The Mars rover Spirit came upon a particular rock in its travels. Scientists studying that rock determined it was volcanic in origin, and a new study now sees the rock as evidence that early Mars was similar to Earth.
The study argues the rock was blown out of a Martiam volcano and fell back to the surface. By measuring the depression, or bomb sag, the rock made when it hit the ground, the study determined it traveled slowly enough that it must have been moving through a much thicker atmosphere than the one Mars has today. That is another line of inquiry pointing to a warmer, wetter Mars early in its history-- one that would have had a thicker atmosphere.
Scientists say the volcano in question erupted 3.5 billion years ago.