Geoscience Reference
In-Depth Information
X-rays, protons,
3
He) and particle fluxes emitted by the Sun during its early gravitational
collapse (T-Tauri phase). The implication is that the accretion of Jupiter and the other giant
planets from the Outer Solar System was by this time largely completed.
The numerous collisions between planetesimals increased the sizes of the resulting plan-
etary bodies, reduced their number, and cleaned up their orbits. The gravitational pull of
the massive outer planets perturbed their neighboring objects, which were either expelled
from the Solar System or kept pouring down on the Sun and the inner planets. The outcome
of these processes, which lasted less than 50 million years, was a small number of planets
The variability of oxygen isotope compositions in the Solar System
The stable isotope compositions of samples from individual planetary objects in the Solar
System follow a mass-dependent fractionation line (
Chapter 3
), which indicates that they
are derived from an isotopically homogeneous reservoir. Among the rare exceptions (Cr, Ti),
oxygen isotopes are remarkably anomalous in defining two separate trends in the
17
Ovs.
δ
such as the terrestrial fractionation line or the SNC (Martian meteorites) trend, and (ii) the
1:1 fractionation line of refractory inclusions and chondrules from carbonaceous chondrites
such as the Allende CV3. The anomalies of the different samples are usually described by
calculating a
δ
17
O parameter, which is approximately equal to
17
O
18
O. Martian
δ
−
0.5
δ
17
O values (they plot above the terrestrial
fractionation line or TFL), whereas refractory inclusions and chondrules from carbonaceous
chondrites have negative
meteorites and ordinary chondrites have positive
17
O values (they plot below the TFL). By analyzing the solar wind
implanted in metal grains from lunar soil and in traps carried by the Genesis mission it was
found that the oxygen isotope composition of the Sun lies on this 1:1 line with a
18
Oof
δ
17
O. Remarkably, ozone produced by electrical
discharge in molecular oxygen shows such fractionation along the 1:1 line (Thiemens and
There is a large consensus on the variety of processes that may have formed the trend
(i) such as vapor-solid fractionation and low-temperature alteration. In contrast, the nature
of the mass-independent process (ii) is not agreed upon. Some scientists believe in the so-
called “self-shielding” of CO dissociation by UV radiation from the young Sun (remember that
CO is particularly abundant at high temperature): the wavelengths of UV absorption by C
16
O,
C
17
O, and C
18
O are slightly shifted with respect to one another. Since the abundances of the
isotopes are very different, it takes different distances for the nebular gas to absorb UV of
a particular radiation: the wavelength of the most abundant C
16
O is smothered much faster
than that of the minor isotopes. Another candidate is the abundant molecule SiO. This idea
is opposed by those who believe that isotopic exchange between the co-existing O and CO
is too fast to make self-shielding an efficient process. Alternative interpretations appeal to
symmetry effects in the kinetics of some gas reactions (think of the differences between the
two ozone molecules
16
O-
16
O-
18
Oand
16
O-
18
O-
16
O). It is remarkable that the processes
that severely affected the isotope compositions of one of the most abundant elements in the
Universe still remain unresolved.
−
40 to
−
60
, i.e. with a very negative
