A New Peek at Pluto
In images taken between 2000 and 2002, Pluto became significantly “redder,” a term astronomers use to indicate that an object reflects more red than blue light.
Scientists think the color changes are due to the shift in seasons as Pluto makes its 248-year journey around the Sun. Pluto’s seasons last a very long time, but the dramatic change in color took place over just two years.
“It’s a little bit of a surprise we’d see this big of a change this fast,” said principal investigator Marc Buie of the Southwest Research Institute in Boulder, Colo.
Ultraviolet light from the distant Sun causes the surface ice – which is composed of nitrogen, carbon monoxide and methane -- to turn into a gas. Scientists think some of this gas migrates to the other side of Pluto and re-freezes into ice. The dark surface colors of Pluto are thought to come from carbon-rich residue that results from this sublimation process.
“‘Redness’ is an interesting common thread throughout the outer solar system,” said Buie. “We think it’s related to carbon. Hydrogen gets chipped off methane molecules, and you’re left with something more carbon-rich than what you’d started with.” Buie said that eventually, this process should turn Pluto completely black.
In actual color, Buie said that Pluto is yellowish orange– not quite as red as the surface of Mars, but more red than the surface of Jupiter’s moon Io. Pluto is not uniform in color, and has patches of white and black terrain. Pluto’s “bright spot”, an area rich in carbon monoxide frost, can be seen in the Hubble images.
It’s difficult to get a good look at Pluto because it is so small and far away. The Hubble images taken by the Advanced Camera for Surveys are a few pixels wide, and only surface variations a few hundred miles across can be seen. Smaller surface features like impact craters aren’t visible.
“The view is comparable to what we see of our Moon with the naked eye,” said Buie.
Buie developed special algorithms and had 20 computers operating continuously and simultaneously for four years on the Hubble images. The new images of Pluto actually were completed several years ago, but Buie said he needed to make sure they were accurate, and that the dramatic 20 percent color change wasn’t an error.
“This business about the color change between 2000 and 2002 had me scared,” he said. “I’m still nervous about it. It could be that I completely screwed this up and got the wrong answer.” He noted, however, that the moon Charon appears in the same images, and its color does not change.
The surface changes on Pluto are due to extreme temperature variations between seasons, said Mike Brown, professor of planetary astronomy at Caltech. While dynamic changes are often seen in the atmospheres of gas planets like Jupiter and Saturn, solid bodies tend to be more static in appearance. “If you look around the entire solar system, the only things that change by that amount are Earth and Mars,” he said.
The Hubble results appear in the March 2010 issue of the Astronomical Journal.
The Fairy Castles of Pluto
“This circulation pattern of moving material from one pole to the next has to have a profound effect on what the surface looks like,” said Buie. He explained that as ice turns into a gas, the texture of the surface material could be turned into “fairy castle structures that may eventually collapse under their own weight.” These structures may be the cause of the brighter appearance of the north.
The atmosphere of Pluto is tenuous, about a million times thinner than the atmosphere of Earth. However, ground-based observations of Pluto taken in 1988 and 2002 show that Pluto’s atmosphere has doubled over that time, possibly due to the amount of ice turning into a gas as Pluto makes its closest approach to the Sun in its orbit. Pluto is the only place in the solar system where we can study an extreme atmosphere that periodically freezes out, said Brown.
The temperatures on Pluto range from 43 Kelvin (minus 382 Fahrenheit) in the coldest regions, to possibly as high as 60 Kelvin (minus 352 F) in the warmest. Brown noted that the temperatures could dip even lower -- we have never observed Pluto at its farthest point from the Sun. It was last at that point long before Clyde Tombaugh discovered the dwarf planet in 1930, and Pluto won’t reach that most-distant point in its orbit again until the year 2113.
Studying Pluto can help us understand how the outer solar system works, said Brown. “The Kuiper Belt objects have a wide variety of orbits, but tend to be characterized by eccentric orbits,” he said. “It’s a big jumble out there.” Brown noted that Makemake and Eris, two other recently discovered bodies in the Kuiper Belt, seem to have surfaces similar to Pluto. Makemake is slightly smaller than Pluto and a little more distant from the Sun, while Eris is slightly bigger and three times the distance of Pluto.
The Hubble images will remain our sharpest view of Pluto until NASA's New Horizons probe is within six months of its Pluto flyby in 2015. New Horizons will pass by Pluto so quickly that only one hemisphere will be photographed in the highest possible detail. With no chance for re-exposures, accurate models for the surface of Pluto are essential in preventing pictures that are either under- or overexposed.
"The Hubble images will help New Horizons scientists better calculate the exposure time for each Pluto snapshot, which is important for taking the most detailed pictures possible," said Buie. He plans to use Hubble's new Wide Field Camera 3 to make more Pluto observations prior to the arrival of New Horizons.