The Face of Pluto--First Look



Hubble imaged nearly the entire surface of Pluto, as it rotated through its 6.4-day period, in late June and early July 1994. These images, which were made in blue light, show an unexpectedly complex object. The two smaller inset pictures at the top are actual images from Hubble. North is up. Each square pixel is more than 100 miles across. At this resolution, Hubble discerns roughly 12 major regions of bright or dark surface. The larger images are from a global map constructed through computer image processing performed on the Hubble data. The tile pattern is an artifact of the image enhancement technique. Opposite hemispheres are seen in these two views.

Credit: Alan Stern (Southwest Research Institute), Marc Buie (Lowell Observatory), NASA,
and ESA

Images from the Hubble Space telescope have given us the first-ever glimpse of Pluto's surface. Taken with the European Space Agency's Faint Object Camera, the images reveal almost a dozen distinctive albedo features, none of which have been seen before. They include a ragged northern polar cap bisected by a dark strip, a bright spot that appears to rotate with the planet, a cluster of dark spots, and a bright linear marking that is intriguing to the scientific team analyzing the images. The images confirm the presence of icy-bright polar cap features, which had been inferred from indirect evidence for surface markings in the 1980s.

"Hubble is providing the first, tantalizing glimpse of what Pluto will be like when we get there," said Alan Stern of Southwest Research Institute's Boulder, Colorado research office. Stern led the imaging team that includes Marc Buie, Lowell Observatory, and Laurence Trafton, University of Texas at Austin. They used the Faint Object Camera to obtain over a dozen high-quality visible and ultraviolet images of Pluto as the planet rotated through a 6.4-day period in mid 1994. These images have now been reduced and analyzed.

"These results and the maps we constructed from them are much better than I ever hoped for," said Buie. "It's fantastic. Hubble has brought Pluto from a fuzzy, distant dot of light to a world which we can begin to map and watch for surface changes. Hubble's view of tiny, distant Pluto is reminiscent of looking at Mars through a small telescope," said Stern.

Some of the sharp variations across Pluto's surface might be caused by topographic features such as basins and relatively fresh impact craters (as on Earth's Moon). However, most of the surface features are likely produced by the complex distribution of frosts that migrate across Pluto's surface with its orbital and seasonal cycles. "The light areas are as bright as fresh Colorado snow, and the darker areas are more reminiscent of the brightness of a dirty snow," said Stern. The darkest regions are probably hydrocarbon residues created by the interaction of ultraviolet sunlight and cosmic rays with the chemically varied surface ices.

Despite its small size and immense distance from the Sun, Pluto experiences these dramatic seasonal changes because of its highly elliptical orbit, which carries it as close as 2.8 billion miles to the Sun (inside Neptune's orbit) and as far as 4.6 billion miles. As Pluto recedes from the Sun, much of its atmosphere of nitrogen, carbon monoxide, and methane freezes out onto its surface. This is thought to explain the abundance of bright ice on the surface. As Pluto warms when its orbit nears the Sun, the surface ices are sublimated into the atmosphere, thickening it and beginning the cycle again; thus the planet is apparently resurfaced with a new layer of ice each 248-year orbit.

Pluto passed its closest point to the Sun in late 1989. As a result, it presently enjoys a relatively balmy surface temperature near -350 degrees F in the dark areas and a cooler -380 degrees F in the bright areas. This difference may create large pressure differences at the surface, resulting in high winds in the thin atmosphere. For astronomers it's a rare and ideal time for viewing Pluto and studying these changes. The last time Pluto was this close to the Sun and the Earth, George Washington was a boy!

The Hubble images suggest much more surface variety on Pluto than on its so-called twin, Neptune's large moon Triton. According to team member Trafton, ". . . the HST images are confirming Pluto's individuality. It isn't a twin of Triton after all."

Pluto is two-thirds the size of Earth's Moon and 1200 times farther away with an apparent size in the sky of 0.1 arcseconds. Viewing such a remote and small body has been so difficult that Pluto's moon Charon wasn't detected until 1978, despite the fact that Pluto itself was discovered by Clyde Tombaugh in 1930. Shortly after its launch in 1990, the Hubble Space Telescope first peered at Pluto and clearly resolved the planet and its satellite (separated by only 1/3000th of a degree) as two distinct objects. However, a detailed look at Pluto's surface had to wait until Hubble's optics were repaired during the 1993 servicing mission.

The Advanced Camera, planned to be installed on Hubble in 1999, should yield slightly better images of Pluto. This will be our best view of the distant planet until space probes eventually make the long trek across the solar system. The images will help pave the way for a proposed Pluto flyby mission early in the next century. Pluto is the only solar system planet not yet visited by a spacecraft.


This is the first image-based surface map of the solar system's most remote planet. It was assembled by computer image processing four separate images taken with the Faint Object Camera. The map, which covers 85% of the planet's surface, confirms that Pluto has a dark equatorial belt and bright polar caps, as inferred from groundbased light curves obtained during the mutual eclipses between Pluto and its satellite Charon in the late 1980s.

Image reconstruction techniques smooth out the coarse pixels in the four raw images to reveal major regions where the surface is either bright or dark. The black strip across the bottom corresponds to the region surrounding Pluto's south pole, which was pointed away from Earth when the observations were made and could not be imaged.

Photo: STScI-PRC96-09a

Credit: Alan Stern (Southwest Research Institute), Marc Buie (Lowell Observatory), NASA,
and ESA