MadSci Network: Earth Sciences

Re: Where did all the ocean water come from?

Date: Thu Oct 28 08:50:58 1999
Posted By: David Kopaska-Merkel, Staff Hydrogeology Division, Geological Survey of Alabama
Area of science: Earth Sciences
ID: 940591099.Es

I can think of two possible ways to answer your question and I don't know 
which kind of answer you are looking for, so I will give you both. 

First, the water in the ocean, and indeed all the water in the world, is 
involved in a continuous process called the water cycle. Most of the water 
that is involved in the water cycle is in the ocean (more than 97%). Water 
leaves the ocean chiefly by evaporation, and it returns to the ocean by 
precipitation, via rivers and streams, and via undersea springs. The total 
amount of water in the ocean doesn't change much. During the ice ages sea 
level fell by more than 100 meters (more than 300 feet), and if all of the 
existing glacial ice melted sea level would rise by an alarming but smaller 
amount. However, from year to year the ocean volume changes little.

Second, one can ask the question: Where did all this water come from in the 
first place? As I understand it, there are two main sources:  primary 
water, and water from comets. When the earth coalesced out of the 
primordial gas cloud that surrounded the young sun, hydrogen and oxygen 
were two of the chemical elements incorporated in the earth. Hydrogen is of 
course by far the most common element everywhere in the universe: you can't 
form a planet without hydrogen. Oxygen is less abundant in the universe, 
but it is a common element too, if you don't consider the "biggies", 
hydrogen and helium. Oxygen and hydrogen are chemically quite reactive, and 
so they both combined with various other elements to form chemical 
compounds. One of these compounds is water, and water quickly became an 
important part of the earth's surficial layer, along with many other 
familiar chemicals (silica - a major component of sand, calcium carbonate - 
what limestone is made of, etc.). If water existed on earth only as a 
solid, then there would be no oceans as we know them, but water can readily 
exist as a liquid under the pressure and temperature conditions that hold 
on many parts of the earth's surface. Liquids run downhill, and so liquid 
water accumulated in low places. There is a lot of water, so some low 
places contain huge amounts of it - we call these oceans. 
     Over the past 4.5 billion years earth has been hit by many comets. We 
know that comets consist chiefly of chemical compounds that are either 
gases or liquids at earth-surface conditions: methane is one common 
component of comets and water is another. A significant amount of earth's 
water has probably been delivered by comets, especially in the first 
billion years or so after earth formed. During this time the solar system 
was filled with debris: small objects that coalesced from the primordial 
gas cloud. After a billion years or so, most of these objects had hit 
something, and airless bodies like our moon show many huge craters dating 
from this period. The Oort cloud of comets out past Pluto and the asteroid 
belt between Mars and Jupiter contain many of the small objects that have 
not yet slammed into planets or the sun.

Hope this helps.

David C. Kopaska-Merkel
Head, Ground Water Section
Geological Survey of Alabama
PO Box 869999
Tuscaloosa AL 35486-6999
(205) 349-2852
FAX (205) 349-2861
web site

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