SUMMIT STATION, Greenland -- The midnight sun is shining on the Greenland ice sheet, and Yeti Robot is out for a spin. The probe's chunky tires crunch a trail through the snow, then jerk to a stop. Yeti's handlers try to diagnose the problem. Seconds later, Yeti is again whizzing toward the horizon. It's the first day of this year's field tests, and the researchers are eager to show off their prize pupil. If all goes well, battery-powered Yeti and its close relative -- a solar-powered version called Cool Robot -- could one day expand scientists' access to Earth's poles and enhance their ability to study how climate change is speeding the melt of Greenland's ice sheet.