December 2, 2008
Comet 96P/Machholz 1's anomalous compositional characteristics help pinpoint its origin to one of three intriguing scenarios. David Schleicher, Lowell Observatory planetary astronomer, measured abundances of five molecular species in the comae of 150 comets and discovered that one comet, 96P/Machholz 1, has an unusual chemistry. The cause of this chemical anomaly remains unknown, but each of three possible explanations yields important but differing constraints on the evolution of comets.
One possible explanation is that Machholz 1 did not originate in our solar system, but escaped from another star. In this scenario, the other star's proto-planetary disk might have had a lower abundance of carbon, resulting in all carbon-bearing compounds having lower abundances. "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," said Schleicher. "Some of these will have crossed paths with the Sun, and Machholz 1 could be an interstellar interloper."
The discovery of comet Machholz 1's anomalous composition reveals the existence of a new class of comets. Astronomers identified two other classes in the 1990s. While Machholz 1 also has strongly depleted C2 and C3 carbon species, what makes it anomalous is that the molecule cyanogen (CN) is depleted. In Machholz 1, cyanogen is missing by about a factor of 72 from the average of other comets. "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," said Schleicher.
Another possible explanation for Machholz 1's anomalous composition is that it formed even farther from the Sun in a colder or more extreme environment than other comets studied. The scarcity of such objects likely is associated with the difficulty of explaining how such comets moved into the inner solar system.
The third possibility is that Machholz 1 originated as a carbon-chain depleted comet but extreme heat altered its chemistry. 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 now takes it to well inside Mercury's orbit every 5 years. (Other comets get even closer to the Sun, but not as often). "Since its orbit is unusual, we must be suspicious that repeated high temperature cooking might be the cause for its unusual composition," said 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."
Although comet 96P/Machholz 1 was first sighted in 1986, compositional measurements only took place during the comet's 2007 apparition. Lowell Observatory's program of compositional studies, currently headed by Schleicher, includes measurements of more than 150 comets obtained during the past 33 years. This research compares and contrasts Machholz 1 against this large database of 150 comets.
In the early 1990s, Lowell Observatory's long-term program first identified the existence of two compositional classes of comets. One class, containing the majority of observed comets, has a composition called "typical." Most members of this typical class have long resided in the Oort Cloud at the fringes of our solar system, but they are believed to have formed amidst the giant planets, particularly among Saturn, Uranus, and Neptune. Other members of this compositional class arrived from the Kuiper Belt, located just beyond Neptune.
The second compositional class of comets has varying depletions in two of the five chemical species measured. Because both depleted molecules, C2 and C3, are composed wholly of carbon atoms, this class was named "carbon-chain depleted." Moreover, nearly all comets in this second class have orbits consistent with their having arrived from the Kuiper Belt. For this and other reasons, the cause of the depletion is believed to be associated with the conditions that existed when the comets formed, perhaps within an outer, colder region of the Kuiper Belt.
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 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. "If additional carbon-bearing species are also depleted, then the case for its origin outside of our solar system would be strengthened," said Schleicher. The next opportunity for observations will be in 2012.