- 1709 - Water from Comets, or , is there another explanation? New data is coming from the Rosetta mission to Comet 67P and the Curiosity Rover mission on Mars. What are the conclusions as to where Earth got its water?
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----------------- 1709 - Water from Comets, or , is there another explanation?
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- Our Solar System is 4,600,000,000 years old. Our planet is the 3rd rock from the Sun. Mercury, Venus, Earth were all too hot in the early evolution of the planets for them to retain liquid water. If the planets had surface H2O it would have all boiled off before the planets could have cooled down.
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- So, 75% of Earth’s surface is covered in water - ice. Where did this water come from?
One theory is that it came from all the asteroids and icy comets that were colliding with the Earth during its early evolution.
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- Evidence of all these collisions can be seen looking at the pock marked surface of our Moon. The Moon has no atmosphere so it would have lost all of its water if it did arrive this way. But, the Earth’s gravity was strong enough to retain an atmosphere. A dense atmosphere could hold the water, recycle it to the surface, and keep it from escaping into space.
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- So, did our water come from icy comets?
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- The Rosetta spacecraft has been visiting Comet 67P since August 6, 2014. One goal in its mission was to sample water if it were there and compare the H2O “fingerprint” with the H2O on Earth. The key is the amount of Deuterium in the water. Deuterium is a form of hydrogen with an additional neutron in the nucleus next to the proton. It is an isotope of normal hydrogen ,”H”, which is an atom with just one proton and one electron.
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- The ratio of Deuterium to Hydrogen is created unique to its time and place in the evolution of the Solar System. That is the “fingerprint”. Common ratios mean the water first formed in a common time and place.
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- The Comet 67P is now orbiting between Earth and Mars. At its furthest elongation of orbit it reaches just beyond Jupiter. The Comet 67P takes 6.5 years to complete one orbit around the Sun.
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- The Comet 67P has a Deuterium / Hydrogen ration that is 3 times greater than that in Earth’s oceans. Another Comet Hartley2 was measured on a different space mission and found to have an even greater D/H ratio. These result make it unlikely that these comets brought their water to Earth.
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- Astronomers have studied meteorites that hit the surface of Earth coming from Asteroids that normally orbit between Mars and Jupiter. These meteorites have mush lower water content, but, the D/H ratio is a close matching that that found in Earth’s oceans. Asteroids are dry, but if there were a large enough number of them striking Earth’s surface they still may be what brought water to Earth.
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- But, could there be another explanation?
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- Maybe the Earth had sequestered a large water supply deep in its interior. Plate tectonics could have slowly released the water to the surface after the surface had cooled down. The oceans may have been fed from below?
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- New rock and curst comes to the surface at the mid-ocean ridges. Could it be delivering water at the same time? We live on the only planet having liquid water on its surface. We also live on the only planet with active plate tectonics.
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- Rocks that appear dry can actually contain water. Hydrogen atoms can become trapped inside natural voids and crystal defects. Of course, oxygen is plentiful in the surrounding minerals. There are certain chemical reactions that free the hydrogen to bond with oxygen to become H2O, water. The subsurface mantle where this could happen is 80% of Earth’s total volume.
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- Science has tried to recreate these same chemical reactions. The elements are put under high pressure using a diamond anvil and high temperatures using a laser to simulate the conditions deep below the Earth’s surface.
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- The mineral bridgmanite which is a form of olivine. The mineral ringwoodite another form of olivine. The mineral garnet have all been tested to simulate conditions 500 miles deep.
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- The conclusions from the experiments were that Earth’s garnet may hold half as much water in its depths as is currently on the surface. This equates to about the volume of water that is in the Pacific Ocean.
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- Another avenue of study is the water on Mars. Is water still there and how did it get there? Ancient Mars maintained a climate that could have produced long-lasting lakes and rivers.
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- NASA’s Curiosity Rover has uncovered evenly layered rocks that are the typical pattern of lake-floor sedimentary deposits. This would suggest repeated filling and evaporation of Martian lakes. This cycle occurred over and over again.
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- The Rover also crossed over a boundary dominated by rivers to one dominated by lakes in its journey across the surface. The water is gone now. But, is it still deep underground?
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- Methane has been detected seeping out of the ground. Could this gas come form a deep underwater reservoir that is still chemically active?
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- How Earth and Mars got their water is a mystery yet to be solved. Without water there is not life as we know it. With water who knows what else we will find. Stay tuned, there is much more to learn.
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