Martian experts have known since 2011 that mysterious, possibly water-related streaks appear and disappear on the planet's surface but are finding it difficult to prove this fact.
Georgia Institute of Technology Ph.D. candidate Lujendra Ojha discovered them while an undergraduate at the University of Arizona. These features were given the descriptive name of recurring slope lineae (RSL) because of their shape, annual reappearance and occurrence generally on steep slopes such as crater walls.
Ojha and Georgia Tech Assistant Professor James Wray looked at 13 confirmed RSL sites using Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) images. They didn't find any spectral signature tied to water or salts.
But they did find distinct and consistent spectral signatures of ferric and ferrous minerals at most of the sites. The minerals were more abundant or featured distinct grain sizes in RSL-related materials as compared to non-RSL slopes.
Ojha said that they still don't have a smoking gun for existence of water in RSL, although we're not sure how this process would take place without water.
He said that just like the RSL themselves, the strength of the spectral signatures varies according to the seasons. The signatures are stronger when it's warmer and less significant when it's colder.
Also Read
The research team also notes that the lack of water-related absorptions rules out hydrated salts as a spectrally dominant phase on RSL slopes. For example, ferric sulfates have been found elsewhere on Mars and are a potent antifreeze.
If such salts are present in RSL, then they must be dehydrated considerably under exposure to the planet's conditions by the time CRISM observes them in the mid-afternoon.
The findings were recently published in Geophysical Research Letters.