Planetary researchers led by Georgia Institute of Technology PhD candidate Lujendra Ojha have examined Martian recurrent slope lineae (RSL) – mysterious, possibly water-related streaks on the planet’s surface.
Recurrent slope lineae are geological features on Mars that might require flowing water. They were first discovered in 2011 on steep, equator-facing slopes in the southern mid-latitudes. These finger-like features form and grow during warm seasons and fade and often completely disappear during colder seasons, but recur over multiple years.
Lujendra Ojha and his colleagues recently discovered the reappearing streaks near the Martian equator, including in the Valles Marineris canyon that lies just south of it.
They then set out to look for minerals that RSL might leave in their wake and to understand their nature – water-related or not.
The scientists examined Compact Reconnaissance Imaging Spectrometer for Mars images of 13 confirmed RSL sites from the southern midlatitudes and the equatorial region. 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.
“We 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. 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,” said Lujendra Ojha, who reported the findings in a paper published in the Geophysical Research Letters.
“The lack of water-related absorptions rules out hydrated salts as a spectrally dominant phase on RSL slopes.”
The team also looked at every image gathered by the High Resolution Imaging Science Experiment from March to October of 2011.
They hunted for areas that were ideal locations for RSL formation: areas near the southern mid-latitudes on rocky cliffs. They found 200, but barely any of them had RSL.
“Only 13 of the 200 locations had confirmed RSL. There were significant differences in abundance and size between sites, indicating that additional unknown factors such as availability of water or salts may play a crucial role in RSL formation,” explained Lujendra Ojha, who is the lead author of another paper, published in the journal Icarus.
Comparing their new observations with images taken in previous years, Lujendra Ojha and his colleagues also found that RSL are much more abundant some years than others.
Lujendra Ojha et al. 2014. Spectral constraints on the formation mechanism of recurring slope lineae. Geophysical Research Letters, vol. 40, no. 21, pp. 5621–5626; doi: 10.1002/2013GL057893
Lujendra Ojha et al. 2014. HiRISE observations of Recurring Slope Lineae (RSL) during southern summer on Mars. Icarus, vol. 231, pp. 365–376; doi: 10.1016/j.icarus.2013.12.021