How did the Earth get its water?

--------- #1304 - How Did the Earth Get Its Water?

- The Earth is a planet that was born in an inferno. No liquid water could have survived the intense heat. So, how did the planet get its precious water?

- 4,500,000,000 years ago the inner Solar System was 400 degrees C, which is hot enough to melt lead and vaporize water. A standing theory for astronomers was that a bombardment of icy comets brought the water to Earth a billion years later, after the Earth had cooled down. However, we have since learned the chemical composition of comet water does not match the water on Earth.

- The accretion disk around the Sun that formed the planets was an excellent system for separating the elements. The heavier elements fell toward the center due to the Sun’s gravity. The rocky planets formed in the inner Solar System and the gaseous planets formed in the outer Solar System. Also, it was hotter closer to the Sun. All the elements solidified or froze at different circumferences as you progress out on the accretion disk. Asteroids are rocky and formed in the inner part of the disk. Comet are frozen snowballs of ice and they formed in the outer part of the disk.

- It turns out that the asteroids are mostly rock, but contain water and their water is a better chemical match to what we find on Earth. The inner planets are Mercury, Venus, Earth, and Mars. The left over rocks that did not form planets are the asteroids. Most of the asteroids lie in a belt orbiting between Mars and Jupiter. This asteroid belt marks the transition between the rocks and the gas in the Solar System. It is estimated that there are over 1 billion asteroids orbiting the Sun with the planets.

- Computer models studying the possible interactions between the two largest planets, Jupiter and Saturn, suggest that gravity traps may have pushed and pulled many of the asteroids out of their natural orbits and sent them hurtling toward the Sun, crossing the orbits of the inner, rocky planets. The outer asteroids were more like icy rocks, they had elongated orbits that may have been what pummeled the Earth with a renewal of water. The recent discovery of water ice on the surface of an outer asteroid (65Cybele) has significantly reinforce this theory.

- The Earth lot all of its water and then regained it with multiple asteroid impacts. If the Earth was perfectly smooth it would be covered in water up to 4 ½ miles deep.

- We tend to think the Earth is a large land mass. If we just count the land on which people can live it is only 12% of the surface of the planet. If we include the inland seas the livable land mass is only 4% of the surface. We are fortunate indeed to have all this liquid water. If our orbit was just 1% farther from the Sun or just 4% closet to the Sun we would have ice or vapor and not liquid water. There are other estimates that say 5% closer or 15% farther but that is still a narrow band for liquid water habitation.

- Theories are just that theories. We are not certain asteroids are the answer on how Earth got its water. Comets were not a good match because they contain too much deuterium, the heavy isotope of hydrogen. The amount of deuterium is twice as much as is found in our oceans today. Asteroids are a good match for the amount of deuterium but they are a bad match for the amount of isotopic abundance of the noble gases, such as Xenon.

- Another theory is that certain molecules on Earth captured the water, withstood the inferno temperatures, and later released the water after things cooled down. Certain molecules such as Olivine, or Magnesium could bind water molecules so tightly that they could withstand temperatures of 600 C. If that were possible water could have arisen out of the cosmic dust after the planet had formed. As the Earth cooled and the rocks crystallized water liquid would be released. A team rich atmosphere would condense onto the surface to form our oceans.

- One of the primary reason astronomers are trying to determine how the Earth got is water is that they need to learn if there could be liquid water on other planets and other moons in our Solar System and other solar systems. Liquid water is necessary for life as we know it. Over 1,200 new planets have been discovered in other solar systems. We would like to learn if any of these planets are habitable. Which planets are water-rich and how did they get that way? How did the Earth itself get that way? We think Earth is water-rich, but, actually it is a desert. 70% of the surface of Earth is covered in water but that represents only 0.02% of the Earth’s mass.

- There may be an ocean ‘s worth of water still locked up in the Earth’s interior. We would like to learn this to apply to the prevalence of water on the extra solar terrestrial planets we discover. To listen to a conference debate on this question visit
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