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More Evidence Points To Impact As Dinosaur Killer

Douglas Isbell
Headquarters, Washington, DC                        March 12, 1998
(Phone: 202/358-1547)

Diane Ainsworth
Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-5011)

RELEASE: 98-42

MORE EVIDENCE POINTS TO IMPACT AS DINOSAUR KILLER 

     Two new impact crater sites in Belize and Mexico add further 
evidence to the hypothesis that an asteroid or comet collided with 
Earth about 65 million years ago, subsequently killing off the 
dinosaurs and many other species on the planet. 

     Researchers Adriana Ocampo of NASA's Jet Propulsion 
Laboratory (JPL), Pasadena, CA, and Kevin Pope of Geo Eco Arc 
Research, La Canada-Flintridge, CA, led an international team that 
discovered the two new sites during a recent expedition sponsored 
by NASA's Exobiology Program and The Planetary Society, Pasadena, CA.  

     "We discovered an important new site in Alvaro Obregon, 
Mexico, about 140 miles (230 kilometers) from the rim of the 
Chicxulub crater.  This crater was formed when a 6-to-8-mile 
diameter (10-to-14-kilometer diameter) asteroid or comet collided 
with Earth," Ocampo said.  

     "The site contains two layers of material, or ejecta, thrown 
out by the impact that flowed across the surface like a thick 
fluid, known as fluidized ejecta lobes," added Pope. "This is the 
closest surface exposure of ejecta to the Chicxulub crater that 
has yet been found and the best example known on Earth from a 
really big impact crater."

     Centered on the coast of Yucatan, Mexico, the Chicxulub 
crater is estimated to be about 120 miles (200 kilometers) in 
diameter.  The impact 65 million years ago kicked up a global 
cloud of dust and sulfur gases that blocked sunlight from 
penetrating through the atmosphere and sent Earth into a decade of 
near-freezing temperatures.  The drop in temperature and related 
environmental effects are thought to have brought about the demise 
of the dinosaurs and about 75 percent of the other species on Earth. 

     The Earth orbits the Sun in a swarm of so-called near-Earth 
objects, whether they are comets or asteroids, yet the science of 
detecting and tracking them is still relatively young.  Only a 
handful of astronomers around the world search for these objects, 
and they estimate that currently only about one-tenth of the 
population of near-Earth objects has been detected.  Chicxulub is 
the only impact event that has been correlated with mass 
extinctions to date.  The site has been dated geologically to the 
boundary between the Cretaceous and Tertiary periods, also known 
as the K/T boundary.

     Local geologist Brian Holland of Punta Gorda, Belize, guided 
the expedition to another new ejecta site about 290 miles (480 
kilometers) from the crater rim.  This Belize site contains tiny 
spheres of altered green glass, called tektites.  Tektites are 
rocks that have been melted to glass by the severe heat of an 
impact.  Expedition member Jan Smit of Free University, Amsterdam, 
noted that the Belize tektites were similar to those found in 
Haiti and northern Mexico.  This finding links the stratigraphy of 
the Belize sites to the more distant Caribbean and Mexican ejecta sites. 

     Alfred Fischer of the University of Southern California, 
Michael Gibson of the University of Tennessee at Martin, and Jaime 
Urrutia and Francisco Vega of the National Autonomous University 
of Mexico helped the team collect 900 pounds (400 kilograms) of 
samples, including drill cores, for paleomagnetic studies.  They 
also collected fossils from the site to help date the deposits and 
add new pieces to the puzzle of what happened at Chicxulub 65 
million years ago.

     Impact ejecta is very rare on Earth, but covers much of the 
surface of Mars because Mars' surface has remained stable and 
unchanged for billions of years, thus preserving debris from these 
rare impact events.  Also, such fluidized ejecta lobes have never 
been observed directly on Earth before and can serve as an 
excellent laboratory for studying the ejecta lobes surrounding 
many Martian craters. 

     "The discovery of these new ejecta sites is very exciting," 
said team co-leader Ocampo.  "It is like seeing a bit of Mars on Earth." 

     The exact nature of these ejecta lobes on Mars remains a 
mystery, Ocampo noted. Some scientists think they were created by 
an abundance of water in the Martian crust, which turned the 
ejecta into a muddy, molasses-like material.  Others suggest the 
fluidized ejecta lobes were enabled by a much thicker atmosphere 
in Mars' early history. As flying ejecta from an impact event flew 
through the Martian atmosphere, it was reduced by friction to a 
very dense, turbulent cloud of debris, which also flowed like 
water.  Study of the Chicxulub fluidized ejecta may help settle 
this debate and shed new light on theories that the Martian 
surface may once have been more hospitable for life.      

     Volunteers who assisted The Planetary Society and the 
scientists in the field have posted their photographs of the 
expedition on The Planetary Society web site at the following URL:

                http://planetary.org

     Information about and images of newly discovered near-Earth 
objects found by JPL's ongoing Near-Earth Asteroid Tracking (NEAT) 
program are available at:           

          http://huey.jpl.nasa.gov/~spravdo/neat.html

     Ocampo and Pope's research was funded in part by the 
Exobiology Program of NASA's Office of Space Science, Washington, 
DC.  NASA's Jet Propulsion Laboratory is a division of the 
California Institute of Technology, Pasadena, CA.

                         -end-