Abstract
Cassini Ion Neutral Mass Spectrometer (INMS) measurements from roughly a hundred Titan encounters over the Cassini mission yield neutral and ion densities systematically lower, by factors approximately 2 to 3, than estimates from several other spacecraft systems, including the Attitude and Articulation Control System, and Navigation system. In this paper we present a new INMS instrument sensitivity model, obtained by re-analyzing (1) the capture and transmission of neutral gas through the instrument, and (2) the detector gain reduction during pre-launch testing. By correcting for an under-estimation of gas leakage out of the instrument into space by the original calibration model, and adjusting for the gain change, the new model brings INMS densities into much closer agreement with the other Cassini systems. Accordingly, the INMS ion densities are revised upward by a constant detector sensitivity correction factor of 1.55±21 %, while the neutral sensitivities have a complex instrument pointing direction dependence, due (mostly) to the effect of the INMS vent and antechamber-to-closed source tube. In the special case of on-ram pointing the neutral densities are revised upward by a constant factor of 2.2±23 %. The corrected neutral and ion sensitivities given here are applicable to all previously published INMS results at Titan, Enceladus and elsewhere in the Saturn system. The new model gives reliable densities at high ram angles, in some cases above 90 degrees, thereby expanding the list of Titan flybys from which INMS densities may be extracted. We apply the model to obtain accurate densities from several off-ram Titan flybys which gave unusual neutral density vs. altitude profiles, or unreasonably high densities, with the original calibration.
Highlights
The primary scientific objectives of the Cassini Ion Neutral Mass Spectrometer (INMS) investigation are: (1) to characterize the composition, structure, and chemical behavior of Titan’s upper atmosphere and its interaction with Saturn’s magnetosphere; and (2) to investigate the neutral and plasma environments of the rings and icy satellites and their magnetospheric interactions (Waite et al 2004)
The ion density extraction process has been discussed in detail by Mandt et al (2012) for ram pointed flybys, and many other aspects of the INMS calibration and instrument response to ions and neutrals have been addressed in literature [see, e.g., Cui et al 2009; Gell and Waite 2015; Magee et al 2009; Perry et al 2010; Teolis et al 2010; Vuitton et al 2008; Waite et al 2004], including (1) compositional extraction from mass spectra, (2) wall adsorption of ‘sticky’ gases such as H2O and NH3, (3) thruster firing contamination, (4) radiation background, (5) high-pressure background effects, (6) residual gas ‘tails’, (7) highsensitivity detector saturation, and (8) on-going work to refine the instrument calibration with the ground-based refurbished engineering unit
The INMS measurements of Titan’s atmospheric neutral mass density were found to be systematically lower than that measured by the Cassini Attitude and Articulation Control System (AACS) and by Navigation (NAV), both of which use spacecraft drag data and aerodynamic models of the spacecraft to derive mass density [as reported by the Titan Atmospheric Working Group [TAMWG], Lee and Hanover 2005]
Summary
B.D. Teolis1 · H.B. Niemann2 · J.H. Waite1 · D.A. Gell1 · R.S. Perryman1 · W.T. Kasprzak2 · K.E. Mandt1 · R.V. Yelle3 · A.Y. Lee4 · F.J. Pelletier4 · G.P. Miller1 · D.T. Young1 · J.M. Bell1,5 · B.A. Magee1 · E.L. Patrick1 · J.
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