SCIENCE UPDATE
Warm summers and dust storms are responsible for the Red Planet losing its rivers and oceans.
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ASTRONOMY Until 3.8 billion years ago, Mars was almost as wet as the Earth, with lakes, rivers and a large ocean that covered around a third of the planet. It has long been a mystery to scientists where all the water went, but German and Russian astronomers have now found an explanation. The Red Planet has been drained of its water by dust storms and warm summers.
Mars has seasons just like Earth, but on the Red Planet they are much more extreme. The Sun is not perfectly at the centre of Mars’ orbit, so the planet is much closer to the Sun during its southern hemisphere’s summer than during the rest of the year. The atmosphere of the southern hemisphere is heated, causing a hole to appear, through which the planet’s water vapour can escape. Normally, vapour is trapped in the atmosphere by a cold air layer located at an altitude of 60-90km. But this layer ‘bursts’ during the southern hemisphere's summer heat.
Scientists have tested their theory by means of computer simulations. These indicate that water will evaporate from the planet’s surface, escape through this hole in the atmosphere’s cold layer, and pass into a higher layer. From there the vapour moves towards the planet’s poles, where it passes back down through the atmosphere to fall as snow.
But not all the water returns. In the upper atmospheric layers, water molecules are subjected to ultraviolet solar radiation that splits them into hydrogen and oxygen. The hydrogen dissipates into space, leaving the oxygen in the atmosphere. So Mars gradually loses water, and what remains collects as ice near the poles.
The computer model also shows that the process is intensified by dust storms that bring tiny dust particles carrying water molecules high into the atmosphere; it is even easier for these particles to reach the upper layers than for the water molecules on their own. This reinforces the effect: the less water on the surface, the more dust is stirred up by the storms, and the more water the planet loses.