Geoscience Reference
In-Depth Information
9
Composition and Internal Dynamics
of Super-Earths
DIANA VALENCIA
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology,
Cambridge, MA, USA
Summary
of the solid Earth interior: composition and
interior dynamics. Although the composition
of super-Earths cannot be uniquely determined
given the degenerate character of the problem, it
is possible to place constraints on the mixing ratio
of compositional end-members - iron rocks, sil-
icate mantles, water/ices, and hydrogen/helium.
Within the dozen of planets for which there are
mass and radius measurements, so far there
are two populations: the high-density planets
that are solid and have a composition similar
to that of Mercury, and the volatile-rich planets
that have a nonnegligible envelope composed of
water/ices and hydrogen/helium. We still have
yet to find an Earthlike analog. In terms of the
interior dynamics, there is a debate on whether
or not rocky super-Earths can lose heat in a plate
tectonic mode. Although a consensus is growing
in favor of plate tectonics, the answer hinges on
better understanding the properties of rocks at
high pressure and moderate temperatures. Also,
studies suggest that rocky super-Earths have
limited time windows to exhibit magnetic fields.
These questions are raised at least in part from
an interest in determining what makes a planet
habitable in the face of exoplanet discoveries.
And while the data will continue to be limited
compared to the solar system planets, there are
exciting perspectives with future observations.
Thanks to the discoveries of extrasolar planets,
the exercise of comparative planetology now
has a new dimension. In particular, due to the
technological advancements towards discovering
smaller and smaller planets, it is possible to place
our knowledge of the Earth, other terrestrial
planets and icy satellites in a broad planetary
context. One advantage of studying extrasolar
planets is that the numbers are large, especially
so for the low mass, small-sized planets as we
have recently discovered that they are more
numerous than their gas giant counterparts.
However, one important limitation is the lack of
detailed data. In fact, the available data consists
mostly of masses, radii, period and calculated
equilibrium temperatures. For some giant planets
and one low-mass planet there are low-resolution
spectra on their atmospheres, and for a few giant
planets there is evidence of luminosity modula-
tion along the orbit, which carries information
about atmospheric heat redistribution around the
planet. Armed with this information, we aim to
characterize these objects by determining their
bulk composition, atmospheric composition and
dynamics, evolution and interior dynamics and
origin. This chapter looks at two aspects of super-
Earths that may be tackled from the perspective
