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Planetary News: Our Solar System (2010)

Hubble Photos Reveal "Dramatic" Surface Changes on Pluto

New maps of Pluto's surface generated from images gathered by the Hubble Space Telescope in 2002 and 2003 have revealed "dramatic" changes when compared to similar maps from 1994 data. "It's a surprise that we've seen this much change, this fast," astronomer Marc Buie said in a press conference held today to discuss the new maps. He demonstrated that Pluto's northern hemisphere has brightened, while its south pole has darkened. At the same time, Pluto's surface has changed color, becoming redder, while Charon's color did not change; and some dark and bright spots observed in the 1994 data have moved or changed in appearance, while other spots have remained the same. These new maps have already proved useful in planning New Horizons' 2015 flyby of Pluto and its moons.

Making the Maps

The new maps were composed of 384 separate Hubble Advanced Camera for Surveys (ACS) photos of Pluto and its moons captured through green and blue filters over 12 different Hubble orbits in 2002 and 2003. As powerful as Hubble is, Pluto is a very faint and distant object, so each individual image of Pluto revealed little about the patterns of brightness and darkness across its surface.

Click to enlarge > Hubble FOC and ACS views of Pluto There have been two cameras onboard HST that can resolve Pluto. The left column of images was taken in 1994 using Hubble's Faint Object Camera (FOC), which actually resolved features on Pluto's surface. In 2002 and 2003, 384 more images were taken using the High Resolution Camera (HRC) on the Advanced Camera for Surveys (ACS). The three columns on the right show 12 views obtained with ACS/HRC, each on a different Hubble orbit. The numbers indicate the longitude of Pluto that was being imaged at the time. Neither the FOC nor the ACS/HRC are functioning on Hubble anymore. Credit: NASA / ESA / M. Buie (SwRI)

The Faint Object Camera data from 1994 were detailed enough that it only took a matter of minutes to generate maps from the four observations. Buie describes the process briefly on his website:

First, you start with a guess for Pluto's map. From this map you project it onto a sphere for the hemisphere of Pluto in each image. Next, you distort the image in the same way that the camera optics distorts the scene and then finally stuff the picture down into the large blocky pixels. When you do this you can then compare your guess against the images. Of course, your first guess won't match the data very well so the trick is to tweak the map, a little darker here, a little brighter there, and try again. You do this over and over until your map creates synthetic images that match the real images.

With only four images to work with, the process proceeded rapidly for the FOC data. But the ACS images lacked the necessary detail. So, instead of taking only one pair of images per orbit, as was done with the 1994 data, he actually took 16 pairs of images, shifting the telescope's point of view very slightly from photo to photo, resulting in a data set consisting of 384 different images. Another issue is that the individual pixels in the ACS instrument are not square; they are not even rectangular. They are skewed. With 50 times as many images to match, and having to calculate the shape of parallelipiped rather than square pixels, generating the new maps was far more computationally intensive than the 1994 maps: "It took about four years running on 20 computers simultaneously and continuously to get these results," Buie said.

The results, when they finally came, were astonishing: the patterns on Pluto's surface had changed markedly in the six years separating the two sets of observations, a period that covered only a tiny fraction of Pluto's year.

Click to enlarge > Comparison between 1994 and 2002-3 maps of Pluto This animation blinks back and forth between two maps of Pluto's surface derived from Hubble Space Telescope observations. One map was generated from four images captured in 1994 using Hubble's Faint Object Camera, while the other was generated from 192 images captured in 2002 and 2003 using Hubble's Advanced Camera for Surveys. During the time that separated the two sets of observations, Pluto's surface changed noticeably. Also, the season advanced, bringing more of the south pole into winter darkness. Credit: NASA / ESA / M. Buie (SwRI) / animation by Emily Lakdawalla

The Most Changeable Surface in the Solar System

The significance of the changes was put into perspective by astronomer Mike Brown, who was not involved in generating the maps: "If you look around the entire solar system, the only things that change their surfaces by any really noticeable amount are the Earth and Mars, where ice caps come and go. That's it.  And then there's Pluto, which has even more dramatic changes. You're looking at the surface in the solar system which has the biggest changes of anything we've ever seen."

The origin of the changes is thought to be seasonal. Spring is advancing in Pluto's northern hemisphere, and as it arrives, volatile ices (in particular, of nitrogen frost) are likely evaporating in the northern hemisphere, to be deposited again in the dark (hence, invisible) winter pole. However, why this evaporation should cause the northern hemisphere to brighten is a bit uncertain. Buie speculated that it may be a transient effect, that the nitrogen may be sublimating in such a way as to create an intricate "fairy castle" texture to Pluto's northern hemisphere surface, which would appear brighter not because it was intrinsically a brighter material but instead because of the way its change structure made it reflect light toward Earth differently.

One spot on the map -- a particularly bright spot located near the equator at about 180 degrees longitude -- was observed to remain fairly constant between the two sets of observations. "This one is unique," Buie said. "We know from spectroscopic studies that at that particular longitude, we see the unmistakable strong signature of carbon monoxide frost. At other longitudes, we don't see that.  There's this nice synergy between imaging data and ground based data. There's got to be a correlation between the two.  It's the brightest spot on the surface and it's singularly rich in carbon monoxide frost, and that's the spot on Pluto we're going to get a really good look at with New Horizons."

Another peculiarity revealed in the new map is that "Pluto is now significantly redder" than it has been in the past. In fact, Buie said, Pluto has had a fairly constant color for 50 years of astronomers' observations, up through the year 2000; the new red color (by which astronomers mean a change in the relative amounts of longer-wavelength rather than shorter-wavelength light reflected by a surface) appeared between 2000 and 2002, an unbelievably short period for such a distant, dark body. Buie still can't quite wrap his mind around this change: "This business about the color change, that has had me scared for a while. I got that result years and years ago.  But it's just so hard to unerstand and believe that I've been checking everything I can think of. I am still nervous about it, I have to admit.  It could be that I've completely screwed this up and got it wrong. But I can't find it.  Charon is on the same images, and Charon's got the same color throughout, but Pluto changed."

If Buie got the result so long ago, why is it only being released now? Part of the delay was Buie's caution about the color change. But it was also because of the discovery, in 2005, of Pluto's second and third moons, Nix and Hydra. "Our [2002 and 2003] images also had data on those two satellites," Buie said, and he dropped his work on the Pluto color maps to focus on deriving precise orbits for Nix and Hydra. "Once I got that taken care of and out of the way, it was time to come back."

Brown, who provided contextual commentary on today's panel, is famous (or, in some circles, infamous) for being the astronomer whose discovery of Eris and other large bodies in the Kuiper belt was the trigger for the "demotion" of Pluto from "planet" to "dwarf planet" status. He emphasized how we can now understand Pluto better because of the context provided by observations of these other worlds: "There are two dwarf planets, Makemake and Eris, that have very similar surfaces to Pluto.  For a long time Pluto was this lonely oddball; now we have Pluto that's close, and Makemake that's a little more distant, and Eris way out, so understanding these all as a new class of objects is fun. Pluto is a fascinating world and it really doesn't care what we call it. It's a great place to study and we'll be learning a lot more about it in the years to come."

Unfortunately, Hubble data won't be able to provide any more maps of the sort that were released today. The Faint Object Camera, used to create the 1994 map, was removed in 2002 and replaced with the Advanced Camera for Surveys (ACS). ACS suffered a series of failures in 2006 and 2007, and though the asteronauts on the Hubble Servicing Mission managed partial restoration of the instrument, the High Resolution Camera remains inoperable. Buie does plan five months of observations of the Pluto system this year using Hubble's newly installed Wide Field Camera 3, but that instrument does not have the spatial resolution of ACS, so it will be impossible to generate maps from the images.

Still, the best is yet to come, Buie said. "These maps have already been used to plan the New Horizons encounter. These maps and these data tell us how long exposures should be, where we should point the spacecraft, things like that." New Horizons will view the Pluto system from January to July 2015, but its closest approach will only produce highest-resolution images covering the hemisphere that is sunlit when it passes by; these maps have helped the team select which hemisphere to place in view during the encounter. The maps produced from New Horizons' data will, of course, be vastly superior in resolution to the ones Buie presented today. But Buie's maps will provide irreplacable historical context for the New Horizons observations, helping scientists to understand how such a distant world with such a slow orbit can have such a dynamic and changeable surface.