Watch for falling rocks

A new investigation of Cerberus Fossae using HiRISE images shows that Mars is probably seismically active now or in the recent past. The clues are tracks left by boulders as they rolled downhill.

WATCH OUT BELOW. A small area of a lava cliff a few tens of meters high spawned a rockfall that scattered boulders into one part of Cerberus Fossae in this HiRISE image. The lingering boulder trails indicate there has been little wind erosion or deposition since the rockfall. The fact that the biggest boulder lies farthest from the source area suggests a high-speed, single event. (Image is taken from Figure 2 in the paper.)

Gerald Roberts (University of London) and colleagues write in the Journal of Geophysical Research that, “Boulders from the youngest sub-population we can resolve have left boulder trails as they rolled and bounced down slopes. These boulder trails have not been erased by eolian processes that we know are active on Mars. This suggests that the fallen boulders, and hence putative marsquakes may be an ongoing feature of Mars.”

Cerberus Fossae is a pair of long fractures that cut across the Athabasca Valles outflow channel in the Elysium volcanic province. The team notes that Cerberus is one of the youngest fracture and graben systems on the Martian surface, perhaps volcanically active in the last 2 million years. (A study published in 2011 suggested that the crack was emitting residual geothermal heat.)

As the Cerberus fractures grew, they developed a 500-meter (1650-foot) offset in the Athabasca channel. Images taken with the HiRISE camera show numerous places where boulders falling from the lava-cliff rim rock spilled into the fracture, leaving bounce marks and trails down the slope. In many cases, these trails appear fresh and unchanged by wind erosion.

The scientists note, “Other rockfall boulders do not have boulder trails leading to them, presumably because the boulder falls are older, so that their boulder trails have been obscured or covered” by ongoing wind erosion and deposition. The team compared the Martian rockfalls with those resulting from a 2009 earthquake near L’Aquila, in central Italy.

What caused the Cerberus rockfalls? After looking at melting ice or thermal stresses caused by solar heating, the scientists say, “Overall, we conclude that ground shaking associated with paleomarsquakes is a plausible mechanism that could produce the boulder size distributions we have described.”

Images of Cerberus showed displaced boulders ranging from 2 to 20 meters (6.5 to 65 feet) in diameter. Their size and number decreased over a distance of 100 kilometers (62 miles) centered at a point along the Cerberus Fossae faults. Roberts says, “This is consistent with the hypothesis that boulders had been mobilized by ground-shaking, and that the severity of the ground-shaking decreased away from the epicenters of marsquakes.”

In the Italian quake, boulder falls occurred up to about 50 km (31 miles) from the epicenter. Because the area of displaced boulders in Cerberus extended across an area approximately 200 km (124 miles) long, the marsquakes likely had a magnitude greater than 7, the researchers estimated.

As to how frequent the quakes were, the scientists write, “We envisage that not just one paleomarsquake, but rather a sequence of paleomarsquakes will have produced the boulder falls we have studied, and ultimately the roughly 500 meters of vertical offset that has accumulated across Cerberus Fossae since formation of the Athabasca Valles outflow channel.”

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