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New test finds no damp soil at Mars’ seasonal dark streaks

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

A new study using data collected by NASA’s Mars Odyssey mission indicates that there is little water to be found in seasonal dark streaks on Mars. The dark streaks, known as recurring slope lineae or RSL, have been a hot topic in planetary exploration since they were first discovered in 2011.

The new study uses ground temperature data collected by Mars Odyssey’s Thermal Emission Imaging System (THEMIS). While these findings do not contradict previous studies that identified the presence 0f hydrated salts at these flows, the temperature measurements identify an upper limit for how much water is present at these darkened streaks: about as much as in the sands of Earth’s driest deserts.

If water is present in the spaces between grains of sand or particles of soil, it will affect how quickly a patch of ground will heat up during the day and cool off at night.

“We used a very sensitive technique to quantify the amount of water associated with these features,” said Christopher Edwards of Northern Arizona University, Flagstaff. “The results are consistent with no moisture at all and set an upper limit at three percent water.”

Artist's concept of 2001 Mars Odyssey spacecraft over Syrtis Major Planum.

Artist’s concept of 2001 Mars Odyssey spacecraft over Syrtis Major Planum. Image Credit: NASA/JPL

RSLs have been identified at dozens of sites on Mars. A darkening of the soil extends downhill during the spring and summer, then fades away in autumn and winter. This pattern then repeats in subsequent years. The mechanism that causes the streaks is still somewhat a mystery.

“Some type of water-related activity at the uphill end still might be a factor in triggering RSL, but the darkness of the ground is not associated with large amounts of water, either liquid or frozen,” Edwards said. “Totally dry mechanisms for explaining RSL should not be ruled out.”

Edwards and Sylvain Piqueux of NASA’s Jet Propulsion Laboratory (JPL) analyzed several years of THEMIS infrared readings of a crater-wall region of the large Valles Marineris canyon system on Mars. Several RSLs are located close to each over in some parts of the study region. The researchers compared nighttime temperatures of patches of ground averaging approximately 44 percent RSL features, to temperatures of slopes nearby that had no RSLs. They found no detectable difference in ground temperatures, even during seasons when RSLs were actively growing. The report of these findings was submitted to the peer-reviewed journal Geophysical Research Letters and is available online.

A study published in 2015 identified hydrated salts in the surface composition of RSL sites, with an increase in the fall and winter season when the streaks are actively growing. Hydrated salts hold water molecules affecting the crystalline structure of the salt.

“Our findings are consistent with the presence of hydrated salts, because you can have hydrated salt without having enough for the water to start filling pore spaces between particles,” Edwards said. “Salts can become hydrated by pulling water vapor from the atmosphere, with no need for an underground source of the water.”

“Through additional data and studies, we are learning more about these puzzling seasonal features – narrowing the range of possible explanations,” said Michael Meyer, Mars Odyssey program scientist. “It just shows us that we still have much to learn about Mars and its potential as a habitat for life.”

Valles Marineris on Mars.

Valles Marineris on Mars. Image Credit: NASA


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.

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