Geoscience Reference
In-Depth Information
to look at average values of all the convective
parameters. In any case, such results need to be
in accordance with numerical models that can
help assess the validity of the assumptions.
One study that moves in that direction is that
of Tackley et al . (2012). They use the numerical
model StagYY in a dimensional version and incor-
porate the effect of pressure as well as temperature
dependency of viscosity. They find that rocky
massive planets are at least just as likely to have
plate tectonics. Judging form Earth where there is
an increase in viscosity and thermal conductivity,
and a decrease in thermal expansivity with depth
(pressure), the expectation is a very low Rayleigh
number and hence very sluggish or no convection
for the lower mantle of super-Earths. However,
Tackley et al . (2012) include two effects that may
keep the viscosity from becoming too large for
convection to happen. At very high pressures, de-
formation by interstitial diffusion may become
more effective than by vacancy diffusion, possi-
bly even decreasing the viscosity with pressure
along an adiabat as suggested by Karato (2011);
and also, that post-perovskite, the dominant min-
eral in super-Earths' mantles appears to have a
viscosity 2-3 orders of magnitude lower than
perovskite according to one density functional
theory study (Ammann et al ., 2010). Under these
conditions, Tackley et al . (2012) find that while
large planets evolve very slowly and get very hot
before convecting, they exhibit plate tectonics
at all sizes. Although enticing, this result is not
definite as it is unclear if the viscosity indeed
decreases for post-perovskite (Ammann et al .,
2010) or increases as suggested by Karato (2010).
For example, and in contrast, Stamenkovic et al .
(2012) use a parameterized scheme that includes
pressure-dependent viscosity and, depending on
the value of the activation volume in the Ar-
rhenius law for viscosity, they obtain a sluggish
convection in the lower mantle that may hinder
mobile-lid convection.
Plate tectonics on rocky super-Earth is still un-
der debate, and it illustrates the importance of
considering planetary processes in a broad con-
text. Interestingly, the diversity of conditions
on exoplanets also provides new ground to test
theories and current models about convection.
For example, many of the short-period planets are
expected to be tidally locked and synchronously
rotating around their host star. This implies that
for rocky short-period planets, the surface temper-
ature condition for convection is bimodal. Van
Summeren et al . (2011) have studied such sce-
nario and they find that the planetary surface
shows a hemispheric dichotomy, with plate-like
tectonics on the night-side and a continuously
evolving mobile lid on the dayside with diffuse
surface deformation and vigorous volcanism.
In addition, because the composition and thick-
ness of the atmosphere is connected to the interior
via ougtassing, studies have been conducted to as-
sess the volcanic rates in extrasolar Earths. Kite
et al . (2009) looked at the rate of volcanism and
degassing on Earth-like planets and found that
volcanism is likely to proceed on massive planets
with plate tectonics over many Gy timescales, in
contrast to stagnant-lid planets where there could
be higher rates of melting in their early thermal
evolution but have shut off melting after a few Gy.
(b) Magnetic fields on super-Earths The condi-
tion for the existence of a global magnetic dynamo
is the presence of a convective metallic core.
Gaidos et al . (2010) use a parameterized convec-
tion model to predict that planets with a mass
larger than 2.5 M E will not develop inner cores.
Planets more massive than Earth will develop
weak magnetic fields due to core cooling only
if plate tectonics is present. They find that the
lifetime for the magnetic field decreases with
planetary mass and increases with higher sur-
face temperature. On the other hand, Tachinami
et al . (2011) use mixing length theory to treat
convection and predict that the lifetime slowly
increases with the planetary mass, independent of
initial temperature gap at the core-mantle bound-
ary until a critical value of the order of O (1 M E )
where it abruptly declines owing to the man-
tle viscosity enhancement from pressure effects.
Even though both studies predict different trends
for the lifetime of the magnetic field, they both
agree that there is a small range of masses and/or
small window of time for the presence of mag-
netic fields on massive terrestrial planets. Fur-
thermore, calculation of the melting temperature
Search WWH ::




Custom Search