The MESSENGER spacecraft is already in space and observations from it support the long-held theory that there is ice on Mercury. How is this possible, you may ask? Mercury is closer to the Sun then Earth and has no atmosphere, this is preposterous! Be that as it may, Mercury defies the obvious due to the tilt of its axis, which is less than one degree. By comparison, the Earth’s axis is tilted at 23.5 degrees, giving many places on Earth the four seasons.
Mercury’s near zero tilt means that there are places that never receive any sunlight. These particular areas are located inside craters at the planet’s poles; and there has been evidence suggesting the existence of water-ice and frozen volatile materials in these craters for years. The Mercury Laser Altimeter (MLA) measured the reflectance of Mercury’s polar deposits in the near-infrared wavelengths. It also mapped the surface topography of Mercury and this data was used to create a detailed model of the planet’s surface and near-surface temperatures. MESSENGER’s contribution was the first measurement of excess hydrogen at the poles.
MESSENGER reached Mercury last year and is imaging the planet with the spacecraft’s Mercury Imaging System. These images have confirmed what has been suspected for years and seen in a limited capacity with radio telescopes on Earth: the radar-bright features at the planet’s poles are located inside craters that are permanently shadowed. The radar reflectance is consistent with that seen from frozen water on our own planet.
MESSENGER uses neutron spectroscopy to derive water-ice concentrations from hydrogen measurements. These measurements indicate that most of the frozen water is beneath a less hydrogen rich layer, as it is too warm for water to stay frozen on most of the planet’s surface. The dark surface is approximately 10 to 20 centimeters thick. The ice underneath is more than tens of centimeters thick and the hydrogen content is consistent with nearly pure water ice. After years of speculation, MESSENGER’s findings have added more certainty to the belief that Mercury harbors water in the form of ice.