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.
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 1 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 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