Could water be streaming down Martian mountainsides? This startling vision may be a reality according to recent results from NASA’s MRO spacecraft. Here is the latest on the waters of Mars.

Image of Martian crater slopes

This image combines orbital imagery with 3-D modelling to show flows that appear in spring and summer on a slope inside Mars' Newton crater. The image has been manipulated to show an aerial view of the slope with a synthetic Mars-like sky. The colour has also been enhanced. The source observation was made 30 May 2011, by the HiRISE camera on NASA's Mars Reconnaissance Orbiter. (Image credit: NASA/JPL-Caltech/Univ. of Arizona)

Although we are exploring the surface of Mars through marvellous rovers like Spirit and Opportunity and their forthcoming big brother Curiousity, we still have so much to learn by simply looking down the planet. Views from orbit can tell us more about the planet’s history, climate and structure than any probe on the surface. The probe which examines Mars in highest resolution is a gold box, about the size of a large people carrier, with solar panels and a huge communications dish.  This is NASA’s Mars Reconnaissance Orbiter (MRO), a sophisticated spacecraft which peers at the red rocks and sands from an altitude of about 300 km. MRO started work in 2006, its purpose to make a detailed study of the geology, mineralogy and weather of Mars. To do this it is equipped with powerful spectrometers, radar and cameras.

One of these cameras makes MRO special. This is the High Resolution Imaging Science Experiment (HiRISE), a huge instrument with a built-in 0.5m reflecting telescope capable of resolving objects a mere 90 cm across on the planet’s surface. This is the most powerful imaging device ever sent to another planet and it means we see the Martian terrain in the sort of detail we would expect from spy satellites over the Earth. HiRISE has given us some extraordinary and beautiful images leading to a whole new perspective on Mars. Through it MRO has witnessed Martian landslides, found new impact craters and even spotted other probes on the surface.  It is an amazing mission which has given us extraordinary insights into Martian landscapes. On 4 August 2011, NASA announced possibly MRO’s greatest discovery.

Using images from HiRISE scientists have discovered a new class of Martian landform which they call Recurring Slope Lineae (RSL). These are narrow (0.5 to 5 m) relatively dark markings running down steep slopes. These seem to appear and gradually get bigger during the Martian spring and summer and fade in the autumn and winter. Apparently starting at bedrock outcrops, these dark streaks follow the slope, seemingly meandering around obstacles.  In some rare locations hundreds of these mysterious markings can be seen.

Image of RSL changes

Warm season flows on a slope in Newton Crater, possible evidence of salty liquid water active on Mars today. (Image credit: NASA)

RSL appear and lengthen on slopes orientated towards the Martian equator. At the times when they appear the peak temperatures at these locations can be between 250 and 300K (-23 and +23° C). The lower end temperature range is cold for pure water but brines (salt solutions) have lower freezing points. Salt deposits are common on Mars indicating brines were abundant in Mars’ distant past. These recent observations suggest brines still may form near the surface today at certain times and places. RSL may be streaks of salty mud flowing downhill.

Image of phoenix-globs

Some have suggested that these mysteriously changing globs on the Phoenix probe's leg are drops of salty water. Sols are Martian days. (Image credit: NASA)

Frozen water is common on and under the surface of Mars but these images are the best evidence to date for the presence of liquid water on the Martian surface today. Claims in 2008 that droplets of brine could be seen on the landing struts of the Phoenix Mars Lander were not taken seriously but it may be time to take another look at them.

The discovery of RSL is encouraging for astrobiologists. Similar phenomena appear in the Dry Valleys of Antarctica. There, during the summer thaw trickles of meltwater dampen slopes to support a population of hardy micro-organisms just beneath the surface. Could something analogous be going on our chilly neighbouring world?


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