MONTHS AFTER PLUTO FLY-BY, DATA STILL YIELDING SURPRISES
On Earth, the only ice is frozen water. On Pluto, nitrogen, methane and carbon monoxide also freeze solid.
The most striking feature that NASA’s New Horizons spacecraft saw when it flew past Pluto in July was a heart-shaped region now named Tombaugh Regio after Clyde Tombaugh, the discoverer of Pluto.
The left half is covered by mostly nitrogen snow; the right side is more methane ice.
Eight months since NASA’s New Horizons spacecraft had its quick, close-up look at Pluto, scientists are reaping the scientific rewards from a bounty of data the spacecraft collected. Mission scientists reported their findings in five articles published Thursday in the journal Science. Among the interesting things they’ve learned:
Not a planet, but not boring
From earlier observations from Earth and the Hubble Space Telescope, scientists knew Pluto was blotchy. Still, they wouldn’t have been surprised if the landscape turned out to be geologically bland.
That’s because the sun, some 4.8 billion kilometres away, provides little energy, and Pluto is so small, smaller than the Earth’s moon, that its interior could have cooled down long ago.
“You’d expect to see a boring cratered ball,” said William M. Grundy of Lowell Observatory in Arizona, who leads the team analyzing the composition of Pluto’s surface.
Others expected Pluto to look somewhat like Triton, a Pluto-size moon captured into orbit around Neptune.
Instead, New Horizons photographed a dazzling variety of landscapes, from soaring mountains to flat plains. Pluto is proving to be far more diverse and quite different from Triton.
“The big surprise is that Pluto turned out so surprising,” said Jeffrey M. Moore of NASA’s Ames Research Center in California, who heads the mission’s geophysics and imaging team.
An ice volcano?
Nitrogen might also flow deep enough to be warmed by the interior and then erupt back at the surface — producing what scientists are surmising might be an ice volcano. They are studying a mountain named Wright Mons that rises three kilometres, spans 145 kilometres across and has a hole at the center.
“It’s not like any feature we’ve seen anywhere else in the solar system,” said John R. Spencer, a planetary scientist at the Southwest Research Institute in Boulder, Colorado.
The varying mix of ices could form different alloys with very different properties, similar to how adding carbon transforms iron into steel, and that could help explain the wide range of topography.
“That’s the new physics that needs to be learned,” Grundy said.
A big, fractured moon
The flyby spotted an enormous gash in Pluto’s largest moon, Charon, that differs from Pluto in the makeup of its surface.
Charon appears to made of just water ice without the other ices seen on Pluto. That matched expectations, because Charon, with less gravity, would have not have been able to hold on to methane, nitrogen and carbon monoxide.
The most striking feature on Charon is the 966-kilometre long gash, longer than the Grand Canyon. Harold A. Weaver Jr., the mission’s project scientist, said the gash was probably formed early in Charon’s history when the surface cracked and material from the still-warm interior oozed out.
Surprise in Pluto’s atmosphere
Another finding indicates that the upper atmosphere of Pluto is much colder, meaning that nitrogen escapes at a rate of about a hundredth of what had been expected.
Frances Bagenal of the University of Colorado, head of the team that performed that analysis, said the calculation runs counter to what mission scientists were saying a week before the flyby, when New Horizons had already detected nitrogen escaping from Pluto.
“We were being fooled by something else,” Bagenal said.
Next up for spacecraft
New Horizons is now headed toward a New Year’s encounter with a smaller body in the outer solar system called 2014 MU69, providing more data about a neighborhood that has been little studied.
So Pluto has now been left behind, and the chances of scientists getting another close-up look in the next few decades are slim.