Our solar system has glamorous planets, ringed Saturn, multicolored Jupiter, and ruddy red Mars. But one of the best recognized and popular (for the public) objects is tiny and distant Pluto, which lies on the dim outer frontier of the solar system.
Since its discovery in 1930, Pluto has been a speck of light in the largest ground-based telescopes. But NASA's Hubble Space Telescope has now mapped the dwarf planet in never-before-seen detail. The new map is so good, astronomers have even been able to detect changes on the dwarf planet's surface by comparing Hubble images taken in 1994 with the newer images taken in 2002-2003. The task is as challenging as trying to see the markings on a soccer ball 40 miles away.
Hubble's view isn't sharp enough to see craters or mountains, if they exist on the surface, but Hubble reveals a complex- looking and variegated world with white, dark-orange, and charcoal-black terrain. The overall color is believed to be a result of ultraviolet radiation from the distant Sun breaking up methane that is present on Pluto's surface, leaving behind a dark molasses-colored, carbon-rich residue.
Astronomers were very surprised to see that Pluto's brightness has changed - the northern pole is brighter and the southern hemisphere is darker and redder. Summer is approaching Pluto's north pole, and this may cause surface ices to melt and refreeze in the colder shadowed portion of the dwarf planet.
The Hubble pictures underscore that Pluto is not simply a ball of ice and rock but a dynamic world that undergoes dramatic atmospheric changes. These atmospheric changes are driven by seasonal changes that are as much propelled by the dwarf planet's 248-year elliptical orbit as its axial tilt, unlike Earth where the tilt alone drives seasons. The seasons are very asymmetric because of Pluto's elliptical orbit. Spring transitions to polar summer quickly in the northern hemisphere because Pluto is moving faster along its orbit when it is closer to the Sun.
The images, taken by the Advanced Camera for Surveys, are invaluable to planning the details of the NASA New Horizons probe mission to fly by Pluto in 2015. New Horizons will pass by Pluto so quickly that only one hemisphere will be photographed in the highest possible detail. Particularly noticeable in the Hubble image is a mysterious bright spot in one hemisphere that is unusually rich in carbon monoxide frost.
The Hubble images are a few pixels wide. But through a technique called dithering, multiple, slightly offset pictures can be combined through computer-image processing to synthesize a higher-resolution view than could be seen in a single exposure. This series of pictures took four years and 20 computers operating continuously and simultaneously to accomplish.
Credit: Ray Villard (Space Telescope Science Institute)