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AST121S: The origin and evolution of the Universe
Handout 10: The formation of planets
If we can view stars as the places within the Universe where atomic (or elemental) complexity is
created, then we can view planets as the locations where conditions are stable and benign enough
that complex chemical reactions can occur that will produce great diversity and complexity at a
molecular level. One can view Life (i.e. you and me) as the ultimate example (or ultimate as far
as we know, but probably not in reality!) of molecular complexity in the Universe. So it is of
interest to understand how planets come about, before we turn to examine the processes by which
molecular diversity and complexity is actually brought about.
Unfortunately, the state of our observational knowledge on planetary formation is very poor. It
is only in the last couple of years that we have had any convinsing evidence of any extrasolar
planets (i.e. planets orbiting other stars). The nine major planets, many more moons, and
countless asteroids and comets in our own Solar System comprise the only planetary system that
we know much about. However, we can at least study our own system fairly well: detailed
observations are possible and sophisticated robot-spacecraft probes have been sent to many of
them (all of the planets, except Pluto, and many of the moons have now been visited). In some
cases, spacecraft have actually landed on the surface (e.g. Mars and Venus), and astronauts have
of course walked on one extraterrestrial object (the Moon!) returning with rock samples, etc.
The formation of a planetary system
The most basic clue regarding the formation of the Solar System is the fact that essentially all of
the major components of the system orbit around the Sun, in the same direction, in a flattened
disk. Most individual objects bodies rotate in the same direction, and most of the moons orbit
their parent planets in this way also. This strongly suggests that the Solar System formed out of a
flattened, rotating disk of gas which has been called the Solar Nebula. It is highly likely that this
Solar Nebula was formed during the formation of the Sun itself.
The initial Solar Nebula would have had the same chemical composition as the Sun, which is the
same as a typical cloud of gas in the Galaxy. Hydrogen is still by far the most abundant element
(roughly 72% by mass), followed by 26% Helium. The total contamination by the other elements
(all produced in stars, as we have seen) is still very low, about 2%. In the case of the inner
"terrestrial" planets, in which the abundance of H and He is now very small, planetary formation
thus represents the concentration of these 2% impurities by an enormous factor.
The formation of the planets from the initial gaseous Solar Nebula is thought to have occurred
through four different phases.
(a) Growth of dust grains through "Condensation"
Except at very high temperatures, gas in the Universe will contain dust grains. These are similar
in size to the grains of solid material found in cigarette smoke, for instance. These dust grains are
thought to form soon after the chemically processed material is ejected from stars during the later
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phases of stellar evolution. The remnant of the famous naked-eye Supernova 1987a is already
forming dust grains, for instance.
As the temperature of the Solar Nebula falls, these grains act as the centers for condensation out
of the gaseous phase. This process is very similar to how steam condenses into water etc. The
surfaces of the grains promote condensation. Different gases have different condensation
temperatures. Furthermore, different parts of the Nebula will have different temperatures (the
inner parts would be expected to be hotter because they are closer to the proto-Sun) and so the
chemical species that condense out of the gas will be different in different places.
Close to the Sun, where the temperature is high, substances with high melting and/or boiling
points will condense out. It will still be too hot for substances with low melting/boiling points
(like familiar water ice) to condense out: they would quickly be vaporized again by the high
temperatures. Substances with low melting and boiling points are called volatile materials. The
substances condensing close to the Sun will be metals, like Iron and Nickel, etc., and metal
oxides, all with very high melting/boiling points. Farther out, we will get primarily Silicon
oxides, while in the outer parts of the Nebulae, where it is coolest, substances with very low
boiling/melting points (i.e. of high volatility) will be able to condense out and survive. These will
include familiar water ice (H20), and also methane ice (CH4) and ammonia ice (NH3).
Note that this condensation is a purely chemical process, with gravity playing no part.
Condensation is important because it explains in a very natural way why the planets in the inner
Solar System are dense "rocky" bodies made of metals and metal oxides. The moons of the Outer
planets, by contrast, are very low density and are composed primarily of the various different ices
of water, methane and ammonia.
(b) The growth of planetessimals through "accretion"
As the grains grow, gravitational effects become more important, and grains are attracted to each
other gravitationally. Once the grains get to be about 1 cm big, this takes over from condensation
as the dominant mechanism for the growth of grains because the mass is proportional to the size
cubed, whereas the surface area (relevant for condensation) increases only as the size squared.
As the "grains" grow past the size of pebbles we refer to them as planetessimals. Note also that
this gravitational growth is unstable, the first few grains to become enlarged then start growing
faster than their neighbors because of their stronger gravitational effect on their neighbours.
Eventually a few planetessimals grow to such enormous sizes that we would call them protoplanets.
(c) The evolution of protoplanets
The protoplanet grows faster and faster as material is attracted to it gravitationally, and is heated
by the energy of this material falling onto its surface. The protoplanet is further heated by the
radioactive decay of heavy elements. This heating leads, at least in the case of the inner terrestrial
planets, to the melting of the originally solid protoplanet.
The melting of the protoplanet allows two important processes to occur. Firstly, heavy material,
such as Iron and Nickel metal, will "sink" to the center of the planet. Lighter material, such as
silicates, will float to the top, and as the planet cools, form a solid crust. Secondly, the molten
planet will release any gases that were originally trapped in its interior. This outgassing will
remove the more volatile materials from the interior. Along with subsequent collisions with
volatile-rich planetessimals (see below), this outgassing is thought to be a source for the
atmospheres of rocky planets like Earth.
(d) The clearing of the Nebula
As might be imagined, this process of planetary formation is not very clean. A large amount of
debris is left over in the form of gas and residual planetessimals. Such material must be cleared
from the young Solar Nebula before it looks like the familiar Solar System. This is achieved by a
number of processes as follows: Residual gas is blown out from the nebula by the pressure of
radiation (i.e. sunlight) and particles (the solar wind) from the Sun. The solid planetessimals,
which will be relatively unaffected by either of these, can be cleared by (a) the sweeping action
of planets clearing a path around their orbits and (b) by being ejected from the solar system when
they suffer a close encounter with a planet.
One of the reasons that comets and asteroids are so interesting is that they are thought to be
fossils of the earlier stages of the Solar System. They are thought to be remnant planetessimals
that have survived within the Solar System without undergoing the subsequent evolution that has
occurred on the more massive planets and moons.
The origin and evolution of planetary atmospheres
The Inner Planets, unlike the Outer (or Jovian) planets, are primarily rocky bodies, with a very
thin atmosphere covering the surface. In the case of the Earth, the atmosphere (including the
oceans in this context of atmosphere) is a just a few tens of kilometers thick, compared with the
6400 km radius of the rocky part of the planet. The atmosphere contains a very small fraction of
the mass of the planet and yet it has played a vital role in the development of Life and complexity
on Earth. The processes through which the Earth's atmosphere evolved can well be seen by
comparing the atmospheres of the different terrestrial planets that have retained significant
atmospheres: Earth, Mars and Venus.
The Earth's first atmosphere
We can be reasonably sure that the Earth actually lost almost all of its first atmosphere, which
would have been comprised of gases from the Solar Nebula. The first atmosphere should have
contained a wide range of relatively inert gases that should have been present in the Solar Nebula
since they are seen in the Sun itself with quite high abundances. As well as Hydrogen, there
should have been Helium, Neon, Argon, Krypton, Xenon. Along with other gases, these were
presumably lost when the Earth went through its high temperature, molten phase. Gases are lost
to space through a leakage effect.
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Leakage of the atmosphere to Space
The temperature of a gas reflects the energy of motion of the individual atoms or molecules. At a
given temperature, the ensemble of molecules or atoms will have a well-defined distribution of
speeds (known as a Maxwellian distribution). At high temperatures, some non-negligible fraction
of the individual molecules will have a speed that exceeds the escape speed of the Earth's
gravitational field, and those molecules will leave the Earth, never to return. These lost
molecules are, however, quickly replenished by the same collisional processes that initially set up
the Maxwellian distribution, so that if the temperature of the gas is maintained, essentially all the
gas can leak away through this "speed window".
The speeds of the molecules in the gas, and therefore the fraction above the escape speed of the
planet depends on the temperature and the mass of the individual atoms or molecules (the
temperature is proportional to the energy of motion, 0.5mv2). Hydrogen and Helium atoms are
very light, so they are quickly lost from the Earth at almost any temperature, even at the relatively
cool present temperature. On the other hand, Argon is heavier than either Nitrogen or Oxygen,
N2 and O2, respectively, which dominate the present-day atmosphere, and Krypton and Xenon
atoms are heavier than even CO2 molecules, one of the heaviest gases that is present today. So if
Argon, Krypton and Xenon are presently found in much below Solar abundances, then we can be
fairly sure that these other familiar species were lost from the early atmosphere because it was
much hotter than at present.
The origin of the Earth's second atmosphere
So Earth's second atmosphere (which evolved into the one we know and breathe), must have
been established after the surface of the planet had cooled enough. Although it is not known for
sure which was dominant, each of the following three origins of gaseous material contributed to
the reestablishment of the atmosphere. The second two are likely the most importan:
(a)
Uncondensed gases from the original solar nebula
(b)
Outgassing of gases released from molten grains in the interior.
(c)
Impact of volatile-rich planetessimals.
The net effect of these is to produce an atmosphere that is rich in those volatile species that had
earlier been able to condense onto dust grains at about our distance from the Sun and hence be
built up into planetessimals. These are primarily methane, CH4, ammonia, NH3, carbon dioxide,
CO2, and water, H2O.
The evolution of Earth's atmosphere
Of these four main constituents, H20 has the highest boiling point. Therefore as the Earth cooled,
the water was the first to condense out of the atmosphere and thus formed the oceans. This was
fortunate, because CO2 gas is soluble in water. The CO2 in the Earth's atmosphere was
"scrubbed" from the atmosphere by rain-droplets and dissolved in the oceans. There it reacted
with things like Calcium and Silicon to make sedimentary rocks on the ocean floors.
The two remaining gases, CH4 and NH3, are both easily dissociated by ultraviolet radiation,
which at that time penetrated to all levels of the atmosphere (since there was no protective ozone
layer). CH4 was broken up into Carbon, forming CO2 which then entered the oceans as before,
and Hydrogen, which escaped off into Space since it is such a light gas and consequently has a
large "escape window". NH3 was broken down into Nitrogen gas (in the molecular form N2) and
Hydrogen, which was again lost.
This process took a considerable amount of time, of order 1-2 billion years, and had the effect of
leaving a Nitrogen rich atmosphere. Note that any free oxygen in the atmosphere was removed,
either by reacting with hydrogen to make water, or by reacting with elements in the solid surface
of Earth so as to make rust and sand etc. This was just as well, because oxygen is a highly
reactive element and its absence from the atmosphere at early times is thought to have been
important in the origin of Life.
However, the Earth's atmosphere is presently 20% Oxygen and 80% Nitrogen. One may then ask
where the oxygen has come from. The presently observed atmospheric oxygen is almost entirely
the product of Life. Particularly with the emergence of land life (only about 500 million years
ago) the oxygen abundance rose rapidly to its present levels. In fact, the O2 abundance rose
gradually over time, and land-based life only became possible once the O2 abundance had risen
enough that a protective ozone (O3) layer had been formed to block harmful ultraviolet radiation
from the Sun. Once land-based life was possible, it rapidly produced a lot more O2 to bring the
abundance up to 20%.
The atmospheres of the other terrestrial planets
Comparative atmospheric data
Composition:
Temperature
Pressure
Earth
80% N2
20% O2
300oK
1 atm
Mars
95% CO2
3%
N2
150-250oK
0.006 atm
Venus
96% CO2
3% N2
700oK
90 atm.
The atmosphere of Mars
Mars is further out from the Sun. Its constituent planetessimals would have been expected to
have been richer in volatiles, and the outgassing process would have been stronger than on Earth.
However, Mars' lower mass and hence weaker gravitational field and lower escape velocity
would have resulted in a greater loss of its atmosphere. Only the very dense CO2 gas is left since
all the other lighter (faster moving) gases have been lost. The temperature at the winter poles of
Mars is about 150oK, below the freezing point of CO2, so the polar caps are made of so-called
dry-ice, or solid CO2. There is evidence from the appearance of the surface of Mars for a period
of liquid flow. This would have required a much more substantial atmosphere than at present
(otherwise liquids would have immediately evaporated into the almost nonexistent atmosphere)
and this may have occurred for a period of time when the stronger outgassing on Mars was able
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to sustain the losses due to the thermal leakage to Space. When outgassing subsequently slowed,
the atmosphere finally leaked away leaving the very tenuous atmosphere that Mars has today.
Venus and the Greenhouse effect
Venus never cooled enough to develop liquid H2O oceans, and therefore could never remove the
CO2 from its atmosphere. This was a misfortune, because CO2 is a so-called greenhouse gas.
The greenhouse effect that is produced in an atmosphere by such gases works as follows.
The Earth is heated by sunlight emitted by the Sun, which has a temperature of 6000oK. Hence
the photons of sunlight arriving at Earth have wavelengths in the optical part of the spectrum that
easily pass through the atmosphere and heats things up to "room-temperature", i.e. about 300oK.
These also radiate energy but at much longer wavelengths, i.e. in the infrared waveband. Since
the temperature of the Earth is stable, we know that there must be a balance between the energy
of the optical sunlight energy that it receives from the Sun and the infrared emission that it
radiates back into space.
Unfortunately, the gas CO2 is transparent to optical light and opaque to infrared light. Therefore
a planet with a dense CO2 atmosphere will heat up since it continues to receive optical sunlight
but is unable to radiate this energy back into space efficiently. It will heat up until the higher
temperature (and stronger thermal radiation) overcomes the inefficient transmission of infrared
radiation back out into space (since Venus is not still warming up, there must again be overall
energy balance). Note that regular glass shares this property with CO2, which is why a car left in
the Sun heats up, and why farmers use green-houses to grow plants which need relatively high
temperatures. This is why Venus is so hot. Note that it only receives twice the sunlight as Earth enough to explain why oceans never formed but not enough to explain the fantastic surface
temperature of about 700oK (hotter than the melting point of lead).