Smithsonian astronomers watched as the "Impactor" probe from NASA's Deep Impact spacecraft hit Comet Tempel 1 earlier this week. They monitored the impact using the ground-based Submillimeter Array (SMA) in Hawaii and NASA's orbiting Submillimeter Wave Astronomy Satellite (SWAS). Results are still coming in, but so far the scientists report seeing only weak emission from water vapor and a host of other gases that were expected to erupt from the impact site. The most conspicuous feature of the blast was brightening due to sunlight scattered by the ejected dust.
"It's pretty clear that this event did not produce a gusher," said SWAS principal investigator Gary Melnick of the Harvard-Smithsonian Center for Astrophysics (CfA). "The more optimistic predictions for water output from the impact haven't materialized, at least not yet."
Astronomer Charlie Qi (CfA) expressed surprise at these results. He explained that short-period comets like Tempel 1 have been baked repeatedly by the sun during their passages through the inner solar system. The effects of that heat are estimated to extend more than three feet beneath the surface of the nucleus. But the Deep Impact indicates that these effects could be much deeper.
"Theories about the volatile layers below the surface of short-period comets are going to have to be revised," Qi said.
As seen from Earth, a comet typically displays a fuzzy round head and a glowing tail. Both the head and tail consist of gases and dust ejected from the comet's nucleus - a frozen chunk of rock and ice about half the size of Manhattan Island.
Five decades ago, Harvard astronomer Fred Whipple developed a model of comet nuclei as "dirty snowballs." He hypothesized that comets consist of mostly ice with some dirt and rock mixed in. Modern astronomers often refer to comets as "icy dirtballs" instead, reflecting the prevailing view that comets contain more dust and less ice than previously believed.
Deep Impact was intended to test these theories by excavating material from the comet's interior, giving scientists clues to its composition and structure. The mission succeeded admirably, pulverizing a section of the comet larger than a house and releasing tons of material into space.
SWAS operators were puzzled by the lack of increased water vapor from Tempel 1. Post-impact measurements showed the comet was releasing only about 550 pounds of water per second-an emission rate very similar to pre-impact values, and less than seen by SWAS during natural outbursts in the weeks before the impact.
SMA measurements corroborate the SWAS findings. Although the SMA wasn't tuned to frequencies of water emission, which are difficult to observe from the ground due to atmospheric water vapor, it watched for other chemicals such as hydrogen cyanide. SMA astronomers saw little increase in production of gases following the impact. Gas production rates remained so low that they could set only an upper limit on the total.
"All we needed was a factor of three boost from the impact to get a definite detection," said Qi. "We didn't see that."
Qi added that the comet might become more active over the following days and weeks. "We're still hoping for a big outgassing from the new active area created by Deep Impact. If we see any signs of that, we'll make more observations."
The researchers will continue their careful and detailed analysis in order to interpret the SMA and SWAS measurements and what they indicate about the comet's composition.
"The big picture will emerge once astronomers meld data from different observatories at different wavelengths," said Melnick.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.