ASTR 1210 (O'Connell) Study Guide
19. EVOLUTION OF THE TERRESTRIAL ATMOSPHERES
Scaled photos of the terrestrial planets.
Why are the atmospheres of the terrestrial planets so astonishingly
different from one another? How have their evolutionary paths
diverged? Given the facts (1) that manmade materials are beginning to
affect Earth's atmosphere and (2) that small changes can make big
differences, this is not merely an academic question. It is essential
to improve our understanding of atmospheric and climate evolution as
quickly as possible.
In the case of the Earth's Moon and Mercury, which have no appreciable
atmospheres, the answer is easy: their gravity is too small to
retain rapidly moving gas molecules near their surface, which therefore
diffuse off into space.
In the case of Venus, Earth, and Mars, we do not yet have a full
understanding of their atmospheric histories, but we have identified
the main processes involved and the likely patterns of evolution.
||VENUS ||EARTH ||MARS
|Relative Planet Mass ||0.8 ||1.0 ||0.1
|Relative Distance from Sun ||0.7 ||1.0 ||1.5
|Relative Atmospheric Mass ||100 ||1.0 ||0.01
|Bulk Atmospheric Composition ||CO2 ||N2, O2
|Relative Water Vapor || 0.0001 || 1.0 [1%] ||0.03
|Mean Surface Temperature ||460oC ||20oC ||-60oC
- There are huge differences in atmospheric mass,
composition, & surface temperature despite a modest range of
distances (only a factor of 2) from the Sun and similar
masses, at least for Venus & Earth.
- The temperature "Goldilocks" syndrome: Venus is (much) too hot; Mars
is too cold; but Earth is "just right" for lifeforms like us
- The habitable zone for Earthlike life is the region
surrounding a star within which liquid water can be stable on
planetary surfaces. For the Sun, it is evidently small: about
0.9-1.4 AU. At present it contains no planet other than the Earth.
The Sun's HZ is a tiny fraction of the total volume of the Solar
System, whose planetary orbits lie in the range 0.4 to beyond 40 AU.
Many different geophysical processes affect atmospheres, acting to
augment, decrease, or change their contents. Important examples:
- Infall of interplanetary material (comets, meteoroids,
This, plus gases captured directly from the solar nebula, was the
source of the earliest atmospheres of the terrestrial planets
but has probably not been important in the last 3.5 billion years.
- Volcanic outgassing (see picture above)
Volcanic activity releases CO2, H2O,
SO2, and many other compounds from the interior into the
atmosphere. This was the dominant source of the present
- Escape of gases to space and stripping by the solar wind
Any molecule near the top of an atmosphere with a velocity higher than
a planet's gravitational
escape velocity (see Study Guide 8)
can escape to space and not return. As a result of collisions with
other molecules, a small fraction of a given kind of molecule will be
moving faster than escape velocity at a given time. The smaller the
mass of the molecule, and the higher the temperature of the atmosphere,
the larger is the fraction of molecules that can achieve escape.
this figure. The
solid symbols show the escape velocities for various planets,
and the dashed lines show molecular velocities at the top of their
atmospheres. Any species whose velocity lies above the symbol
for a given planet will escape to space over time.
Most important molecules have already escaped from the Moon and
Mercury, which are effectively without atmospheres. This is also
true for most of the other small bodies in the solar systems (like the
asteroids & the satellites of the outer planets), though it is easier
to retain an atmosphere if the temperature is lower (as in the case of
Saturn's satellite Titan). On Earth, Venus, and Mars, most hydrogen
and helium have escaped.
The thin but high-speed "solar wind," a continuous outflow of gas from
the Sun, can also strip molecules if it can impinge on the
can protect it from the wind, and the Earth, where circulation in the
hot interior maintains a strong magnetic field, is shielded this way. But
Mars' magnetosphere diminished as its interior cooled off, and
stripping is thought to have importantly reduced the Martian
- "Carbonate-silicate cycle"
See the illustration above. This cycle involves a transfer of carbon
between the Earth's surface, its atmosphere, and its crust.
CO2 is washed out of the atmosphere by liquid
H2O precipitation and is deposited (through chemical
reactions involving silicate rocks) in limestone on the
seabeds. Deposition over the last billion years has been assisted by
sea animals leaving their shells behind.
Volcanic outgassing during recycling of the
crust returns CO2 to the atmosphere.
The net effect of the cycle depends on the relative strengths of the
deposition and return branches. On the Earth, because
the deposition rate is larger than the return rate, this
process has tied up massive amounts of CO2 in
terrestrial surface rocks (about 100 times the current
- Photo-Destruction of H2O
UV radiation dissociates H2O molecules in the upper atmosphere,
allowing H to escape to space (as described above). O2
combines with other molecules, and the water is lost. Since
almost all water is trapped at low altitudes in Earth's atmosphere,
this is not an important process there; but it has had a major
effect on Venus.
- The Greenhouse Effect
Absorption of sunlight is the primary source of heat for the
atmosphere and surface of planets. But secondary heating of
the surface and lower atmosphere is caused by the partial trapping
of infrared radiation from a planet's surface by certain gases
(H2O, CO2, CH4, O3).
though these are only "trace gases" in Earth's atmosphere (for
instance, CO2 constitutes only 0.04% of the volume of the
atmosphere), they account for
almost all of the infrared blocking of radiation from the
Earth's surface. They constitute a powerful "choke-point" in the flow of
atmospheric heat, so they have a major effect on the temperature
balance. For more details, see the diagram above (click for
enlargement) and Study Guide
This diagram shows the
relative heat flows (averaged over a year) involved in direct and
trapped solar radiation. Note that infrared radiation reflected by
the Greenhouse Effect provides almost twice as much heating for
the Earth's surface as does direct solar radiation. The large contribution
is due in part to the fact that Greenhouse heating occurs at night as
well as during the day and during all seasons.
The cycle rates for geophysical processes affecting the
atmosphere can be very fast in geological time:
For example, in 50 Myr, which is only 1% of Earth's age...
Key concept: the characteristics of an atmosphere are determined by
the balance point or "equilibrium" among all
- 1 "bar" (= the present mass of Earth atmosphere) of CO2
can be outgassed from Earth's interior
1 bar is 2500x the present CO2
content of the atmosphere.
- 1 bar of CO2 can be washed out of Earth's atmosphere by
precipitation of CO2-bearing liquid water or ice
- 1 ocean of H2O can be evaporated & dissociated on Venus
If the rates don't balance, there would be rapid evolution toward more
All significant processes must be considered. In complex
systems like planetary atmospheres it is easy
to overlook important factors.
Feedback mechanisms are critical: they can stabilize the system ("negative
accelerate change ("positive feedback")
Example: the carbonate-silicate cycle provides negative
feedback to help regulate the temperature of Earth's surface.
If the atmospheric temperature rises, there will be increased evaporation from
the oceans, which washes more CO2 from the atmosphere, which
decreases the Greenhouse trapping and hence the temperature. If the temperature
falls, the reverse situation occurs. This
feedback occurs only over long periods by human standards
(about 400,000 years), however, and is never able to keep the climate
perfectly stable. But it helps prevent a runaway Greenhouse on the one hand or
a permanent ice age ("snowball Earth") on the other.
The existing atmospheres were probably outgassed from the
interior in all cases, amounting to probably 100 bars on Earth
and Venus but less on Mars. An alternative subsidiary source of water
and atmospheric gases: comet impacts.
Earth: "It's the water"....
- Earth's distance from the Sun is "just right." A moderate Greenhouse,
driven by the four main Greenhouse gases, elevates the average
temperature about 30oC (54oF) to above the
freezing point of water. This much Greenhouse warming
is essential to having a robust biosphere on Earth.
- Earth's mean surface temperature is in the range such that outgassed water
stays liquid near the surface.
- There is a "cold trap" at modest altitude in Earth's
atmosphere where the temperature drops below the freezing point of
water. (See this
temperature profile.) Because of the trap, rising water vapor
condenses into droplets or ice crystals and ultimately falls back to
the surface. This prevents escape of water to the stratosphere and
therefore destruction by solar UV radiation. Water is retained near
- Water washes most CO2 out of the atmosphere,
depositing it in minerals on the ocean floors. The remaining
CO2 constitutes only about 0.04% of the bulk atmosphere
- With water vapor condensed into oceans and CO2 scrubbed
from the atmosphere, outgassed nitrogen becomes the dominant
gas in the atmosphere.
- Liquid H2O provided the environment necessary for
life to develop (about 3-3.5 billion years ago). Organisms
then process atmospheric gases to
release free oxygen (O2). By about 2 billion years
ago, significant amounts of oxygen were present in the atmosphere; the
oxygen density rapidly increased about 500 Myr ago, coinciding with an
explosion in the diversity of lifeforms. Free oxygen is the most
important global tracer of the presence of Earth-like life.
oxygenation of Earth's atmosphere also importantly changed the
kinds of minerals present on its surface.
- Earth's surface temperature oscillates by about 10o
degrees C (18o F) over periods of about 100,000 years,
driven by the astronomical Milankovitch cycles (see Study Guide 4) and other factors but mediated
by the carbonate-silicate feedback cycle. The mean temperature on
Earth over the past 2 million years has been lower than in earlier
- Because it is nearer the Sun and had a hotter surface,
H2O deposited by outgassing readily evaporated from its
- Because of the higher temperatures, there was no cold
trap. This allowed water vapor molecules to rise to high
altitudes, where they were destroyed by solar UV light.
- The absence of the carbonate-cycle cleansing by liquid
H2O precipitation permitted the CO2 atmosphere to
- The heavy H2O and CO2 atmospheric blankets
trapped infrared radiation from the surface ==> T rose further ==> more
liquid water evaporated ==> Greenhouse trapping increased.
- This positive feedback produced a "runaway" greenhouse effect
- All water was eventually removed from the atmosphere
- The end product of the runaway is the massive, hot, dry
CO2 atmosphere Venus has today.
- Continuous volcanic outgassing of materials like sulfur
dioxide in the absence of water cleansing
produced sulfuric acid clouds
- Without its abundant water, Earth would probably be like
Venus. In fact, if the Earth as it is today were moved
to Venus' orbit, it would probably evolve quickly toward the
hellish Venusian state.
- Mars was more distant from Sun and therefore colder.
- Its smaller mass supported less
outgassing from its interior and therefore a smaller Greenhouse Effect
- Early, wet atmosphere & oceans? There is considerable
evidence supporting this interpretation. The water era probably
ended over 1 billion years ago. See Study
Guide 16 and the Mars image page for
- An early biosphere? There is some, but marginal,
evidence in favor. See Study Guide
- But Mars is a small planet compared to Earth. It has a large
surface area compared to its volume, and therefore
its interior cooled off more quickly. Sufficient
CO2 was not resupplied from the interior; so
the Greenhouse failed. Water froze out and was presumably
deposited in subsurface permafrost.
- The lack of an active interior also left Mars with a weak
magnetic field, possibly allowing the solar wind to strip
much of its protective atmosphere away.
- Atmospheric pressure at the surface is now too low to prevent
the evaporation of liquid water if any appeared.
- The end product is a thin, cold, dry atmosphere with only a trace
of water vapor.
- Mars presents a harsh surface but one where, with difficulty,
human colonies could live. By contrast, we could never live on Venus.
E. Lessons Learned for Atmospheric Evolution
- Little differences can have huge consequences
Our favorable environment is due mainly to our
distance from the Sun and secondarily to the size of our planet.
- Biospheres are fragile on Earth-like planets
F. Climate Change: Natural and Unnatural
So far, we have discussed the bulk properties of the
terrestrial atmospheres: mass and composition as they change over many
millions of years. "Climate" refers to the behavior
of surface temperature, precipitation, and wind flow over the much
shorter timescales of interest to human beings. Climate changes
on the Earth have major practical consequences.
There are two distinct branches of the study of climate
change: measurements of the temperature and composition histories of
Earth's atmosphere and modeling of those histories so that
its future properties can be realistically predicted.
The most conspicuous climate events of the last 2 million years have
been the ice
ages, when a drop in the mean surface temperature allowed
great expansions of the polar ice caps. The last ice age ended about
10,000 years ago. To see the temperature and ice volume histories,
click on the thumbnail below. Note that a drop in the mean surface
temperature of only about 3oC was sufficient to
precipitate ice ages.
Astronomical effects (e.g. the "Milankovitch Effect", see Study Guide 4)
including small changes in the Earth's orbit and the tilt of the its
axis, which affect the amount of insolation in the polar regions, are
important in causing the small temperature changes that trigger ice
Intensive studies have also been made of the Earth's temperature history
over the past 1000 years. Except for the period since 1900, such
studies must rely on the use of various "proxies" for actual thermometric
measures. The profiles show several major climate events:
a "medieval warm period" (about 1000 AD) and a "little ice age" (about
1600 AD). But the most important change is a rapid increase in
Earth's mean temperature since 1900.
Remember, we're talking about global surface temperature
here, meaning averages over the entire surface of the Earth,
including the oceans and the southern hemisphere. So our local
weather is only a tiny component of the whole. For instance, despite
abnormally cold temperatures in the eastern US in January 2014,
this was globally the fourth warmest January on record since
1880. Here is a world map
showing 25-year temperature trends to 2000.
The global temperature increase coincided with a rapid increase in the
average atmospheric concentration of carbon dioxide, which was
discovered through independent
spectroscopic studies of atmospheric composition at Mauna Loa
observatory in Hawaii (see left hand panel below). There is no
question that this is due, in turn, to human use of fossil fuels. The
present CO2 concentration is 60% higher than the average
over the preceding 600,000 years (and 23% higher than the
maximum); see this plot.
Click on the thumbnails below for enlarged plots of changes in the
CO2 concentration and the Earth's surface
The plots above refer to the temperature in the lowest layers of the
Earth's atmosphere. There are many other geophysical markers of
global heating; changes
in seven indicators over 50-100 years are shown here.
Perhaps of greatest long-term consequence for the
climate is the increasing heat content of the oceans. The
chart below shows the huge rise in ocean heat content that has taken
place since 1960.
CO2 Concentration since 1960
Surface temperature since 1880
Surface temperature since 800
How are we to interpret these changes, and is there a link between
our use of fossil fuels and global heating?
The basic physical principles that govern the structure of planetary
atmospheres have been well understood for a century. The problem is
putting these together to model the myriad of processes and
environments that affect a system as complex as the real terrestrial
atmosphere. Those include ocean currents, mountain ranges, cloud
shielding, and solar energy input.
A major technical difficulty is that the
atmosphere is a strongly "non-linear" system: output is not
simply proportional to input.
Mathematically, such systems can exhibit "chaotic" behavior,
i.e. divergent results for small changes in the starting point.
This kind of behavior is exemplified by
the "butterfly" effect: a butterfly flapping its wings off the coast
of Africa can, in principle, produce a hurricane over Florida several
Here is a nice BBC video
showing how small disturbances are amplified by atmospheric
instabilities into large storms.
Such complexity makes it very difficult to study the effects that
humans may be having on the atmosphere and climate---and contributes
to the major scientific and political controversies surrounding
Nonetheless, the rapid
(exponential!) growth of the human species (see Study Guide 9) coupled
with our use of technology will inevitably affect Earth's
atmosphere unless we take deliberate actions to avoid this.
There is no doubt that human-induced changes have begun, with the
partial destruction of the ozone (O3) layer, which
shields Earth's surface from solar UV radiation, and the rapid rise
of CO2 content in the atmosphere.
In the absence of any other changes, the added CO2 (double
the pre-industrial amount by mid-21st century) would create
significant additional global warming through the Greenhouse Effect.
Recall that the pre-industrial complement of Greenhouse gases accounted
for a 30o C, or 54o F, increase in Earth's
surface temperature over its "bare" equilibriium temperature. This
much warming was beneficial, and the Earth's surface has long since
adjusted to it.
The problem is that only a small further global temperature increase
(2o C) over a period as short as a century would force a
readjustment that would have dramatic effects on weather, water
distribution, and growing seasons on a human scale.
sounds ridiculously small and inconsequential, yes? But recall that a
small 3o C change in temperature in the other direction
could induce an ice age, with clearly catastrophic consequences for
Fortunately, computer models of the atmosphere and climate change have
rapidly become more sophisticated and realistic as supercomputer power
has accelerated. Most atmospheric physicists agree that the models
are capable of distinguishing human-induced effects from the
atmosphere's continuous natural change. Nonetheless, debate has raged
over the extent to which a human Greenhouse warming component is
The Scientific Consensus
The scientific consensus, based on thousands of studies since the
1950's, is that some human-induced warming has occurred
(probably 40% of the temperature rise over the last 50 years) and that
significant additional warming is expected over the next 100 years.
The conclusion of the 2007 United Nations Panel on
Climate Change was that, with 90% confidence, humans are the main cause
of climate warming since 1950. Here is a December 2007 summary of the
situation from the American Geophysical Union:
"During recent millennia of relatively stable climate, civilization
became established and populations have grown rapidly. In the next 50
years, even the lower limit of impending climate change---an
additional global mean warming of 1 degree C above the last
decade---is far beyond the range of climate variability experienced
during the past thousand years and poses global problems in planning
for and adapting to it. Warming greater than 2 degree C above 19th
century levels is projected to be disruptive, reducing global
agricultural productivity, causing widespread loss of biodiversity,
and---if sustained over centuries---melting much of the Greenland ice
sheet with an ensuing rise in sea level of several meters. If this 2
degree C warming is to be avoided, then our net annual emissions of
CO_2 must be reduced by more than 50 percent within this century.
With such projections, there are many sources of scientific
uncertainty, but none are known that could make the impact of climate
change inconsequential. Given the uncertainty in climate projections,
there can be surprises that may cause more dramatic disruptions than
anticipated from the most probable model projections."
Here is a historical perspective on the predicted impacts of global
warming. The UN Panel will be producing another summary of the status
of climate change in 2014.
Such conclusions have been disputed by the fossil fuel industries and
their political allies and by a small subset of climate scientists,
though these are retreating in the face of growing evidence for
climate change. One formerly contrarian group at UC Berkeley has
recently independently reanalyzed surface temperature records and
earlier published trends.
In assessing the controversy, it's useful to remember that scientists
do not easily reach a consensus. There are tremendous incentives for
scientists to contravene the "conventional wisdom," to be able to
demonstrate convincingly that their peers are misguided. That's how
scientists become famous. No one earns great credit for merely
confirming what people already know. If scientists have reached a
consensus in the case of global warming, this means that contrary
evidence is unconvincing both in quality and quantity.
What to do? There is no doubt that humans can adjust to whatever
(nonlinear) changes occur over 100 years...so we will survive. But
the robustness of our economy depends on the stability of
climate patterns, not variations in them. The costs of dislocations
produced by major climate change could be enormous. Hurricanes
Katrina (2005) and Sandy (2012) are good examples of the scale of
the economic disruptions that climate change could produce, even
though it is difficult to determine whether those storms were
directly caused by such change. Such dislocations could easily favor
nations other than the US (the southwestern quarter of which, for
instance, could suffer severe drought), so climate change becomes
our national economic
The prudent course is to take steps to reverse the
increase in Greenhouse gases until there is a better understanding
of what we are doing to the atmosphere.
Reading for this lecture:
Study Guide 19
Bennett textbook, Chapter 10.
Reading for next lecture:
Study Guide 20
Bennett textbook, Chapter 11
August 2014 by rwo
Carbon cycle figure copyright © 2008
Pearson/Addison-Wesley. Text copyright © 1998-2014 Robert W.
O'Connell. All rights reserved. These notes are intended for the
private, noncommercial use of students enrolled in Astronomy 1210 at the
University of Virginia.