Mercury''s poles icier than thought

Washington, September 20

There may be much more ice on Mercury's scorching hot surface than thought, say scientists who have found evidence of frozen water hidden away on crater floors that are permanently shadowed from the Sun's blistering rays.

A study, published in the Journal Geophysical Research Letters, found three new craters near Mercury's north-pole that appear to harbour large surface ice deposits.

The research also shows evidence of smaller-scale deposits scattered around Mercury's north-pole, both inside craters and in shadowed terrain between craters.

Those deposits may be small, but they could add up to a lot more previously unaccounted — for ice, researchers said.

"The assumption has been that surface ice on Mercury exists predominantly in large craters, but we show evidence for these smaller-scale deposits as well," said Ariel Deutsch, PhD candidate at Brown University in the US.

"Adding these small-scale deposits to the large deposits within craters adds significantly to the surface ice inventory on Mercury," said Deutsch.

The idea that Mercury might have frozen water emerged in the 1990s, when Earth-based radar telescopes detected highly reflective regions inside several craters near the planet's poles.

The planet's axis does not have much tilt, so its poles get little direct sunlight, and the floors of some craters get no direct sunlight at all.

Temperatures in those eternal shadows have been calculated to be low enough for water ice to be stable.

NASA's MESSENGER probe, which entered Mercury's orbit in 2011, detected neutron signals from the planet's north-pole that were consistent with water ice.

For the new study, researchers looked specifically at readings from the spacecraft's laser altimeter. The device is mostly used to map elevation, but it can also be used to track surface reflectance.

The addition of those craters to Mercury's ice inventory is significant. Deutsch estimates the total area of the three sheets to be about 3,400 square kilometers.

Researchers also looked for patches that were smaller than the big crater-based deposits, but still large enough to resolve with the altimeter. They found four, each with diameters of less than about five kilometers.

"These four were just the ones we could resolve with the MESSENGER instruments," Deutsch said.

"We think there are probably many, many more of these, ranging in sizes from a kilometer down to a few centimeters," he said.

Knowing that these small-scale deposits exist, and that they are likely the source of the slightly brighter surface outside craters, could dramatically increase the ice inventory on Mercury. — PTI