Messenger Probe Finds Seasonal Changes On Mercury

NASA’s  Messenger space probe has discovered what appear to be seasonal changes on the planet Mercury, according to the Associated Press.

The probe made the observations during its third flyby of Mercury on Sept. 29, when it took a host of measurements and images of the innermost planet’s surface and atmosphere. Only about half of the planned measurements were made because of a data glitch that affected the spacecraft during the flyby.

The $446 million probe’s third flyby brought it within 142 miles  of Mercury’s surface to cover more uncharted terrain, leaving 98 percent of the planet now mapped. The flyby was also a gravity assist meant to guide the spacecraft into orbit around the planet in 2011.

Mercury’s atmosphere is what scientists call an “exosphere,” and is made up of atoms kicked up from the surface. It is very tenuous and has a very low density, meaning atoms in the atmosphere rarely run into each other. It also has a tail that streams away from the planet in the opposite direction of the sun.

The probe looked at differences in three atoms in the exosphere — sodium, calcium and magnesium — between the probe’s three flybys. They detected much less sodium during the third flyby than they had during the second.

“While this is dramatic, it isn’t totally unexpected,” Vervack said. This is because radiation pressures from the sun change as Mercury moves through its orbit, which changes the amount of sodium liberated from the surface.

In essence, Mercury’s atmosphere experiences seasonal effects during the planet’s orbit.

Calcium and magnesium showed less variation between “seasons” than did sodium, showing that different atoms “are going to have their own unique seasonal varations,” Vervack said.

Understanding these seasonal differences will help scientists understand how surface material is lost and how the surface has changed over time.

Mercury’s atmosphere is “the end product of a few billion years of these processes, they never stop,” said mission scientist Ronald Vervack, Jr., of The Johns Hopkins University Applied Physics Laboratory (JHUAPL) in Laurel, Md.