Question

How would you go about finding out, when Mars lost it's Warm, wet environment, with a substantial atmosphere?

The general approach is to locate evidence for liquid water and determine its geological context. Thus, for example, the “valley networks” that are generally interpreted as evidence of rainfall (with an implied larger and warmer atmosphere) are associated with the ancient cratered highlands and are classed as Noachian in age, meaning generally older than 3.5 billion years. The huge outflow channels, which may indicate episodic presence of water on the surface, are Hesperian in age (roughly 2 to 3 billion years), as are the major volcanoes and the Tharsis Bulge. The most recent period is Amazonian, and this seems to have been a time when Mars was cold and dry, as it is today. Similar analysis is possible with the evidence for liquid water found on the ground by the current Mars rovers. Perhaps the greatest uncertainty is assigning absolute ages to these geological periods. We infer ages from the numbers of impact craters, but we have no Mars samples that can be used to give us precise ages in years, as we do for Earth and Moon. David Morrison
NAI Senior Scientist
December 26, 2007