What Hit Odessa? UT Researcher Computes the Origin of the Odessa Crater

Odessa, Texas, received its ironic name in 1881 from showed that early estimates of the size and trajectory Russian railroad laborers, who must have longed for of the Odessa meteor were wrong and that a much the temperate Black Sea resort in their hot, prairie larger object coming in at a nearly grazing angle had exile. The Texan Odessa lies in the Permian Basin, site made the crater. “Our work has both validated the of some of the country’s richest oilfields. About eight code we used and enabled us to reconstruct the actual miles southwest of the town (which is now, thanks to event,” Littlefield says, “and it is also an interesting oil, a metropolis of nearly 100,000), the barren pas- story about the progress of science.” tures boast the second-largest meteor crater in the United States. Old Data Await New Methods

Chances are that your personal catalogue of meteor Drilling to investigate the crater and four surround- craters begins and ends with the largest meteor crater, ing smaller craters, begun in 1939 by geologist Glen the Great Crater near Winslow, Arizona, also called Evans, was completed in the 1940s by E. G. Sellards the Barringer Crater or the Canyon Diablo crater, of UT Austin’s Bureau of Economic Geology. A shaft which is three quarters of a mile across and 600 feet was sunk in the center, in hopes of finding a large deep. The site has been open to the public since the core remnant of the nickel-iron meteorite, but only 1920s, astronauts trained to walk on the Moon there, fragments were discovered, and the diggers reluctant- and its picture has become the canonical crater photo. ly concluded that the collision had caused it to dis- integrate. Core samples from the excavations helped The Odessa Crater is far smaller, only 160 m (175 delineate the stratigraphy of the impact area, and the yards) in diameter, and the original depth of 30 m data found their way to the Texas Memorial Museum (~100 feet) is now only 2 m owing to the gradual ac- on the UT Austin campus. An artist’s rendition of a cumulation of sediments since the meteor struck an cross section through the center of the crater (Fig. 1) estimated 25,000 years ago. With its rocky rim ris- was exhibited by the museum, but it was only in 2000 ing another 2 to 5 m above the plain, it is far from that Evans and Charles Mear published a detailed unimpressive--but still, few have heard of it. Until five paper on the geology--in a museum bulletin issued by years ago, the site was basically uncared-for. Baylor University.

What hit Odessa? How big was the meteor and what Littlefield and Bauman used the original stratigraphic was the impact like? Answers have been a long time data, which were supplied by Dr. Ann Molineux, cu- in coming. The details are only now emerging, thanks rator of non-vertebrate paleontology at the Texas Me- to new computer simulations, performed at the Texas morial Museum. Molineux believes that Littlefield’s Advanced Computing Center (TACC), based on early subsequent calculations at TACC validate the fun- data from the site. “This crater is finally getting some damental museological idea. “The work David and respect,” says Dr. David L. Littlefield of the Institute Paul did is a wonderful example,” she said, “because for Computational Engineering and Sciences at The it shows how old data are never exhausted--data that University of Texas at Austin (UT Austin), who carried have been saved can be re-used in new contexts at out the calculations together with graduate student later dates, when more is known or better methods of Paul T. Bauman. analysis are developed.”

The simulations of the impact, done on TACC’s As Littlefield points out, it was not only new methods 224-processor IBM Power4 cluster, called Longhorn, that were developed, but also a revolution in scientific

Texas Advanced Computing Center | Feature Story For more info, contact: Faith Singer-Villalobos, Public Relations, [email protected], 512.232.5771 Page 1 of 3 thinking about the Earth’s encounters with meteors. meteor craters, but not around volcanoes. The idea Because other geologic processes tend to erase the evi- that a meteor had been responsible for ending the dence, meteor craters are not the obvious suspects in dominance of dinosaurs some 65 million years ago, the constitution of the Earth’s crust. Yet there was an as it gained acceptance, completed the transformation epoch when the Earth must have been bombarded by of our thinking. The Earth was not a protected corner meteors as often as--or more often than, as it presents of the Universe, but subject from time to time to the a larger cross section to meteors--the Moon. Tectonic most catastrophic events. processes have disposed of those craters, and since the current geological epoch began, the Earth has New Calculations in a New Context suffered only occasional hits. Most have gone into the oceans, which cover 6/7 of the Earth’s surface. Those Littlefield has been at UT Austin since 1996, using that hit in the tropics were quickly overgrown. computational methods for predicting the effects of high-velocity impacts in solid and fluid media. He has Very few meteor falls have occurred throughout computed at TACC for many years. “We deal with human history. When Yale professor Benjamin Silli- problems that have high strain rates, large deforma- man and a colleague authenticated a fall in Weston, tions, and strong shocks--mostly highly transient Connecticut, in 1807, President Thomas Jefferson--an events,” he explains. Much of his funding has come Enlightenment scientist himself--was extremely skep- from the Department of Defense, for investigations tical. “I would rather believe two Yankee professors of the effects of explosions or of the penetration of would lie,” he reportedly said, “than that stones have armor by projectiles. “Planetary impacts are a new fallen from the heavens.” Arguments among geolo- departure, but they are also of greater interest in the gists over whether crustal features were volcanic or context of the new ‘catastrophism’ of astronomy and meteoric in origin were often heated, and astronomers geology,” he says. seemed to have little interest in meteors once they had landed. By the 1950s, the world list of craters con- When he learned that detailed geological data were tained only about 20 sites. available for the Odessa crater, including measure- ments of the crater and rim and a stratigraphy of the Thanks to efforts made by some astronomers, how- layers of soil and rock that the meteor hit, Littlefield ever, the list was gradually lengthened. The pioneers saw an opportunity to test his computational codes included Robert Dietz in the United States and Car- and validate them in a new regime, with a broader lyle Beals in Canada, and a second generation led by range of input parameters. Robert Grieve in Canada and the late and very well known Eugene Shoemaker in the United States. By Littlefield and Bauman used CTH, which he describes 1989, then, meteoritics expert H. Jay Melosh of the as a code embodying “Eulerian finite-volume con- University of Arizona could look at a list of about 150 tinuum mechanics,” in which calculations are made known craters and write, “As recently as 1950 most on a mesh of points representing the physical domain. astronomers believed that the lunar craters were gi- CTH is widely used in the academic, government, and ant volcanoes and all but a few geologists derided industry communities for high-velocity impact analy- the idea that the earth’s surface had been scarred by sis. “What we needed for input was material models impact structures kilometers in diameter. . . . geo- of the constitution of the meteor and of the surface it chemists are just now beginning to realize that nearly impacted,” Littlefield says, “plus some guesses about all of the material now residing in planets has been the size and speed of the meteor and the angle of processed through high-velocity impacts.” (Melosh, attack.” The output was a redistribution of the mate- 1989: Impact cratering: A geologic process, Oxford rial in the model, showing the crater and the material Monographs on Geology and Geophysics, p. v.) ejected from the site, with gaseous material removed.

The plate tectonics revolution of the 1960s and the Littlefield and Bauman’s preliminary calculations trips to the Moon helped validate this view, Littlefield used impact conditions suggested by the geological notes, as did a new means of distinguishing between studies: an iron meteor 4 m in diameter coming in at volcanoes and impact craters: shocked quartz crys- an angle of 60 degrees from the vertical at a velocity tals are found in the ejecta and in the breccia under of 12 km/s. These could not produce a crater volume

Texas Advanced Computing Center | Feature Story For more info, contact: Faith Singer-Villalobos, Public Relations, [email protected], 512.232.5771 Page 2 of 3 great enough, or one of the right dimensions, so larger than originally believed. “As we worry about further runs were made. “We used a lower bound of the near-Earth and Earth-crossing asteroid orbits,” he 12 km/s for the velocity and an upper bound of 26-27 says, “we need to know what effects to expect from km/s,” Littlefield says, “with the original meteor -di a hit the size of Odessa--which probably occurs with ameter of 4 m and a larger diameter of 6.87 m, coming much greater frequency than the dinosaur-killing in at a lower angle, 77 degrees from vertical.” impacts.

These calculations also failed to reproduce the Odessa “We needed the speed of a machine like Longhorn at crater morphology, and in particular the evidence TACC to do these calculations at all,” Littlefield notes, that the meteor dug up and turned over about 7 m of “and many of our runs used 64 processors at a time. the subsurface in creating the crater rim. Finally, the I think we could easily scale the calculations to more calculations were repeated with much larger meteor processors on faster machinery.” diameters (9 and 15 m) and a near-grazing angle of 84 degrees from the vertical. These inputs produced a The mythic giant jackrabbit of Odessa may have been model crater of the right shape, although still smaller the only witness 25,000 years ago (although the dig than the Odessa crater (Figs. 2 and 3). did turn up the bones of a Pleistocene mammoth), but Littlefield’s simulations can inform our present won- “We’re homing in on the answers,” Littlefield says, der--and our hopes never to be near such an event! as the calculations continue. “What we have shown is the great sensitivity of the code to the variation of As for the crater itself, interest is also increasing. Dal- inputs and thus the need for accurate material mod- las attorney Tom Rodman took its part and shepherd- els to take best advantage of the code’s flexibility. ed it into the modern era: a museum was opened at Because we have an answer to shoot for in the case the site in 2000, and the land is now in the care of the of Odessa, at least in terms of the size and shape of Park Service. Molineux of the Texas Memorial Mu- the crater and the amount of material upended and seum will incorporate a movie of the new computer ejected, we can come a lot closer to a better estimate of simulations into an educational CD; contact her for the velocity and trajectory of the meteor at impact.” more information [email protected].

Littlefield and Bauman have already shown that the meteorite impact energy was more than 50 times

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