Geoscience Reference
In-Depth Information
Venus shows steep topography despite its high
surface temperature, and on Venus, the surface
topography and gravity have strong positive cor-
relation (Solomon
et al
., 1991; Solomon & Head,
1991). These observations indicate that the near
surface layers including the lithosphere on Venus
are stiffer than those on Earth. Indeed, there is no
evidence for plate tectonics on Venus, and rather
the stagnant lid mode of mantle convection with
cyclic resurfacing is inferred. The high strength
of the lithosphere on Venus despite its high near
surface temperature may be partly due to the lack
of water (Kaula, 1990). It may also be due to the
shut-off of shear localization due to high near
surface temperature. This is either due to high
healing rate (Landuyt
et al
., 2009) or to the sup-
pression of diffusion creep relative to dislocation
creep due to the difference in activation enthalpy
(Jin
et al
., 1998). Alternatively, the stagnant lid
mode of convection could preserve more water in
the deep mantle that leads to the deeper depth of
dehydration and the resultant thicker lithosphere.
Some of the near surface tectonics on a planet
depends on the lithosphere thickness. One ma-
jor factor controlling the lithosphere thickness
is the temperature-depth profile that depends
critically on the mode of heat transfer (plate tec-
tonics versus stagnant lid convection, e.g., Breuer
and Moore (2009). The lithosphere thickness is
also controlled by the depth at which dehydra-
tion hardening occurs (Karato, 1986; Hirth &
Kohlstedt, 1996). The depth at which dehydra-
tion hardening occurs is determined by the depth
at which the adiabat (during the process of crust
formation) cuts the dry solidus. This depth is in-
versely proportional to the gravity, and is large
for a small planet (Grott
et al
., 2013) (Figure 4.24).
The inferred thick lithosphere on Mars (Phillips
et al
., 2008) may be partly due to the role of
dehydration hardening.
Recently a large number of planets have been
discovered outside of the solar system (see
Chapter 9, this volume, below). These exoplanets
include Earthlike planets but some of their mass
exceeds that of the Earth (to
depth (km)
Earth Venus
Mars
1
100
60, 66
2
200
(dry) solidus
90, 100
3
300
120, 132
4
400
150, 165
5
adiabat
500
180, 200
1600
1800
1200
1400
Temperature,
°
C
Fig. 4.24
A diagram showing the depth of dehydration
(from Grott
et al
., 2013). Substantial dehydration will
occur when mantle materials undergo a large degree
(
10% or more) of partial melting. This leads to a large
change in viscosity (Karato, 1986; Hirth & Kohlstedt,
1996). The depth at which this occurs corresponds to
the depth at which dry solidus intersects the adiabat.
This depth is essentially controlled by the pressure and
temperature. Because the pressure is linearly
proportional to the gravity (which depends on the mass
of a planet), the depth at which this dehydration occurs
depends strongly on planetary mass. Reproduced with
permission of Springer.
∼
dynamics of these planets have been studied as
a function of mass (e.g., Tachinami
et al
., 2011;
Valencia & O'Connell, 2009; Papuc & Davies,
2008; Kite
et al
., 2009). Because the convective
heat transfer controls the evolution of planets,
rheological properties of these planets are the
key to understand their evolution. However,
because the pressures in these planets reach
∼
1TPa, the pressure effect is potentially very
large but it is difficult to make reliable estimates
of their rheological properties. In some studies,
pressure dependence of viscosity was ignored
(e.g., Korenaga, 2010; Valencia & O'Connell,
2009). In other studies, a model of a constant
V
∗
was used (e.g., Papuc & Davies, 2008; Tachinami
10
M
⊕
). Given a
broad range of planets with a similar composition
but different total mass, plausible evolution and
∼
