Escape of an atmosphere to space

Atmospheric escape calculations play a role in determining how a planet got to be the way we see it today, and what kind of atmosphere it may have had in its past; escape calculations can also indicate how long a planet can maintain a habitable climate, and can inform investigations into the mechanisms needed to maintain an atmosphere in a given state. For example, does there need to be an source of N2 outgassing to maintain Titan's largely N2 atmosphere? The answer to that hinges primarily on how rapidly a small, cold body like Titan can lose a relatively heavy gas like N2. Similar questions apply to maintainance of CH4 on Titan, since the processes which turn CH4 into ethane and other compounds which accumulate on the surface liberate H2, which must escape to space or accumulate in the atmosphere. Likewise, if Venus even had an ocean comparable to Earth's, and went into a runaway state, both the hydrogen and oxygen in the water must have been lost somehow, since there is very little water in Venus' atmosphere today. Could the hydrogen escape to space at a sufficient rate? The oxygen? The escape of water to space is what makes a runaway greenhouse essentially irreversible, so it enters into the habitability question for Venus and other planets subject to a runaway greenhouse. One picture of the climate of Early Mars posits a warm, wet climate supported in part by the greenhouse effect associated with a CO2 atmosphere with a surface pressure of around 2 bars. How much of such an atmosphere could be lost to space in the available time? How much would have to be lost into chemical reaction with surface rocks instead? Is it possible that a body as small as Mars could remain habitable for billions of years, or does escape to space inevitably doom such a planet to the long chill? The contrast between Mars and Titan is stark. We expect atmospheres to be in some sense bound by gravity, but why is it then that Titan, with far weaker gravity than Mars, retains a 1.5 bar atmosphere while Mars retains practically none? One feels it must have something to do with the lower temperature of Titan, but that notion begs to be quantified. Even for Earth, escape mechanisms are important: before the rise of atmospheric oxygen, there is the possibility that large amounts of hydrogen could accumulate in the atmosphere, if it does not escape rapidly to space. This is important because hydrogen and carbon dioxide can serve as feedstocks for synthesis of many prebiotic organic molecules.

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