季节决定冥王星的颜色?

New Pluto Pictures Unveiled; Hubble's Sharpest Yet

Swaths of white, dark orange, and charcoal gray dance across the surface of Pluto in the sharpest view yet of the dwarf planet, made possible by pictures from the Hubble Space Telescope.

The new Hubble shots, taken from 2002 to 2003, show rapid changes on Pluto's surface driven by the world's extreme seasons, lead investigator Marc Buie, of the Southwest Research Institute in Boulder, Colorado, said today at a press briefing.

In a project that took four years and 20 computers running simultaneously, Buie and colleagues combined 384 Hubble pictures of Pluto to create what he calls "my best guess at a true-color appearance" for the tiny world.

"If you were sitting in a spacecraft puttering around Pluto and looking out the window, this is what you'd see, but in higher resolution."

The pictures reveal that Pluto is a much more dynamic body than commonly thought, and they offer astronomers ideas of what to focus on when the New Horizons spacecraft reaches the dwarf planet in 2015.

Pluto at Its Most Extreme

As seen in the new pictures, Pluto's orange and gray hues could be the result of methane on the surface being broken down by sunlight, leaving a carbon-rich residue, the scientists say. (Related: "Pluto Has 'Upside Down' Atmosphere.")

While the Hubble pictures aren't detailed enough to make out surface features, the striking differences in dark and bright regions suggest that Pluto has highly diversified terrain.

And by comparing the new pictures to previous images, astronomers can tell that some parts of Pluto, including the southern hemisphere, became significantly darker and redder between 2000 and 2002, while the northern hemisphere got brighter.

It's likely these changes are due to ice melting and refreezing as Pluto's seasons change, astronomer Mike Brown, of the California Institute of Technology in Pasadena, said at the briefing.

Pluto takes about 248 years to make a full orbit around the sun, traveling on an extremely elliptical path in a region of the solar system called the Kuiper belt.

Pluto's elongated orbit means that at its closest, the tiny world gets about 2.7 billion miles (4.4 billion kilometers) from the sun, while at its farthest Pluto is about 4.5 billion miles (7.3 billion kilometers).

Such extreme swings cause Pluto's surface to undergo the most dramatic changes of any known body in the solar system, Brown said.

"Places that have weather change dramatically, because it's easy to blow things around," Brown said, referring to the constant changes seen on planets with atmospheres, such as Jupiter and Saturn. "But rapid surface changes are more rare."

When Pluto goes from spring to fall, "it's as if on Earth you had a nice springtime day of 60 or 70 degrees [Fahrenheit, or 15.5 to 21 degrees Celsius] and in the fall ... it dropped to something like -90 degrees [Fahrenheit, or -67.7 degrees Celsius]," he said. "It's a ridiculously extreme place to be."

New Horizons Getting Closer to Pluto

Hubble's latest Pluto pictures were actually taken by older cameras on the recently upgraded space telescope. Following the installation of the Wide Field Camera 3 in 2009, Hubble can now be used to take even more detailed pictures of Pluto. (See some of the first pictures taken by the newly upgraded Hubble.)

But the best Pluto portraits are likely to come from New Horizons. Launched in 2006, the probe is more than halfway to Pluto and is due to be the first spacecraft to orbit the dwarf planet, offering new insights into the largely mysterious objects that exist in the Kuiper belt.

"Pluto isn't the biggest thing out there," Brown noted.

"But it is the closest ... and it is going to be the one we learn the most about to help us interpret all the other things in the outer solar system."