Geochemistry at 4 Vesta: Observations Using Fast Neutrons

Dawn is currently in orbit around the asteroid 4 Vesta, and one of the major objectives of the mission is to probe the relationship of Vesta to the Howardite, Eucrite, and Diogenite (HED) meteorites. As Vesta is an example of a differentiated planetary embryo, Dawn will also provide fundamental information about planetary evolution in the early solar system [1]. To help accomplish this overall goal, the Dawn spacecraft carries the Gamma-Ray and Neutron Detector (GRaND). GRaND uses planetary gamma-ray and neutron spectroscopy to measure the surface elemental composition of Vesta and will provide information that is unique and complementary to that provided by the other Dawn instruments and investigations. Gamma-ray and neutron spectroscopy is a standard technique for measuring planetary compositions [2], having successfully made measurements at near-Earth asteroids, the Moon, Mars, Mercury and now Vesta. GRaND has made the first measurements of the neutron spectrum from any asteroid (previous asteroid measurements were only made with gamma-rays). Dawn has been collecting data at Vesta since July 2011. The prime data collection period for GRaND is the Low-Altitude Mapping Orbit (LAMO), which started on 12 December 2011 and will last through spring 2012. During LAMO, the Dawn spacecraft orbits at an average altitude of ~210 km above the surface of Vesta, which allows good neutron and gamma-ray signals to be detected from Vesta. A description of the overall goals of GRaND and a summary of the initial findings are given elsewhere [3,4]. The subject of this study is to present the information that will be returned from GRaND using fast neutron measurements. Here, we discuss what fast neutrons can reveal about Vesta s surface composition, how such data can address Dawn science goals, and describe fast neutron measurements made in the early portion of the Vesta LAMO phase.