[meteorite-list] 'Rogue' Asteroids May Be The Norm

[Ron Baalke]

http://web.mit.edu/press/2014/rogue-asteroids-may-be-the-norm-0129.html

>From MIT News: January 29, 2014
contact: Sarah McDonnell, MIT News Office
email: s_mcd at mit.edu phone: 617-253-8923

'Rogue' asteroids may be the norm

A new map of the solar system's asteroids shows more diversity than previously 
thought.

CAMBRIDGE, Mass. - To get an idea of how the early solar system may have 
formed, scientists often look to asteroids. These relics of rock and dust 
represent what today's planets may have been before they differentiated 
into bodies of core, mantle, and crust. 

In the 1980s, scientists' view of the solar system's asteroids was essentially 
static: Asteroids that formed near the sun remained near the sun; those 
that formed farther out stayed on the outskirts. But in the last decade, 
astronomers have detected asteroids with compositions unexpected for their 
locations in space: Those that looked like they formed in warmer environments 
were found further out in the solar system, and vice versa. Scientists 
considered these objects to be anomalous "rogue" asteroids. 

But now, a new map developed by researchers from MIT and the Paris Observatory 
charts the size, composition, and location of more than 100,000 asteroids 
throughout the solar system, and shows that rogue asteroids are actually 
more common than previously thought. Particularly in the solar system's 
main asteroid belt - between Mars and Jupiter - the researchers found 
a compositionally diverse mix of asteroids. 

The new asteroid map suggests that the early solar system may have undergone 
dramatic changes before the planets assumed their current alignment. For 
instance, Jupiter may have drifted closer to the sun, dragging with it 
a host of asteroids that originally formed in the colder edges of the 
solar system, before moving back out to its current position. Jupiter's 
migration may have simultaneously knocked around more close-in asteroids, 
scattering them outward. 

"It's like Jupiter bowled a strike through the asteroid belt," says Francesca 
DeMeo, who did much of the mapping as a postdoc in MIT's Department of 
Earth, Atmospheric and Planetary Sciences. "Everything that was there 
moves, so you have this melting pot of material coming from all over the 
solar system."

DeMeo says the new map will help theorists flesh out such theories of 
how the solar system evolved early in its history. She and Benoit Carry 
of the Paris Observatory have published details of the map in Nature. 

>From a trickle to a river

To create a comprehensive asteroid map, the researchers first analyzed 
data from the Sloan Digital Sky Survey, which uses a large telescope in 
New Mexico to take in spectral images of hundreds of thousands of galaxies. 
Included in the survey is data from more than 100,000 asteroids in the 
solar system. DeMeo grouped these asteroids by size, location, and composition. 
She defined this last category by asteroids' origins - whether in a warmer 
or colder environment - a characteristic that can be determined by whether 
an asteroid's surface is more reflective at redder or bluer wavelengths. 

The team then had to account for any observational biases. While the survey 
includes more than 100,000 asteroids, these are the brightest such objects 
in the sky. Asteroids that are smaller and less reflective are much harder 
to pick out, meaning that an asteroid map based on observations may unintentionally 
leave out an entire population of asteroids. 

To avoid any bias in their mapping, the researchers determined that the 
survey most likely includes every asteroid down to a diameter of five 
kilometers. At this size limit, they were able to produce an accurate 
picture of the asteroid belt. The researchers grouped the asteroids by 
size and composition, and mapped them into distinct regions of the solar 
system where the asteroids were observed. 

>From their map, they observed that for larger asteroids, the traditional 
pattern holds true: The further one gets from the sun, the colder the 
asteroids appear. But for smaller asteroids, this trend seems to break 
down. Those that look to have formed in warmer environments can be found 
not just close to the sun, but throughout the solar system - and asteroids 
that resemble colder bodies beyond Jupiter can also be found in the inner 
asteroid belt, closer to Mars. 

As the team writes in its paper, "the trickle of asteroids discovered 
in unexpected locations has turned into a river. We now see that all asteroid 
types exist in every region of the main belt."

A shifting solar system

The compositional diversity seen in this new asteroid map may add weight 
to a theory of planetary migration called the Grand Tack model. This model 
lays out a scenario in which Jupiter, within the first few million years 
of the solar system's creation, migrated as close to the sun as Mars is 
today. During its migration, Jupiter may have moved right through the 
asteroid belt, scattering its contents and repopulating it with asteroids 
from both the inner and outer solar system before moving back out to its 
current position - a picture that is very different from the traditional, 
static view of a solar system that formed and stayed essentially in place 
for the past 4.5 billion years.

"That [theory] has been completely turned on its head," DeMeo says. "Today 
we think the absolute opposite: Everything's been moved around a lot and 
the solar system has been very dynamic."

DeMeo adds that the early pinballing of asteroids around the solar system 
may have had big impacts - literally - on Earth. For instance, colder 
asteroids that formed further out likely contained ice. When they were 
brought closer in by planetary migrations, they may have collided with 
Earth, leaving remnants of ice that eventually melted into water. 

"The story of what the asteroid belt is telling us also relates to how 
Earth developed water, and how it stayed in this Goldilocks region of 
habitability today," DeMeo says.

Written by: Jennifer Chu, MIT News Office