Geoscience Reference
In-Depth Information
primordial terrestrial water is best represented by water outgassed from oceanic basalts.
This water was depleted in deuterium by about 80 per mil or more with respect to SMOW
(standard mean ocean water). Seawater therefore has a separate origin that some scientists
do not hesitate to attribute to the outer Solar System. Comet showers are suspected to have
been frequent during some particular geological periods and thus were considered first.
Comets are celestial bodies originating in the outermost Solar System (the Kuiper belt)
whose highly elliptical orbits bring them close to the Earth. They are made of abundant ice
surrounding a small rocky nucleus. The measurement of the D/H ratio of Halley's comet
by the Giotto probe and of the Wild2 comet minerals retrieved by the Stardust mission
revealed a D/H ratio twice that of seawater, therefore limiting the input of cometary water
to less than 15% of the ocean. Alternatively, water may have been added to the Earth as
frost from the abundant planetary bodies that, right after the Solar System formed, were
cruising in the outskirts of Jupiter. These planetesimals were ejected very early from their
orbit by the gravitational pull of this planet, which acted just like a giant slingshot. In
either case, we would expect the terrestrial abundances of heavy rare gases, such as xenon,
to be higher than they actually are. A mixed source, mantle outgassing, plus a contribution
from distant planetesimals and comets, would probably account reasonably well for these
observations.
Convection in the core powered by gravitational energy and radioactive decay sustains
the Earth's magnetic field. Geophysicists working on the dynamo theory would like, how-
ever, to see core convection powered by more radioactive decay than is currently accepted
by geochemists. Experiments of elemental partitioning between pure molten iron and sil-
icate have so far been unable to incorporate substantial amounts of K into the metal. The
class of iron meteorites that are believed to best represent the core of proto-planets are
devoid of potassium. The two points of view could be reconciled if the high abundance of
certain light elements such as Si, S, or O, which the velocity of seismic waves in the core
requires, increased K solubility in molten iron alloy compared with that of pure iron.
12.5 The early Solar System
Contrary to an often-heard assertion, the Earth itself has never been dated as there is no
unmodified, inherited sample from the earliest times of our planet. In an atmosphere rife
with religious prejudice, the age of the Earth was one of the great debates of the nineteenth
and twentieth centuries between geologists, who claimed much time was required for sed-
imentary strata to be deposited and organisms to evolve, and physicists. Using the theory
of heat conduction, Lord Kelvin calculated an age of the Earth of 94 million years (see
box). One of the more imaginative attempts at dating the Earth was made in 1899 by the
British astronomer Joly who calculated the time required for an ocean initially made of
fresh water to become as saline as it is today if its only salt input was from rivers. This age
τ
can be calculated as the ratio of the mass of chlorine of seawater to the flux of chlorine
from rivers, which using the data from
Appendices A
and
G
gives:
