Geology Reference
In-Depth Information
angular momentum causes them to rotate at increas-
ing velocities, like a pirouetting ballerina. As a
result, the shapeless cloud flattens out into a disk
(rather like pizza dough being spun by a
pizzaiolo
). It
was from this
protoplanetary disk
of gas and dust
(examples of which have recently been discovered
around many young stars) that the present planets
eventually accreted.
Planetary mass/10
24
kg
Density/kg dm
-3
Mercury
0.335
5.4
Atmosphere ~ 0.01%
Venus
4.87
5.2
Hydrosphere and atmosphere ~ 0.03%
Earth
5.98
5.5
Given the heating caused by gravitational collapse,
and the large variations observed in the abundances of
the volatile elements in the Solar System (Figure 11.5),
it is natural to postulate that the inner parts of the solar
nebula became very hot and entirely gaseous.
11
The
equilibrium condensation theory
regards the solid constit-
uents of the inner Solar System as
condensates
from a
cooling gaseous nebula whose initial temperature may
have been as high as 1500 °C. As cooling progressed,
elements would
condense
into solids in a predictable
'condensation sequence' which - given one or two
assumptions about the density and composition of the
gas - can be worked out thermodynamically.
First to condense would be the most refractory
elements and compounds (the platinum metals and
oxides of Ca, Al, Ti, and so on), appearing as solids at
about 1300 °C (~1600 K). Other elements would follow
at progressively lower temperatures, broadly in the
descending order shown in Figure 11.5.
According to this model, the planets that ultimately
formed from these condensates would differ in their
content of volatile elements according to (i) the dis-
tance from the Sun at which they accreted, and/or
(ii) the stage in this condensation sequence at which
they accreted to planetary size. Planets accreting
early would fail to incorporate moderately volatile
elements, not yet available in solid form; whereas
accretion at a later stage or in a cooler part of the neb-
ula would lead to assimilation of lower-temperature
condensates too, producing bodies of more primitive
composition like carbonaceous chondrites. In its
infancy, the Sun would also have radiated a much
more intense
solar wind
- the outward flux of protons
radiating from the Sun - than it does today, and this
could have swept uncondensed volatile components
Moon
?
0.074
3.3
Mars
0.642
3.9
Core of rock and ice ~ 5%
1900
1. 3
Jupiter
0
20
40
60
80
100
Percentage of mass of planet as:
Metal
Silicate
'Atmosphere'
Figure 11.6
Mass proportions of metal, silicate and 'atmos-
phere' in the terrestrial planets and Jupiter, from astronom
-
ical data. Note the rough correlation with the mean density
of each body (right-hand figures). It is uncertain in geophys-
ical terms whether the Moon has a metallic core or not
(Wieczorek
et al
., 2006).
present Solar-System average composition shown in
Figure 11.2. The heavy elements present in the cloud
were the accumulated products of successive cycles of
stellar nucleosynthesis in earlier stars, each star's con-
tribution having been recycled into the interstellar
medium by the supernova that ended the star's life.
Gravitational collapse of such clouds has two
outcomes:
• Mass is accelerated toward the cloud's centre of
gravity. The potential energy released by gravita-
tional collapse appears in the form of heat, leading
eventually to temperatures at the centre so high that
thermonuclear fusion can begin, igniting (in the
Solar System case) the infant Sun.
• As the dispersed outer parts of the gently rotating
cloud also contract under gravity, conservation of
11
It is clear from the low-temperature assemblages preserved in
carbonaceous chondrites that the solar nebula could not have
been hot throughout.
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