Loess in the lowlands

A team of geologists led by James A. Skinner, Jr. (U.S. Geological Survey) has discovered and mapped a previously unidentified unit in the Martian northern lowlands. The unit appears to give evidence of a major climate shift long ago in Martian history. Their report appears in the December 2012 issue of Geology.

YESTERYEAR'S REMNANTS. A newly identified unit of soft sediments (gray areas) in Mars' northern lowlands offers evidence of a major climate change that occurred approximately 3.5 billion years ago. (Image taken from Figure 1 in the paper.)

Currently the unit, which lies in several patches and discrete outcrops, has a total area larger than 3 million square kilometers, or about 4.5 times the area of Texas. When originally deposited, the team says, its area was probably five times larger still. Today it averages 32 meters (105 feet) thick.

The age of the unit (Middle Amazonian, or about 3.5 billion years old) and physical details of its outcrops lead the researchers to suggest that the unit is made of materials eroded from the basal unit that underlies the north polar ice cap.

The scientists write, “Our observations are consistent with the widespread emplacement of a loess-like deposit tens of meters thick in the Martian northern lowlands during the Middle Amazonian due to climate-driven erosion of the north polar plateau.”

Loess is a friable compact silt-like sediment that accumulates from wind-blown rock dust. On Earth loess deposits can reach many tens of meters thick. The researchers noted that the draping and layering characteristics of the newly identified Martian unit fit an origin as fallout from the air.

The team used a Mars Orbiter Laser Altimeter (MOLA) digital elevation model; MOLA-derived slope, aspect, and roughness data sets; and Thermal Emission Imaging System (THEMIS) infrared images. They determined ages by crater-counting using Context Camera (CTX) images.

“Results of this study reveal that the Martian lowlands underwent a previously unrecognized phase of widespread sedimentation during the Middle Amazonian,” the scientists report. The episode lasted roughly a billion years and resulted in the burial of about 16 million square kilometers of Late Hesperian and Early Amazonian plains by a unit tens of meters thick.

While the polar caps reveal layering caused by periodic climate cycles, scientists can model these swings only over the last 10 million years. The newly identified unit is far older than this. The researchers suggest that deposition of the unit interrupted more than two billion years of inactivity in the lowlands, and it may have been caused by a major change in the climate due to a prolonged, large tilt of the Martian axis.

“In this scenario,” they write, “climate-induced downcutting of the paleo-polar plateau resulted in the southward transport and deposition of fine sediments and volatiles from the polar basal unit.” This produced an ice-enriched, loess-like deposit at least a few tens of meters thick on average.

When the climate changed again, this soft and friable unit began to erode, and its sediments traveled back north. There they were deposited as the Planum Boreum upper-layered deposits. Computer models for the remaining parts of the unit suggest that while it is porous, it has become partly or wholly dried out, with little or no subsurface ice remaining.

The team concludes, “We stress that the hemisphere-scale mapping presented herein is subject to refinement at larger (local) scales in order to more completely assess the history of lowland sedimentation during the Amazonian Period.”

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