The New Face of Pluto

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.

Four years of processing by 20 computers have turned a set of Hubble Space Telescope images of Pluto, each only a few pixels wide, into this map of its complex surface. Numbers show the longitude of the central meridian. Click on the animation to see three enlarged frames with a scale bar and coordinate grids.
NASA / ESA / Mark Buie
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.

The surface features of Pluto altered dramatically between its Hubble portrait sessions in 1994 and in 2002-03. Most of the changes are probably due to the migration of nitrogen frost between its northern and southern hemispheres.
NASA / ESA / Mark Buie
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.

13 thoughts on “The New Face of Pluto

  1. Wally

    Yep, pretty much a no-brainer. As the planet moves outwards from its perihelion, “weather” will happen as the tenuous atmosphere cools down.

    Nice to see the composite animation of this in full color. Good work!

  2. Glenn Reish

    I beleave Pluto is a planet ( “Marc Buie” Distant World ).

    Pluto orbits our sun. Pluto has a moon Sheron.

    I would like to have Pluto re-entered as our 9th planet.

    Yours truly,
    Glenn A. Reish, Jr.

  3. Yale W.

    Pluto has been warming recently, due to a delay in temperature rise after passing within the orbit of Neptune. It cannot be the sun, because that would leave the Earth boiling even to heat Pluto 1C. This is probably why the frost has been melting and changing.

  4. Anthony B

    Anyone notice that one side of Pluto? There’s a bright spot, surrounded by a huge ring around the edge of the image. I wonder if that’s a giant impact crater? It’s probably just illusion, but it is interesting! Just a thought.

  5. Laurel Kornfeld

    Pluto is still a planet. Only four percent of the IAU voted on the controversial demotion, and most are not planetary scientists. Their decision was immediately opposed in a formal petition by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto.

    Pluto is a planet because it is spherical, meaning it is large enough to be pulled into a round shape by its own gravity–a state known as hydrostatic equilibrium and characteristic of planets, not of shapeless asteroids held together by chemical bonds. These reasons are why many astronomers, lay people, and educators are either ignoring the demotion entirely or working to get it overturned. Using this broader definition gives our solar system 13 planets and counting: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris.

  6. Fred from Laurel, Md

    First, this is a marvelous teasing of real science out of a thimbleful of data. It gives us a bit of a sneak peak behind the veil of distance that conceals so much about Pluto, before the arrival of New Horizons, which will give us lots more detail, but we’ll have to wait 5+ years to get it. So the Hubble observations mesh well with the New Horizons mission.

    Second, this whole Pluto is/is not a planet thing is premature. I suppose with a term like ‘planet,’ which has been in use since dirt was young, there’s an understandable feeling that we need to have a precise definition for it. Well just maybe WE’re too young, in the sense that we still have only begun to explore the greater solar system. As the article by David Jewitt in the March issue of S&T points out, an Earth-sized planet would escape the reach of current instruments at or beyond 600 a.u., as would a Neptune-sized planet at or beyond 1200 a.u. Who knows what we’ll find out there when we get the capability? Any of a zillion possible discoveries could completely gut any definition we attempt now, maybe even calling for a new term to include what we now call planets, along with newly-discovered objects. Perhaps we just need to keep the term, ‘planet,’ informal for now. I think that, while it’s important to bat around some ideas about it, still, it’s important to recognize our current lack of knowledge (an essential for science!), and that the attempt to define it at this stage is a fool’s errand.

  7. Jim in the unstarlit city

    This is a fascinating article, provoking interesting discussion and anticipation for New Horizons images in a few years. For those seeking some relaxation from the Pluto planet/nonplanet debate, check out Clare and the Reasons lamenting Pluto’s demotion:
    http://www.youtube.com/watch?v=ptgSD2ilzEo
    A good song, by a good group, on a good topic. I saw them sing this in November. Support them when they come to your neighborhood.

  8. carsten

    Pluto is a dwarf what? Say the word – PLANET! In plain english Pluto is still a planet, albeit a dwarf one. No one disputes the fact that it is small. Honestly,I really don’t understand what the fuss is all about!

  9. Rick Blair

    Although the axis of rotation and mean orbital plane of Pluto are known, I don’t think any meridian lines have been set. The recent data showing the changing brightness and colors near the surface makes it impossible to have a stable landmark on which to base a prime meridian (0 degrees). New Horizons will give us that and gloriously so much more!

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