Examples of each type of anomalous RSL studied: fading, recurrence, incremental lengthening.
Credit: Lark, Huber, Head

On Mars, it looks like there’s a cascading truism about Recurring Slope Lineae – what’s spurring these odd features continue to be a recurring controversy.

Recurring Slope Lineae, RSL for short speak, grow incrementally, fade when inactive and return annually. To be classified as RSL, many streaks in the same site must be observed to incrementally lengthen downslope, fade away, and happen again, roughly in the same location.

There are those that see RSL as granular flows, a product of sand and dust movement. On the other hand, some scientists suggest, the appearance and growth of RSL resemble seeping liquid water.

If water, that has implications for habitability of the Red Planet. RSL may be the result of water or brine seeping through the sub-surface over an impermeable layer; the surface darkens where liquid wicks up.

Examples of collinear RSL, both in Valles Marineris.
Credit: Lark, Huber, Head

Orbital imagery

A new look at RSLs has been led by Laura Lark, a research scientist at Brown University in Providence, Rhode Island. Lark, along with advisors and co-authors, Christian Huber and Jim Head of Brown, authored the paper – “Anomalous recurring slope lineae on Mars: Implications for formation mechanisms” – published in the journal Icarus.

Three types of anomalous RSL, categorized into three types — early faders, collinear RSL, and those which emerge at featureless locations – were investigated using imagery from the powerful High Resolution Imaging Science Experiment (HiRISE) onboard NASA’s Mars Reconnaissance Orbiter. This spacecraft cranks out the only available images that can resolve features as narrow as RSL.

“The role of water in the processes responsible for their formation remains undetermined,” they write. “RSL in close proximity (meters to tens of meters apart) are likely to be expressions of the same underlying processes; therefore, differences in behavior between neighbors can provide new constraints on potential mechanisms for initiation, lengthening and fading.”

The researchers evaluated the feasibility of two specific proposed mechanisms: dry granular flow and flow of liquid through porous regolith.

On extended duty: NASA Mars Reconnaissance Orbiter yields unmatched views of layered materials, gullies, channels, and other science targets and also characterizing possible future landing sites for robotic and human missions.
Credit: NASA

Bottom line: “Our observations are more immediately compatible with the liquid flow mechanism,” they conclude.

What next?

“I don’t believe that any currently proposed dry mechanisms can produce the behavior we report in the paper, seeping liquid could, Lark told Inside Outer Space. “However, this doesn’t rule out a not-yet-imagined dry mechanism and there are other arguments against the existence of liquid seeps,” she added.

As far as what’s next, “that’s the real challenge,” Lark responded.

“In some sense, as long as the formation mechanism of RSL is inconclusive, orbital data is our only option,” Lark said.

Indeed, if RSL might indicate liquid water, “we can’t risk contaminating them with surface-based exploration,” Lark added. “We need to keep thinking about the possible underlying physical process: when do two candidate processes predict different outcomes, and how do those predictions compare to what we observe?”

Lark said that, since a lot of these predictions are time-related, the repeated targeting of RSL sites by the NASA Mars Reconnaissance Orbiter’s HiRISE camera system has been very helpful, “but at some point we’re limited by the orbit of the Mars Reconnaissance Orbiter and by the number of images HiRISE can send back to Earth.”

To access this informative paper — “Anomalous recurring slope lineae on Mars: Implications for formation mechanisms” – go to:



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