Download *Poster author, .

(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
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1. Motivation
- University of California, Berkeley
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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.