Tom’s post yesterday about the asteroid hitting Jupiter got me looking at the Jovian system, and since my mind was on impact craters (I’ve been looking at Mercury) I went straight to Callisto.  Here’s a really cool image of Callisto taken by Cassini December 7, 2000.  Callisto is the lower left, and that’s Europa by the Great Red Spot.

NASA/JPL Cassini 12/07/2000 Jupiter, Europa, Callisto

What’s with Callisto, anyway?  From a distance she doesn’t look too bad, but close up the poor thing looks beat to pieces.  Battered though Callisto may be, it’s easy to see the ghostly beauty of Jupiter’s 4th Galilean moon.

NASA/JPL Galileo 05/2001 Callisto

As it happens, Callisto’s surface is extremely old.  Scientists estimate its surface age is about 4 billion years old.  It doesn’t experience plate tectonics or vulcanism, which would work to smooth out impact craters (like it does on Earth).  Its surface is a pretty faithful record of its battered past.  Callisto has the oldest surface in the Solar System.

Now, before you start thinking of Callisto as a dead and boring hunk of rock (with a lot of potholes), get this; NASA has good reason to believe there is a subsurface salt-water ocean on Callisto.

It also has a thin atmosphere of carbon dioxide and molecular oxygen.

NASA/JPL Icy spires on Callisto surface

Callisto is distant enough from Jupiter that it doesn’t receive as much radiation from the planet as the other Galilean moons.  If we ever establish a residential base in the Jovian system, it’ll probably be on Callisto.  Can you imagine exploring the surface?  That’s a 4 billion year old record of activity in our solar system.

NASA/JPL CalTech Callisto surface structures - Galileo

The enlargement of this image is spectacular, by the way.

In spite of its liquid ocean, Callisto is not considered to be a good candidate to support life.  We’ll still look to Europa for that, and we’ll probably be sitting on Callisto while we’re looking.

I’d say it’s far from being a dead, boring hunk of rock.