We’ve known for years that Jupiter’s moon Europa almost certainly has an ocean of liquid water deep under its frozen surface. For one thing, the surface is almost all water ice. We also know that it’s covered in thousands of cracks that look very much like the type we see in ice floes floating on liquid water here on Earth. And we have a heating mechanism: tides from Jupiter as well as from the other moons flex Europa, causing its interior to warm up.

A nagging question has been how thick is the solid ice shell over that ocean: is it many kilometers thick, or much thinner? Evidence supports both arguments, which is maddening. However, that problem may now be solved: astronomers studying Europa’s terrain think the ice shell is generally very thick, but — and this is the cool part — may have vast underground lakes of water!

This picture is from observations of Europa made by the Galileo spacecraft, which orbited Jupiter for many years. It’s a combination of optical images and photoclinometry — using pictures to measure the heights of surface features. Purple and red is elevated terrain, and you can see that this looks like a depression in the surface. It’s filled with what’s called "chaotic terrain" for obvious reasons. Most of the surface of Europa has larger scale structure, and is more organized, as you might expect from a thick shell of ice. But these smaller regions are a mess, and it looks like this is from pockets of liquid water under the surface, giant lakes the size of North America’s Great Lakes, completely buried in the ice.

This artist’s view shows how this works; the lake is completely embedded in the ice shell. In general, the ice is very thick, explaining the usual look of Europa’s surface. But in some spots, just below the ice, the ice has melted. The ice above this underground lake is much thinner, perhaps only 3 km (about 2 miles) thick, explaining the chaotic surface in those localized spots.

That’s pretty nifty, but why is this so important? (more…)

Last year, the Cassini spacecraft found solid (haha) evidence for the existence of lakes of liquid methane and ethane on the giant moon Titan. Of course, Titan is barely a moon at all — bigger than Mercury, it would be a planet in its own right if it weren’t orbiting Saturn. It has an atmosphere with almost twice the surface pressure as Earth’s, which is mostly nitrogen and a trace of hydrocarbons.

But that trace is important: because Titan is so cold, methane and ethane can rain from the Titanian sky, forming river systems and lakes. But there’s a problem: the north pole of the moon has far more lakes than the south pole. Seven times as many!

Why?

First, methane on Titan goes through cycles something like water does on Earth. During Titan’s summer, the northern lakes lose methane to evaporation, and the gas gets transported to the colder south pole, where it recondenses. The opposite happens in the southern summer. But that should balance out, so that each pole over the course of time has the same amount of liquid. They don’t. So, clearly, something else is going on. And it isn’t topography; the north and south polar regions of the moon have roughly the same overall shape, so you don’t expect liquids to flow into or out of one of those regions more than the other.

However, some scientists think they may now know what it is. Their idea, not yet proven but still very interesting, is that the reason is due to the seasons on Titan, coupled with Saturn’s elliptical orbit.

Saturn orbits the Sun once every 29 or so years. Its orbit is decidedly elliptical; it varies from about 1.35 to 1.5 billion kilometers from the Sun, a variation of 150 million or so kilometers! When Titan’s north pole is in summer, Saturn is farther from the Sun, and the southern summer is when Saturn is closest. That means that summers are cooler in the north, lowering the methane transport to the southern pole… and summers are warmer for the south pole, increasing methane transport.

In other words, Titan’s south pole is selling low and buying high. That’s not what you want to do if you want to keep yourself rich in hydrocarbon lakes.

Interestingly, again like the Earth, the geometry of Saturn’s orbit changes slightly over time. Over the course of tens of thousands of years, it changes such that the situation is reversed: Titan’s north pole will experience summer when Saturn is closest to the Sun, and the south pole when Saturn is farthest. If this idea of asymmetrical seasonal flow is correct, this will reverse that flow, putting more lakes at the Titanian south pole. On the Earth, these orbital variations are called Milankovitch cycles, and are most likely tied to very long term (like thousands of years long) global climate change. It’s possible, too, that the Milankovitch cycle may be tied to regions on the Earth periodically becoming deserts and then turning more humid once again.

Titan is sometimes seen as an analog of the young Earth, with methane taking the place of water in its geology and chemistry. It’s interesting to see that perhaps we have even more in common than we thought!