INTERLUDE 13-1Surface Detail on Planet Pluto
Pluto is the only planet for which we have very little detailed surface information, because Pluto is the only planet to date that hasn't been visited by a robot spacecraft. Now new models of Pluto's surface have been constructed, based mainly on images taken by the Hubble Space Telescope. The first figure shows two samples of a dozen snapshots of Pluto, taken at visible and ultraviolet wavelengths throughout the planet's 6.4-day rotation period. The raw images are the small inserts at the upper left of each frame. The clearer ball of Pluto shown in each of these frames is a modeled view, that is, a computer-generated sketch of the planet's surface based on modeling of all available data.

The second figure is a complete surface map of Pluto, created by carefully collating and analyzing all the data. It is good confirmation of rougher maps made over the previous decade using ground-based observations of eclipses of Pluto by its moon, Charon. Clearly, we have now progressed beyond the stage of seeing Pluto merely as a fuzzy, distant

dot of light. With maps like these, astronomers can monitor its surface detail about as well as you can study nearby Mars with a small backyard telescope.

These models reveal quite a lot of large-scale contrast–in fact, surprisingly for a body composed almost entirely of ice, more contrast than on any other planet except Earth. About a dozen "provinces" can be identified, including icy-bright polar cap regions, another bright spot seen rotating with the planet itself, a cluster of dark spots, and a few peculiar linear markings. Some of these spots could turn out to be craters or impact basins, such as on Earth's Moon, but much of Pluto's contrast is probably caused by frosts and snows that migrate across the planet as it goes through its seasonal cycles. The reflectivities of the lighter areas resemble that of fresh snow, whereas the darker areas are more akin to the subdued brightness of dirty snow. Let's just hope that people don't start talking about the odd linear markings as being reminiscent of canals!