Mercury Not Too Hot For Polar Water Ice?

Mercury is the closest planet to the sun; it must therefore be the least likely place to find a reservoir of water ice.

However, according to observations made by the Mercury Dual Imaging System (MDIS) aboard NASA's MESSENGER spacecraft and radio telescope data from the 1990s, a supply of water may be held inside Mercury's shady craters in the planet's polar regions.

MESSENGER (short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging) has been in Mercury orbit for a year and it is giving us an unprecedented view of the tiny, barren world.

On Wednesday, during the Lunar and Planetary Science Conference in Houston, the MESSENGER science team announced the weird discovery that the planet's surface is being shaped by a mystery mechanism not related to asteroid impacts or volcanism. In short, Mercury is swelling. The planet's core is also more massive than previously believed.

On Thursday, scientists also announced the possibility that water ice may exist on Mercury's surface. However, this potential discovery comes with some conditions.

"Bright patches" had been detected on Mercury in the past - namely by radar observations by the Arecibo radio telescope - but there was little supporting evidence to suggest these highly radar-reflective regions could be proof of water ice.

But with MESSENGER's help, a comprehensive global map is being constructed. By using the spacecraft's high-resolution imagery of Mercury's craters, the earlier Arecibo observations could be overlaid.

It turns out that these bright patches appear inside Mercury's shady craters, a potential indicator that water ice is present.

"All radar-bright features near Mercury's north pole are confined to shadowed areas in MDIS images to date, consistent with the water-ice hypothesis," said Johns Hopkins University planetary scientist Nancy Chabot in the conference proceedings (PDF).

But there's a catch. For water ice to remain stable inside these craters, an insulating layer of regolith - the thin, pulverized rock dust that forms on planetary bodies after eons of meteorite impacts - needs to blanket the surface, keeping the ice in a frozen state (preventing it from sublimating into space). For the ice to be preserved, around 20-30 centimeters (8-12 inches) of overlaying regolith needs to be present.

Should the presence of water inside the shady craters of Mercury's poles be confirmed, many would be surprised. But the discovery of Mercurian H2O, the stuff that supports all life as we know it, will undoubtedly provide important clues as to the ubiquitous nature of water throughout the solar system. Although the question of life doesn't extend to an environment as extreme as Mercury, the source of the water locked in its craters will keep scientist busy for years to come.

Image: Mercury north polar "bright patches" as observed by Arecibo in 1999 (yellow) overlaying the MESSENGER MDIS monochrome map. Credit: Chabot et al., 2012/Johns Hopkins University