Post by O. Bourgeois, M. Gourronc, D. Mège and S. Pochat – Laboratoire de Planétologie et Géodynamique, Université de Nantes, France
The current climate on Mars does not allow for significant accumulations of surface ice at low latitudes. Therefore ice is only found at the two polar ice caps and in a number of ice-filled craters scattered at northern and southern latitudes (> 70°).
Theoretical considerations on the stability of water ice and climate numerical simulations however predict that surface ice accumulation areas may have shifted repeatedly between high and low latitudes in the past, in response to changes in Martian orbital and atmospheric characteristics (Forget et al., 2006 ; Wordsworth et al., 2013).
Observational evidence of past glaciations has also been described at Martian polar to tropical latitudes (Kargel and Strom, 1992). These include currently debris-covered glaciers and landforms interpreted as remnants of extensive glacial landsystems. All these glacial landforms are thought to be related to Amazonian (< 3 Ga old) cold-based glaciations (Dickson et al., 2008; Head et al., 2010).
However, little morphological evidence of past glacial activity has been described in the equatorial regions of Mars. A reapraisal and cartographic compilation of landforms in Valles Marineris (Gourronc et al., 2014) has revealed that this giant valley system that stretches over 2000 km along the martian equator, was entirely covered by a wet-based glaciated valley system during Late Noachian to Early Hesperian times, 3.5 Gy ago, and still contains huge volumes of fossil ice inherited from this ancient glaciation (Image 1). This supports the idea that massive wet-based ice deposits have formed at the Martian equator during the Late Noachian or Early Hesperian and have been preserved for their larger part until the present day.
The upper parts of valley walls in Valles Marineris display a spur-and-gully morphology typical of terrestrial periglacial environments, whereas their lower parts generally comprise smooth basal escarpments resembling glacially trimmed surfaces. Mège and Bourgeois (2011) interpreted the contact between these two distinct morphologies as a glacial trimline, i.e. a morphological boundary marking the maximal elevation of an ancient glacial fill. The morphological boundary has been spectacularly preserved along valley walls and stands several hundreds to several thousands meters above valley floors (Image 2). It winds around topographic obstacles and displays long-wavelength variations in elevation. This boundary is underlined by lateral benches, hanging valleys and truncated spurs, which are diagnostic landforms of glaciated valley margins.
Valles Marineris valley floors are covered by various kinds of terrains, including hummocky terrains, platy terrains, lateral banks, layered benches and a draping mantle (Image 3). Sublimation landforms observed in these terrains and their cartographic relationships with the interpreted trimline suggest that all these terrains correspond to various disintegration stages of a fossil glacial fill, currently protected by a superficial cover of ablation till (Image 3).
Partial disintegration of the Valles Marineris glaciated valley system caused gravitational spreading of the intervening rocky basement ridges, thus leading to the development of sackung, crestal grabens and uphill-facing fault scarps on basement ridges (Image 4, Mège and Bourgeois, 2011). In some instances, paraglacial collapse of rocky basement ridges triggered giant landslides (Image 1). These landslides have singular morphologies, unexpectedly long runout distances, and cover unexpectedly wide areas comprised in the range between those of dry subaerial landslides and those of wet submarine. All these characteristics are consistent with the hypothesis that the landslides were emplaced supraglacially (De Blasio, 2011), over fossil ice bodies that have been preserved on Valles Marineris valley floors until the present day.
Altogether, landforms and terrains in Valles Marineris thus compose a full glacial landsystem, consistent with the former presence of wet-based glaciers, as thick as 1000 to 3000 m, that were able to flow and slide over their beds. This glacial system was most probably fed by ice accumulating at low elevations directly from the atmosphere onto valley floors and walls, with only minor contributions from tributary glaciers flowing down from higher elevations. Similar fossil glacial landsystems dating back from the early Martian history are to be expected in many other low-latitude troughs such as chasmata, chaos, valleys, impact craters and other basins.
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