Magmatism on Super-Earths: What do we expect to see?

Edwin Kite & Michael Manga (UC Berkeley) Eric Gaidos (U. Hawaii)

exoplanet.eu, 12/2/2009

Queloz et al., A&A, 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, 2009Valencia & O’Connell, EPSL, 2009

Papuc & Davies, Icarus, 2008

Thermal model

Melting model

Parameterized convectionModels tuned to reproduce 7kmthick oceanic crust on today’sEarth

Tν = 43K

Assumptions: Melting with small residual porosity, melts separate quickly, and suffer relatively little re-equilibration during ascent.

.X(T,P) from:McKenzie & Bickle, 1988Katz et al., 2003pMELTS (Asimow et al.,2001)

Competing effects of greater planet mass

ΔT

P/ρgk(Tp – Ts)/Q

Mantle parcel ascendingbeneath stagnant lid

Mantle parcel ascendingbeneath mid-oceanridge

Melt fraction

Plate tectonics

Stagnantlid

Results: Plate tectonics versus stagnant lid

PLATES

STAGNANTLID

Katz et al., 2003productivity model

Kite, Manga & Gaidos, ApJ, 2009

Is plate tectonics possible?Valencia & O’Connell (EPSL, 2009) show that faster plate velocities on super-Earths don’tlead to buoyant plates - provided that Tc < 0.16 Tl at the subduction zone.

We find that this limit is comfortablyexceeded, and plates arepositively buoyant at the subductionzone when M ≥ 10 Mearth

Differing results related tochoice of tν.

Galactic cosmochemical evolution

Time after galaxy formation (Gyr)Time after galaxy formation (Gyr)

[X]/

[Si],

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mal

ized

to E

arth

10

1

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.

Effec

t of o

cean

s

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

Stellar heating dominates:

Knutson et al., Nature, 2007

HD 189733b(1.13 MJup)

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Detectability of ponds with isothermal surface temperature

Tem

perature

Tem

perature

Tem

perature

Atmospheres have wavelength-dependent phase curve shapeMagma ponds have wavelength-independent phase curve shape

Tidal heating dominates:Barnes et al., ApJL, 2009

Tidal habitable zoneInsolation habitable zoneCombined habitable zone

Minimum heating: 0.04 W/m2

Maximum heating: 2 W/m2 (Io)

Barnes et al., ApJL, 2009

Q’ is fixed (500).

Open question: Can tidal heating initiate a runaway greenhouse?

Hemming et al., ApJ, 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)