The Origin of the Solar System 33
important effect. Protoplanetary disks are thought to be
flared, so that their vertical thickness grows more rapidly
than their radius. As a result, the surface layers are always
irradiated by the central star. For this reason, the surface
layers of the outer solar nebula may have been warmer than
the midplane.
The nebula cooled over time as the viscous accretion rate
declined and dust was swept up by larger bodies, reducing
the optical depth. In the inner nebula, cooling was proba-
bly rapid. Models show that at the midplane at 1 AU, the
temperature probably fell to about 300 K after 10^5 years.
Because the energy generated by viscous accretion and so-
lar irradiation declined with distance from the Sun, disk
temperatures also declined with heliocentric distance. At
some distance from the Sun, a location referred to as the
ice line, temperatures became low enough for water ice to
form. Initially, the ice line may have been 5–6 AU from the
Sun, but it moved inward over time as the nebula cooled.
Some asteroids contain hydrated minerals formed by reac-
tions between water ice and dry rock. This suggests water
ice was present when these asteroids formed, in which case
the ice line would have been no more than 2–3 AU from
the Sun at the time.
Meter-sized icy bodies drifted rapidly inward through
the solar nebula due togas drag(see Section 5). When
these objects crossed the ice line, they would have evapo-
rated, depositing water vapor in the nebular gas. As a result,
the inner nebula probably became more oxidizing over time
as the level of oxygen from water increased. When the flux
of drifting particles dwindled, the inner nebula may have
become chemically reducing again, as water vapor diffused
outward across the ice line, froze to form ice, and became
incorporated into growing planets.
3. Meteorites and the Origin of the Solar
System
Much of the above is based on theory and observations of
other stars. To find out how our own solar system formed, it
is necessary to study meteorites and interplanetary dust par-
ticles (IDPs). These are fragments of rock and metal from
other bodies in the solar system that have fallen to Earth and
survived passage through its atmosphere. Meteorites and
IDPs tend to have broadly similar compositions, and the dif-
ference is mainly one of size. IDPs are much the smaller of
the two, typically 10–100μm in diameter, while meteorites
can range up to several meters in size. Most such objects
are quite unlike any objects formed on Earth. Therefore, we
cannot readily link them to natural present-day processes as
earth scientists do when unraveling past geological history.
Yet the approaches that are used are in some respects very
similar. The research conducted on meteorites and IDPs
is dominated by two fields: petrography and geochemistry.
Petrography is the detailed examination of mineralogical
and textural features. Geochemistry uses the isotopic and
chemical compositions. This combined approach to these
fascinating archives has provided a vast amount of informa-
tion on our Sun and solar system and how they formed. We
know about the stars and events that predated formation of
the Sun, the nature of the material from which the planets
were built, the solar nebula the timescales for planetary ac-
cretion, and the interior workings and geological histories
of other planets. Not only these, meteorites provide an es-
sential frame of reference for understanding how our own
planet Earth formed and differentiated.
The geochemistry of meteorites and IDPs provides evi-
dence that the Sun’s protoplanetary disk as well as the plan-
ets it seeded had a composition that was similar in some
respects to that of the Sun itself (Fig. 5). In other respects
however, it is clear the disk was a highly modified residuum
that generated a vast range of planetary compositions. The
composition of the Sun can be estimated from the depths
of lines associated with each element in the Sun’s spectra
(although this is problematic for the lightest elements and
the noble gases). The Sun contains almost 99.9% of the total
mass of the solar system. A sizable fraction of this material
passed through the solar nebula at some point, which tells
us that the composition of the original nebula would haveFIGURE 5 The abundances of elements in our Sun and solar
system are estimated from the spectroscopic determination of
the composition of the Sun and the laboratory analysis of
primitive meteorites called carbonaceous chondrites—thought
to represent unprocessed dust and other solid debris from the
circumsolar disk. To compare the abundances of different
elements, it is customary to scale the elements relative to one
million atoms of silicon. The pattern provides powerful clues to
how the various elements were created. See text for details.
(Based on a figure in W. S. Broecker “How to Build a Habitable
Planet,” with kind permission.)