Scientists who got an unprecedented look at a disintegrating comet were surprised to find the comet's chemistry was consistent throughout — in contrast to popular ideas that a comet's outside is chemically different than its inside.
A team led by Neil Dello Russo of The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., studied the two largest fragments from comet 73P/Schwassmann—Wachmann 3, which had split into nearly 70 pieces by the time it passed within about 7 million miles of Earth in spring 2006. Using ground-based telescopes equipped with powerful spectrometers to measure the composition of the gases released by the comet, the researchers discovered a remarkable chemical similarity between the two largest remaining pieces of the comet's nucleus. The surface of the larger fragment was once buried hundreds of meters below the surface of the original nucleus and was revealed only when SW3 broke apart. The smaller fragment studied broke off from the original nucleus and is comprised at least partly from material near the surface of the original nucleus.
"This was a unique observation; you very rarely get the opportunity to measure the chemistry in the deep interior of a comet and in two distinct fragments from the same parent comet," says Dello Russo, whose team publishes its findings in the July 12 issue of the journal Nature. "There's an expectation that comets have different compositions at different layers, as their exteriors are exposed to heat, cosmic rays and other processes that can alter their compositions during their lifetimes. But that's not what we found for this particular comet — and it's going to make us ask new questions about how comets formed and evolved and the nature of the early solar system."
Deep-freeze relics of the early solar system, cometary nuclei are porous and fragile mixes of dust and ices. From the homogeneous composition and the amounts of water ice and simple chemicals such as methanol, ammonia, formaldehyde, hydrogen cyanide, acetylene, ethane, and carbon monoxide in Schwassmann—Wachmann 3's largest fragments, Dello Russo's team realized it was seeing primordial materials, unchanged from when the comet formed billions of years ago in the distant Kuiper Belt region, beyond the orbit of Neptune.
"This is the best evidence we've seen that we were actually sampling pristine materials from the early solar system, when this comet formed, and not processed layers that had been altered over time," Dello Russo says.
Schwassmann—Wachmann 3, discovered in 1930 by German astronomers Arnold Schwassmann and Arno Arthur Wachmann, approaches Earth every 5.4 years. In 1995 astronomers discovered that its nucleus had split into several large pieces, and since then, amateur and professional astronomers around the world have been tracking the comet's disintegration. APL scientist Hal Weaver led a team that observed SW3 with the Hubble Space Telescope in April 2006; those images (available online at http://www.jhuapl.edu/newscenter/pressreleases/2006/060427.asp) revealed a spectacular destruction process in which fragments broke into smaller chunks and zipped down the comet's long tail, or vaporized altogether.
Dello Russo's team — which includes Weaver, Ronald Vervack and Carey Lisse of APL and Nicholas Biver, Dominique Bockelée-Morvan and Jacques Crovisier of the Observatoire de Meudon-Paris, France - made its observations in May 2006 at the NASA Infrared Telescope Facility and Keck II telescope on Mauna Kea, Hawaii, with funding from NASA's Planetary Atmospheres and Planetary Astronomy programs.