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| A Mosaic of MESSENGER Images of Mercury's North Polar Region [IMAGE: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/National Astronomy and Ionosphere Center, Arecibo Observatory] |
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.
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."