Pluto: A Complex and Changing World : Big Pic

Our solar system has glamorous planets: ringed Saturn, multicolored Jupiter, and ruddy red Mars. But arguably one of the most popular worlds for the public is the tiny and distant Pluto.

And, it has much more of a kinship with the other major planets than you might imagine as revealed today in high resolution pictures from NASA's Hubble Space Telescope.

Since its discovery in 1930, Pluto has been a speck of light in the lenses of the largest ground-based telescopes. From Earth, the disk of Pluto is ten times smaller than the typical resolution limit of a ground-based telescope. In other words, apart from that speck of light, no features on the dwarf planet's surface can be seen.

But using the power of Hubble, Marc Buie of the Southwest Research Institute in Boulder Colorado has persevered to assemble the most detailed view of Pluto ever.

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Completing an intensive four years of image processing on 20 computers, Buie used Hubble images to assemble a photomap of the planet in unprecedented detail, a task as challenging as trying to see the markings on a soccer ball 40 miles away.

The Pluto photomap comparison. Top panel shows the surface features during a 1994 observing campaign and the bottom panel shows the same surface during a 2002/2003 observing campaign. Notice the changes in shading in the north and south poles (click to enlarge).

The new map is so good that 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.

Hubble's view isn't sharp enough to see impact craters that presumably pockmark Pluto's surface. But the space telescope reveals a complex-looking and variegated world with white, dark-orange and charcoal-black terrain. The mottled appearance, in terms of resolution, is comparable to our naked-eye view of the full moon. We see splotches of light and darker material on the moon, but no craters (except for the young impact features Copernicus and Tycho).

Pluto's coloring 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. This material, called "Tholin" (Greek for "mud"), is found on other icy minor bodies but not Earth.

Astronomers were very surprise to seen that Pluto's brightness has changed over a few years. The northern pole is brighter and the southern hemisphere darker and redder. Summer is approaching Pluto's north pole and this may cause surface ices to melt and refreeze in the colder shadowed southern pole of the planet.

Only two other solar system bodies go through a comparable range of visible surface changes based on the melting or sublimation of ices: Earth and Mars.

The Hubble pictures underscore that Pluto is not simply a ball of ice and rock but a dynamic world that undergoes dramatic changes. These are driven by seasonal changes that are as much propelled by the planet's 248-year elliptical orbit as its axial tilt, unlike Earth where the tilt alone drives seasons.

You would think that each season is 62 years long, except that Pluto is in a very elliptical orbit. When it is closest to the sun it is moving fastest along its orbit. Therefore, spring transitions to summer quickly in the northern hemisphere.

The Hubble images are invaluable to planning the details of the NASA New Horizons mission to flyby Pluto in 2015. New Horizons will pass by Pluto so quickly that only one hemisphere will be photographed in the highest possible detail. The target hemisphere, identified in the Hubble map, has a mysterious bright spot that is unusually rich in carbon monoxide frost. Adjoining it is a coal-black region.

"What makes you think the bright spot is interesting," quipped one NASA scientist, "maybe the black region is more interesting."

Hubble's map is the best view of Pluto we'll have until the New Horizons probe is six months from close encounter.