Gravity Assist Podcast: Jupiter, with Jared Espley

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 is joined by Dr Jared Espley, who is a planetary scientist at NASA’s Goddard Space Flight Center, and also the program scientist on Juno, one of planetary science’s fantastic missions, currently orbiting Jupiter.

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

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

Juno’s orbit enables it to pass over both Jupiter’s poles. Image: NASA/JPL–Caltech.

Jim Green: Let’s talk about the big guy on the block – Jupiter. It’s the “800-pound gorilla” of all our planets. Jared, why is it so big? Why did it get the way it is?

Jared Espley: We actually don’t know that, Jim. That’s one of the really cool things in science, we don’t know exactly how Jupiter formed and how planets in general form throughout the Universe. So, that’s what the Juno mission is designed to do, to try and help us understand how planets form, in general.

Part of the answer, of course, is that gas condensed into the solar nebulae to form Jupiter, but under what circumstances that happened and why Jupiter is the largest planet in our Solar System, we just literally don’t know.

Jim Green: We think that as the planets were created and Jupiter obtained most of the gases, that it would have or could have rivaled the Sun at one time in terms of being able to turn on and become another sun. Since a large percentage of stars in our Galaxy are double stars, perhaps Jupiter was a failed sun.

Jared Espley: Yeah, that’s right. I mean, basically, you just need to get enough mass to ignite nuclear fusion, of course. Jupiter didn’t quite get there.

Jim Green: I think the calculation is that it takes about 80 times the mass of Jupiter to get it to the point where it’d become a sun.

One of the most iconic and fascinating features of Jupiter is its Great Red Spot. Juno’s really learned a lot about the planet as it’s passed over very close [to its atmosphere], and it passed over the Great Red Spot in summer 2017. What did we find out?

Jared Espley: We’ve seen a lot of images of the Great Red Spot over the years, but Juno has all these instruments that are specifically designed to look underneath the clouds of Jupiter, and we used those instruments to look at the Great Red Spot. What we found, in particular with the microwave radiometer that can see the microwave emissions coming from deep down in the atmosphere, was that it was warmer very deep beneath the Great Red Spot, way down into the deep atmosphere, hundreds of kilometers deep. So clearly [the storm] has roots, there’s sources of atmospheric turbulence down below, the details of which are being worked out. But it means that not only is it an iconic storm at the surface but it goes deep down into the atmosphere.

Jim Green: Optical astronomers have been using telescopes to observe Jupiter for hundreds of years and they have been finding that the Great Red Spot is shrinking.  What’s been happening? Do we know yet?

Jared Espley: I don’t think we really do. Again, that’s one of the awesome things about science: there is always mysteries. Like you say, observationally, we can see clearly that at the surface it’s shrinking. Whether the roots deep down have been growing or shrinking, we have no idea because, of course, we only have those recent measurements from Juno.

But, we want to keep tracing that and to be able to see the evolution of this gigantic storm over the next decade or so and see how that compares with the past few hundred years.

upiter’s Great Red Spot. Image: NASA/JPL–Caltech/SwRI/MSSS/Björn Jónsson.

Jim Green: One of the really exciting instruments on Juno that I really love is the Waves instrument. That’s an instrument that makes measurements of electromagnetic waves, and we’ve got a process by which they take those electromagnetic waves and convert them into sound, and that’s really been a fascinating opportunity for us to listen to the sounds of Jupiter, listen to the sounds of space. What do those sounds tell us?

Jared Espley: The waves that we are typically recording are electromagnetic waves produced by the different energetic particles that are found at the different planets, and they produce different types of waves. In some cases they can be also be related to lightning activity. We have whistler waves at Earth. We have these at Jupiter, as well.

But like you say, the audio version just makes this really eerie space music.

Jim Green: As the big guy on the block, Jupiter also seems to have most of the moons. How many moons does Jupiter have?

Jared Espley: We think it has dozens, 50, 60 moons [67 confirmed moons], but it has four major satellites, the Galilean satellites that were discovered by Galileo hundreds of years ago. Those moons are particularly interesting to us because they are full worlds in their own right and have really interesting features: oceans, ice, tenuous atmospheres. So, they’re really fascinating moons.

Jim Green: Yeah, they’re huge. I think Europa, which is the smallest of the four, is just a little smaller than our own moon. And Ganymede is the largest moon in the Solar System, plus it has its own magnetic field. One of the things that the moons do, of course, is that as they orbit the planet, they really connect to the magnetic field of Jupiter, and that produces all kinds of effects. What are some of those things?


Dr Jared Espley, program scientist on the Juno mission to Jupiter. Image: NASA.

Jared Espley: I’s really neat how the magnetic field does that connection between the moons and Jupiter itself, because one of the main things that we think is happening is that material from some of the moons, Io in particular, is being lofted into space. Giant volcanoes on Io are blowing material into space. That material becomes ionized, loses an electron and then gets entrained in the magnetic field and driven back into Jupiter. So, there’s a literal physical connection between a moon, Io, with volcanoes blowing stuff into Jupiter. When that happens it produces aurorae that we can see at Jupiter, those northern and southern lights. So it’s just an awesome connection between geology, space physics and Jupiter itself.

Jim Green: Jupiter’s magnetic field is really quite different to the magnetic fields that we’re used to around some of the other planets, but it also gives us an opportunity to study where the magnetic field is generated. What are we learning about Jupiter’s magnetic field?

Jared Espley: As I’ve said, Juno is designed to look inside Jupiter and one of the ways it does that is through the magnetic field, like you mentioned. With our instruments on board Juno we are learning that the magnetic field at Jupiter is even more complex than we originally thought. It’s got this global planetary magnetic field that we knew something about to begin with, but it looks like there’s even more structure going on there. So, there may or may not be multiple places where the magnetic field’s originating, something deep inside and something maybe a little closer to the surface. We’re starting to just disentangle that now.

Jim Green: Perhaps that is related to the core of Jupiter, and Juno is designed through its gravity measurement to make some of those measurements. What are we finding out about the size of Jupiter’s core?

Jared Espley: Yeah, exactly; both the gravitational and the magnetic fields are ways to probe Jupiter’s deep interior. We’re starting to revise the classic picture that we’ve always had in our mind where we thought there might be a dense rocky core in the center surrounded by really, really condensed hydrogen and helium, a metallic layer [hydrogen under enough pressure takes on ‘metallic’ properties, such as becoming an electrical conductor] and then the lighter layers of gas above that.

The gravity measurements are starting to really upend this classical view, and it looks like it may just be more well mixed down there. There may not be the classic rocky core that we thought. But, honestly, there’s a lot of controversy and discussion going on right now within the science team, which is what you want. You want to have scientists arguing because that’s what’s fun.

A complex, writhing storm system in Jupiter’s northern hemisphere. Image: NASA/JPL–Caltech/SwRI/MSSS/Gerald Eichstädt/Seán Doran.

Jim Green: One of the things that Jupiter’s got that are just absolutely spectacular are the beautiful cloud structures. What is Juno finding out about these?

Jared Espley: While we have a lot of interest to look inside [Jupiter], we also happen to have this instrument, the JunoCam instrument, which is creating these amazing pieces of data and art at the same time. Some of those images are just beautiful to look at, and some of them are really revealing new science. A lot of that’s in the polar regions because we’re getting really good imagery at the poles that we didn’t have before. And we’re starting to see all these storms, these vortices that are swirling around the polar regions. JunoCam’s really starting to tease out how the polar atmosphere is working.

Jim Green: How come we didn’t see these before? Is this something new?

Jared Espley: It is because almost all of our imagery either comes directly from Earth, of course, where we have the Hubble Space Telescope or even the backyard telescopes that the listeners might have, where you can look at Jupiter [from an angle almost level with its equator]. So we’re able to see the side view of Jupiter, and it’s almost impossible to see the top view unless you happen to have a spacecraft there flying directly over that top view, which we do in the shape of Juno.

You can read the second part of the transcript here.