Seasonal dark streaks on Mars — called recurring slope lineae by planetary researchers — could be the result of water boiling whilst flowing under the low pressure of the thin Martian atmosphere, according to a study published online this week in the journal Nature Geoscience.
This image of an impact crater in the Sirenum Fossae region of Mars was taken by NASA’s Mars Reconnaissance Orbiter on March 30, 2015. The crater is approximately 3,300 feet (1-km) wide and appears relatively recent as it has a sharp rim and well-preserved ejecta. The steep inner slopes are carved by gullies and include possible recurring slope lineae on the equator-facing slopes. Image credit: NASA / JPL / University of Arizona / Alfred McEwen.
Recurring slope lineae appear in several locations on Mars when temperatures are above minus 10 degrees Fahrenheit (minus 23 degrees Celsius), and disappear at colder times. These features often have been described as possibly related to liquid water.
“On Mars, where the atmosphere is much thinner than on Earth, water can boil at temperatures as low as 32 degrees Fahrenheit (0 degrees Celsius),” said study co-author Dr. Manish Patel, a scientist at the Open University, and his colleagues from the United States, France and the UK.
“During the Martian summer, when the subsurface water ice begins to melt and emerge at the surface, where the mean temperature reaches 68 degrees Fahrenheit (20 degrees Celsius), it immediately starts to boil.”
“And it’s this sudden boiling during the flow of the water which is the key process that could be causing these channels on the Martian surface,” Dr. Patel said.
The scientists used their unique Mars simulation chamber to conduct experiments of water flowing down a slope of Martian surface material, under Martian atmospheric temperatures and pressures.
A block of ice was placed on top of a sandy slope and under Earth-like conditions, little change was observed to the slope as the ice melted and trickled downwards.
Using the team’s simulation chamber, the experiment was repeated but under simulated Mars conditions.
With the lower air pressure, the melting water rapidly boiled as it flowed and disrupted the sand by explosively ejecting the surface material, with grains piling up and forming small channels which resembled those observed on the surface of Mars.
“This discovery has the potential to change how we interpret these kinds of geomorphological flow features on the surface of Mars, and clearly shows us that there are important differences in how water-related debris flows occur on Earth and Mars,” Dr. Patel said.
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M. Massé et al. Transport processes induced by metastable boiling water under Martian surface conditions. Nature Geoscience, published online May 2, 2016; doi: 10.1038/ngeo2706
