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Study of Martian canyons yields clues about possible water

Blue dots on this map indicate sites of recurring slope lineae (RSL) in part of the Valles Marineris canyon network on Mars.

Blue dots on this map indicate sites of recurring slope lineae (RSL) in part of the Valles Marineris canyon network on Mars. RSLs are seasonal dark streaks that may be indicators of liquid water. The area mapped here has the highest density of known RSLs on Mars. Image Credit: NASA/JPL-Caltech/Univ. of Arizona

Scientists studying seasonally recurring dark streaks on Martian slopes that may be indicators of water on the surface of Mars have published their findings on Thursday, July 7, in the Journal of Geophysical Research: Planets. The study investigated thousands of these dark streaks in the Valles Marineris canyon network near the Red Planet’s equator. 

The dark streaks are called recurring slope lineae, or RSL. First discovered in 2011, Martian RSLs have become a hot topic in planetary exploration and are one of the strongest forms of evidence for water on the surface of modern Mars. RSLs appear as dark lines extending downslope during a warm season, then fade during the colder parts of the Martian year, then repeat the cycle in the following year. The presence of water, in the form of hydrated salts, was confirmed at some RSL sites in 2015.

Valles Marineris on Mars.

Valles Marineris on Mars. (Mouseover for details; click to enlarge) Image Credit: NASA

The recently published study presented findings from 41 RSL sites in central and eastern portions of Valles Marineris, the largest canyon system in the Solar System. Each site is defined as the size of a single image from the High-Resolution Imaging Science Experiment (HiRISE) camera on NASA’s Mars Reconnaissance Orbiter (MRO): about 3.4 miles by 8 miles (5.4 by 12 kilometers). The number of individual lineae (flows) in each site ranges from a few to more than 1,000.

“There are so many of them, it’s hard to keep track,” said Matthew Chojnacki of the University of Arizona’s Lunar and Planetary Laboratory, Tucson, and lead author of today’s report in the Journal of Geophysical Research: Planets. “The occurrence of recurring slope lineae in these canyons is much more widespread than previously recognized. As far as we can tell, this is the densest population of them on the planet, so if they are indeed associated with contemporary aqueous activity, that makes this canyon system an even more interesting area than it is just from the spectacular geology alone.”

The presence of water, in either liquid or frozen form, at or near the surface of Mars, would have important have important ramifications for investigating whether life currently exists on Mars. Water on or close to the surface would also be an important resource for future human missions to Mars. Evidence of water ice has been if found near the poles of Mars, but if RSLs are found to be water related, near-surface water may be present much closer to the planet’s equator.

In order to determine the mechanism involved in forming RSLs, Chojnacki and his team examined the geological context of canyonland RSL sites and calculated how much water would need to be present if the streaks are due to liquid water seeping through a thin surface layer to darken the ground.

Many sites where RSLs were previously found are on the inner walls of impact craters. At this type of site, a conceivable explanation could be that an extensive underground layer holding water was punctured by the crater-forming impact long ago and still feeds warm-season flows. No such underground layer fits the ridge or peak shapes at several of the RSL sites in the most recent study.

Another suggested mechanism for forming RSLs is that some types of salts pull water vapor out of the Martian atmosphere to form brine on the planet’s surface. The new study did find some evidence to support the connection between salts and RSLs. Some sites bear bright, persistent streaks near the dark, seasonal ones. The bright streaks might result from salt left behind after evaporation of brine.

“There do seem to be more ways atmosphere and surface interact in the canyons than in blander topography, such as clouds trailing out of the canyons and low-lying haze in the canyons,” he said. “Perhaps the atmosphere-surface interactions in this region are associated with the high abundance of recurring slope lineae. We can’t rule that out, but a mechanism to make the connection is far from clear.”

Other mechanisms for producing RSLs involving little or no flowing water may also be possible. Water-free mechanisms do produce other flow features on Mars. There is also the possibility of a “damp” mechanism that requires much less water than suggested by flowing water mechanisms.

While the recent study suggests some support for the idea that significant amounts of near-surface water may exist, more research will be required for a definitive answer and for an understanding of the mechanisms that result in RSLs. Other studies are currently in progress, including laboratory experiments simulating RSLs on Earth. NASA is also considering to use the Curiosity Mars rover to study an ISL site if the rover’s mission is extended. A decision on Curiosity’s extended mission is expected in the next several months.

Coprates Montes area in Valles Marineris

The white arrows indicate locations in this scene where numerous seasonal dark streaks, called “recurring slope lineae”, have been identified in the Coprates Montes area of Mars’ Valles Marineris by repeated observations from orbit. Image Credit: NASA/JPL-Caltech/Univ. of Arizona


Jim Sharkey is a lab assistant, writer and general science enthusiast who grew up in Enid, Oklahoma, the hometown of Skylab and Shuttle astronaut Owen K. Garriott. As a young Star Trek fan he participated in the letter-writing campaign which resulted in the space shuttle prototype being named Enterprise. While his academic studies have ranged from psychology and archaeology to biology, he has never lost his passion for space exploration. Jim began blogging about science, science fiction and futurism in 2004. Jim resides in the San Francisco Bay area and has attended NASA Socials for the Mars Science Laboratory Curiosity rover landing and the NASA LADEE lunar orbiter launch.

Reader Comments

It’s taken them a long time to find the evidence, but finally they seem to be turned on to the idea that likely sites for water and evidence of life are in and around the deepest parts of Valles Marineris. Hopefully we’ll send a mission there at some point in the future to check it out.

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