Magmatism on Super-Earths: What do we expect to see? Edwin Kite & Michael Manga (UC Berkeley)  Eric Gaidos (U. Hawaii) Detected planets are shown, minimum mass on the y axis, semimajor axis on the left. Planets detected both in transit and We can see a couple of dozen Super-Earths, including one – CoRoT-7b – that was confirmed this August to have a density of 6 g/cc, indicating a rock/metal composition. Queloz et al., A&A, 2009 exoplanet.eu, 12/2/2009

Radiogenic heating , stellar insolation, and tidal forcing In the first part of the talk I will discuss magmatism on Earth like planets where radiogenic heating (and secular cooling) are the dominant heat sources. Then I will move into the close in regime. The motivation here is that transit searches for Earth like planets, Corot and Kepler, are more sensitive to close in planets. These planets will have magma ponds on their star-facing hemispheres. Extrasolar planetary systems tend to be dynamically packed and many have high eccentricities. So finally I will discuss tidal heating.

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 Melting 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 re-equilibration 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 Stagnant lid ΔT 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 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.  Acts to reduce the effect of aging. [X]/[Si], normalized to Earth 1 Time after galaxy formation (Gyr)

Effect of oceans Kite, Manga & Gaidos, Astrophysical Journal, 2009; Ocean and planet masses (black dots) from accretion simulations of Raymond et al., Icarus, 2006

Stellar heating dominates: ESO (artist’s impression) HD 189733b (1.13 MJup) Now I’m going to discuss rocky planets close enough to the star that melting should occur at the substellar point The most basic questions we can ask are, What is the size of the magma pond? And, will it be detectable? As the planet passes through transit, the cold dark side faces towards Earth. As it moves around the orbit towards secondary eclipse, the hot star-facing side rotates into view. The resulting phase curve has already been measured for hot Jupiters – shown on the right – and provides constraints on atmospheric transport. I’m going to assume that close-in rocky planets have lost their volatiles, so that the primary means of transporting thermal energy away from the subtellar point is magma flow (or a silicate atmosphere, if it exists). I’m also going to assume that, like the Galilean satellites, these planets will be tidally locked. http://www.jach.hawaii.edu Knutson et al., Nature, 2007

Detectability of ponds with isothermal surface temperature If there is efficient horizontal mixing within the magma pond, then the pond will have an isothermal surface temperature If the whole planet has an isothermal surface, there will be no flux variation with position in phase curve 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

Minor effect of planet mass on crustal thickness 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)