(6) Earth in space and time. The student knows the evidence for how Earth's atmospheres, hydrosphere, and geosphere formed and changed through time. The student is expected to: (a) analyze the changes of Earth's atmosphere that could have occurred through time from the original hydrogen-helium atmosphere, the carbon dioxide- water vapor-methane atmosphere, and the current nitrogen-oxygen atmosphere; (b) evaluate the role of volcanic outgassing and impact of water-bearing comets in developing Earth's atmosphere and hydrosphere;

Atmosphere Hydrosphere Geosphere The atmosphere is a layer of gases surrounding the planet that is held close to us by our gravitational field. It protects organisms by absorbing the relatively dangerous part of the EM spectrum known as ultraviolet radiation. The atmosphere helps keep the Earth’s surface warm through retention of thermal energy, which helps reduce temperature extremes between day and night. The atmosphere is seen here as the blue haze above the planet The hydrosphere describes the combined mass of water found on, under, and over the surface of a planet. It includes liquid water in the oceans, rivers, lakes, clouds, soil, and groundwater; solid water in snow and ice found in cold regions and in ice caps; gaseous water found in the atmosphere. The geosphere includes the solid Earth portion of the Earth Systems. Rocks and soil (regolith) at the surface, and all the deep interior portions of the Earth. It differs from the Lithosphere, which only includes the planet’s crust.

Chemical Composition Today: Nitrogen (N 2 )- 78%, Oxygen (O 2 )- 21%, Trace Gases-Argon, CO 2, H 2 O and others…

First Atmosphere’s composition: - Probably H 2, He These gases are relatively rare on Earth compared to other places in the universe and were probably lost to space early in Earth's history because Earth's gravity is not strong enough to hold lighter gases Earth still did not have a differentiated core (solid inner/liquid outer core) which creates Earth's magnetic field (magnetosphere = Van Allen Belt) which deflects solar winds. Once the core differentiated the lighter gases could be retained

Second Atmosphere Origin and Composition: Produced by volcanic out gassing. Gases produced were probably similar to those created by modern volcanoes (H 2 O, CO 2, SO 2, CO, S 2, Cl 2, N 2, H 2, NH 3 (ammonia) and CH 4 (methane) No free O 2 at this time (not found in volcanic gases). Uniformitarianism

How are the geosphere and the lithosphere different? Why do scientists think the first atmospheric gases were H 2 and He? How would Earth’s first atmosphere get “lost” in space? How was Earth’s 2 nd atmosphere likely produced? What gas was likely not present in Earth’s 2 nd atmosphere?

Ocean Formation - As the Earth cooled, H 2 O produced by out gassing could exist as liquid in the Early Archean, allowing oceans to form. Evidence - pillow basalts, deep marine beds in greenstone belts.

The Earth in its earliest years was a horribly hot and violent place. Asteroids, comets, and other chunks of space debris left over from the solar system's formation continually bombarded the young planet, releasing huge amounts of heat. The decay of radioactive elements inside the Earth also generated great quantities of heat. At the same time, frequent volcanic eruptions may have covered much of the planet's surface in red-hot flows of lava. The early Earth's surface was hot enough to turn any liquid water instantly into steam. Nonetheless, the planet eventually cooled enough and obtained enough water to fill a vast ocean.

Some of the water in the Earth's oceans came from condensation following the outgassing of water vapor from volcanoes on the surface of the planet, while some was delivered by impacting comets. An important question in recent years has been the relative importance of these two sources.

According to one school of thought, comets may have supplied the bulk of oceanic water during the heavy bombardment phase of the solar system, between about 4.5 and 3.8 billion years ago. If this is true, it increases the chances that the delivery of organic matter, (also found in comets) played an important part in the origin of life on Earth. However, cosmochemists found that comet Hale-Bopp contains substantial amounts of heavy water, which is rich in the hydrogen isotope deuterium. If Hale-Bopp is typical in this respect and if cometary collisions were a major source of terrestrial oceans, it suggests that Earth's ocean water should be similarly rich in deuterium, whereas in fact it is not. While studies suggest that most of Earth's water probably did not have a cometary origin, there is contradictory data as well. It is hotly debated to this day! Shoemaker-Levy

1.What evidence is there that there was water on the Earth during the early Archaen? 2.What evidence is there that water was delivered by comets? 3.What evidence is there that water was formed from volcanic outgassing? 4.Which theory has more evidence today?

Sunlight Today, the atmosphere is 21% free oxygen. How did oxygen reach these levels in the atmosphere? Let’s look at processes that contribute to the cycling of O 2 on our planet: Oxygen Producers: Photochemical dissociation - breakup of water molecules by ultraviolet radiation  Produced O 2 levels approx. 1-2% current levels  At these levels O 3 (Ozone) can form to shield Earth surface from UV Photosynthesis - CO 2 + H 2 O C 6 H 12 O 6 + O 2 produced by cyanobacteria, and eventually higher plants – probably supplied the rest of O 2 to atmosphere. Oxygen Consumers Chemical Weathering - through oxidation of surface materials (early consumer) Animal Respiration (much later) Burning of Fossil Fuels (much, much later)

Evidence from the Rock Record includes Iron (Fe), which is extremely reactive with oxygen. If we look at the oxidation state of Fe in the rock record, we can infer a great deal about atmospheric evolution. Archean – minerals that only form in non-oxidizing environments in these sediments: Pyrite (Fools gold; FeS 2 ), Uraninite (UO 2 ). These minerals are easily dissolved out of rocks under present atmospheric conditions. Banded Iron Formation (BIF) - Deep water deposits in which layers of iron-rich minerals alternate with iron-poor layers, primarily chert. These are common in rocks B.y. old, but do not form today. Red beds are never found in rocks older than 2.3 B. y., but are common during later times. Red beds are red because of the highly oxidized mineral hematite (Fe 2 O 3 ) Conclusion – the amount of O 2 in the atmosphere has increased with time.

The primordial atmosphere had 1,000 times more CO 2 than it does now. Where did it all go? H 2 O condensed to form the oceans. CO 2 dissolved into the oceans and precipitated out as carbonates (e.g., limestone). Most of the present-day CO 2 (the largest carbon sink) is locked up in crustal rocks and dissolved in the oceans. By contrast, N 2 is chemically inactive, and stayed a gas in the atmosphere and become its dominant constituent.

1.How are BIFs and Red beds evidence of Earth’s oxygen atmosphere? 2.What are two oxygen producers? 3.What are two oxygen consumers? 4.Why is ozone important for the origin of life? 5.Where is the Earth’s largest carbon sink today? 6.Why is N 2 the largest part of Earth’s atmosphere today?