The Meaning Ofa Moon

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For almost fifty years, finding out anything at all about Pluto was devilishly difficult, but finding out that it had a moon took just two days.

That's how much time astronomer James Christy set aside at the U.S. Naval Observatory in Washington to look through photographic plates showing Pluto's position among the stars at various times. The plates had been made at the Naval Observatory's astronomical facility in Flagstaff, not far from the Lowell Observatory.

The pictures of Pluto on some of the plates seemed to be oddly distorted. Perhaps the plates were defective. Maybe the Naval Observatory's 61-inch telescope had failed to track the stars properly, or maybe it was just that the "seeing" wasn't good on those particular nights. Christy had developed an expert eye in the course of analyzing tens of thousands of plates for the observatory's extensive survey of double stars, and so he brought out his microscope to double-check these troublesome Pluto plates.1

Sure enough, Christy saw that ten of the pictures of Pluto were slightly elongated. But then he noticed something else: The images of the surrounding stars were perfect points. The misshapen views of Pluto were revealing something real about the planet. Moreover, when Christy compared the plates from different times, that "something" seemed to be moving from one side of Pluto to the other.

At first Christy wondered whether the elongation could be a mountain sticking up from the surface, but no mountain could be that tall. Could it be the eruption of a volcano into space? He dropped that idea as well: An eruption that big, from a planet that small, couldn't possibly last a month. Then Christy seized on the right answer. "What?" he thought to himself. " Pluto has a moon? "

The next morning, he dug into the files and checked images of Pluto going back to 1965. The pictures, and the calculations made by Christy and his colleagues, confirmed that the elongation rounded the planet every 6.387 days. The best explanation for the observations was a moon that was gravitationally locked in orbit with Pluto.

Astronomer James Christy discovered Pluto's largest moon, Charon, in 1978 by comparing these fuzzy images. Christy noticed that some images were elongated, like the one at left, while others were not. After close examination, Christy concluded that the elongations were caused by the presence of a previously unknown moon orbiting Pluto.

Astronomer James Christy discovered Pluto's largest moon, Charon, in 1978 by comparing these fuzzy images. Christy noticed that some images were elongated, like the one at left, while others were not. After close examination, Christy concluded that the elongations were caused by the presence of a previously unknown moon orbiting Pluto.

It took years longer to make all the observations required to confirm what Christy saw that first day, but in the end, Pluto was recognized as having something that two bigger planets in the solar system—Mercury and Venus—did not: an orbital companion.

Why did it take so long to figure out Pluto had a moon? One reason was that the photographic exposure settings had to be just right to bring out the elongation in Pluto's image. Pluto was gradually coming closer to Earth, and thus the images taken in the 1970s were sharper than the images from the 1930s or 1950s. But the biggest factor behind the find was Christy's ability to see a discovery where others just saw defects.

Once the moon's existence was confirmed to the International Astronomical Union's satisfaction, Christy was given the honor of naming it. At first he told his wife, Charlene, that he'd name it after her: It would be called "Char-on," like a proton or neutron. Then he realized that the name had to follow the Roman or Greek god pattern in order to conform to the IAU's rules. He picked up a dictionary, flipped to the Ch- section, and started looking for mythological names. There, to his amazement, he found a reference to the ferryman of Greek lore whose boat carried the dead across the River Styx to Pluto's dark realm: Charon!

The story sounds too good to be true, but in any case, Christy found a way to keep his wife as well as the IAU happy. Classical scholars may pronounce the mythological ferryman's name as "Care-on," but in accordance with Christy's wishes, most astronomers today call Pluto's moon "Shar-on."2

Every once in a while, a puzzle fan experiences a dazzling moment when the addition of one key jigsaw piece, or one crossword entry, opens up a cascade of opportunities for solving the puzzle. That was the kind of moment that astronomers experienced once they learned about Pluto's moon.

Having two data points was the key to working out the mass and the motions of two worlds that could just barely be seen as more than a single point. "Within a week of Charon's discovery there were roughly a dozen major conclusions made concerning the true nature of Pluto," Christy recalled.3

The angle of the elongations in the pictures of Pluto told Christy and his colleagues that the moon was tracing a nearly north-to-south orbit—a cockeyed circuit that had been seen at only one other celestial location: Uranus, the planet found by William Herschel almost two centuries earlier.

Meanwhile, the length of the elongations revealed how far away Charon was in its orbit. Assuming that Pluto was much more massive than Charon, astronomers could figure out Pluto's mass, based on equations that factored in the time it took Charon to circle Pluto (6.387 days) as well as the distance between them (12,000 miles).

The answer was shockingly small: Pluto was only 0.2 percent as massive as Earth, or about one-sixth as massive as Earth's moon. If Lowell's ghost still haunted anyone wondering whether Pluto could affect the big planets' orbits, Charon exorcised it.

To make the maximum use of their observations, astronomers employed computing power far more advanced than the four human "calculators" Lowell had hired to search for Planet X. But finding the best ways to mine the data required scientists who were extraordinarily savvy—or extraordinarily lucky. Swedish-born astronomer Leif Andersson was both.

Based on the rough readings of Charon's orbit, Andersson divined that Pluto and Charon should go through a series of mutual occultations, during which they would repeatedly pass in front of each other. By analyzing how the light dipped and flared over time—the detailed light curve for the two paired worlds—astronomers could compare their sizes and their compositions.

Pluto makes a close approach to the sun only once in the course of its 248-year orbit, and the season for seeing Pluto-Charon occultations comes only once every 124 years. As luck would have it, the prime-time season was just about to begin, although astronomers didn't know the orbits quite well enough to determine exactly when.

So the world's astronomers kept watch for the telltale pattern of dimming and brightening that would signal the start of the cosmic pas de deux. At last, in 1985, the dance began. In the beginning, each icy world cast a shadow just slightly grazing the edge of its orbital partner. As the dance continued, each eclipse covered more of each disk: Charon darkening Pluto, then Pluto darkening Charon. The shadows reached their deepest when Charon's orbit took it directly over Pluto's disk. After the climax, the orbital angle widened and the shadows ebbed.

The shadow dance took six years to go from start to finish, and during all that time, the light curve served as a cosmic CAT scan, tracing the outlines of the planet and its moon.

Pluto and Charon turned out to be the most evenly matched planet and moon known in the solar system: Charon was one-seventh as massive as Pluto, and a little more than half its diameter. If you were standing on Pluto, Charon would loom above you, looking seven times wider than Earth's moon as seen from our home planet.

Like our own moon, Charon always turns the same face toward Pluto. But unlike our moon, Charon holds a fixed place in the Plutonian heavens. Viewed from one half of Pluto, Charon would stand still while the sun and other stars whirl dimly beyond. Viewed from the other half, the moon would be perpetually missing from the skies.

The occultations were good for much more than orbital mechanics. By analyzing the changing spectrum of the light from Pluto and Charon, scientists could improve upon Lampland's observation back in 1930 that Pluto's hue was "yellowish." They could even tease out the differences in the elemental composition of the two worlds. The spectral data showed that Charon's surface was dominated by grayish water ice rather than the yellowish methane ice seen on Pluto.

One of the most amazing computational feats involved analyzing light-curve data going all the way back to 1954, and matching those readings against computer models for the shadings of Pluto and Charon. Two teams of researchers came up with "maps" of Pluto based on the models that fit the data best, and although there were some differences, there were amazing similarities as well: Both maps showed a bright, methane-rich south polar cap, and a dirty, dark region around Pluto's midriff.

Those bright spots of methane frost supported the idea that Pluto had an atmosphere that came and went with the planet's seasons: The frozen methane would break down over time, chemically changed by ultraviolet radiation from the far-off sun. Astronomers even saw evidence that some of the frost was darkening, which gave Pluto its yellowish-brownish hues. If there was fresh frost, the likeliest explanation was that it froze out of the atmosphere. Putting all the evidence together, scientists concluded that Pluto's atmospheric methane turned to frost during the winter, and then rose back into the air during the summer.4

Knowing the sizes and the masses of Pluto and Charon gave scientists an opening to calculate their density and guess at what kind of stuff might lie beneath the surface. Pluto's density was somewhere around 2 grams per cubic centimeter— midway between rock and water ice. That meant Pluto was no mere ball of ice, but more likely a mixture of 30 percent ice and 65 percent light minerals by mass, with traces of heavier minerals at its core. Charon's density was less than Pluto's, at about 1.2 grams per cubic centimeter. That implied that the moon was significantly icier.

Thanks to the discovery of Charon, more and more of Pluto's puzzle pieces were being fitted into place. Astronomers no longer saw Pluto as just a single point of light, but as an actual world spinning on its poles in the oddest way, with an actual terrain that could be mapped in light and dark, possessing an atmosphere that froze and thawed with the seasons, traveling in tandem with a moon hanging in the black sky.

Pluto was no longer alone on the solar system's edge. It had a partner. But that drew attention to a huge gap in the jigsaw puzzle: How did Charon get there? And were there still more icy worlds out there, waiting to be discovered?

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