Trick Allows Scrutiny of Pluto's Moon
From left to right is the before, during and after of the occultation of C313.2 by Charon on July 11. The brightest object in the center of the frame is Pluto. Below and to the left is the merged light from Charon and the background star. During the occultation (center image), only Charon is visible. The images are from the 6.5-meter Clay telescope at Las Campanas Observatory in Chile.
Credit: MIT and Williams College

Near midnight of July 11, several telescopes in Chile caught a rare and wonderful sight:  the shadow of Pluto's moon, Charon, as it passed in front of -- or occulted -- a distant star.

The observations, now being analyzed, may pin down the size of the moon and whether or not it has an atmosphere.  Preliminary indications from one group seem to suggest little or no gaseous envelope.

Charon blocked the light of the relatively faint star C313.2, casting a shadow that was roughly the same size as the moon itself -- around 630 miles wide.  A previous occultation by Charon of a different background star was observed in 1980, but only one telescope -- with limited precision -- managed to observe that event. 

To have eight major telescopes and three separate astronomy teams recording this most recent alignment is considered very fortuitous -- especially since a year ago no one knew that this shadowy event would happen at all.

"Originally, we thought this would be an occultation by Pluto," said James Elliot of MIT.  "But last August, we said, 'Wait a minute, this looks like Charon.'" 

Proven technique

Elliot was one of the astronomers who discovered the rings of Uranus using an occultation by the planet in 1977.  He and his colleagues have a catalogue of potential occultations for several outer solar system bodies, but knowing exactly where the shadow will pass over on the Earth requires continuous refinements. 

"A few days before, the predictions were still bouncing around several 100 kilometers north and south," said Jay Pasachoff of Williams College.  "It was a considerable relief that the actual path went over our telescopes." 

A similar sort of 11th hour scrambling occurred during an occultation by Pluto in 2002.  The reason for the uncertainty in these events is the fact that the position and velocity of celestial bodies are not perfectly known, and that really matters when you are waiting for them to perfectly align.

The shadows, of course, are not visible to the naked eye. But powerful telescopes can record the starlight, note its absence, and also record dimming if it passes through a distant object's atmosphere. The technique can also reveal the constituants of an atmosphere if there is one.

"It's amazing that people in our group could get in the right place at the right time to line up a tiny body 4 billion miles away," Pasachoff said.  "It's quite a reward for so many people who worked so hard to arrange and integrate the equipment and to get the observations."

Sizing up

Charon was discovered in 1978, 48 years after its planet Pluto.  Recent measurements by the Hubble Space Telescope have determined that Charon has about 10 percent the mass of Pluto, making it the largest moon -- relative to its planet -- in our solar system.   

From the 1980 occultation, the radius of Charon was estimated to be greater than 370 miles (600 kilometers).  But in the mid-1980's, the orbit of Charon was such that -- as seen from Earth -- the planet and moon passed in front of each other.  During these mutual eclipses, astronomers measured a Charon radius of 360 miles (580 kilometers). 

This discrepancy in size could be because the moon is not round.  Having several telescopes watching the occultation from different vantage points should help sort out the shape of Charon, Pasachoff told SPACE.com.

Even if the moon turns out to be spherical, a more precise determination of the radius is crucial for saying how dense Charon is and, therefore, what it is likely made of.

"At this point the uncertainty in the density is coming from the radius," said Leslie Young of the Southwest Research Institute.  "If we know the density, we can say whether it is half rock and half ice, or two-thirds rock and one-third ice." 

Pluto -- which has a density twice that of water -- is believed to be about 70 percent rock and 30 percent ice.  Comparing this to the make-up of Charon may give scientists a clue as to how the moon was formed. 

One of the more popular theories for Charon's origin is that it was created after something smashed into Pluto -- similar to the collision that likely created Earth's Moon. 

Gassing out

During an occultation, the atmosphere around a planet or moon will scatter the starlight at the split second transition between light and shadow.  If there is no atmosphere around the foreground object, then observers will see an abrupt snuffing out of the background star's light. 

Two occultations by Pluto in 2002 revealed that its atmosphere -- which is about 10,000 times thinner than Earth's -- was warmer than expected.  The highly elliptical orbit of Pluto causes the temperature of its atmosphere to oscillate between --274 and --391 degrees Fahrenheit, as the planet's distance to the Sun varies between 30 AU and nearly 50 AU. 

An AU is the Earth-Sun distance of 93 million miles. 

On the surface of Charon, which is just as cold as Pluto, astronomers have detected frozen water.  But this ice will be "as hard as steel," Elliot said, and therefore, one should not expect any water vapor around the moon. 

In contrast, other ices, like carbon monoxide and nitrogen, could have escaped -- or sublimated -- off the surface.  But the planet's gravity is probably not strong enough to hold onto these gases for very long. 

"In our data there is no indication for an atmosphere," Young said. "This tells us that ices on the surface must have sublimated and escaped the system."

Elliot and Pasachoff, who are part of a separate observing group, are not yet willing to say whether the data show an atmosphere or not.  According to Elliot, an atmosphere on Charon could be made of heavier gases, like argon, or it could have been recently refreshed by out gassing from the surface. 

On moon watch

The Charon shadow moved from east-northeast to west-southwest, passing over most of South America.  Because the light rays from a distant star travel along nearly parallel lines, the shadow was about the same size as Charon.

A total of eight telescopes reported seeing the occultation for about 30 to 40 seconds.  All of these sites were in Chile, where the weather was apparently perfect for viewing.  Attempts to see the event in Brazil were foiled by clouds. 

Elliot and Pasachoff were part of a group that detected the occultation with the Clay Telescope, the DuPont Telescope, the telescope of Cerro Armazones Observatory, and the giant 8-meter Gemini South project on Cerro Pachon.

Young led another group that obtained data from the 4-meter and 0.9-meter telescopes at the Cerro Tololo Inter-American Observatory, as well as from the Southern Astrophysical Research telescope.  A third group led by Bruno Sicardy saw the event with the European Southern Observatory's Very Large Telescope.

In the future, there should be several more opportunities for occultations, as Pluto and Charon will be passing in front of the heart of the galaxy where the majority of stars are located.

"We are going to be busy in the next decade," Young said.

One instrument that should be available soon is SOFIA -- a 747 aircraft with an infrared camera on board.  Because it is mobile, SOFIA will be able to go where the shadow is -- as long as it is "politically feasible," remarked Elliot.  It will also be able to fly above weather, so astronomers won't have to worry about clouds.