K-T Asteroid is Smaller than Previous Estimates? (2.5 - 3.7 miles versus 9.3 - 12 miles in diameter)

University of Hawaii at Manoa geology and geophysics associate professor Greg Ravizza, left, and doctoral student Francois Paquay developed a method to use osmium isotope records from ocean sediments to determine the impact sizes of meteorites (Source: Star Bulletin).

Summary of the article appears below (as well as link to Science article)...

A University of Hawaii doctoral student has found that the huge asteroid crash believed to have wiped out the dinosaurs 65 million years ago was only about half the size of previous estimates.

Francois Paquay developed a method using osmium isotopes in deep-ocean sediments to determine sizes of meteorites that have collided with the Earth.

The asteroid believed to have created the Chicxulub Crater buried under Mexico's Yucatan Peninsula and caused the dinosaurs' extinction had been estimated to be 9.3 to 12 miles in diameter from crater simulations.

But by Paquay's calculations, the asteroid was about 2.5 to 3.7 miles in diameter.

Paquay's research with his adviser Gregory Ravizza was reported in yesterday's issue of the journal Science.

"UH student downsizes cataclysmic asteroid: Deep-sea sediments yield insight into the end of the dinosaurs"

Helen Altonn
12 April 2008
Star Bulletin

Link: Star Bulletin Article

Link: Space.com article

Link: NewScientist Article

Link: National Geographic Article

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Science 11 April 2008:
Vol. 320. no. 5873, pp. 214 - 218
Reports

Determining Chondritic Impactor Size from the Marine Osmium Isotope Record

François S. Paquay,1* Gregory E. Ravizza,1 Tarun K. Dalai,1+ Bernhard Peucker-Ehrenbrink2

Decreases in the seawater 187Os/188Os ratio caused by the impact of a chondritic meteorite are indicative of projectile size, if the soluble fraction of osmium carried by the impacting body is known. Resulting diameter estimates of the Late Eocene and Cretaceous/Paleogene projectiles are within 50% of independent estimates derived from iridium data, assuming total vaporization and dissolution of osmium in seawater. The variations of 187Os/188Os and Os/Ir across the Late Eocene impact-event horizon support the main assumptions required to estimate the projectile diameter. Chondritic impacts as small as 2 kilometers in diameter should produce observable excursions in the marine osmium isotope record, suggesting that previously unrecognized impact events can be identified by this method.

1 Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822–2225, USA.
2 Department of Marine Chemistry and Geochemistry, Woods Hole Oceano-graphic Institution, Woods Hole, MA 02543, USA.
+ Present address: Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, India.

Link: Science Magazine Article