Earth Formation and Accretion Hypothesis

Study Notes

Accretion hypothesis explains Earth’s formation via gravity attracting matter.
Process starts with the formation of planetesimals, which combine to create protoplanets.
Homogeneous accretion refers to Earth forming from materials of the same composition post-condensation.
Heterogeneous accretion involves materials of varied compositions during formation, leading to a differentiated planet.
Homogeneous accretion's weaknesses include inability to explain the abundance of certain elements in the mantle.
Heterogeneous accretion's challenges include the required rapid accretion rate and inconsistencies with observed planetary compositions.

Hydrosphere encompasses all water on Earth, divided into saltwater (97.5%) and freshwater (2.5%).
Freshwater is vital for life and has negligible salt content compared to saltwater.
Water's maximum density occurs at 3.98°C, contributing to habitat stability.
Water is hypothesized to originate from comet collisions or could have been trapped within Earth as volatiles during magma formation.

The Sun provides the necessary energy for processes on Earth, including supporting photosynthesis in plants.
Albedo affects how much solar energy is absorbed or reflected; values range from 0 to 1.
Earth’s energy budget consists of the balance between energy absorbed from the Sun and energy reflected, with about 30% reflected.
Factors influencing Earth’s energy budget include surface color, radiation received, axial tilt, and greenhouse gas presence.
The greenhouse effect plays a significant role in regulating the planet's temperature.

Earth’s atmosphere consists primarily of nitrogen (78%) and oxygen (21%), with argon (0.9%) and trace gases (0.1%) in minor amounts.
The atmosphere has five major layers: troposphere, stratosphere, mesosphere, thermosphere, and exosphere.
Troposphere is the lowest layer where human activity and biosphere predominantly exist; thickness varies from 5 to 9 miles.
Stratosphere, about 35 km thick, houses the protective ozone layer which absorbs UV radiation.
Mesosphere is where meteorites disintegrate, characterized by extreme cold temperatures.
Thermosphere can reach 1500°C and extends approximately 600 km; it's a layer of high temperatures.
Exosphere, the outermost layer extending about 10,000 km, interfaces with outer space.
The atmosphere plays a crucial role in the hydrologic cycle, acting as a reservoir for significant amounts of water and facilitating global water movement.