The clays detected by Curiosity have since been shown to be of a type called smectites. Typically these form in the sediments of lakes. "It is therefore quite likely that the entire Gale Crater was once flooded, possibly to a depth of more than a kilometre," adds Dietrich, who is also head of planetary sciences at the
For the project's engineers and scientists, the discovery was particularly satisfying because it revealed the power of Curiosity and its array of instruments. Tens of thousands of tests have now been carried out on Mars by the rover, threatening to engulf
"In the past, experiments that produced ambiguous results on Mars could not be repeated or compared with tests carried out by other instruments," says Pan Concord, a deputy principal investigator for the Sample Analysis at Mars device. "Here, we can go on and on until we are absolutely sure of our results. That is what we did at Yellowknife. It was wonderful. And I am sure we will do it again."
However, it is the wider implications of Curiosity's discovery at
Finding the nature of that catastrophe will be a future task for Curiosity. Most scientists suspect Mars's low mass and its lack of a magnetic field may have doomed it. On Earth, our relatively strong gravitational field plus our relatively intense magnetic field protect our atmosphere from battering by the solar wind, a constant stream of particles that pours from the sun. Without such fields, Mars could not hold on to its atmosphere or its water, which were swept into space. Earth remained blue and watery while Mars turned to dust. Over the next few years, Curiosity's Sam detector will sniff out delicate isotope variations in the painfully thin remnants of Mars's atmosphere for clues to the timing and nature of this disaster. The key question is: did life get a chance to make its appearance before catastrophe struck?
For Meyer, this issue is of critical importance. "We know life appeared on Earth. But we do not know how easy that process was. It could have been straightforward or it could have been a highly fraught business filled with all sorts of unlikely contingencies."
Mars provides the perfect place to solve that mystery. If its ancient watery, organic soils eventually led to the appearance of primitive living beings before catastrophe struck, we can conclude that life could be relatively commonplace in the cosmos, he argues. "Life appearing separately on two neighbouring worlds would suggest it is a straightforward phenomenon."
But if the sands of Mars turn out never to have supported life, despite their initially attractive properties, life will look a far less likely outcome in the universe. "From that perspective, life on Earth - including humans - may turn out to be a cosmic improbability," adds Meyer. Finding out which version is the right one underscores the importance of Curiosity and future rover missions, which will be designed specifically to answer such questions.
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