LATE NOACHIAN ICY HIGHLANDS CLIMATE REGIME: A HYDROLOGICAL SYSTEM CONCEPTUAL MODEL BASED ON THE McMURDO DRY VALLEYS

Introduction: In contrast to a " warm and wet " early Mars climate scenario [1,2], recent Late Noachian GCMs produce mean annual temperatures (MAT) well below 0°C. Above a few tens of millibars, atmospheric-surface coupling yields adiabatic cooling [3] and creates a highlands cold trap leading to a Late Noachian Icy Highlands (LNIH) climate model [4]. Melting to account for the abundant LN fluvial/lacustrine features comes from extreme variations in spin-axis/orbital paramenters and/or punctuated heating melting events such as impact or volcanism. The McMurdo Dry Valleys (MDV) provide a similar hyperarid, hypothermal environment, a-20°C mean annual temperature, adiabatic effects, and very limited melting. We examine the MDV hydrological system and cycle to gain insight into the possible configuration of the LNIH, and develop a conceptual model for Late Noachian Mars under MDV-like conditions [5]. MDV Conditions and Application to the LNIH: In the MDV, MAT are well below 0°C producing a regional permafrost layer and a horizontally stratified hydro-logic system (Fig. 1). Snow and glacial ice provide the meltwater source. Top-down heating and melting of these ice deposits takes place when peak seasonal (PST) and peak daytime (PDT) temperatures are >0°C. Here we assume that the peak seasonal/daytime temperatures in the Late Noachian icy highlands model can rise above 0°C for limited periods and we explore the consequences of this scenario. The extended summer season on Mars approximately doubles the time when melting could take place. This meltwater forms a perched aquifer above the ice table aquilude. The resulting fluvial activity is ephemeral, but repeated yearly events can carve valleys and form lakes. Lakes are semi-permanent due to ice cover, and thus meltwater is stored there during periods when temperatures fall below 0°C. Variations in the hy-drologic system are introduced by landscape-induced snow and ice dynamics (e.g., volume, altitude, and inso-lation geometry of flowing ice) and by small fluctuations in input parameters to solar insolation (e.g., spin ax-is/orbital parameters). In the LNIH scenario, the southern hemisphere up-lands would be blanketed by a layer of ice and snow tens to hundreds of meters thick [6] and snow would concentrate at high altitudes above the +1.0 km surface ice stability line (Figs. 2-3) [4]. Below this altitude, snow and ice could also accumulate on local highs depending on local and regional topography and atmospheric circulation patterns [7]. In a steady-state situation, any surface liquid water at low-altitudes (equatorial and northern lowlands) would …