Pluto has nitrogen glaciers flowing down from its distinctive, icy heart. And the dwarf planet's thin atmosphere may have begun to freeze out onto its surface—a change long expected, as Pluto moves farther away from the Sun, but never before seen.

Scientists with NASA's New Horizons mission unveiled the findings, and a raft of new images, at a press conference on July 24, just ten days after the spacecraft flew by Pluto.

A radio-science instrument aboard the New Horizons probe measured the surface pressure at Pluto for the first time, in what amounts to a measure of the mass of the atmosphere above it. What scientists found puzzled them.

“The mass of Pluto's atmosphere has decreased by a factor of two in two years,” said Michael Summers, a team member and planetary scientist at George Mason University in Fairfax, Virginia. “That's pretty astonishing.”

Pluto's atmosphere glows in this image taken as the dwarf planet passed between Earth and a distant star.

Credit: NASA/JHUAPL/SWRI

Measurements taken from Earth, starting in the late 1980s when Pluto was closer to the Sun, suggested that Pluto's atmosphere had actually gotten denser in the past couple of decades. That went against some theories that the nitrogen-dominated atmosphere would freeze out and condense on the surface as Pluto moved farther from the Sun.

Summers cautions that the new measurement is just one data point and still needs to be confirmed.

At the same time, he and his colleagues have spotted layers of haze in Pluto's thin atmosphere. The haze appears in bands extending up to 160 kilometres above the surface, which is roughly five times higher than scientists had predicted, Summers says.

Pluto's atmosphere is replenished by ices that sublimate off its surface. New Horizons has identified three major types of ice—nitrogen, methane, and carbon monoxide—all within the bright 'heart' feature called Tombaugh Regio.

Nearby, close-up images of the edges of fractured plains called Sputnik Planum reveal the nitrogen glaciers. At Pluto's frigid temperatures—about -235 °C, 38 degrees above absolute zero—water ice is too brittle to flow. But nitrogen can, which means the features must be made of nitrogen, says William McKinnon, a team member and a planetary scientist at Washington University in St Louis, Missouri. “To see evidence for recent geological activity is really a dream come true,” he adds.

So far, the New Horizons spacecraft has sent back about 5% of the data that it has collected.

Patterns of light and dark on Pluto's surface appear to signal flowing nitrogen ice.

Credit: NASA/JHUAPL/SWRI

This article is reproduced with permission and was first published on July 24, 2015.