Astronomers have confirmed that ice exists on one of the least likely places in the solar system—Mercury.
The planet is among the solar system’s hottest—only Venus has higher average temperatures—thanks to the sun’s searing proximity, which raises temperatures to as high as 800˚ F. But hidden in the frigid shadows—where temperatures can sink as low as -280˚ F—are frozen patches that are the likely remains of icy comets which have blasted the rocky terrain over the last 100 million years.
Scientists have long suspected that Mercury is home to solid H2O, but recent photos taken by the MESSENGER probe confirm it. Here’s Michael Lemonick, reporting for Time:
These are the first optical images, and nobody is entirely sure how the ice got there. One idea is that it was released from water-bearing rock in Mercury’s crust. But the leading theory suggests it arrived instead in the form of impacts from icy comets, which may well be the same way Earth got its oceans. “It’s a fair amount of ice,” Chabot said, “about equivalent to the water in Lake Ontario, so if it was one comet, it was a pretty sizable one.” More likely, she said, it would have been a series of smaller comets, falling over billions of years.
Mercury’s extreme temperature fluctuations are thanks to its extremely thin atmosphere. The planet’s small mass means it has a hard time holding onto gas particles, and the sun’s intense rays do their best to blast away what little that does cling to the rocky world. If the planet had a denser atmosphere, it would more closely resemble Venus, where the thick sky keeps average temperatures above 860˚ F.
Mercury’s thin atmosphere gives scientists another unique opportunity. Next spring, MESSENGER will be able to fly just 12 miles above the surface without burning up. During that pass, it will take extraordinarily high-resolution images of the planet, giving scientists a unique window into a world of extremes.