What’s going on on Europa?

The bizarre surface of Europa

Europa is an intriguing little place. It’s the second innermost of the 4 Galilean satellites (there are actually more than 60 moons around Jupiter but these are all much, much smaller than the 4 big, Galilean, ones) and it has a very interesting and bizarre surface.

When we look at most moons in the Solar System we find that there are an awful lot of craters on them. Some of these are very small whilst others are very big. What’s useful about this is that through looking at the craters we can tell how old the surface is. When we look at the cratering record on the Moon we find that it is about 4 billion years old. Europa is a different story altogether. In the image above you can see very few craters, if any at all. What this means is that the surface of Europa is very, very young. After careful analysis it appears that the surface of Europa is, on average, only 65 million years old (that’s considered very young in geologic terms). Therefore there must be a process on Europa that is working to erase the evidence of these craters. What could it be?

The secret to Europa’s resurfacing lies in Jupiter’s massive gravitational field and interactions with 3 of the other big Galilean satellites. Through this process (tidal heating) the interior of the inner two moons are kept warm. This leads to the spectacular volcanoes of Jupiter’s innermost moon, Io. Io looks kind of like a pizza and is the most volcanically active body anywhere in the Solar System. Europa is under these same processes but to a lesser extent.

The Galileo mission to Jupiter, launched in 1989, found that Europa had a magnetic field about it. This inferred that Europa either has a salty ocean beneath the ice or that there is motion in its core. It is now accepted that this is probably due to the presence of an ocean. Further evidence for the existence of this ocean comes from one of Europa’s few impact craters, Pwyll. The crater is unusual in that its base isn’t any lower than the surrounding terrain and shows the hallmarks of the impact having been into thin ice (about 20km thick). It would be useful here to note that lots of different models come up with different thicknesses for the ice shell, some as little as 3km and some up to 100km. This is where the problems starts. Some features on Europa are explained by there being a thin ice shell and others only by a thick ice shell.

A new theory, published in the journal Nature, sets out to put aside some of these problems and explain how what’s called chaos terrain can form. It was previously suspected that you need a very thin layer of ice for them to form, but this theory explains how shallow sub-surface lakes may be responsible.

Thera Macula (false color) is a region of likely active chaos production above a large liquid water lake in the icy shell of Europa. Color indicates topographic heights relative to background terrain. Purples and reds indicate the highest terrain. Credit: Paul Schenk/NASA

The new theory states that phenomena similar to mantle plumes here on Earth heat the base of the ice. Convection occurs and ice lower in impurities slowly rises and melts forming a lens of water a few kilometers below the surface. This would cause the surface to subside and lead to cracks forming from the top of the lens and surface. Eventually these cracks would end up creating rafts, water would then freeze in between and the lens would slowly refreeze and then causing a dome to form at the surface. This process would take millions of years.

It has been speculated that these lakes would be ideal habitats for life. I’m not so sure though. As these lakes would eventually freeze over the only life they could sustain would be some kind of extremophile. In my opinion it would seem that life would be better off living in the ocean beneath the ice. I don’t know enough on this area of astrobiology though to make a truly sound argument.

Europa is a bizarre, intriguing and beautiful place but the only way we’re going to learn more about it is sending a mission there. A review said a mission to Europa should be NASA’s second highest priority (I guess succumbing only to a Mars sample return mission?). Previous missions have been cancelled, like the Jupiter Icy Moons Orbiter (JIMO). Work going on in Antarctica at Lake Ellsworth and Lake Vostok will help us with a possible lander mission to Europa. These lakes in Antarctica are hidden beneath deep layers of ice, drilling technology being used will help us when we come to drill and venture beneath Europa’s icy shell.

Four step process for building “chaos terrains” on Europa

If I haven’t explained any of this adequately (and I probably haven’t, I’ve been up since 5am) or purely just if you’re interested I’ve put some links below this and also the video from the initial conference explaining the theory.

Video: Jupiter Moon’s Subsurface Ocean of Water

NASA Probe Data Show Evidence of Liquid Water on Icy Europa

Active formation of ‘chaos terrain’ over shallow subsurface water on Europa

Greenberg et al (1999)
Schmidt et al (2011)
An Introduction to Astrobiology – C4 Europa – Dr David Rothery (OU study book)