Obituary: Eugene Shoemaker

Eugene Shoemaker was acknowledged by most scientists as the father of planetary geology.

Shoemaker discovered that meteorite and cometary impact is a major process in the solar system. This discovery revived the largely discarded geological concept of catastrophism, which holds that short-term catastrophic events help shape the surface of the planets and on Earth have contributed to species extinction, and thus to evolution. For this concept, Shoemaker will be remembered as one of the more important geologists of the 20th century.

The recognition of impact as a significant geological process allowed the establishment of the widely held hypothesis that a major meteorite impact caused the extinction of dinosaurs and many other life forms, and that major impacts threaten the earth in the future - a matter of concern today both to civil defence authorities and screenwriters.

Shoemaker was heeded by both sides during the Cold War, when he warned that a large impact could be mistaken for a nuclear explosion. His articulation that impact is a continuing process was graphically demonstrated to the world in 1994, when the comet Shoemaker-Levy, discovered by Shoemaker and David Levy, broke into pieces that crashed spectacularly into Jupiter.

Gene Shoemaker was concerned with both the process of impact and the population and flux of objects in the solar system (asteroids, comets, and meteoroids) that can impact planets, especially the Earth. To this end, he studied the geology of, and counted the density of, impacts on the Earth and the Moon, and looked for comets and asteroids through the telescope and calculated their orbits. As a team, he and his wife and co-worker, Carolyn, discovered 32 comets and 1,125 asteroids, which is a record.

Shoemaker was interested in the Moon at an early age and his ambition was to be a geologist-astronaut on the Moon, but Addison's Disease prevented this, much to his chagrin. He was giving talks on the geology of the Moon in the late 1950s, although many other geologists considered this odd, to say the least.

He realised that the geology of the lunar surface could be interpreted in terms of stratigraphy, that is in terms of sequence and correlation, one layer upon the other, and very early recognised the importance of impact and volcanism in surface processes. He began the systematic mapping of the Moon by telescope. Using impact crater density and assumed impacting body fluxes, he established a method of determining relative age and approximate absolute age for the lunar surface. His methods of planetary geology are used today in studying other bodies in the solar system.

Shoemaker established the Branch of Astrogeology at the US Geological Survey in Flagstaff, Arizona; its buildings and their occupants are a living memorial to him. He was involved in the National Aeronautics and Space Administration (Nasa)'s early unmanned lunar exploration, and headed the Lunar Geology Team for the lunar landing on Apollo 11. He resigned that position from subsequent Apollo missions because he was unhappy with the relative lack of emphasis by Nasa on science. This was courageous, as studying the Moon was his great love. During the Apollo programme, Shoemaker played an important role in training the astronauts before they went to the Moon.

Gene Shoemaker was active in many other areas of geology (Colorado Plateau geology, uranium deposits, paleomagnetism and others), and in each of these he would have been distinguished had he done nothing else.

Shoemaker was born in Los Angeles in 1928. He received bachelor's and master's degrees in geology from the California Institute of Technology (Caltech) in 1947 and 1948 respectively, and a PhD from Princeton in 1960. His professional life was spent with the US Geological Survey (USGS), except for three years as Chairman of the Division of Geological and Planetary Sciences from 1969 to 1972. He retired from the USGS in 1993, but the retirement was only in name.

Many honours, medals, and awards deservedly came to him. Some of the more important are the US National Medal of Science (the highest scientific award in the US), the Bowie Medal of the American Geophysical Union, several honorary doctorates from US universities, membership in the US National Academy of Sciences, and the Day Medal of the Geological Society of America.

Gene Shoemaker was unusual among scientists in that he took time to educate the public about the excitement of planetary geology and the impact process through highly articulate lectures and television interviews. He also was a wonderful mentor to students; he was just as happy talking science with a young student as with a distinguished colleague. He was an excellent scientific supervisor when it came to enthusiasm, inspiration, and critical review, but left a lot to be desired when it came to the pushing of papers and observing bureaucratic protocols. He was loved for this failing as he was for his inspiration of others.

Shoemaker was eminently decent and simple. His scientific ethics were an example to all, and his wonder at, and the fun he got from, his science were infectious. Most things were fun to him, even, with his wife, building his house in Flagstaff from volcanic rock. He laughed at many things and he clearly enjoyed the magnificent trip he was on in Australia when he was killed. The vehicle he was driving crashed into a truck in desolate country north-west of Alice Springs. (His wife was injured and is recovering in the Alice Springs Hospital.)

His modesty, exuberance, and warmth endeared him to his colleagues around the world and the thousands who knew him through his lectures.

A memorial service will be held on the rim of Meteor Crater, Arizona, the impact structure on which he did his first definitive work. The date has not been determined.