Here’s something to consider re the recent Pluto news: The Hubble maps of the tiny world that were released yesterday show a resolution of roughly 300 miles per pixel. When New Horizons flies by Pluto/Charon in 2015, it will send images with a resolution of 300 feet per pixel. And we’ve been reminded once again that every time we look deeper into something hitherto unexplored, we’re likely to be surprised. The surprise in this case was the significant reddening of the dwarf planet and the time frame in which it occurred, a mere two years.

I thought the liveliest part of the teleconference on Pluto yesterday was Marc Buie’s response to what had appeared in his datasets. Buie (Southwest Research Institute) was looking at imagery collected by the Hubble Space Telescope from 2002 to 2003 and comparing it with the results of earlier ground-based observations, as well as with Hubble pictures taken in 1994. The dramatic reddening seems to have occurred between 2000 and 2002, even as the illuminated northern hemisphere continued to get brighter.

Asked about his reactions to the newer Hubble imagery, Buie was candid:

“The color change in such a short period had me scared, because it’s so hard to understand. I’ve been checking absolutely everything I can think of, wondering if I screwed this up somehow and got the wrong answer. If I did, I can’t find the mistake.”

Another key point: In the Hubble imagery, the color of Charon remains the same throughout, whereas the reddening of Pluto is pronounced. Have a look at the images below, which represent the most detailed view of Pluto taken to date:

Image: 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. The center disk (180 degrees) has a mysterious bright spot that is unusually rich in carbon monoxide frost. This region will be photographed in the highest possible detail when NASA’s New Horizons probe flies by Pluto in 2015. Credit: NASA, ESA and M. Buie (SwRI).

Buie noted that the images represent his best guess at the true color appearance of the dwarf planet. The reddening seems to be caused by ultraviolet radiation from the Sun, which breaks up the methane found on Pluto’s surface and leaves behind a dark red, carbon-rich residue. The surface we’re looking at is doubtless a consequence of seasonal changes, with ice melting on the sunlit pole and refreezing on the dark, southern pole. Pluto’s elliptical orbit contributes to a relatively quick transition between spring and polar summer in the northern hemisphere. Previous observations have shown the mass of Pluto’s atmosphere doubling in the period between 1988 and 2002, evidently because of the warming and melting of nitrogen ice. Even so, this much surface change in a short period is surprising.

Kuiper belt specialist Mike Brown (Caltech) pointed out at the teleconference that the changes on Pluto are more extreme than anything previously seen in the Solar System:

“We see Pluto in these images about as well as we see the Moon with the naked eye. Now imagine the Moon changing by that much in such a short period of time. If we look around the Solar System at the surfaces we can observe, we see changes to the ice caps of Earth and Mars and that’s about it. But Pluto offers up more dramatic changes than anything else. There’s a good reason for this: The Kuiper belt features objects in extreme orbits. Right now it’s in the spring of its year, but by 2108 it will be at its furthest from the Sun. Pluto in winter will be a colder place on which things will freeze out and re-condense.”

This second image gives an idea of the brightening found in the northern hemisphere:

Image: Two Hubble photo maps of the dwarf planet Pluto, as seen in 1994 and 2002-2003. The white areas are surface frost, and the dark areas are a carbon-rich residue caused by sunlight breaking up methane that is present on Pluto’s surface. A comparison of the maps shows that Pluto’s brightness has changed between 1994 and 2003. The northern pole is brighter and the southern hemisphere is darker. Summer is approaching Pluto’s north pole, and this may cause surface ices to melt and refreeze in the colder shadowed portion of the planet. Credit: NASA, ESA and M. Buie (SwRI).

Twelve orbits of Hubble were dedicated to Pluto between 2002 and 2003, using the ACS high resolution camera to make 16 images in each of two filters, one blue, one green, for a total of 384 images. Using dithering techniques and specially developed algorithms to reconstruct a higher-resolution image, the pictures are the result of intense processing requiring twenty computers operating continuously for four years. But they won’t retain pride of place as our best photos of Pluto for long. Buie intends to use Hubble’s Wide Field Camera 3 to make additional observations before New Horizons arrives. The more the better, for the current maps are already in use for planning the brief encounter.

When New Horizons does arrive, it will only have time to photograph one hemisphere in detail. The bright spot seen in the Hubble images, known to be rich in carbon monoxide frost, is a prime target for investigation. Doubtless further surprises await.

The papers are Buie, et al., “Pluto and Charon with the Hubble Space Telescope: I. Monitoring global change and improved surface propertices from light curves,” and Buie et al., “Pluto and Charon with the Hubble Space Telescope: I. Resolving changes on Pluto’s surface and a map for Charon.” Marc Buie’s page on Pluto at SwRI has links to both. They were released on February 4, the 104th birthday of Pluto discoverer Clyde Tombaugh.