Had you searched the sky with a telescope just a few hundred thousand years ago, you would have struggled to find a red planet. Instead, you would have seen a gleaming-white ice ball where Mars should be. A team of astronomers led by Isaac Smith, an astrophysicist at the Southwest Research Institute in Boulder, Colorado, has collected the first concrete evidence that Mars has just exited an extreme ice age, one so intense it would have put Earth's recent frosty foray to shame. Using cameras and a radar-pinging device on board NASA's Mars Reconnaissance Orbiter, Smith's team deduced this history...