<p>The key finding why we think Earth's water came from Asteroids (big rocks) and not Comets (small rocks) is the Deuterium/Hydrogen ratio that we can measure in several sources.<br>
When a star forms, it has an initial value of D/H that came from the nucleosynthesis in its progenitor nebular / star.</p>

<p>In a protoplanetary disc, as dust grows to rocks grow to planets your nebular gas will be trapped with initial D/H ratios in gas giant atmospheres. But the way to get Water onto Asteroids and Comets (those don't have substantial mass to retain atmospheres themselves!) is sublimation and maybe adsorption.<br>
The latter two processes are strongly sensitive to the gas-mass and therefore different D/H ratios from the protosolar ones are expected.
And in fact we find them to be different:</p>

<p><img src="https://i.sstatic.net/cmT0G.jpg" alt="D/H ratios from various sources in the solar system"></p>

<p>This was in the news lately, as ESA managed to touch down and measure 67P's D/H ratio which gave another hint on the asteroidic origin of water on earth.  </p>

<p>This finding, however does not resolve the question: </p>

<ul>
<li>There maybe other isotopical tracers like D/H that give a hint on the history of water.</li>
<li>There could have been a decrease in D/H and subsequential increase again, or the other way round.</li>
<li>We know that the protosolar nebula must have had massive amounts of water (Oxygen is the third-most abundand element in normal stellar fusion!), so again the question is, why did Earth retain so much water, while the <em>lighter</em> planets Venus and Mars retained the <em>heavier</em> element $CO_2$
...</li>
</ul>

<p>I could continue this for quite some time, but the bottom line is: We only have hints, not definitive answers.</p>

<p>To refer to the rest of your question: A hot plume in a colder atmosphere will not necesarily leave the whole gravitational potential well.</p>

CC BY-SA 3.0
2015-03-27T12:46:08.170Z
2015-03-27T12:46:08.170Z
AtmosphericPrisonEscape
<p>I don't believe there is factual answer just yet, though many solid theories. Here are some sources I've found to be helpful. I don't necessarily have an opinion myself and while certainly everyone is entitled to one, I wouldn't put too much stock in any one source of information.</p>

<p>It was "always" here:
<a href="http://news.nationalgeographic.com/news/2014/10/141030-starstruck-earth-water-origin-vesta-science/" rel="nofollow noreferrer">http://news.nationalgeographic.com/news/2014/10/141030-starstruck-earth-water-origin-vesta-science/</a></p>

<p>Comets:
<a href="http://space.com/27969-earth-water-from-asteroids-not-comets.html" rel="nofollow noreferrer">http://space.com/27969-earth-water-from-asteroids-not-comets.html</a></p>

<p>Meteors:
<a href="http://youtube" rel="nofollow noreferrer">http://youtube</a> .com/watch?v=_LpgBvEPozk</p>

<p>ps: had to break the links due to reputation. :/</p>

CC BY-SA 3.0
2017-03-16T15:39:49.533Z
To note: Stack Exchange prefers answers to be self-contained. It's ok to post links to your sources, but a good answer also includes a summary of the content so that if the links die, the answer is still useful as an answer.
Asher
CC BY-SA 3.0
2015-03-27T10:37:43.547Z
2017-03-16T18:39:10.893Z
2017-03-16T18:39:10.893Z
GingerBeard
<p>I recently read that water more than likely got here by comets (carrying water) hitting the Earth. However it also it says the impact of a comet hitting the Earth is much greater than an atomic bomb. So if comets brought water to Earth how was it not vaporized when the comet hit the atmosphere or in the massive explosion when it hit the Earth itself?</p>

CC BY-SA 3.0
2015-03-27T05:23:28.040Z
Well, of course the water would have been vaporized on impact. The result is water vapor, which eventually becomes rain.
David H
CC BY-SA 3.0
2015-03-27T07:46:48.207Z
Seems a perfectly sensible question to me. If you have a very hot gas it would escape the Earth's gravity nonetheless. The question is how hot or can it cool quickly enough as it expands.
ProfRob
CC BY-SA 3.0
2015-03-27T10:45:10.370Z
In order for Earth's gravitational field to keep most of the vaporized gas, the rms velocity of the water molecules shouldn't exeed [roughly 1/6 of Earth's escape velocity](http://astronomy.stackexchange.com/questions/10256/gravitational-pull-needed-to-keep-a-gas-in-atmosphere/10258#10258). This corresponds to a temperature of ~2500 K. A quick [Google search](https://www.google.dk/#q=temperature+of+meteor+impact&start=10) gives me a couple of popular sources saying that the temperature of a meteor impact is ~2000 K, so it seems that the water vapor would stay in Earth's atmosphere.
pela
CC BY-SA 3.0
2015-03-27T12:20:17.607Z
This number of 1/6 you're citing is only valid if the whole atmosphere has the temperature. In case of a hot plume of this temperature launched in the troposphere, this will be easily trapped in the lower atmosphere, and can maybe wirl up dust higher up, like we know it from volcanic eruptions. However like @Rob-Jeffries said, it's important how fast the plume cools.
AtmosphericPrisonEscape
CC BY-SA 3.0
2015-03-27T14:07:29.613Z
Ah, thanks @AtmosphericPrisonEscape. Good point. Should I delete my comment, so as not to confuse readers, or should I let it be?
pela
CC BY-SA 3.0
2015-03-27T16:38:29.010Z
@pela: I think we can leave it be, as a little discussion is sometimes illuminating, as in this case too.
AtmosphericPrisonEscape
<earth><comets><water>
How did water get on Earth
TheDudeAbides
# How did water get on Earth

(Asked by: username_0 on 2015-03-27)

I recently read that water more than likely got here by comets (carrying water) hitting the Earth. However it also it says the impact of a comet hitting the Earth is much greater than an atomic bomb. So if comets brought water to Earth how was it not vaporized when the comet hit the atmosphere or in the massive explosion when it hit the Earth itself?

**Tags:** earth, comets, water

> **Comments:**
> **username_1**: Well, of course the water would have been vaporized on impact. The result is water vapor, which eventually becomes rain.  
> **username_2**: Seems a perfectly sensible question to me. If you have a very hot gas it would escape the Earth's gravity nonetheless. The question is how hot or can it cool quickly enough as it expands.  
> **username_3**: In order for Earth's gravitational field to keep most of the vaporized gas, the rms velocity of the water molecules shouldn't exeed [roughly 1/6 of Earth's escape velocity](http://astronomy.stackexchange.com/questions/10256/gravitational-pull-needed-to-keep-a-gas-in-atmosphere/10258#10258). This corresponds to a temperature of ~2500 K. A quick [Google search](https://www.google.dk/#q=temperature+of+meteor+impact&start=10) gives me a couple of popular sources saying that the temperature of a meteor impact is ~2000 K, so it seems that the water vapor would stay in Earth's atmosphere.  
> **username_4**: This number of 1/6 you're citing is only valid if the whole atmosphere has the temperature. In case of a hot plume of this temperature launched in the troposphere, this will be easily trapped in the lower atmosphere, and can maybe wirl up dust higher up, like we know it from volcanic eruptions. However like @Rob-Jeffries said, it's important how fast the plume cools.  
> **username_3**: Ah, thanks @username_4. Good point. Should I delete my comment, so as not to confuse readers, or should I let it be?  
> **username_4**: @username_3: I think we can leave it be, as a little discussion is sometimes illuminating, as in this case too.  

---

## 2 answers

### Answer by username_4 on 2015-03-27. Score: 10

The key finding why we think Earth's water came from Asteroids (big rocks) and not Comets (small rocks) is the Deuterium/Hydrogen ratio that we can measure in several sources.  
When a star forms, it has an initial value of D/H that came from the nucleosynthesis in its progenitor nebular / star.

In a protoplanetary disc, as dust grows to rocks grow to planets your nebular gas will be trapped with initial D/H ratios in gas giant atmospheres. But the way to get Water onto Asteroids and Comets (those don't have substantial mass to retain atmospheres themselves!) is sublimation and maybe adsorption.  
The latter two processes are strongly sensitive to the gas-mass and therefore different D/H ratios from the protosolar ones are expected.
And in fact we find them to be different:

![D/H ratios from various sources in the solar system](https://i.sstatic.net/cmT0G.jpg)

This was in the news lately, as ESA managed to touch down and measure 67P's D/H ratio which gave another hint on the asteroidic origin of water on earth.

This finding, however does not resolve the question:

* There maybe other isotopical tracers like D/H that give a hint on the history of water.
* There could have been a decrease in D/H and subsequential increase again, or the other way round.
* We know that the protosolar nebula must have had massive amounts of water (Oxygen is the third-most abundand element in normal stellar fusion!), so again the question is, why did Earth retain so much water, while the *lighter* planets Venus and Mars retained the *heavier* element $CO\_2$
  ...

I could continue this for quite some time, but the bottom line is: We only have hints, not definitive answers.

To refer to the rest of your question: A hot plume in a colder atmosphere will not necesarily leave the whole gravitational potential well.


---

### Answer by username_5 on 2015-03-27. Score: 3

I don't believe there is factual answer just yet, though many solid theories. Here are some sources I've found to be helpful. I don't necessarily have an opinion myself and while certainly everyone is entitled to one, I wouldn't put too much stock in any one source of information.

It was "always" here:
<http://news.nationalgeographic.com/news/2014/10/141030-starstruck-earth-water-origin-vesta-science/>

Comets:
<http://space.com/27969-earth-water-from-asteroids-not-comets.html>

Meteors:
<http://youtube> .com/watch?v=\_LpgBvEPozk

ps: had to break the links due to reputation. :/

> **Comments:**
> **username_6**: To note: Stack Exchange prefers answers to be self-contained. It's ok to post links to your sources, but a good answer also includes a summary of the content so that if the links die, the answer is still useful as an answer.  

---

