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by density functional theory predicts that planets
with masses larger than 2M
E
may not even have
liquid cores, shutting all possibilities of magnetic
field generation due to the large internal pres-
sures. To exceed the temperature of the melting
curve of iron, temperatures in excess of
of planets and the level of insolation that would
yield liquid water, it may indicate the presence
of a similar negative feedback cycle such as
the carbon-silicate cycle on Earth. Although
it remains to be proven that plate tectonics is
the
only
possible or most suitable way for this
cycle to operate on billions of years timescales.
Conversely, if we find that CO
2
levels do not
correlate with insolation from the star, then it
is clear that there is no negative feedback cycle
involving carbon dioxide.
However, measuring the absolute levels of CO
2
or other constituents in the atmosphere is a
formidable task. So far, spectra have been taken in
transmission, so that only the very top of the up-
per atmosphere is sampled (e.g. at the
∼
10 000
K are required (Morard
et al
., 2011).
Many questions are still unanswered, especially
in terms of the deep interior of super-Earths.
For example, it is not resolved if the mantle is
convecting in a layered mode as a consequence
of either very different viscosity properties be-
tween perovskite and post-perovskite, a very pro-
nounced Clapeyron slope, or perhaps other phases
that may alter the behavior of convection as well.
The significance of addressing these questions,
aside from being theoretical interest, is that the
effect of outgassing, ingassing and thermal cool-
ing may have a detectable impact as well as a
profound effect on habitability. And habitability,
in planetary sciences, stands as a central question.
1 millibar
level), yielding no information about the deeper
regions. (This is the reason why Venus was first
thought to have an Earth-like tropical climate be-
low an atmosphere with a modest level of CO
2
.)
This means that to obtain the amount of CO
2
responsible for the greenhouse effect, one would
need to ''measure'' the thickness of the atmo-
sphere, which means ''seeing'' the ground. Indeed,
an example of this has been the clever experiment
of looking at the Earthshine reflected from the
Moon. Two features that indicated there is a solid
surface beneath our atmosphere are the reflection
from vegetation, and the glint from the ocean
(Pall e, 2010). When extrapolating to other Earths,
the red-edge signature may not unambiguously
indicate vegetation (Tinetti
et al
., 2006), while
the large changes in apparent reflectivity from an
ocean-bearing planet would be a signature of the
specular reflection from the ocean (Williams &
Gaidos, 2008). Therefore, perhaps it will be pos-
sible to put constraints on the amount of CO
2
and other atmospheric constituents by fitting the
spectrum with the additional constraint that the
atmosphere needs to be thin enough for a ground-
signature to be possible. As can be seen, the
problem of ''observing'' the mode of convection
on another planet is convoluted and filled with
obstacles and anticipated degeneracies. However,
this is an active field of research that may yield
more robust observables in the next few years.
∼
(c) Interior dynamics observables
Behind all
these theoretical studies, one main goal is to
connect the results to observations. This is
not an easy task. For the case of detecting the
mode of convection of a planet, one can think
of ''Earth as an extrasolar planet'' and ask what
features would be indicative of plate tectonics
from remote observations. Given that there
would be no resolution of the planet's surface,
it would not be possible to directly ''see'' any
of the familiar features, such as the mid-ocean
ridges and convergent zones, that are evident
in space pictures (with clouds removed) and are
representative of Earth's mobile-lid tectonism.
Thus, we need to turn to indirect evidence. It has
been argued that on Earth (Walker
et al
., 1981;
Kasting
et al
., 1993) plate tectonics has helped
regulate the amount of CO
2
in the atmosphere
so that the planet maintains a temperature at
the surface that allows for liquid water, and that
the CO
2
content in the atmosphere must have
been higher in the past to make up for the fact
that the Sun was 70% less bright (i.e. the faint
young Sun paradox). If we find a correlation
between the amount of CO
2
in the atmosphere
