DIX Planetary Science Seminar

Interior to the orbits of Jupiter's iconic Galilean moons are four minor, regular satellites, which occupy a unique niche within the Jovian system. Multiple lines of evidence suggest that these bodies formed at a more distant location in Jupiter's circumplanetary disk before coming to reside at their current short-period orbits. Nonetheless, how these moons dynamically evolved to such orbits has yet to be formally explained in the emerging paradigm of giant planet satellite formation. In this talk, I will present a quantitative model for the origin of the largest of these inner moons, Amalthea, that can be extended to its neighbor, Thebe, as well as other small bodies around their host planet or star. The basic idea is that Amalthea formed concurrently with the Galilean satellites from a reservoir of material located at a large joviancentric distance. As the innermost Galilean, Io, migrated inward from this reservoir, it captured the satellitesimal, Amalthea, into resonance and shepherded the small body to its modern orbital neighborhood. During migration through the disk, however, dissipative forcing due to aerodynamic drag induces overstable librations in the Io-Amalthea resonance, such that only a narrow range of nebular parameters can accommodate the requisite long-range transport. Thus by exploring this dynamical evolution, we can set unique constraints on the primordial environment of the Jovian system, and deliver key insights into the more general phenomenon of satellite migration in astrophysical disks.

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Between August 1976 and April 1977, the Apollo 17 Lunar Seismic Profiling Experiment (LSPE) deployed four vertical-component geophones to record continuous seismic data. Over 12000 moonquakes were detected, resulting in the most comprehensive seismicity catalog to-date for Apollo 17. However, studies have yet to constrain the exact source mechanisms of these events. From analysis of waveform shape, possible correlation with temperature, and incident azimuth, we categorize these events based on their possible source mechanism. Roughly a quarter of these moonquakes originate in the direction of the Lunar Module, the spacecraft left behind on the lunar surface by the Apollo 17 crew. These moonquakes are characterized by hours-long "bursts" of near-identical high-amplitude signals, each spaced five minutes apart. These bursts also consistently occur at the start of sunrise, the time-of-day with the steepest temperature increase, strongly suggesting a thermally- driven source. We attribute these events to stresses accumulated and released within the Lunar Module, in turn as a result of rapid thermal expansion at sunrise. The remaining moonquakes also occur at sunrise, as well as at other times-of-day characterized by steep temperature gradients (e.g. sunset), once again suggesting a thermal source. It is generally agreed upon that thermal moonquakes are caused by regolith sliding along crater walls or by cracks propagating in boulders, both of which are the result of thermal expansion and contraction. By comparing signal onset and amplitude between the four geophones, and assuming these factors correlate with source-receiver distance, we pinpoint a number of moonquakes that may originate from known boulders at the Apollo 17 site. Our findings have implications for lunar surface processes, as well as future lunar seismic deployments such as Farside Seismic Suite.