Geology of Mercury - High-albedo Polar Patches and Possible Presence of Ice

High-albedo Polar Patches and Possible Presence of Ice

The first radar observations of Mercury were carried out by the radiotelescopes at Arecibo (Puerto Rico) and Goldstone (California, United States), with assistance from the U.S. National Radio Astronomy Observatory Very Large Array (VLA) facility in New Mexico. The transmissions sent from the NASA Deep Space Network site at Goldstone were at a power level of 460 kW at 8.51 GHz; the signals received by the VLA multi-dish array detected points of radar reflectivity (radar luminosity) with depolarized waves from Mercury's north pole.

Radar maps of the surface of the planet were made using the Arecibo radiotelescope. The survey was conducted with 420 kW UHF band (2.4 GHz) radio waves which allowed for a 15 km resolution. This study not only confirmed the existence of the zones of high reflectivity and depolarization, but also found a number of new areas (bringing the total to 20) and was even able to survey the poles. It has been postulated that surface ice may be responsible for these high luminosity levels, as the silicate rocks that compose most of the surface of Mercury have exactly the opposite effect on luminosity.

In spite of its proximity to the Sun, Mercury may have surface ice, since temperatures near the poles are constantly below freezing point: On the polar plains, the temperature does not rise above -106°C. And craters at Mercury's higher latitudes (discovered by radar surveys from Earth as well) may be deep enough to shield the ice from direct sunlight. Inside the craters, where there is no solar light, temperatures fall to -171°C.

Despite sublimation into the vacuum of space, the temperature in the permanently shadowed region is so low that this sublimation is slow enough to potentially preserve deposited ice for billions of years.

At the South Pole, the location of a large zone of high reflectivity coincides with the location of the Chao Meng-Fu crater, and other small craters containing reflective areas have also been identified. At the North Pole, a number of craters smaller than Chao-Meng Fu have these reflective properties.

The strength of the radar reflections seen on Mercury are small compared to that which would occur with pure ice. This may be due to powder deposition that does not cover the surface of the crater completely or other causes, e.g. a thin overlying surface layer. However, the evidence for ice on Mercury is not definitive. The anomalous reflective properties could also be due to the existence of deposits of metallic sulfates or other materials with high reflectance.