The part I especially am wondering about is how it goes from the atmosphere to space, and what form it takes there? Is the atmosphere waning at the same time? Is this a relatively slow process, with tiny drops of ice accumulating around the planet and eventually aggregating? Why wouldn’t they remain in orbit?
Thanks!
I think the mechanism is this: water vapor gets dissociated into Hydrogen and Oxygen in the upper atmosphere, due to UV radiation, and then the hydrogen escapes.
Water evaporates into the atmosphere and in the long term is roughly uniformly spread around. High in the atmosphere water molecules are dissociated into hydrogen and oxygen by sunlight, gamma rays, etc.
Meanwhile the atmosphere is more or less in thermal equilibrium at some temperature. At a given temperature molecules have, on average, a given kinetic energy. Kinetic energy is proportional to mass and velocity squared so at a given temperature, hydrogen with a much lower mass is traveling much faster than the oxygen (and nitrogen and whatever else is in the atmosphere). But not every molecule of hydrogen (or oxygen) has the same speed. Some are going much faster and actually reach escape velocity. This happens much more for hydrogen than the other gasses since its average speed is higher.
So in the long run hydrogen “leaks” out into space and it’s no longer there to combine with the oxygen to reform into water. (Which is probably a good thing or I think we wouldn’t have enough of the latter to breathe.)
both hydrogen and helium can be leakers. helium though not part of water is valuable.
I built and calibrated one of the instruments on this mission. The purpose of MAVEN (Mars Atmosphere and Volatile EvolutioN) is to test theories on how Mars has lost it’s atmosphere, especially CO2 but water also, over its history. If you Google around MAVEN, you can find some good stuff that talks about the theory (think magnetic fields, ionizing radiaton, solar wind, etc…).
Any gas will leak from the atmosphere into space as I explained above. Hydrogen leaks the fastest as it’s the lightest. Helium is next lightest and forms virtually no compounds so whatever is in the atmosphere doesn’t recombine with anything to come out of it. Neon with an atomic weight of 20 is probably the next most likely to leak as the potentially lighter gases, nitrogen, oxygen, and chlorine would not be monatomic so their molecules would mas more.
Oh, great explanations! Makes it very clear. Okay, then; they hypothize that our oceans on earth were deposited here by ice-bearing comets … where did the water on those comets come from? Can hydrogen and oxygen combine to form water in subzero temperatures?
Oxygen, at least, exists primarily in its atomic form at orbital altitudes, with the O2 molecules split apart by UV.
There seems to be significant atomic nitrogen also, but all the links on this Google Scholar search go to Wiley.com, which is down for maintenance right now.