September 10, 2013
Don Quixote’s coma and tail (left) as seen in infrared light by NASA’s
Spitzer Space Telescope. After image processing (right), the tail is
more apparent. Image courtesy NASA/JPL-Caltech/DLR/NAU
For 30
years, a large near-Earth asteroid wandered its lone, intrepid path,
passing before the scrutinizing eyes of scientists while keeping
something to itself: 3552 Don Quixote, whose journey stretches to the
orbit of Jupiter, now appears to be a comet.
The discovery
resulted from an ongoing project led by researchers at Northern Arizona
University using the Spitzer Space Telescope. Through a lot of focused
attention and a little bit of luck, they found evidence of cometary
activity that had evaded detection for three decades.
“Its orbit resembled that of a comet, so people assumed it was a comet that had gotten rid of all its ice deposits,” said
Michael Mommert,
a post-doctoral researcher at NAU who was a Ph.D. student of professor
Alan Harris at the German Aerospace Center (DLR) in Berlin at the time
the work was carried out.
What Mommert and an international team
of researchers discovered, though, was that Don Quixote was not actually
a dead comet—one that had shed the carbon dioxide and water that give
comets their spectacular tails.
Instead, the third-biggest
near-Earth asteroid out there, skirting Earth with an erratic, extended
orbit, is “sopping wet,” said NAU associate professor
David Trilling.
The implications have less to do with potential impact, which is
extremely unlikely in this case, and more with “the origins of water on
Earth,” Trilling said. Comets may be the source of at least some of it,
and the amount on Don Quixote represents about 100 billion tons of
water—roughly the same amount found in Lake Tahoe.
Mommert said
it’s surprising that Don Quixote hasn’t been depleted of all of its
water, especially since researchers assumed that it had done so
thousands of years ago. But finding evidence of CO2, and presumably
water, wasn’t easy.
During an observation of the object using
Spitzer in August 2009, Mommert and Trilling found that it was far
brighter than they expected. “The images were not as clean as we would
like, so we set them aside,” Trilling said.
Much later, though,
Mommert prompted a closer look, and partners at the Harvard-Smithsonian
Center for Astrophysics found something unusual when comparing infrared
images of the object: something, that is, where an asteroid should have
shown nothing. The “extended emission,” Mommert said, indicated that Don
Quixote had a coma—a comet’s visible atmosphere—and a faint tail.
Mommert said this discovery implies that carbon dioxide and water ice also might be present on other near-Earth objects.
This
study confirmed Don Quixote’s size and the low, comet-like reflectivity
of its surface. Mommert is presenting the research team’s findings this
week at the
European Planetary Space Conference in London.
