Surely the ghost of John Wesley Powell is out there somewhere, shouting a hurrah for his two new companions in the annals of heroic one-armed geology. When Powell led a three-boat expedition through the Grand Canyon more than a century ago, planets were but fuzzy dots in telescopes and space travel rarely even a dream. But the Civil War veteran, who lost half his right arm at the Battle of Shiloh, would grasp exactly what two NASA rovers and their earthbound handlers have done since January 2004, when the machines alighted on Mars to hunt for signs of ancient water.
In Powell's day geologists puzzled over how water sculpted the pink-hued Colorado Plateau into a canyon maze; their counterparts today wonder about water's role in carving landforms on red-hued Mars. Just as Powell's ragtag company defied expectations in 1869 by surviving nearly a hundred days of savage rapids, the rovers, expected to conk out well before the end of last year, were going strong months later. Powell was a field geologist, cracking rocks and taking notes with his single arm. So too each rover uses its three-jointed arm to wield equipment including a camera and a tool for grinding into Mars rocks.
Rock-breaking brings up the essential reason to muse over these robots and old-timer Powell: ground truth. This is not a casual term among geologists. It evokes the dust-on-boots conclusions that arise from personally grabbing samples of rock, walking formations, and exposing fresh stone. Generations of geologists have placed their literal, bedrock faith in ground truth.