Statistics – Computation
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsm22b0249c&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #SM22B-0249
Statistics
Computation
2720 Energetic Particles, Trapped, 2732 Magnetosphere Interactions With Satellites And Rings, 2756 Planetary Magnetospheres (5443, 5737, 6030), 5421 Interactions With Particles And Fields, 5440 Magnetic Fields And Magnetism
Scientific paper
Perhaps the most exciting discovery from Galileo Orbiter multiple flybys of Galilean moons has been presence of permanent (Ganymede) and induced (Europa, Ganymede, and Callisto) dipole moments from magnetometer data. Two polar flybys of Io ruled out a permanent dipole but the complex plasma environment has made determinations for an induced dipole, e.g. from a magma ocean, more elusive. However, a magnetometer can only measure local field vectors and magnitude at the spacecraft, not global field line configuration around a moon. The initial discovery of the Ganymede dipole in the magnetometer data was, however, complemented by detection of surface loss cones, consistent with dipole field geometry, in magnetic pitch angle distributions of energetic particles measured by the Energetic Particles Detector (EPD). It is of historic interest to note that Voyager 2 discoveries of the large tilts of planetary dipoles by magnetometer observations at Uranus and Neptune were also quickly confirmed by energetic particle measurements. This is because trapped energetic particles have predictable motions in smoothly varying magnetic fields, so precise predictions from magnetic field models can be made about positions of intensity and anisotropy variations due to moon interactions. In the Voyager data at Uranus and Neptune the moon sweeping signatures increased in depth at MeV energies for electrons and ions. Due to large gyroradii of high energy heavy ions, the Heavy Ion Counter (HIC) intensities of these iogenic ions at MeV/nucleon energies showed strongly anisotropic intensity variations, due to surface absorption, during many Galileo flybys of Io and Europa. Analysis of the HIC flyby data continues but lack of a permanent dipole at Io is consistent with the equatorial flyby observations. At Europa the HIC results to date indicate strong sensitivity to the induced dipole. Value of energetic ion anisotropy measurements for the moon magnetic environments could be enormously improved by increased energy, mass, and angular resolution at keV to MeV energies covered by EPD and at MeV/nucleon energies covered by HIC. These resolution increases, also allowing isotopic measurements of ions for composition objectives, could be enabled by substantially greater power, data rate, instrument mass, and near-moon observation times to be provided by the Jupiter Icy Moons Orbiter (JIMO) mission. Improved energy and directional resolution for energetic electrons above 10 keV to tens of MeV are also needed to model global distributions of incidence through magnetic environments onto the moon surfaces for computation of depth-dependent radiation dosages relevant to surface chemistry and for astrobiology applications. The latter include production rates to meter depths in the icy regoliths for oxidants and other chemical resources for life in the putative subsurface oceans, assuming efficient geologic transport downward through the overlying ice crust at least for Europa, and destruction rates to the same depths for biochemical signatures of life that may be found on the moon surfaces by remote and in-situ measurements. Our ability to resolve the potential oceanic abode of life within Europa, and to confirm the presence of oceans within Ganymede, Callisto, and perhaps (extended mission?) Io, should vastly improve from measurements provided by energetic particles as complementary to those from magnetic fields, orbital gravimetric analysis, laser/radar altimetry, and the newer techniques for planetary missions of active radio sounding for ionospheric imaging, local magnetic field line tracing via guided echoes, and determination of subsurface stratigraphy.
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