ARTEMIS observations of terrestrial ionospheric molecular ions at the Moon: implications for lunar exospheric and volatile inventories

The Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) spacecraft observes multiple instances of outflowing molecular ionospheric ions at lunar distances in the terrestrial magnetotail. The heavy ion fluxes are observed during geomagnetically disturbed times and consist of mainly molecular species (N2+, NO+, and O2+, approximately masses 28-32 amu) on the order of 1e5-1e6 /cm2/s at nearly identical velocities as concurrently present protons. By performing backwards particle tracing in time-dependent electromagnetic fields from the magnetohydrodynamic Open Global Geospace Circulation Model (OpenGGCM) of the terrestrial magnetosphere, we show that the ions escape the inner magnetosphere through magnetopause shadowing near noon and are subsequently accelerated down-tail to lunar distances. As these ions stream past the Moon in the magnetotail, they contribute significantly to exospheric neutral sputtering while also delivering potentially significant amounts of oxygen and nitrogen to the lunar volatile inventory. Furthermore, the terrestrial nitrogen ions may become embedded in lunar grains upon impact, potentially representing an on-going source of embedded lunar nitrogen, in addition to the hypothesis of Ozima et al., Nature, [2005] that most of the observed lunar nitrogen was emplaced during the early evolution of Earth when terrestrial atmospheric escape was potentially much greater.