Transcript slides
Magmatism on Super-Earths: What do we expect to see? Edwin Kite & Michael Manga (UC Berkeley) Eric Gaidos (U. Hawaii) Queloz et al., A&A, 2009 exoplanet.eu, 12/2/2009 Radiogenic heating , stellar insolation, and tidal forcing Radiogenic heating dominates: How does melt flux vary with time and planet mass? Is plate tectonics possible on Super-Earths? What is the role of galactic cosmochemical evolution? What is the role of oceans? Kite, Manga & Gaidos, Astrophysical Journal, 2009 Valencia & O’Connell, EPSL, 2009 Papuc & Davies, Icarus, 2008 Thermal model Parameterized convection Models tuned to reproduce 7km thick oceanic crust on today’s Earth Tν = 43K Melting model Assumptions: Melting with small residual porosity, melts separate quickly, and suffer relatively little reequilibration during ascent. .X(T,P) from: McKenzie & Bickle, 1988 Katz et al., 2003 pMELTS (Asimow et al.,2001) Competing effects of greater planet mass k(Tp – Ts)/Q P/ρg Plate tectonics ΔT Stagnant lid Melt fraction Mantle parcel ascending beneath mid-ocean ridge Mantle parcel ascending beneath stagnant lid Results: Plate tectonics versus stagnant lid PLATES Katz et al., 2003 productivity model STAGNANT LID Kite, Manga & Gaidos, ApJ, 2009 Is plate tectonics possible? Valencia & O’Connell (EPSL, 2009) show that faster plate velocities on super-Earths don’t lead to buoyant plates - provided that Tc < 0.16 Tl at the subduction zone. We find that this limit is comfortably exceeded, and plates are positively buoyant at the subduction zone when M ≥ 10 Mearth Differing results related to choice of tν. Galactic cosmochemical evolution [X]/[Si], normalized to Earth 10 Eu is a spectroscopic proxy for r –process elements such as U & Th. Eu/Si trends indicate that the young Galaxy is Si – poor. Effects on present-day conditions: Including cosmochemical trends in [U] and [Th] lowers mantle temperature (Tm) by up to 50 K for young planets, while raising Tm by up to 40 K for old stars, compared to their present-day temperature had they formed with an Earthlike inventory of radiogenic elements. 1 Time after galaxy formation (Gyr) Acts to reduce the effect of aging. Effect of oceans Kite, Manga & Gaidos, Astrophysical Journal, 2009; Ocean and planet masses (black dots) from accretion simulations of Raymond et al., Icarus, 2006 http://www.jach.hawaii.edu ESO (artist’s impression) Stellar heating dominates: HD 189733b (1.13 MJup) Knutson et al., Nature, 2007 Temperature Detectability of ponds with isothermal surface temperature Temperature Temperature Atmospheres have wavelength-dependent phase curve shape Magma ponds have wavelength-independent phase curve shape Tidal heating dominates: Barnes et al., ApJL, 2009 Minimum heating: 0.04 W/m2 Maximum heating: 2 W/m2 (Io) Tidal habitable zone Insolation habitable zone Combined habitable zone Q’ is fixed (500). Open question: Can tidal heating initiate a runaway greenhouse? Hemming et al., ApJ, 2009 Barnes et al., ApJL, 2009 Summary Minor effect of planet mass on crustal thickness Provided plate tectonics operates; buoyancy may be a problem Galactic cosmochemical evolution probably less important Si accumulates over galactic evolution, U & Th reach steady state Massive oceans suppress volcanism Important, e.g., for migrating planets (“ocean planets”) Magma ponds may be probe of composition Not known if ponds are close to isothermal Stable to TPW? Tidal heating can drive geodynamics and perhaps climate See recent Henning et al. paper on arxiv Backup slides (removed from online version)