the gas envelope surrounding Earth, held by gravity
composition: 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% CO2, trace gases, water vapor
layers: troposphere (weather), stratosphere (ozone), mesosphere, thermosphere, exosphere
troposphere contains 75% of atmospheric mass and nearly all water vapor and weather
the ozone layer in the stratosphere absorbs UV radiation, shielding life
greenhouse gases (CO2, CH4, N2O, H2O) trap infrared radiation, maintaining habitable temperature
driven by solar energy: differential heating creates wind, pressure systems, and climate zones
medium for the water cycle (evaporation, cloud formation, precipitation)
reservoir in the carbon cycle and nitrogen cycle
the magnetic field protects the atmosphere from solar wind stripping
thermodynamics of atmospheric convection drives all weather systems