In an e-mail exchange, Roberts explains that the inspiration for using boulders as surrogates for estimating quake activity came during a conversation he was having with a colleague. The colleague asked about detecting earthquakes on Mars and Roberts recalls regaling him with the Viking story. The colleague then asked: "Can you not see evidence of landslides triggered by marsquakes?"
Earthquakes, for instance, can trigger boulder falls that show a distinctive pattern. They dislodge a large number of bigger boulders on a slope at the epicenter. The number and size of dislodged boulders drop with distance along the fault on either side of the epicenter, where shaking is weaker.
For the Roberts' team, the quintessential examples involved rock falls associated with a 1981 quake in Greece, which occurred far enough from populated areas that the boulder falls the quake triggered have remained undisturbed, and rockfalls from the 2009 L'Aquila earthquake in central Italy.
Prior to the arrival of NASA's Mars Reconnaissance Orbiter at the red planet in March 2006, the answer to the colleague's question about spotting boulders and boulder trails in the dust would have been: No. Cameras aboard previous orbiters weren't able to produce images of sufficient detail to spot boulder-sized features. The HiRISE camera aboard MRO, however, can detect objects as small as 8 inches across.
"They are really amazing images," Roberts says.
The team analyzed MRO images taken along a nearly 200-mile stretch of the parallel graben. Sloped walls on either side of these large gashes can rise as high as 1,600 feet. The researchers focused on walls that showed evidence of being fault scarps. The team found boulder falls that not only fit the pattern seen in quake-triggered boulder falls on Earth. In one location, boulder trails were clearly visible in the Martian dust the boulders rolled through. The deep graben could be sheltered from Martian winds, allowing old trails to look like new for long periods of time. Or the trails could indicate recent activity. The team sites other geological evidence that, combined with the trails, suggests recent activity.
One alternate explanation for the boulder falls centers on melting ice near the surface along graben rims, the researchers acknowledge. Such slides have been seen elsewhere on Mars. Slumping soil following the melt would send boulders tumbling down slope. But the distribution of sizes along the length of the graben would be far more random than the distribution the team saw.
In addition, an enormous impact crater sits in the vicinity of Cerberus Fossae – a crater estimated to have formed some 2 million years ago. But the boulders taper off in size and number along a line in the crater's direction. If a smack to the crust had triggered the rockfalls, bigger rocks would appear more frequently in the sections of the grabens closest to the impact site, Roberts says. Since this tapering off indicates weak shaking, the boulder falls would have occurred after the impact that formed the crater.
The team concludes that a series of earthquakes during the past 2 million years are a plausible explanation for the distribution of boulders they see in the graben, with quakes occurring at different times and places along the length of the southernmost graben of the two they examined.
Brown University's Dr. Head says that quake activity needn't be tied to active volcanism. He notes that quakes could come from long-term cooling of the conduits whose roofs collapsed to form the grabens.
But Roberts and his colleagues note that if the faults along Ceberus Fossae are still active, and if the activity is driven by the movement of magma through subsurface dikes, Mars could still be volcanically active – at least at this corner of the planet.