In case you're getting lost, this is where we've been trying to get to all along. We took a picture of Pluto to see if it is still the same or different than when we looked in 1994. We need to start with a review of the 1994 results.
The 1994 map of Pluto.
In 1994, Dr. Alan Stern (Southwest Research Institute), Dr. Laurence Trafton (University of Texas, Austin), and myself teamed up to create a global map of the surface of Pluto. We took a total of 12 images at 4 distinct longitudes in visible light and 8 images in the ultraviolet. Our results were announced in March 1996 and you may have seen some news report on the maps. If you're interested, you can check out the actual news release we made. To recap, here's the 1994 map of Pluto's surface:
Comparison to the new map.
Take a moment to look at these two maps. Are you confused yet? At my first look I was really confused too. At first glance, these two maps don't really look all that much alike, do they? Did we just discover something changing on Pluto? If you haven't been reading all my journals, you might want to review how I attack problems like this. We are looking for change, however, this is an enormous difference and I just don't believe it.
Map comparison as images.
Before we get too far, let's look at the data in a different way. These images show the maps wrapped onto a "globe" at the orientation as when the new image was taken.
On the left, is the 1994 map image and on the right is the 1996 map image. This still looks confusing, maybe even more confusing. The 1996 image looks really dark. Why? Well, to display image, I usually set the display to show the brightest area as bright white. In the 1996 the brightest area is a very small region pointed to on the lower left part of the image.
Now, here's where my judgement comes into play. The process of pulling out the "perfect" image of Pluto works best in the center of Pluto. Near the edge of the planet, the extraction is not as accurate. So I want to adjust the map to make those really bright areas near the south pole a little darker so they aren't as prominent. Here's what it looks like along with the 1994 image:
Here's the comparison of the new "modified" map against the 1994 map.
I think this looks much better. The change I made was relatively small but it let's us see the other areas of the map much more clearly. Here's where it gets interesting. The basic appearance of the maps is similar. Both maps show a cluster of three bright regions near the center. Just left of center both maps show a dark region. In fact, the similarites are pretty strong. However, there are differences. The north pole is darker in the new map, the south pole is brighter. The change in the south is quite strong, especially considering that I made two small areas darker in the previous step.
The question now is: Has Pluto changed? Or, is this process of getting maps from HST images not entirely repeatable? These questions can be addressed by examining how we got these images and by looking at past work on Pluto.
At the north pole, there are quite a few previous maps that predict its brightness. Half of them say the north pole is dark and half of them say the north pole is bright. All the work was well done but the answer still eludes us. The lack of agreement between previous work means we shouldn't get too excited about a lack of agreement in the HST maps.
The south polar area is more interesting. I'm still not entirely convinced that these differences are due to a real change on Pluto. However, this new map does makes me wonder. This is an area where I'd expect to see change. If we repeat this experiment and see it continue to brighten then I'd say we are finally seeing a change in the surface. If a repeat shows a dark area again, then I'd say that we have the same problem in the south as everyone has had with the north pole.
Marc W. Buie, Southwest Research Institute
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