New Evidence for Ice on Mercury
Nov. 29, 2012: New data from NASA's MESSENGER spacecraft suggest that there's enough ice on Mercury to encase the entire US capitol.
"The new data indicate the water ice in Mercury's polar regions, if spread over an area the size of Washington, D.C., would be more than 2 miles thick," said David Lawrence, a MESSENGER participating scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., and lead author of one of three papers describing the findings in the online edition of Science Express.
Given its proximity to the Sun, Mercury would seem to be an unlikely place to find ice. But the tilt of Mercury's rotational axis is almost zero -- less than one degree -- so there are pockets at the planet's poles that never see sunlight. Scientists suggested decades ago that water ice might be trapped in those shadowed areas at Mercury's poles.
The idea received a boost in 1991, when the Arecibo radio telescope in Puerto Rico detected unusually radar-bright patches at Mercury's poles, spots that reflected radio waves in the way one would expect if there were water ice. Many of these patches corresponded to the location of large impact craters mapped by the Mariner 10 spacecraft in the 1970s. But researchers weren't sure if the radar-bright patches detected by Arecibo corresponded to shadowy places in the craters.
MESSENGER's arrival at Mercury last year changed that. Images from the spacecraft's Mercury Dual Imaging System taken in 2011 and earlier this year show that radar-bright features at Mercury's north and south poles are within shadowed regions on Mercury's surface.
Now, the newest data from MESSENGER confirm that water ice is the major constituent of Mercury's north polar deposits. In the coldest places, the ice is exposed on the surface. In slightly warmer spots, some kind of dark insulating material appears to cover the ice.
MESSENGER uses neutron spectroscopy to measure average hydrogen concentrations within Mercury's radar-bright regions. Ice concentrations are derived, in turn, from the hydrogen measurements. This is possible because water, or H2O, is two parts hydrogen.
"The neutron data indicate that Mercury's radar-bright polar deposits contain, on average, a hydrogen-rich layer more than tens of centimeters thick beneath a surficial layer 10 to 20 centimeters thick that is less rich in hydrogen," says Lawrence. "The buried layer has a hydrogen content consistent with nearly pure water ice."
Data from MESSENGER's Mercury Laser Altimeter (MLA) -- which has fired more than 10 million laser pulses at Mercury to make detailed maps of the planet's topography -- corroborate the ice hypothesis, writes Gregory Neumann of the NASA Goddard Flight Center. In a second paper, Neumann and his colleagues report that the first laser measurements of the shadowed north polar regions reveal irregular dark and bright deposits near Mercury's north pole.
Neumann suggests that both the dark and bright materials were brought to Mercury by comets or asteroids, a finding corroborated in a third paper led by David Paige of the University of California, Los Angeles.
Stay tuned to Science@NASA for answers.
MESSENGER Home Page -- NASA
MESSENGER Home Page -- Johns Hopkins Applied Physics Lab
MESSENGER was designed and built by APL. The lab manages and operates the mission for NASA's Science Mission Directorate in Washington. The mission is part of NASA's Discovery Program, managed for the directorate by the agency's Marshall Space Flight Center in Huntsville, Ala.