2. Mars: Ancient Highlands
The meteorite ALH 84001, where possible traces of martian life have been found, is one of the oldest rocks ever studied, and probably came from the oldest regions on Mars. These ancient parts of Mars, most of its southern hemisphere, are covered with asteroid impact craters, like this area in the bright region of Noachis. This part of Mars is as cratered as the Moon’s highlands, and so is about the same age, more than 4 billion years old.
The biggest crater here is Proctor, named for a nineteenth-century British astronomer. The dark splotches inside Proctor and many other craters are fields of sand dunes. In high-resolution images, these linear dunes look like waves on the ocean, but actually show how dry and desolate Mars is now.
Viking Orbiter mosaic (MDIM), NASA. From 45°S to 50°S and 325°W to 337°W, view approximately 500 kilometers across. Image processing by Brian Fessler and Allan Treiman (Lunar and Planetary Institute).