Article written by: Tolis Christou

Astronomers now know many hundreds of planets orbiting other stars in our Galaxy. These show an incredible amount of diversity in their basic properties such as size and temperature with no two planets being quite the same. But the Earth is still unique among planets within or outside our solar system in its ability to support life. It is also the only astronomical body we know of, able to sustain liquid water on the surface. This is probably not a coincidence, as water is thought to be a necessary ingredient in the emergence and proliferation of living organisms.

The Red Planet, named after the Roman God of War: Credit: NASA

Now scientists have announced the discovery, for the first time, of a cache of liquid water on a planet other than our own. The discovery was made using an instrument on board Mars Express, a probe launched by the European Space Agency back in 2003, now in orbit around the Red Planet for more than a decade.  The instrument is used to shoot radar pulses down at the planet. The pulses penetrate up to several miles below the surface before they are reflected back to the spacecraft. By measuring the travel time and strength of the echo, one can work out the structure of the subsurface.

Image of Mars from HST

Image of Mars captured by the HST in 2003 at its closest approach to Earth in 60 000 years. Mars will be this close again in 2287. (Image credit: NASA)

The cache was found under a mile-deep layer of water ice near the south pole of Mars, in an area called Planum Australe. Scientists had noticed that this area gave off particularly strong radar echoes but it took many orbital passes over several years to map it out and see it as a distinct “patch” on the ground. Although the radar data cannot tell us precisely how thick it is, a depth of at least a metre is necessary to give off as strong an echo as detected by Mars Express. The layer extends over an area 20 km across – about the size of Lough Neagh – and is probably mixed up with mineral salts to create a sort of thick “sludge” or brine.  The presence of salts and the pressure from the overlying ice layer is helping to keep the water liquid, despite the freezing temperatures, some 70 degrees below zero.


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