1 December 2010
I love pictures of a planet’s limb (jargon for the horizon of a planet seen from space). In the typical overhead views of planets that we get most of the time, it’s easy to forget that we’re looking at another planet from outer space. On the other hand, when you can see the terrain stretching off into the distance, and the darkness of space above it, it somehow seems more vast and more real.
I especially like this photo of Mars and its little moon Phobos, taken by Mars Express last month and featured as today’s Astronomy Picture of the Day. You can see the cratered surface of Mars, and some sinuous features that might be fluvial channels, but what’s really striking is how much darker Phobos is than Mars. In most pictures of rocky objects like asteroids or the moon, they are seen against a black background so they look grayish-white, but in fact a lot of the rocks up there, including the moon, are very dark colored. Part of the reason for this is a process called space weathering, where solar wind, cosmic rays and micrometeorites bombard the surface and cause little blobs of glass to form. These glassy agglutinates have nanophase iron particles in them, which tend to be very dark colored. That’s why fresh impact ejecta on airless bodies like the Moon and Mercury look lighter than the surroundings: they haven’t been space-weathered as much!