During Cassini's initial orbit, we observed a dynamic magnetosphere composed primarily of a complex mixture of water-derived atomic and molecular ions. We have identified four distinct regions characterized by differences in both bulk plasma properties and ion composition. Protons are the dominant species outside about 9 RS (where RS is the radial distance from the center of Saturn), whereas inside, the plasma consists primarily of a corotating comet-like mix of water-derived ions with approximately 3% N+. Over the A and B rings, we found an ionosphere in which O2+ and O+ are dominant, which suggests the possible existence of a layer of O2 gas similar to the atmospheres of Europa and Ganymede.

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The Cassini spacecraft commenced its tour of the planet Saturn on 1 July 2004 (GMT). During the insertion orbit, the Cassini magnetometer (MAG), radio/plasma wave experiment (RPWS), and plasma spectrometer (CAPS) obtained in situ measurements of the magnetic field and plasma conditions associated with Saturn's environment. Analysis of the magnetic field data indicate that Cassini repeatedly crossed a mainly quasi‐perpendicular bow shock boundary on both the inbound (post‐dawn) and outbound (predawn) legs. Modeling of the bow shock and magnetopause crossing positions shows evidence for a magnetospheric compression during Cassini's immersion in the magnetosphere. The magnetic signatures of the bow shock crossings show the clearly defined “overshoot” and “foot” regions associated with the quasi‐perpendicular geometry. The duration of the shock foot, considered in combination with the RPWS and CAPS solar wind electron parameters upstream of the bow shock crossings, indicates that the length scale for the bow shock ramp at Saturn is about an ion inertial length. This is consistent with multispacecraft observations of the spatial scale of the Earth's shock foot region. The data are generally consistent with Saturn bow shock velocities up to ∼400 km s−1 and shock structures governed by ion dynamics.