<p>Your assumption that there is not a lot of water elsewhere in the solar system is incorrect.  According the <a href="https://solarsystem.nasa.gov/yss/display.cfm?ThemeID=23" rel="noreferrer">this article</a> on NASA's website;</p>
<blockquote>
<p>Missions in recent years have overturned our view of a dry solar system, returning mounting evidence of ample water from a vast array of locations.</p>
<p>Comets from the remote corners of our solar system are made of water and other ices. Orbiters, landers, and rovers reveal Mars as a watery world in the distant past -- a world that today may contain entire underground oceans of frozen water.</p>
</blockquote>
<p>It also points out that</p>
<blockquote>
<p>Jupiter's moon Europa has a frozen crust of water, covering a thick global ocean. By current estimates, it has twice as much water as all of Earth's oceans and rivers!</p>
</blockquote>
<p>The perception that the rest of the solar system does not have a lot of water is probably due to the fact that the Earth is in the <a href="http://en.wikipedia.org/wiki/Habitable_zone" rel="noreferrer">'Goldilocks zone'</a> where water can exist in all three phases <em>at the surface</em> of the planet.  The temperatures on the rest of the planets in the solar system are either so hot that it can only exist in a vapor phase or so cold that it can only exist in ice at the surface or liquid beneath.</p>

CC BY-SA 3.0
2014-04-16T16:39:03.887Z
2014-04-16T16:39:03.887Z
2020-06-18T08:25:19.090Z
Chris Mueller
<p>An important issue to realize is that water must have been very abundant in the protosolar disc, as tobias already stated.
To expand on that I just want briefly touch upon the atomic abundances that we measure in the corona of the sun, as presented by a wikimedia-commons graph:</p>

<p><a href="https://i.sstatic.net/hlTxQ.png" rel="noreferrer"><img src="https://i.sstatic.net/hlTxQ.png" alt="enter image description here"></a>
(Sidenote: Those abundances compare well to the revised <a href="http://adsabs.harvard.edu/abs/2009ARA%26A..47..481A" rel="noreferrer">Asplund2009</a>-abundances)<br>
We think that those numbers are representative of the bulk composition of the Sun, as it is only burning Helium since 4.567 Gyrs. Thus the composition is generally assumed to be primordial, or what the solar system started out with.  </p>

<p>Now let us imagine this atomic mixture accumulating around the young terrestrial planets and focus on the most abundant elements H, He, C, N and O. In a thick planetary envelope, shielded by UV, equilibrium chemistry will set in. Then a lot of $H_2$ will form, He will stay inert and C, N, O will try to react with Hydrogen, simply because the number of encounters is much higher with that, than inside of the CNO group. Some CO will form, as this is a very stable molecule.
But as C becomes depleted, and our protosolar nebula has $C/O \approx 0.5$, there is still a lot of Oxygen left. Thus it will inevitably combine to $H_2O$.</p>

<p>The result is, that we expect there to be really <em>a lot</em> of water around in Planet-forming discs. </p>

<p>The escape or destruction of water after this period however, is apparently also very efficient, and other answers have touched upon the retention of water on Earth. So in fact Astronomers at the moment rather wonder "where did all the water go?"</p>

CC BY-SA 3.0
2016-01-14T18:02:12.550Z
2016-01-14T18:02:12.550Z
AtmosphericPrisonEscape
<p>Oceans <em>did</em> form on other rocky planets - at least Venus and Mars, and moreover, many moons of Jupiter and Saturn as well.</p>

<p>The problem is that of the two other terrestrial "<em>uberplanets</em>" who <em>had</em> oceans - i.e. the aforementioned Venus and Mars - they lost them, but in rather different ways. </p>

<p>On Venus, what happened appears to have been that, synthesizing the best theories and evidence so far, as the Sun steadily grew hotter as a result of the natural main-sequence stellar evolution, the planet essentially left the habitable zone. The oceans evaporated and were lost to space. The way that works is that as the planet heats up, the water vapor content in the atmosphere increases, and some of that vapor rises to the top of the atmosphere. At the top, the UV flux begins to break apart (photodissociation) the water molecules, releasing the hydrogen, which then easily escapes as its gravitoionization energy is very low (for Venus, with an escape velocity of 10.36 km/s, it is similar to Earth - stripping one <span class="math-container">$\mathrm{H}_2$</span> molecule, a mass of ~3.4 yg, costs 0.18 aJ or 1.1 eV, which can easily be supplied by photons, stellar wind, or other factors. Stripping nascent [monoatomic] hydrogen, if such could happen without molecularization, takes half this.). The end result is an oxygen-rich atmosphere (nearly pure oxygen) at very high pressure, though chemical reactions with the rock below may strip some of this.</p>

<p>But as you may note, the current atmosphere is mostly carbon dioxide, with a very high surface temperature, and thus while the above is fairly plausible, one must wonder how this atmosphere obtained. Well, here's the thing. Much like Earth, Venus likely had plate tectonics early in its history when it had oceans. Ocean water seems to be a necessary component of plate tectonic activity on planets at least around the size of Earth, acting as a sort of "lubricant", and without it, plate tectonics cannot operate. Hence, once the water evaporated from Venus, any tectonics it had would have stopped.</p>

<p>The cessation of plate tectonics, while it may not seem like much to the uninitiate (so what? The continents stop moving, and you stop getting earthquakes?), is actually <em>catastrophic</em> for this kind of planet. You see, plate tectonics does more than just move continents, mountains, and earthquakes. What it most <em>crucially</em> does is to serve as a sort of "release valve" for the heat that is being generated inside the planet as a result of the decay of primordial radionuclides mixed with the mantle rocks (mostly <span class="math-container">$^{238}\mathrm{U}$</span>). This is because that, at spreading zones, hot mantle material oozes out (Iceland is a good Earthbound example of where this process happens to just come above water and can be seen with the naked eye - youtube it if you're too poor to travel), and at subduction zones, volcanoes are created, like in the Pacific Ring of Fire on Earth. The released material transfers the heat to the atmosphere and thus ultimately it gets radiated into space, hence shedding the internal heat. Without this process, that heat is shed far less efficiently - essentially only by conduction through the ground, which is terrible.</p>

<p>As a result, once you lose tectonics, the radioactive heat begins to build within the interior, and the mantle temperature starts to rise considerably. And when that starts, eventually, something has to give. And what "gives" is not quite well-understood but it seems to be either that tectonics restarts for a "short" time "with a vengeance", or that there commence widespread basaltic flood eruptions, or some combination of the two. In any case, the result is a rapid - on a geological scale of course, actually about 100 million years - "global resurfacing event" resulting in either or a combination of total subduction/reemplacement of the old crust (hence that is a considerably faster rate of tectonic motion than on Earth - to recycle all its crust over 100 Myr would take plate motion on the order of 20 cm/yr, where the fastest plate movement [India] in recent time was only around 4-6 cm/yr), or its progressive burial in flows similar to those from the Siberian Traps, likely from multiple vent sources, extending over such a long enough period as to cover all low-lying areas at least. In both cases there occurs the widespread release of volcanic gases which chiefly includes carbon dioxide, and that quickly converts the atmosphere to a "runaway greenhouse" state like we see on the planet today. The last GRE on Venus was finished at about 500 Ma, and there may have been more before that until the time when it last had oceans, and it is possible that the current thickness was the result of such a chain of episodes instead of just one.</p>

<p>The oceans on Mars, on the other hand, seem to have suffered a rather quieter demise. In this case, it seems the chief problem was the planet was simply too small, and the cause was not the result of external changes in the form of those in the Sun, but rather internal ones. As a result of its lack of size and hence of mass, there was both less primordial heat within the core and a smaller radionuclide load to maintain the internal temperature. The interior would have cooled to the point its iron inner core froze completely, and that shut down the planetary magnetic dynamo, which we know existed in the past thanks to there still being today small areas of "fossilized" magnetization in the crust due to rich concentrations of magnetizeable iron minerals.</p>

<p>And this is a problem because the magnetic field on a terrestrial planet serves the purpose of deflecting the solar wind away from the atmosphere. Without it, the wind directly strikes the atmosphere and begins to "sputter", or blow, it away. Mars thus lost its atmosphere in a comet tail-like effect, and with that also went the oceans thanks to the resulting pressure drop. What was left was the small amount of heavy (and hence hard to remove) carbon dioxide. No runaway greenhouse was produced because the thermal situation in the interior was just the opposite - it wasn't overheating, but rather, it was cooling down.</p>

<p>Generally, as long as the material from which you form planets - i.e. the star's proplyd - has sizeable quantities of water, as typically is the case, the planets will form oceans thanks to the chemical differentiation process leading to accumulation of a surface reservoir. Indeed, in many cases, they can form much <em>more</em> ocean as we've seen by observation of extrasolar planetary systems with planets that could have "ocean" layers hundreds of km thick (technically, this would only be liquid <em>ocean</em> proper down to about 100 km or so - at that point, the pressure becomes high enough [though the precise depth depends on the local gravity] to freeze the water by compression [pressure approx 1 GPa, for comparison, the Marianas on Earth, at 10 km depth is around 100 MPa], and the rest of the layer below that point is various high-pressure phases of ice) hence no bare land. It all depends on the water content of the material that forms it, and that is quite variable.</p>

<p>(One could, actually, consider certain moons of Jupiter and Saturn as actually Solar System examples of this kind of planetary composition, just at a much-reduced size as the extrasolar examples are north of one Earth mass (<span class="math-container">$6000\ \mathrm{Yg}$</span>).)</p>

CC BY-SA 4.0
2019-07-22T12:01:01.263Z
This is a nice, detailed answer, but take into account that the "magnetic field protects atmosphere"-story is not supported by current data. See a recent paper http://adsabs.harvard.edu/doi/10.1093/mnras/stz1819 and refs in the introduction. Also question: Are you aware of any coupled geological+atmospheric evolution models that can quantitatively reproduce the stories as presented by you about Mars/Venus? Without quantitative underpinning, they're after all just that...stories.
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-07-22T12:27:23.433Z
@AtmosphericPrisonEscape : Your linked article doesn't seem to really destroy the general thrust of the idea, but it does suggest it's a little bit simplistic and adds more detail, more interesting. Reading the link, a strong enough magnetic field _will_ deflect the stellar wind indeed, and as it weakens this protection falters, just as I've been saying. But the wrinkle is that apparently, per the paper, once it drops past a certain strength, but before it has hit zero, it actually acquires an _antagonistic_ effect where it _assists_ the stellar wind in stripping the atmosphere.
The_Sympathizer
CC BY-SA 4.0
2019-07-22T12:28:55.363Z
Thus if you think about a curve that has "magnetic field strength" on the x-axis and "protective capability" on the y-axis, it would look like a "bowl curve" - starts zero, goes _down_ into _negative_ capability (i.e. is harmful), reaches a minimum, then turns around and starts to climb again to and then past zero to positive protective (actually protecting) capability.
The_Sympathizer
CC BY-SA 4.0
2019-07-22T12:30:16.460Z
Moreover, the precise when and where of this happening, i.e. the shape of the bowl, depends on the strength of the wind. Of course, it could also be that perhaps Mars _started_ with field in the antagonistic range, and I believe the Solar wind in the primeval Solar System was stronger, too - I don't have the full paper accessible atm.
The_Sympathizer
CC BY-SA 4.0
2019-07-22T12:36:17.410Z
@AtmosphericPrisonEscape : Also found this stuff: https://www.lpi.usra.edu/vexag/meetings/archive/vexag_4th/nov_2007/presentations/bullock.pdf talking about Venus, gives an outline at least similar to here though with only one resurfacing event. But not sure of any formation-to-present simulation of any model, if you have references on any that have been done that you know of, perhaps with a _different_ scenario, I'd like to see it and then I could perhaps revise the answer with something better supported by stronger evidence.
The_Sympathizer
CC BY-SA 4.0
2019-07-22T12:43:43.727Z
Also, that post feels like it had been made a _lil_ bit barbish with the _repeated_ doubling-down on "stories" and "after all just that ... stories". You can (and should!) challenge my *arguments*, but I won't take that kind of junk, where it seems you're playing up the *tone*, putting a lil point on it like that, using a lathe and chisel a lil on it like that, when there's no need for that. Of course, maybe I am wrong on that assessment of your intent and if so I will retract said accusation, but that's how it seems.
The_Sympathizer
CC BY-SA 4.0
2019-07-22T12:56:16.730Z
My tone was as neutral as it gets, it's up to you to misread it. "Story" is a particular way of viewing how one writes a scientific article, being aware of a certain close relation to journalistic 'stories' - hence that choice of words. Yes, the article I've linked tries to convey that the mass-loss as function of planetary magnetization has a global maximum, in this case at 30 nT, for a Mars-like body. I think that's a simpler wording of what the article does. It does destroy the notion of "no magnetic field -> massive loss", which is still heard in many popscience debates.
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-07-22T12:58:27.280Z
Something that is not paywalled is the work on H. Gunell's website https://www.herbertgunell.se/research_escape.php (also has a nice comic) where I recommend reading the article, it's kept very simple, but gives the fundamental physics in a very nice manner. Concerning the 'global modelling', I'm only aware of work from the Vienna group, http://adsabs.harvard.edu/abs/2018A%26ARv..26....2L, and I know they got trouble with the geochemical constraints in their models, that's why I've asked.
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-07-22T12:58:30.143Z
@AtmosphericPrisonEscape Thanks for the first bit bit it was the "are just that ... stories" part at the end, _combined_ with the prior references, that kinda made me think something. It was not in any one use, but came across that way in the whole. Anyway, let's drop that and go for the good stuff - do you have data and graphs like, say, what then the magnetic field would have been at the inception of Mars, and how it evolved over time? E.g. if it started at more than your 30 nT figure, and dropped to that and then below over time?
The_Sympathizer
CC BY-SA 4.0
2019-07-22T13:00:02.633Z
As if it started much higher, say closer to Earth's, then it still seems the general outline works, but if it started already in the antagonistic regime from the get-go, then yes, the story is rejected on dint of empirical evidence.
The_Sympathizer
CC BY-SA 4.0
2019-07-22T13:09:07.110Z
I'm unfortunately not very familiar with the literature on early terrestrial dynamos. But the few articles I'm familiar with tell me that models and geological evidence don't mach up so far. I don't have much time to read many articles at the moment, but another name that could help you find reasonable numbers is B. Ehlmann. She does a lot of work on the geological history of Mars, and has this great overview chart in Ehlmann et al. 2011 (fig. 4, also googlable).
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-07-22T13:09:42.537Z
@AtmosphericPrisonEscape : thx.
The_Sympathizer
CC BY-SA 4.0
2019-07-22T11:34:39.507Z
2019-07-22T11:34:39.507Z
The_Sympathizer
<p>Water actually was never present on Earth before its creation and that is for sure.</p>

<p>Astronomers realized that there are two ready-made sources: comets and asteroids, the solar system’s gravel strewn among planetary boulders. The primary difference between the two is that comets typically have a greater concentration of ingredients that vaporize when heated, accounting for their iconic gaseous tails. Both comets and asteroids can contain ice. And if, by colliding with Earth, they added the amount of material some scientists suspect, such bodies could easily have delivered oceans’ worth of water. Accordingly, each has been fingered as a suspect in the mystery.</p>

<p>Adjudicating between the two is a challenge, and over the years scientific judgment has swung from one to the other. Nevertheless, recent observations of their chemical makeups are tipping the scale toward asteroids. Researchers reported last year, for example, that the ratios of different forms of hydrogen in asteroids appear to better match what we find here on Earth. But the analyses are based on limited samples, meaning there’s a good chance we’ve not yet heard the final word</p>

CC BY-SA 3.0
2016-05-08T14:03:21.687Z
References/ Citations would improve this answer
Fred
CC BY-SA 3.0
2016-05-09T11:49:22.097Z
"Water actually was never present on Earth before its creation and that is for sure." Well that's a statement that I can't dispute... :-)
Semidiurnal Simon
CC BY-SA 3.0
2016-05-10T21:54:17.727Z
Downvote: "Water actually was never present on Earth before its creation and that is for sure." Water was and is everywhere in the universe, as O and H are very abundant and H2O forms readily.
AtmosphericPrisonEscape
CC BY-SA 3.0
2017-03-29T16:40:51.327Z
earth was not present before the creation,but all the buildingblocks including water was present in the dust cloud that became the solar system.
trond hansen
CC BY-SA 3.0
2016-05-08T12:22:53.800Z
2016-05-08T12:22:53.800Z
Prince Mahad
<p>The water was already present when the Earth assembled itself out of the accretionary disk. Continued outgassing of volcanoes transferred the water into the atmosphere which was saturated with water. And rain transferred the water onto the surface.</p>

<p>Compared to other planets and smaller solar system object Earth has a big advantage. It is large enough to prevent water molecules to leave the gravitational field, and it has a magnetic field which prevents atmospheric erosion <a href="https://en.wikipedia.org/wiki/Atmospheric_escape" rel="noreferrer">(Wikipedia).</a> This is due to the Earth's outer core being liquid (Moving charged liquid = magnetic field). Mars probably had oceans until its outer core solidified so much, that the convection was stopped. After the magnetic field disappeared a few million years of solar radiation removed all of the atmosphere and the oceans.</p>

CC BY-SA 3.0
2014-04-16T16:09:44.857Z
2016-07-17T14:04:21.330Z
2016-07-17T14:04:21.330Z
tobias47n9e
<p>Earth is the only planet in our solar system that has copious amounts of water on it.  Where did this water come from and why is there so much water on Earth compared to every other planet in the solar system?</p>

CC BY-SA 3.0
2014-04-19T21:44:48.763Z
+1 for correctly referring to the name of the planet as Earth, not *the* Earth. Just like we don't say *the* Mars. :)
kwknowles
CC BY-SA 3.0
2014-04-16T08:37:17.890Z
Isn't there more water on one of Jupiter's moons? Yeah, [Europa](http://io9.com/theres-more-water-on-jupiters-moon-europa-than-there-5913104).
naught101
CC BY-SA 3.0
2014-04-16T22:55:40.717Z
@naught101, I didn't refer to moons in my question I referred to planets.
Kenshin
CC BY-SA 3.0
2014-04-17T01:16:43.067Z
I don't see why the process would be any different. Also, there are likely to be planets in other solar systems with surface water oceans, so a more general answer could be quite useful.
naught101
CC BY-SA 3.0
2017-10-26T03:28:45.537Z
keep in mind all four the gas giants have a lot of water.
John
CC BY-SA 4.0
2018-05-21T04:24:33.607Z
It actually is thought that both Venus and Mars had oceans just like Earth does but that they went away over time for different reasons. Venus was incredibly volcanic and CO2 and other volcanic gases kept building a thick atmosphere and the water simply evaporated into the atmosphere. With Mars, it had to do with the magnetic field turning off and exposing everything to the solar wind and because it only had 1/10 the gravity of earth, it already had a thin atmosphere to begin with. The water got to its triple point and either evaporated into space or froze into ice.
Caters
<geology><oceanography><geochemistry><planetary-science>
How and why did the oceans form on Earth but not on other planets?
Kenshin
# How and why did the oceans form on Earth but not on other planets?

(Asked by: username_0 on 2014-04-16)

Earth is the only planet in our solar system that has copious amounts of water on it. Where did this water come from and why is there so much water on Earth compared to every other planet in the solar system?

**Tags:** geology, oceanography, geochemistry, planetary-science

> **Comments:**
> **username_2**: Isn't there more water on one of Jupiter's moons? Yeah, [Europa](http://io9.com/theres-more-water-on-jupiters-moon-europa-than-there-5913104).  
> **username_0**: @username_2, I didn't refer to moons in my question I referred to planets.  
> **username_2**: I don't see why the process would be any different. Also, there are likely to be planets in other solar systems with surface water oceans, so a more general answer could be quite useful.  
> **username_1**: +1 for correctly referring to the name of the planet as Earth, not *the* Earth. Just like we don't say *the* Mars. :)  
> **username_3**: keep in mind all four the gas giants have a lot of water.  
> **username_4**: It actually is thought that both Venus and Mars had oceans just like Earth does but that they went away over time for different reasons. Venus was incredibly volcanic and CO2 and other volcanic gases kept building a thick atmosphere and the water simply evaporated into the atmosphere. With Mars, it had to do with the magnetic field turning off and exposing everything to the solar wind and because it only had 1/10 the gravity of earth, it already had a thin atmosphere to begin with. The water got to its triple point and either evaporated into space or froze into ice.  

---

## 5 answers

### Answer by username_12 on 2014-04-16. Score: 26 | Accepted answer

The water was already present when the Earth assembled itself out of the accretionary disk. Continued outgassing of volcanoes transferred the water into the atmosphere which was saturated with water. And rain transferred the water onto the surface.

Compared to other planets and smaller solar system object Earth has a big advantage. It is large enough to prevent water molecules to leave the gravitational field, and it has a magnetic field which prevents atmospheric erosion [(Wikipedia).](https://en.wikipedia.org/wiki/Atmospheric_escape) This is due to the Earth's outer core being liquid (Moving charged liquid = magnetic field). Mars probably had oceans until its outer core solidified so much, that the convection was stopped. After the magnetic field disappeared a few million years of solar radiation removed all of the atmosphere and the oceans.


---

### Answer by username_5 on 2014-04-16. Score: 31

Your assumption that there is not a lot of water elsewhere in the solar system is incorrect. According the [this article](https://solarsystem.nasa.gov/yss/display.cfm?ThemeID=23) on NASA's website;

> Missions in recent years have overturned our view of a dry solar system, returning mounting evidence of ample water from a vast array of locations.
>
> Comets from the remote corners of our solar system are made of water and other ices. Orbiters, landers, and rovers reveal Mars as a watery world in the distant past -- a world that today may contain entire underground oceans of frozen water.

It also points out that

> Jupiter's moon Europa has a frozen crust of water, covering a thick global ocean. By current estimates, it has twice as much water as all of Earth's oceans and rivers!

The perception that the rest of the solar system does not have a lot of water is probably due to the fact that the Earth is in the ['Goldilocks zone'](http://en.wikipedia.org/wiki/Habitable_zone) where water can exist in all three phases *at the surface* of the planet. The temperatures on the rest of the planets in the solar system are either so hot that it can only exist in a vapor phase or so cold that it can only exist in ice at the surface or liquid beneath.


---

### Answer by username_6 on 2016-01-14. Score: 6

An important issue to realize is that water must have been very abundant in the protosolar disc, as tobias already stated.
To expand on that I just want briefly touch upon the atomic abundances that we measure in the corona of the sun, as presented by a wikimedia-commons graph:

[![enter image description here](https://i.sstatic.net/hlTxQ.png)](https://i.sstatic.net/hlTxQ.png)
(Sidenote: Those abundances compare well to the revised [Asplund2009](http://adsabs.harvard.edu/abs/2009ARA%26A..47..481A)-abundances)  
We think that those numbers are representative of the bulk composition of the Sun, as it is only burning Helium since 4.567 Gyrs. Thus the composition is generally assumed to be primordial, or what the solar system started out with.

Now let us imagine this atomic mixture accumulating around the young terrestrial planets and focus on the most abundant elements H, He, C, N and O. In a thick planetary envelope, shielded by UV, equilibrium chemistry will set in. Then a lot of $H\_2$ will form, He will stay inert and C, N, O will try to react with Hydrogen, simply because the number of encounters is much higher with that, than inside of the CNO group. Some CO will form, as this is a very stable molecule.
But as C becomes depleted, and our protosolar nebula has $C/O \approx 0.5$, there is still a lot of Oxygen left. Thus it will inevitably combine to $H\_2O$.

The result is, that we expect there to be really *a lot* of water around in Planet-forming discs.

The escape or destruction of water after this period however, is apparently also very efficient, and other answers have touched upon the retention of water on Earth. So in fact Astronomers at the moment rather wonder "where did all the water go?"


---

### Answer by username_7 on 2019-07-22. Score: 4

Oceans *did* form on other rocky planets - at least Venus and Mars, and moreover, many moons of Jupiter and Saturn as well.

The problem is that of the two other terrestrial "*uberplanets*" who *had* oceans - i.e. the aforementioned Venus and Mars - they lost them, but in rather different ways.

On Venus, what happened appears to have been that, synthesizing the best theories and evidence so far, as the Sun steadily grew hotter as a result of the natural main-sequence stellar evolution, the planet essentially left the habitable zone. The oceans evaporated and were lost to space. The way that works is that as the planet heats up, the water vapor content in the atmosphere increases, and some of that vapor rises to the top of the atmosphere. At the top, the UV flux begins to break apart (photodissociation) the water molecules, releasing the hydrogen, which then easily escapes as its gravitoionization energy is very low (for Venus, with an escape velocity of 10.36 km/s, it is similar to Earth - stripping one $\mathrm{H}\_2$ molecule, a mass of ~3.4 yg, costs 0.18 aJ or 1.1 eV, which can easily be supplied by photons, stellar wind, or other factors. Stripping nascent [monoatomic] hydrogen, if such could happen without molecularization, takes half this.). The end result is an oxygen-rich atmosphere (nearly pure oxygen) at very high pressure, though chemical reactions with the rock below may strip some of this.

But as you may note, the current atmosphere is mostly carbon dioxide, with a very high surface temperature, and thus while the above is fairly plausible, one must wonder how this atmosphere obtained. Well, here's the thing. Much like Earth, Venus likely had plate tectonics early in its history when it had oceans. Ocean water seems to be a necessary component of plate tectonic activity on planets at least around the size of Earth, acting as a sort of "lubricant", and without it, plate tectonics cannot operate. Hence, once the water evaporated from Venus, any tectonics it had would have stopped.

The cessation of plate tectonics, while it may not seem like much to the uninitiate (so what? The continents stop moving, and you stop getting earthquakes?), is actually *catastrophic* for this kind of planet. You see, plate tectonics does more than just move continents, mountains, and earthquakes. What it most *crucially* does is to serve as a sort of "release valve" for the heat that is being generated inside the planet as a result of the decay of primordial radionuclides mixed with the mantle rocks (mostly $^{238}\mathrm{U}$). This is because that, at spreading zones, hot mantle material oozes out (Iceland is a good Earthbound example of where this process happens to just come above water and can be seen with the naked eye - youtube it if you're too poor to travel), and at subduction zones, volcanoes are created, like in the Pacific Ring of Fire on Earth. The released material transfers the heat to the atmosphere and thus ultimately it gets radiated into space, hence shedding the internal heat. Without this process, that heat is shed far less efficiently - essentially only by conduction through the ground, which is terrible.

As a result, once you lose tectonics, the radioactive heat begins to build within the interior, and the mantle temperature starts to rise considerably. And when that starts, eventually, something has to give. And what "gives" is not quite well-understood but it seems to be either that tectonics restarts for a "short" time "with a vengeance", or that there commence widespread basaltic flood eruptions, or some combination of the two. In any case, the result is a rapid - on a geological scale of course, actually about 100 million years - "global resurfacing event" resulting in either or a combination of total subduction/reemplacement of the old crust (hence that is a considerably faster rate of tectonic motion than on Earth - to recycle all its crust over 100 Myr would take plate motion on the order of 20 cm/yr, where the fastest plate movement [India] in recent time was only around 4-6 cm/yr), or its progressive burial in flows similar to those from the Siberian Traps, likely from multiple vent sources, extending over such a long enough period as to cover all low-lying areas at least. In both cases there occurs the widespread release of volcanic gases which chiefly includes carbon dioxide, and that quickly converts the atmosphere to a "runaway greenhouse" state like we see on the planet today. The last GRE on Venus was finished at about 500 Ma, and there may have been more before that until the time when it last had oceans, and it is possible that the current thickness was the result of such a chain of episodes instead of just one.

The oceans on Mars, on the other hand, seem to have suffered a rather quieter demise. In this case, it seems the chief problem was the planet was simply too small, and the cause was not the result of external changes in the form of those in the Sun, but rather internal ones. As a result of its lack of size and hence of mass, there was both less primordial heat within the core and a smaller radionuclide load to maintain the internal temperature. The interior would have cooled to the point its iron inner core froze completely, and that shut down the planetary magnetic dynamo, which we know existed in the past thanks to there still being today small areas of "fossilized" magnetization in the crust due to rich concentrations of magnetizeable iron minerals.

And this is a problem because the magnetic field on a terrestrial planet serves the purpose of deflecting the solar wind away from the atmosphere. Without it, the wind directly strikes the atmosphere and begins to "sputter", or blow, it away. Mars thus lost its atmosphere in a comet tail-like effect, and with that also went the oceans thanks to the resulting pressure drop. What was left was the small amount of heavy (and hence hard to remove) carbon dioxide. No runaway greenhouse was produced because the thermal situation in the interior was just the opposite - it wasn't overheating, but rather, it was cooling down.

Generally, as long as the material from which you form planets - i.e. the star's proplyd - has sizeable quantities of water, as typically is the case, the planets will form oceans thanks to the chemical differentiation process leading to accumulation of a surface reservoir. Indeed, in many cases, they can form much *more* ocean as we've seen by observation of extrasolar planetary systems with planets that could have "ocean" layers hundreds of km thick (technically, this would only be liquid *ocean* proper down to about 100 km or so - at that point, the pressure becomes high enough [though the precise depth depends on the local gravity] to freeze the water by compression [pressure approx 1 GPa, for comparison, the Marianas on Earth, at 10 km depth is around 100 MPa], and the rest of the layer below that point is various high-pressure phases of ice) hence no bare land. It all depends on the water content of the material that forms it, and that is quite variable.

(One could, actually, consider certain moons of Jupiter and Saturn as actually Solar System examples of this kind of planetary composition, just at a much-reduced size as the extrasolar examples are north of one Earth mass ($6000\ \mathrm{Yg}$).)

> **Comments:**
> **username_6**: This is a nice, detailed answer, but take into account that the "magnetic field protects atmosphere"-story is not supported by current data. See a recent paper http://adsabs.harvard.edu/doi/10.1093/mnras/stz1819 and refs in the introduction. Also question: Are you aware of any coupled geological+atmospheric evolution models that can quantitatively reproduce the stories as presented by you about Mars/Venus? Without quantitative underpinning, they're after all just that...stories.  
> **username_7**: @username_6 : Your linked article doesn't seem to really destroy the general thrust of the idea, but it does suggest it's a little bit simplistic and adds more detail, more interesting. Reading the link, a strong enough magnetic field _will_ deflect the stellar wind indeed, and as it weakens this protection falters, just as I've been saying. But the wrinkle is that apparently, per the paper, once it drops past a certain strength, but before it has hit zero, it actually acquires an _antagonistic_ effect where it _assists_ the stellar wind in stripping the atmosphere.  
> **username_7**: Thus if you think about a curve that has "magnetic field strength" on the x-axis and "protective capability" on the y-axis, it would look like a "bowl curve" - starts zero, goes _down_ into _negative_ capability (i.e. is harmful), reaches a minimum, then turns around and starts to climb again to and then past zero to positive protective (actually protecting) capability.  
> **username_7**: Moreover, the precise when and where of this happening, i.e. the shape of the bowl, depends on the strength of the wind. Of course, it could also be that perhaps Mars _started_ with field in the antagonistic range, and I believe the Solar wind in the primeval Solar System was stronger, too - I don't have the full paper accessible atm.  
> **username_7**: @username_6 : Also found this stuff: https://www.lpi.usra.edu/vexag/meetings/archive/vexag_4th/nov_2007/presentations/bullock.pdf talking about Venus, gives an outline at least similar to here though with only one resurfacing event. But not sure of any formation-to-present simulation of any model, if you have references on any that have been done that you know of, perhaps with a _different_ scenario, I'd like to see it and then I could perhaps revise the answer with something better supported by stronger evidence.  
> **username_7**: Also, that post feels like it had been made a _lil_ bit barbish with the _repeated_ doubling-down on "stories" and "after all just that ... stories". You can (and should!) challenge my *arguments*, but I won't take that kind of junk, where it seems you're playing up the *tone*, putting a lil point on it like that, using a lathe and chisel a lil on it like that, when there's no need for that. Of course, maybe I am wrong on that assessment of your intent and if so I will retract said accusation, but that's how it seems.  
> **username_6**: My tone was as neutral as it gets, it's up to you to misread it. "Story" is a particular way of viewing how one writes a scientific article, being aware of a certain close relation to journalistic 'stories' - hence that choice of words. Yes, the article I've linked tries to convey that the mass-loss as function of planetary magnetization has a global maximum, in this case at 30 nT, for a Mars-like body. I think that's a simpler wording of what the article does. It does destroy the notion of "no magnetic field -> massive loss", which is still heard in many popscience debates.  
> **username_6**: Something that is not paywalled is the work on H. Gunell's website https://www.herbertgunell.se/research_escape.php (also has a nice comic) where I recommend reading the article, it's kept very simple, but gives the fundamental physics in a very nice manner. Concerning the 'global modelling', I'm only aware of work from the Vienna group, http://adsabs.harvard.edu/abs/2018A%26ARv..26....2L, and I know they got trouble with the geochemical constraints in their models, that's why I've asked.  
> **username_7**: @username_6 Thanks for the first bit bit it was the "are just that ... stories" part at the end, _combined_ with the prior references, that kinda made me think something. It was not in any one use, but came across that way in the whole. Anyway, let's drop that and go for the good stuff - do you have data and graphs like, say, what then the magnetic field would have been at the inception of Mars, and how it evolved over time? E.g. if it started at more than your 30 nT figure, and dropped to that and then below over time?  
> **username_7**: As if it started much higher, say closer to Earth's, then it still seems the general outline works, but if it started already in the antagonistic regime from the get-go, then yes, the story is rejected on dint of empirical evidence.  
> **username_6**: I'm unfortunately not very familiar with the literature on early terrestrial dynamos. But the few articles I'm familiar with tell me that models and geological evidence don't mach up so far. I don't have much time to read many articles at the moment, but another name that could help you find reasonable numbers is B. Ehlmann. She does a lot of work on the geological history of Mars, and has this great overview chart in Ehlmann et al. 2011 (fig. 4, also googlable).  
> **username_7**: @username_6 : thx.  

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### Answer by username_8 on 2016-05-08. Score: -2

Water actually was never present on Earth before its creation and that is for sure.

Astronomers realized that there are two ready-made sources: comets and asteroids, the solar system’s gravel strewn among planetary boulders. The primary difference between the two is that comets typically have a greater concentration of ingredients that vaporize when heated, accounting for their iconic gaseous tails. Both comets and asteroids can contain ice. And if, by colliding with Earth, they added the amount of material some scientists suspect, such bodies could easily have delivered oceans’ worth of water. Accordingly, each has been fingered as a suspect in the mystery.

Adjudicating between the two is a challenge, and over the years scientific judgment has swung from one to the other. Nevertheless, recent observations of their chemical makeups are tipping the scale toward asteroids. Researchers reported last year, for example, that the ratios of different forms of hydrogen in asteroids appear to better match what we find here on Earth. But the analyses are based on limited samples, meaning there’s a good chance we’ve not yet heard the final word

> **Comments:**
> **username_9**: References/ Citations would improve this answer  
> **username_10**: "Water actually was never present on Earth before its creation and that is for sure." Well that's a statement that I can't dispute... :-)  
> **username_6**: Downvote: "Water actually was never present on Earth before its creation and that is for sure." Water was and is everywhere in the universe, as O and H are very abundant and H2O forms readily.  
> **username_11**: earth was not present before the creation,but all the buildingblocks including water was present in the dust cloud that became the solar system.  

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