[meteorite-list] A New Compositional Class of Comets: from Fire, Ice, or Beyond?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 2 Dec 2008 10:58:03 -0800 (PST)
Message-ID: <200812021858.KAA26143_at_zagami.jpl.nasa.gov>

http://www.lowell.edu/media/releases.php?release=20081202

FOR IMMEDIATE RELEASE
December 2, 2008

A New Compositional Class of Comets: from Fire, Ice, or Beyond?
Lowell Observatory Astronomer Confirms New Class of Comets

Flagstaff, Ariz. -- Comet 96P/Machholz 1 shows extremely anomalous
compositional characteristics helping 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 extremely unusual chemistry. The exact cause of this chemical
anomaly remains unknown, but each of three possible explanations will
yield important but differing new constraints on the formation or
evolution of comets. The study is published in the November issue of the
Astronomical Journal.

The discovery of comet Machholz 1's extremely 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 extremely depleted. In Machholz 1 CN 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," said Schleicher.

One possible explanation is that Machholz 1 did not originate in our
Solar System, but instead 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."

Another possible explanation for Machholz 1's anomalous composition is
that it formed even further from the sun in a colder or more extreme
environment than another other comet we have studied thus far. 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.

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 now takes it 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,"
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 and orbits the
sun with a period of slightly over five years, compositional
measurements only took place during the comet's recent 2007 apparition.
Lowell Observatory's program of compositional studies, currently headed
by Schleicher, includes measurements of over 150 comets obtained during
the past 33 years. This research is unique because it 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 very fringes of our Solar System but are
believed to have originally formed amidst the giant planets,
particularly between 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. Since both depleted molecules, C2
and C3, are wholly composed 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 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. "If additional carbon-bearing species are
also strongly 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.

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

The study is published in the November issue of the Astronomical
Journal. <http://www.iop.org/EJ/abstract/1538-3881/136/5/2204>

FOR MORE INFORMATION

Scientific contact: David Schleicher (dgs at lowell.edu) (928) 233-3228

See a pdf of the report, Lowell Observatory Comet 96/P Machholz 1
Background
<http://www.lowell.edu/media/content/release_supplements/Mach1_backgrd.pdf>


About Lowell Observatory

Lowell Observatory is a private, non-profit research institution founded
in 1894 by Percival Lowell. The Observatory has been the site of many
important findings including the discovery of the large recessional
velocities (redshift) of galaxies by Vesto Slipher in 1912-1914 (a
result that led ultimately to the realization the universe is
expanding), and the discovery of Pluto by Clyde Tombaugh in 1930. Today,
Lowell's 20 astronomers use ground-based telescopes around the world,
telescopes in space, and NASA planetary spacecraft to conduct research
in diverse areas of astronomy and planetary science. The Observatory
welcomes more than 75,000 visitors each year to its Mars Hill campus in
Flagstaff, Arizona for a variety of tours, telescope viewing, and
special programs. Lowell Observatory currently has four research
telescopes at its Anderson Mesa dark sky site east of Flagstaff, and is
building a 4-meter class research telescope, the Discovery Channel
Telescope, in partnership with Discovery Communications.

CONTACT

Steele Wotkyns
steele at lowell.edu
(928) 233-3232

end
Received on Tue 02 Dec 2008 01:58:03 PM PST