148 Encyclopedia of the Solar System
a “primitive solar nebula.” The presumed composition of
the nebula was that of the Sun, mostly hydrogen and he-
lium with a small sprinkling of heavier elements. It is these
impurities that must have condensed into dust and ice parti-
cles and accreted to form the planets. Evidently, the Jovian
planets were also able to retain a substantial amount of the
gas as well, but the terrestrial planets and many satellites
must have been made from the solids. [SeeTheOrigin of
theSolarSystem.]
An intermediate stage in the accretion was the forma-
tion of “planetesimals,” Moon-sized objects that merged to
form the final planets. For the terrestrial planets (Mercury,
Venus, Earth, Moon, and Mars), the number was probably
about 500. These objects would not remain in near-circular
orbits, and the ones in the inner solar system might end up
as part of any of the terrestrial planets. One would there-
fore expect them to begin with similar atmospheric com-
positions, and indeed those of Venus and Earth have many
interesting resemblances, as mentioned in Section 1. The
smaller bodies appear to have lost all or most of their orig-
inal gas (or never possessed much in the first place).
Many of the differences between the atmospheres of
Earth and Venus can be traced to the near-total lack of water
on Venus. These dry conditions have been attributed to the
effects of a runaway greenhouse followed by massive escape
of hydrogen. A runaway greenhouse might have occurred on
Venus because it receives about twice as much solar energy
as the Earth. If Venus started with a water inventory simi-
lar to that of the Earth, the enhanced heating would have
evaporated additional water into the atmosphere. Because
water vapor is an effective greenhouse agent, it would trap
some of the thermal radiation emitted by the surface and
deeper atmosphere, producing an enhanced greenhouse
warming and raising the humidity still higher. This feed-
back may have continued until the oceans were gone and
the atmosphere contained several hundred bars of steam.
(This pressure would depend on the actual amount of wa-
ter on primitive Venus.) Water vapor would probably be the
major atmospheric constituent, extending to high altitudes
where it would be efficiently dissociated into hydrogen and
oxygen by ultraviolet sunlight. Rapid escape of hydrogen
would ensue, accompanied by a much smaller escape of
the heavier deuterium and oxygen. The oxygen would re-
act with iron in the crust, and also with any hydrocarbons
that might have been present. Although such a scenario is
reasonable, it cannot be proved to have occurred. The en-
hanced D/H ratio certainly points in this general direction,
but it could have been produced from a much smaller en-
dowment of water (as little as 1%) than is in the Earth’s
oceans.
It used to be thought that Venus was a near twin of the
Earth, perhaps a little warmer but perhaps able to sustain
Earth-like life. It is still possible that the large divergences
we now see could have arisen from different evolutionary
paths; alternatively, the two planets may always have been
very different.
Two important minor gases in the atmosphere are likely
to be variable in time: water vapor H 2 O and sulfur diox-
ide SO 2. Each one is an infrared absorber that contributes
to the greenhouse effect, and together they make up the
material of the clouds, which also are involved with the
greenhouse and which reflect some of the solar energy that
would otherwise help heat the planet. Both are likely to be
released from large-scale volcanic flows and eruptions, and
water may also be brought in by the impact of a large comet.
Water is dissociated by solar ultraviolet radiation, and the
light H atoms escape from the top of the atmosphere while
the oxygen, as well as the sulfur dioxide, react chemically
with materials of the surface.
These processes have been studied by Bullock and
Grinspoon (2001) who find that the present situation is un-
stable and that after a billion years the clouds may disappear
altogether. The predicted surface temperature may fall by
about 50◦C; although the planet will absorb more of the
incoming solar energy, the effectiveness of the greenhouse
will also be reduced. Rapid supply of gases from a volcanic
event could raise the surface temperature by as much as
100 ◦C for half a billion years, followed eventually by a re-
turn to conditions similar to present ones. A large number
of other scenarios can be imagined, depending on the rate
and timing of the events that might supply extra gases and
the ratio of water to sulfur dioxide in each event. For exam-
ple, the impact of a large comet would supply mostly water
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