November 29, 2012
Water Ice Deposits And Organic Material Found On Mercury
Lee Rannals for redOrbit.com — Your Universe OnlineUCLA planetary scientists crafted the first accurate thermal model of the solar system's innermost planet, pinpointing the extremely cold regions where ice has been found on or below the surface.
Researchers wrote in the journal Science that the newly discovered black deposits are a thin crust of residual organic material brought to the planet over the past several million years through impacts by water-rich asteroids and comets.
The research helps shed light on the long-standing issue of ice on Mercury. Several independent lines of evidence reveal that the planet has extensive water ice deposits at its poles.
Scientists were surprised to find in the early 1990s that areas near Mercury's poles were unusually bright when observed with radar from Earth.
"Mercury is the innermost planet in the solar system, and, arguably, it's among the least explored," David Paige, a professor of Earth and space sciences at UCLA, said in a statement. "The surface of Mercury exhibits the most extreme range of temperatures of any body we know of in the solar system."
Paige said these "natural freezers" exist within the shadowed areas of polar-crater rims, which never experience direct sunlight due to the low angle of the sun at high latitudes.
The scientists used the first detailed topographic map of Mercury's north polar region produced by MESSENGER to generate an accurate thermal model of the probe. They found that the planet's sub-surface temperatures are a near-perfect match to Earth-based radar observations and surface-brightness measurements made by the Mercury Laser Altimeter (MLA) instrument onboard the orbiting spacecraft.
Where the team's temperature model predicts water ice should be stable on the surface, the MLA always measures unusually bright particles, indicating surface ice deposits.
"This stuff we find covering the ice is darker than the rest of Mercury, which is already a really dark planet. That's amazing," Paige said in a statement. "At the very least, it means there is something out of the ordinary going on inside these permanently shadowed areas where the ice has accumulated."
The dark substance likely arrived on Mercury as part of the comets and asteroids that periodically crash into the planet, Paige said. The only place water and organics survive is within permanently shadowed craters.
"There are areas on the surface where it is too hot for ice to exist, but radar data from Earth show something bright reflecting from these areas, so we're pretty sure that there's water ice buried underneath," co-author Matthew Siegler, a researcher at the Jet Propulsion Laboratory and a UCLA alumnus, said in a statement. "You need some kind of insulating layer to keep that heat from getting down to the ice."
For the water ice and black organic layers to remain exposed on Mercury's ancient surface, the deposits must have formed recently in the planet's geological history.
"Billions of years ago, the Earth acquired a layer of water and other volatile material that formed atmospheres, oceans and even the first organic molecules that started life," Paige said. "Understanding the origin of that material is a very important problem and is essential to finding out about the potential habitability of planetary systems around other stars."