Bad Astronomy
The entire universe in blog form
March 9 2009 1:45 PM


HiRISE view of Deimos

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!  


That is so cool! Deimos is the smaller of the two moons, a lumpy ball 15 x 12 x 10 km in size (Phobos is 27 x 22 x 18). Deimos has incredibly weak gravity; its escape velocity is only about 20 km/hr, so you could throw a baseball right off into space, and biking without taking an unintentional EVA would be difficult.

There is also another picture showing Deimos from two slightly different angles (though the angle to the Sun had changed enough to provide a different lighting angle). The resolution is such that you can see features as small as 60 meters across, smaller than a football field.

You can see some small impact craters on its surface, and it also has lots of smooth areas. Those latter are probably covered in regolith, rocks ground into fine ground powder from eons of micrometeorite impacts (much like the surface of the our own Moon). As the HiRISE page points out, you can see subtle color differences, with areas around craters being lighter, and smoother areas redder in color. As material like that is exposed to ultraviolet light from the Sun, it undergoes photolysis -- having its chemical structure changed -- and it becomes redder. So you can get an idea of relative ages of the surface just by looking at the color! As you might expect, regions around craters are younger, and tend to be lighter in color.

The one exception is the crater to the right of the picture; it's possible that the regolith has flowed "downhill" even in the low gravity, covering the crater.

Pictures like these remind me that the solar system is not just a collection of lights in the sky; it's composed of worlds to explore. And the more we reach out and look, the more fascinating, beautiful, and exciting it becomes.