Inquiring minds want to know if Earth has the same amount of H2O it had

[snagglepuss]

200 million years ago or has it diminished over time?

[izquierdista]

1. Probably not

There is one process to lose H2O, that of molecules high in the atmosphere getting enough of a kick from other molecules to knock them past escape velocity and off they go into the cosmos. Then there is a process of gaining H2O, from comets and meteors and other foreign bodies that enter the atmosphere. Oh, they may lose the water to the atmosphere as they enter since they will be very hot, but the water comes in with them.

What you have to do is figure out which rate is higher. Given the amount of water on the planet, I would guess that it is the second one.

[snagglepuss]

3. So the water we guzzle is the same water guzzled by dinosaurs?

[izquierdista]

6. Most of it

You can calculate those two rates I noted, but when divided by the amount of mass firmly held on the planet, it's going to be a very, very small number.

[snagglepuss]

11. Thanks for your explanations. It's staggering to think that we drink something

that is hundreds of millions years old.

[Chemisse]

10. And peed out by dinosaurs!

A fact that never fails to delight/horrify my chemistry students.

[arcane1]

2. We've probably lost all the water contained in Appolo astronout urine n/t

[marasinghe]

4. while i don't know the answer to your question -

from high-school chemistry:
although it is a pretty stable compound, water will react with halides -- Flourine, Chlorine, etc..
it will also be removed from it's free state - by bonding with crystals, and, in forming other types of non-chemical bonds (i think).
and, i have no idea how much water -- if any -- man-made synthetic manufacture will use up to form other compounds.
the expanding human population, all use water to form their digestive juices - Hydrochloric acid.
so, there is a possibility that the amount of water within the Earth's eco-system, has diminished.

not to mention the quantity of potable fresh water, available.

[Salviati]

5. Water is also broken down during photosynthisis to provide all the hydrogens to attach to carbon

and is created when we metabolize sugars when those hydrogens are reunited with the oxygen that we breath.

[1620rock]

7. Escape velocty?

"molecules high in the atmosphere getting enough of a kick from other molecules to knock them past escape velosity and off they go into the cosmos."

Not doubting you here, but escape velosity...isn't that around 24000 MPH?

[Dead_Parrot]

8. Possibly more

we keep scooping up extra from comets (and thier tails).

[eppur_se_muova]

9. There is a very slow loss of water due to photodissociation in the upper atmosphere ...

this results in free atomic H and H2, which are volatile enough to escape Earth's gravity. This is a very slow process, however, and probably has been more than compensated for by the infall of meteoritic dust and cometary debris.

http://books.google.com/books?id=hWa0LcQNZhcC&pg=PA5&lpg=PA5&dq=photodissociation+of+water+in+upper+atmosphere&source=bl&ots=9JBa0qvKlI&sig=6S5ee0Ydm4nw7ihOE1NoUJW2bAs&hl=en&sa=X&ei=uBVbT5u-F5CItwfsxsiFDA&ved=0CEcQ6AEwAw#v=onepage&q=photodissociation%20of%20water%20in%20upper%20atmosphere&f=false

Mars, with its lower gravity and no ozone shield to block UV, would be more subject to this effect, and the loss of water from Mars by this process has probably been much more significant.

[Warpy]

12. It's probably got more

because of all the icy space junk raining down through the atmosphere every year.