Debris aprons and valley fill hold ancient ice several hundred million years old

Along the edge of the crustal dichotomy, where the southern highlands step down onto the northern plains, lies a landscape of mesas, buttes, and valleys. Valleys appear choked with linear streams of debris, and on the flanks of many mesas are gently sloping aprons of debris that make each resemble a half-melted ice cream sandwich.

ROCKY PRESERVATIVE. As the regional ice sheet covering Deutronilus disappeared, it uncovered mesas and valley walls of rock. Once exposed, these shed rocky debris that eventually covered the ice, taking roughly half a million years to do so. The rock-covered icy aprons have remained mostly unchanged for the last several hundred million years. (Image taken from Figure 2 in the paper.)

Both features are probably caused by the slow disappearance of regional sheets of ice, says a group of researchers led by James Fastook (University of Maine). Their work, published in Icarus, examines three existing models for the origin of “lobate debris aprons” and “lineated valley fill” in the Deuteronilus region. Using SHARAD ground-penetrating radar, previous theoretical work, and computer-modeling, the team concludes that the features are most likely remnants of a retreating regional ice sheet. This would have buried the mesas and buttes over a broad area along the dichotomy, much as ice sheets covered parts of Europe, Asia, and North America during the Pleistocene.

“As a regional ice sheet collapses,” they write, “the surface drops below the cliff and massif bedrock margins,” exposing rock and regolith, which fall onto the ice surface. The rocky debris reduces the amount of sublimation by armoring the ice. This “produces a surface slope and consequent ice flow that carries the armoring debris away from the rock outcrops.”

As the collapse and ice retreat continue, the debris cover eventually reaches the ground surface at the front of the glacier, thus leaving the entire apron covered and armored by rocky debris. Based on crater-counts, the aprons are at least several hundred million years old.

From the SHARAD data, the debris aprons’ profiles, and ice-flow modeling (including temperature), the scientists were able to estimate how long it took to make a fully formed debris apron.

“We find that for the population examined [in Deuteronilus],” they say, “typical temperatures are in the range of –85° to –40° C [–120° to –40° F] and typical sublimation rates lie in the range of 6 to 14 millimeters per Earth year.” They add that lobate debris aprons will form — from the point of bedrock exposure to complete debris cover — in around 400,000 to 500,000 Earth years.

Similarly, the lineated valley fill is consistent with an extensive flow of glacial ice from plateau ice fields. The ice streams would have acquired a debris cover in the waning stages of the regional ice sheet’s retreat as bedrock scarps and mesa regolith became exposed.

The team says there’s a long term climate finding, which also presents an opportunity. “We conclude that lobate debris aprons along the dichotomy boundary could never have experienced temperatures near or above the ice melting point and still retain their current shape.”

The valley fill and debris aprons have thus been preserved at subzero temperatures for the last several hundred million years. The scientists note, “One might then expect to potentially find, preserved below the sublimation lag deposit, ancient ice that contains a record of the climate of Mars when the debris aprons formed.”

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