Lawrence Livermore scientists and collaborators have analyzed a Martian meteorite (see image below) and determined that a significant topographic and geophysical feature of Mars—its crustal dichotomy, or divide, that separates the northern hemisphere lowlands and southern hemisphere highlands of the planet—likely formed within the first hundred million years of planetary history. The study was published in the May 23, 2018, issue of Science Advances.
Northwest Africa (NWA) 7034, discovered in the Sahara Desert, is the oldest Martian meteorite that has been discovered to date. The sample is a breccia, containing a variety of crustal rocks that were mixed together and sintered during a meteoroid impact on Mars. Using radioisotopic dating techniques and other data, the team determined that the crustal rocks incorporated into NWA 7034 were emplaced near the Martian surface more than 4.4 billion years ago in a regional terrain spanning hundreds of square kilometers, and that this terrain remained relatively undisturbed throughout the planet’s history. The rocks were brought together by a small impact event 2 to 3 million years ago.
The retention of ancient volcanic terrains near Mars’ surface indicated to the researchers that the Martian crustal dichotomy likely formed within the first 100 million years of planetary formation—a time when many planetesimals were impacting the inner planets. The data are consistent with a giant impact hypothesis for its formation, although other mechanisms cannot be dismissed.
The results of this research have important implications for understanding when and how one of the oldest, and most distinctive, global geologic features on Mars was formed. Cassata says, “This study demonstrates that multiple radioisotopic dating systems that are reset by different metamorphic processes can be used to tease out the thermal history of a sample over billions of years.”
Contact: William Cassata (925) 423-2812 (cassata2@llnl.gov).
Scientists from Lawrence Livermore and Texas A&M University have made significant progress in understanding the formation of radiation defects. The team’s research, which appears in the May 25, 2018, issue of Physical Review Letters, explains how the density of collision cascades—collisions of atoms induced by energetic particles—significantly affects defect interaction dynamics in silicon.
The team calculated cascade densities with a computer model that accounted for the fractal nature of collision cascades and, using a novel pulse ion-beam method, systematically studied the temperature dependencies of the rate of defect interaction in silicon bombarded with ions in a wide range of masses, creating collision cascades with vastly different densities. The results demonstrate that the complex dependence of defect dynamics on irradiation conditions can be reduced to a deterministic effect of collision cascade density. The density determines the lifetime of point defects, which is the time it takes for them to annihilate or interact to form stable damage. Defects in denser cascades take longer to decay, and a change in the dominant process of defect interaction occurs at higher temperatures.
“The new results can be used to predict radiation defect dynamics in silicon and provide a blueprint for future studies of radiation defect dynamics in other technologically relevant materials,” says Sergei Kucheyev, the Livermore project lead and co-author of the paper. Understanding radiation damage is important for nuclear materials performance and for electronic materials. The results from this study can be used to predict the ranges of process conditions where the dynamic defect interaction phenomena are pronounced or less important.
Contact: Sergei O. Kucheyev (925) 422-5866 (kucheyev1@llnl.gov).
A team of scientists from Lawrence Livermore and Lawrence Berkeley national laboratories as well as from the University of California at Berkeley and other institutions have calculated, with unprecedented precision, a quantity that is central to the understanding of a neutron’s lifetime, nucleon axial coupling (gA). The research appears in the May 30, 2018, issue of Nature.
The complex theory of quantum chromodynamics (QCD) describes the strong interaction between quarks and gluons—the building blocks for larger particles, such as neutrons and protons. Using supercomputers from Lawrence Livermore and Oak Ridge national laboratories, the research team numerically simulated QCD on a four-dimensional grid of points called a lattice. The simulation method, called Lattice QCD, helps to determine the mass of a neutron and proton as well as the value of gA. The team’s theoretical determination of gA was based on a simulation that represented only a tiny piece of the universe—the size of a few neutrons in each direction. They effectively simulated a neutron transitioning to a proton inside this section of the universe to predict what happens in nature. Livermore scientist Pavlos Vranas, who co-led the research, says, “Our study indicates that Lattice QCD is now capable of calculating gA with high precision, opening the door for a wealth of Lattice QCD calculations of importance to nuclear physics.”
The scientists’ work builds upon decades of research and advances in computational resources by the Lattice QCD community. The team is the first to calculate gA to a precision within 1 percent and aims to drive the uncertainty margin down to approximately 0.3 percent.
Contact: Pavlos Vranas (925) 422-4681 (vranas2@llnl.gov).