A gigantic dust storm has enveloped about half of Mars, recent NASA spacecraft images show. “This is by far the largest storm we’ve seen during the Mars Global Surveyor mission,” said Philip Christensen of Arizona State University in Tempe, principal investigator for the probe’s thermal emission spectrometer. “We expect that the storm will continue to grow — perhaps becoming a global storm of the type that was seen during the Mariner 9 and Viking missions in the 1970s.”