"EPIC" Earth Image by NASA (July 06, 2015)

Where Earth’s water came from?

Earth is a blue marble in the space: the water, gives our planet its blue color: about 71 percent of the Earth’s surface is water-covered. There is roughly 326 million cubic miles (1.332 billion cubic kilometers) water on the Earth’s surface. Almost 97% of that water is salty (ocean water). But where all that water came from?

Earth and water
All Earth’s water, liquid fresh water, and water in lakes and rivers
Spheres showing:
1. All water (sphere over western U.S., 860 miles/1384.04 in diameter)
2. Fresh liquid water in the ground, lakes, swamps, and rivers (sphere over Kentucky, 169.5 miles/272.78 km in diameter), and
3. Fresh-water lakes and rivers (sphere over Georgia, 34.9 miles/56.17 in diameter).
Credit: Howard Perlman, USGS; globe illustration by Jack Cook, Woods Hole Oceanographic Institution (©); Adam Nieman.

There are two possible sources: the space, and the Earth itself. Scientists think most of Earth’s water came from the space.

The Oort Cloud

Previously, it’s been thought that the comets from Oort cloud. The Oort cloud, sometimes called the Öpik–Oort cloud, is a theoretical cloud of predominantly icy planetesimals believed to surround the Sun to as far as somewhere between 50,000 and 200,000 AU (0.8 and 3.2 light year). AU stands for “astronomical unit” here, a unit of measurement equal to 149.6 million kilometres, the mean distance from the centre of the earth to the centre of the sun.

But there is a problem with that theory: the water on Earth contains two types of water: every 9,997 of 10,000 molecules are “normal water”, the H2O. But the remaining three are Heavy water (deuterium oxide), 2H2O or D2O. Deuterium, also known as heavy hydrogen is a hydrogen isotope. The nucleus of deuterium, called a deuteron, contains one proton and one neutron, whereas the far more common hydrogen isotope, protium (or simply hydrogen), has no neutron in the nucleus.

To see if the comets are the main source of Earth’s water, in 1986, the Giotto robotic spacecraft of ESA (European Space Agency) flew by and studied Halley’s Comet (which is coming from the Oort cloud). The mission succeeded in approaching Halley’s nucleus at a distance of 596 kilometers, and in doing so became the first spacecraft to make close up observations of a comet. Analysis showed the comet formed 4.5 billion years ago from volatiles (mainly ice) that had condensed onto interstellar dust particles.

Halley was made by 80% water, but it turned out the comet had twice the amount of heavy water compared to the water on Earth.

The Kuiper Belt

Philae on Churyumov-Gerasimenko
Rosetta’s robotic lander Philae on comet 67P/Churyumov–Gerasimenko.

There’s another source of the comets: the Kuiper Belt, a circumstellar disc in the Solar System beyond the planets, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. In 2011, ESA’s Herschel Space Observatory revealed that Kuiper Belt comet 103P/Hartley 2 had a deuterium-to-hydrogen ratio “that matched terrestrial water’s perfectly”. So, it seemed the problem probably has been solved.

But it’s not: the European Space Agency’s Rosetta mission, launched on 2 March 2004, rendezvoused with 67P/Churyumov–Gerasimenko on 6 August 2014 and entered orbit on 10 September 2014. Rosetta’s lander, Philae, touched down on its surface on 12 November 2014, becoming the first spacecraft to land on a comet nucleus.

Philae didn’t live long, but it send a large amount of invaluable data. And we learnt that the amount of heavy water on 67P/Churyumov–Gerasimenko was even greater than the Halley comet: three times compared to the water on Earth. If the Kuiper Belt objects were the main source of the Earth’s water, even if most of them were like comet 103P/Hartley 2, deuterium-to-hydrogen ratio on Earth would be significantly higher than it is today.

The Asteroids

There is a third possible explanation: the asteroids. Today’s asteroids have very little water, but probably a few billion years ago they had more. Approximately 4.1 to 3.8 billion years ago, a very large number of asteroids apparently collided with the early terrestrial planets in the inner Solar System, including Mercury, Venus, Earth, and Mars. This event known as the Late Heavy Bombardment (abbreviated LHB and also known as the lunar cataclysm). At that time, the asteroids could have much more water than today. They lost most of their water because of the sun and the heat. Future analysis of the ice-rich asteroids can tell us if they are the main source of Earth’s water.

67P/Churyumov–Gerasimenko again

In the future, 67P/Churyumov–Gerasimenko will come closer to the Sun, and with the heat, it will get warmer and become more active. The ESA is planning to fly Rosetta through a jet of gas that the comet to see if the deuterium-to-hydrogen ratio seen from the water near the comet’s surface is the same as that from near its core.

Other possible sources

A small amount of water may have come from volcanism: water vapor in the atmosphere that originated in volcanic eruptions may have condensed to form rain, slowly filling the Earth’s oceanic basins.

Some terrestrial water may have had a biochemical origin, during the Great Oxygenation Event, via redox reactions and photosynthesis.

In the early 1930s, Cornelis Van Niel discovered that sulfide-dependent chemoautotrophic bacteria (purple sulfur bacteria) fix carbon and synthesize water as a byproduct of a photosynthetic pathway using hydrogen sulfide and carbon dioxide:

CO2 + 2H2S -> CH2O + H2O + 2S

Few modern organisms use this method of photosynthesis, making their water contribution negligible.

Sources

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