An esker still physically associated with its parent glacier in Phlegra Montes, Mars

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.

Image 1

Image 1. A longitudinal section through part of an esker ridge in Ireland. Esker sediments are layered and often extremely coarse. The structures of the layers represent highly variable depositional settings within subglacial meltwater tunnels. The coarse sedimentary calibre represents extremely powerful meltwater flows.

Image 2

Image 2. (Left) A subglacial meltwater river emerges from debris-rich glacial ice Alaska. Note the ice-bridge crossing the ice-confined channel down-flow of the glacial meltwater portal – the channelised reach represents the unroofing of the subglacial conduit. (Right) Glacial collapse and partial unroofing reveals a large diameter (ca. 10 m) glacial meltwater tunnel in the Kennicott Glacier, Alaska. When meltwater-borne sediments fill conduits like this, and the glacier eventually melts, eskers are left behind as casts of the meltwater tunnel(s).

 

Image 3_Kinnity esker_digital globe image

Image 3. Vertical air photograph of the Kinnitty (or Knockbarron) Esker, Ireland. This small esker system is a complex of sinuous ridges, which were deposited within one of the lowland ice sheets that covered Ireland during the Earth’s last glaciation. Meltwater flow was from lower left to upper right along several adjacent (but not necessarily contemporaneous) tunnels. Note houses and buildings for approximate scale. Image credit: DigitalGlobe.

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.

Image 1_Phlegra Zone 1 to Zone 5 CTX mosaic complete_esker in context

Image 4. Southern Phlegra study region, 6m/pixel CTX mosaic. The different zones described in Gallagher and Balme (2015) are marked, as are the inferred flow directions of the lineated valley fill (LVF) occupying the east-west trending trough. The location of the system is shown in Box A, Fig. 2. Image credits NASA/JPL/MSSS/MOLA science team. North is up in this and all other images.

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.

Image 2_Phlegra location map_MOLA_THEMIS

Image 5. Location of Phlegra Montes (excerpt from USGS coloured relief map). White box A shows the location of the glacier and esker system shown in Image 1. Boxes A, B and C also indicate the location and strike of the regional graben.

Image 6_Phlegra_Zone 1_CTX

Image 6. The slope and distinctly lineated surface of the glacier (Zone 1, Image 1) indicates flow from west to east (L-R). The lineated surface presently terminates at a cross-valley band of hectometer-scale pits (Zone 2, Image 1). Image credit NASA/JPL/MSSS.

Image 7_Phlegra_Zone 2_CTX

Image 6. The slope and distinctly lineated surface of the glacier (Zone 1, Image 1) indicates flow from west to east (L-R). The lineated surface presently terminates at a cross-valley band of hectometer-scale pits (Zone 2, Image 1). Image credit NASA/JPL/MSSS.

Image 8_Phlegra_Zone 4b_ESKER_CTX

Image 8. Sinuous ridges (arrowed in Image 1) revealed by the degradation of the older piedmont glacial terminus. Image credits NASA/JPL/MSSS.

Image 9_Phlegra_Zone 4b_ESKER_detail_CTX

Image 9. Close-up showing details of the sinuous ridges, including ice margin remnants crossing over the ridges. This esker system is of a similar scale to the Kinnitty Esker, shown in Image 3. Image credit NASA/JPL/MSSS.

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.

Published article:

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.

Further reading:

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.

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