Newly released images from the Mars Reconnaissance Orbiter (MRO) have shown that the surface of Mars is a far more dynamic place than previously suspected. Aside from spotting new meteor impact craters and planet-wide dust-storms that we already knew about, the MRO has spotted long dark features that grow seasonally down slopes on the Martian surface. They first appear in the spring and grow to lengths of a kilometer or more in the summer, before then retreating to vanish entirely come winter. Scientists speculate that these features could be explained by salt-water flowing downhill, wetting the soil and making it appear darker.
The High Resolution Imaging Science Experiment (HiRISE) camera is a sophisticated imaging system mounted on the Mars Reconnaissance Orbiter (MRO). Not only does it return a seemingly unending parade of gorgeous views of the red planet, but it allows planetary scientists to collect valuable scientific data that has advanced their understanding of Mars enormously. Over the years it has accumulated enough data that they can not only spot new features, such as meteorite impacts, but can also discover seasonal variations in the conditions on various parts of the planet. This would not be possible without long-running orbital missions like HiRISE.
Dr Alfred McEwen of the University of Arizona in Tucson first recorded a series of seasonal surface flows two years ago. These features, visible as dark lines in the image above, are less than 5 meters wide but extend to over 1200 meters at their maximum length. They have been studied in five locations in Valles Marineris, the largest canyon in the solar system. They are found on both north and south facing slopes, and grow during the seasons when sunlight shines on their slope. “The explanation that fits best is salty water is flowing down the slopes when the temperature rises,” McEwen said. “We still don’t have any definite identification of water at these sites, but there’s nothing that rules it out, either.”
This is not a far-fetched idea. We’ve known for some time that there is a lot of water on Mars, mostly frozen in the polar caps which grow and shrink with the changing seasons. But the chance dragging of a seized wheel on the Spirit rover revealed evidence of water seeping down through the surface, showing that Mars has liquid water as well (even if only in thin films of dew). Fresh water has a hard time staying liquid under the cold, low-pressure environment of Mars, but if it carries enough dissolved salts and minerals, the freezing point can be lowered substantially, allowing it to persist for long enough to soak down into the ground or even flow as a short-lived stream.
There is another phenomenon which can be observed by comparing past and present images of the changing landscape: impact craters. Mars has a much thinner atmosphere than Earth, which offers a great deal less protection from meteorites. This means that rocks falling in from space have a much higher chance of surviving their passage through the atmosphere, and hit the ground at much higher speeds, which leads to Mars having many more fresh impact craters. HiRISE found five craters containing fresh ice in 2009 and has found another fifteen since then. Some of these icy deposits have been found at latitudes as low as 39°, closer to the equator than the poles.
“The more we find, the more we can fill in a global map of where ice is buried,” said Colin Dundas of the U.S. Geological Survey in Flagstaff, Ariz. “We’ve now seen icy craters down to 39 degrees north, more than halfway from the pole to the equator. They tell us that either the average climate over several thousand years is wetter than present or that water vapor in the current atmosphere is concentrated near the surface. Ice could have formed under wetter conditions, with remnants from that time persisting today, but slowly disappearing.”
As the MRO and other robotic vehicles continue to collect their data, we will answer more and more about Mars. What these discoveries have revealed is the importance of a continued presence at a world if we want to fully understand how it works, since sometimes the truth is revealed not in a single detailed snapshot, but in the changes recorded over time.