The Origin of the Solar System 45
The abundances of the highly siderophile elements in
Earth’s mantle are higher than one would expect to find af-
ter the planet differentiated because most siderophile ma-
terial should have been extracted into the core. The most
likely explanation for these high abundances is that Earth
continued to acquire some material after its core and man-
tle had finished separating. This late veneer amounted to
about 1% of the total mass of the planet.
The origin of Earth’s water is the subject of much de-
bate at present. Earth’s oceans contain about 0.03% of the
planet’s total mass. A roughly comparable amount of water
exists in the mantle (with an uncertainty of a factor of 3
in either direction). Earth may have also lost an unknown
fraction of its water early in its history due to reactions with
iron. Temperatures at 1 AU are currently too high for water
ice to condense, and this was probably also true for most of
the history of the solar nebula (pressures were always too
low for liquid water to condense). As a result, Earth proba-
bly received most of its water as the result of collisions with
other embryos or planetesimals that contained water ice or
hydrated minerals in their interiors.
Planetesimals similar to modern comets almost certainly
delivered some water to Earth. However, a typical comet
has a probability of only about one in a million of colliding
with Earth, so it is unlikely that comets provided the bulk
of the planet’s water. The deuterium–hydrogen ratio (D/H)
seen in comets is twice that of Earth’s oceans, which sug-
gests comets supplied at most about 10% of Earth’s water.
However, D/H has been measured in only 3 comets to date,
so this conclusion is tentative. Planetesimals from the Aster-
oid Belt are another possible source of water. Carbonaceous
chondrites are especially promising because they contain up
to 10% water by mass in the form of hydrated silicates, and
this water would be released upon impact with the Earth.
Calculations suggest that if the early Asteroid Belt was sev-
eral orders of magnitude more massive than today, it could
have supplied the bulk of Earth’s water. This water must
have arrived before core formation was complete however,
because carbonaceous chondrites and Earth’s mantle have
different osmium isotope ratios. As a result, the delivery
of water to Earth and its acquisition of a late veneer were
separate processes that occurred at different times in its
history.
The origin of Earth’s atmospheric constituents is also
somewhat uncertain. When the solar nebula was still
present, planetary embryos probably had thick atmospheres
mostly composed of hydrogen and helium captured from
the nebula. Most of this atmosphere was lost subsequently
by hydrodynamic escape as hydrogen atoms were acceler-
ated to escape velocity by ultraviolet radiation from the Sun,
dragging other gases along with them. Much of Earth’s cur-
rent atmosphere was probably outgassed from the mantle
at a later stage. Some noble gases currently escaping from
Earth’s interior are similar to those found in the Sun, which
suggests they may have been captured into Earth’s man-
tle from the nebula or were trapped in bodies that later
Total Earth
has chondritic
Hf/W but
non-chondritic
U/Pb Core
Rich in W and Pb
Poor in Hf and USilicate Earth
or Primitive Mantie
Rich in Hf and U
Poor in W and Pb(^180) Hf/ (^184) W and (^238) U/ (^204) Pb fractionation in the Earth
(^180) Hf/ (^184) W
total Earth =
(^180) Hf (^184) W
solar system~1.3
(^238) U/ (^204) Pb
Silicate Earth = 8−^9
(^238) U/ (^204) Pb
core = 0
(^180) Hf/ (^184) W
Silicate Earth = 15−^20
(^180) Hf/ (^184) W
core = 0
(^238) U/ (^204) Pb
total Earth ~0.7 >
(^238) U/ (^204) Pb
solar system~0.14
FIGURE 19 Hafnium–tungsten chronometry provides insights
into the rates and mechanisms of formation of the solar system
whereas U–Pb chronometry provides us with an absolute age of
the solar system. In both cases the radioactive parent/radiogenic
daughter element ratio is fractionated by core formation, an early
planetary process. It is this fractionation that is being dated. The
Hf/W ratio of the total Earth is chondritic (average solar system)
because Hf and W are both refractory elements. The U/Pb ratio
of the Earth is enhanced relative to average solar system because
approximately>80% of the Pb was lost by volatilization or
incomplete condensation mainly at an early stage of the
development of the circumstellar disk. The fractionation within
the Earth for Hf/W and U/Pb is similar. In both cases, the parent
(Hf or U) prefers to reside in the silicate portion of the Earth. In
both cases the daughter (W or Pb) prefers to reside in the core.
collided with Earth. Most of the xenon produced by ra-
dioactive decay of plutonium (half-life 83 Ma) and^129 I has
been lost, which implies that Earth’s atmosphere was still
being eroded 100 Ma after the start of the solar system,
possibly by impacts.
Radioactive isotopes can be used to place constraints on
the timing of planet formation. The hafnium–tungsten sys-
tem is particularly useful in this respect because the parent
nuclide^182 Hf is lithophile (tending to reside in silicate man-
tles) while the daughter nuclide^182 W is siderophile (tend-
ing to combine with iron during core formation) (Fig. 19).
Isotopic data can be used in a variety of ways to define
a timescale for planetary accretion. The simplest method
uses a model age calculation, which corresponds to the cal-
culated time when an object or sample would have needed
to form from a simple average solar system reservoir, as rep-
resented by chondrites, in order generate its isotopic com-
position. For the^182 Hf–^182 W system, this time is given as
tCHUR=
1
λ
ln
[( 182
Hf
(^180) Hf
)
BSSI
×
⎛
⎝
( 182
184 W
W
)
SAMPLE
−
( 182
184 W
W
)
( CHONDRITES
(^180) Hf
(^184) W
)
SAMPLE
−
( 180
Hf
(^184) W
)
CHONDRITES
⎞
⎠
⎤
⎦
(13)