: Any student who would like to work during the summer should visit, and do a search on summer employment. [Also, see the NASA Pathways program:]