In my previous post about Pluto I included three maps, not images, released in 2010 and laboriously made by a team led by Marc Buie of the Southwest Research Institute, Colorado. Fortuitously, the latest New Horizons image also shows three views of Pluto. How do the 2010 maps compare to the 2015 images?
The maps and images show Pluto with a similar tilt, north pole towards us, of about 30 degrees. However, the central longitudes are different making a direct comparison difficult. But the following graphic shows the full set of 2010 maps.
And now we can compare them.
Let’s first look at the left-hand 2015 image, at 133-degrees. Comparing it to the 2010 map at 120-degrees we see clear matches between the dark feature along the equator (at the bottom of the disc), the bright “square” feature on the right and the dark region surrounding the “square”.
Now the middle 2015 image at 63-degrees. We compare this to the 60-degree 2010 map. The correspondence in features is less obvious. The 2015 image shows the dark equatorial region continues, but its form does not appear to closely resemble the 2010 map data. The 2015 image shows a subtle dark region across the middle of the disc. In the 2010 map data this region appears more pronounced and connects to the equatorial dark region. Maybe this region has frosted over in the intervening years? It was already clear in 2010 that some features of Pluto were changing with time. The clearest correlation is a dark region ‘chord’ on the upper-right of the 2015 image that also appears in the 2010 map data.
Finally, the right-hand 2015 image at 19-degrees. We can compare these to the 0 and 30-degree 2010 maps. Here the correspondence between features is least convincing. There may be a match between the intriguing equatorial dark spots in the 2015 images and the large dark circular region on the 2010 maps. The 2015 image shows a ‘donut’ and a bright spot in the lower half, and these may correspond to one or two bright spots in the 2015 maps.
In conclusion, the new images show features that do appear to correspond to features revealed five years ago in the 2010 maps. Yet, as we expected, Pluto’s surface is more complex and interesting than the old maps suggested.
It is worth noting that the data for the 2010 maps was collected by the Hubble Space telescope in Earth orbit – a distance of around 5.7 billion km with Pluto appearing about 0.1 arc-second in diameter. New Horizons was only 14-million km, and Pluto appeared about 1 arc-minute across (60x larger), when the 2015 images were taken. The remarkable correspondence between the two set of data gives us confidence in the image processing techniques used to make the 2010 maps. I am sure Buie’s team feels the years of effort & computer time they put in to make the 2010 maps was all worth it, and are well pleased with their results.
UPDATE, Jul 09 2015
Shorty after I posted the above images a new map of Pluto was released. I have combined the New Horizons image/map with two black & white HST maps from 1994 and 2002-3 so they are easy to compare. I have cut and rearanged the New Horizons map to match the odd longitude scale (beginning at 270 degrees) of the HST maps. Again there are similarities but also changes happening over timescale of a few years.