Ion composition in Titan's exosphere via the Cassini Plasma Spectrometer I: T40 encounter

Abstract

We investigate the complex interaction between Saturn's magnetosphere and Titan's upper ionosphere using ion data acquired by the Cassini Plasma Spectrometer (CAPS) during the T40 encounter. Bounds on ion-group abundances at altitudes between~2733 and~12,541 km are determined by fitting mass spectra with model functions derived from instrument calibration data. The spectra are dominated by H + , H 2 + , H 3 + , and two hydrocarbon groups with mass ranges 12-19 and 24-32 amu, respectively. Notably, this constitutes the first reported observation of H 3 + in Titan's exosphere. These measurements are discussed in the context of data from the CAPS electron spectrometer (ELS) and the Ion and Neutral Mass Spectrometer (INMS), which fortuitously sampled the ionospheric outflow during the T40 encounter at altitudes between~2225 and~3034 km. The CAPS data reveal a composition that is constitutively similar to that sampled by INMS, with hydrocarbon ions first observed as far as~11,000 km from Titan and increasing in density by more than an order of magnitude along Cassini's inbound trajectory.In addition, we juxtapose the CAPS ion data with numerical results from three different interaction models and show that it is consistent with the location of the field-draping boundary described by Ulusen et al. (2012) and the Saturnward ion tail predicted by Sillanpää et al. (2006).

Titan: Earth-like on the Outside, Ocean World on the Inside

“…Cassini identified the compositions of neutral and positive ions up to 99 Da but could only detect the presence of negative ions with a mass-to-charge ratio so large as to be similar to that of terrestrial proteins (Coates et al 2007;Waite et al 2007;Wellbrock et al 2013;Woodson et al 2015). These molecules grow larger during polar winter and with decreasing altitude (Wellbrock et al 2019), but their ultimate fate remains unknown.…”

A High-Resolution Mass Spectrometer for the Experimental Study of the Gas Composition in Planetary Environments: First Laboratory Results

“…Mass spectrometry is one of the most efficient techniques for in situ experimental studies of the chemical composition of samples of astrochemical relevance. In situ measurements carried out during recent space missions to Saturn and its moons (Cassini-Huygens [1][2][3][4]) and the 67P/Churyumov-Gerasimenko comet (Rosetta [5][6][7]) have provided unique data on the composition of the environment and surface of the Solar System's bodies. These missions unveiled the complexity of the chemical composition of both the atmosphere of Saturn's moon Titan [3,4,[8][9][10] and of comets, which are considered to be relatively primitive small space bodies.…”

Suprathermal Magnetospheric Atomic and Molecular Heavy Ions at and Near Earth, Jupiter, and Saturn: Observations and Identification

“…Although a Titan torus has not yet been clearly distinguished in Cassini data (Smith & Rymer, 2014), work to characterize plasmas near Titan (Woodson et al, 2015) continues. Early predictions of Saturn's ion populations envisioned a torus derived from N 2 -rich Titan near its orbit, similar to the Enceladus watergroup torus.…”

“…Early predictions of Saturn's ion populations envisioned a torus derived from N 2 ‐rich Titan near its orbit, similar to the Enceladus water‐group torus. Although a Titan torus has not yet been clearly distinguished in Cassini data (Smith & Rymer, 2014), work to characterize plasmas near Titan (Woodson et al., 2015) continues. An attractive aspect of HCNH + is that its dissociation could easily yield comparable numbers of N‐rich and C‐rich fragments contributing to the comparable abundances of suprathermal C + and N + observed in Saturn's magnetosphere (Mauk et al., 2009; Christon, Hamilton, Difabio, et al, 2013; Christon, Hamilton, Mitchell, et al., 2013; DiFabio, 2012).…”