Geology Reference
In-Depth Information
out of the inner parts of the solar nebula, amplifying
their depletion in the inner planets.
Observational evidence for this condensation
sequence is provided by
calcium-aluminium-rich
inclusions
(CAIs) that are preserved as minute white
specks in some carbonaceous chondrite meteorites.
They consist of oxides and silicates of Ca, Al and Ti,
precisely those refractory substances predicted by
thermodynamics to condense at the highest temper-
atures (Figure 11.5). They also contain tiny refractory
metal nuggets rich in platinum metals and other
refractory metals like tungsten (W) and molybdenum
(Mo). CAIs have long been recognized geochron-
ologically as the earliest solid bodies known in the
Solar System: their absolute ages fall in a narrow
interval of 4567.30 ± 0.16 Ma (Connelly
et al.
, 2012),
12
and so it seems likely that they were the earliest con-
densates from a cooling solar nebula (Figure 11.5).
The chondrules characteristic of chondrite meteorites
(Plate 5) also had an early, high-temperature origin -
dated between 4567.3 and 4564.7 Ma (Connelly
et al.
,
2012) - but their igneous textures (Plate 5) seem to indi-
cate formation by re-melting of dust particles rather
than direct condensation from a cooled solar nebula.
The processes of re-melting are still poorly understood.
It was only later (at least 1.0-1.5 Ma later) that CAIs
and chondrules were incorporated into the meterorites
that host them today.
have been frequent in the disk, breaking up some
bodies but adding to others. As time went by the particle-
size distribution evolved through metre-scale and
kilometre-scale bodies to 100-km-scale
planetesimals
,
similar to bodies known to exist today in the asteroid
belt. By sweeping up smaller bodies in their path, such
orbiting bodies would generally grow larger and fewer
in number (forming what are aptly referred to today as
'oligarchs') and eventually coalesce into bodies resem-
bling the present planets.
This main accretionary stage in the formation of
the planets probably lasted only 30-40 Ma (Wood
et al.,
2006). Yet it is clear from the density of cratering on the
Moon - and from radiometric dating of impact melts
collected by Apollo missions - that intensive bombard-
ment of the terrestrial planets by smaller planetesimals
continued until about 3800 Ma ago,
13
700 million years
after the planets had originally been formed.
Some of these impacts were evidently huge. The
anomalously large metallic core of Mercury, for example,
has been attributed to a so-called
giant impact
that ejected
a large proportion of its original silicate mantle. Indeed,
it is widely accepted today that the Earth itself suffered a
similar catastrophic impact - perhaps 50-150 Ma after
the formation of the Solar System - in which the mantle
of the colliding planetary body was ejected, forming an
orbiting debris disk from which our present Moon
accreted. Such an origin for the Moon would account for
its tiny core and low density in relation to other inner-
Solar System bodies (Figure 11.6) if, as modelling sug-
gests, the impactor's core was captured by the Earth. An
impact origin for the Moon would also explain its
extreme depletion in volatiles (Figure 11.5), as the impac-
tor mantle is likely to have been vaporized by the coll-
ision. New evidence suggests that the Earth may have
been affected by more than one giant impact, only the
last of which led to the formation of the present Moon.
Planet formation
The planets of the Solar System originated as dust and gas in
the young Sun's protoplanetary disk. The mechanisms of initial
growth towards large bodies are poorly understood but, whether
by gravitational instability or simple 'sticking together' of
aggregates, the process must have formed a large number of
10-km-sized objects rapidly. … Once bodies reached this critical
size, gravitational perturbation became the dominant mecha-
nism for further accretion through collision.
(Wood
et al
., 2006)
How did a protoplanetary disk of dust (including
finely dispersed condensates) and gas surrounding the
early Sun aggregate into planets, specifically the rocky
terrestrial planets familiar to us today? The standard
model, accepted since the 1960s, is that the disk became
progressively more 'lumpy' as small particles collided
and agglomerated into larger ones. Collisions would
Chemical evolution of the Earth
The core
If the Earth was formed by the aggregation of
large planetesimals as discussed above, the energy of
their collisions would have been sufficient to cause
12
A slightly older age of 4568.2 ± 0.2 Ma was determined by
Bouvier and Wadhwa (2010).
13
Possibly culminating in a so-called 'Late Heavy Bombardment'
between 4.0 and 3.8 Ga.
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