Hidden valley at the north pole

The north polar cap of Mars has a wide, deep trough called Chasma Boreale that slices into the polar cap for 560 kilometers (350 miles).

HIDDEN VALLEY. Radar profiles from the MARSIS instrument have uncovered a large trough (yellow arrow) in the Basal Unit that lies beneath the north polar ice cap. (The red area at left shows where the Basal Unit is thickest.) Image is from Figure 2 in the paper.

The floor of the chasma exposes a layer of sand and dust cemented with water ice. This layer, known to scientists as the Basal Unit, underlies the entire ice cap and rests upon the bedrock of the northern plains.

New, high-quality data from the MARSIS radar sounding instrument on the Mars Express orbiter has let scientists explore the shape and internal structure of the Basal Unit. The data have revealed another trough nearly as large as Chasma Boreale. However this new valley, which measures about 400 km long by 100 km wide (250 by 60 mi), lies entirely within the Basal Unit itself and is hidden from view, being covered by the polar ice deposits.

At the 43rd Lunar and Planetary Science Conference in The Woodlands, Texas, a team of MARSIS scientists led by Alessandro Frigeri (Istituto di Astrofisica e Planetologia Spaziali of INAF, Italy) presented a 3D computer reconstruction of the Basal Unit. The reconstruction revealed internal layering of the Basal Unit and the new trough.

The team built the computer model using about 160 MARSIS sounding radar profiles across the whole polar region. The profiles were extracted from those made between May and December 2011, under conditions that produced data of unprecedently high quality.

The researchers started by making a 3D computer model of the entire polar cap, including the Basal Unit. This let them explore reflections from within the Basal Unit and from its contacts with the Northern Plains and the upper, more icy, polar cap deposits. Then they linked up radar reflections within the ice that come from the unseen top of the Basal Unit to draw a picture of the unit’s upper surface as well as its bottom layer.

“The result of this analysis,” says Frigeri, “are three-dimensional maps of the echoes from the Basal Unit that are starting to reveal its internal structure as well as its overall morphology.”

The modeling was complicated by the fact that the researchers had to assume an average value for the radar transparency of the ice cap and Basal Unit. In reality, the transparency likely varies both from place to place and from layer to layer.

The team plans to improve the model by using more realistic figures for radar propagation speeds. They will also integrate the work of colleagues who are analyzing the same area using data from MARSIS’ companion radar instrument, SHARAD, which is on board the Mars Reconnaissance Orbiter.

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