November 30, 2012

Mercury probe confirms ice at poles

Raphael Jaffe
staff writer

Instruments aboard NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft studying the planet Mercury have provided compelling support for the long-held hypothesis the planet harbors abundant water ice within its permanently shadowed polar craters.

There is also evidence of a dark insulating material above some of the ice, which is believed to be condensed organic material.

The first ever map of the maximum surface temperature of Mercury shows an astounding range, from about 800 degrees Fahrenheit on the equator, to minus 370 degrees Fahrenheit in the ever shadowed craters. Recall that liquid nitrogen boils at minus 327 degrees Fahrenheit.

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 ó less than one degree ó so there are pockets at the planet’s poles that never see sunlight. Scientists suggested decades ago that there might be water ice and other frozen volatiles trapped at Mercury’s poles. The idea received a boost in 1991 when the Arecibo radio telescope in Puerto Rico detected radar-bright patches at Mercury’s poles.

Three independent lines of evidence support the finding of water ice. The first measurements of excess hydrogen at Mercury’s north pole with MESSENGER’s Neutron Spectrometer, the first measurements of the reflectance of Mercury’s polar deposits at near-infrared wavelengths with the Mercury Laser Altimeter, and the first detailed models of the surface and near-surface temperatures of Mercury’s north polar regions that utilize the actual topography of Mercury’s surface measured by the MLA. These findings were discussed at a NASA press conference on Nov 29; scientific papers have been published in Science magazine.

Images from the MESSENGER spacecraft’s Mercury Dual Imaging System taken in 2011 and earlier this year confirmed that radar-bright features at Mercury’s north and south poles are within shadowed regions of craters on Mercury’s surface, findings that are consistent with the water-ice hypothesis.

Now the newest data from MESSENGER strongly indicate that water ice is the major constituent of Mercury’s north polar deposits, that ice is exposed at the surface in the coldest of those deposits, but that the ice is buried beneath an unusually dark material across most of the deposits, areas where temperatures are a bit too warm for ice to be stable at the surface itself.

MESSENGER uses neutron spectroscopy to measure average hydrogen concentrations within Mercury’s radar-bright regions. Water-ice concentrations are derived from the hydrogen measurements. “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 superficial layer 10 to 20 centimeters thick that is less rich in hydrogen,” writes David Lawrence, a MESSENGER Participating Scientist based at The Johns Hopkins University Applied Physics Laboratory and the lead author of one of the papers. “The buried layer has a hydrogen content consistent with nearly pure water ice.”

Data from MESSENGER’s Mercury Laser Altimeter which has fired more than 10 million laser pulses at Mercury to make detailed maps of the planet’s topography ó corroborate the radar results and Neutron Spectrometer measurements of Mercury’s polar region, writes Gregory Neumann of the NASA Goddard Space Flight Center.

In a second paper, Neumann and his colleagues report that the first MLA measurements of the shadowed north Polar Regions reveal irregular dark and bright deposits at near-infrared wavelength near Mercury’s North Pole. The MLA also recorded dark patches with diminished reflectance, consistent with the theory that the ice in those areas is covered by a thermally insulating layer. Neumann suggests that impacts of comets or volatile-rich asteroids could have provided both the dark and bright deposits, a finding corroborated in a third paper led by David Paige of the University of California, Los Angeles Paige and his colleagues provided the first detailed models of the surface and near-surface temperatures of Mercury’s north Polar Regions that utilize the actual topography of Mercury’s surface measured by the MLA. The measurements “show that the spatial distribution of regions of high radar backscatter is well matched by the predicted distribution of thermally stable water ice,” he writes According to Paige, the dark material is likely a mix of complex organic compounds delivered to Mercury by the impacts of comets and volatile-rich asteroids, the same objects that likely delivered water to the innermost planet. The organic material may have been darkened further by exposure to the harsh radiation at Mercury’s surface, even in permanently shadowed areas.

This dark insulating material is a new wrinkle to the story, says Sean Solomon of the Columbia University’s Lamont-Doherty Earth Observatory, principal investigator of the MESSENGER mission. “For more than 20 years the jury has been deliberating on whether the planet closest to the Sun hosts abundant water ice in its permanently shadowed polar regions. MESSENGER has now supplied a unanimous affirmative verdict
ìBut the new observations have also raised new questions,” adds Solomon. “Do the dark materials in the polar deposits consist mostly of organic compounds? What kind of chemical reactions has that material experienced? Are there any regions on or within Mercury that might have both liquid water and organic compounds? Only with the continued exploration of Mercury can we hope to make progress on these new questions.”

MESSENGER is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and entered orbit about Mercury on March 18, 2011 (UTC), to begin its primary mission ñ a yearlong study of its target planet. MESSENGER’s extended mission began on March 18, 2012. Dr. Sean C. Solomon, of the Carnegie Institution of Washington, leads the mission as Principal Investigator. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA. 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.

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