Ice Found on Mercury

NASA's Mercury-orbiting probe, MESSENGER, is providing evidence that water ice exists near the north pole of Mercury. Most of the ice is covered by a thin layer of material that blankets and protects the ice, but in a few areas where sunlight never reaches, some ice lies exposed on the surface.

The findings are based on reflectance measurements made by the spacecraft's laser altimeter and hydrogen concentrations from the neutron spectrometer as well as new computer modeling that explains how ice could persist for eons on the planet closest to the sun.

"Twenty years ago, radar astronomers made the startling proposal that there is abundant water ice at the poles of our solar system's innermost planet," says Sean Solomon of Columbia University. "But alternative explanations of the radar observations have since been suggested, and the MESSENGER team set a high priority on settling this question with orbital observations. The jury has at last returned a clear verdict."

Despite Mercury's proximity to the sun, the north and south polar regions remain cold because they never tilt toward the sun.

Evidence for ice deposits at both poles has been building since the early 1990s, when radar studies first spotted areas that looked very bright at radar wavelengths. These "radar-bright" regions suggested that some kind of reflective material lay on or very near the surface, and the locations turned out to be inside impact craters. More recently, extensive imaging by MESSENGER
(short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging) made it possible to match up the radar-bright areas with permanently shadowed regions in craters near the north and south poles.

The new studies focused on Mercury's north pole and the surrounding high latitudes. When researchers measured the reflectance at near-infrared wavelengths using the altimeter, they found more than a hundred dark regions, which reflected less than half the typical amount of light. A few exceptionally bright areas also were detected in the craters named Prokofiev and Kandinsky.

"Nobody had seen these dark regions on Mercury before, so they were mysterious at first," says Greg Neumann, an instrument scientist for the Mercury Laser Altimeter.

The ice seems to be the result of a geologically recent or ongoing process, perhaps delivered by comets and other small bodies from the outer solar system. Organic chemicals from those same bodies might make up the material that covers the ice in most places.

"These new findings let us discard the proposals that the radar might be picking up other reflective materials instead of ice," says Neumann. "Now we can focus on understanding the processes that are emplacing volatiles at Mercury’s poles."


Elizabeth Zubritsky
NASA's Goddard Space Flight Center, Greenbelt, Md.