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

[1] 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 (RS) 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.