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A new class of comet?

Posted: 04 December, 2008


Schleicher measured abundances of five molecular species in the comae of 150 comets and discovered that one comet, 96P/Machholz 1, stood out from crowd, bearing an extremely unusual chemistry that doesn’t fit into any existing class of comet. ‘Typical’ comets reside either in the Kuiper Belt just beyond Neptune, or in the Oort Cloud at the very fringes of our Solar System, but the latter are believed to have originally formed amidst the giant planets. A second compositional class of comets has varying depletions in two of the five chemical species measured, C2 and C3. Since these species are wholly composed of carbon atoms, this class was named ‘carbon-chain depleted’ and applies to comets that originated in the Kuiper Belt.

SOHO captured Comet Machholz 1 close to the Sun on 8 January 2002. Its orbit brings it close to the Sun every five years. Image: SOHO/LASCO (ESA/NASA).

While Machholz 1 also has strongly depleted C2 and C3 carbon species, it is also highly depleted in the molecule cyanogen, CN, which is missing by about a factor of 72 from the average of other comets, i.e., only a little above one percent of normal. "This depletion of CN is much more than ever seen for any previously studied comet, and only one other comet has even exhibited a CN depletion," says Schleicher. The exact cause of this chemical anomaly remains unknown, but there are three possible explanations that would each yield important but differing new constraints on the formation and evolution of comets.

The first possible explanation is that Machholz 1 formed in another solar system – one where the star's proto-planetary disc might have had a lower abundance of carbon, resulting in all carbon-bearing compounds having lower abundances – and later found its way into our own Solar System. "A large fraction of comets in our own Solar System have escaped into interstellar space, so we expect that many comets formed around other stars would also have escaped," says Schleicher. "Some of these will have crossed paths with the Sun, and Machholz 1 could be an interstellar interloper."

Another possible explanation is that Machholz 1 formed further from the Sun in a colder or more extreme environment than any other comet we have studied to date. If this was the case, then the scarcity of such objects is likely associated with the significant difficulty of explaining how such comets moved into the inner Solar System where they can then be discovered and observed.

“I just don't know whether or not very low temperature chemistry would be expected to cause a depletion of HCN, the primary parent of CN,” Shleicher tells Astronomy Now. “Hopefully the planetary astronomers who concentrate on the formation chemistry in the early Solar System can shed some light on this. If said chemistry would be expected to tie up the CN into other, unseen molecules, then I would favor the cold Kuiper disc origin. If not, then extra-solar origin rises to the top.”

A third possibility is that Machholz 1 originated as a carbon-chain depleted comet but that its chemistry was subsequently altered by extreme heat. While no other comet has exhibited changes in chemistry due to subsequent heating by the Sun, Machholz 1 has the distinction of having an orbit that brings it well inside Mercury's orbit every five years. Although other comets get even closer to the Sun, they do so much less often. "Since its orbit is unusual, we must be suspicious that repeated high temperature cooking might be the cause for its unusual composition," says Schleicher. "However, the only other comet to show depletion in the abundance of CN did not reach such high temperatures. This implies that CN depletion does not require the chemical reactions associated with extreme heat."

Click for link to NASA/ESA YouTube video of Comet Machhoz 1.

Comets are widely thought to be the most pristine objects available for detailed study remaining from the epoch of Solar System formation. As such, comets can be used as probes of the proto-planetary material that was incorporated into our Solar System. Differences in the current chemical composition among comets can indicate either differences in primordial conditions or evolutionary effects.

Although the location of origin cannot be definitively determined for any single comet, Machholz 1's short orbital period means that astronomers can search for additional carbon-bearing molecular species during future apparitions. “Since the comet returns in 2012 there is hope that some of the other carbon-bearing species can be measured in the near-UV and the IR portions of the spectrum. Any additional constraints on the observed abundances should provide clues to the chemists.”

Only time will tell. The study is published in the November issue of the Astronomical Journal.

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