[samstave]

If the moon had ancient volcanic activity and thus, why is it, after observable impact from various bodies - shown in the craters all over the surface, and the sumizing of the fact that water was deposited to earth from extra-orbital impacts... does the moon have zero water or atmosphere?

Where did water originate?

(I didn't know how to format that question any better, so please forgive the fumbly format)

[spdustin]

The moon's core isn't made of the swirling molten iron dynamo like Earth's is. No swirling molten iron, no magnetic field. No magnetic field, nothing deflects the solar wind that then, literally, blows away any fledgling atmosphere.

It's believed that Mars' atmosphere was considerably more dense than it is today, but along the way, its core cooled, and it lost its protective magnetic shield, allowing the solar wind to strip away much of its atmosphere.

[drdrey]

Even with a magnetic field, would the moon's gravity be sufficient to retain an atmosphere?

[simulate]

Yes, it is possible that with a magnetic field the moon's gravity would be sufficient to retain an atmosphere. Titan has an atmosphere denser than earth's atmosphere despite having a lower gravity than the moon. There are a few hypotheses why and one of them is that, although Titan has no magnetic field of its own, it orbits inside Saturn's magnetosphere. https://astronomy.stackexchange.com/questions/8345/how-does-...

[SimbaOnSteroids]

No, for that matter Mars doesn't have the gravity to retain an atmosphere either. I can't cite a source on this because I don't remember where I read it but I believe having a magnetic field surrounding a celestial body is a relatively minor factor in atmosphere retention compared to mass. If someone who specializes in planetary atmosphere retention knows better feel free to eviscerate me.

[pavel_lishin]

Someone replied mentioning that Titan has a denser atmosphere than Earth, despite being lighter than the Moon: https://news.ycombinator.com/item?id=15531798

[logfromblammo]

Temperature is also a factor.

PV=nRT. T is temperature.

Titan is not only protected by Saturn's magnetic field, but it is also very cold. Some of the molecules that are gaseous on Earth are liquids on Titan. Cold gases have lower pressure, and less likelihood of bouncing a light molecule high enough up in the atmosphere that the solar wind can grab it and blow it away.

Earth has a hard time holding on to light molecules like H2 and He, but its He is replenished somewhat by alpha decay, and it takes a long time to get from the inside of a rock to the upper reaches of the atmosphere. Most of the hydrogen is attached to heavier molecules. But it happens eventually, and even the Earth's magnetic field and gravity can't keep them. Venus is almost as massive as Earth, but it is hotter than Mercury and has no core-generated magnetic field. So most of its water has already thermally dissociated (which happens slowly starting at around 800 degC) and the H, H2, He, and monoatomic O bounces high up into the atmosphere, ionizes, and blows away. So now Venus has about 90 bar of CO2 and barely any water left.

One of the terraforming proposals for Venus is to transport a large quantity of hydrogen from Jupiter to Venus, and use Fe catalyst to react it with the CO2, to get graphite C, H2O, and O2. That would strip off much of the greenhouse blanket, but the planet would still have to be cooled off and protected from the solar wind to keep all that hydrogen around on a geologic time scale.

[eradicatethots]

>feel free to eviscerate me

Off topic but I’d like it quite a lot if people could be informative without being eviscerating lol. I suppose this doesn’t usually happen, but it should happen more. That’d be so much better, so much more tolerable and happier. It’d be good for all of us probably lol.

[averagewall]

I think eviscerating comes when people present false statements as facts. Qualifying it with uncertainty as the OP did probably won't anger anybody. But telling lies because you don't know what you're talking about justifiably does.

[brw12]

You're pointing to an important difference: between being uninformed but trying to get towards being informed, and being uninformed but fiercely protective of your certainty.

What Would Feynman Do?

In this case, I'm imagining he'd say these are some of the things we think might explain bodies' keeping or losing atmosphere, but we have to remember that almost everything we think we know about planets and moons comes from looking at the light that bounces off of them and making guesses about the underlying rules, but we still don't know do much about the underlying rules of physics, so there's probably more that we don't know than what we do know about atmosphere retention.

[u801e]

I've read that it's the same case with Venus' core, but its atmosphere is far more dense than either Earth's or Mars'.

[Scaevolus]

Venus has an induced magnetic field that may protect its atmosphere. https://www.astrobio.net/also-in-news/a-magnetic-surprise-fr...

[nonbel]

Apparently so does Mars though? http://www.sciencedirect.com/science/article/pii/S0019103509...

[kirillkh]

Then we have no hope of ever engineering Mars to have enough air for us to breath?

[LeifCarrotson]

A Martian atmosphere (not sure about a lunar atmosphere) would be lost on a timescale of tens of millions of years. That's very quick in cosmological timescales, but slow enough that any human effort to create or replenish it could be very successful.

[waegawegawe]

That said, the earth's atmosphere has a mass of about five billion billion tons. For reference, as a species, we produce about ten billion tons of concrete each year. This is just to give a sense to the scale of effort involved in replacing a planetary atmosphere.

[larkeith]

Depending on the volume of surface ice (especially at the poles), might it be possible to produce atmosphere on a massive scale via orbital lenses or mirrors? With recent advances in solar sail technology, I can't imagine the implementation would be too far removed from current capabilities.

[waegawegawe]

Think about how large of a lens you're talking about. Even if it were one hundred meters across and was able to collect 100% of the sun's energy passing through it, the amount of energy produced would be utterly insignificant compared to the problem we're discussing. And how would you get a one hundred meter lens to mars orbit? Even after that, you have to consider that we want an oxygen atmosphere, not one made of water vapors.

I don't know what you mean by "too far removed from current capabilities", but I doubt we'll even start working on the problem for two or three centuries.

No, it seems more likely to me that we'd use our growing knowledge of genetics and psychology to hack out the part of ourselves that needs to be outside, and opt for a purely enclosed existence on Mars.

[hutzlibu]

With lenses you could also burn the soil to produce gases that add up to he atmosphere ... but we want breathable atmosphere. With lot's of oxygen and very little co2. And that is a bit harder ...

So you also couldn't just vaporize the ice, you need to split it up. Solar heat could be sufficient, but the water will then be missed everywhere else on mars where life wants to grow.

And Mars is dry.

So I would first use the water in enclosed habitats. And then after, if there is plenty of water left, one could start to think about smoking that up ..

But there might be other options, once you have lot's and lot's of autonomous machines and rockets available and allmost unlimited fuel (sun?). But without that? Not a chance ...

[kobeya]

Also many thermoforming efforts consider adding an artificial dynamo (using, say, superconducting rings) to protect both the atmosphere and life on the surface. The energy cost of maintaining an artificial dynamo would be less than the incremental maintenance cost of maintaining atmosphere and dealing with health risks from radiation.

[jlebrech]

we could also use the resources on mars or the moon more sparingly, making dome bases would provide that.

we could also bore into those tunnels and put caps on the hole afterward, have some of those channel light into those tunnels too.

[icanhackit]

NASA proposes building artificial magnetic field to restore Mars’ atmosphere: https://www.universetoday.com/134052/nasa-proposes-magnetic-...

[extrapickles]

Interesting, it only needs a 1-2 Tesla magnetic field. MRI machines go up to 3T, which means this isn't too outlandish with known technology. The main issue is that they probably want a Mars sized field at 1-2T.

[andai]

Does that mean it needs a bigger coil?

[larkeith]

I don't know the rate of atmospheric loss, but it might be possible to introduce air at a high enough rate to outstrip the losses, making for a sustainable atmosphere. It would presumably depend on the availability of ice and other volatiles.

[abandonliberty]

Unlikely because you can't use a noun as a verb :)

Someday though we may be able to breathe on Mars

[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.

[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?

[wyager]

The moon’s gravity isn’t strong enough to hold water vapor or any other atmospheric gas. It would either get siphoned off by the earth or just fly off into the solar system.

The same is true of earth and helium.

[NegativeLatency]

Also it doesn't have magnetic fields like the earth does to shield its gas from being pushed off by solar wind.

[samstave]

Ok, and I like the comment about helium...

But where did water originate?

[dsr_]

https://cosmologyandspace.wordpress.com/tag/oxygen/

answers your question.

If you are trying to go back to "why is there anything?", then the answer is "we don't know precisely, but we do know that if those things hadn't happened, we wouldn't be asking the question, so so we have no basis of knowing whether it's random, rare, or a near-certainty for a new universe."

[lrdd]

I think the short answer is we don't really know, but the simplest explanation is muddy icy comets colliding

[samstave]

If that's the case, why haven't we been hit by any in recorded history that gave us more water?

How many comets have hit us that were rife with water supplies to provide the amount of water earth has given that water isn't also abundant on planets that are much larger, like Jupiter?

Why do we see planets with atmospheres of say sulfer, and earth doesn't have an issue with sulfer in the atmosphere?

[Turing_Machine]

"If that's the case, why haven't we been hit by any in recorded history that gave us more water?"

Most of the comets whose orbit intersects with Earth's orbit collided eons ago.

Some comets still enter the inner Solar System, of course, but it's going to be extremely rare for one to be on the right path to hit the Earth.

[Brybry]

I saw a recent study that said the moon probably did have atmosphere from volcanic activity... but only for around 70 million years. It dissipated into space.

http://www.sciencedirect.com/science/article/pii/S0012821X17...