# How did water get on Earth

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?

• Well, of course the water would have been vaporized on impact. The result is water vapor, which eventually becomes rain. – David H Mar 27 '15 at 5:23
• 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 Mar 27 '15 at 7:46
• 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. This corresponds to a temperature of ~2500 K. A quick Google search 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 Mar 27 '15 at 10:45
• 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 Mar 27 '15 at 12:20
• @pela: I think we can leave it be, as a little discussion is sometimes illuminating, as in this case too. – AtmosphericPrisonEscape Mar 27 '15 at 16:38

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:

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.

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.