MARS: SPECULATIONS REGARDING THE EVOLUTION OF THE NOACHIAN HYDROSPHERE AND CLIMATE

The resemblance of the Martian valley networks to terrestrial runoff channels, and their almost exclusive occurrence in the planet's ancient cratered terrain, has suggested to some that the networks are the relics of a substantially warmer and wetter greenhouse climate that may have existed throughout much of the Noachian. Here we also consider another possibility: that the valley networks, as well as other frequently cited geomorphic evidence of a warm early Mars, are simply the byproducts of the geologic evolution of a water-rich planet - created by a variety of processes, including rainfall, that occurred under ambient global atmospheric and climatic conditions no different than those we observe on Mars today (subject to enormous local, regional and global perturbations due to impacts and other energetic geologic processes known to have occurred during the Noachian).

A recent analysis of the likely initial distribution and evolution of water on Mars suggests that, following the development of the global dichotomy, an ice-covered ocean may have occupied the northern plains, with numerous lakes and seas residing in low-lying elevations elsewhere on the planet. The progressive crustal assimilation of these early surface reservoirs of water appears to have been a natural consequence of the planet's subsequent climatic and geothermal evolution, with the instability of H2O at low-latitudes leading to its sublimation and ultimate cold-trapping at the poles. Eventually, this process would have resulted in polar deposits that were thick enough to undergo basal melting - introducing the water into the subsurface at both poles. In the northern plains, the former presence of a primordial ocean, followed by repeated episodes of eolian deposition, volcanism, impacts, catastrophic flooding and high-obliquity sublimation, is likely to have resulted in a complex stratigraphy of segregated ice deposits sandwiched between layers of varying lithology and pore saturation. Geophysical investigations, conducted over the next decade, are likely to provide significant insights regarding the validity of this analysis.