Other
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p24a..09r&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P24A-09
Other
[5770] Planetary Sciences: Fluid Planets / Tidal Forces, [6270] Planetary Sciences: Solar System Objects / Pluto And Satellites
Scientific paper
The internal state of Pluto is currently poorly understood [1]. It was probably struck by a Charon-forming impactor early in its history [2]; both bodies then evolved to the end state in which we find them now, synchronously locked to each other. Here we develop a thermal evolution model for Pluto including the despinning process to constrain the present-day thermal state and the likelihood of a subsurface ocean. We assume an initially differentiated Pluto with an icy mantle above a rock core. The thermal evolution of Pluto is determined using two coupled codes. For the icy mantle we use the numerical tool OEDIPUS [3] in which the thermal convection equations are solved for a fluid with temperature-dependent viscosity in a 3D spherical geometry. The thermal evolution of the core is computed using a 1D conductive model with internal heat generation. We coupled these thermal codes with a despinning model [4], using two different viscoelastic models (Burgers and Maxwell), to compute the evolution of the spin period and the rotational energy dissipated inside Pluto. We modified the code to include melting of the ice shell, and validated this modification with analytical results given by the Stefan problem. We tested the sensitivity of our model results to different initial temperature profiles, initial spin periods, potassium content of the core and ice viscosity at the melting point. The two most important parameters are the ice reference viscosity and the potassium content of the core. The ice viscosity limits the total rate at which heat can be transferred across the shell; if the radiogenic heating rate exceeds this value, the ice shell must melt [5]. The viscosity also controls whether convection occurs and, if so, the rate at which the ocean re-freezes. For a low viscosity (10^13 Pa s at the melting point) hot plumes form at the base of the ice shell and allow convection to occur without the ice shell melting. However, for a hot initial temperature profile or a low core potassium content convection still occurs but cannot prevent ice shell melting. Higher viscosities or higher potassium contents result in the formation of a sub-surface ocean. Despinning occurs over periods of a few to a few hundred million years, depending on the exact parameters adopted. In most cases, present-day Pluto consists of an ice shell ~100 km thick with an ocean ~200 km thick beneath. These simulations suggest that Pluto likely possesses an ocean at the present day [cf. 6,7,8]. The New Horizons spacecraft will be able to look for evidence of such an ocean, for instance by measuring the shape and Love number of Pluto. [1] McKinnon et al. (1997) Pluto and Charon [2] Canup (2005) Science [3] Choblet et al. (2007) Geophys. J. Int. [4] Robuchon et al. (2010) Icarus [5] McKinnon (2006) Icarus [6] Collins & Barr (2008) AGU [7] Hussmann et al. (2006) Icarus [8] Desch et al. (2009) Icarus
Nimmo Francis
Robuchon Guillaume
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