Simulation of Callisto's exosphere as measured by JUICE/NIM Audrey Vorburger (1, 2), Peter Wurz (2), André Galli (2), Olivier Mousis (3), Stas Barabash (4), Helmut Lammer (5) Dept. of Earth and Planetary Sciences, Division of Physical Sciences, American Museum of Natural History, New York, USA (avorburger@amnh.org), (2) Physikalisches Institut, University of Bern, Bern, Switzerland, (3) Institut UTINAM, Université de Franche-Comté, OSU THETA, France , (4) Swedish Institute of Space Physics, S-981 28 Kiruna, Sweden , (5) Austrian Academy of Sciences, A-8042 Graz, Austria Abstract Whereas Callisto's surface has been mapped as early as in 1980 by the two Voyager missions, Callisto's tenuous atmosphere, actually an exosphere, was not directly observed for almost another two decades. In 1999, during the Galileo mission, the Near-Infrared Mapping Spectrometer finally conducted the first and so far only directly measurement of a constituent in Callisto's exosphere: A layer of CO2 molecules reaching up to 100 km above the surface [2]. During the same mission, an ionospheric layer was discovered above Callisto's sunlit trailing hemisphere [5]. The photo-ionization of the observed neutral CO2 atmosphere is insufficient to produce the observed electron densities, though. The existence of a neutral exosphere consisting primarily of O2 was thus proposed, models of which agree well with O2 upper limits derived from Hubble Space Telescope measurements [8]. The Neutral Ion Mass Spectrometer of the Particle Environment Package on board the planned JUpiter ICy moons Explorer mission will conduct the first-ever direct sampling of the exospheres of Europa, Ganymede, and Callisto. We present here density profiles of all primary constituents expected to be present in Callisto's exosphere, and mass spectra as we expect them to be recorded by NIM. 1. JUICE - NIM The JUpiter ICy moons Explorer mission (JUICE) of ESA to the Jovian system is currently implemented. The mission’s goal is to investigate in detail Jupiter and its system, with focus on the three Galilean moons Europa, Ganymede and Callisto. Before entering orbit around Ganymede, JUICE will conduct several flybys of the other two Galilean moons, Europa and Ganymede. The Neutral Ion Mass Spectrometer (NIM), one of the sensors of the Particle Environment Package (PEP), will conduct the first-ever direct sampling of the exospheres of Europa, Ganymede, and Callisto. NIM is capable of detecting exospheric neutral gas and thermal plasma at the Galilean moons with very high mass resolution and unprecedented sensitivity. The mass resolution is M/∆M > 1100 in the mass range 1–1000 amu and NIM’s energy range is ≤ 5e V for neutrals and <10 eV for ions. The detection level for neutral gas is 1·10 −16 mbar for a 5-second accumulation time [10], which corresponds to a particle density of about 1 cm −3 . With such a detection limit, NIM will be capable of recording mass spectra during the satellites' flybys starting at 10 5 km altitude, which means that during a single flyby NIM will record more than 10'000 mass spectra, or, collect several hours of data. 2. Callisto's surface composition The surface of Callisto consists of an icy and a rocky constituent. We introduce two alternative surface compositions describing both the icy and the rocky constituent separately. For the ice component we implement two different compositions that find analogues in the initial gas phase conditions of the solar nebula: One composition represents the oxidizing state and the other represents the reducing states of the gas [3, 7]. For the non-ice component we implement the compositions of typical CI as well as L type chondrites [1, 6]. 3. Callisto's exosphere composition – Monte Carlo Simulation results We present density profiles for all species present on Callisto's icy and rocky surface, as computed by our Monte-Carlo Model. The Monte-Carlo model used is an updated version of the model first presented in [9]. EPSC Abstracts Vol. 9, EPSC2014-596, 2014 European Planetary Science Congress 2014 c  Author(s) 2014 E P S C European Planetary Science Congress