Jupiter System Observer Mission Study: Final Report - Lunar and ...

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01 NOVEMBER 2007 2007 JUPITER SYSTEM OBSERVER MISSION STUDY: FINAL REPORT SECTION 2— JUPITER SYSTEM SCIENCE GOALS AND OBJECTIVES Task Order #NMO710851 is an intricate patchwork of smooth bright material and material with closely-spaced parallel ridges and troughs, termed grooves (Figure 2.2-5). The grooves are dominated by extensional tectonic features, and morphologically have much in common with terrestrial rift zones (Parmentier et al., 1982; Pappalardo et al., 1998b). Bright terrain exhibits the full range of extensional tectonic behavior, from wide rifting, to narrow rifting, to possible examples of crustal spreading (much like the smooth bands on Europa). Though the ultimate driving mechanism behind groove formation is still unknown, there are many intriguing possibilities that may be tied to the internal evolution of Ganymede and the history of orbital evolution of the Galilean satellite system. Ganymede’s surface composition is dominated by water ice of varying grain sizes (McKinnon and Parmentier, 1986). Near the poles, there is a pronounced boundary between frosty polar caps and the rest of the surface. This surface boundary appears to follow the magnetospheric boundary between open and closed field lines (Khurana et al., 2007), and provides a unique opportunity to examine differences in space weathering processes on the same surface under different conditions. Figure 2.2-5. High-resolution topography overlaid on image of Ganymede’s surface acquired during Galileo’s G28 orbit. Red is high, blue is low. (NASA/JPL/Brown U.) 2-20 Outside of the polar caps and toward the equatorial regions, there are also dark non-ice materials on the surface, which may be hydrated brines similar to those on Europa. Other minor constituents of Ganymede’s surface include CO2, SO2, and some sort of tholin material exhibiting CH and CN bonds (McCord et al., 1998). There is also evidence for trapped O2 and O3 in the surface, as well as a thin molecular oxygen atmosphere, with auroral emissions concentrated near the polar cap boundaries (McGrath et al., 2004), but there are yet no ionospheric indications from Galileo radio occultation data of an equatorial atmosphere. To the extent that surface composition may reflect magnetospheric irradiation effects that depend on the intrinsic magnetic field, global distributions of some species may inform us on age and variability of the present magnetic configuration. Analysis of Galileo data from several close flybys indicate that Ganymede’s moment of inertia is 0.31 MR 2 , which is the smallest measured value for any solid body in the solar system (Anderson et al., 1996). Three-layer models, constrained by plausible compositions, indicate that Ganymede is differentiated into an outermost ~800-km thick ice layer and an underlying silicate mantle of density 3000–4000 kg/m 3 . A central iron core is allowed, but not required, by the gravity data. The existence of Ganymede’s magnetic field, however, supports the presence of such a metallic core. Galileo gravity data also indicate that Ganymede has internal mass anomalies, possibly related to topography on the ice-rock interface or internal density contrasts (Anderson et al., 2004; Palguta et al., 2006). Galileo magnetometer data provide tentative evidence for an inductive response at Ganymede, which again suggests the presence of a salty internal ocean within 100–200 km of Ganymede’s surface. However, the inference is less robust than at Europa and Callisto, because the existing flyby data are equally well explained by an intrinsic quadrupole magnetic field (superposed on the intrinsic dipole), whose orientation remains fixed in time, rather than an induced dipole (Kivelson et al., 2002). Additional flybys by a future spacecraft are needed to resolve this ambiguity. The Ganymede surface is more
2007 JUPITER SYSTEM OBSERVER MISSION STUDY: FINAL REPORT 01 NOVEMBER 2007 Task Order #NMO710851 SECTION 2— JUPITER SYSTEM SCIENCE GOALS AND OBJECTIVES cratered and ancient than Europa’s, consistent with a much thicker outer shell of solid ice. Although Europa shows stronger geologic and magnetic manifestations of a subsurface ocean, potentially (along with Ganymede) fulfilling the liquid water requirement we associate with life on Earth, the intrinsic magnetic field of Ganymede offers an alternative astrobiological parallel to Earth in partial protection of the equatorial surface and low-altitude orbital environment from magnetospheric irradiation. Importantly for potential long-term extensions of the Ganymede orbiter phase of this mission, this intrinsic field deflects much of the magnetospheric particle flux away from portions of low-altitude orbits within the region of dipolar field lines. Magnetometer data acquired during several close flybys show that Ganymede has an intrinsic magnetic field strong enough to generate a mini-magnetosphere embedded within the Jovian magnetosphere (Figure 2.2-6) (Kivelson et al., 1996). A model with a fixed Ganymede-centered dipole superposed Figure 2.2-6. Ganymede’s magnetosphere, simulated by X. Jia, UCLA, 2007. Field lines are green; current perpendicular to B is represented by color variation. Note intense currents flow both upstream on the boundary between Jupiter’s field and the field lines that close on Ganymede, and downstream in the reconnecting magnetotail region. 2-21 on the ambient jovian field provides a good first-order match to the data and suggests equatorial and polar field strengths of ~719 and 1438 nT, respectively; these values are 6– 10 times the 120-nT ambient jovian field at Ganymede’s orbit. Detection of numerous electromagnetic and electrostatic waves and measurements of energetic particles close to Ganymede confirm the inference of a magnetosphere. The most plausible mechanism for generation of Ganymede’s intrinsic field is dynamo action in a liquid-iron core. Dynamo action in a salty ocean is unlikely because unrealistically large convective velocities of ~1 m/sec are required to trigger a dynamo; in contrast, the required velocities in the core are only ~10 –5 m/sec (Schubert et al., 1996). Remnant magnetization in the uppermost layers of Ganymede’s silicate mantle is also a possible explanation for the intrinsic magnetic field, but to magnetize these layers, Ganymede must have exhibited strong dynamo action in the past (with field strengths at least ~30 times the present value). In addition, such an explanation would require unrealistically high levels of silicate magnetization. Whether or not Ganymede’s intrinsic field has secular variation like that of Earth cannot be determined from the presently available data, but primary and potentially extended orbital mission measurements by JSO would be valuable in attempting to set limits on this variation. Ganymede’s magnetosphere is one of the most important targets for exploration by the JSO. This unique magnetosphere within a magnetosphere is of particular interest because it exists in a parameter regime that can be studied in no other known magnetospheric system. The pressure in the surrounding plasma is dominated by magnetic pressure, a situation described as being a low plasma. (Near the Earth, the solar wind plasma is close to 1.) The flow upstream of the magnetosphere is sub-Alfvénic and submagnetosonic, so there is normally no bow shock upstream in the flow. The diversion of flow and the interaction with the upstream plasma is mediated by magnetohydrodynamic waves, with some contribution from interaction with ions newly produced from neutrals sputtered off of Ganymede (such ions
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