ASTR 1210 (O'Connell) Study Guide

Scaled photos of the terrestrial planets.

A. Comparison

• There are huge differences in atmospheric mass, composition, & surface temperature despite similar masses for Venus & Earth and a modest range of distances (only a factor of 2) from the Sun.

• 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 (in terms of distance from the Sun) within which liquid water can be stable on planetary surfaces. It is evidently small: about 0.9-1.4 AU. At present it contains no other planet than the Earth. Compare the HZ to the distribution of planetary orbit sizes in the Solar System from 0.4 to beyond 40 AU.

B. Processes

• Infall of interplanetary material (comets, meteoroids, asteroids)
  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)
  Releases CO₂, H₂O, SO₂, and many other compounds into atmospheres. This was the dominant source of the present terrestrial atmospheres.

• 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 can 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.
  See 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, and they 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 atmosphere. A planet's "magnetosphere" 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 atmosphere.

  
• "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.
  Because the deposition rate is larger than the return rate, this process has tied up massive amounts of CO₂ in the Earth's surface rocks (about 100 times the current atmosphere's mass).

• 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
  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). Even though these are only "trace gases" in Earth's atmosphere, they account for almost all of the infrared blocking of radiation from the Earth's surface, so they have a major effect on the temperature balance. For more details, see the diagram above (click for enlargement) and Study Guide 15.
  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.

C. Equilibrium

For example, in 50 Myr, which is only 1% of Earth's age...
• 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 extreme conditions.
All significant processes must be considered. In complex systems like planetary atmospheres it is easy to overlook important factors.

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.

D. Histories

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 30^oC (54^oF) 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 & therefore destruction by solar UV radiation. Water is retained near the surface.

• Water washes most CO₂ out of the atmosphere, depositing it as minerals on ocean floors. The remaining CO₂ constitutes only about 0.04% of the bulk atmosphere today.
  Even at this tiny level, CO₂ is still a major contributor to blocking the escape of IR radiation from the Earth's surface and hence in maintaining the Greenhouse warming.

• 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 capable of photosynthesis then release free oxygen (O₂). By about 2 billion years ago, significant amounts of oxygen were present in the atmosphere. Free oxygen is the most important global tracer of the presence of Earth-like life.

• Earth's surface temperature oscillates by about 10^o degrees C (18^o F) over periods of about 100,000 years, driven by the astronomical Milankovitch cycles (see Study Guide 4) and other factors and mediated by the carbonate-silicate feedback cycle. The mean temperature on Earth over the past 2 million years has been lower than in earlier epochs.

Venus

• Because it is nearer the Sun and had a hotter surface, H₂O readily evaporated from surface

• Because of the higher temperatures, there was no cold trap. This allowed water vapor to rise to high altitudes, where it was destroyed by solar UV

• The absence of the carbonate-cycle cleansing by liquid H₂O precipitation permitted the CO₂ atmosphere to grow

• CO₂ trapped infrared radiation from surface ==> T rose further ==> more liquid water evaporated ==> Greenhouse trapping increases.
  • This positive feedback produced a "runaway" greenhouse effect

  • All water was eventually removed from the atmosphere

  • The end product 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

• Mars was more distant from Sun and therefore colder.

• Its smaller mass supported less outgassing from 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 more discussion.

• An early biosphere? There is some (so far marginal) evidence in favor. See Study Guide 17.

• But Mars is a small planet compared to Earth, and 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

1. Little changes can have huge consequences
   Our favorable environment is due mainly to our distance from the Sun and secondarily to the size of our planet.

2. Biospheres are fragile on Earth-like planets

F. Climate Change: Natural and Unnatural

Temperature Histories

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 large snowfall here in Virginia in the winter of 2009-10, the period January-November of 2010 was (globally) the warmest 11-month period since records began in 1880. Here is a world map showing 25-year temperature trends to 2000.

CO2 Concentration since 1960

Surface temp since 1880

Surface temp since 800

Climate Modeling

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 weeks later.
Here is a nice BBC video showing how small disturbances are amplified by atmospheric instabilities into large storms.

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.

Recall that the pre-industrial complement of Greenhouse gases accounted for a 30^o C, or 54^o 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 (2^o 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.
(Two degrees sounds ridiculously small and inconsequential, yes? But recall that a small 3^o C change in temperature in the other direction could induce an ice age, with clearly catastrophic consequences for human civilization.)

The Scientific Consensus

"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."

Study Guide 19
Bennett textbook, Chapter 10.

Study Guide 20
Bennett textbook, Chapter 11

Web Links:

State of the Climate: Global Analysis (monthly updates from NOAA)
The Discovery of Global Warming (Comprehensive history by S. Weart, American Institute of Physics)
Atmospheric Evolution & Climate Regulation (J. Schieber, Indiana U)
Ice Ages (Wikipedia)
Lecture on Ice Ages (J. Frogel, OSU)
Java Demonstration of the Butterfly Effect (Exploratorium)
Full text of the December 2007 statement from the American Geophysical Union
Ozone Depletion(Cambridge)
Responses to GW Critics (J. Cook)
A Global Warming Primer (J. Bennett, UCol)
National Research Council Report on "Climate Change Science" (2001)
Climate Guide (UK MetOffice)
Climate Change (New Scientist site, articles & resources)
The Pentagon's Weather Nightmare. (There is some disturbing evidence that climate change can occur much more abruptly than is usually supposed. This article describes the implications of a possible "climate collapse.")

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Last modified May 2013 by rwo

Carbon cycle figure copyright © 2008 Pearson/Addison-Wesley. Text copyright © 1998-2013 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.