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Messenger Spacecraft Finds Evidence of Ice on Planet Closest to Sun

Shown in red are areas of Mercury’s north polar region that are constantly in shadow in every image that the MESSENGER spacecraft has acquired to date. These areas are rich in water ice, despite the planet’s close proximity to the sun. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/National Astronomy and Ionosphere Center, Arecibo Observatory

NASA’s MESSENGER spacecraft has detected evidence that the tiny planet Mercury could contain substantial amounts of water ice. This new data backs up a long-held hypothesis that the planet holds frozen water and other volatiles in the permanently shadowed craters located on Mercury’s polar region.

MESSENGER is an acronym for MErcury Surface, Space ENvironment, GEochemistry, and Ranging. Given that Mercury was the messenger of the gods, the name fits.

The instruments onboard MESSENGER have provided views of Mercury that were not possible before. Since the spacecraft’s arrival in March of 2011, researchers are able to better define how the inner terrestrial planets gained their water as well as some organic elements.

“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). Lawrence is also the lead author on one of three papers that detail the findings. The papers were posted online in Thursday’s edition of Science Express.

The MESSENGER spacecraft, depicted in this artist’s rendering, began studying Mercury last year. It was designed and built by the Johns Hopkins Applied Physics Laboratory. Image Credit: NASA

MESSENGER’s instruments have finished their initial assessment of excess hydrogen at Mercury’s north pole. The spacecraft has also made initial measurements of the reflectivity of deposits at near-infrared wavelengths which allowed the first-ever comprehensive models to be produced. These included the temperatures of Mercury’s north pole.

Mercury is just 35,980,000 miles (57,910,000 km) out from the sun, a fact that would make the presence of water, in any form, unlikely. The fact that ice has been detected on Mercury is due to the tilt of the tiny world’s rotational axis, which is less than one degree. As such, some portions of Mercury’s poles never see the light of the sun.

“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,” said Sean Solomon of Columbia University’s Lamont-Doherty Earth Observatory in Palisades, N.Y. “MESSENGER now has supplied a unanimous affirmative verdict.”

The concept of ice existing in the permanently-shadowed regions of Mercury’s north pole is nothing new. It was first suggested decades ago. Photo Credit: NASA

The concept that Mercury might have ice on its surface is nothing new. According to a NASA press release, scientists suggested decades ago that Mercury’s poles could contain water ice.

The idea received support in the 90s when the Arecibo radio telescope, located in Puerto Rico, noted radar-bright patches at Mercury’s poles. Arecibo’s findings overlapped well with the large impact craters that the Mariner 10 probe discovered back in the 70s. However, Mariner 10 only mapped about 50 percent of the planet’s surface, leaving our understanding of the world somewhat lacking.

MESSENGER’s 2011 imagery matched the radar-bright regions with shadowed areas on Mercury’s surface. This appears to further validate the water ice hypothesis.

New findings by MESSENGER suggest that a major component of Mercury’s north pole deposits is water ice. Measurements also show that ice is exposed on the planet’s surface in the coldest of these deposits.

MESSENGER used its neutron spectrometer to study the average hydrogen concentrations within the radar-bright regions.

“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.

MESSENGER conducted detailed topographical surveys of Mercury’s terrain, which corroborated the radar data as well as the neutron measurements that Lawrence highlighted.

NASA’s release also mentioned another interesting discovery. In a second paper by Gregory Neumann, measurements of the shadowed north polar regions reveal irregular bright and dark deposits that were detected in near-infrared wavelengths at Mercury’s north pole.

Mariner 10 scanned about 50 percent of Mercury’s surface. The MESSENGER mission is providing much more data about the planet. Image Credit: NASA / JPL

“Nobody had seen these dark regions on Mercury before, so they were mysterious at first,” said Gregory Neumann with NASA’s Goddard Flight Center in Greenbelt, Md.

Whereas some sections of the region surveyed by MESSENGER showed some areas with far less reflectivity than others. This indicates that the ice in these regions is covered by a layer of material that dims the highly-reflective ice.

Neumann thinks that either asteroids or comets could have deposited the material on Mercury’s surface, but the materials were not pure. This would explain the varied surface at Mercury’s north pole.

“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,” said David Paige with the University of California at Los Angeles, one of the authors of the third paper about these recent findings.

MESENGER was produced by APL. The mission is operated by the lab for NASA and is part of NASA’s Discovery Program.

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