Mars’ bizarre dark streaks may not be so astrobiologically intriguing after all.
Those streaks, known as recurring slope lineae, were discovered in 2011 by scientists studying imagery captured by the powerful High Resolution Imaging Experiment (HiRISE) camera aboard NASA’s Mars Reconnaissance Orbiter (MRO).
As their name suggests, recurring slope lineae, or RSL for short, are found on Red Planet slopes. The marks creep down steep inclines, especially in Mars’ southern hemisphere, during warm times of the year and fade away as the weather cools.
These characteristics led scientists to speculate that the dark marks could be caused by salty liquid water flowing or seeping through the red dirt, in spots that get warm enough for some of Mars’ plentiful subsurface ice to melt. That exciting hypothesis got a boost in 2015, when data gathered by MRO’s Compact Reconnaissance Imaging Spectrometer instrument, or CRISM, revealed the apparent signature of hydrated salts at some RSL locales. That’s just what you’d expect to see after briny liquid had evaporated away.
But the liquid-water explanation has been losing favor over the last few years. For example, a 2018 study cast serious doubt on the CRISM find, suggesting that the supposed hydrated-salt fingerprint was actually an artifact of data processing. And researchers have been finding more and more evidence, from both experimental and modeling work, that dry landslides are etching the dark marks into warm Martian slopes.
Another such study came out this past January. HiRISE imagery revealed a big increase in RSL activity in the wake of the global dust storm that killed NASA’s Opportunity Mars rover in 2018, researchers report in the paper, which was published in Journal of Geophysical Research: Planets.
The team, led by HiRISE principal investigator Alfred McEwen, counted 150 active RSL sites during the year of the dust