Pluto’s atmosphere may be changing before our eyes. Measurements with NASA’s New Horizons spacecraft have revealed that Pluto’s atmosphere has an unexpectedly low surface pressure compared to that derived from previous observations. One explanation for the low pressure is that about half of Pluto’s atmosphere may have recently frozen onto the planet’s surface. If confirmed, it could indicate that further decreases in pressure may soon be in store.

The pressure measurement is the first ever obtained for the surface of Pluto. It was made by REX, the spacecraft’s radio experiment, about one hour after New Horizons’ closest approach to Pluto on July 14, 2015.  In a carefully-planned observation that had never before been attempted, two radio dishes on Earth — part of NASA’s Deep Space Network — beamed radio waves precisely timed to reach Pluto just as New Horizons passed behind the dwarf planet.

The radio waves traveled through Pluto’s atmosphere en route to the spacecraft and were bent, or refracted, by the atmospheric gases. The amount of bending — which appears as a shift in the frequency of the radio waves — revealed that the gas pressure at Pluto’s surface was only 1/100-thousandth that of the pressure on the surface of Earth. That’s about half the amount calculated from previous Earth-based observations.  

“For the first time, we have ground truth, measuring the surface pressure at Pluto, giving us an invaluable perspective on conditions at the surface of the planet,” said New Horizons researcher Ivan Linscott of Stanford University. “This crucial measurement may be telling us that Pluto is undergoing long-anticipated global change.”

New Horizons is expected to transmit a wider variety of REX measurements of Pluto’s atmospheric pressure in the next few weeks.