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Question
As I understand it, the requirements for an atmosphere to form are a gravitational field strong enough to hold onto the gases and a magnetic field to protect them from solar wind. Are there any other factors that determine if a planet can retain an atmosphere?
The factors that determine whether a planet or moon can retain an atmosphere are primarily temperature and gravity (mass). Also, of course, the availability of a suitable gas is critical, and this is related to the chemical interactions between the atmosphere and the crust. Excluding the gas giants, there are four objects in the solar system with substantial atmospheres: Earth, Venus, Mars, and Titan. Venus has about 100 times the Earth’s atmosphere, Titan about the same, and Mars about 100 times less. Magnetic fields do not play a major role, as you can see from the fact that neither Venus, Mars, nor Titan has a significant magnetic field. Of the four objects with substantial atmospheres, only Earth has an internal magnetic field.
David Morrison
NAI Senior Scientist
October 23, 2009
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