Geodynamics and rate of volcanism on massive Earth-like planets (1) Edwin S. Kite* and Michael Manga - University of California, Berkeley Eric Gaidos - University of Hawaii, Manoa 4. Plate Tectonics: Can It Work On Massive Earths? (14-15) 3. Melt-Column Productivity: Plates & Stagnant Lids 5. Deep Oceans Needed To Suppress Melting Also, if continent production scales with ocean crust production (16), massive Earths will enshroud themselves in nonsubductible material: *Poster author, [email protected]. Stagnant lid Plate tectonics arXiv:0809.2305v1 [astro-ph] 1. Motivation 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 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. crustal thickness in m Potential temperature too high for meaningful model output Potential temperature too high for meaningful model output 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 long- lived 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) 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/m 3 . Dash-dot lines are for a crustal density of 3000 kg/m 3 , 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. a) b) c) Acknowledgements 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. 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, G 3 4(9), 1073, 2003. 9) Smith & Asimow, G 3 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, 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. where f area is the area covered by continents, f radio is the fraction of the planet's radiogenicity contained within continents, A is planet surface area, Z crust is maximum continental crustal thickness (limited by crustal flow, and scaling as the inverse of gravity), and M is planet mass. 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. crustal thickness in m 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.
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Geodynamics and rate of volcanism on massive Earth-like planets(1)
Edwin S. Kite* and Michael Manga - University of California, Berkeley
Eric Gaidos - University of Hawaii, Manoa
4. Plate Tectonics: Can It Work On Massive Earths? (14-15)
3. Melt-Column Productivity: Plates & Stagnant Lids 5. Deep Oceans Needed To Suppress Melting
Also, if continent production scales with ocean crust production (16), massive Earths will enshroud themselves in nonsubductible material:
*Poster author, [email protected].
Stagnant lidPlate tectonics
arXiv:0809.2305v1 [astro-ph]
1. MotivationNext-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
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.
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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 long-lived 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)
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.
a) b) c)
AcknowledgementsThis 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.
References1) 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,
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.
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, andscaling as the inverse of gravity), and M is planet mass.
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.
crustal thickness in m
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.
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