Image: Comet Machholz 1
This image taken by the ESA-NASA sunwatching spacecraft SOHO reveals Comet Machholz 1 close to the sun on Jan. 8, 2002. SOHO's coronograph hid the bright sun, the size of which is shown by the inner ring.
updated 12/2/2008 8:45:33 PM ET 2008-12-03T01:45:33

The bizarre chemical make-up of a comet suggests the blob of ice is an interloper, possibly flung into our solar system from beyond, astronomers now say, adding that the wacky comet is forcing them to create a new category for such objects.

The comet, called Machholz 1, was discovered in 1986 by Donald Machholz of Loma Prieta, Calif. Since then, the icy denizen has made a few appearances, including one in 2007.

"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 David Schleicher, a planetary astronomer at Lowell Observatory in Arizona. "Some of these will have crossed paths with the sun, and Machholz 1 could be an interstellar interloper."

The oddball, described in the November issue of the Astronomical Journal, could shed light on the formation and evolution of comets. These blobs of dust and frozen gases may be the oldest, most primitive bodies in the solar system. In fact, they hold the earliest record of material from the nebula that formed the sun and planets.

Head measurements
Schleicher measured the amounts of certain carbon and other compounds in the coma or head of Machholz 1 in 2007. He compared the composition with information from 150 other comets, finding that Machholz 1 had an odd make-up. Particularly, the comet contains much less of a carbon-nitrogen molecule called cyanogen, by a factor of about 72, compared with the average found in other comets. The comet also contained much less of two molecules called C2 and C3 (which have two and three atoms of carbon in their structures, respectively) than the average comet.

Schleicher puts forth three scenarios that could be responsible for the odd chemistry, although no one explanation stands out as right on. "I'm not real thrilled with any of the three [scenarios]," Schleicher told

One possible explanation is that Machholz 1 did not originate in the solar system, but instead escaped from another star's gravity. In this scenario, the other star's proto-planetary disk (the disk of debris from which planets are thought to form) might have had a lower abundance of carbon, resulting in all carbon-bearing compounds having lower abundances.

Several comets previously in our solar system have gotten the boot when their orbits came into contact with that of Jupiter, whose gravity kicked them out.

Another possible explanation for Machholz 1's anomalous composition is that it formed inside the solar system but even farther from the sun in a colder or more extreme environment than other comets.

A third possibility is that Machholz 1 originated as a so-called carbon-chain depleted comet, but that this chemistry was subsequently altered by extreme heat and so cyanogen was also depleted. While no other comet has exhibited changes in chemistry due to subsequent heating by the sun, Machholz 1's orbit takes the comet to well inside Mercury's orbit every five 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," Schleicher said.

However, he added, the only other comet, called Yanaka, to show less cyanogen than is typical never reached such high temperatures.

"This implies that [cyanogen] depletion does not require the chemical reactions associated with extreme heat," Schleicher said.

Class of its own
The results place Machholz 1 into a new class of comets. Until now, astronomers have clumped comets into one of two classes based on their chemical compositions.

Most comets, considered members of the "typical" class, have long resided in the Oort Cloud at the fringes of our solar system but are thought to have originally formed amidst the giant planets, particularly between Saturn, Uranus and Neptune. Other members of this class arrived from the Kuiper Belt, a region of icy objects beyond Neptune (but closer in than the Oort Cloud.)

The second class includes comets with varying depletions of certain carbon molecules, including the C2 and C3 molecules. Nearly all of these comets (called carbon-chain depleted comets) have orbits that suggest they arrived from the Kuiper Belt. For this and other reasons, scientists think the carbon depletion is associated with conditions that existed when the comets formed, perhaps within an outer, colder region of the Kuiper Belt.

The new class of comets, which would include Machholz 1 and possibly Yanaka, would be characterized by low levels of at least three carbon compounds — C2, C3 and cyanogen. And as of now, these comets would be considered of unknown origin, Schleicher said.

The comet, now bound in an orbit around the sun, is expected to come into view again in 2012, at which time Schleicher expects astronomers will point other instruments toward it to measure even more carbon-bearing molecules.

This research was supported by NASA's Planetary Astronomy and Planetary Atmospheres Programs.

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