Evolution of the Atmosphere

The Prebiotic Atmosphere:

*Reduced: electron hungry; easily combines with other atoms, molecules

Major Constituents---->90%

CO2---->80%

Source: volcanic outgassing, impact degassing due to infalling planetesimals Sink: minimal; negative feedback loop with silicate weathering normally reduces CO2; at this time, fewer continents present and less of a sink; more CO2 remains in atmosphere

N2---->10%

Source: volcanic outgassing, impact degassing due to infalling planetesimals Sink: nitrogen is inert, (does not react readily with other molecules/atoms), thus stays put in the atmosphere

Minor Constituents---->10%

CO

Source: volcanic outgassing, impact degassing due to infalling planetesimals Sink: today combines with O2; higher concentrations then because there was little or no O2 in atmosphere to combine with

H2

Source: volcanic outgassing, impact degassing due to infalling planetesimals Sink: today combines with O2; higher concentrations then because there was little or no O2 in atmosphere to combine with

Trace Constituents

Methane, CH4

Source: hydrothermal vents

Prebiotic Oxygen

Source: Photolysis, (splitting apart of molecules by UV or visible radiation), produces O2 by the following: Step 1: Split H2O and CO2 molecules H2O à H + OH, hydrogen and hydroxyl CO2 à CO + O, carbon monoxide and oxygen Step 2: Combine OH and O OH + Oà O2 + H Figure 9-1, The Earth System, page 174 Sink: reactions (recombining) with reduced chemicals from volcanic eruptions such as CO and H2 Net Free Oxygen: close to nil Not enough free oxygen at the surface for: Respiration (breathing) To form an ozone *weakly reducing atmosphere: type of atmosphere that contains small concentrations of reduce gases and virtually no free O2 *photochemical reactions: chemical reaction induced by light or other electromagnetic radiation

Origin of Life

There is a general consensus among scientists that life originated by prebiotic synthesis. However, everything else surrounding this theory is debatable. *Prebiotic synthesis: complex organic compounds forming from chemicals and chemical compounds; life from raw building materials QUESTIONS: Where did life originate? On the Earth, elsewhere? What were the first organisms made of? Were those components available in the Earth's early atmosphere? What chemicals were available in the early atmosphere? How would such life have survived in this environment? What would keep those complex compounds from breaking back down??? Attempting to answer the latter questions, several theories have been proposed. The first three theories assume that life originated on the planet Earth somehow. The fourth assumes life originated elsewhere and came to Earth in the heavy bombardment period early in Earth's history.

Background Biology

Why are proteins important to us?

Our genetic material (DNA) is simply code for the making of proteins. All else follows suit.

How we use proteins

Catalyzing chemical reactions in the body (enzymes) Structural (literally building blocks in our bodies) Transport (cell to cell, through the blood, etc) Regulation of bodily processes (hormones): form countless chemically based feedback loops that do things like maintain homeostasis and signal the formation of secondary sex characteristics in humans

Earth-originating Life Theories

Theory One The Miller-Urey experiment (Harold Urey, geochemist and graduate student Stanley Miller from University of Chicago) Predating and in contrast to what we just learned about the composition of the early atmosphere, the Miller-Urey experiment is based on a different theory of early atmospheric composition. Modeled after Jupiter's atmosphere, thought at the time, 1952, to be similar to Earth's early atmosphere, a flask containing methane, ammonia, water vapor, and molecular hydrogen was sparked with electric discharge simulating lightning. The reaction produced many organic compounds, including the protein-building blocks, amino acids. What we got out of this: Basic compounds on which life depends can be synthesized by a simple mechanism that could have occurred in nature What has been proven otherwise since then: The early atmosphere probably wouldn't have had much methane and ammonia RNA WORLD: It is now believed that RNA preceded DNA and proteins were NOT the earliest structural elements of life

Theory Two RNA WORLD

(Thomas Cech and Sydney Altman from the University of Colorodo in the mid-80's) Most modern life depends on a complex interaction between DNA, RNA, and proteins for cell replication and carrying out the building of proteins. Because this interrelationship is so complex (and requires it's own products to reproduce and produce more products), it is evolutionarily illogical to think that the first organisms utilized such a complex system. It makes more sense to think that something simpler originated and evolved over time into the modern system we are familiar with today. Evidence that RNA preceded DNA: (as the genetic material carrier) -RNA (some types) can self-replicate unassisted, unlike DNA -RNA carries essentially the same genetic information that DNA does; slightly less stable form What we need to build life: RNA, among other things Early Atmospheric components: CO2, N2, CO, H2, trace CH4 How to get RNA from the latter compounds and elements: RNA is made of ribose, four nitrogenous bases, and phosphate How to get ribose: CO2 + H2 : H2CO Five molecules of H2CO, (formaldehyde), spontaneously react in water solution to form various sugars including ribose How to get four nitrogenous bases, (for our purposes, how to get the simplest base, adenine) Trace methane, CH4, from hydrothermal vents combined with N atoms in the stratosphere form HCN Five molecules of HCN, (hydrogen cyanide) form adenine How to get Phosphate: weathering of rocks by oceans Conclusion: Thus, it would have been possible though circumstantially tough to form RNA from available elements in the early Earth.

Theory Three Hydrothermal Vents

What happens at a vent:

Seawater flows about a kilometer down into the cracks in the rock. The water heats up and rises back out to the surface, carrying reduced substances it picks up along the way. The water picks up: H2, hydrogen H2S, hydrogen sulfide Fe2+, ferrous iron Ferrous iron is a reduced form of iron that is soluble in seawater. When the hot seawater (350 degrees Celsius) rises up and hits the cold seawater, the substances that were dissolved in the hot water precipitate out as the water becomes cooler. (Colder water holds fewer molecules than hot water…similar to condensation in the atmosphere). What is available chemically at a vent: H, H2S, Fe2+ Could organic molecules form from these elements: yes Problems: It is debatable whether or not life could have survived at such high temperatures. Even if complex molecules formed from the latter reduced materials, the structure of the organic molecules would not have been stable, (would have easily broken down again). Supporting Evidence: Current Hydrothermal Vent Life Live bacteria have been found in off-axis vents, (slightly cooler than main-axis vents), in water temperatures up to 130 degrees Celsius. They are called thermophilic bacteria, which means that they are heat-loving. Modern bacteria appear to have remained mostly unchanged in the past 3.5 billion years so they probably resemble primitive life. Conclusion: Thus, it is probable that life originated at hydrothermal vents. However, just because life can live under these circumstances doesn't mean it originated there.

Non-Earth-originating Life Theory

Theory Four Interplanetary Dust Particles

Why some think life may not have originated on Earth: While it is true that the constituents of life were available on Earth, concentrating them enough to allow reactions to occur would have been difficult. Also, early Earth was a hostile environment and most complex organic compounds that formed would not have lasted long. Some theorize that organic compounds formed outside of Earth and subsequently "arrived" on Earth during the heavy bombardment period. Source of Organic Compounds: interplanetary dust particles, IDPs Interplanetary dust particles are small particles in our stratosphere of extraterrestrial origin. They are known to contain amino acids and other complex organic compounds (as are meteors). They come from interstellar dust clouds.

How these Organic Compounds (might have) formed

--Some believe that organic molecules could have formed in the very cold (10K) interstellar dust cloud environment from ions and neutral molecules present there.

How these interstellar-cloud-borne compounds would have gotten to Earth

--When the pre-solar system cloud collapsed, forming our Sun and solar nebula, molecules formed in the interstellar environment that survived the collapse of the cloud, would have been incorporated into solid materials and formed asteroids and comets. These solid materials might have been delivered to Earth in large numbers during the heavy bombardment period of Solar system history, between 4.5 and 3.8 billion years ago.

Effect of Life on the Early Atmosphere

First Evidence of Life

Despite the fact that we are unsure as to how life began, we do know that it had begun at least by 3.5 b.y.a because we have fossilized evidence of that age in the form of microfossils and stromatolites. Microfossils are preserved remains of single-celled organisms. Stromatolites are the fossilized remains of bacteria that form layered, sedimentary mats, (stacks of horizontal layers over time).

Biota Affecting the Atmosphere

Methane: Immediate Affect

The most dramatic impact that biota could have had on the early atmosphere would have been through methane production. Methanogenic bacteria, methanogens, produce energy from chemical reactions that produce methane as a byproduct, (just as photosynthesis produces oxygen as a byproduct of an energy-creating metabolic process). H2 + CO2à CH4 + H2O Evidence suggests (through RNA sequencing) that these organisms are ancient and might have existed on early Earth. Assuming that methane was produced at the same rate as it is today, methane concentrations would have been greater than 1000ppm, 600 times its present level, due to the slower breakdown of methane in the absence of oxygen.

How this might have affected the atmosphere

CO2

As a greenhouse gas, the presence of 1000ppm methane in the atmosphere would have warmed the early atmosphere by about 15 degrees Celsius. Warmer temperatures accelerate the rate of silicate weathering, thus reducing the CO2 levels in the atmosphere (negative feedback) by a factor of 30. Increased methane from biota would have decreased CO2 levels in the atmosphere. The Huronian glaciation: The first glaciation documented was about 2.3 billion years ago, around the same time that oxygen first rose to significant levels. An increase in oxygen would have decreased methane concentrations in the atmosphere dramatically. Because methane is a greenhouse gas, a big reduction in methane concentrations could have cooled the planet enough to bring on this ice age.

Nitrogen Cycle Immediate Affect

Biological Nitrogen Cycle

Nitrogen is removed from the atmosphere via nitrogen fixation. Nitrogen is returned to the atmosphere via denitrification. Denitrification is a process by which some organisms reverse nitrification and release nitrogen back into the atmosphere. Thus, nitrogen cycles through the biosphere and atmosphere.

Oxygen Not An Immediate Affect