New Class of Comets May Be the Source of Earth's Water

http://www.ifa.hawaii.edu/%7Ehsieh/mbc-release.html

Embargoed until Thursday, March 23, 2006, at 2:00 PM EST (9:00 AM HST)

Contacts:

Mr. Henry Hsieh
Institute for Astronomy
University of Hawaii
2680 Woodlawn Drive
Honolulu, Hawaii 96822
1-808-778-6943


Dr. David Jewitt
Institute for Astronomy
University of Hawaii
2680 Woodlawn Drive
Honolulu, Hawaii 96822
1-808-956-7682


Ms. Karen Rehbock
Assistant to the Director
Institute for Astronomy
University of Hawaii
1-808-956-6829



New Class of Comets May Be the Source of Earth's Water

Three icy comets orbiting among the rocky asteroids in the main
asteroid
belt between Mars and Jupiter may hold clues to the origin of Earth's
oceans.

The newly discovered group of comets, dubbed "main-belt comets" by
University of Hawaii graduate student Henry Hsieh and Professor David
Jewitt, has asteroid-like orbits and, unlike other comets, appears to
have formed in the warm inner solar system inside the orbit of Jupiter
rather than in the cold outer solar system beyond Neptune.

The existence of these main-belt comets suggests that asteroids and
comets are much more closely related than previously thought and
supports the idea that icy objects from the main asteroid belt could be
a major source of Earth's present-day water. This work appears in the
March 23 edition of Science Express
http://www.sciencemag.org/cgi/content/abstract/1125150 (pdf
http://www.ifa.hawaii.edu/~hsieh/HJ06.pdf) and will also appear in an
April print edition of Science.

The crucial observations were made on November 26, 2005, using the
8-meter Gemini North Telescope on Mauna Kea. Hsieh and Jewitt found
that
an object designated as Asteroid 118401 was ejecting dust like a comet.
Together with a mysterious comet (designated 133P/Elst-Pizarro) known
for almost a decade but still poorly understood, and another comet
(designated P/2005 U1) discovered by the Spacewatch project in Arizona
just a month earlier, "Asteroid" 118401 forms an entirely new class of
comets.

"The main-belt comets are unique in that they have flat, circular,
asteroid-like orbits, and not the elongated, often tilted orbits
characteristic of all other comets," said Hsieh. "At the same time,
their cometary appearance makes them unlike all other previously
observed asteroids. They do not fit neatly in either category."

http://www.ifa.hawaii.edu/~hsieh/mbcs6a.gif
http://www.ifa.hawaii.edu/~hsieh/mbcs6c.gif
Orbits of the 3 known main-belt comets (red lines), the 5 innermost
planets (black lines; from the center outward, Mercury, Venus, Earth,
Mars, and Jupiter), a sample of 100 main-belt asteroids (orange lines),
and 2 "typical" comets (Halley's Comet, and Tempel 1, target of the
recent Deep Impact mission) as blue lines. Positions of the main-belt
comets and planets on March 1, 2006, are plotted with black dots.
[click
each image to enlarge]
Image courtesy of Pedro Lacerda (Univ. Hawaii; Univ. Coimbra, Portugal)

In both 1996 and 2002, the "original" main-belt comet,
133P/Elst-Pizarro
(named after its two discoverers), was seen to exhibit a long dust tail
typical of icy comets, despite having the flat, circular orbit typical
of presumably dry, rocky asteroids. As the only main-belt object ever
observed to take on a cometary appearance, however, 133P/Elst-Pizarro's
true nature remained controversial. Until now.

"The discovery of the other main-belt comets shows that
133P/Elst-Pizarro is not alone in the asteroid belt," Jewitt said.
"Therefore, it is probably an ordinary (although icy) asteroid, and not
a comet from the outer solar system that has somehow had its comet-like
orbit transformed into an asteroid-like one. This means that other
asteroids could have ice as well."

The Earth is believed to have formed hot and dry, meaning that its
current water content must have been delivered after the planet cooled.
Possible candidates for supplying this water are colliding comets and
asteroids. Because of their large ice content, comets were leading
candidates for many years, but recent analysis of comet water has shown
that comet water is significantly different from typical ocean water on
Earth.

Asteroidal ice may give a better match to Earth's water, but until now,
any ice that the asteroids may have once contained was thought to
either
be long gone or so deeply buried inside large asteroids as to be
inaccessible for further analysis. The discovery of main-belt comets
means that this ice is not gone and is still accessible (right on the
surfaces of at least some objects in the main belt, and at times, even
venting into space). Spacecraft missions to the main-belt comets could
provide new, more detailed information on their ice content and in turn
give us new insight into the origin of the water, and ultimately life,
on Earth.

As conventionally defined, comets and asteroids are very different.
Both
are objects a few to a few hundred miles across that orbit throughout
our solar system. Comets, however, are thought to originate in the cold
outer solar system and consequently contain much more ice than the
asteroids, most of which are thought to have formed much closer to the
Sun in the asteroid belt between Mars and Jupiter.

Comets also have large, elongated orbits and thus experience wide
temperature variations. When a comet approaches the Sun, its ice heats
up and sublimates (changes directly from ice to gas), venting gas and
dust into space, giving rise to a tail and a distinctive fuzzy
appearance. Far from the Sun, sublimation stops, and any remaining ice
stays frozen until the comet's next pass close to the Sun. In contrast,
objects in the asteroid belt have essentially circular orbits and are
expected to be mostly baked dry of ice by their confinement to the
inner
solar system. Essentially, they should be just rocks. With the
discovery
of the main-belt comets, we now know this is not the case, and that, in
general, the conventional definitions of comets and asteroids are in
need of refinement.

This work is supported by a grant from the NASA Planetary Astronomy
Program of the Science Mission Directorate.

More information: http://www.ifa.hawaii.edu/~hsieh/mbcs.html

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UH 2.2-meter Telescope: http://www.ifa.hawaii.edu/88inch/

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