8.4. THE CONDENSATION SEQUENCE 115
with nothing. ese two lightest elements are gaseous unless compressed to a liquid by
high pressures. But hydrogen and helium can be used to make planets only if there is
enough gravity to hold them together.
2. Ices: carbon, nitrogen, and oxygen will combine with hydrogen to make molecules that can
be either gaseous or solid. Water, H
2
O, is the most abundant ice in the solar system, and
it is what both hydrogen and oxygen will form if given the chance. If there is not enough
hydrogen, the remaining oxygen can then form molecules such as CO
2
(carbon dioxide).
If on the other hand there is not enough oxygen, the remaining hydrogen can form ices
such as methane (CH
4
) or ammonia (NH
3
).
3. Rock: rocky materials are primarily made of combinations of oxygen, silicon, aluminum,
iron, magnesium, and calcium. Olivine, a magnesium iron silicate, is a good example; it
is the primary component of Earth’s rocky mantle, and is also found in meteorites. e
greenish-yellow crystals in the meteorite on the right in Figure 8.7 are made of olivine.
4. Metal: that is to say, real metals such as iron and nickel that can form metallic alloys,
sometimes in combination with sulfur.
e condensation sequence means that ices could condense out of the solar nebular only at
temperatures much lower than for rocky materials, which condense out at temperatures less than metals
such as iron and nickel. us, very close to the Sun, only rock and metal condensed out of the
solar nebula.
Far from the Sun, ices could condense, especially the most abundant—ordinary water ice.
Rock and metal condensed out too, just as it did close to the Sun. But the fact that water could
condense out in the outer solar system means that massive proto-planets—probably several times
the mass of Earth—quickly developed out of this mostly icy material. ese icy pre-planetary
bodies would have likely had cores of rock and metal, but the bulk of the mass was ice.
And so we have a scenario in which the Jovian planets began with these large proto-
planetary cores that had enough mass to pull in their own self-gravitating piece of the solar
nebula—creating what looks like mini-versions of the solar system as a whole. us, each Jovian
planet formed from its own self gravitating concentration of mostly hydrogen and helium at the
center of an orbiting disk of gas and dust. e major satellites of Jupiter, Saturn, Uranus, and
Neptune then formed from the disks around these newly formed planets.
is process of a cloud of gas pulling together under its own self gravity is strongly de-
pendent on temperature; if the temperature is too high, the cloud will be instead supported by
gas pressure and will not collapse. And so the inner planets, with smaller cores forming in a hot
part of the solar nebula from only much-less-abundant rock and metal, could not use gravity to
pull in their own massive envelopes of hydrogen and helium.
116 8. EVOLUTION OF THE SOLAR SYSTEM
8.5 THE LATE HEAVY BOMBARDMENT
ere are many individual exceptions to the overall properties of the solar system, as outlined in
Section 8.2. Uranus rotates, and its moons orbit, at an angle nearly perpendicular to the plane
of the solar system. Venus rotates very slowly, and backward. Earth has a large Moon, unlike
the other terrestrial planets.
How might these exceptions have come about? A glance at the surface of our own Moon
provides a clue. It is covered by impact features; nearly every visible mark on the Moon is at least
indirectly due to high-speed collisions with smaller bodies. With only a few exceptions, all of
the smaller solid bodies (and most of the larger ones) in the solar system bear the scars of impact
features.
e most obvious impact features are craters—nearly circular marks that are either bowl-
shaped (if they are small), or consist of a circular rim of material surrounding a shallow depression
(sometimes with a central mountain peak). is is just the feature one expects from an impact
with an object moving at high velocity. e kinetic energy of the impacting body creates an
enormous explosion, and it is this explosion that makes the crater. e crater thus formed is far
larger than the impacting body, which is destroyed in the impact.
If the impacting body is large and energetic enough, a huge impact basin may be formed.
e large darker areas on the Moon that mark its visible face-like features are the remains of
such impact basins. ese lunar maria regions are darker, flatter and show fewer craters than the
lunar highlands regions that surround them. e highlands, on the other hand, show evidence
of craters on top of craters, covering still more craters.
And so we can hypothesize a period of intense bombardment that formed the highlands
regions of the Moon’s surface. Near the end of this period, in the late heavy bombardment, the
Moon suffered several giant impacts that created the lunar maria, destroying the earlier im-
pact craters in those regions. After that, the rate of impacts decreased dramatically, leaving the
younger (but still very old) lunar maria regions relatively free of craters.
e Apollo missions to the Moon brought back many rocks from mostly the lunar maria,
but also the highlands—and these rocks date from the early solar system, roughly 4.5 billion
years ago. e most plausible scenario for the formation of the Moon is the giant impact theory:
the Moon formed as a result of a giant impact between proto-Earth and a (no longer existing)
Mars-sized proto-planet.
us, we have an early solar system plagued by collisions between the newly formed bod-
ies. is would have caused a period of intense bombardment of the surviving solar system
objects—both planets and moons. But eventually, most everything that could collide, would
have already collided, making collisions increasingly rare. Models of this process suggest the pe-
riod of intense bombardment would have mostly ended with a few very large impacts, in rough
agreement with the observations of the lunar maria.
Today, impacts occur at a far smaller rate—fortunately! Since the vast majority of the
impacts happened billions of years ago, early in the solar system’s formation, only bodies with
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