The deep interior of Venus, Mars, and the Earth: A brief review and the need for planetary surface-based measurements

Research areas:
10, SI
A comparison of the internal structure of Earth-like planets is
unavoidable to understand the formation and evolution of the solar
system, and the differences between Earth's, Mars', and Venus'
atmospheres, surfaces and tectonic behaviors. Recent studies point at
the role of core structure and dynamics in the evolution of the
atmosphere, mantle and crust. On Earth, the crust thickness and the
radius and physical state of the cores are known for almost one century,
since the advent of seismological observations, but the lack of
long-term surface-based geodetic, electromagnetic and seismological
observations on the other planets, results in very large uncertainties
on the crust thickness, on the temperature and composition of their
mantle, and on the size and physical state of their cores. According to
the currently available geodetic data, Mars' dimensionless
mean-moment-of-inertia ratio is equal to 0.3653 +/- 0.0008. When
combined with geochemical observations and with the inputs of laboratory
experiments on planetary materials at high pressure and high
temperature, this result constrains a narrow range of density values for
Mars' mantle and favors a light {[}6200-6765 kg m(-3)] sulfur-rich core,
but it still allows for a 1600-1750 km range for the core radius, i.e.
an uncertainty at least ten times larger than the precision obtained in
1913 by Gutenberg for the Earth's core. Mars' mantle density
distribution may be explained by a large range of temperatures and
mineralogical compositions, either olivine- or pyroxene-rich. The
unknown mean thickness of Mars' crust makes necessary a number of
working assumptions for the interpretation of gravimetric and magnetic
data. The situation is worse for Venus, and the most conservative model
of its deep interior is a transposition of the Earth's structure scaled
to Venus' radius and mass. The temperature conditions at the surface of
this planet hardly make possible long-term ground-based measurements,
but this is indeed feasible at the surface of Mars. Precise measurements
of Mars' crust thickness, core radius and structure, and the proof of
the existence or absence of an inner core, would put tight constraints
on mantle dynamics and thermal evolution, and on possible scenarios
leading to the extinction of Mars' magnetic field about 4.0 Ga ago.
Long-lasting surface-based geodetic, seismological and magnetic
observations would provide this information, as well as the
distributions as a function of depth of the density, elastic and
anelastic parameters, and electrical conductivity. Current studies on
the structure of Earth's deep interior demonstrate that the latter data
set, when constrained by laboratory experiments, may be inverted in
terms of temperature, chemical, and mineralogical compositions. (C) 2010
Elsevier Ltd. All rights reserved.