Geoscience Reference
In-Depth Information
internal dynamical state of super-Earths is due to
its influence on habitability and future detectabil-
ity. These connections are described below.
The composition of the atmosphere is a re-
flection of outgassing from the interior, escape
driven by thermal and nonthermal processes, and
life affecting its constituents. It is well known
that on Earth, the amount of oxygen in the atmo-
sphere is a consequence of biotic processes taking
place. In their absence, oxygen and ozone levels
would drop to negligible amounts. Therefore, oxy-
gen has been suggested to be a viable biosignature
(Leger
et al
., 2011a).
The carbon-silicate cycle is Earth's long-term
thermostat. Carbon dioxide is a greenhouse gas
that when present in the atmosphere, heats up
the planet's surface. Through the interaction with
silicate rocks, rained out carbon can be removed
from the atmosphere, reducing its temperature,
and can be kept sequestered in the crust and
plates. Conversely, the addition of CO
2
into the
atmosphere from volcanism can increase the tem-
perature at the surface. This negative feedback cy-
cle regulates the surface temperature. It has been
proposed to operate on Earth on a timescale of
billions of years thanks to plate tectonics (Walker
et al
., 1981; Kasting
et al
., 1988) through the sub-
duction of the plates that carry the sequestered
carbon (closing the cycle), the continuous volcan-
ism and, exposure of fresh rock that may continue
to interact with the CO
2
in the atmosphere. In
addition, a planet that has evolved with plate tec-
tonics may have an atmosphere different than if
it had evolved with a stagnant lid.
As for the existence of a magnetic field and
its connection to life, it is unclear if one is
needed. Earth's magnetic field, produced by large-
scale motions of liquid conductive iron in the
outer core, protects the atmosphere from high-
energy solar particles. And while the absence of a
magnetic field does not imply the absence of an
atmosphere (as Venus demonstrates), as with the
mode of convection, a planet with or without one
may have evolved different atmospheres.
Some progress has already been done in the
areas of plate tectonics and magnetic fields of
rocky super-Earths. A summary of the results
follows.
(a) Plate tectonics on super-Earths
Earth is the
most natural starting point when investigating
the interior dynamics of massive analogs. The
Earth is the only Solar System planet losing heat
through plate tectonics, at present time. Even
though plate tectonics is a complex process, the
basic principles are well understood. Plates are
generated at mid-ocean ridges, they spread and
move away coherently from the ridge, cooling
and thickening towards subduction zones, where
they founder. For mobile-lid tectonics to take
place, subduction has to happen and is the out-
come of two competing forces: the strength of the
fault where deformation of the plate happens, and
the driving forces available from convection that
may cause deformation. By studying the Earth,
we have learned that the rheological properties
are the key to control the mode of convection,
and thus on the viability of plate tectonics. It
determines convective vigor and the strength of
the plate.
A commonly used parameter to represent the
rheological properties is ''viscosity.'' Viscosity is
very important for rocky super-Earths because it
can vary by many orders of magnitude and may
dramatically influence the mode of convection. In
mantle material, viscosity highly depends on tem-
perature, as well as pressure and grain size (Karato
& Wu, 1993; Karato, 2008; see also Chapter 4, this
volume, above). A rough estimate can be obtained
by using a conventional Arrhenius formula that
depends on pressure (
P
) and temperature (
T
):
=
η
0
exp
E
+
PV
RT
E
+
P
0
V
RT
0
η
(
T
,
P
)
−
(9.2)
where
η
is viscosity,
E
is the activation energy,
V
is the activation volume,
R
is the gas constant,
and subscripts zero indicate quantities evaluated
at some reference point. With a somewhat
small activation volume of 1 cc/mol, an energy
activation of 2
10
5
J
/
mol, and a constant tem-
perature of 4000 K, the viscosity increases by
×
10
orders of magnitude between 100 to 1000 GPa (or
∼
