Geophysical and cosmochemical constraints on properties of mantles of the terrestrial planets
1981
Basalts are understood to be derived by partial
melting from the mantles of their planets, and properties
of the basalt samples we have been able to obtain
(from Earth, the Moon, and the parent planets of the
basaltic achondrite meteorites) have been an invaluable
source of information about the composition and
nature of the mantles of these bodies (Chapter 3). There
is little doubt that basaltic volcanism has also been
active on the planets that we have not been able to
sample; in several cases, the surface morphologies and
photometric properties of these planets strongly indicate
the action of basaltic volcanism (Chapters 5 and 2).
The purpose of this chapter is to infer as much as we
can, by means other than sampling, about the compositions
of planetary mantles. Our estimates of mantle
compositions provide a basis for experimental petrologists
to project the possible compositions of basalts that
would be erupted on unsampled planets (Chapter 3).
The clues we have to mantle compositions, apart
from directly sampling them and the basalts that issue
from them, are (I) the gross geophysical properties of
the planets in question and (2) our estimates of their
bulk compositions. Fragmentary chemical information
comes to us from sources such as spectral studies of
planetary atmospheres and surface regoliths, but most
of our chemical constraints are based on broader considerations
of the overall composition of the solar system
and the early processes that appear to have caused
the terrestrial planets to incorporate varying proportions
of the elements.
The geophysical constraints are reviewed in section
4.2, below; the admittedly speculative question of chemical
processes that affected the origin and bulk compositions
of the planets (cosmochemistry) is treated in
section 4.3. Section 4.4 describes the process of planetary
modelling by which the most probable mantle
compositions, or compositional profiles, are derived
from these inputs. Finally, section 4.5 carries out this
modelling exercise for the terrestrial planets and Earth's
moon, discussing the most likely mantle properties for
each body.
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