Kandinsky crater on Mercury.
Kandinsky crater on Mercury. (Courtesy of NASA)

A spacecraft circling Mercury has captured the first photos of ice deep inside polar craters on the planet, strengthening confirmation that such pockets exist and helping to further explain how they have evolved.

Scientists working on NASA's Messenger mission, which is managed from the Johns Hopkins Applied Physics Laboratory in Laurel, say the images reveal that the ice was deposited relatively recently, possibly through some recurring process. Their research was published online Wednesday in the journal Geology.


"Those images show extensive regions with distinctive reflectance properties," lead author Nancy Chabot, an instrument scientist on the mission and a planetary scientist at the Hopkins lab, said in a statement. "A location interpreted as hosting widespread surface water ice exhibits a cratered texture indicating that the ice was emplaced more recently than any of the underlying craters."

Messenger is moving closer to Mercury as it heads toward a crash in March, enabling scientists to get an increasingly close-up view of the planet's volcanoes, craters and magnetic field. Proving the presence of ice in Mercury's craters, a long-held theory about the planet closest to the sun, was a key part of its mission.

The scientists captured images of Prokofiev, the largest crater around Mercury's north pole, and four other craters, Chesterton, Tolkien, Tryggvadottir and Kandinsky, where instruments had shown deposits of ice. Though the ice is permanently in shadow because of Mercury's orientation toward the sun, the scientists were able to use low levels of light reflecting off crater walls to get a glimpse.

In some cases, the texture of the ice suggested it had collected in existing craters. In other areas, water ice was covered by a thin layer of frozen organic material in a way that suggested the deposits were relatively young, geologically speaking.

"Understanding the age of these deposits has implications for understanding the delivery of water to all the terrestrial planets, including Earth," Chabot said.