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

[Ron Baalke]

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 Poincaré Fellow in the Planetology Group in the 
Cassiopée Laboratory of CNRS at the Côte 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