Ice, Ice Mercury

Water ice deposits in craters around Mercury's north pole are seen in this mosaic image taken by the MESSENGER spacecraft, superimposed with a radar image. Courtesy: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/National Astronomy and Ionosphere Center, Arecibo Observatory

It’s rare that astronomers declare news with great certainty, given the awesome scope of their work and the level to which they must rely on data gleaned from objects so far away that superlatives quickly run out.

So Thursday’s announcement about the confirmation of water ice (as opposed to, say, carbon-dioxide or methane ice) in Mercury’s poles was, as the scientists put it, an “exclamation point” rather than a plain old period.

Three separate research papers, published in Science Express this week, all arrived at the same conclusion: that the planet closest to the Sun harbors a wealth of water ice, plus dark deposits believed to be organic compounds delivered by comets and asteroids.

The amount of ice is astounding—something on the order of 100 billion to a trillion metric tons.  Or, as David Lawrence of the MESSENGER Mercury mission team says, the rough equivalent of layering Washington, D.C. with 2 to 2.5 miles of ice.


The evidence derives from data gathered by the MESSENGER spacecraft that’s orbiting around Mercury.  Part of its mission has been to confirm theories about the presence of water ice, first ventured 20 years ago when radar measurements from Earth discovered “radar bright” areas in Mercury’s polar regions.

With MESSENGER actually orbiting the tiny planet, scientists could apply three key tests: a neutron spectrometer, which tests for the presence of hydrogen; the reflectance of the polar deposits at near-infrared wavelengths; and thermal models of the surface based on the planet’s actual topography.

“To have them all come together in this way is like getting a key that turns, and the door opens,” says Lawrence, senior scientist at Johns Hopkins University Applied Physics Laboratory.

The confirmation of a long-held theory has an even larger ramification—Mercury as a planet of astrobiological interest, says Sean Solomon, principal investigator of the MESSENGER mission.

“Messenger has revealed a very important chapter in the story of how water ice and other materials have been delivered to the inner planets we think by comets and asteroids,” he says.  “It’s amazing this history is so well-preserved on the planet closest to the sun.

“This isn’t to say we expect to find life on Mercury, but in the book of life there are some early chapters, and Mercury may indeed tell us something about those early chapters.”

Changing Planet