[meteorite-list] Study Puts Solar Spin on Asteroids, their Moons & Earth Impacts

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 9 Jul 2008 17:39:44 -0700 (PDT)
Message-ID: <200807100039.RAA07591_at_zagami.jpl.nasa.gov>

Office of University Communications
University of Maryland

Contacts:
Lee Tune, 301-405-4679

For Immediate Release: July 9, 2008

Study Puts Solar Spin on Asteroids, their Moons & Earth Impacts

COLLEGE PARK, Md. -- Asteroids with moons, which scientists call binary
asteroids, are common in the solar system. A longstanding question has
been how the majority of such moons are formed. In this week's issue of
the journal Nature, a trio of astronomers from Maryland and France say
the surprising answer is sunlight, which can increase or decrease the
spin rate of an asteroid.

Derek Richardson, of the University of Maryland, his former student
Kevin Walsh, now Poincar? Fellow in the Planetology Group in the
Cassiop?e Laboratory of CNRS at the C?te d'Azur Observatory, France, and
that group's leader, co-author Patrick Michel outline a model showing
that when solar energy "spins up" a "rubble pile" asteroid to a
sufficiently fast rate, material is slung off from around the asteroid's
equator. This process also exposes fresh material at the poles of the
asteroid.

If the spun off bits of asteroid rubble shed sufficient excess motion
through collisions with each other, then the material coalesces into a
satellite that continues to orbit its parent. Because the team's model
closely matches observations from binary asteroids, it neatly fills in
missing pieces to a solar system puzzle. And, it could have much more
down-to-earth implications as well. The model gives information on the
shapes and structure of near-Earth binary asteroids that could be vital
should such a pair need to be deflected away from a collision course
with Earth.

Finally, the authors say, these findings suggest that a sample return
mission to such a binary asteroid could bring back exposed pristine
material from the poles of the parent asteroid, providing a chance to
probe the internal composition of an asteroid without having to dig into it.

Solar Spin Power

It's estimated that about 15 per cent of near-Earth and main-belt
asteroids with diameters less than 10 kilometers have satellite
Scientists have determined that these small binary asteroid pairs were
not formed at the beginning of the solar system, indicating that some
process still at work must have created them. "It was at first thought
the moons in these asteroid pairs probably formed through collisions
and/or close encounters with planets," said Richardson, an associate
professor of astronomy at the University of Maryland. "However, it was
found that these mechanisms could not account for the large number of
binary asteroids present among near-Earth and inner main belt asteroids."

Recent studies have outlined a thermal process -- known as the YORP
effect after the scientists (Yarkovsky, O'Keefe, Radzievskii, Paddack)
who identified it -- by which sunlight can speed up or slow down an
asteroid's spin. Widespread evidence of this mechanism can be seen in
the
notable abundance of both fast and slow rotators among [near-Earth
asteroids] and small main belt asteroids,
 Walsh, Richardson and Michel
write in the Nature paper.

The trio modeled different types of 'rubble pile' asteroids -- chunks of
rock held together by gravity. This work, supported by the National
Science Foundation and NASA, as well as the European Space Agency and
the French National Planetology Program, is the first to show how the
slow spinup of such asteroids leads over millions of years to mass loss
that can form binaries. "Our model almost exactly matches the
observations of our test case, binary asteroid KW4, which was imaged
incredibly well by the NSF-supported Arecibo radio telescope in Puerto
Rico," Walsh said.

Asteroid Deep Impacts

"Based on our findings, the YORP effect appears to be the key to the
origin of a large fraction of observed binaries," said Michel. "The
implications are that binary asteroids are preferentially formed from
aggregate objects [rubble piles], which agrees with the idea that such
asteroids are quite porous. The porous nature of these asteroids has
strong implications for defensive strategies if faced with an impact
risk to Earth from such objects, because the energy required to deflect
an asteroid depends sensitively on its internal structure," he said.

Doublet craters formed by the nearly simultaneous impact of objects of
comparable size can be found in a number of places on Earth, suggesting
that binary asteroids have hit our planet in the past. Similar doublet
craters also can be found on other planets. The authors say that their
current findings also suggest that a space mission to a binary asteroid
could bring back material that might shed new light on the solar
system's early history.

The oldest material in an asteroid should lie underneath its surface,
explained Richardson, and the process of spinning off this surface
material from the primary asteroid body to form its moon, or secondary
body, should uncover the deeper older material. "Thus a mission to
collect and return a sample from the primary body of such a binary
asteroid could give us information about the older, more pristine
material inside an asteroid, just as the University of Maryland-led Deep
Impact misssion gave us information about the more pristine material
inside a comet," Richardson said. Michel added, "Bringing back pristine
material is the goal of our proposed Marco Polo mission, which is
currently under study by the European Space Agency, in partnership with
JAXA in Japan."

IMAGE CAPTIONS:

[Movie 1:
http://www.astro.umd.edu/%7Ekwalsh/BinaryFormation.mpg (9.8MB)]
Watch an animated model of the spin-up and binary formation from two
views, on the left is an overhead view. The right pane of the movie
looks at the equator of the primary body, which is also the plane in
which the asteroid's satellite is formed. Courtesy of the authors.

[Movie 2:
http://echo.jpl.nasa.gov/%7Eostro/kw4_2001_060830.S3M.320.mov (4.6MB)]
Animation of the KW4 system as viewed from Earth during May/June 2001
(with the actual star background and simulated solar illumination).
Courtesy NASA

[Image:
http://www.newsdesk.umd.edu/images/Binary/clearwater_lakes.jpg (9KB)]
These twin circular lakes in Quebec, Canada were formed by the impact of
an asteroidal pair which slammed into the planet approximately 290
million years ago. Courtesy NASA
Received on Wed 09 Jul 2008 08:39:44 PM PDT