Candidate and confirmed RSL sites in the hydrated sulfate layered deposits or in craters near the layered
deposits. Background map is Mars Orbiter Laser Altimeter (MOLA) topography.
Credit: Stillman, et al.

One of the more intriguing findings on Mars of late has been spotting recurring slope lineae in certain areas of the Red Planet.

These dark fingers of mystery – RSL in Mars shorthand — emerge from steep, rocky exposures. They incrementally grow, fade, and reform on a seasonal basis.

But what RSL truly represent is controversial. Some researchers say they are suggestive that liquid water occurs on or near the surface of Mars today.

More to the point are RSL created via brine or just small rock/debris falls?

Opportunity rover observation

The upcoming 48th Lunar and Planetary Science Conference (LPSC), to be held March 20-24 in The Woodlands, Texas, promises to be a meeting of the minds on enigmatic RSL.

(Left) This HiRISE image shows the darkest and longest candidate RSL at Cape Victory on the SW rim of Victoria crater. Other HiRISE images show these candidate RSL recurring and fading.
(Right) False color Pancam image from Opportunity (MER-B) at Cape Verde on 04 November 4, 2006. This image shows the candidate RSL beneath Cape Victory as viewed by the rover. The arrow points to the same candidate RSL in (a) and (b).
Credit: Stillman, et al.

For example, new research is being presented that NASA’s Opportunity rover has detected candidate RSL sites in Cape Victory on the southwest side of Victoria crater and on the east side of Endeavour Crater.

Opportunity is a true veteran of Mars exploration. The wheeled robot just cruised past 13 years of service on the Red Planet; it landed on Mars on January 24, 2004.

Superimposed Opportunity rover on Rim of Victoria Crater (Artist Concept).
Credit: NASA/JPL-Solar System Visualization Team

The rover is now heading south, across rough and steep terrain, along the rim of Endeavour Crater. Its current scientific objective is to investigate a gully about a kilometer south of its current location.

As of Sol 4623 (Jan. 24, 2017, the total odometry for Opportunity since landing is 27.26 miles (43.87 kilometers).

Opportunity traverse map.
Credit: NASA/JPL

Serendipitously imaged

Research led by David Stillman of the Southwest Research Institute in Boulder, Colorado, is to be presented at the LPSC – including a look at the first image of a candidate RSL recorded from the surface of Mars.

This feature was serendipitously imaged via the Panoramic Camera (Pancam) on Opportunity at Cape Verde back in November 2006, Stillman and his colleagues report.

The candidate RSL in Cape Victory is roughly 40 feet (12 meters) long and has been seen in over a dozen Mars Reconnaissance Orbiter HiRISE (High Resolution Imaging Science Experiment) images. Those images show recurrence and fading of the feature, but no incremental lengthening.

Confirmed, candidate sites

In introducing their research, Stillman and his fellow researchers note: “To gain insight into the formation and recharge mechanism(s) of recurring slope lineae (RSL), we have been using HiRISE images to search for candidate RSL sites. As of the December 2016 HiRISE release, we have cataloged 577 candidate RSL sites, of which 74 have been confirmed to exhibit RSL.”

Built by Ball Aerospace, the HiRISE camera system on the Mars Reconnaissance Orbiter yields unmatched views of layered materials, gullies, channels, and other science targets and also characterizes possible future landing sites for robotic and human missions.
Credit: NASA

Confirmed sites show recurrence, incremental lengthening, and fading of dark lineae, the researchers explain, while candidate RSL sites have dark lineae that look like RSL but do not possess all three of the characteristics in available imagery.

Close to Curiosity

Stillman and his research colleagues also note in their LPSC research – “Dark Lineae on the Equatorial Layered Deposits; Are these Recurring Slope Lineae (RSL) or Small Debris Flows?” — that candidate RSL near the Curiosity Mars rover have been detected to fade and recur.

This site is roughly 3 miles (5 kilometers) from Curiosity’s current locale, with plans for the rover to drive much closer to the feature, enabling future imaging of the candidate RSL.

In their research, Stillman adds that candidate RSL are detected in most HiRISE images covering layered deposits with hydrated sulfates.

Self image taken by NASA’s Curiosity Mars rover.
Can it probe makeup of RSL? Credit: NASA/JPL-Caltech

Exploration implications

“If additional HiRISE images are able to confirm that RSL are being formed at locations where groundwater reached and altered the surface billions of years ago, this finding would support the hypothesis that RSL are formed and recharged via groundwater and that discharge groundwater is accessible to search or extant life or to be used as a resource for human exploration,” the researchers conclude.

Alternatively, the research team adds, “if these dark lineae never show incremental lengthening, fading, and recurrence, then we conclude that dark lineae in the hydrated sulfate layered deposits are debris flows. This would suggest these layered deposits are weak and could be used to chemically extract water from as a resource for human exploration.”

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