Atmospheres are held in place by the force of gravity. Lighter gases like helium and hydrogen escape into space (what happens if you let go a helium balloon?)
Where do atmospheres come from?
Atmospheric gases come from three major sources:
Gases released from volcanoes (outgassing)
Evaporation and sublimation of water
Bombardment (micrometeorites, solar wind particles)
There are five ways atmospheres can lose their atmosphere:
Thermal escape (into space)
Solar wind stripping
Chemical reactions with surface materials
Evolution of the Earth's atmosphere
Today, the Earth's atmosphere is composed of nitrogen (78%) and oxygen (21%) with traces of water vapour, carbon dioxide, argon and other gases. But it wasn't always so.
Scientists are unable to determine the Earth's first atmosphere, but by four billion years ago the atmosphere was largely composed of nitrogen, carbon dioxide, methane, ammonia and water vapour.
It was not until two and a half billion years ago that oxygen appeared in the atmosphere. Oxygen gas (O2) is highly reactive, and will quickly disappear if it is not continually replenished.
Within a billion years oxygen levels had built up to 21% (today's level). Where did the oxygen come from? Photosynthesis. Plants provided and continue to provide oxygen for planet Earth.
Recycling on Earth - active carbon and water cycles
Water, carbon and rock cycles on Earth
TASK 20: Create a flow diagram that shows the relationship between the carbon and water cycles and the atmosphere. Include processes (eg, evaporation, photosynthesis etc) and products (shells, limestone, oxygen etc).
Major volcanic activity (and outgassing) ceased on Mars more than 3 billion years ago, and without an active core solar winds began to strip away the atmosphere. Today, there is no evidence of an active carbon or water cycle on Mars. The average atmospheric pressure and temperature on Mars means that liquid water cannot exist on the surface. Any water on Mars is locked up as permafrost below the surface. Carbon is locked up as carbon dioxide in the atmosphere, and perhaps in rocks. But the discovery of carbonate rocks or minerals on Mars might prove that the carbon cycle was once active.
Next we'll look at how plate tectonics recycles rocks, and how rocks tell the story of the Earth's past.