Bouncing Boulders Point to Quakes on Mars

If a rock falls on Mars and no one is around to see it, does it leave a mark? Yes, and it’s a beautiful herringbone pattern, new research reveals. Scientists have now spotted thousands of tracks on the Red Planet created by falling rocks. Delicate chevron-shaped piles of Martian dust and sand frame the tracks, the team showed, and most fade over the course of a few years.

Rockfalls have been spotted elsewhere in the solar system, including on the moon and even on a comet. But a big open question is the timing of these processes on other worlds – are they ongoing or have they mostly happened in the past?

A study of such ephemeral features on Mars, published last month in Geophysical Research Letters, indicates that such boulder traces can be used to pinpoint recent seismic activity on the Red Planet. This new evidence that Mars is a dynamic world flies in the face of the idea that all of the planet’s exciting geology happened much earlier, said Ingrid Daubar, a planetary scientist at Brown University who didn’t not participated in the study. “For a long time we thought Mars was this cold, dead planet.”

To arrive at this discovery, Vijayan, a planetary scientist at the Physical Research Laboratory in Ahmedabad, India, who uses a single name, and his colleagues pored over thousands of images of the equatorial region of Mars. The images were captured from 2006 to 2020 by the High Resolution Imaging Science Experiment (HiRISE) camera aboard NASA’s Mars Reconnaissance Orbiter, and revealed details as small as 10 inches in diameter.

“We can discriminate between individual rocks,” Dr. Vijayan said.

The team manually searched for chain-like features – a telltale signature of rock rolling downhill – on the tilted walls of the impact craters. Dr. Vijayan and his collaborators spotted more than 4,500 such rock tracks, the longest of which stretched over a mile and a half.

Sometimes the tracks change direction and sometimes new tracks suddenly branch off, Dr Vijayan said. These changing trajectories are likely evidence that a boulder disintegrated in mid-fall and its offspring continued to bounce down the slope.

About a third of the tracks the researchers studied were missing from the first images, meaning they must have formed since 2006. The bounce marks of all these young tracks are framed by a chevron-shaped pile of Martian regolith . This material, which Dr. Vijayan and his colleagues have dubbed “rock fall ejecta,” is expelled whenever a rock hits the surface, the researchers propose.

And this rockfall material is transient: By tracing the same tracks in images obtained at different times, the team found that rockfall ejecta tend to remain visible for only about four to eight years. The researchers suggest that the winds continually sweeping the surface of Mars are redistributing dust and sand and wiping out ejecta.

Because ejecta from falling rocks fade so quickly, it implies that a rock was recently dislodged, the team suggests. And a common cause of rockfalls, on Earth and elsewhere, is seismic activity.

Dr. Vijayan and his collaborators found that about 30% of the boulder traces in their sample with ejecta from falling rocks were concentrated in the Cerberus Fossae region on Mars. This is much more than expected, according to the researchers, since this region covers only 1% of the study area. “The surrounding craters have a lot of rockfall,” Dr Vijayan said. “Some of them even have multiple falls in one place.”

That makes sense, said Alfred McEwen, a planetary geologist at the University of Arizona and principal investigator of HiRISE, not involved in the research. The geography near Cerberus Fossae, namely the Tharsis volcanic region, predisposes the area to seismic activity. “These giant masses of dense, charged rock at the surface create stresses throughout the surrounding crust of Mars,” Dr McEwen said.

Since 2019, hundreds of Marsquakes have been detected by NASA’s InSight lander, and two of the largest occurred last year in the Cerberus Fossae region.

In the future, Dr. Vijayan and his collaborators plan to extend their analysis to the polar regions of Mars. The HiRISE camera will hopefully oblige, Dr McEwen said, although the instrument is well past its design life. “HiRISE is still going strong.”