Group members preparing summary: Karen Bandhauer, Deborah Frundle, Krystina Schwartz and Sue VansiceOnline Summary Information
Learning Unit 1-2 incorporated two main objectives: to allow the student to gain an understanding of how the composition of the Earth's atmosphere evolved to its current state and to look at the agents and events which have caused this evolutionary process. This task was accomplished by demonstrating some basic physics to help in the comprehension of the long-term characteristics of the atmosphere.
The Earth's atmosphere is constantly evolving. One major point in the atmosphere evolution is when the Earth cooled and water condensed. This water was later photochemically broken down into oxygen. The oxygen blocked ultra violet rays, which allowed life to begin on Earth. The Earth's gravitational field and escape velocity (Ve = (2gRp)^1/2) determines the amount and type of atmosphere it will retain. Absorption properties (an Albedo (A) of 0.29) and radiation laws of the chemicals in the Earth's atmosphere determine the Earth's surface temperature (59*F), and the Earth's effective radiating temperature (also 59*F). The surface temperature allows water to be present in all three states. Gases absorb some fraction of radiant energy. The range of absorptivity of various gases ranges from 0 to 1.0, for various wavelengths of energy in both the visible and infrared spectrum. Ozone absorbs below .3 microns (ultraviolet light) Radiated energy from the Earth are absorbed by: H20 - (water vapor), CO2 absorb centered around 10 microns and other gases such as: CH4, N20, O2 and O3 absorb somewhere in between. The Atmosphere acts like a one-way blanket, allows solar radiation to enter the Earth's atmosphere but holds in (absorbs) outgoing infrared energy radiated from the earth. Note: All energy emitted by the earth does eventually escape to outer space - but the spherical shell of the atmosphere interrupts the flow of energy and radiating it back. The end result is an increase increasing the temperature of the earth and causing more energy to be emitted. Two of the four layers of the atmosphere are relevant to global change - the troposphere, stratosphere. They exist in then lower 30km. This contains 99% of the atmosphere and 99.9% is below 50km. Within the atmosphere are temperature gradients, which define the layers of atmosphere - from about 15C at the surface to about -55 C at an altitude of 10km is the troposphere of which the top portion is called the tropopause. Above the tropospause is a 10 km thick region of constant temperature and above this layer is a region of constant temperature, and above this layer the temperature increases with height to about 0c at 50km to make up the stratosphere. Within the lower 50 km of the atmosphere atmospheric gases are quite homogenous due to convection and turbulent processes. Nitrogen and oxygen decrease exponentially with height through out the troposphere. Ozone is produced by photochemical processes in the stratosphere with concentrations highest at around 30km above this mono-tomic oxygen and monatomic hydrogen exist in low concentrations. 00 million years ago two main processes gave way to the formation of the oxygen. 1) Nitrogen was fixed and 2) CO2 was absorbed by plants giving way to biological expansion. Note: Large amounts of O3 ozone were present relative to today's atmosphere. The oxygen cycle is made up of four reservoirs of oxygen: atmosphere, surface organic matter, sedimentary rocks, and fossil fuels. The atmosphere is large with 10^19 moles but sedimentary rocks are even larger with 10^21 moles however this is not free and is chemically bound. The greatest flow of oxygen is photosynthesis and respiration/decay with 10^16 moles per year. Burning of fossil fuels represents the greatest loss of oxygen. Oxygen is gained by weathering of rocks and from photolysis of water vapor.
Online Discussions
Dr. Takle proposed the following question at the start of Unit 1-2 dialog, "Why is it likely the first plants in an oxygen-deficient atmosphere were ocean plants rather than land plants?" Kurtis Cecil was the first of five respondents to this question. He brainstormed with the idea that perhaps it was a function of the abundant CO2 in the oceans, not the deficiency of oxygen in the atmosphere that was the answer. Dr. Takle followed up this line of reasoning with a response that the early atmosphere was very high in CO2, and therefore CO2 limitation was not the problem. Gregory Cillo brainstormed as well and hypothesized that perhaps the land was too hot and the gases were still too harmful for plants outside of the ocean, therefore initially isolating life to the oceans. Ryan Kardell articulated that perhaps the ultraviolet radiation entering unobstructed (without ozone) through the earth's atmosphere would have prevented plants from living on land. Mark Kochen brainstormed with logic similar to Ryan's. He proposed that the chemical composition of the atmosphere would have been "pre-livable" due to incoming radiation. Noor Salem responded to Mark by reacting that he though Mark's explanation made since, but he wondered how the ocean acted as a shield to reflect harmful ultraviolet radiation and harmful chemicals in the atmosphere. Mark analyzed Noor's question, and responded that after searching the Internet he found information that UV radiation decreases approximately 14% per meter of depth. He then posed the question of what becomes of the harmful gases that end up in the oceans. Noor once again articulated that through his research, plants lived near the top of the ocean, where sunlight is present and the temperature is warmer (higher temperature water is less soluble), therefore limiting the harmful gases entering the ocean, but filtering some of the UV radiation. Once enough oxygen was produced through photosynthesis, ozone was created and land plants came into existence. The final response to Dr. Takle's initial question was from Todd Pederson, who articulated that the answer to Dr. Takle's question could be answered by the results of Stanley Miller's experiment in the 1950s. The experiment illustrated that with conditions similar to those after the earth formed, with the addition of electrical impulses; organic material was formed in pure water. This experiment demonstrates that life on earth began in water, not land.
Lecture Material
The lecture material, presented on 1/10/01 by Dr. Richard Seagrave, focused mainly on the understanding the basic physics behind how the earth's atmosphere was formed, and the various models that were used to mathematically estimate the earth's surface temperature. It was stressed at the end of the lecture that once you know the temperature of the sun, its distance and the size of the earth you can predict many things, that chemicals selectively absorb, and that the atmosphere is not in equilibrium but at steady state.