he Red Planet was once warm, wet and life-friendly – or so we thought. The closer we look, the muddier the water becomes
Mars has received its fair share of visitors over the years, each one arriving with ever more sophisticated instruments and promising to uncover ever more of the Red Planet's secrets. Right now, spacecraft are circling it and rovers are trundling across its surface - the most recent being NASA's Curiosity. Yet, when it comes to the question of Mars's ancient climate, they only seem to be muddying the waters.
For over a century, we have blown hot and cold, or perhaps wet and dry, on the Martian climate. Then after decades of observing geological features, such as sinuous networks reminiscent of river valleys, one view came to prevail. It goes something like this... Way back when, just as life was getting started on Earth, Mars, too, was relatively wet and hospitable. It had a thick atmosphere, an active rainfall cycle and a system of rivers and lakes. The Red Planet may even have had a huge and long-lasting ocean.
But what if we've got the story all wrong? Several strands of evidence seem to be pointing in a different direction. If they are correct, Mars has never had a lasting hydrological cycle of open water, evaporation and rainfall, similar to Earth's. Instead, it has always been a cold, arid desert, punctuated by a smattering of brief episodes of warmer, wetter conditions. It's a radical view that - if it holds up - will re-write our understanding of Mars's climate and cause us to reassess the types of life that may have once flourished there.
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