A NASA spacecraft has found compelling evidence to support the long-held hypothesis that Mercury 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.
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.
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, New York.
“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 Messenger now has supplied a unanimous affirmative verdict,” Solomon said.
The papers were published online in Thursday’s edition of Science Express.