THE CLIMATE HISTORY OF MARS AS RECORDED BY THE GEOLOGIC RECORD

A number of investigations have demonstrated that early Mars supported rainfall as well as an advanced hydrologic cycle, and Jeff Moore has been involved in many of the important benchmark studies. Although valley networks are the most commonly cited features for evidence that liquid water was once stable on the martian surface, Irwin et al. [2015] demonstrated that their formation was restricted towards the end of the Noachian/beginning of the Hesperian when climatic conditions become optimum. They showed that valley networks are not well-integrated with the surrounding cratered landscape, which would not be the case if conditions favoring valley network development persisted throughout the Noachian. Quantitative evaluation of features, such as large alluvial fans by Moore and Howard [2005], indicate that intense fluvial conditions only lasted for hundreds of thousands of years.

If valley networks do not represent the climatic conditions that occurred through the Noachian, then modified impact craters do. Modified impact craters occurred at all crater diameters; however, the degree of modification is independent of size. This indicates that modification processes were continuously (if not episodically) operating as new craters were forming on early Mars, as demonstrated by results of landscape evolution simulations presented by Barnhardt et al. [2008]. Unlike valley networks, which are restricted to a band near the equatorial region of Mars, modified impact craters also occur at higher latitudes suggesting that the erosional processes—and thus the associated climatic conditions—were global. This suggests that potentially early conditions were controlled by a primordial steam atmosphere that slowly collapsed and precipitated into the Martian regolith, eventually resulting in lakes and even an ocean. Unlike the Earth where most of the rock record from earlier conditions has been lost to erosion and plate tectonics, the spatial and temporal variations of the ancient climate as represented by the geology are preserved on Mars. The implications are not only for understanding the history of water on Mars, but the origin of life on Earth as well as habitable zones and extrasolar planets.