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

[Eugen Leitl]

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 any 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.


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

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