If a rock falls on Mars and no one is there to see it, does it leave a trail? Yes, and it’s a beautiful herringbone-like pattern, new research reveals. Scientists have now spotted thousands of tracks on the red planet created by tumbling boulders. 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.
Rocky outcrops have been seen elsewhere in the solar system, including on the moon and even a comet. But a big open question is the timing of these processes on other worlds – are they ongoing or have they mainly occurred in the past?
A study of these ephemeral features on Mars, published last month in Geophysical Research Letters, says such boulders could be used to locate recent seismic activity on the red planet. This new evidence that Mars is a dynamic world goes against the idea that all of the planet’s exciting geology happened much earlier, said Ingrid Daubar, a planetary scientist at Brown University who was not involved in the study. “For a long time we thought Mars was this cold, dead planet.”
To arrive at this finding, Vijayan, a planetary scientist at the Physical Research Laboratory in Ahmedabad, India, who uses a single name, and his colleagues studied 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, revealing details as small as 10 inches wide.
“We can discriminate against individual boulders,” said Dr. Vijayan.
The team manually looked for chain-like features — a telltale signature of a rock rolling down a slope — on the sloping walls of impact craters. dr. Vijayan and his associates saw more than 4,500 such boulders, the longest of which stretched for a mile.
Sometimes the tracks change direction, and every once in a while new tracks branch out suddenly, said Dr. Vijayan. Such changing tracks are likely evidence that a boulder disintegrated in mid-fall and its offspring bounced further down.
About a third of the tracks the researchers studied were absent from early images, meaning they must have formed since 2006. The bouncing spurs of all these young spurs are framed by a chevron-shaped pile of Martian regolith. That material that Dr. Nicknamed the “boulder fall ejecta,” Vijayan and his colleagues are kicked out every time a boulder hits the surface, the researchers suggest.
And that material from boulder fall is transient: By tracing the same trails in images acquired at different times, the team found that the ejected boulder remains visible for only about four to eight years. The researchers suggest that winds that continuously sweep across the surface of Mars redistribute dust and sand and erase the ejecta.
Because a boulder’s excrement fades so quickly, the team suggests it points to a boulder recently dislodged. And a common cause of rock chips, on Earth and elsewhere, is seismicity.
dr. Vijayan and his collaborators found that about 30 percent of the boulder tracks in their boulder-excreted sample were concentrated in the Cerberus Fossae region of Mars. That’s much more than expected, the researchers say, given that this region only covers 1 percent of the study area. “The surrounding craters have a lot of boulders,” said Dr. Vijayan. “Some of them even have multiple waterfalls in the same location.”
That makes sense, said Alfred McEwen, a planetary geologist at the University of Arizona and the principal investigator of HiRISE, not involved in the study. The geography near Cerberus Fossae, namely the Tharsis volcanic area, makes the area prone to seismicity. “These gigantic masses of dense rock charged on the surface create tensions in the surrounding crust of Mars,” said Dr. McEwen.
Since 2019, hundreds of Marsquakes have been detected by NASA’s InSight lander, and two of the largest occurred in the Cerberus Fossae region last year.
In the future, Dr. Vijayan and his collaborators plan to extend their analysis to the polar regions of Mars. The HiRISE camera will hopefully help, said Dr. McEwen, despite the instrument significantly past its design life. “HiRISE is still going strong.”
