The MESSENGER spacecraft has made a compelling case for the presence of water in the form of ice on the surface of the Solar System's smallest and innermost planet, Mercury. The case is supported by three independent groups of evidence from different sensors aboard the Mercury orbiter.
MESSENGER's Neutron Spectrometer detects low-energy neutrons colliding with hydrogen-rich matter in the top 40 cm (16 in) of Mercury's surface. Readings from the spectrometer point to an "excess" of hydrogen in some form at the planet's north pole.
"The neutron data indicate that Mercury's radar-bright polar deposits contain, on average, a hydrogen-rich layer more than tens of centimeters thick beneath a surface layer 10 to 20 centimeters thick that is less rich in hydrogen," writes MESSENGER scientist David Lawrence of Johns Hopkins University Applied Physics Laboratory. "The buried layer has a hydrogen content consistent with nearly pure water ice."
The presence of water ice is reinforced by MESSENGER's Mercury Laser Altimeter which maps Mercury's topography by reflecting infrared lasers from the planet's surface. Since entering Mercury's orbit early in 2011, MESSENGER has bombarded the planet with over 10 million laser pulses.
The Laser Altimeter has recorded "reflectance anomalies" concentrated on slopes facing the planet's poles, with the presence of both bright and dark deposits. "Correlation of observed reflectance with modeled temperatures indicates that the optically bright regions are consistent with surface water ice," writes Gregory Neumann of the NASA Goddard Flight Center, while the dark patches reinforce the idea that the ice is buried under an insulating layer in some regions.
Lastly there are the simulations of temperatures at and near to Mercury's surface, based upon data provided by the Mercury Laser Altimeter. These models reinforce the conclusions drawn from the Altimeter data, with the lighter regions detected coinciding with the distribution of water ice predicted in the thermal models.
Both the University of California's David Paige, who led the thermal modeling study, and Neumann believe that the dark insulating material detected could have been delivered by comet or "volatile-rich" asteroids that could also have brought the water with them. Paige writes that the material is likely to include complex organic compounds.
"Do the dark materials in the polar deposits consist mostly of organic compounds?" asks Columbia University's principal MESSENGER investigator Sean Solomon. "What kind of chemical reactions has that material experienced? Are there any regions on or within Mercury that might have both liquid water and organic compounds?"
Despite Mercury's relative proximity to the Sun, the possibility of ice on the planet has long been speculated. Because Mercury's rotates on an axis of less than one degree, there are localized sheltered pockets near the planet's poles that sunlight never penetrates.
On the question of water on Mercury, Solomon seems convinced. "For more than 20 years the jury has been deliberating on whether the planet closest to the Sun hosts abundant water ice in its permanently shadowed polar regions," he said. MESSENGER has now supplied a unanimous affirmative verdict."