Electromagnetic Sounding of the Moon from ARTEMIS

ARTEMIS is a twin-satellite, two-year lunar orbital mission, formed by retasking two of the THEMIS constellation (Angelopoulos, Space Sci. Rev.2010). The two spacecraft achieved lunar orbit in summer 2011. Although conceived for heliospheric science, investigations of the exosphere, crustal magnetic fields, and interior are enabled by the electromagnetic (EM) instruments of ARTEMIS (Sibeck et al., Space Sci. Rev, 2011). EM sounding of the interior will be improved over Apollo-era investigations due to the larger bandwidth, longer mission duration, and geographic coverage. Science objectives include (1) structure and heterogeneity of the outermost 500 km (crust and upper mantle), a region that may contain key information on the lunar magma ocean and the origin of the anomalous Procellarum KREEP Terrane (PKT); (2) tighter bounds on the conductivity of the lower mantle (500-1400 km depth), in order to constrain the temperature and nature of trace elements that control electrical conduction, particularly water; and (3) size of the metallic core, and whether a surrounding layer of molten silicate is present. EM sounding from ARTEMIS can be performed in at least two ways. In the transfer-function (TF) method derived during Apollo, the magnetic fields at a distant platform are compared to a (near) surface sensor to derive the source and sum of source and induced fields, respectively. From these data the internal conductivity structure giving rise to the induced field can be derived. However, source-field heterogeneity disturbs TF responses > 0.01 Hz. These high frequencies are necessary to resolve the crust and upper mantle. In contrast, the magnetotelluric (MT) method derives internal structure from the horizontal components of electric and magnetic fields at a single near-surface sensor, and therefore does not depend strongly on source-field geometry. MT has been used for more than a half-century in terrestrial exploration, but ARTEMIS marks its first planetary application. Both TF and MT are optimally applied when the Moon is in the lobes of the geomagnetic tail and the spacecraft are in daylight, where plasma effects are minimized. Periapsis passages at altitudes of a few hundred km or less with this geometry appear regularly in Nov and Dec. Periapses in the diamagnetic wake cavity are the next choice for EM sounding. The current layer that develops on the day side when the Moon is exposed to the solar wind screens EM sounding from orbit, but ARTEMIS will determine the thickness of this layer. ARTEMIS will advance our understanding of the lunar interior in ways that are complementary to the GRAIL gravity mission, and will provide a baseline for long-integration EM sounding from a surface geophysical network.