Scientists at The University of Texas at Austin Sunday (Dec. 17) will present a report offering new geophysical clues to a cataclysmic event that may have killed off the dinosaurs.

This report on Mexico's Chicxulub crater will be presented by Dr. Gail Christeson, a research associate at UT Austin's Institute for Geophysics (UTIG), at the fall meeting of the American Geophysical Union in San Francisco. UT Austin's involvement in the project was sponsored by the National Science Foundation.

The Chicxulub structure was formed 65 million years ago when a large celestial body -- a comet or an asteroid -- slammed into the Yucatan Peninsula with a force that makes a nuclear blast seem like a firecracker.

The impact produced fires, acid rain and tsunami-like destructive waves. The collision gouged a crater nearly eight miles deep and sent 12,000 cubic miles of rock, dirt and debris spinning into the earth's atmosphere. The material blocked the sun, causing extreme changes in the Earth's climate, which many scientists believe resulted in mass extinctions.

The collision marked the abrupt end of the Cretaceous period in geologic time and the start of the Tertiary period. And many scientists currently believe that the event wiped out 80 percent of all living species in the ocean. It also may have destroyed many terrestrial species, including the dinosaurs.

Christeson and UTIG senior research scientists, including Dr. Richard T. Buffler and Dr. Yosio Nakamura, worked with an international team of scientists to survey the Chicxulub crater, which remains as an unusual circular feature buried beneath 1,000 meters of sediments under the northern Yucatan Peninsula and the Gulf of Mexico. Co-authors on Christeson's presentation are Jo Morgan and Mike Warner from Imperial College in London, and Colin Zelt from Rice University in Houston.

The aim of the researchers was to determine the Chicxulub crater's actual size and to characterize its internal structure. Such details should make it easier for scientists to understand how the crash actually could have caused mass extinctions. It should also allow them to assess the present-day risk posed by the thousands of comets and asteroids that cross earth's orbit.

The team collected seismic reflection, refraction, gravity and magnetic data over the crater. This research has provided the first direct evidence of a crater with the multi-ring basin shape that is typical of the largest impact craters on the moon and Venus.

The impact was so enormous it changed the shape of the earth's crust -- 22 miles below the surface of the planet. The Chicxulub crater is the first location where deformation at the base of the crust has been found in a terrestrial impact crater.

The scientific team concluded that the Chicxulub crater is about 125 miles in diameter, and that 12,000 cubic miles of debris was blasted out of the earth by the impact. The impact carved out a cavity about 7.5 miles below sea level. Mount Everest, in comparison, is 5.5 miles high. Prior to this research, the size and morphology of the Chicxulub crater had been in dispute, with estimates of its diameter ranging between 180 and 300 kilometers.

Such a large discrepancy in size translates to a factor of ten differences in the energy of the impact with quite different consequences for the Earth's environment. The energy released by the impact that blew out the Chicxulub crater was equivalent to about 100 million megatons, many orders of magnitude greater than the nuclear explosion at Hiroshima, a 15-kiloton blast.

To collect the seismic data in the Gulf of Mexico, the scientists deployed an array of Ocean Bottom Seismograph (OBS), instruments which had been developed at UT Austin's Institute for Geophysics for undersea projects such as this one. The OBS instruments were deployed from the UT Austin Marine Science Institute vessel RV Longhorn based out of Port Aransas.

Additional analysis of the OBS data revealed that a region at the center of the crater about 22 miles in diameter has been uplifted by about 11 miles as a result of the impact and removal of overlying material.

During the AGU meeting, Christeson will be a panelist at a press conference organized by AGU on large impact events.

During routine scanning with the Spacewatch 36-inch telescope on 2000 November 28, observer R. S. McMillan was manually blinking the displayed scans in real time and noticed this relatively slow-moving object. Its rate of motion is too slow for the real-time software to detect; normally the slower objects such as this one are found with another computer program that processes the data off-line.

Lunar meteorite ages present new, strong evidence for the "lunar cataclysm," a 20-to-200 million-year episode of intense bombardment of the moon and the Earth at 3.9 billion years ago -- when the first evidence of life appeared on Earth, planetary scientists report in the Dec. 1 issue of Science.

Whether or not there was life on Earth at the beginning of the bombardment, such cataclysmic pounding would have enormous consequences for life on this planet, whether by destroying existing life or organic fragments or by delivering molecules and creating conditions suitable for life, the researchers add.

Barbara Cohen of the University of Tennessee -- Knoxville analyzed the lunar meteorite ages for her dissertation research at the University of Arizona in Tucson. Timothy D. Swindle and David A. Kring of the UA collaborated on the study and are co-authors on the Science article. Swindle supervised Cohen's research. Kring is an expert in impact cratering and one of the discoverers of the K/T boundary Chicxulub impact site.

Moon rocks returned by the Apollo and Luna missions in the 1970s suggested that Earth's moon was blasted in a maelstrom of solar system debris at 3.9 billion years ago. A great swarm of asteroids or comets pounded the lunar surface during a brief pulse in geologic time, melting rocks, excavating vast craters and resurfacing Earth's natural satellite.

But for safety and communications reasons, both manned and robotic spacecraft were landed near the moon's equator, on the side facing Earth. No one could say if just this part of the moon or the entire moon had been hammered.

Cohen, Swindle and Kring bring the most significant data in nearly 30 years to bear on this question. They used an argon-argon dating technique in analyzing impact melt ages of four lunar meteorites -- rocks ejected at random from the moon's surface and that landed on Earth after a million or so years in space. They find from the ages of the "clasts," or melted rock fragments, in the breccia meteorites that all of the moon was bombarded 3.9 billion years ago, a true global lunar cataclysm.

Further, although the moon may have been bombarded before 3.9 billion years ago, the scientists find no evidence for it. If there were no earlier bombardment, scientists must jettison theoretical models that assume a steady falloff in the lunar and inner solar system cratering rate through time.

"Given the model of what was going on in the solar system, there is no obvious reason why you should suddenly have a bunch of things banging on the moon 4 billion years ago and not 4.2 billion years ago," Swindle said.

But the most dramatic implication is what happened during this event on Earth.

"The Earth is a much bigger target than the moon, " said Kring, associate professor at the UA Lunar and Planetary Lab. "Earth would have been bombarded by at least 10 times as many impact events as the moon, and these impact cratering processes are immense. The Chixculub crater that we identified, which is related to the mass extinction of dinosaurs and other life 65 million years ago, is puny by comparison to the scheme we are talking about. Here we are talking about impacts that are 10 times larger, impacts that blasted craters rim-to-rim the size of continents on Earth today."

"The bombardment would have charged the atmosphere with silicate vapor and vaporized the oceans, so if there was life on Earth before the bombardment, the question is what, if anything, survived," Swindle said. Perhaps some genetically primitive "extremeophiles" survived, he added. This kind of life is found on Earth today deep in rocks or living at the ocean vents.

What did the bombarding? More likely asteroids than comets, based on some evidence from meteoritic trace constituents involved in the impacts and on other studies on what was happening at the time in the asteroid belt, Kring suggests.

"When we first started this research, the goal was to find something older than 3.9 billion years," Cohen said. "We were very surprised at the evidence presented by seven different impacts, which pointed to 3.9 billion years."

Swindle said, "Going into this study, I would have bet that we wouldn't have found these results. I would have bet that we would have seen impacts earlier than 3.9 billion years ago."

Kring said, "I've quit being surprised at what impact cratering processes can do."

Kring, director of the Lunar and Planetary Lab's Space Imagery Center, has just added new web pages on impact cratering, the lunar cataclysm and origin of life, the moon and lunar meteorites at the Space Imagery Center web sites.

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Ordinarily the discovery of a 20th-magnitude blip wouldn't be much cause for excitement, but the one found early on November 28th is a special case. If preliminary calculations are borne out by further observations, the object now designated 2000 WR106 may prove to equal the size Ceres, the largest asteroid. Located about 1.5 deg. south of the star Epsilon Geminorum, the new find was spotted first by Robert S. McMillan and later by Jeffrey A. Larsen (University of Arizona) with the 0.9-meter Spacewatch telescope on Kitt Peak. They noticed its shifting position by eye in a computer display of successive frames -- the motion was too slow to be picked up by Spacewatch's automatic-detection software.

The object's actual size remains very uncertain in part because astronomers aren't yet sure of its distance from the Sun. Right now the best estimate is 43 astronomical units (6.4 billion kilometers), which means it is a Kuiper Belt object (KBO) beyond the orbit of Pluto. An object this far away takes nearly three centuries to circle the Sun, so astronomers will need to observe it over many weeks or months for its motion to betray the orbit's true character. However, according to Brian G. Marsden (IAU Minor Planet Center), the assumed distance is unlikely to change very much.

Another unknown is the albedo, or reflectivity, of the body's surface. If 2000 WR106 is bright, like Pluto or Charon, then its diameter might not exceed 250 km, something akin to Vesta in size. But just the opposite might be true. "Many people think KBOs have albedos closer to comet nuclei -- very dark," William J. Romanishin (University of Oklahoma) told members of the Minor Planet Mailing List. In that case, the Spacewatch discovery could exceed 1,200 km in diameter. Ceres, discovered almost exactly 200 years ago, is roughly 950 km across.

Apparently the object has escaped detection until now because it spent many years lurking among the stars of the northern Milky Way. With modern electronic detectors, it is actually within the detection range of many backyard telescopes.