July 2019 Letter from the Director
DTM in Space
On July 21, 1969 at 2:56 UTC, Neil Armstrong became the first human to set foot on the Moon. In July 2019, the world and DTM celebrated the 50th anniversary of this remarkable achievement. At DTM, we reflected on how much we learned about the process of planet formation as a result of the study of lunar samples. Our former Director, George Wetherill, was one of the early proponents of the giant impact model for Moon formation based on his theoretical studies of the mechanisms of planet growth. Our departed colleague, Erik Hauri, developed the analytical techniques that allowed us to finally measure the abundance of water in lunar volcanic rocks, dispelling the decades-old belief that the Moon’s interior is completely devoid of water. My own work focusses on determining the age of the lunar crust and the timing of the magma ocean event that separated crust from mantle. As I emphasized in my review article published in Science magazine’s special edition for Apollo 11’s 50th anniversary, the analysis of the returned lunar samples dramatically changed our thinking about planet formation from one characterized by slow, gentle growth and cold starting conditions, to one dominated by huge, violent impacts that melt large portions of the planet. In the case of the Earth, the impact was large enough that it ejected enough material into orbit to reassemble into a largely molten initial state for the Moon.
Perhaps the strongest argument in support of planetary sample return missions is recognition that the paradigm shift in our understanding of planet formation originated from the presence of a small number of grains of anorthite feldspar in the soil samples collected by Apollo astronauts Armstrong and Aldrin. All prior thinking about the Moon’s origin was forever changed by that first inspection of lunar dirt. The study of the returned lunar samples that followed these initial analyses then provided a level of detail about the Moon’s history and origin that to this day would be impossible to deduce by remote observation or robotic landers. Though robotic missions are improving dramatically in their capabilities, and orbital analyses provide the better global view, neither can compete with the level of analysis possible from samples returned to Earth-based laboratories like those at DTM. As an example of the continuing need for lab-based analysis of planetary materials, this summer we are pleased to host the visit of Professor Tetsuya Yokoyama of the Tokyo Institute of Technology. Professor Yokoyama is the team leader for the Hayabusa2 wet geochemistry analysis team. He is working in the DTM isotope laboratory to perfect the chemical separation and mass spectrometry techniques that will be used to analyze the samples of the Ryugu asteroid that will be returned in 2020 by the Japanese Hayabusa2 mission. DTM staff scientist Larry Nittler and I are honored to have been selected by the Hayabusa2 leadership to participate in the analyses of the Ryugu samples after they reach Earth.Top (L): From left: Molly Tatel, Merle Tuve, Winifred Whitman, Judy Tatel, and David Tatel at the dedication of the Howard E. Tatel Telescope, October 16, 1958. Credit: DTM archives. Bottom (L): From left: Meredith MacGregor, Edith Tatel, David Tatel, Rick Carlson, Alycia Weinberger, Alan Boss, and Frank Drake, former Director of the Carl Sagan Center for the Study of Life in the Universe, in front of the Tatel Telescope, July 22, 2019. Credit: Paul W. Vosteen, Green Bank Observatory. Right: The 85-foot Howard E. Tatel telescope in Green Bank. Credit: NRAO/AUI/NSF.
Another space-oriented activity that occurred in July is the participation of several DTM’ers in the Astrobio2019 workshop “Moonshots and Earthshots in the Search for Life Beyond Earth,” held at the Green Bank Observatory. DTM staff scientist Alan Boss served as a member of the organizing committee that put together a diverse program that included talks by DTM staff scientists Alycia Weinberger and John Chambers, as well as DTM’s NSF Postdoctoral Fellow Meredith MacGregor, Boss, and myself. A highlight of the workshop was the participation of David and Edith Tatel, who are long-time DTM supporters. Howard Tatel, David’s father, was a staff scientist at DTM from 1947 until his untimely death in 1957. Although Howard Tatel was hired as a seismologist to assist with DTM’s early work in explosion seismic imaging of Earth’s crust, he also played a leading role in the development of the radio astronomy effort that was pursued at DTM from 1953 through 1965. He designed components of the 85-foot radio telescope that, in 1959, was the first to be put into operation in the newly commissioned National Radio Astronomy Observatory site in Green Bank, WV. The telescope was later named the Howard E. Tatel Telescope in his honor.
Another out of this world event in July was the passage of the bill to rename the Large Synoptic Survey Telescope the Vera Rubin Survey Telescope by the U.S. House of Representatives. The bill still requires the approval of the Senate and President, but it is past its first hurdle.
Not all of DTM had its head in the skies in July. DTM staff scientist Lara Wagner was the recipient of a large grant from the NSF Frontiers of Earth Science Program to lead a multidisciplinary program to examine the nature of oceanic plate subduction beneath Colombia. This is an area where the subducting oceanic plate initially plunges beneath the continent, but then bends to travel horizontally at about 100 km depth beneath Colombia before again sinking steeply into Earth’s interior. This type of subduction, known as flat-slab subduction, was first discussed in the 1960s by DTM seismologists Selwyn Sacks and David James. Flat-slab subduction has many significant geologic consequences compared to normal dip plate subduction. It cuts off volcanism above the flat slab because hot mantle cannot flow into the narrow gap between the subducting oceanic plate and the overlying continental plate and thereby instigate volcanism triggered by water release by the oceanic plate. The flat slab also transmits the forces of plate convergence well inland from the generally off-shore trench that marks the plate boundary, leading to earthquakes and mountain-building well removed from the plate boundary. Examples of the long-distance effects of flat-slab subduction include the damaging Mexico City earthquake of 2017, and the uplift of the Rocky Mountains over 50 million years ago. The type of seismic imaging to be used by Wagner will provide high-resolution images of the plate as it descends beneath Colombia. The work is likely to address many key questions about the process of subduction, from the strength of down going plates, to the forces that cause flat-slab subduction, to the mechanisms that carry water with the plate deep into Earth’s interior.