Observability of Callisto’s exosphere with MAJIS/JUICE

<p>The most direct evidence that the icy Galilean satellite Callisto is able to sustain a significant neutral exosphere dates back to the detection of the CO₂ non-LTE emission at 4.3 &#956;m wavelength, measured by the NIMS instrument onboard the NASA Galileo spacecraft [1]. Other exospheric emissions have been observed in the UV spectral range, basically tracing the ionised exospheric component ([2], [3]). The analysis of such emissions in the framework of exospheric models (see e.g. [4], [5]) allowed to establish an overall composition dominated by O₂ and H₂O, with minor contributions by CO₂ and CO. However, direct observations of neutral species other than CO₂ are still missing, and their actual abundances, as well as spatial and temporal variability, are poorly constrained.</p> <p>The MAJIS (<em>Moon And Jupiter Imaging Spectrometer</em>, [6]) instrument, on board the ESA JUICE spacecraft, is expected to contribute in this field, by searching for non-LTE emissions falling in its spectral range, from 0.50 to 5.54 &#956;m. In particular, we evaluate the chance of detection of signals at the satellite&#8217;s limb emitted by the CO₂ complexes at 4.3 &#956;m and 2.3 &#956;m, by the H₂O complex at 2.3 &#956;m, by O₂ at 1.27 &#956;m, and by the CO bands at 4.7 &#956;m and 2.3 &#956;m. We calculate the populations of molecular levels by using the GRANADA algorithm [7], then the emissions intensities, for reference abundances of the molecular species and for limb-viewing geometry, by taking advantage of the KOPRA algorithm [8].</p> <p>Detection limits for all the abovementioned species are obtained in the approximation of horizontal uniformity of exospheric layers and adopting a vertical scaling compatible with the scale height in [1]. Surface density detection limits around 6.2 ^.10⁶ cm^-3, 6.6 ^.10⁶ cm^-3, 3.4 ^.10⁹ cm^-3, 3.4 ^.10⁷ cm^-3 are found for CO₂, H₂O, O₂, and CO respectively. For both CO₂ and H₂O, these results indicate a high detection probability during the Callisto flybys planned in the current JUICE trajectory version (crema 5.0, [9]). Detection of O₂ could also be possible if appropriate observing strategies are adopted. Detection of CO is instead very challenging, being its expected abundance well below the detection limit.</p> <p><strong>&#160;</strong><strong>Acknowledgements</strong></p> <p>This work is supported by the Italian Space Agency (ASI-INAF grant 2018-25-HH.0). IAA researchers acknowledge financial support from the State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa" award to the Instituto de Astrof&#237;sica de Andaluc&#237;a (CEX2021-001131-S/fund by MICIN/AEI/10.13039/501100011033).</p> <p><strong>References</strong></p> <p><strong>[1]</strong>&#160;Carlson,R.W.,1999, <em>Science</em> 283 (5403): 820&#8211;21. <strong>[2]</strong> Kliore,A.J., 2002, <em>Journ.Geophys.Res</em>. doi: 10.1029/2002ja009365. <strong>[3] </strong>Cunningham,N.J. et al., 2015, <em>Icarus</em>. doi:10.1016/j.icarus.2015.03.021. <strong>[4]</strong> Vorburger,A., et al., 2015, <em>Icarus</em>. doi:10.1016/j.icarus.2015.07.035. <strong>[5]</strong> Liang, M., 2005, <em>Journ.Geophys.Res</em>. doi:10.1029/2004je002322. <strong>[6]</strong> Guerri I., et al., 2018, Proc.of SPIE Vol.10690 106901L-1. doi: 10.1117/12.2312013.<strong> [7] </strong>Funke,B., et al., 2012, <em>Journ.Quant.Sp.Rad.Tran</em>. doi:10.1016/j.jqsrt.2012.05.001. <strong>[8] </strong>Stiller,G.P., et al., 2002. <em>Journ.Quant.Sp.Rad.Tran</em>. doi:10.1016/s0022-4073(01)00123-6. <strong>[9]</strong> ESA SPICE Service, JUICE Operational SPICE Kernel Dataset, doi:10.5270/esa-ybmj68p.</p>