Planetary Cores Flows Driven by Mantle Libration

Details Planetary Cores Flows Driven by Mantle Libration Planetary Cores Flows Driven by Mantle Libration

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p41a0211n&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #P41A-0211

Physics

1507 Core Processes (1213, 8115), 5440 Magnetic Fields And Magnetism, 5464 Remote Sensing, 5724 Interiors (8147)

Scientific paper

We investigate, via a set of laboratory and numerical experiments, the flow induced inside a spherical fluid cavity by torsional oscillation of the outer shell. Our goal is to produce models of libration-driven flows within planetary cores and subsurface oceans. Such models will improve our understanding of a number of planetary bodies including Mercury, Europa, Io, Callisto, Ganymede and the Earth's Moon. Here we focus on the case of a spherical shell with either a small inner core or no inner core; moderate planetary rotation rate (Ekman number E = 10- 4); and libration frequency equal to the planetary rotation frequency ("synchronous libration"). We vary only the non-dimensional amplitude of libration α, defined as α=Δ φ (2 π flib) / Ømega, where Δ φ is the total angular displacement, flib is the libration frequency and Ømega is the background angular rotation rate. Different core flow regimes are observed as α is increased. For a small amplitude of libration (α \ll 1)), the oscillatory motion of the outer boundary drives laminar flows that are well described as inertial modes and waves. For α ~ 0.5, azimuthal roll instabilities periodically develop and decay along the outer shell boundary during each libration cycle. These instabilities tend to develop when the outer shell is decelerating and decay when it is accelerating. By further increasing α, the flow pattern transitions from axisymmetric rolls (m=0) to wavy rolls (m ≠ 0), and then to turbulent flow. Extrapolating our present results to Mercury suggests that mantle libration can drive large-scale instabilities in its liquid metal core. The authors wish to the thank NASA's PG&G and PME Programs for reasearch funding under grant #NNG0697G.

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