One of the best places on Earth to test spacecraft destined for Mars or the moon is the Atacama desert in Chile. Visiting there certainly feels like a trip to Mars, but it is definitely still planet Earth.
The Atacama is known as the driest place on Earth; a 1,000 km-long plateau, nestled in the Andes Mountains, that only receives an average of 15 mm of rain a year, with some regions getting no rain at all. This super aridity produces a beautiful, alien-looking landscape of salt flats and multi-coloured cliff faces streaked with gypsum, magnesium and pyrite while dark sand dunes migrate through the valleys.
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These dried-up conditions are similar to Mars, which used to be a warm, wet world with rivers, lakes and possibly oceans billions of years ago and has since turned into the dry desert world we see today.
Since Earth and Mars are made of the same basic rocky materials, engineers use places like the Atacama to test landers and rovers and see how they will perform in those conditions. They test the performance of vehicles that must drive across sand dunes without getting stuck, navigate around boulders and climb loose slopes, as well as seeing how well robotic drills and other instruments can penetrate rocks.
No spacesuit required
But while the similarities to Mars are striking, the differences between the Atacama and the Red Planet are just as dramatic.
First of all, the reddish ground is a good Mars analogue, but the blue sky above is the wrong colour. Martian skies are tinted salmon-orange by fine dust constantly stirred up by winds and a lack of water in the atmosphere.
The second big difference is the fact that you can walk around on the desert here without wearing a spacesuit. Mars will kill you in seconds, because Martian air is almost entirely carbon dioxide, with only traces of oxygen, and is so thin that, even if you could breathe it, your blood would boil from the extremely low air pressure. So, a spacesuit is needed every time you take a walk on Mars.
Then there is the temperature. The Atacama can be a hot place, with daytime temperatures well above 30 C. Pictures from the Mars rovers look like hot desert scenes, but, in fact, the temperature seldom climbs above the freezing point. Even on the equator, during summer at high noon, a thermometer on Mars could still read -20 C, dropping to -70 C at night.
That constant extreme cold is seldom portrayed in movies about Mars exploration and is something future Mars colonists will have to take very seriously.
(That's one reason another Mars testing ground is on Devon Island in the Canadian High Arctic.)
Animals have adapted to desert life
The earthly desert is also a good analogue for Venus, another rocky planet, often referred to as our sister world because it is the same size as Earth. There, we would also see a landscape of ruddy rocks and sandy-coloured sky. But the temperature is a scorching 460 C — the temperature of a pizza oven. It's another killer planet.
So, while the Atacama provides a good analogue for other worlds, it also underlines how special this one is. The Earth is just right: not too cold like Mars, not too hot like Venus, with plenty of water above and below the ground.
In fact, in the northern region of the desert, around San Pedro de Atacama, there is a lot of water underground in the form of aquifers and subterranean streams that are fed by glacial runoff from the Andes mountains. Some streams encounter hot volcanic magma and erupt as geysers that feed small rivers. So, while the desert gets very little rain from above, the water below supports a variety of life. It was a surprise to see different species of llama wandering around, munching on desert plants. Flamingos wade through briny streams, and even foxes have adapted to the unique conditions.
Mars may have harboured life billions of years ago during its wet past, and perhaps extremophiles — organisms that thrive in extreme conditions — still exist there in underground aquifers today, but no evidence for life has been found on Mars... yet. In fact, no life has been found anywhere in space... yet
Future travellers to Mars will be astounded by the dramatic landscapes: steep cliffs, many kilometres high, towering over canyon floors, volcanoes standing taller than Mt. Everest. But they will always be reminded that they are on an alien world and, like most travellers, will think about their little blue home planet and just how special it really is.