Geoscience Reference
In-Depth Information
of solar ultra-violet radiation is strong enough to ionize the atmospheric gases partially
and create a plasma (a mixture of positively charged ions and electrons): this is the iono-
sphere, which acts as a shield against most of the deadly solar wind and cosmic rays.
The composition of the upper atmosphere and the temperature it creates therefore condi-
tion the preservation or the loss of atmospheric gases from planetary atmospheres and the
well-being of life.
Loss may be amplified by hydrodynamic escape, an entrainment mechanism that cattle
would describe as molecular stampede. Let us assume that the atmosphere of a small planet
is essentially composed of hydrogen molecules and contains traces of N
2
. On average, the
hydrogen molecules travel faster upwards, down the pressure gradient, and keep pounding
over and over against the slower nitrogen molecules. If the atmosphere is thin enough
for the collision between nitrogen molecules to be infrequent, these remain in thermal
disequilibrium and some of them will soon find themselves moving faster than their escape
velocity and leave the gravity field. Hydrodynamic escape has been invoked in particular
to explain why Venus' atmosphere is devoid of nitrogen. This effect can be felt for the
isotopes of elements as heavy as xenon.
Loss of gases, such as oxygen and hydrogen, from the Earth, Mars, and Venus' iono-
sphere, has clearly shaped the evolution of their atmospheres. The internal magnetic field
of our planet creates a protective shell, known as the magnetosphere, that protects the atmo-
sphere from erosion by the solar wind; yet leakage through the polar (“auroral” zone) is
still substantial. Today, the Earth loses tens to hundreds of moles of hydrogen and oxygen
per second and these numbers fluctuate as a function of the strength of the terrestrial mag-
netic field and of the solar activity. The atmospheres of Mars and Venus, being devoid of
an internal magnetic field, are less well protected than Earth. Since light isotopes are pref-
erentially lost with respect to the heavy ones, this process is most clearly identified in the
isotope compositions of light gaseous elements. The D/H and
15
N/
14
N ratios of terrestrial
planets is higher than those of the Sun or the giant planets and characterize the residual
character of their atmospheres. They let us dream of the days Mars had an atmosphere not
so different from that of our planet. If gravity had helped, another planet might also have
received the gift of life.
Exercises
1.
Figure 12.21
shows the portion of the chart of the nuclides in the Ce-Gd range.
a. Draw the path of the
s
process in that range.
b. Assign each stable nuclide to the most probable process (
s
,
r
,
p
or a mixture of
these).
c. Use pure
s
Sm nuclides and the data of
Table 12.1
to calculate the relative contribu-
tions of the
s
and
r
processes to
147
Sm and
149
Sm (neglect the
p
contributions).
2. Show that for the
182
Hf-
182
W chronometer (
T
1
/
2
=
8.9 My) the “isochron” diagram
(
180
Hf/
183
W)
(
182
W/
183
W)
x
=
,
y
=
(where
stands for now) has a slope
s
=
∞
∞
∞
(
182
Hf/
180
Hf)
t
and an intercept
i
(
182
W/
183
W)
t
, where
t
is the time at which the last
=
