Landed Science At A Lunar Crustal Magnetic Anomaly
Introduction: The Moon does not currently possess a global, internally generated magnetic field, but the lunar crust does contain areas of magnetized rocks ("magnetic anomalies", Fig. 1). The crustal magnetic anomalies are correlated with unusual, high-reflectance markings referred to as lunar swirls. Lunar magnetic anomalies present a natural laboratory for at least five major areas in planetary science: a) Planetary magnetism: What is the origin of the magnetized material? Current hypotheses include magnetization imprinted during a recent comet impact, basin ejecta magnetized by plasma effects, and a core dynamo field that magnetized an igneous intrusion or iron impactor material. By measuring the strength and structure of the surface field, it will be possible to con-strain the size and the depth of the magnetic source. A surficial anomaly would support a comet or iron-impactor origin. A deep source might indicate the presence of a magnetized intrusion or a deposit of magnetized basin ejecta, with important implications for an ancient internal dynamo and lunar thermal evolution. b) Space plasma physics: How does the magnetic anomaly interact with the incident plasma to form a standoff region? How important are electric fields? What are the fluxes (energy and species) that actually reach the surface? How does the solar wind/magnetic field/surface interaction change with time of lunar day? c) Lunar geology: What are the nature and origin of lunar swirls, and how are they related to the magnetic anomalies? Are swirls ancient or recent? Is levitated dust or cometary material responsible for the albedo markings? d) Space weathering: Space weathering alters the optical and chemical properties of airless surfaces, and it is important to develop a complete understanding of space weathering on the Moon as a framework for weathering of other bodies. A key issue relates to the relative importance of plasma vs. micrometeoroid bombardment and what roles these agents play in mod-ifying the surface. The lunar magnetic anomalies offer control on one of the key variables, solar wind expo-sure, because micrometeoroids will generally not be affected by the presence of the magnetic field. e) Lunar water cycle: It is known from orbital data that the high-reflectance portions of swirls exhibit weaker hydroxyl absorptions at 2.82 m than the back-ground, consistent with either a lower flux of solar wind protons reaching the surface or a difference in retention or lifetime of the OH. How does this hydration feature vary on the lunar surface, and with location/magnetic field strength and time of day? A Rover Mission: To answer these important questions, in situ measurements from an instrument package traversing the surface across a lunar swirl are required. The leading candidate is the Reiner Gamma swirl, on the nearside. A long-lived rover would carry the relevant payload elements to define the nature of the magnetic anomaly and to characterize the surface, including a vector magnetometer, solar-wind spectrometer, mast-mounted multispectral imager, a UV-VIS-NIR spectrometer, a microscopic spectral imager, an XRF/XRD, a Mössbauer spectrometer, and a traverse gravimeter. Exploration SKGs: Measurements within a magnetic anomaly would also address Strategic Knowledge Gaps (SKGs) for human exploration. SKG Themes include: Theme I, Resource Potential: I-D, temporal variability and movement dynamics of surface-correlated OH and H2O. Theme II, Lunar Environment: II-B, radiation at the lunar surface. Theme III, Living and Working on the Lunar Surface: III-B-1, lunar geodetic control. III-C-2, lunar surface trafficability. III-E, near-surface plasma environment. Fig. 1 caption: Map of magnetic anomalies derived from Lunar Prospector magnetometer data [Richmond and Hood, 2008 JGR]. The strongest anomaly (28 nT at 30 km altitude) is that near crater Gerasimovich (Crisium basin antipode region). The strongest nearside anomaly is at Descartes (24 nT), in the highlands south of the Apollo 16 landing site. Reiner Gamma's strength is 22 nT.