[kurthr]

The same place that water came from on Earth... comet impacts and residual moisture from the impact that split the Earth-Moon system. Really the same place all planetary water and gas has come from.

There's not much left because it mostly boils off due to low pressure, but in dark crevasses it's likely to stay cold enough to remain condensed. It's been measured spectroscopically.

[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.

[samstave]

Where did the comets get the water they impacted on earth?

[JumpCrisscross]

> Where did the comets get the water they impacted on earth?

Hydrogen comes from primordial nucleosynthesis [1] and makes up most of the interstellar medium [2]. Oxygen is produced when neutron stars collide and stars explode [3] as well as when some stars burn [4]. These freely combined in the gas disk from which our solar system formed, condensing into planets, moons, comets and other things [5].

[1] https://en.m.wikipedia.org/wiki/Big_Bang_nucleosynthesis

[2] http://casswww.ucsd.edu/archive/public/tutorial/ISM.html

[3] https://www.chemistryworld.com/news/heavy-elements-forged-by...

[4] https://en.m.wikipedia.org/wiki/Oxygen-burning_process

[5] https://www.lpi.usra.edu/books/MESSII/9028.pdf

[pjungwir]

Thank you for this answer! But then why does the water have to come from comets? Why couldn't the Earth have formed with its own water?

Also, we seem to have a lot of water. It's really all from comets, a little here, a little there?

[kobeya]

There is a "frost line" in the middle of the asteroid belt where you see rocky objects closer and icy objects further. The reason is that closer the Sun has enough energy to sublimate the icy objects into a comet with a tail, where the tail is the icy material being blown off into space. Over billions of years that results in objects with ice-free rocky surfaces inside the frost line, and big, gaseous, volatile-rich outer solar system objects.

So at the time of the formation of the solar system, the whole area was a big gaseous cloud of supernova debris with lots of ice. The ignition of the sun started the frost line, and pushed volatiles out of the inner solar system. However some had already been trapped in the formation of the planets, and rose to the surface as they cooled. That's where Earth's ocean came from, and we know Mars and Venus had oceans too. Presumably also Mercury, although I'm sure that was short-lived.

So yes, it's all from cometary material. But then ALL of the Earth is from cometary material, and the oceans only make up a small amount of the Earth's total mass.

[JumpCrisscross]

> Why couldn't the Earth have formed with its own water?

We’re not sure from where Earth’s water came [1]. Some evidence suggests the Earth was born with all its water, some that most came from comets.

[1] https://en.m.wikipedia.org/wiki/Origin_of_water_on_Earth

[njarboe]

Earth likely had some water during early creation (accretion) but the best theory on how the Moon formed is that a Mars sized object hit the early Earth at the end of accretion. This blasted much of the Earth's rock part (mantle) and any water it might have had into orbit. The rock part quickly fell back to Earth and the rest condensed and collapsed to form the moon. The water and other volatilizes that were on Earth were blown away by the solar wind at that time. The surface of the Earth and Moon would have been boiling lava at this point and had to accumulate water from new impacts of comets and icy asteroids.

The water on Earth may seem to be a lot to us on the surface but it is only about 0.02% by mass [1].

https://www.universetoday.com/65588/what-percent-of-earth-is...

[kobeya]

There's actually a lot of water in the mantle of the Earth. There's something like 3x the current ocean volume trapped in the mantle, and there are theories that pressure differentials cause this water to escape as gaseous water escapes from the atmosphere. It would explain why the ocean happens to be approximately the height of the continental shelf, and always has been. Under this theory the water simply emerged as the Earth cooled from the impact. The Moon, on the other hand, would be missing this water as the debris was mostly stripped of water content as it accreted.

[sdenton4]

Clever question, but it's comets all the way down, I'm afraid.

[comicjk]

Stars produce oxygen (along with all elements up to iron) during nuclear fusion. If the star is big (several times bigger than our sun), it eventually explodes and the oxygen then winds up in interstellar gas and dust. The oxygen reacts with hydrogen to make water. Eventually you wind up with a lot of dirty snowballs floating around (comets).

Oxygen is pretty common and hydrogen is everywhere, so water (as ice) is not scarce in the universe. The only place where water is uncommon is near a star, like us, where the water boils off into space unless a planet has enough gravity to hold it in.

[gjjrfcbugxbhf]

Water is just hydrogen plus oxygen. Both are fairly common elements in the universe.

[samstave]

Under what circumstances does one need to have in order to make them combine into water, organically? (Meaning without any device or machine, assume you have two big clouds of each element floating in space - if they collide, do the naturally just form into water molecules?)

[kurthr]

At reasonable temperatures, yes, the clouds turn into water and release energy, hydrogen burns. Water is entropically preferred as a lower energy state than separate atoms/molecules because of lower enthalpy at reasonable temperatures. Depending on density and ignition sources the reaction rate may vary, but it doesn't have to explode in a ms when there is a billion years available.

[comicjk]

That's not what "entropically preferred" means. If something is a lower-energy state, it's energetically preferred, but may or may not be entropically preferred. Water is NOT - entropy would prefer simpler molecules over more structured molecules.

I think the concept you're thinking of is free energy, which determines the final destination of a process. The equation relating these things is (change in free energy) = (change in energy) - temperature × (change in entropy). Entropy only becomes the dominant component when temperature is high. And, as expected, water molecules dissociate at high temperature.

[kurthr]

No, since this was chemistry, I was talking about enthalpy of an exothermic reaction (negative enthalpy), and interstellar space is pretty close to constant pressure, but not constant entropy for Gibbs free energy.

In any case: dH=TdS + Vdp note dp is small in space but V can be large and dS is the change in entropy.

...and enthalpy of ideal (interstellar H & O) gases does not depend on pressure, unlike entropy and Gibbs energy. If you really just mean free energy U, then they are basically the same thing in open space (but not in a plasma), a distinction without difference.

For further pedantics I recommend Wikipedia, since I doubt we are helping anyone else.

[Retric]

They randomly form and are destroyed over time due to collisions at various energies. It's also more common for ions to form in space which makes such simple chemical reactions easier.

[comicjk]

Yes, oxygen and hydrogen mixed together will eventually react to form water. To react, atoms or molecules simply need to collide hard enough to overcome their initial repulsion and form bonds. In space it is a slow process, because the pressure and temperature are both low - this means the atoms rarely hit each other, and when they hit, they don't hit very hard.

[logfromblammo]

H2O is probably the second, third, or fourth most common molecule in the observable universe. H2 is definitely the most common, and all you need is a good explosion from a CNO-cycle star (viz. heavier than 1.3 solar masses) to liberate all those fusion-catalyzing oxygen nuclei, and they readily combine with any H2 they may meet.

So in a stellar accretion disc, much of the water will end up inside the new star, and dissociate, but quite a lot of it will gather in the planets, moons, and comets. Europa, for example, has about 2 or 3 Earth-oceans worth of water. Uranus and Neptune likely have solid cores composed mainly of ices that include water ice.

[spdustin]

Water is formed naturally when the oxygen created by the fusion process in stars combines with the hydrogen from those same stars, often after a supernova explosion.

Water is fairly abundant in the universe. As are alcohols.

[gpderetta]

"Humanoids are the galaxy’s way of trying to get rid of all that alcohol."

-- Ian Banks

[kurthr]

The big bang?