Mars: Always Cold, Sometimes Wet?

and Introduction: A synthesis of a diverse suite of observations of H2O-related landforms that are possible Mars analogs from terrestrial polar regions (Devon Island in the Arctic; the Dry Va lleys of Antarctica) put into question any requirement for extended episode(s) of warm and wet climate in Mars’s past. Geologically transient episodes of localized H2O cycling, forced by exogenic impacts, enhanced endogenic heat flow, and/or orbit-driven short-term local environmental change under an otherwise cold, low pressure (=10 mbar) global climate, may be sufficient to account for the martian surface’s exposed record of aqueous activity. A Mars that was only sometimes locally warm and wet while remaining climatically cold throughout its history is consistent with results (difficulties) encountered in modeling efforts attempting to support warm martian climate hypotheses [1], [2]. Possible analogs from terrestrial cold climate regions for the recent gully features on Mars also illustrate how transient localized aqueous activity might, under specific circumstances, also occur on Mars under the present frigid global climatic regime. Early Mars: Wet and Warm? Two main observations of martian surface landforms are at the core of classical interpretations of Early Mars (Late Noachian) being wet and warm, in contrast to the later dry and cold climate: a) the state of advanced degradation and erosion of impact craters in the martian highland terrain compared to lunar craters; b) the ubiquitous presence of small valley networks dissecting the same heavily cratered highlands, with the interpreted implication of relatively moderate discharges and hence extended formation times; meanwhile small valleys are largely absent in younger units with the exception of some recent volcano flank surfaces. Crater Degradation and Erosion Rates on Late Noachian Mars: Lessons from Haughton Crater. While ancient highland craters on Mars are indeed significantly more degraded and heavily eroded than lunar craters, they remain remarkably well preserved compared to terrestrial impact structures that have experienced only moderate erosion by terrestrial standards. Figures 1 and 2 provide a comparison between Northport Crater in the martian ancient highlands and Haughton Crater on Devon Island, Canadian Arctic. The two structures are of similar diameter (~20 km) and display evidence for substantial aqueous erosion. Although Northport is greater in age than Haughton by at least two orders of magnitude, Northport exhibits less cumulative degradation and erosion than Haughton: Northport presents a well-defined rim, remnant ejecta blanket material, limited dissection of surrounding uplands, and an unfilled well-incised V-shaped crater-cutting valley; Haughton in contrast has lost all original rim and extracrater ejecta materials, and exhibits substantial dissection from glacial peneplanation, extensive meltwater drainage, and limited rain.