Post by Dr. Mikhail Kreslavsky1 and Prof. James Head2
1Assistant Research Planetary Scientist, UC Santa Cruz, USA. 2Planetary geosciences group, Brown University, Providence, Rhode Island, USA.
On Earth, cold-based glaciers (glaciers deforming internally, without basal melting and basal sliding) are found in the coldest environments (e.g., Antarctica, Marchant et al., 1993). Unlike the majority of glaciers, cold-based glaciers do not scour their substrate and leave pre-glacier topography unaffected. When cold-based glaciers advance and then dynamically stabilize (the ice flow is balanced by frontal ice ablation); debris carried forward by the glacier drops out at the glacial fronts as sublimation of the ice occurs; the dropped material produces so-called drop moraines.
In three locations at high northern latitudes of Mars, overlapping small ridges of arcuate planforms associated with slopes were interpreted as drop moraines left by extinct cold-based glaciers (Garvin et al., 2006; Kreslavsky and Head, 2011). Image 1 shows one of these locations, where a presumable glacier was formed at south-eastern part of an impact crater rim. The shapes of the extinct glacial lobes around the central peak of the crater suggest a few hundred meters thickness of the glacier.
Image 2 shows part of the largest of the three sets of drop moraines in another location. The slope of polar layered deposits associated with the moraines is located ~ 20 km to the east, outside the image. This slope made of water-ice with minor admixtures had retreated since formation of the moraines due to sublimation. In each of the three locations, overlapping of the moraines suggests 5 – 7 episodes of advance and retreat of cold-based glaciers. Ages of these features are poorly constrained, however, their morphological freshness and superposition relationships suggest ages at millions or tens of millions years.
Comparison of these extinct glaciers with known ice-flow landforms on Mars shows that these features differ in important aspects; in particular the morphology of the moraines suggested they flow easier.
These glaciers were interpreted to be due to perennial accumulation and flow of solid carbon dioxide during recent periods of low spin-axis obliquity (Kreslavsky and Head, 2011). Obliquity of Mars spin axis varies in a wide range; a dozen of a few-thousand-years long periods of low-obliquity occurred 0.5 – 3.5 millions years ago (Laskar et al., 2004). During these low-obliquity epochs carbon dioxide, the main component of Martian atmosphere, condensed in the cold polar areas of the planet (Kieffer and Zent, 1993) and might form glacial features in association with colder slopes (Kreslavsky and Head, 2003).
Garvin, J. B. and others (2006) High-latitude cold-based glacial deposits on Mars: Multiple superposed drop moraines in a crater interior at 70°N latitude. Meteoritics and Planetary Science 41, 1659-1674.
Kieffer, H. H. and A. P. Zent (1992) Quasi-periodic climate change on Mars, In: Mars, Univ. Arizona Press, Tuscon, AZ, p. 1180-1218.
Kreslavsky, M. A. and J. W. Head (2005) Mars at very low obliquity: Atmospheric collapse and the fate of volatiles, Geophys. Res. Lett. 321, L12202
Kreslavsky, M. A. and J. W. Head (2011) Carbon dioxide glaciers on Mars: Products of recent low obliquity epochs (?), Icarus 216, 111-115.
Laskar, J., and others (2004) Long term evolution and chaotic diffusion of the insolation quantities of Mars, Icarus 170, 343-364.
Marchant D. R., and others (1993) Miocene glacial stratigraphy and landscape evolution of the western Asgard Range, Antarctica. Geografiska Annaler 75A, 303-330.