It's uncanny. Dark finger-like streaks form on dozens of Martian crater rims and hillsides as local spring warms into summer. The streaks lengthen down the slopes until summer's height, then stop growing and fade as fall comes on.

What causes these seasonal streaks has been a mystery since they were first discovered in 2011. Thin, dark streaks appear every Martian summer and crawl down the inside rim of Garni Crater and numerous other hillsides on Mars. Scientists using a Mars-orbiting instrument operated at ASU have just discovered the streaks are not caused by flows of water, as scientists have thought. Instead the streaks are extremely dry, and their cause remains unknown. Photo by NASA/JPL-Caltech/University of Arizona Download Full Image

Most scientists have assumed the seasonal streaks — known as recurring slope lineae, or RSLs for short — form as summer temperatures melt buried ice near the tops of the slopes, and the water flows downhill making dark streaks. Later in the year, as temperatures drop, the flow halts and the dark streaks fade as dust falls out of the atmosphere to cover them. Supporting this idea were observations that the streaks contain hydrated salts.

Now a discovery made using an instrument operated at Arizona State University's School of Earth and Space Exploration throws strong doubt on this "wet streak" model for RSLs.

Christopher Edwards, of Northern Arizona University in Flagstaff, and Sylvain Piqueux, of NASA's Jet Propulsion Laboratory in Pasadena, California, used the Thermal Emission Imaging System (THEMIS) on NASA's Mars Odyssey orbiter to measure temperatures of a large group of RSLs in a Martian crater. Their work, published in the journal Geophysical Research Letters, found that the group of dark RSLs contained essentially no water. (Both Edwards and Piqueux were PhD graduate students at ASU.)

"We used a very sensitive technique to quantify the amount of water associated with these features," said Edwards. "The results are consistent with no moisture at all and set an upper limit at 3 percent water."

Temperature tells all

Edwards and Piqueux analyzed several Mars years of THEMIS infrared observations of RSLs within the large Valles Marineris canyon. The area has many RSL features close together in parts of the study region. The researchers compared nighttime temperatures of patches of ground — one covered to about 44 percent by RSLs, and the other on nearby slopes having no RSLs.

They found no detectable temperature difference, even during seasons when the RSLs in the crater were actively growing.

The observations have an error margin of 1 degree Celsius, or 1.8 degrees Fahrenheit. Calculating the maximum possible water that could be concealed in that temperature difference led them to the 3 percent upper limit for water.

"These results show that THEMIS has a unique way of measuring the water content of RSLs," said Philip Christensen, of ASU's School of Earth and Space Exploration. He is the designer and principal investigator for the THEMIS instrument.

"Water in the soil has a strong effect on the surface temperature of that soil. So measuring the temperature accurately can put tight constraints on the quantity of water the soil holds," he explains.

Edwards notes that some type of water-related activity at the uphill end still might be a factor in triggering RSLs. "But the darkness of the ground is not associated with large amounts of water, either liquid or frozen," he said. "Totally dry mechanisms for explaining RSLs should not be ruled out."