Oblique view of Newton crater recurring slope lineae (RSL). This image has been reprojected to show a view of the RSL-covered slope as would be seen from a helicopter inside the crater, with a synthetic Mars-like sky.
Credit: NASA/JPL/University of Arizona
SHROPSHIRE: Dark, narrow streaks have been discovered running down the slopes of Mars - features that could be caused by flows of salt water.
A team of astronomers have published their findings in the current issue of Science, revealing that the surface lines grow throughout the warm spring and summer months before fading during the Martian winter.
"We have discovered a new type of current activity on Mars, which has very specific behaviour in terms of environmental conditions," said lead author Alfred McEwen from the University of Arizona's Lunar and Planetary Laboratory. "Given the observed temperatures and what we know about Mars, briny water is the best candidate for explaining these features."
An ancient water world
Today, Mars is nicknamed the 'Red Planet' because of the thin dusting of iron oxide that coats its surface. Millions of years ago, however, the planet may not have looked quite so rusty.
There is growing evidence that liquid water once flowed freely over the surface of Mars. Space orbiters and rovers have revealed riverbeds, valleys and canyons, as well as widespread deposits of chloride and sulfate minerals, all of which point to the one-time existence of water.
Although present-day Mars seems barren in comparison, it was announced in 2002 that large quantities of ice still exist below the Martian surface, in addition to the ice caps at each pole.
And now, this new study offers a tantalising hint that channels of salty water may still exist on the Martian surface.
Streaky slopes
Whilst studying images taken by NASA's Mars Reconnaissance Orbiter (MRO), Alfred McEwen and colleagues noticed dark, finger-like markings on several of the steep slopes in Mars' southern hemisphere.
These surface features - christened 'recurring slope lineae' ¬- were found at seven different mid-latitude sites, each containing as many as 1000 individual flows. The markings are much narrower than any previously reported gullies, with widths ranging from just 50 cm to 5 m.
By using images spanning several Martian seasons, the team found that the streaks grow during the summer, fade away during wintertime months, and then return again the following year. Typical summertime afternoon temperatures on these slopes range from approximately -25°C to 25°C, positively balmy compared to the average Martian surface temperature of -55°C.
Explaining the lines
The shape, position, temperature preferences and rapid evolution of these streaks all point to the presence of a volatile - a substance with a low boiling point.
Two possible volatiles - carbon dioxide frost and pure water - were discounted because surface temperatures are either too warm or too cold for these to remain as liquids. The most likely volatile is salty water, which has a lower freezing point than pure water. This means that a brine can remain liquid at temperatures below 0°C.
However, when the team analysed the dark streaks with the orbiter's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), they failed to detect water. This may be because the features dry quickly, or because the flows are situated just below the surface.
Another mystery is the exact mechanism that causes the streaks to appear dark. One possibility is that briny water flows rearrange the surface grains in such a way that darkens the Martian slopes.
Life on Mars?
Despite these puzzles, the salt water theory is currently the best candidate for explaining these intriguing streaks.
"The brine hypothesis seems like a good one at this stage," said Adrian Brown, a planetary scientist working at the SETI Institute in California. "I'd love to watch these features evolve in person on the ground! It would be nice to monitor their temperature and humidity using visible near-infrared data. We have such an instrument in orbit ¬- CRISM - but these features are too small to be seen by it."
And confirmation of this hypothesis would be especially big news for those scientists engaged in the perpetual search for Martian life. "Brines exposed to the surface could be a potential habitat," said Brown. "These lines may just be providing a great clue for where future astrobiologists will be looking for hungry and freezing Martian microbes."
