BioEd Online

Pluto has nitrogen glaciers flowing down from its distinctive, icy heart. And the dwarf planet's thin atmosphere may have begun to freeze out and drift as snow 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 these findings, and a raft of new images, at a press conference on 24 July, just ten days after their spacecraft flew by Pluto.

During that historic journey, the New Horizons craft measured Pluto's surface pressure for the first time. Scientists were able to use that reading to estimate the mass of the dwarf planet's atmosphere — and what they found puzzled them.

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

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 argued against the idea that the nitrogen-dominated atmosphere would freeze out and condense on Pluto's surface as it moved farther from the Sun.

Strange behaviour

Summers cautions that the latest observation is just one data point. It comes from a one-off experiment in which radio antennas in NASA's Deep Space Network beamed a powerful signal from Earth to the spacecraft as it passed behind Pluto. By measuring how much those radio waves bent along that journey, New Horizons was able to measure the surface pressure of Pluto's atmosphere.

That pressure reading is about half of what ground-based astronomers saw in 2013, when they also measured Pluto's atmosphere. The New Horizons team isn't sure why the atmosphere apparently started to freeze out around the time that their spacecraft buzzed by the dwarf planet.

“It would be an amazing coincidence,” says the mission's principal investigator, Alan Stern of the Southwest Research Institute in Boulder, Colorado. “But there are some on our team who would say, I told you so.”

Another recent experiment could yield some answers. On 29 June, Pluto passed in front of a star — at least to viewers on Earth — and teams of astronomers observed that 'occultation', which can reveal the density of Pluto's atmosphere. They are currently crunching through the data to see whether it agrees with the New Horizons suggestion of a lightweight atmosphere.

“I'm sceptical,” says Michael Skrutskie, an astronomer at the University of Virginia in Charlottesville who is helping lead the analysis. Among other things, he notes, the occultation measurements study the atmosphere tens of kilometres above Pluto's surface, whereas the New Horizons radio measurement is done right at its surface. “It's kind of apples and oranges,” he says.

Outlook: hazy

The New Horizons team has also spotted layers of haze in Pluto's remaining atmosphere. These bands extend up to 160 kilometres above the dwarf planet's surface — roughly five times higher than scientists had predicted, Summers says.

Such haze could persist even if much of Pluto's atmosphere has already dissipated, because the particles that make up the fog are tiny and easily lofted.

That atmosphere comprises gases created when ices sublimate off of Pluto's surface — such as the nitrogen, methane and carbon monoxide ices that New Horizons has spotted in the heart-shaped feature called Tombaugh Regio.

Nearby, close-up images of the edges of fractured plains called Sputnik Planum reveal the nitrogen glaciers, which reach into a more rugged, cratered terrain to the north. At Pluto's frigid temperatures — about -235 °C, 38 degrees above absolute zero — water ice is too brittle to flow. But nitrogen ice can, if its roughly a kilometre thick and heated by radioactive decay leaking out from Pluto's interior, says William McKinnon, a team member and a planetary scientist at Washington University in St Louis, Missouri.

Phoning home

So far, the New Horizons spacecraft has sent back about 5% of the data that it has collected. On 31 July, it will move into a new mode where it begins spinning, so that engineers can turn off many of the navigation systems and use that electrical power instead to turn on a second antenna. Using both antennas to radio the information back to Earth will speed up the data download, says Kimberly Ennico-Smith, a planetary scientist at NASA's Ames Research Center in Moffett Field, California, and deputy project scientist for the mission.

No new images will be sent back until mid-September, although images already on Earth will continue to be released publicly. Between now and September, New Horizons will focus on transmitting datasets from other instruments such as those that study the solar wind and space environment.

“The one-shot nature of it has been very exciting,” says Carly Howett, a planetary scientist at the Southwest Research Institute. “And just a little bit scary.”