Cassini INMS observations of neutral molecules in Saturn's E-ring

Abstract

In 2008, the Cassini ion neutral mass spectrometer (INMS) investigation made in situ measurements of neutral species near Saturn's equatorial plane within 0.5 Saturn radii (R_S) of the orbit of Enceladus. After removing the large background and modeling to interpret instrumental effects, the data provide rough constraints on the neutral distribution and composition. These data show an azimuthal asymmetry in the neutral densities and provide measurements used to compare to simulations of neutral H₂O emitted from Enceladus. Far from Enceladus, the neutral water densities, at a few times 10³ molecules/cm³, are near the detection limit of INMS. Near Enceladus, but outside of the plumes and north of the equatorial plane, the INMS detects particles within 5000 km of Enceladus, with the density increasing to approximately 10⁵ molecules/cm³ at the equatorial plane. The observations also show CO₂ in the form of its dissociated product, CO. On the basis of the spatial distribution of CO₂ counts, the scale height of the neutral cloud above and below the equatorial plane is less than 7000 km. Far from Enceladus, the concentration of CO₂ with respect to H₂O increases, a consequence of the predicted decline in H₂O density. Relatively high counts at 2 amu are infrequently observed. These measurements indicate H₂ released during ice-grain impacts and provide a constraint on the frequency and distribution of small ice grains.