NANTES, France—The shape of Comet Hartley 2 has inspired a number of colorful descriptors—it's been called a pickle, a peanut, a dog bone and a bowling pin.
But none of those shape analogues shares what may be a defining characteristic of Hartley 2—its two lobes now appear to be made of different material. The end pieces of the comet may have formed separately before merging to create the odd-shaped object, researchers ventured here this week at a joint meeting of the American Astronomical Society Division for Planetary Sciences and the European Planetary Science Congress. [Read more news from the meeting here.]
The analysis comes from data gathered by NASA's Deep Impact/EPOXI spacecraft, which buzzed Comet Hartley 2 in November 2010. At that time the comet was near Earth and about as close to the sun as it gets on its elliptical orbit, which carries it out past the orbit of Jupiter. During its flyby, EPOXI photographed volatile compounds such as water and carbon dioxide sublimating from ice to vapor and streaming off the sunlit, smaller end of the comet (the top of the bowling pin, if you will). But 8.5 hours later, when the Hartley 2 had rotated so that its larger end aimed sunward, the comet's activity did not shift accordingly. "Where the larger lobe is pointed toward the sun, we do not see the carbon dioxide turned on or much water outflow," said mission scientist Lori Feaga of the University of Maryland, College Park. "The outgassing is definitely coming from the small lobe and not from the larger lobe."
The implication is that the smaller end of the comet is rich in volatiles, which escape when the comet draws near the sun, whereas the larger end is relatively volatile-poor, at least near the surface. "We have a very heterogeneous nucleus," Feaga said. "The lobes are different."
One explanation is that the two ends developed independently and then joined in a sort of protoplanetary merger. "I think you get two different compositions in two different parts by forming gradually," said Michael A'Hearn, also of U.M. and principal investigator for Deep Impact's extended EPOXI mission. (The spacecraft's original mission, to slam an impactor into another comet, known as Tempel 1, was completed in 2005.) "Bringing the two ends together gently, almost like a contact binary [star], gives you a natural way," he added.
Such a merger is "perfectly possible," noted Paul Weissman, a senior research scientist at the NASA Jet Propulsion Laboratory in Pasadena, Calif., during a question-and-answer period following Feaga's presentation. But the details are not straightforward. After the talk, Weissman mused that getting two different compositions out of the comet-spawning Kuiper Belt might be tricky, because the belt is expected to have been fairly homogenous. (It is thought that most short-period comets originated in this broad orbital band, located beyond the orbit of Neptune.) "How you'd get what looks like a volatile-rich object linked up to a volatile-poor object is a bit of a puzzle," he said.
Shapely considerations aside, Hartley 2 seems to be unique in at least one other way, Hal Weaver of the Johns Hopkins University Applied Physics Laboratory reported at the meeting. Weaver and his colleagues used a spectrograph on the Hubble Space Telescope to measure the comet's carbon monoxide (CO) abundance. Carbon monoxide varies widely from comet to comet, but Hartley 2's reserves are puny. The Hubble data revealed "the lowest CO abundance of any comet measured to date," Weaver said.
One explanation could be that Hartley 2's frequent passes through the inner solar system—it completes an orbit which brings it close to the sun every 6.4 years—have stripped it of highly volatile carbon monoxide. But the comet retains other volatiles, such as water and carbon dioxide, and Weaver points out that the comet became an inner solar system regular only following an 1875 encounter with Jupiter, so it has not had much time to shed its volatiles. And some comets on much longer orbits still display low abundances of carbon monoxide, although their encounters with the sun are relatively rare.
"I favor the primordial effect that this particular comet formed from material…depleted in carbon monoxide," Weaver said. Perhaps Hartley 2 formed in a part of the solar system where carbon monoxide was rare, for instance, or where it was too warm for the compound to solidify into ice.
Either way, Hartley 2's volatile activity today may reflect the conditions that brought it into being billions of years ago, just as its asymmetric release of gases points to an ancient merger. It seems planetary scientists have much to learn from the odd-shaped comet—whatever colorful descriptor they prefer for it.