Seasonal dark streaks in Palikir Crater on Mars Credit: NASA/JPL/University of Arizona - Seasonal dark streaks in Palikir Crater on Mars
A data-processing artifact may be responsible for evidence cited in a 2015 report that cold salty waters are responsible for forming seasonally dark streaks on the surface of Mars, according to a new study from Caltech. The study, published online on November 9 by the journal Geophysical Research Letters , shows that a filtering step in the processing of data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter (MRO) can, in rare circumstances, lead to signatures that mimic the appearance of certain minerals, including perchlorate-a salt whose presence would imply the existence of cold, salty waters at the surface of Mars. MRO has been orbiting Mars since 2006, and during that time, CRISM has been capturing images of visible and infrared light reflected from the planet's surface. Different minerals absorb light at different wavelengths and so the fingerprints of the reflected light provide clues about what minerals might be present at a given location. In 2015, scientists analyzing CRISM images found absorption patterns in a few pixels of the images that appeared to indicate concentrations of perchlorate. The pixels where CRISM detected perchlorate were associated with seasonally changing dark streaks on the slopes of Martian craters (formally named "recurring slope linae," or RSLs), sparking the idea that the dark streaks were, in fact, caused by salty flowing liquid water. The existence of perchlorate brines on Mars has been the subject of speculation and debate ever since.
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