For decades, researchers looking for life elsewhere in the solar system have stuck to a simple rule: "follow the water". The stuff is essential to life on Earth, and its peculiar properties mean those hunting aliens think the same will be true elsewhere. This is one reason that Mars has received so much of their attention. Mars has water vapour in its atmosphere, and water ice in the shining cap at its north pole. Liquid water-which is the form life needs-may well exist underground, and may still sometimes be seen at the surface, where dried-up rivers and lakes suggest it was once commonplace. Orbiting spacecraft have seen mysterious dark channels appear on crater walls; recent results from Curiosity, the newer of the two American rovers now exploring Mars, suggest these could be trickles of brine, kept liquid in the bitter cold by their high salt content.
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