Late noachian icy highlands: Patterns of ice accumulation and flow

J. L. Fastook and J. W. Head. Late noachian icy highlands: Patterns of ice accumulation and flowLunar and Planetary Science XXXXV, #1115, 2014.

The record of non-polar ice deposits throughout the Amazonian suggest a cold and dry climate not significantly different from the present climate observed on Mars [1], where latitudinal movement of ice is largely in response to the varying obliquity component of the spin-axis/orbital parameters [2]. When one looks further into the past to the Noachian, the record is less clear and the characterization of the early martian climate is less certain. The existence of liquid water that flowed across the surface [3, 4, 5, 6] as well as open- and closed-basin lakes [5] has lead many to suggest that early Mars was “wet and warm” [7, 8]. Others argue that the early martian climate was more likely much colder and considerably drier [9, 10] based on a number of lines of evidence that include proposed phyllosilicate formation mechanisms [11], low erosion rates [12], poorly integrated valley networks with the open-basin lakes that suggest short-term episodic fluvial formation rather than long-term pluvial activity [13], as well as the possibility that the form of most precipitation might have been nivial [14]. The existence of a late-Noachian south circumpolar ice sheet [15] for which modeling studies suggest a mean annual temperature well below freezing [16] also argues for a “cold and dry” early Martian climate. Terrestrial analogs from the Antarctic Dry Valleys demonstrate that fluvial activity can take place at temperatures well below freezing [17, 18].

Recent estimates of the rate of water lost to space [19] has reduced the amount of water available for a “wet” Mars, and evidence from Martian meteorite requiring lower atmospheric pressures [20], when coupled with the faint young Sun, makes it very difficult for atmospheric modelers to produce a “warm” Mars [21]. “Extreme” events such as meteorite impacts have been proposed, whereby an ephemeral “steam” atmosphere lasting a few thousand years might exist and produce the observed landforms [22, 23, 24], however, landform evolution modeling seems to contradict these findings [25]. Recent atmospheric modeling results [26, 27], demonstrate that moderate pressures accompanied by a full water cycle produce an atmosphere where temperature declines with elevation following an adiabatic lapse rate, in contrast to the current situation on Mars, where temperature is almost completely determined by latitude. Lower temperatures at higher elevations encourage the movement of water from the warmer lowlands to the colder southern highlands, where it is sequestered in the form of regional ice sheets above an ice stability line that occurs close to 1000 m elevation. This ice, effectively “stored” at higher elevations can then be released by “extreme” events, such as meteorite impacts or volcanism, without the need to invoke the “steam” atmosphere. Further geological implications of this “cold and icy” scenario are explored in [18]. Any meltwater produced seasonally during these episodes would flow naturally to the lowlands in exactly the areas where the geologic record requires liquid water to be present and flowing across the landscape [18]. As the climate cooled again, water frozen below the ice stability line would sublime and return to the highlands as snowfall [27]. We analyze the nature and development of the “icy highlands” using glacial flow models.