[meteorite-list] Scientists Find That Mars, Not Earth, Shakes Up Some Near-Earth Asteroids

[Ron Baalke]

http://web.mit.edu/press/2013/asteroids-mars.html

For Immediate Release: November 19, 2013
contact: Sarah McDonnell, MIT News Office
email: s_mcd at mit.edu phone: 617-253-8923

Scientists find that Mars, not Earth, shakes up some near-Earth asteroids.

CAMBRIDGE, Mass. - For nearly as long as astronomers have been able to 
observe asteroids, a question has gone unanswered: Why do the surfaces 
of most asteroids appear redder than meteorites - the remnants of asteroids 
that have crashed to Earth? 

In 2010, Richard Binzel, a professor of planetary sciences at MIT, identified 
a likely explanation: Asteroids orbiting in our solar system's main asteroid 
belt, situated between Mars and Jupiter, are exposed to cosmic radiation, 
changing the chemical nature of their surfaces and reddening them over 
time. By contrast, Binzel found that asteroids that venture out of the 
main belt and pass close to Earth feel the effects of Earth's gravity, 
causing 'asteroid quakes" that shift surface grains, exposing fresh grains 
underneath. When these "refreshed" asteroids get too close to Earth, they 
break apart and fall to its surface as meteorites. 

Since then, scientists have thought that close encounters with Earth play 
a key role in refreshing asteroids. But now Binzel and colleague Francesca 
DeMeo have found that Mars can also stir up asteroid surfaces, if in close 
enough contact. The team calculated the orbits of 60 refreshed asteroids, 
and found that 10 percent of these never cross Earth's orbit. Instead, 
these asteroids only come close to Mars, suggesting that the Red Planet 
can refresh the surfaces of these asteroids. 

"We don't think Earth is the only major driver anymore, and it opens our 
minds to the possibility that there are other things happening in the 
solar system causing these asteroids to be refreshed," says DeMeo, who 
did much of the work as a postdoc in MIT's Department of Earth, Atmospheric 
and Planetary Sciences. 

DeMeo and Binzel, along with former MIT research associate Matthew Lockhart, 
have published their findings in the journal Icarus. 

Asteroid roulette

The idea that Mars may shake up the surface of an asteroid is a surprising 
one: As Binzel points out, the planet is one-third the size of Earth, 
and one-tenth as massive - and therefore exerts a far weaker gravitational 
pull on surrounding objects. But Mars' position in the solar system places 
the planet in close proximity with the asteroid belt, increasing the chance 
of close asteroid encounters. 

"Mars is right next to the asteroid belt, and in a way it gets more opportunity 
than the Earth does to refresh asteroids," Binzel says. 'So that may be 
a balancing factor."

DeMeo, who suspected that Mars may have a hand in altering asteroid surfaces, 
looked through an asteroid database created by the International Astronomical 
Union's Minor Planet Center. The database currently consists of observations 
of 300,000 asteroids and their orbits; 10,000 of these are considered 
near-Earth asteroids. 

Over the past decade, Binzel's group has tracked the brightest of these 
asteroids, measuring their colors to determine which may have been refreshed 
recently. For this most recent paper, the researchers looked at 60 such 
asteroids, mapping out the orbit of each and determining which orbits 
had intersected with those of Earth or Mars. DeMeo then calculated the 
probability, over the last 500,000 years, that an asteroid and either 
planet would have intersected, creating a close encounter that could potentially 
generate asteroid quakes. 

"Picture Mars and an asteroid going through an intersection, and sometimes 
they'll both come through at very nearly the same time," Binzel says. 
"If they just barely miss each other, that's close enough for Mars' gravity 
to tug on [the asteroid] and shake it up. It ends up being this random 
process as to how these things happen, and how often."

Refreshing the face of an asteroid

>From their calculations, the researchers found that 10 percent of their 
sample of asteroids only cross Mars' orbit, and not Earth's. DeMeo explored 
other potential causes of asteroid refreshing, calculating the probability 
of asteroids colliding with each other, as well as the possibility for 
a phenomenon called "spin-up," in which energy from the sun causes the 
asteroid to rotate faster and faster, possibly disrupting its surface. 
>From her calculations, DeMeo found no conclusive evidence that either 
event would significantly refresh asteroids, suggesting that "Mars is 
the only game in town," Binzel says.

Although 10 percent of 60 asteroids may not seem like a significant number, 
DeMeo notes that given Mars' small size, the fact that the planet may 
have an effect on one out of 10 asteroids is noteworthy. "Mars is more 
powerful than we expected," she says. 

The researchers add that now that Mars has been proven to refresh asteroids, 
other planets, such as Venus, may have similar capabilities - particularly 
since Venus is closer in mass to Earth. 

"You think about these asteroids going around the sun doing their own 
thing, but there's really a lot more going on in their histories," says 
DeMeo, who is now a postdoc at Harvard University. "This gives you a dynamic 
idea of the lives of asteroids."

This work was supported by the National Science Foundation.

Written by: Jennifer Chu, MIT News Office