Thermal Structure and Composition of Saturn's Upper Atmosphere From Cassini/Ion Neutral Mass Spectrometer Measurements

Abstract

Analysis of measurements of the H 2 density in Saturn's equatorial thermosphere indicates temperatures from 340 to 370 K. The deepest measurements, obtained during Cassini's final plunge into the atmosphere, measure the thermospheric temperature profile. The measurements are well fit by a Bates profile with an exospheric temperature of 354 K and a temperature gradient at 1.2 × 10 −4 Pa of 0.4 K/km, corresponding to a thermal conduction flux of 7.3 × 10 −5 W/m 2 . The helium profiles are consistent with diffusive equilibrium. The CH 4 profiles are not in diffusive equilibrium but instead have a roughly constant mixing ratio relative to H 2 . We interpret this as the signature of a downward external flux of ∼ 10 13 m −2 s −1 . Saturn's rings are the most likely source of this external material. Plain Language SummaryThe mass spectrometer on the Cassini spacecraft measured the densities of molecular hydrogen, helium, and methane, among other species, along the spacecraft track through the atmosphere. Analyzing these data determines the temperature structure of the upper atmosphere. We find temperatures of 340 to 370 K. We also find that the methane in the upper atmosphere comes from Saturn's rings rather than the lower atmosphere.

The Ion Composition of Saturn's Equatorial Ionosphere as Observed by Cassini

“…The analysis indicated that a neutral molecular (M) species exists that reacts with H + and H 3 + . The surprisingly high mixing ratios are associated with high downward diffusive fluxes of 10 8 -10 9 cm À2 /s of heavy neutrals such as CH 4 (Koskinen & Guerlet, 2018;Perry et al, 2018;Yelle et al, 2018). The high-mass species mixing ratios from INMS neutral data are similar (W18; Perry et al, 2018).…”

Atmospheric Waves and Their Possible Effect on the Thermal Structure of Saturn's Thermosphere

“…The thermosphere of Saturn has been observed remotely with ultraviolet occultations by the Voyager and Cassini spacecraft (Koskinen et al, 2013(Koskinen et al, , 2015Vervack & Moses, 2015) and in polar (auroral) regions by observations of infrared radiation by the Cassini spacecraft and ground-based telescopes (Stallard et al, 2004;Melin et al, 2011). These and recent in situ observations (Yelle et al, 2018) have shown exospheric temperatures on Saturn to reach values of 340-470 K at low to middle latitudes and 470-530 K at latitudes poleward of 40 • latitude. When calculating exospheric temperatures on Saturn from solar extreme ultraviolet heating alone, values reach around 180 K over the equator (Müller-Wodarg, Mendillo, Yelle, et al, 2006;Yelle & Miller, 2004), with solar cycle changes being responsible for variations of less than 10 K (Müller-Wodarg, Mendillo, , illustrating that direct solar heating represents only a minor heat source in the upper atmosphere of Saturn.…”

“…(upper) Temperatures at the top of Saturn's thermosphere, as observed from stellar (circles) and solar (triangles) ultraviolet occultations (Koskinen et al, , ) and Cassini Ion and Neutral Mass Spectrometer in situ measurements (crosses; Yelle et al, ). Lines are zonally averaged exospheric temperatures from the Saturn Thermosphere Ionosphere General Circulation Model (Müller‐Wodarg, Mendillo, Yelle, et al, ; Müller‐Wodarg et al, ) for equinox conditions.…”

“…Temperatures at the top of Saturn's thermosphere, as observed from ultraviolet occultations (Koskinen et al, , ; round symbols), Cassini Ion and Neutral Mass Spectrometer in situ measurements (Yelle et al, ; crosses) and calculated by the Saturn Thermosphere Ionosphere General Circulation Model (Müller‐Wodarg, Mendillo, Yelle, et al, ; Müller‐Wodarg et al, ), assuming the drag term of equation with the Λ( θ ) from simulation C (Figure , lower panel). Red and blue lines are for southern hemisphere summer and winter conditions, respectively, while the green line is for equinox (simulation C).…”

“…To date, studies have examined the effects of direct heating by dissipating gravity or acoustic waves (O'Donoghue et al, ; Schubert et al, ; Young et al, ) but found that even under optimal conditions, wave heating could only provide part of the missing energy at low latitudes (Hickey et al, ; Matcheva & Strobel, ; Strobel, ; Yelle & Miller, ). Motivated by temperature retrievals from Cassini observations (Koskinen et al, , ) as well as recent in situ observations by Cassini's Ion and Neutral Mass Spectrometer (Yelle et al, ), we revisit the question and investigate the role of additional horizontal Rayleigh drag on the global circulation and temperatures of the upper atmosphere using the Saturn Thermosphere Ionosphere General Circulation Model (Müller‐Wodarg, Mendillo, Yelle, et al, ; Müller‐Wodarg et al, ).…”