Keeping an Atmosphere
For a moon to be friendly to advanced life, it certainly needs an atmosphere. To have enough gravity to hold onto an atmosphere, it obviously must be larger than Earth's own airless Moon, which has a mass 0.012 times that of Earth. But how much larger?
Darren Williams, James Kasting, and Richard Wade at Penn State University have examined this problem in detail. They found that a number of processes allow a world's atmosphere to escape.
The first is well known. Some of the gas atoms at the top of an atmosphere will get kicked by random thermal collisions to faster than the planet's escape velocity and fly away. For an atmosphere to last long, this process must be kept very slow. Either the temperature at the top of the atmosphere must be low, or the world must be massive enough to have a high escape velocity.
For a body with a Mars-like density and an Earth-like atmospheric temperature structure, calculations show that the mass must be at least 7 percent of Earth's to retain most of its atmosphere for 4.6 billion years (Earth's current age).
The escape of some atmosphere is not necessarily fatal. On Earth, carbon dioxide can be replenished from the vast stores locked up in carbonate deposits and available to be weathered out. However, the loss of some other biologically important gases, such as nitrogen, is irreversible.
A major loss mechanism for nitrogen is called dissociative recombination. This process starts when a positively charged nitrogen molecule at the top of the atmosphere combines with an electron to produce a pair of free nitrogen atoms. The energy released by this reaction gives the atoms enough of a kick to escape. Estimates based on Mars's nitrogen-loss rate indicate that dissociative recombination becomes negligible for a world with more than 12 percent of Earth's mass at a distance of 1 a.u. from a Sun-like star.
A potentially greater threat to a moon's atmosphere is sputtering. This process occurs when energetic charged particles hit the atmosphere and kick molecules into space. The gas giants in our solar system, and presumably others as well, have magnetospheres with radiation belts potent enough to completely erode the atmosphere of an orbiting Earth-like moon in only a few hundred million years.
One way to blunt this type of atmospheric loss is shielding by a moon's own strong magnetic field. Measurements by NASA's Galileo spacecraft hint that large moons might have magnetospheres of their own with the required strength. Galileo has detected a strong Earth-like magnetic field around Jupiter's moon Ganymede, which has a mass only 2.5 percent of Earth's.
Researchers once believed that small bodies like Jupiter's major moons could not possess such strong fields at all. But these moons orbit deep inside Jupiter's own powerful magnetosphere. According to models developed by Graeme Sarson (University of Exeter) and his colleagues, a strong ambient field helps initiate the circulation needed to produce a vigorous dynamo effect in the core of an even slightly active moon, leading to a strong field for the moon itself. Taken together, these observations and models hint that planet-size moons can maintain protective magnetic fields that they would not have in isolation.
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