NASA Spacecraft Finds New Evidence for Water Ice on Mercury

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Nov. 29, 2012

George Diller
Kennedy Space Center, Fla.
321-867-2468
george.h.diller@nasa.gov

Dwayne Brown
Headquarters, Washington
202-358-1726
dwayne.c.brown@nasa.gov

Paulette Campbell
Johns Hopkins University Applied Physics Laboratory, Laurel, Md.
240-228-6792
paulette.campbell@jhuapl.edu

RELEASE: 12-411

NASA SPACECRAFT FINDS NEW EVIDENCE FOR WATER ICE ON MERCURY

WASHINGTON -- A NASA spacecraft studying Mercury has provided 
compelling support for the long-held hypothesis the planet harbors 
abundant water ice and other frozen volatile materials within its 
permanently shadowed polar craters.

The new information comes from NASA's MErcury Surface, Space 
ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. Its 
onboard instruments have been studying Mercury in unprecedented 
detail since its historic arrival there in March 2011. Scientists are 
seeing clearly for the first time a chapter in the story of how the 
inner planets, including Earth, acquired their water and some of the 
chemical building blocks for life.

"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. The papers were published online in 
Thursday's edition of Science Express.

Spacecraft instruments completed the first measurements of excess 
hydrogen at Mercury's north pole, made the first measurements of the 
reflectivity of Mercury's polar deposits at near-infrared 
wavelengths, and enabled the first detailed models of the surface and 
near-surface temperatures of Mercury's north polar regions.

MESSENGER launched at 2:15:56 a.m. EDT on Aug. 3, 2004, aboard a 
Boeing Delta II rocket from Cape Canaveral Air Force Station in 
Florida. The countdown and launch were managed by NASA's Launch 
Services Program based at Kennedy Space Center.

Given its proximity to the sun, Mercury would seem to be an unlikely 
place to find ice. However, the tilt of Mercury's rotational axis is 
less than one degree, and as a result, there are pockets at the 
planet's poles that never see sunlight.

Scientists suggested decades ago 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. Many of these 
patches corresponded to the locations of large impact craters mapped 
by NASA's Mariner 10 spacecraft in the 1970s. However, because 
Mariner saw less than 50 percent of the planet, planetary scientists 
lacked a complete diagram of the poles to compare with the radar 
images.

Images from the spacecraft taken in 2011 and earlier this year 
confirmed all radar-bright features at Mercury's north and south 
poles lie within shadowed regions on the planet's surface. These 
findings are consistent with the water ice hypothesis.

The new observations from MESSENGER support the idea that ice is the 
major constituent of Mercury's north polar deposits. These 
measurements also reveal ice is exposed at the surface in the coldest 
of those deposits, but buried beneath unusually dark material across 
most of the deposits. In the areas where ice is buried, temperatures 
at the surface are slightly too warm for ice to be stable.

MESSENGER's neutron spectrometer provides a measure of average 
hydrogen concentrations within Mercury's radar-bright regions. Water 
ice concentrations are derived from the hydrogen measurements.

"We estimate from our neutron measurements the water ice lies beneath 
a layer that has much less hydrogen. The surface layer is between 10 
and 20 centimeters [4-8 inches] thick," Lawrence said.

Additional data from detailed topography maps compiled by the 
spacecraft corroborate the radar results and neutron measurements of 
Mercury's polar region. In a second paper by Gregory Neumann of 
NASA's Goddard Flight Center in Greenbelt, Md., measurements of the 
shadowed north polar regions reveal irregular dark and bright 
deposits at near-infrared wavelength near Mercury's north pole.

"Nobody had seen these dark regions on Mercury before, so they were 
mysterious at first," Neumann said.

The spacecraft recorded dark patches with diminished reflectance, 
consistent with the theory that ice in those areas is covered by a 
thermally insulating layer. Neumann suggests 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 at Los Angeles.

"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," Paige said.

This dark insulating material is a new wrinkle to the story, according 
to MESSENGER principal investigator Sean Solomon of Columbia 
University's Lamont-Doherty Earth Observatory in Palisades, N.Y.

"For more than 20 years, the jury has been deliberating whether the 
planet closest to the sun hosts abundant water ice in its permanently 
shadowed polar regions," Solomon said. "MESSENGER now has supplied a 
unanimous affirmative verdict."

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.

For more information about the Mercury mission, visit:

http://www.nasa.gov/messenger 

	
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