Strange Spots on Pluto May be Tar and Frost
Images of the dwarf planet Pluto taken by NASA's Hubble Space Telescope.
Credit: NASA, ESA, and M. Buie (Southwest Research Institute)

When scientists got an unprecedented up-close view of Pluto from the Hubble telescope recently, they found mysterious bright and dark spots mottling the dwarf planet's surface. Now researchers think they have a better guess at what's causing those weird spots.

The Hubble images, released in February, revealed Pluto as a molasses-colored world on the fringe of the solar system with surprising variations in brightness across its surface. Based on closer analysis, scientists say the darker spots may represent parts of the ground covered in a tar of primordial organic compounds.

"We know there's methane on Pluto," said dwarf planet expert Mike Brown of Caltech. "Here's what we think happens: Sunlight hits the methane and breaks it apart into its chemical components ??hydrocarbons. Over millions of years this process makes a dark reddish-brown oil or tar-like substance that sticks to the ground. These darker areas spread larger as they absorb more sunlight and cause additional frost to sublimate."

The bright spots, in turn, are thought to be related to areas covered in carbon monoxide frost.

These recent views of Pluto reveal a different picture from what astronomers observed in past images, partly because the dwarf planet's appearance is changing with the seasons. But seasons are extremely long on Pluto. The reason: It takes the world 248 Earth-years to make a full trip around the sun.

"Until the mid-1980s, Pluto's northern hemisphere was tilted away from the sun for over 100 years, accumulating a substantial amount of frost," said study leader Marc Buie of the Southwest Research Institute. "Now the northern hemisphere is coming into sunlight and appears, as shown in the Hubble images, to have been growing brighter."

Right now, Pluto is a relatively balmy minus 385 degrees Fahrenheit (minus 232 degrees Celsius), following its closest approach to the sun in the late 1980s. But Pluto is in for some colder times in the future.

And when temperatures get frigid enough, scientists think the gas in Pluto's wispy atmosphere will actually freeze and fall to the ground.

"Now, Pluto is headed away from the sun again," says Brown. "It will gradually get colder and colder and its atmosphere will refreeze to its surface. In fact, that should have already started happening, but apparently it has not. It's a mystery."

If Earth ever got cold enough for its atmosphere to freeze, it would create a layer 30 feet (9 meters) thick. Luckily, our planet is a tropical paradise compared to Pluto. And Pluto's atmosphere is so thin, when it freezes it will make only a frosty film of nitrogen and methane.

The new Hubble views of Pluto are just the tip of the iceberg for scientists studying the frigid world. NASA's New Horizons spacecraft is currently speeding toward Pluto on a decade-long trek across the solar system.

NASA launched New Horizons in January 2006. It zoomed past the moon in 10 hours and swung around Jupiter a year later, making it NASA's fastest spacecraft ever sent to another planet.

New Horizons is hitting a series of midpoint milestones on its trip to Pluto this year. The probe is due to fly by Pluto and its three moons ? Charon, Nix and Hydra ? in July 2015.