could earth's early magnetic field have been strong enough to trap hydrogen within the atmosphere?
How would a magnetic field trap neutral hydrogen atoms?
if they were hydrogen ions emitted as solar wind
Hydrogen ions, let's just call them protons, have a charge and a mass. When they encounter a magnetic field they will be deflected onto a different trajectory by a Lorentz_force. http://en.wikipedia.org/wiki/Lorentz_force
As a proton spirals down the converging magnetic field lines toward a magnetic pole, it may collide with ions in the upper atmosphere and so could be slowed and trapped.There is significant coupling between the solar wind and the Earth's magnetic field.
Interesting question, Daniel. Are you wondering if this could have given the early atmosphere reducing properties, a condition often thought necessary for abiogenesis?
Not any longer, since early mantle convection is now known to give local environments with the necessary redox differences. (Often around hydrothermal vents as they pass the differentiated and serpentinized minerals.) Instead a near neutral atmosphere would be conducive for life emergence, since the CO2 generates a slightly acidic ocean that contrasts to the mantle convection alkaline conditions and makes (local) organic production effective. Such conditions may seem constraining (less global organic production), but in fact increase the likelihood for life elsewhere since especially ice moons with oceans but also early cold, wet bodies like Mars have them.
Perhaps there were a slightly reducing excess due to hydrogen escape from the hot planet (and perhaps remaining and, yes, captured hydrogen). But too much of an excess and the upper atmosphere goes into hydrodynamic escape, hydrogen vents preferably. (But can remove some somewhat more massive molecules with the outflow.) I think the upper limit of hydrogen excess has been limited to 0.5 billion years from such considerations.
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