Fitting, isn't it, that the most dramatic views ever made of distant Pluto should be made public on the 104th birthday of its discoverer, the late Clyde Tombaugh?
Today Marc Buie, a devoted Plutophile at the Southwest Research Institute, unveiled maps of Pluto's surface that he derived from Hubble Space Telescope images taken over a 13-month stretch in 2002 and 2003. When compared to a comparable set of Hubble snapshots from 1994, it's clear that the icy surface of this distant world has gone through some big changes.
The new views reveal a surface with three distinct coatings: nearly white areas that are likely a mixture of methane and nitrogen frosts, and terrains that look dark orange and charcoal black, which probably contain complex organic compounds created by constant exposure to radiation.
Most remarkable, Buie says, is that these color changes have taken place rapidly, over just a few years. "That had me scared for a while," he admits. "It's so hard to believe." But because Pluto's moon Charon appears in the same Hubble frames and showed no color shift, Buie finally convinced himself that the alterations were real.
One curiosity is a a large, bright spot straddling the equator, at a longitude of roughly 180°, that has not changed much. Ground-based spectra show this side of Pluto to be capped with a frosting of carbon-monoxide ice. Buie note that evidence for the spot's existence goes back to the 1950s, and it'll be well placed for close-up scrutiny when the New Horizons spacecraft gets there in mid-2015.
In one sense, planetary scientists have been anticipating changes on Pluto. Pluto passed through perihelion in 1989, and it's been gradually chilling out ever sense. It's also in the midst of a gradual seasonal shift that is exposing parts of its northern hemisphere to sunlight for the first time in more than a century, a consequence of this little world's steep axial tilt and 248-year-long orbit.
As the thinking goes, the weak sunlight should cause nitrogen frost to sublimate into gaseous wisps where its warming up north, making that hemisphere darker, and freezing out in the fading sunlight down south, making it brighter (but hidden by shadow). "These changes have to be a consequence of nitrogen ice moving around," says Buie.
Yet few expected such dramatic changes on a place where the temperatures vary little, from –382°F (43 K) in the darkest recesses to about –360°F (55 K) in the dim glow of full sunlight. One who had a hunch is Caltech astronomer Michael Brown, who in 2002 predicted that Pluto might undergo brightness and color shifts much like those now revealed. Yet even he finds the rapidity of Pluto's frosty face-lift surprising. "You're looking at the surface in the solar system which has the biggest changes of anything we've ever seen."
So why are images taken so many years ago only now making their debut, you might ask? Buie explains that the reconstructions were incredibly challenging because even the optical prowess of Hubble's Advanced Camera for Surveys only barely resolved Pluto's disk. It then took four years of crunching by a bank of 20 computers to extract the subtle detail, and Buie admits getting sidetracked by the discovery of two more Plutonian moons, Nix and Hydra, in 2005.
The resulting maps are probably the best views we will have of distant Pluto until the arrival of New Horizons. That's because the ACS's high-resolution camera, used in the 2002-03 campaign, no longer works, and Hubble's new Wide-Field Camera 3 lacks the resolution to pick out any details.