The first surface photos of Pluto, the only solar system planet never visited by spacecraft, show it has icy polar caps, just like Earth, with clusters of dark spots and bright features.
Pluto, a frozen world now 29 times farther from the sun than is the Earth, resembles a soccer ball in the fuzzy pictures taken by the Hubble Space Telescope and released on Thursday by NASA.
Bright patches of white are probably caused by nitrogen that has frozen in extreme temperatures and snowed onto the surface, said astronomer Alan Stern of the Southwest Research Institute in Boulder, Colo.
“We can see from the images that Pluto is the most variegated, contrasty object in the outer solar system,” said Stern. The images reinforce the belief by astronomers that Pluto is one of the strangest of the nine planets orbiting the sun, he said.
“Pluto never fails to surprise,” said Bruce Margon, an astronomy professor at the University of Washington.
“It is like an icy little dwarf on the outskirts of the solar system,” said Anne L. Kinney of the Space Telescope Institute in Baltimore.
Discovered just 66 years ago, Pluto was the last of the solar system planets to be identified. Only in 1978 was it found that Pluto has a moon, Charon, and an atmosphere was discovered 10 years later.
Pluto is so strange that some astronomers have argued that it isn’t really a planet, but some other object, such as an asteroid.
“It is a planet,” said Stern. “It is round, it has a satellite (a moon), it has an atmosphere. There’s only a minority view that it is not a planet.”
Pluto is turned on its side, in relation to its orbit. This the result, astronomers believe, of a gigantic collision with a comet or asteroid that caused the small planet to tilt.
The planet is about two-thirds the size of the Earth’s moon and is so far from the sun that sunlight on its surface is about as intense as moonlight is on Earth. Temperatures on Pluto drop to about minus 380 degrees Fahrenheit when the planet is at its farthest from the sun. Pluto made its closest approach to the sun in 1989 and temperatures rose to a balmy level of minus-350.
Stern said the slight thaw has allowed some nitrogen and other gases that are normally frozen to evaporate into the atmosphere. He said that over the next few years Pluto will again cool and the gases will “snow” back on the surface, perhaps covering some the dark spots seen in the Hubble photos.
Margon said astronomers, using Hubble images, should be able to monitor these dramatic changes as they occur.
Most astronomers believe the dark spots of Pluto are frozen hydrocarbons originating from the methane gas. But Stern said it is impossible from the images to tell if Pluto has mountains and valleys like those on the Earth and Mars.