An interview with Kelsi Singer, Washington University in St. Louis.
– You seem to be interested mostly in moons. Why such an interest? What is so special about moons?
The moons around planets are a very diverse and interesting population. They expand what we know about the geology that is possible. For example, we see really interesting features called „chaos” on Jupiter’s moon Europa that we don’t see anywhere else in the solar system. Chaos look sort of like a broken up ice sheet on Earth, and although we don’t know the exact formation mechanism, we know that it is related to Europa’s subsurface ocean. Studying moons also expands our possibility of finding life outside of Earth, as many of the moons have signs of liquid water.
– Can research like this tell us how thick is Europa’s ice shell? Will it be ever possible to drill through it and reach the ocean beneath?
As I mentioned above, we see a unique kind of geologic feature on Europa called „chaos”. We have models of how these features form, and by comparing what we actually see (like how big the features are, if they are higher or lower than the surrounding terrain, etc.) to the models, we can get some idea of how thick Europa’s ice shell is.
Looking at surface features is an indirect way of determining the ice shell thickness. There is still debate about how thick the ice shell is, but likely around 10 km thick. This would be a lot of ice to drill through! Although drilling through Europa’s crust won’t happen anytime soon, the next best step would be to peer through Europa’s ice shell with ice-penetrating radar. Depending on how thick the shell is and the properties of the radar instrument, we may see through the ice shell to the bottom. Even if we don’t see the total thickness, it would still be a thrilling way to gain insight into geologic processes in the shell. An ice penetrating radar will likely fly on any future spacecraft to visit Europa.
– You are also interested in other Jupiter’s moon – Ganymede.
On Ganymede I was measuring the depth of impact craters to try to understand how much heat is coming out of the moon. Readers may be familiar with impact craters from the moon, but we also have them on Earth. The reason we do not see as many on Earth is because geologic processes have erased many of them. The same thing happens on Ganymede, although the geologic processes are slightly different.
The great thing about impact craters is that they form with a characteristic shape – small crater have bowl shapes, and larger craters have more of a pie pan shape (you can think about cratering next time you are in the kitchen!). So if we see a crater that is not that shape, that tells us something interesting. If a surface is warm enough, the craters will want to flatten out as the material very slowly flows. If you have ever noticed that old windows are thicker at the bottom, this is because even solid material flows over long timescales. And the hotter the material is the faster if flows. On Ganymede we see flattened craters and some differences between different areas, telling us that the heat flow on Ganymede was fairly high and may have varied some over the surface.
– There is also a lot of heat flow on Enceladus – a Saturn’s moon. What the craters of Enceladus can tell us?
As with Ganymede, my research was looking at how the appearance of craters can help us understand how warm Enceladus has been in the past. Enceladus is known to have an inner rocky core, and an outer icy shell. The amazing tectonic features at the south pole (called the tiger stripe region) and the existence of plumes there indicate that Enceladus may also have a subsurface ocean, either at a global or local scale. It is still somewhat of a mystery how Enceladus gets all of its heat, but it could be related to something special about its orbit. It was not predicted to be this active! This is why it is great to send spacecraft to study our solar system, because we find all sort of interesting things that are not predicted.
– There is another extraordinary moon orbiting Saturn – Iapetus. It has a very strange shape, with a equatorial ridge. Iapetus looks like a big walnut.
The equatorial ridge on Iapetus is a very unusual feature, there is no other body in the solar system with a ridge quite like it. It reaches 20 km high at its highest point. The ridge is very cratered and thought to be a relatively ancient feature. It is so neatly placed at the equator that it is probably not tectonic. Tectonic features, like faults on Earth, usually do not form such straight lines. If Iapetus had a ring around it some point (perhaps from a moon that broke up in the past, or from a large impactor hitting Iapetus) then this ring could have gradually gravitationally come closer to Iapetus and eventually fallen to the surface to create the ridge. There are still many open questions about the ridge though.
Iapetus also has some of the solar systems largest landslides – some reach lengths of 80 km! We also have very large landslides on Earth, and it has been somewhat of a mystery why these very large landslides occur. Having examples on another world helps us understand how such a process works. For example, if we go to other bodies in the solar system, we can see landslides forming in different gravities (Iapetus has gravity that is 1/45th of Earth’s) and in different surface materials (Iapetus has a mostly water ice surface).
In general: I had an impressions that most of your research are about looking at a surface to determine what is beneath. And moons give you possibility to look at a naked globe (with no atmosphere, biosphere etc.) so the geological structures are clearly visible. Is that right?
That is a good description of my research – I work with both modeling of the geologic processes, and images of the geologic features on the surface. We try to match the models to what we actually see. And yes, there are no clouds or trees in the way of seeing the geology on the worlds I study! As you can tell, we have learned a lot about our solar system by sending spacecraft to study these worlds either by flying the spacecraft by them, or actually going into orbit about them, but we still have a lot of questions remaining.
Pluto is the last of the traditional set of planets for humanity to visit. The New Horizons spacecraft will undoubtedly reveal many interesting geologic features that we cannot even imagine now, just like when we visited the moons of Jupiter, Saturn, Uranus, and Neptune for the first time. I am very excited for the next few months leading up to July 14th, when the New Horizons spacecraft will make its closest approach of Pluto. I hope everyone will join in my excitement, as this will be the last “first visit” to a planet and everyone can follow along as the pictures come down and we get to see what Pluto and its large moon Charon looks like :)
Interview by Łukasz Kaniewski
Publishing date: 28 April 2015