Geoscience Reference
In-Depth Information
Chapter 2
Comparative planetology
The sun, with all those planets
revolving around it and dependent on
it, can still ripen a bunch of grapes as
if it had nothing else in the universe
to do.
the giant impacts, and is still being added,
including material much richer in the noble
metals and noble gases than occur in the
crust or mantle.
(6) The difference in composition of the atmo-
spheres of the terrestrial planets shows that
the original volatile compositions, the extent
of outgassing -- or the subsequent processes of
atmospheric escape -- have been quite differ-
ent.
(7) We now know that plate tectonics, at least the
recycling kind, is unique to Earth. The thick-
ness and average temperature of the litho-
sphere and the role of phase changes in basalt
seem to be important. Any theory of plate tec-
tonics must explain why the other terrestrial
planets do not behave like Earth.
Galileo Galilei
Before the advent of space exploration, Earth sci-
entists had a handicap almost unique in science:
they had only one object to study. Compare
this with the number of objects available to
astronomers, particle physicists, biologists and
sociologists. Earth theories had to be based
almost entirely on evidence from Earth itself.
Although each object in the solar system is
unique, we have learned some lessons that can
be applied to Earth.
Although the Earth is a unique body, and is the
largest of the terrestrial planets, we can apply
lessons learned from the other objects in the
solar system to the composition and evolution
of the Earth. The Earth is also an average terres-
trial planet; if we take one part Mercury, one part
each of Venus and Mars, and throw in the Moon,
we have a pretty good Earth, right size and den-
sity, and about the right size core. The inner solar
system has the equivalent of two Earths.
(1) Study of the Moon, Mars and the basaltic
achondrites demonstrated that early melting
is ubiquitous.
(2) Although primitive objects, such as the car-
bonaceous chondrites, have survived for the
age of the solar system, there is no evidence
for the survival of primitive material once it
has been in a planet.
(3) The
magma-ocean
concept
proved
useful
when
applied
to
the
Earth,
taking
into
account
the
differences
required
by
the
Planetary crusts
higher pressures on the Earth.
(4) The importance of great impacts in the early
history of the planets is now clear.
(5) Material was still being added to the Earth
and Moon after the major accretion stage and
The total crustal volume on the Earth is anoma-
lously small, compared with other planets, and
compared with its crust-forming potential, but
