Earth's Atmosphere and Hydrosphere Origins

Earth's Atmosphere and Hydrosphere Origins

• Water exists as ice and vapor in other solar system bodies, while Earth's liquid surface water is unique, due to its distance from the sun.
• The hydrosphere (oceans, lakes, rivers, streams) and atmosphere are discussed. 
• Early Earth's atmosphere and hydrosphere underwent substantial changes, primarily during the Pre-Cambrian Era (4.6 billion to 540 million years ago).
• These changes stabilized later.
• Key learning outcomes include understanding the origin of the atmosphere and hydrosphere, as well as the early conditions which may have fostered life.

Early Composition and Climate

• Earth’s early atmosphere and hydrosphere were dramatically different from today's.
•  The initial atmosphere was potentially from outgassing, volatiles from small planetesimals, with the force of impacts vaporizing surface water. This caused a dense steam atmosphere, and high temperatures that may have melted rocks.
• Impacts from comets and asteroids (4.5 to 3.8 billion years ago), perhaps created the moon, and removed this steam atmosphere. 
• Early atmospheres likely consisted of hydrogen and helium, which Earth, due to its size, may not have held for long.
• Other abundant gases were also present. Hydrogen exists today in compounds such as water, methane, and ammonia.
• The 'iron catastrophe' (formation of the iron-core) further modified the atmosphere composition.

The Role of Life

• Stromatolites (fossil mats from early bacteria) suggest liquid water existed 3.5 to 3.8 billion years ago, which is earlier than previously thought.
• Volcanic outgassing and other exogenic sources may have contributed to ocean formation.
• The early sun was less bright, yet early Earth was warm due to high CO2 levels, which decreased over time with increasing solar luminosity, maintaining the climate balance.
• Early Earth's oxygen levels increased gradually. 

Oxygen's Rise and Life

• Oxygen accumulation was hindered by minerals and rocks incorporating it (e.g., banded iron formations - BIFs).
• The evolution of photosynthesis in microorganisms (3.5 to 2.5 billion years ago) led to oxygen buildup, correlating with the end of the Archean Eon (2.5 billion years ago).
• Cyanobacteria (first photosynthesizers) are credited with producing atmospheric oxygen.
• Early oxygen buildup was in stages as discussed by Kasting (1993).
• Stage I involved minimal oxygen, with rusted rocks reacting with iron.
• Stage II occurred when free iron ions became depleted, signifying oxygen increase.
• Stage III showed further oxygen increase enabling multicellular life development.

Formation of Non-living Spheres

• Factors contributing to atmosphere and hydrosphere emergence were combined.
• Defining precise timing is difficult due to speculative and limited information about the Earth's first hundred million years, and predominantly based on Hadean rocks and undifferentiated meteorites.
• Scientific investigations continue to build the model for understanding the Earth's early history.
• The non-living spheres (atmosphere, hydrosphere, and geosphere) are prerequisites for life.

A Note on Atmosphere vs. Hydrosphere

• Information on atmosphere formation is more readily available than on the hydrosphere's development.
• pinpointing the beginning of liquid water on Earth is hard.