We were reminded recently of DTM’s rich scientific legacy through two events, the donation by Charles and Robert Hunter of an amazingly complete collection of news clippings about the proximity fuze, DTM’s contribution to the war effort during World War 2. Their father, George B. Hunter, worked on the fuze at DTM in 1942.
The second was the sad event of the passing of Ms. Eleanor Gish Crow. Ms. Crow was employed as a “computer” at DTM in the early 1940’s, charged with carrying out mathematical calculations with just pen, paper, and her brain in the pre-computer era. Ms. Crow was the daughter of Oliver H. Gish, a DTM staff scientist from 1922-48. Dr. Gish led DTM’s investigations into the electrical properties of Earth’s atmosphere including electrical phenomena that accompany volcanic eruptions and thunderstorms.
DTM is celebrating its centennial anniversary this year on the Broad Branch Road campus. To commemorate, Janice Dunlap and Shaun Hardy put together two beautiful posters. One lists, decade by decade, the major scientific discoveries arising from DTM research, and the other depicts our role in development of unique scientific instruments.
In science, past discoveries and new instruments often combine to form the building blocks for future discovery. This is certainly true of the ongoing research at DTM. In the 1970s, staff scientists Stan Hart, Al Hofmann and postdoctoral fellow Bill White used some of the first computer-controlled mass spectrometers, built at DTM, to map chemical variability in Earth’s interior. At a time when the theory of plate tectonics was young, they suggested that ancient subduction of oceanic plates deep into Earth’s interior gave rise to the chemical variations they observed in modern volcanic rocks. In the 1980s, Julie Morris, Louis Brown, Fouad Tera and Selwyn Sacks used the new technique of accelerator mass spectrometry to detect radioactive 10Be, formed initially in Earth’s upper atmosphere, in the lavas erupted at convergent margin volcanoes, where subduction of oceanic plates instigates volcanism in the overriding plate. Though present at concentrations of only a few million atoms per gram of rock, finding 10Be in these young lavas proved conclusively that even the surface sediments on subducted plates make it to the 100+ km depths of magma genesis. Steve Shirey and myself, working with postdocs Richard Walker and Graham Pearson, developed techniques in the 1990s that used a mass spectrometer we built at DTM with Louis Brown to determine the age of tens-of-micron-sized mineral inclusions contained within diamonds. With this, we could track the timing of ancient subduction of sediments deeper into the mantle. Shirey summarized these results, along with other fascinating insights into deep Earth chemistry extracted from inclusions in diamonds, at his recent packed-house Neighborhood Lecture. Just last year, DTM postdoc Matt Jackson working with staff scientist Erik Hauri and colleagues used a high-resolution ion microprobe to measure the sulfur isotopic composition of micron-sized sulfides in a recent volcanic rock from the central Pacific. They showed that the sulfur incorporated in this mineral had been a gas high in Earth’s atmosphere over 2.5 billion years ago, but had spent the time since migrating around Earth’s interior carried along with the slow convective motion of the mantle, confirming the thesis of Hart, Hofmann and White.
Off the Earth, Vera Rubin used an image intensifier designed by Kent Ford in the 1960s to show that the rotation rate of distant galaxies did not fall off as expected with distance from the galaxy center. In the process, she provided the key evidence for the existence dark matter, which along with dark energy constitute something like 95% of the total content of the universe. Paul Butler and colleagues in the 1990s designed an instrument that when fitted to a telescope can resolve minute variations in the velocity of distant stars relative to Earth and hence detect the “wobble” due to the gravitational tug of orbiting planets. With this instrument, Butler discovered not only that planetary systems around other stars are common, but that many are quite different from our own, for example with Jupiter-sized planets orbiting their star closer than Mercury orbits the Sun. In just the last year, Scott Sheppard has been using the tremendous light-gathering capabilities of the Magellan telescopes to search for faint objects in our Solar System. He has found both active comets in the asteroid belt and large objects in the Kuiper Belt whose orbital characteristics suggest the presence of a yet unseen planet, perhaps one much larger than Earth.
In 1996, John Horgan published a book entitled “The End of Science” whose main thesis was that everything important has already been discovered. He is not the first to conclude this. In 1900, more than a decade before the discovery of the proton, Lord Kelvin (famous for a number of major discoveries, but also for his 20-400 million year age estimate of the Earth, which is now known to have formed from 4,450 to 4,568 million years ago), is quoted as saying, “There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.” We are doing our best to prove both wrong. Resolving the dynamics of the solid Earth that operate over billion-year time scales, discovering the evidence for dark matter, planets around other stars, and the possible impending discovery of a large, distant, planet in our own Solar System suggests that we are doing a pretty good job of it!
Director, Terrestrial Magnetism
Carnegie Institution for Science