[ealloc]

The reason water exists on earth, and not on mars, may be that life on earth saved the water by discovering photosynthesis.

The discovery of photosynthesis caused the "great oxygenation event" which pumped oxygen into the atmosphere. Oxygen reacts with atmospheric hydrogen to form water. Without the oxygen, the very light hydrogen molecules would float to the top of the atmosphere and are easily blown away by solar winds, which is what happened on Mars. But with high oxygen concentrations on Eath, hydrogen molecules react to form heavier water molecules before they have a chance to be blown away, and thus hydrogen and water are retained.

I read about this in the book "Oxygen" by Nick Lane.

[DougWebb]

That seems like it must be incorrect. Life on earth evolved in the oceans, so the oceans had to exist for eons before life evolved and the eons it took for life to evolve photosynthesis and produce significant amounts of oxygen in the atmosphere. If what you're saying is correct, then the earth either started out with much more water than it has now, or the hydrogen escaped a lot more slowly than I'd expect it to.

[ealloc]

The book was written in 2002, and I just looked up Nick lane's latest 2016 take on it here: [1]

His point in this new article is that instead of one big "oxygenation event" there may have been multiple. But he sticks to his story that the creation of an ozone layer by photosynthesis was the key step in saving the oceans. He argues both Mars and Earth had oceans originally (confirmed by Mars Satellite observations), which were gradually diminished by a process in which ultraviolet light splits atmospheric water, minerals on the surface absorbed the oxygen (rusting, making Mars red) leaving the hygrogen to blow away. But life on earth pumped extra oxygen into the atmosphere, faster than minerals could aborb it, creating the reactive ozone layer which prevented hydrogen from blowing away, thus saving the oceans from their fate on Mars.

[1] nick-lane.net/wp-content/uploads/2016/12/Oxygen-and-life.pdf

[kobeya]

I've been in planetary science and never encountered this theory. I have a default position of skepticism towards it for that reason, but I don't have a reason to object on the face of it. The question is not where the oceans came from (that's settled -- it came out of the mantle as the Earth cooled, and ultimately before that from cometary impact), but how the oceans did not boil or evaporate off like they did everywhere else. I could understand a theory that they were kept from boiling by being in the habitable zone AND by some combination of life processes, and that this hydrogen-capture mechanism helped replenish the ocean as hydrogen escaped from natural gas upwellings. But I have no sense of the scales and magnitudes involved to see if inputs approximately match outputs without seeing the underlying paper.

[chaotic_clanger]

<quote>but how the oceans did not boil or evaporate off like they did everywhere else.</quote>

gravity and magnetic field? if it were hydrogen or helium it would be stripped off by solar wind (storms) like everywhere else, but water is quite heavy due to the oxygen.

also 'everywhere else' means practically mercur, mars and asteroids (moon). no idea about venus. ice giants keeps their water also due to gravity and far apart on pluto it's frozen like rock.

[narag]

but how the oceans did not boil or evaporate off like they did everywhere else.

You forgot freeze.

[kobeya]

Not inside the frost line.

[joshuahedlund]

> Life on earth evolved in the oceans, so the oceans had to exist for eons before life evolved

I can't intelligently contribute to the overall discussion here, but I know one of the most interesting things about the currently understood timeline is the apparent lack of eons between the earliest conditions conducive to life after its initial cooling and the earliest evidence of life. Don't quote me on exact numbers, but within margins of error, as I understand it, it's like in the range of millions of years, not billions (which has all sorts of interesting implications for both the Fermi paradox and religious thinkers) Though, as far as I know, you'd still be correct on the distance to photosynthesis.

[dragonwriter]

> Life on earth evolved in the oceans

Or not; the “land theory” (that it began in shallow, possibly volcanic, terrestrial pools”) and “sea theory” (that it began in oceans, possibly at hydrothermal vents) have been competing theories forever, essentially.