A NASA spacecraft has detected what appear to be changing seasons on Mercury as well as much more iron on the surface of the small, rocky planet than previously thought.

The Messenger probe made the observations during its third flyby of Mercury 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 228 kilometers 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,” composed 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.

Messenger observed three types of atoms in the exosphere — sodium, calcium and magnesium — during three flybys. According to scientists, the probe detected much less sodium during the third flyby than during the second.

“While this is dramatic, it|isn’t totally unexpected,” said mission scientist Ronald Vervack of the Johns Hopkins University’s Applied Physics Laboratory. 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 than sodium, showing that different atoms “are going to have their own unique seasonal variations,” Vervack said.

Studying these seasonal differences will help scientists understand how Mercury’s surface has changed over time.

Mercury’s atmosphere is “the end product of a few billion years of these processes; they never stop,” Vervack said.