Post contributed by Colman Gallagher, University College Dublin.
Eskers are sinuous ridges composed of deposits (Image 1) laid down from meltwater flowing in tunnel-like conduits beneath glaciers (Image 2). On Earth, eskers are common components of deglaciated landscapes (Image 3) but eskers also can be observed emerging from the margins of intact glaciers.
Sinuous ridges exist on Mars and several have been interpreted to be eskers. However, none of these previously identified possible eskers is associated with an extant glacier. This is not because there are no glaciers left on Mars – actually, glaciers are common across the mid-latitudes of Mars. However, these present-day martian glaciers are thought to be incapable of producing sufficient meltwater for eskers to form within them. This is a consequence of both the present climate and, therefore, the glaciers being extremely cold. Hence, previously observed isolated sinuous ridges interpreted to be martian eskers are thought to have formed beneath melting glaciers that existed in earlier, warmer periods over 3 billion years ago (3 Ga) but which have long since disappeared.
These Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) images show an assemblage of sinuous ridges emerging from the degraded piedmont terminus of a lineated valley fill (LVF) of Amazonian-age (Image 4). The Amazonian is the most recent geological period on Mars, spanning 3 Ga to present. The system is located in the southern Phlegra Montes region of Mars (Image 5). The characteristics of the LVF indicate that it is a trough-confined glacier, with a surface patterned by flow lineations and a present terminus characterised by bands of cross-valley pits (Images 6 to 7). The spatial density of craters on the glacier surface indicates it is ca. 150 million years old (150 Ma). The sinuous ridges have the hallmarks of being an esker system emerging from an older, degraded terminus of the glacier (Images 8 to 9), which had extended out of the uplands onto the piedmont before retreating to its present limits within the upland trough.
These observations are the first identification of a martian esker system that is still physically associated with its parent glacier (Gallagher and Balme, 2015). Significantly, the eskers and their contextual landform assemblage, both on the LVF and along its piedmont glacial reach, are evidence of a wet-based glacial regime. This is difficult to reconcile with the prevailing cold, hyper-arid martian environment, in which only cold, dry-based glaciers should exist. However, the glacial system and the eskers are confined to a well-defined, regionally significant graben (Image 5). On Earth, grabens are often zones of significantly elevated geothermal heat flow, e.g. as recorded along the Rheingraben in Germany. In Phlegra Montes, therefore, the presence of the eskers in a graben, but the absence of eskers elsewhere in the region outside the graben, suggests that sub-glacial melting occurred as a response to enhanced geothermal heat flux, rather than climate-induced warming. If so, glaciers on Mars apparently can behave like temperate glaciers on Earth, despite the climate of Mars favouring only cold/dry-based glaciation, provided that sufficient heat is received from non-climatic sources. This offers important new insights to the non-climatic forcing of glacial behaviour on Mars, including the production of significant quantities of meltwater triggered by geothermal heating. These issues are of importance in the domain of ice stream generation and glacial stability on Earth and, we now realise, in the context of geothermally-induced cryosphere destabilisation as an amplifier of climate on Mars.
Colman Gallagher and Matthew Balme (2015). Eskers in a complete, wet-based glacial system in the Phlegra Montes region, Mars, Earth and Planetary Science Letters, 431, 96-109.
Banks, M.E., Lang, N.P., Kargel, J.S., McEwen, A.S., Baker, V.R., Grant, J.A., Pelletier, J.D., Strom, R.G., 2009. An analysis of sinuous ridges in the southern Argyre Planitia, Mars using HiRISE and CTX images and MOLA data. J. Geophys. Res. 114, E09003.
Bernhardt, H., Hiesinger, H., Reiss, D., Ivanov, M., Erkeling, G., 2013. Putative eskers and new insights into glacio-fluvial depositional settings in southern Argyre Planitia, Mars. Planet. Space Sci. 85, 261–278.
Fastook, J.L., Head, J.W., Marchant, D.R., Forget, F., Madeleine, J.-B., 2012. Early Mars climate near the Noachian–Hesperian boundary: independent evidence for cold conditions from basal melting of the south polar ice sheet (Dorsa Argentea Formation) and implications for valley network formation. Icarus 219, 25–40.
Head, J.W., 2000. Tests for ancient polar deposits on Mars: origin of esker-like sinuous ridges (Dorsa Argentea) using MOLA data. Lunar Planet. Sci. Conf. Abstr. XXXI, 1116.
Hubbard, B., Souness, C., Brough, S., 2014. Glacier-like forms on Mars. Cryosphere 8, 2047–2061.
Kargel, J.S., Strom, R.G., 1991. Terrestrial glacial eskers: analogs for martian sinuous ridges. Lunar Planet. Sci. Conf. Abstr. XXII, 683–684.
Milliken, R.E., Mustard, J.F., Goldsby, D.L., 2003. Viscous flow features on the surface of Mars: observations from high resolution Mars Orbiter Camera (MOC) images. J. Geophys. Res. 108 (E6), 5057.
Shreve, R.L., 1985. Esker characteristics in terms of glacier physics, Katahdin esker system, Maine. Geol. Soc. Am. Bull. 96, 639–646.
Souness, C., Hubbard, B., Milliken, R.E., Quincey, D., 2012. An inventory and population-scale analysis of martian glacier-like forms. Icarus 217, 243–255.
Thompson, W.B., 2014. Maine’s eskers. Maine Geological Survey Website. Geological Site of the Month, January 2014. Accessed 23 April 2015.
Warren, W.P., Ashley, G.M., 1994. Origins of the ice-contact stratified ridges (eskers) of Ireland. J. Sediment. Res. A 64, 433–449.