A billion km distant ice mountain against the black

A billion km distant ice mountain against the black

A billion km distant ice mountain against the black

Bad Astronomy
The entire universe in blog form
July 14 2010 3:03 PM

A billion km distant ice mountain against the black

Carolyn Porco just tweeted about a beautiful image from Cassini, showing the icy moon Tethys hanging in space:


Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


How forbidding and lovely!

Tethys is big, about 1100 km (660 miles) across (about 1/3 the diameter as our own Moon). Its density is actually a bit less than that of water, so it's most likely predominantly composed of water ice. The surface is bombarded with craters, including the big one at the bottom called Melanthius. It's 250 km across (150 miles!) and sports a massive central peak, common in larger craters. The crater itself is from a gigantic impact on the moon, and the central mountain forms when material is first displaced by the impact, then flows back. Under those titanic stresses, solid material can actually flow as the impact shock wave passes through, so these peaks are seen on lots of big objects in the solar system.

Cassini was 670,000 km (415,000 miles) from Tethys when it took this shot, which is nearly twice the Earth-Moon distance. The Sun is shining on Tethys from the left (the angle between the Sun, Tethys, and Cassini was about 41°, for those keeping track at home). You can see just how beaten Tethys is by looking at the terminator, the day/night dividing line, where shadows highlight the cratering. Even so, sandblasting by particles in Saturn's rings has smoothed the moon's surface, making it highly reflective -- it's shiny!

If there ever comes a day when I tire of seeing pictures like this, write my obituary. I'll be done.

Image credit: NASA/JPL/Space Science Institute