Frozen material at the planet’s poles likely came from comet or asteroid impacts

By Tanya Lewis

Web edition: November 30, 2012
Print edition: January 12, 2013; Vol.183 #1 (p. 17)

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Ice spotted

New data from NASA’s MESSENGER spacecraft reveal bright spots (shown in yellow) on Mercury that are almost certainly ice.

Credit: NASA, Johns Hopkins Univ. Applied Physics Laboratory, Carnegie Institution of Washington, National Astronomy and Ionosphere Center, Arecibo Observatory

The sun-scorched surface of Mercury may be the last place you’d expect to find ice. But NASA’s MESSENGER spacecraft has found the strongest evidence yet of frozen water — and carbon-rich material — on the planet closest to the sun.

While Mercury itself couldn’t support life, the findings provide clues about how water and other vital ingredients ended up on Earth, perhaps delivered by comets or asteroids. “Studying this stuff elsewhere in the solar system is really relevant for the origin of life,” says UCLA planetary scientist David Paige.

He and other scientists describe the findings in three studies published online November 29 in Science.

In the early 1990s, Earth-based radar measurements hinted at the presence of ice when they found mysterious bright spots near Mercury’s poles. The new studies reveal that the spots are indeed water ice. Darker areas may be carbon-rich material blanketing the ice and insulating it from the sun.

“The combination of results has presented a beautiful case for water ice with hydrocarbon infusion,” says Ann Sprague, a planetary scientist at the University of Arizona who was not involved in the studies.

New observations of Mercury’s northern polar regions come from MESSENGER’s laser altimeter, which maps the planet’s topography and measures how reflective the surface is. Simulations of Mercury’s surface temperatures match the laser measurements. Inside shadowy, steep-walled craters temperatures are below -173° Celsius, so frozen water can exist, Paige and colleagues report. Bright areas coincide with regions predicted to be cold enough for ice to be stable, while the dark areas coincide with warmer regions thought to harbor ice only beneath the surface.

“It matched perfectly,” Paige says. “We actually see bright ice every place we expect to see it.” Temperature models also suggest the ice was once much more extensive but has retreated, leaving behind the darker deposits.

Another line of evidence for frozen water comes from MESSENGER’s neutron spectrometer, which detects how much hydrogen (and therefore, presumably, water) is present. Cosmic rays constantly bombard Mercury’s surface, breaking apart atomic nuclei and scattering neutrons. When the neutrons collide with hydrogen atoms, they lose energy and grind to a halt like a cue ball hitting a billiard ball, says planetary scientist David Lawrence of Johns Hopkins University.

The dearth of neutrons detected near Mercury’s north pole suggests a lot of hydrogen lies just below the planet’s surface, almost certainly as pure ice. The mass of ice could be up to a trillion metric tons.

Scientists think the ice and carbon-rich material probably arrived via comets and asteroids, in a constant pummeling that should mix up the top layers of the crust. “It’s like you’ve got a bunch of people with shovels, reworking the surface,” says Paige. The fact that the bright and dark spots have not yet been mixed in suggests the spots formed fairly recently, in geological terms.