Until 3 billion years ago, Mars was a wet planet much like our own. It had land, an extensive atmosphere and an ocean that covered two thirds of the northern hemisphere as well as smaller bodies of water. But significant climate change caused the water and atmosphere to boil away, leaving Mars a dry and arid planet. Or so we thought.
In 2011 a graduate student at the University of Arizona, USA, was studying images taken by the Mars Reconnaissance Orbiter (MRO) that showed dried-up river beds. These gullies were evidence that there was once water on the red planet, but what Lujendra Ojha discovered was even more interesting. He noticed that in some images there were long, dark streaks on crater walls, and that these streaks seemed to appear and disappear with the Martian seasons; they were dark and clear during the spring and summer but faded away in the autumn and winter. Lujendra published a paper with Alfred McEwan that cautiously suggested these streaks (called recurrent slope lineae or RSL) might be evidence that there was still some sort of water on Mars.
Today they were declared right. Lujendra, Alfred and their colleagues used a spectrometer aboard the MRO to examine the landscape where the RSL could be seen. This technique is called spectroscopy and enables scientists to view objects in many different kinds of light, including infra-red. The way the light interacts with the surface tells researchers what chemicals it is made of. When the RSL were obvious the spectrometer detected several kinds of hydrated salts (don’t panic, I’ll explain that in a moment!), and when the RSL had faded the spectrometer was unable to detect anything out of the ordinary.
A hydrated salt is a crystalline form of salt molecule that is loosely attached to water molecules. It’s different to the salt that we eat with our chips – in this case the salts detected were magnesium perchlorate, magnesium chlorate and sodium perchlorate. As Mary-Beth Wilhelm explained at today’s press conference, these particular salts are able to absorb moisture from air and form liquid water, a process called deliquescence. The Mars rovers have already reported that the air there is more humid than had been expected so it may be that this is what scientists are seeing, or it could be that there is water below the surface that wells up during the Martian spring and summer, and retreats during the autumn and winter. The salts in the water would act as a kind of antifreeze, which explains why the RSL appear when the surface temperature rises above -23 Celsius; pure water still would be frozen solid. RSL have already been examined at 4 sites on the surface of Mars and these hydrated salts were found at all of them, which suggests that there will be water elsewhere on the planet as well.
Whatever the source of the water is, its discovery is momentous. Not just for what it may tell us about the history of Mars and what the planet is like today, but also for the possibilities for future missions. If there is water on Mars there may well be life, although it’s more likely to be microbial (single cell organisms) than little green men. Of course, as John Grunsfeld, a physicist and former NASA astronaut, pointed out this afternoon it’s quite likely that microbes from Earth have been transported to Mars aboard the rovers and other exploratory craft despite our best attempts to keep them sterile. Earth microbes, John said with a wry smile, are tenacious. He also pointed out that even if we did find alien life we wouldn’t necessarily understand how it developed; after all, we don’t even know how life began on our own planet.
As well as opening up the possibility of alien life, today’s announcement has fired the imagination of many people who are picturing astronauts setting up greenhouses and growing crops on Mars. Although this ties in rather neatly with a newly released Hollywood film, it’s almost certainly not going to happen in our lifetime. Although the RSL are large, with some more than 7 kilometres wide, the amount of liquid water observed so far is still miniscule when compared to the planet’s surface area. It may well be there there are greater sources of water below the surface but we will have to wait to find out. The current Mars rovers are unable to tackle the terrain near the RSL and although there are future missions planned (including the INSIGHT explorer in 2016) none of the space agencies have yet built a rover that could tackle difficult terrain as easily as an astronaut can. However, if all goes well, humans will set foot on the red planet within the next 15 years…