Bouncing Boulders on Mars?
by Ken Rock, MSDC Editor
Sources: primarily The New York Times, Katherine Kornei, 1/22/2022; also WION Web Team, New Delhi, Published 1/27/2022
A New York Times article in January noted that scientists have been able to detect thousands of tracks on Mars 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.
The open question about these rockfalls detected not only on Mars, but also on the moon and a comet, is whether these processes are ongoing or if the occurred mostly in the past.
A study of these ephemeral features on Mars, recently published in Geophysical Research Letters, says that such boulder tracks can be used to pinpoint recent seismic activity on the red planet. This new evidence that Mars is a dynamic world runs contrary to the notion 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 that Mars was this cold, dead planet,” Daubar said.
To arrive at this finding, scientists at a research lab in India reviewed thousands of images captured from 2006 through 2020 by the High Resolution Imaging Science Experiment (HiRISE) camera onboard NASA’s Mars Reconnaissance Orbiter and revealed details as small as 10 inches across, allowing them to discriminate individual boulders.
When a boulder tumbles on Mars, it creates a characteristic V-shaped pattern at the location of impact. And a bouncing boulder means that a series of these 'V's is sketched on the surface on points the bouncing boulder makes contact. The broad portion of the V-shape points at the direction of the slope.
The team manually searched for these chain-like features on the sloped walls of impact craters and spotted more than 4,500 such boulder tracks, the longest of which stretched more than a mile and a half. Sometimes the tracks change direction and occasionally new tracks suddenly branch off, suggesting that a boulder may have disintegrated mid-fall and that its smaller offspring continued bouncing downslope.
Roughly one-third of the tracks the researchers studied were absent in early images, meaning that they must have formed since 2006. The bounce marks of all of these young tracks are framed by a chevron-shaped pile of Martian regolith. By tracing the same tracks in images obtained at different times, the team found that boulder fall ejecta tends to remain visible for only about four to eight years. The researchers suggest that winds continuously sweeping over the surface of Mars redistribute dust and sand and erase the ejecta.
Because boulder fall ejecta fades so rapidly, seeing it implies that a boulder was dislodged recently. And a common cause of rockfalls, on Earth and elsewhere, is seismic activity.
The researchers found that roughly 30% of the boulder tracks in their sample with boulder fall ejecta were concentrated in the Cerberus Fossae region of Mars. That’s far more than expected, the researchers say, since this region encompasses only 1% of the study’s area.
That makes sense, said Alfred McEwen, a planetary geologist at the University of Arizona and the 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 rock loaded up on the surface creates stresses throughout the surrounding crust of Mars,” McEwen said.
Since 2019, hundreds of marsquakes have been detected by NASA’s InSight lander, and two of the largest occurred in 2021 in the Cerberus Fossae region.
In the future, the team plans to extend its analysis to Mars' polar regions. The HiRISE camera will hopefully oblige, McEwen said, despite the instrument being significantly past its design lifetime.