The space rock called the Tissint meteorite, that landed in the Moroccan desert last year, (with a fireball and double sonic boom), was knocked off Mars (NOTE: Subscribers can still listen to this show) in a cosmic collision around 700,000 years ago. This means that the rock was flung into space and began its journey to Earth when the shared ancestors of modern humans and Neanderthals were still living in Africa.
The deserts of Utah and Nevada have not always been dry: Between 14,000 and 20,000 years ago, valleys throughout the desert southwest filled with water to become large lakes, which–at their maximum–covered about a quarter of both those states. And a dry stream bed has been found on Mars–was this "red desert" planet ALSO once covered with water? This transformation may have happened on Mars as well!
The theory is that dark matter is made up of particles that can’t interact with the electromagnetic force, and thus can’t be revealed with light. But they do interact gravitationally–in fact, it’s the gravitational pull of dark matter that stops galaxies from flying apart as they rotate. Astronomers think there is five times as much dark matter in the universe as there is ordinary matter, even though we can’t see it. Since both the dark and the visible forms of matter are affected by gravity, they tend to cluster together.
If we can’t see dark matter, how is it detected? By looking at the distorting effect that dark matter has on the light emitted by nearby galaxies.