Pluto’s thin air may be falling to the ground.

The total mass of the atmosphere on Pluto appears to have fallen by half in just two years, scientists working on NASA’s New Horizons mission reported during a news conference on Friday.

“That’s pretty astonishing, at least to an atmospheric scientist,” said Michael Summers, a professor of planetary science and astronomy at George Mason University in Virginia and a member of the New Horizons science team. “That’s telling you something is happening.”

Pluto reached its closest approach to the sun in 1989, and the expectation had been that as it moved farther away along its elliptical orbit, temperatures would drop and its atmosphere, mostly nitrogen, would begin to freeze and eventually disappear.

That was a driving motivation for the rush to send New Horizons to Pluto. “We wanted to get there while there was still an atmosphere to study,” said S. Alan Stern, the principal investigator.