THE LATE NOACHIAN CLIMATE OF MARS: EVIDENCE FROM GEOMORPHOLOGY, STRATIGRAPHY, CLIMATE MODELING AND TERRESTRIAL ANALOGS

The Late Noachian geology of Mars displays significant evidence at orbital and surface exploration scales for fluvial and lacustrine processes, relatively intense crater degradation, a vertically integrated hydrologic system, the occurrence of phyllosilicates in terrains of Noachian age, and perhaps pluvial processes and the presence of a northern ocean. These lines of evidence have been interpreted to mean that the Late Noachian climate of Mars was “warm and wet” or “warm and arid” throughout this period, with global Mean Annual Surface Temperatures (MAST) typically exceeding ~273 K. In contrast to the geological evidence, however, recent global climate models with a faint young Sun have been unable to reproduce warm and wet conditions, instead predicting extremely cold temperatures (MAST ~225 K); furthermore, with atmospheric pressure above a few tens of millibars, atmospheric-surface thermal coupling is induced and adiabatic effects cause water vapor to be deposited in the highlands as snow and ice, leading to the “Late Noachian Icy Highlands” (LNIH) climate model. Advocates of the LNIH model call on episodic climate excursions to cause melting of the icy highlands to explain the abundant geological evidence for flowing and ponding water, but can these processes adequately explain the extensive geologic evidence? We review the geologic evidence from the perspectives of geomorphology, stratigraphy, mineralogy, global water budget, climate modeling and terrestrial analogs to assess the “warm and wet” and “cold and icy” hypotheses for Late Noachian Mars. We also trace the aftermath of these two hypothesized Late Noachian climate scenarios into and through the Hesperian and Amazonian, and examine the implications for the evolution of the Late Noachian climate to its current hyperarid, hypothermal condition.