Even the Best Images are Fuzzy

The images taken by Hubble Space Telescope and from the best telescopes on Earth show how Pluto and Charon are mere specks in the sky - and we see very little detail of their surfaces. To illustrate how "fuzzy" our view is of Pluto, below is a comparison with an image of Moon degraded to the same resolution.

But astronomers were lucky that a quirk of nature - mutual eclipses of Pluto and Charon -provided an opportunity to make better maps of Pluto. They were clever enough to seize the rare opportunity.


Mutual Eclipses of Pluto and Charon



Pluto is tipped on its side with its spin axis close to the plane of its orbit. Like most satellites, Charon orbits in Pluto's equator. From 1985 through 1990, Pluto's equator and Charon's orbit plane were aligned with the line-of-sight from Earth, with Charon passing in front of and behind Pluto every 6.4 days. This means that from 1985 through 1990 Pluto and Charon eclipsed each other every Pluto day.

These eclipses turned out to be very important, since observations of the eclipses (called "mutual events") led to the first accurate determination of Pluto's and Charon's sizes, and subsequently to surprising maps of the reflectance of the surface. While Charon moved in front of and then behind Pluto every 6.4 days, the location of Charon - and Charon's shadow - varied slowly over the 1985-1990 eclipse season.

Astronomers stared at Pluto as Charon moved in front and carefully measured the dimming of sunlight reflected by the combined objects due to Charon blocking some of Pluto's surface. At the beginning and end of the eclipse season Charon only blocked out a very little of the light and there was little change in the "light curve." Around the peak of the eclipses, in 1987, the reflected sunlight dimmed by more than 30%.

Actually, the view from Earth is more complicated because Earth's motion around the Sun makes Charon's orbit around Pluto appear to "wobble." This graphic shows the changing geometry of the mutual eclipses and illustrates this subtle effect of the Earth's motion.

Turning Light Curves into Maps

Several maps of Pluto's albedo (surface reflectivity) have been made based on the mutual event light curves. By keeping careful track of the decrease in brightness observed when part of Pluto is covered by Charon, we can solve for the brightness of the covered part. Eventually we can piece together a mosaic of Pluto's Charon-facing hemisphere.

This map shows a bright south polar region, which we now know to be frost made mostly of nitrogen, with some methane. The dark features have not been identified, although they may be colored by methane's photochemical byproducts.