Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmmr14a..01m&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #MR14A-01
Astronomy and Astrophysics
Astronomy
[1221] Geodesy And Gravity / Lunar And Planetary Geodesy And Gravity, [5430] Planetary Sciences: Solid Surface Planets / Interiors, [5450] Planetary Sciences: Solid Surface Planets / Orbital And Rotational Dynamics, [6949] Radio Science / Radar Astronomy
Scientific paper
Rotation studies coupled with gravity measurements provide powerful probes of planetary interiors (e.g., Munk and MacDonald 1960, Peale 1976, Lambeck 1980, Wahr 1988, Dickey et al 1994). In the absence of seismology data, measurements of the bulk density and polar moment of inertia of a planetary body provide critical constraints on interior models and core size. Depending on the rotation state, the moment of inertia can be derived from spin precession (Venus, Earth, Mars) or from obliquity measurements (Mercury, Europa, Titan), combined with second-degree gravity coefficients. Measurements of length-of-day (LOD) variations can reveal dynamical interactions between layers as well as the state of the core. For instance, the amplitude of spin rate variations at Mercury together with spacecraft determinations of the gravity field indicate that the mantle of Mercury is decoupled from a molten outer core (Margot et al 2007). Earth-based monitoring of decadal LOD signatures could inform us about core-mantle interactions at Mercury. With improved gravity field determinations from MESSENGER and BepiColombo, the rotation measurements can yield an estimate of the size of the core. Departures from the expected spin orientation can inform us about core properties and dynamics. Such an offset in the spin orientation of the Moon has been used to quantify dissipation in the lunar interior, with both dissipation due to solid-body tides and dissipation at a liquid core/solid body boundary playing a role (Yoder 1981, Williams et al 2001). Continued observations of Mercury are therefore important as they may reveal a departure from the expected orientation. Venus apparently maintains a large (~2 degree) free obliquity that should damp on million-year timescales due to dissipation between a liquid core and solid mantle (Yoder 1997), so the spin state and core-mantle interactions are not well understood. Earth-based measurements of the spin state of Venus can refine our understanding of its interior structure and processes. There are currently no data constraints on the polar moment of inertia of Venus. The values that appear in the literature are based on analogy with Earth and scaling relationships. Although the predicted precession rate of 45 arcseconds/year is similar to that of Earth, motion of the pole in inertial space is only 2 arcseconds/year due to the small obliquity. Telemetry from multiple landers would provide the necessary data, as in the case of Mars (Folkner et al 1997), but the technical challenge and cost make this prospect tenuous in the near future. Because the long-lived Magellan spacecraft measured the pole location with uncertainties of about an arcminute (Sjogren et al 1997), much larger than the precision required, prospects from orbiting spacecraft are also limited. Earth-based observations spanning a decade have the potential to pinpoint the spin orientation to a few arcseconds, and provide the best short term hope for measuring the moment of inertia. Monitoring of the spin state of Mars can provide information about volatile transport and core properties (Zuber and Smith 1999, Yseboodt et al 2004). The measurements can be obtained in conjunction with seismology packages (e.g. Lognonne et al 2000, Banerdt et al 2004, Dehant et al 2004).
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