ASTR 121, O'CONNELL. Study Guide 19 [Spring 2008]

ASTR 121 (O'Connell) Study Guide

19. EVOLUTION OF THE TERRESTRIAL ATMOSPHERES

Scaled photos of the terrestrial planets.
(Radar surface map in case of Venus.)

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. This is 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).

In the case of Venus, Earth, and Mars, we have only a partial understanding of the main processes involved and the likely patterns of evolution.

A. COMPARISON

	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	CO₂
Relative Water Vapor	0.0001	1.0 [1%]	0.03
Surface Temperature	460^oC	20^oC	-60^oC

BIG DIFFERENCES in atmospheric mass, composition, & surface temperature despite similar masses for Venus & Earth and a modest range of distances from Sun.
Temperature "Goldilocks" syndrome: Venus (much) too hot; Mars too cold; but Earth is "just right" for lifeforms like us
The habitable zone for Earthlike life is evidently small: 0.9-1.4 AU, compared to a distribution of planetary orbit sizes from 0.4 to 39 AU.

B. PROCESSES

Many different geophysical processes affect atmospheres, e.g.

Volcanic outgassing
"Carbonate-silicate cycle"
Photo-Destruction of H₂O
The Greenhouse Effect

C. EQUILIBRIUM

The cycle rates for these processes can be FAST in geological time:

1 bar (= present mass of Earth atmosphere) of CO₂ is outgassed from Earth.
1 bar of CO₂ is washed out of Earth atmosphere by precipitation of CO₂-bearing liquid water or ice
1 ocean of H₂O is evaporated & dissociated on Venus

Key concept: the characteristics of an atmosphere are determined by the BALANCE POINT or equilibrium among all processes.

Feedback mechanisms are critical: can stabilize system OR accelerate change

D. HISTORIES

Atmospheres were probably OUTGASSED from the interior in all cases; probably 100 bars on Earth and Venus; less on Mars. An alternative source of water and atmospheric gases: comet impacts.

EARTH: "It's the water"....

Distance from Sun just right. Moderate Greenhouse elevates average temperature about 30^oC to above the freezing point of water. (This much Greenhouse warming is actually essential to having a robust biosphere on Earth.)
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 captures most CO₂ and washes it out of the atmosphere, depositing it in mineral form on ocean floors. The remaining CO₂ constitutes only about 0.04% of the bulk atmosphere today.
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 tracer of the presence of Earth-like life.

VENUS

Nearer Sun, hotter ===> H₂O readily evaporates from surface
Because of its proximity to the Sun, there is no cold trap ===> water vapor rises to high altitudes, where it is destroyed by solar UV
Absence of cleansing action by H₂O precipitation permits CO₂ atmosphere to grow
CO₂ traps infrared radiation from surface ==> T rises ==> more liquid water evaporates ==> enhances Greenhouse further
- This "positive feedback" produces a "RUNAWAY" GREENHOUSE EFFECT
- All water is eventually removed from the atmosphere
- Yields a massive, hot, dry CO₂ atmosphere
Continuous volcanic outgassing of materials like sulfur dioxide without water cleansing produces sulfuric acid clouds
Without its abundant water, Earth would probably be like Venus.

MARS

More distant from Sun, colder.
Smaller mass implies less outgassing from interior, therefore a smaller Greenhouse Effect
Early, wet atmosphere/oceans? Considerable evidence in favor. Wet era probably ended over 1 billion years ago. See Study Guide 16.
Early biosphere? Some evidence in favor. See Study Guide 17.
But CO₂ was not resupplied from interior; so the greenhouse fails; water freezes out ==> in subsurface permafrost?
Atmospheric pressure at surface is now too low to prevent evaporation of liquid water
===> Thin, cold, dry atmosphere

E. LESSONS LEARNED FOR ATMOSPHERIC EVOLUTION

Little changes can have huge consequences
Biospheres are fragile on Earth-like planets

F. CLIMATE CHANGE: NATURAL AND UNNATURAL

So far, we have discussed bulk properties of the atmospheres: mass and composition as they change over many millions of years. Climate refers to the behavior of surface temperature, precipitation, and windflow, over timescales of more interest to human beings. Climate changes on the Earth have major practical consequences.

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. Astronomical effects (e.g. the "Milankovitch Effect", see Study Guide 4) including small changes in the tilt of the Earth's axis and orbit, are important in causing the small temperature changes that trigger ice ages.

A major technical difficulty in studying climate change is that the atmosphere is a strongly "non-linear" system: output is not simply proportional to input.

"chaotic"

"butterfly" effect

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 controversy surrounding "global warming."

Simple mathematics demonstrates that 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.

partial destruction of the ozone (O₃) layer

rapid rise in the CO₂ content of the atmosphere

₂

But the atmosphere is sensitive to many different geophysical effects (e.g. ocean currents, cloud shielding, solar energy input) and is undergoing continuous natural change. It is difficult to realistically model the effects of Greenhouse pollution. Debate has raged over the extent to which a human Greenhouse warming component is detectable.

Click on the thumbnails below for time histories of CO₂ and the Earth's surface temperature.

The scientific consensus, based on thousands of studies, 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 in 100 years. The most recent (2007) statement by the United Nations Panel on Climate Change is 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 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. 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. 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. Most global warming skeptics don't understand the weight of the scientific evidence.

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 and Rita (2005) are good examples of the scale of the economic disruptions that climate change could produce, even though we don't yet know 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 important to our national economic security.

The prudent course is to take steps to reduce the increase in Greenhouse gases until there is a better understanding of what we are doing to the atmosphere.

Reading for this lecture:

Reading for next lecture:

Web Links:

Atmospheric Evolution & Climate Regulation

Lecture on Ice Ages

Java Demonstration of the Butterfly Effect

Full text of the December 2007 statement from the American Geophysical Union

Ozone Depletion

A Global Warming Primer

History of discovery of global warming

National Research Council Report on "Climate Change Science" (2001)

NRC Overview of Global Warming

Climate Change

The Pentagon's Weather Nightmare

Congress confronts scientists.

Current articles on climate change

Previous Guide

Guide Index

Next Guide

Last modified July 2008 by rwo

Text copyright © 1998-2008 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 121 at the University of Virginia.