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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)