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 SPACE.com.
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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