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Planet Mercury News: What’s Hiding in those Polar Shadows?

MESSENGER’s highly eccentric orbit, which passes low over Mercury’s north polar region, enables higher-resolution views of Mercury’s surface in the north than in the south. Of particular note are the craters hosting radar-bright features at low latitudes, extending southward to 67° N, and the many small craters that host radar-bright deposits. Low-latitude and small craters provide thermally challenging environments for water ice to persist. A thin (few tens of centimeters thick) layer of insulation is likely required to cover and to lower the temperature of these deposits if they are water ice. However, the smallest craters and the lowest-latitude locations may prove a challenge for water ice stability over extended periods of geologic time even with such cover. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

THE WOODLANDS, Texas – NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging – long hand for the MESSENGER spacecraft – has provided surprising new looks at the planet Mercury.

Details of MESSENGER’s findings are being presented this week here at the 43rd Lunar and Planetary Science Conference in The Woodlands, Texas.

A chief goal of MESSENGER’s primary mission was to understand the nature of the radar-bright deposits at the poles of Mercury. The leading proposal since the deposits were discovered has been that radar-bright material consists dominantly of frozen water ice.

Given Mercury’s closeness to the Sun, that’s a puzzle.

“We’ve never had the imagery available before to see the surface where these radar-bright features are located,” says Nancy Chabot, instrument scientist for MESSENGER’s Mercury Dual Imaging System (MDIS) at the Johns Hopkins University Applied Physics Laboratory (APL).

“MDIS images show that all the radar-bright features near Mercury’s south pole are located in areas of permanent shadow, and near Mercury’s north pole such deposits are also seen only in shadowed regions, results consistent with the water-ice hypothesis.”

Not definitive proof

However, this finding is not definitive proof that those deposits are water ice.

Some of the radar-bright deposits are located in craters that provide thermally challenging environments to the water-ice theory. For instance, for the radar-bright material in many of the craters to be water ice would require that there be a thin layer of insulation to keep it colder than the surface, Chabot adds.

More work is ahead on this issue.

The MDIS images, combined with ongoing analysis of data from other instruments on MESSENGER will provide a more complete picture of the nature of the deposits.

MESSENGER’s second year at Mercury will build upon these and other results from the primary mission phase.

Discovery-class mission

The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.

MESSENGER is the first space mission designed to orbit the planet closest to the Sun. The spacecraft was launched on August 3, 2004, and entered orbit about Mercury on March 18, 2011 to begin its primary mission – a yearlong study of its target planet.

MESSENGER completed its one-year primary mission on March 17.

Since moving into orbit about Mercury a little over one year ago, the spacecraft has captured nearly 100,000 images and returned data that have revealed new information about the planet, including its topography, the structure of its core, and areas of permanent shadow at the poles that host the mysterious polar deposits.

By Leonard David

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