Geoscience Reference
In-Depth Information
Venus actually suggests that no surface is older than about 800 Ma. The nature of this
young resurfacing is not well understood.
The composition of Venus' atmosphere is ascribed to a runaway greenhouse effect: the
enormous amount of CO
2
led to a rapid increase of the temperature, to the complete vapor-
ization of water, and to its photolysis by ultraviolet radiation in the upper atmosphere. The
recent mission, Venus Express, did observe that oxygen and hydrogen are lost by the upper
atmosphere in 1:2 atomic proportions.
12.9 Planetary atmospheres
A nagging issue of planetary sciences is why some planets, such as the Earth and Venus,
are endowed with an atmosphere, while others such as Mars, Mercury, and the Moon have
none or almost none. Some planets are too small for their gravity field to hold back gases
with a light molecular weight, but this is not argument enough to explain why Mars has
such a thin atmosphere (700 Pa), whereas Venus seems only to have preserved an enormous
amount of carbon dioxide. The most important factor is the atmospheric temperature: if
some atmospheric molecules travel faster than the escape (“Jeans”) velocity (see below),
they will be lost and the atmosphere will disappear. The answer is a little more complicated
and the discussion can use a little bit of physics.
A first idea is to compare the kinetic energy
2
M
u
2
carried by a gas of molecular weight
M
at temperature
T
with mean squared velocity
u
2
with the potential energy of the gas
in the gravity field of the planet (we assume that the atmosphere is thin and therefore that
its distance to the center is equal to the radius
R
E
of the planet). We start from the law of
gravitational attraction to express that the potential is minus the derivative of the attraction
and get:
1
2
Mu
2
G
M
E
M
R
E
=
(12.16)
where
M
E
stands for the mass of the planet and
G
for the universal gravity constant.
Newton's law of mechanics relates the gravitational force and acceleration of gravity
through
G
M
E
M
R
E
=
Mg
(12.17)
and the escape velocity
u
e
is:
2
gR
E
u
e
=
(12.18)
This expression gives the value of 11 km s
−
1
for the Earth and 2 km s
−
1
for the Moon. We
now relate the velocity to the temperature through
1
2
Mu
e
=
3
2
RT
(12.19)
