- Some planets (and planetary satellites) have atmospheres while
others do not.
- Gravity and temperature play important roles in determining whether a planet can retain an atmosphere.
- If gravity is too weak or temperature is too high individual
gas atoms can achieve escape velocity and leave the planet forever
(e.g. Mercury, the Moon, and to some extent Mars).
- Escape velocity is the speed an object requires to leave the
surface of a planet and never fall back to the ground.
- Alternatively a small body with weak gravity can retain
an atmosphere if the temperature is low enough (e.g. Titan, Triton, and Pluto)
- Some planets accumulated their atmospheres at the time of their
formation while others produced (or accumulated) an atmosphere afterward.
- During the formation of the Solar System the Jovian planets grew
large enough to attract gas gravitationally from the protoplanetary
- As a result, Jupiter and Saturn
are composed largely of Hydrogen and Helium -- the dominant
materials in the protoplanetary disk.
- Interesting question... does this constitute an atmosphere?
- The smaller terrestrial worlds (and the Jovian satellites) were not
massive enough to retain gas from the protoplanetary disk.
- The terrestrial worlds obtained their initial
atmospheres (and oceans) via
- Other processes may subtantially modify a planet's
atmosphere over time. For example, on Earth
- liquid water oceans have removed a substantial
amount carbon dioxide from the atmosphere.
- plant life has contaminated the atmosphere
with a corrosive gas -- oxygen
- human civilization is now in the process of altering
the atmospheric composition.
Updated April 1, 2012