January 2018 Letter from the Director

For many at DTM, the end of the year is marked by the AGU Fall Meeting, held this year, for the first time, in New Orleans. The meeting this year brought together over 22,000 Earth scientists to present their latest discoveries. DTM and GL scientists were well represented as the video abstracts, web posts, and daily social media feeds during the meeting produced by Roberto Molar Candanosa and Michelle Scholtes documented. Besides the interesting science, the AGU meeting also gives us the chance to mingle with Carnegie alumni at the reception we host just prior to the meeting. The event always allows the opportunity to catch up on the latest news from our alumni. The sizeable attendance also provides clear documentation of the major role that the Carnegie departments play in the careers of those who now constitute an impressively large fraction of the solid Earth scientists at the very large AGU conference.

Carnegie AGU Reception, December 10, 2017.

In addition to the usual flurry of getting ready for the meeting, the evening of November 30 provided us, and everyone in the immediate area, a geophysical reminder in the form of a magnitude 4.1 earthquake. The earthquake, just to the east of Dover, Delaware, is an exceedingly rare event for the area with no other recorded earthquake occurring anywhere nearby. The earthquake allowed Diana Roman, Lara Wagner, and DTM postdoc Helen Janiszewski the opportunity to put the DTM-developed "Quick Deploy Box" or QDB to good use the morning after the quake. The QDB, designed by Roman and Wagner, built by DTM Instrument Maker Tyler Bartholomew, and funded by grants from the Brinson Foundation, reduces the equipment needed for a seismometer deployment to a single checked baggage-size suitcase. Previously, the necessary components to install a single field seismic station filled the bed of a good sized pick-up truck. Their efforts even made it to the evening news in Philadelphia. Along with colleagues from several local universities and federal agencies, the group set up a total of 17 seismometers near the epicenter to capture information on the aftershocks. The location, magnitude, and direction of fault movement of the aftershocks can provide critical information on the causes of the main quake that may well relate to the rifting of Europe and North Africa away from the North American eastern seaboard some 200 million years ago.

DTM's Diana Roman setting up a Carnegie Quick Deploy Seismic Station, December 1, 2017. Photo courtesy Diana Roman.
 
In another corner of the galaxy, former DTM postdoc Nan Liu, working with Larry Nittler, Conel Alexander, and Jianhua Wang were able to show that some rare carbon-rich mineral grains contained in primitive meteorites formed more than 2 years after the constituent elements were ejected into space by a massive supernova. Although the grains must be over 4.5 billion years old to have been swept up during the meteorite formation in the early history of our Solar System, the grains clearly originated from elsewhere, as their isotopic compositions are far removed from the values typically found in Solar System materials. Astronomical observations of supernovae indicate that these exploding stars produce large amounts of dust.

 


 

An electron microscope image of a micron-sized supernova silicon carbide, SiC, stardust grain (lower right) extracted from a primitive meteorite. Such grains originated more than 4.6 billion years ago in the ashes of Type II supernovae, typified here by a Hubble Space Telescope image of the Crab Nebula, the remnant of a supernova explosion in 1054. Laboratory analysis of such tiny dust grains provides unique information on these massive stellar explosions. (1 μm is one millionth of a meter.) Credits: NASA and Larry Nittler.
 
The incredible temporal resolution of the Liu discovery is made possible because stars are continually producing new elements through nuclear reactions in their interior. A fraction of these elements contain radioactive isotopes – some, like uranium, with decay half-lives similar to the age of the Earth, but others with very short half-lives, on the order of seconds to a few years. When the star explodes, these newly created elements are ejected into space, and as they cool from the incredibly high temperatures of their nucleosynthesis, the elements will begin to condense into mineral grains – the dust that surround the supernova.

The DTM team examined grains of silicon-carbide – a mineral made up of two abundant elements, and one that condenses at very high temperatures. They measured the titanium isotopic composition of these grains and were able to come up with their longer than 2-year estimate for grain formation because one of the titanium isotopes is produced by the radioactive decay of an isotope of vanadium. Both elements are produced by nucleosynthesis in the star. If the grains formed within a couple of half-lives of the vanadium isotope, and given their ratio of vanadium to titanium, they should have contained an excess of the titanium-49 isotope, the decay product of vanadium-49. They did not. They would have, given the 330-day half-life of vanadium-49, if the grains formed within 2 years of the cessation of nucleosynthesis when the star exploded.
 
The Delaware earthquake and the supernova grain discovery both serve to highlight an intriguing aspect of the research conducted at Carnegie – we use modern measurements to understand events that happened long ago. Minor Earth movements today may provide us with information on the assembly, or disassembly, history of North America that occurred hundreds of million years ago. Compositional features of micron-size dust grains are used to deduce the expansion and cooling rates of stellar explosions 22 orders of magnitude larger in size that were taking place on a few-year time scale over 4.5 billion years ago. As stated at the top of the DTM web page, our charter is to address "broad questions about nature." The examples above provide a pretty good picture of the breadth in time and space of the work conducted at DTM.
 
 
Richard Carlson, Director, DTM
Carnegie Institution for Science
 
January 2018 Newsletter
 

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