[meteorite-list] New Research Reveals a Large Asteroid Breakup to be Likely Source of Impactor That Caused Mass Extinction 65 Million Years Ago

[Ron Baalke]

http://www.swri.org/9what/releases/2007/Bottke.htm

Contact:
Maria Martinez
maria.martinez at swri.org
(210) 522-3305

New research reveals a large asteroid breakup to be the likely source of 
the impactor that caused a mass extinction event on Earth 65 million years ago

Embargoed until 1 p.m. EST, Sept. 5

Boulder, Colo. -- Sept. 6, 2007 -- The impactor believed to have wiped
out the dinosaurs and other life forms on Earth some 65 million years
ago has been traced back to a breakup event in the main asteroid belt.
A joint U.S.-Czech team from Southwest Research Institute (SwRI)
and Charles University in Prague suggests that the parent object of 
asteroid (298) Baptistina disrupted when it was hit by another large 
asteroid, creating numerous large fragments that would later create the
Chicxulub crater on the Yucatan Peninsula as well as the prominent Tycho
crater found on the Moon.

The team of researchers, including Dr. William Bottke (SwRI), Dr. David
Vokrouhlicky (Charles University, Prague) and Dr. David Nesvorny (SwRI),
combined observations with several different numerical simulations to
investigate the Baptistina disruption event and its aftermath.
A particular focus of their work was how Baptistina fragments affected
the Earth and Moon.

At approximately 170 kilometers in diameter and having characteristics
similar to carbonaceous chondrite meteorites, the Baptistina parent
body resided in the innermost region of the asteroid belt when it was hit
by another asteroid estimated to be 60 kilometers in diameter. This
catastrophic impact produced what is now known as the Baptistina asteroid
family, a cluster of asteroid fragments with similar orbits. According to
the team's modeling work, this family originally included approximately
300 bodies larger than 10 kilometers and 140,000 bodies larger than 1
kilometer.

Once created, the newly formed fragments' orbits began to slowly evolve due 
to thermal forces produced when they absorbed sunlight and re-radiated the 
energy away as heat. According to Bottke, "By carefully modeling these 
effects and the distance traveled by different-sized fragments from the 
location of the original collision, we determined that the Baptistina 
breakup took place 160 million years ago, give or take 20 million years."

The gradual spreading of the family caused many fragments to drift into
a nearby "dynamical superhighway" where they could escape the main asteroid
belt and be delivered to orbits that cross Earth's path. The team's
computations suggest that about 20 percent of the surviving 
multi-kilometer-sized fragments in the Baptistina family were lost in 
this fashion, with about 2 percent of those objects going on to strike 
the Earth, a pronounced increase in the number of large asteroids 
striking Earth.

Support for these conclusions comes from the impact history of the Earth
and Moon, both of which show evidence of a two-fold increase in the
formation rate of large craters over the last 100 to 150 million years.
As described by Nesvorny, "The Baptistina bombardment produced a prolonged
surge in the impact flux that peaked roughly 100 million years ago. This
matches up pretty well with what is known about the impact record."

Bottke adds, "We are in the tail end of this shower now. Our simulations
suggest that about 20 percent of the present-day, near-Earth asteroid
population can be traced back to the Baptistina family."

The team then investigated the origins of the 180 kilometer diameter
Chicxulub crater, which has been strongly linked to the extinction of the
dinosaurs 65 million years ago. Studies of sediment samples and a meteorite
from this time period indicate that the Chicxulub impactor had a 
carbonaceous chondrite composition much like the well-known primitive 
meteorite Murchison.  This composition is enough to rule out many potential 
impactors but not those from the Baptistina family. Using this information 
in their simulations, the team found a 90 percent probability that the 
object that formed the Chicxulub crater was a refugee from the Baptistina 
family.

These simulations also showed there was a 70 percent probability that the
lunar crater Tycho, an 85 kilometer crater that formed 108 million years 
ago, was also produced by a large Baptistina fragment. Tycho is notable for 
its large size, young age and its prominent rays that extend as far as 1,500
kilometers across the Moon.  Vokrouhlicky says, "The probability is smaller
than in the case of the Chicxulub crater because nothing is yet known about
the nature of the Tycho impactor."

This study demonstrates that the collisional and dynamical evolution the 
main asteroid belt may have significant implications for understanding the
geological and biological history of Earth.

As Bottke says, "It is likely that more breakup events in the asteroid belt
are connected in some fashion to events on the Earth, Moon and other 
planets.  The hunt is on!"

The article, "An asteroid breakup 160 Myr ago as the probable source of the
K/T impactor," was published in the Sept. 6 issue of Nature.

The NASA Origins of Solar Systems, Planetary Geology and Geophysics, and
Near-Earth Objects Observations programs funded Bottke's and Nesvorny's
research; Vokrouhlicky was funded by the Grant Agency of the Czech 
Republic.

Editors: Images to accompany this story are available at

http://www.swri.org/press/2007/asteroid.htm .