NASA Mars Reconnaissance Orbiter’s HiRISE image of recurring slope lineae (RSL) in Melas Chasma, Valles Marineris. Arrows point out tops and bottoms of a few lineae.
Credit: NASA/JPL-Caltech/University of Arizona


They are called recurring slope lineae – RSL for Mars shorthand.

Spacecraft orbiting the Red Planet have focused on these perplexing features. RSL are spotted as narrow dark features on Mars that incrementally lengthen down steep slopes, fade in colder seasons, and recur annually.

These qualities seemingly imply that liquid water is flowing in the shallow subsurface of Mars today.

Real-time liquid water

Indeed, recent observation of hydrated salts associated with recurring slope lineae in several locations on Mars has created excitement over the possibility of “contemporary” (real-time) liquid water flows on that cold, distant planet.

Hydrated salts are crystalline solids such as sodium chloride dihydrate; although in the solid phase, these salts can form by precipitating from aqueous solutions.

Features called recurrent slope lineae (RSL) have been spotted on some Martian slopes in warmer months. Some scientists think RSL could be seasonal flows of salty water. Red arrows point out one 0.75-mile-long (2 kilometers) RSL in this image taken by NASA’s Mars Reconnaissance Orbiter.
Credit: NASA/JPL-Caltech/Univ. of Arizona

Another hypothesis

But new research by Raina Gough and Margaret Tolbert at the University of Colorado at Boulder suggest another hypothesis for RSL on Mars.

At the recent meeting of the American Geophysical Union (AGU) they reported it is also possible that such solid salts can form via absorption of atmospheric water vapor by anhydrous or less hydrated salts.

“Such a pathway would not require liquid water on any scale on current Mars, and therefore the hypothesis that RSL are due to recent liquid water would be weakened,” they suggested.

Lab experiments

At the AGU gathering in San Francisco, the researchers reported on the results of laboratory experiments that address the likelihood of these two hydrate formation pathways specifically for perchlorate and chloride salts observed in Martian RSL.

They used a Raman microscope and environmental cell to study hydrated salt formation under Mars-relevant temperature and humidity conditions.

Moreover, the scientists attempted to form these Mars-relevant hydrated salts by recrystallizing perchlorate or chloride brines and also by increasing the humidity around their anhydrous salts.

They identified which particular hydrated salts, if present, would be the best markers of recent liquid water in the shallow subsurface because their water vapor-induced hydration is kinetically or thermodynamically hindered.

To date, their findings remain subject to peer-review.


However, David Stillman, a senior research scientist at the Southwest Research Institute (SwRI), also in Boulder, Colorado, advised Inside Outer Space that perchlorates are everywhere on Mars – as indicated by the Viking and Phoenix Mars landers, as well as the Curiosity rover now wheeling about on the Red Planet.

Selfie taken earlier of NASA’s Curiosity Mars rover. It too found a carpet of perchlorate on the Red Planet.
Credit: NASA/JPL-Caltech

“Hydrated perchlorates have only been detected officially at four other locations. These locations all are RSL sites. Plus, the hydration of the perchlorates goes away when RSL are not found at these sites,” Stillman said.

In response to the Gough and Tolbert research, Stillman said:

“So yes, hydrated perchlorates do not guarantee liquid water, but since no other place on Mars has hydrated perchlorates, why would RSL locations be the only place where perchlorate became hydrates especially if the source of the water was atmospheric?”

Messages from Valles Marineris

In a forthcoming paper in the scientific journal, Icarus, Stillman and colleagues Timothy Michaels of the SETI Institute, and Robert Grimm, also of SwRI, have charted the characteristics of the numerous and widespread recurring slope lineae within the Red Planet’s huge canyon system, Valles Marineris.

Valles Marineris RSL are found in every major canyon except Echus Chasma.
Credit: D.E. Stillman et al.,

The researchers detail seasonality for RSL in Valles Marineris and that RSL occur on sulfate-rich (evaporite-deposits) layered deposits.

Making use of the Mars Reconnaissance Orbiter’s High Resolution Imaging Science Experiment (HiRISE), Stillman and his associates have identified numbers of candidate and confirmed RSL sites within all the major canyons of Valles Marineris, with the exception of Echus Chasma. Hundreds of RSL sites have been detected on relatively dust-free steep slopes.

The work was performed under a NASA Mars Data Analysis Grant.

Global distribution of RSL sites and the four RSL regions from 60°N to 60°S.
Credit: D.E. Stillman et al.

Annual recharge

Still to be understood are the mechanism(s) by which RSL are recharged annually.

The team reports that there are always RSL lengthening within Valles Marineris, regardless of the season. “If RSL are caused by water, such a long active season at hundreds of VM RSL sites suggests that an appreciable source of water must be recharging these RSL,” they write.

Candidate spur-and-gully Valles Marineris RSL site in SE Candor Chasma that has slope streaks nearby. Shown are candidate RSL that merge downhill and incrementally lengthen, highlighted with arrows. (c) Overview of site. Slope streaks cover the W-facing slopes of the brighter (further east) spur, while candidate RSL are only seen on a small fraction of the darker spur. The center of this image is located at 8.596°S, 294.282°E. (d) Image of the most recent slope streak, roughly 1,148 feet (350 meters) in length.
Credit: D.E. Stillman et al.

Overall, understanding more fully how RSL form and recur can benefit the search for existing life on Mars and could provide details about an in situ water resource, the team writes.

So what next for RSL studies?

For one, these features could come under enhanced scrutiny by a proposed Mars 2022 orbiter the researchers suggest, provided it could acquire more frequent images of specific sites and have greater imaging coverage.

Also, higher-resolution thermal imaging could explore the brine content of the RSL.

Melas Chasma’s Ancient Lake. A small basin (center foreground) lies below the southern rim of Melas Chasma, part of Valles Marineris. The basin likely holds ancient lakebed sediments, which earned it a place on the list of potential landing sites for NASA’s 2020 rover. This view looks west down Melas Chasma (no vertical exaggeration).
Credit: NASA/JPL-CALTECH/Arizona State University, R. Luk

Candidate RSL are near the SW Melas, one prospective landing site of eight now in the running for NASA’s Mars 2020 rover. SW Melas is part of Valles Marineris.


Year-round access to water

“Landers or penetrators could provide even greater detail for individual RSL and best determine the RSL flow mechanism and salinity. Determining how RSL recharge and flow is important, as these features could be the most accessible location to determine if Mars has extant life,” the team writes in their Icarus paper.

Candidate RSL near the potential SW Melas Mars 2020 rover landing site – one of eight now under review.
Credit: D.E. Stillman et al.

Perhaps RSL are too briny to allow life? If so, then planetary protection could be eased for human exploration.

If RSL are found to be sourced by local or regional aquifers, drilling sites could be located on nearby flat terrain and access water year-round. Such aquifers could be detected using surface-based geophysical exploration and then accessed via drilling, the RSL investigators suggest.

One Response to “Flowing Water on Mars…Today? Debate and Discussion!”

  • Kye Goodwin says:

    It sure looks like SOMETHING is coming out of the ground on those slopes. I think that the water hypothesis may prove to have been a dead end when we finally understand this, but I’ve no doubt that this is a phenomenon worthy of study. There are hundreds of examples of a smaller scale but otherwise similar process at work on slopes right in front of Curiosity (and both MERs). There’s no sign of liquid water but in many cases it appears that sand is emerging from the sub-surface. This line of evidence is not being investigated.

    One more conjecture: The fine textured slopes on which RSLs typically run out and terminate are created by the accumulation of RSL contents over geologic time.

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