In 1936, as a young
girl, Carolyn Shoemaker first saw Meteor Crater near Flagstaff, Ariz. She was
awestruck. Years later, she would visit the site again, this time as a newlywed,
with her husband Gene. Gene Shoemaker at Meteor Crater in Arizona in the mid-1960s. Image courtesy of Astrogeology Team, U.S. Geological Survey, Flagstaff, Ariz.
“We were on our way home to Grand Junction in Colorado, coming through Flagstaff, and I said, ‘Gene, you really ought to look at that crater. We have just enough time,’” Carolyn fondly recalls.
At the time, in 1952, the true origins of this crater were still unknown. The name itself referred only to pieces of meteorites found in the surrounding area. It was Gene’s first time visiting the crater. And at first sight, he was sure he knew how it formed, Carolyn says. He knew that it was the work of a meteorite. “I remember the feeling of wonderment that something could come down and hit Earth and make a hole that big,” she says.
That moment, looking out over the enormous hole in the ground, was a defining one for the Shoemakers. In many ways, it was the start of a lifelong journey to understand impacting bodies and the scars they leave behind.
Now 74 and an astronomer at the Lowell Observatory in Flagstaff, Carolyn has discovered more comets than anyone else alive today. Under the tutelage of her late husband, Carolyn learned how to identify these objects both in the sky and on the ground. Her foray into science began in 1980 at age 51, but she says that she slid into astronomy easily with Gene’s help. “He was a great teacher, always,” she says. “He loved to explain things and he didn’t mind explaining them many times over if need be,” she says, laughing.
Although Carolyn’s focus is on sky observation, she clocked many hours in the field with Gene to characterize and understand impact structures. “I was interested in seeing the places where these bodies that I study in the sky end up. And so I started going out with him.” Investigating structures from Meteor Crater to dozens of craters in Australia, the Shoemakers were a revolutionary force in the field of impacts.
Although Carolyn was not involved directly in Gene’s research on Meteor Crater, she knows the history well. When they visited Meteor Crater in 1952, the only person on record convinced of the extraterrestrial origin of the 1.2-kilometer-wide crater was Daniel Moreau Barringer, a mining geologist who owned the crater land. Instead, most scientists believed the opinion of Grove Karl Gilbert, a legendary 19th-century geologist and explorer, who looked for — but did not find — a large mass of iron meteorite beneath the crater to confirm its meteoric origin. He concluded the crater must be volcanic in origin.
“This was before anyone really knew just what happened when something comes down and hits the ground like that,” Carolyn says. “In any crater that size or larger, you’re not going to find one great big meteorite; it’s going to disperse and break up and melt.”
When Gene returned to conduct research at Meteor Crater, he was hoping to find a comparison with the nuclear blast craters he was studying at the Nevada Test Site. He hypothesized that Meteor Crater would just be a scaled-up version of those craters. Based on his fieldwork there and working with colleagues Beth Madsen and Edward Chao, he published an article in Science in July 1960, providing evidence that Meteor Crater was indeed the result of an impact.
This work, followed shortly thereafter by a surprising impact finding in Germany and by research on lunar craters, spurred a new worldwide search to find other impact sites. Today, 40 years later, scientists have identified about 170 craters and structures formed by impacts throughout the world. “It’s not that other craters or structures had not been found; they had, but for a while they were called cryptovolcanos, because they were different from your regular volcano, but it wasn’t quite understood that they were caused by an asteroid or a comet hitting Earth,” Carolyn says. “That took a while to work out.”
In fact, it wasn’t until the 1980s that people began to believe more widely that these strange structures were the result of impacts from space, Carolyn says. About that time, in 1984, Carolyn and Gene began what was to become a more than decade-long tradition of visiting the Australian Outback every summer to search for impact structures.
Australia, Carolyn says, was an ideal spot for their research for several reasons. The rocks there start off at the same age as the oldest rocks at the bottom of the Grand Canyon. “And if you want to learn about what happened in an earlier period in our world, you go to where you find the oldest rock,” she says. Additionally, large sections of Australia are relatively undisturbed by large-scale weather and tectonic events. But, there was another incentive too, she says. “Gene always said, ‘It’s an ideal place for an old geologist,’” Carolyn says, laughing. The lack of topographic relief made navigating the outback by foot or car a breeze.
Over the 13 years they spent studying impacts in Australia, they identified 22 structures. Carolyn says that it was an intense time of both learning and discovery. They would search for distinct impact indicators — impact glass, from the melting of the impacting body with the surface rock, and broken rocks with shocked quartz. And they’d map the structure itself. “Gene always said you could tell an impact structure just from the structure itself, even if you didn’t find any remnants of the impacting body,” she says.
Each structure, she says, presented a new experience. “The knowledge that each one was different, so that if I had to pick a favorite, I’d have a hard time … that was special,” Carolyn says.
She was amazed to see the varying paths that an asteroid or comet could take in creating the unique spots they visited. “It tells you about what was going on in the world at the time that those craters or structures were formed,” she says, for example, whether the collision occurred on land or sea, and whether life on Earth existed at that time. It was storytelling at its finest.
Carolyn remembers camping up on a ridge above the Henbury craters near Alice Springs in the center of Australia. Overlooking this field of strewn craters, she would imagine an object coming over her head and landing in the field, using what they knew about the direction the object was heading. “I pictured in my mind just what was happening,” she says.
The Shoemakers’ trips to the outback coincided with their work at the Palomar Observatory in California, where Carolyn discovered a record-breaking 32 new comets. It was the perfect marriage of in-sky and on-Earth studies. “When I am observing comets and looking at the sky, I think about the fact that impacts represent one of the biggest processes throughout the solar system, throughout the universe,” she says. “And even though distances are enormous, things hit other things and our solar system was probably built upon collisions of small bodies hitting larger bodies and becoming part of it. It’s totally awesome.”
Indeed
in the summer of 1994, the Shoemakers witnessed just such a process in action.
A year after they and colleague David Levy discovered a new comet (later named
Shoemaker-Levy 9), they watched it smash into Jupiter, leaving Earth-sized scars
still seen today. “We could look at that and say yes, for the first time,
we have absolute proof that bodies like comets or asteroids do impact planets,”
she says.Carolyn and Gene Shoemaker watch the first Hubble Space Telescope image of the Shoemaker-Levy 9 comet impacting Jupiter in 1994 — the first time people witnessed a comet impact a planet. Photo by John Aiello, NASA/JPL.
If something could hit Jupiter, then something could hit Earth, Carolyn says. And, in 1998, Congress agreed, asking the scientific community to discover all of those near-Earth objects 1 kilometer in size or larger within 10 years (see page 5). “We’re not going to reach that point, but we’ve discovered a lot — thousands of bodies, with lots of them asteroids, but quite a few comets,” she says. “So far, we haven’t found anything that could impact Earth in less than 100 years.”
The next big discoveries in astronomy, she says, are going to come from the Kuiper belt (beyond Neptune’s orbit) and determining what role the bodies there play in the history of Earth’s solar system. The big discoveries in Earth-based studies of impact structures are going to come in the area of mass extinctions, she thinks. “We need to learn a lot more about the characteristics of the impacts to see if we can tie them to the disappearance of life.” Also, she says, studying impact structures here on Earth will help researchers learn more about what to look for on other planets.
Right now, Carolyn is taking a break from her sky observing, although she plans to return soon. She is working on pulling together the Australian field work. “Gene always said that when he retired, he could do the maps and reports,” she says. “Well, he didn’t get to the maps and the reports, except in a very small fashion.” Gene died in a car crash near Alice Springs in 1997, while they were working in the field.
“I think it’s important that that work be published,” she says. “It’s important that we describe the discoveries that we made in each place so that people can carry on with that as a basis for more work.”
