Reprint - Earth & Planetary Sciences - University of California, Santa ...

134 suggested endogenic activities. However, the nature of these activities was not well understood. For
135 a more detailed discussion of our knowledge before Cassini, see Dougherty et al. (2009) and Orton
136 et al. (2009). In terms of geology, the Cassini mission was designed to provide significantly better
137 area coverage, spatial resolution, and spectral range, resulting in numerous new discoveries. A
138 detailed description of the Cassini mission can be found in Dougherty et al. (2009) and Seal et al.
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(2009). Maps of the Saturnian satellites are included in Roatsch et al. (2009).
141 21.2 Cassini’s Exploration of Saturn’s Icy Satellites
142 The Cassini spacecraft is equipped with instruments tailored to investigate the surfaces,
143 environments and interiors of icy satellites. The optical remote sensing (ORS) instrument suite
144 includes cameras and spectrometers designed for high spatial and spectral resolution covering
145 wavelengths between 0.06µm and 1000µm. With the imaging subsystem (ISS) (Porco et al., 2004),
146 morphologic, stratigraphic and other geological surface properties can be observed at spatial
147 resolutions down to a few meters (locally) or a few hundred meters (globally), depending on flyby
148 distances. Moreover, as many as 33 color and polarization filter combinations permit mapping
149 geologically diverse terrain spatially at wavelengths between ~0.3µm and ~1.1µm. The visible and
150 infrared mapping spectrometer (VIMS; 0.4µm to 5.1µm) provides chemical and compositional
151 spectral information with spatial resolutions from a few kilometers (globally) to better than one
152 hundred meters (locally) (Brown et al., 2004; Jaumann et al., 2006). The composite infrared
153 spectrometer (CIRS) determines global and regional surface temperatures and thermal properties on
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a kilometer scale (Flasar et al., 2004). The ultraviolet imaging spectrograph (UVIS) provides
information about thin atmospheres and volcanic plume structures as well as about water ice and
156 other minor constituents on the surface (Esposito et al., 2004), operating in the 60nm - 190nm
157 wavelength range. A suite of magnetosphere and plasma science (MAPS) instruments characterizes
158 the satellites’ environments by in-situ methods. Micron-sized dust grains and neutral molecules
159 released from the surface by active or passive processes (volcanoes/sputtering) carry surface
160 composition information to distances as far as hundreds of kilometers above the surface. Released
161 material affects the plasma surrounding the satellite and is registered by variations in the magnetic
162 field, ion density, and neutral gas and dust density as well as gas and dusty composition.
163 The primary task of the radio science subsystem (RSS) is to determine the mass of the moons (Tab.
164 1) by tracking deviations in Cassini's trajectory. RADAR data can provide unique information about
165 the upper sub-surface (Elachi et al., 2004), though few close-up RADAR SAR observations were
166 performed during satellite flybys. The lack of a scan platform for the remote sensing instruments
167 prevents the radio science, RADAR and remote sensing systems from operating simultaneously
168 during a flyby because the antenna and remote sensing instruments are oriented 90° apart on the
169 spacecraft. Even joint surface scans by the ORS instruments require significant compromises
170 between individual observations due to data rate and integration time constraints. While the ISS
171 instrument needs short 'dwell' times for mosaicking, the VIMS depend on long 'dwells' for
172 improved signal-to-noise ratios. On the other hand, the CIRS and UVIS instruments contain line-
173 scanning devices, which depend on either slow or fast slews to scan the surfaces. Thus, reaching
174 satisfactory compromises is a major challenge in the planning process.
175 Since the Cassini spacecraft orbits the planet rather than individual satellites, the moons can only be
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observed at various distances and illumination conditions, ideally during very close-targeted flybys.
Depending on the distance of a moon to Saturn, the flybys occur at very different velocities. In
178 March 2008, for instance, Enceladus was passed at ~14 km/s, while the Iapetus flyby in September
179 2007 took place at a leisurely 2.4 km/s. The flyby geometry can also vary significantly. The closest-
180 ever approach was during the Enceladus flyby on October 9, 2008, when the spacecraft skimmed as
181 low as 25 km over Enceladus’ surface. Other, more typical targeted flyby altitudes occur between
182 100 km and 2000 km. A flyby can be polar, equatorial, or in between, and at the closest approach,
183 the sub-spacecraft point over a moon can be located either over its illuminated or over its unlit side.
184 The MAPS instruments measure densities or field gradients over time. Sputtering processes lead to
185 high dust and neutral-gas densities above the surface, so that environmental in-situ instruments rely
186 on measurements as close to the surface as possible. Further plasma density variations caused by
187 satellites moving in the magnetosphere are empty flux tubes (wakes) and the drop of plasma along
188 the related L-shell. Crossing those regions is of high value for in-situ plasma investigations that
189 provide indirect information about the satellite surface and the density of its neutral and plasma
190 environment.
191 During the nominal mission, Cassini performed nine targeted as well as numerous close flybys of
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icy satellites (Tab. 2).
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