Spitzer peers inside

Comet Holmes
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: October 15, 2008

Comet Holmes unexpectedly exploded in 2007, and new observations from NASA’s Spitzer Space Telescope only deepen the mystery as to why the comet put on such a show.

"The data we got from Spitzer do not look like anything we typically see when looking at comets," says Bill Reach, lead investigator of the Spitzer observations. "The comet Holmes explosion gave us a rare glimpse at the inside of a comet nucleus."

Spitzer captured the picture on the left of comet Holmes in February 2008, four months after the comet suddenly erupted, revealing fine dust particles in the outer shell (coma) of the comet. The nucleus is within the bright whitish spot in the centre, while the yellow area shows solid particles that were blown from the comet in the explosion.
The contrast-enhanced picture on the right shows the comet's outer shell, and strange filaments, or streamers, of dust. The horizontal line is a trail of debris that travels along with the comet in its orbit. Image: NASA/JPL-Caltech/XMM/NTT/MPIA.

Every six years, comet 17P/Holmes heads away from Jupiter and in toward the Sun, travelling the same route typically without incident. However, twice in the last 116 years, in November 1892 and October 2007, comet Holmes exploded as it approached the asteroid belt, and brightened a million-fold overnight.

Spitzer observed the comet in infrared in November 2007 and March 2008, in order to gain insight into the composition of Holmes' solid interior. The telescope’s spectrograph breaks up infrared light into its component parts, creating ‘fingerprint’ spectra that are unique to an individual mineral. The November 2007 observation campaign revealed a lot of fine silicate dust, smaller than grains of sand. Reach noted that this particular observation revealed materials similar to those seen around other comets where grains have been treated violently, including NASA's Deep Impact mission, which smashed a coffee table-sized projectile into comet Tempel 1 to observe the ejected material; NASA's Stardust mission, which swept particles from comet Wild 2 into a collector at 21,000 kilometres per hour and returned the samples to Earth, and the outburst of comet Hale-Bopp in 1995.

"Comet dust is very sensitive, meaning that the grains are very easily destroyed,” says Reach. "We think the fine silicates are produced in these violent events by the destruction of larger particles originating inside the comet nucleus."

When Spitzer repeated the spectrographic observations again in March 2008, the fine grained silicate dust was gone, leaving only larger particles. "The March observation tells us that there is a very small window for studying composition of comet dust after a violent event like comet Holmes' outburst," says Reach.

Another mystery surrounds the curious streamers that were seen emanating from the shell of dust around the nucleus shortly after the explosion. Scientists had initially suspected that the streamers were small dust particles ejected from fragments of the nucleus, or from jets on the nucleus. Initially the streamers were pointing away from the Sun, normal behaviour since radiation pressure from the Sun pushes the material out, shifting in orientation as a comet follows its orbit around the Sun. However, when Spitzer imaged the same streamers in March 2008, they were surprised to find the streamers still pointing the same direction as five months before, even though the comet had moved and sunlight was arriving from a different location.

"We have never seen anything like this in a comet before,” says Jeremie Vaubaillon, a colleague of Reach's at Caltech. Vaubaillon also realised that the shell surrounding the comet was also acting very peculiarly, in that the shape of the shell did not change in the time elapsed between the two observations. Vaubaillon suggests that this is because the dust grains seen in March 2008 are relatively large, approximately one millimetre in size, and thus harder to move. "If the shell was comprised of smaller dust grains, it would have changed as the orientation of the Sun changes with time," he says. "This Spitzer image is very unique. No other telescope has seen comet Holmes in this much detail, five months after the explosion."

Comet Holmes has been studied for around 116 years, but still astronomers are far away from truly understanding what went on inside the comet last year. But combined with amateur and professional observations, the Spitzer observations have allowed astronomers to get one step closer to unlocking the comet’s secrets.