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(1) Geodynamics and rate of volcanism on massive Earth-like planets Next-decade observatories promise to measure atmospheric mass, and perhaps composition, on rocky exoplanets (2). Geologically sustained volcanism can maintain gases with short lifetimes against photochemical decay, and replenish atmospheres that would otherwise be lost to stellar winds (e.g., 3). Other workers have looked at early degassing (4); we focus on geologically sustained degassing. 2. Method a) b) c) While we impose a core mass fraction and mantle composition similar to that of the known terrestrial planets, at least 3 modes of mantle convection are possible: a) Earth-like - Plate tectonics: Using a mass-radius relationship valid up to 25 Earth masses (5), we couple a standard parameterization of whole-mantle convection (6) to three melting models including pMELTS (7-9). Each mantle radioisotope is tracked seperately, but we ignore core cooling and tidal heating . We tune mantle temperature to match today's Earth. Plate spreading rate adjusts to balance the heat flux at the top of the mantle, and melting columns are integrated to the surface to mimic mid-ocean ridges. b) Venus-like - Stagnant lids: Our treatment is similar to plates, but with a stagnant-lid convection parameterisation (10), and our melt columns are truncated at the base of the lithosphere. c) Io-like - Magma pipes: We do not explicitly model mush ocean geodynamics. Instead, we track the lithosphere's Peclet number (i.e., the ratio of magmatically advected to lithospherically conducted energy) (11), and monitor melt fraction beneath the lithosphere. We find that mush oceans are not expected after 2 Gya on even the largest planets. *Poster author, [email protected]. arXiv:0809.2305v1 [astro-ph] 3. Melt-Column Productivity: Plates & Stagnant Lids 5. Deep Oceans Needed To Suppress Melting A silicate mantle cannot degas if a deep ocean layer inhibits melting. If ocean mass scales as planet mass then melt suppression will be a small effect, but volatile exsolution may be inhibited at lower pressures. Plate tectonics Po fo ten r m tia ea l te nin m gf per ul at m ure od el too ou hi tp gh ut 1. Motivation - University of California, Berkeley Pote n for m tial temp e eani ngfu rature to l mo del o o high utpu t Edwin S. Kite* and Michael Manga Eric Gaidos - University of Hawaii, Manoa crustal thickness in m Stagnant lid crustal thickness in m Our model output shows that decompression melting of passively upwelling mantle on old planets requires plate tectonics. This conclusion does not change when we account for galactic cosmochemical evolution of the principal longlived radionuclides and Si (12). However, compositionally layered mantle convection, which we do not model, may allow volcanism to persist on old stagnant-lid planets (such as Mars?) (13) 4. Plate Tectonics: Can It Work On Massive Earths? (14-15) Our model output suggests not. Plate tectonics requires subduction, but hot (massive) planets make thick crust but thin lithosphere, resulting in bouyant plates that are hard to subduct. Buoyancy stresses as a function of thermal evolution and planet mass. Positive values denote plate denser than underlying mantle, favoring subduction; negative values denote plate more dense than underlying mantle, retarding subduction. Solid lines connect buoyancy values for planets of different masses 2.5 Gyr, 5 Gyr 7.5 Gyr and 10 Gyr after planet formation, for constant crustal density of 2860 kg/m3. Dash-dot lines are for a crustal density of 3000 kg/m3, as might be the case for partial amphibolitization. Dotted lines are possible lower limits to plate tectonics based on Earth's (disputed) geological record; arguably, subduction must be possible on planets whose buoyancy forces plot above these lines. The Earth symbol is the model calculation for present day conditions on Earth. Also, if continent production scales with ocean crust production (16), massive Earths will enshroud themselves in nonsubductible material: To show effect on melting of a volatile overburden whose mass scales with planet mass, M. Crustal thickness in meters. Thick lines correspond to results with a volatile overburden; thin lines correspond to results without a volatile overburden. References 1) Kite, Manga & Gaidos, in revision. 2) Beckwith, ApJ 604, 1404, 2008. 3) Murray-Clay, Chiang & Murray, astro-ph 0811.0006. 4) Elkins-Tanton & Seager, ApJ 685, 1237, 2008. 5) Seager et al., ApJ, 669, 1279. 6) Schubert, Turcotte & Olsen, Mantle Convection in The Earth and Planets, Cambridge, 2001. 7) McKenzie & Bickle, J. Petrol. 29, 342,1988. 8) Katz, Spiegelman & Langmuir, G3 4(9), 1073, 2003. 9) Smith & Asimow, G3 6, Q02004, 2005. 10) Grasset & Parmentier, J. Geophys. Res. 103, 18171, 1998. 11) Moore, Icarus, 154, 548, 2001. 12) Pont & Eyer, MNRAS, 351, 487, 2004. 13) Elkins-Tanton, Zaranek, & Parmentier, EPSL 236, 1, 2005. 14) Valencia et al., ApJL 670, L45, 2007. 15) O'Neill & Lenardic, GRL 34, L19204, 2007. 16) Condie & Pease (eds.), When did Plate Tectonics Begin on Planet Earth?, GSA, Acknowledgements where farea is the area covered by continents, fradio is the fraction of the planet's radiogenicity contained within continents, A is planet surface area, Zcrust is maximum continental crustal thickness (limited by crustal flow, and scaling as the inverse of gravity), and M is planet mass. This project began as a 2004 summer project at Caltech, mentored by Dave Stevenson. Our thinking has also benefited from conversation with Norm Sleep, Nick Butterfield, Brook Peterson & Rhea Workman.