- 3377 - EARTH - where did the water come from? One of the most difficult questions about Earth regards the origins of its water. Where did it come from? The early Earth was too hot to hold water. It would have all evaporated.
--------------------- 3377 - EARTH - where did the water come from?
- One widely-held theory gives comets the honor of bringing water to Earth. Another one says that Earth’s water came when a proto-planet crashed into early Earth, not only delivering a vast quantity of water, but creating the Moon.
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- Now a new study shows that the minor planet Vesta got its water from space dust. Could that help explain the origin of Earth’s water? This research shows that you can transport small micrometeoroids that contain ice to dry bodies that formed without water.”
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- Most of the matter in our Solar System came from other long-dead stars. When our Sun and Solar System formed, that material was vaporized, then recondensed into new material. But some tiny portion escaped that rebirth scenario. Some of those tiny grains were never vaporized, making them older than our Solar System itself.
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- By identifying and examining that material, scientists can learn some things. This study looked at the minor planet (or asteroid) Vesta (4 Vesta). Some of those ancient grains of stardust made it to Vesta, in the form of micro-meteroids, and some of those brought water to the rocky body. How do scientists know that?
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- Like our Earth Vesta has a core, a mantle and crust. These features formed as the materials that make up Vesta melted, differentiated and coalesced into a single planet-like object. The same thing happened with Earth. Like Vesta, Earth was also pummeled by micrometeoroids.
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- If icy micro-meteoroids delivered water to the inner solar system when the Earth was still forming, this could be one way that the Earth ended up with enough water to support life.
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- These things are all linked by an event from 1942. In that year, the Kapoeta meteorite came to Earth at Sudan. It was World War II then, and luckily, the meteorite landed on the road in front of a British Army convoy.
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- The Kapoeta meteorite is a “Howardite” meteorite. There are about 200 of them, and they’re named after Edward Howard. Howard was primarily a chemist. He invented a method of refining sugar. But he was also very interested in meteorites and their compositions.
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- All of the Howardite meteorites came from Vesta, though we didn’t know that until NASA’s Dawn spacecraft arrived at the minor planet in 2011. That visit revealed the link between Vesta and the Howardite meteorites, as well as two other families of meteorites.
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- Researchers studying the Kapoeta meteorite found small dark inclusions that looked out of place. They look completely different from surrounding material. The tiny inclusions were micro-meteorites, smaller than the thickness of a human hair.
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- “NanoSIMS” stands for “Nanoscale Secondary Ion Mass Spectrometry“. NanoSIMS uses an ion gun to create a beam of ions. That beam is aimed at a sample, in this case the sample of the Kapoeta meteorite.
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- The ion beam creates atomic collisions, and some of those collisions produce secondary ion particles. Those secondary ions are then fed through a mass spectrometer, which identifies the ions. As of 2018, there were just over 40 of these machines in the world.
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- The spectrometry centered around 14 tiny samples from the Kapoeta meteorite, the tiny inclusions are called “clasts“. They were all from one edge of a thin section of the sample. When they analyzed it, they found that 2 of the 14 contained magnetite “embayments“, in between the host meteorite and the carbonaceous clasts.
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- An embayment is a shaped void, and their presence indicates “aqueous alteration” occurring on Vesta as a result of melting the ice embedded in the C-clasts.
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- The NanoSIM spectrometry revealed a different ratio of isotopes than expected in the clasts, and showed that some of what they were seeing was “pre-Solar material“, material that existed long before our Solar System did. They found two pre-Solar silicate grains, and four pre-Solar silicon-carbide grains.
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- These micrometeorite grains arrived at Vesta, or at the pieces of rock that became Vesta, some time after the Late Heavy Bombardment. The team found minerals and textures that were linked to interactions between rock and water from melted ice lead them to the question of Earth’s water, and its source.
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- We need a mechanism to bring ice from the outer solar system, where the temperature is low. The inner Solar System was too hot for water to form, so it had to be transported from cooler regions in the outer Solar System. This research shows that you can transport small micrometeoroids that contain ice to dry bodies that formed without water.
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- The problem with studying ancient Earth is the planet’s level of activity. Erosion and plate tectonics have erased ancient evidence. So the links between Vesta, Kapoete, and the ions may help explain how Earth got its water.
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- If icy micrometeoroids delivered water to the inner solar system when the Earth was still forming, this could be one way that the Earth ended up with enough water to support life.
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- And if it happened in our Solar System, it could happen in others. Habitable planets around other stars may have acquired their water through similar means.
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- Comets may have brought some of it. Asteroids may have brought some. Icy planetesimals may have played a role by crashing into the young Earth and depositing their water. Hydrogen from inside the Earth may have contributed, too. Another hypothesis states the collision that formed the Moon gave Earth its water.
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- There’s evidence to back up all of these hypotheses. But new research suggests that the Sun and its Solar Wind may have helped delivered some water, too.
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- Earth’s oceans contain about 1.37 × 10^21 kg of water by mass. (That’s one sextillion three hundred seventy quintillion kilograms.) Lakes, rivers, ice, groundwater, and water vapour amount to another 5.0 × 10^20 kg, which is five hundred quintillion kilograms.
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- All that water covers about 70% of the Earth’s surface. A few moons in our Solar System have sub-surface oceans, but among planets, Earth is unique. And without all that water, of course, we wouldn’t be here and there’d be no life. So the source of that water is an enduring mystery in science.
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- Earth formed out of the proto-planetary disk, the material swirling around the Sun after it became a star. It’s the same material that all the other planets and moons formed from. Earth formed close to the Sun, where temperatures were relatively higher, so Earth’s water didn’t form at the same time the planet did.
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- 90% of Earth’s mass is iron, oxygen, silicon, sulphur, and magnesium. But water has a much lower condensation temperature than those materials so any water available at the time would’ve been vapor.
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- Further from the Sun, water would’ve condensed into icy asteroids, planetesimals, and comets. That’s why the idea of an extra-planetary source for Earth’s water endures.
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- One line of evidence shows that after the planets formed, Jupiter migrated inward toward the Sun, sending icy asteroids from the outer Solar System toward the inner Solar System where some of them crashed into Earth. After a long period of time, enough water arrived by asteroids and comets to account for Earth’s water.
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- There’s a problem with the asteroid water-delivery theory, though. The idea is centered on “C-type asteroids” like 2004 EW95 which contain hydrated minerals. Research has shown a distinct difference between the isotopic composition of water on C-type asteroids and Earth’s water. There must be a missing source of water that is isotopically different than C-type asteroids.
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- This theory that water was carried to Earth in the final stages of its formation on C-type asteroids has previous testing of the isotopic ‘fingerprint’ of these asteroids didn’t match with the water found on Earth meaning there was at least one other unaccounted for source.
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- These results are based on samples from asteroid Itokawa. The Hayabusa mission returned samples from near-Earth asteroid Itokawa to Earth in 2010. Those samples showed that Itokawa contained abundant water. That discovery fortified the idea that Earth got its water from asteroids.
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- Itokawa is an S-type asteroid, which means it formed much further from the Sun than Earth did, out in the cold reaches of the Solar System where water would freeze rather than vaporize.
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- The researchers examined the tiny samples from Itokawa with “Atom Probe Tomography“. What they found is that H+ (Hydron, or hydrogen ions) in the solar wind irradiates silicate minerals on the surface of the asteroid and creates water molecules.
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- This new solar wind theory is based on meticulous atom-by-atom analysis of miniscule fragments of an S-type near-Earth asteroid known as Itokawa, samples of which were collected by the Japanese space probe Hayabusa and returned to Earth in 2010.
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- Taking an incredibly detailed look inside the first 50 nanometers of the surface of Itokawa dust grains, contained enough water that, if scaled up, would amount to about 20 liters for every cubic meter of rock.
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- The atom probe tomography directly observed an average enrichment in water and hydroxyls in the solar-wind-irradiated rim of an olivine grain from the S-type asteroid Itokawa. We also experimentally confirm that H+ irradiation of silicate mineral surfaces produces water molecules.
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- These results suggest that the Itokawa regolith could contain solar-wind-derived water and that such water reservoirs are probably ubiquitous on airless worlds throughout our Galaxy.
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- This discovery helps explain how Earth got all its water. If the researchers are correct, we have a much more complete picture of the early Earth and how it came to be life-sustaining.
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- The production of this isotopically light water reservoir by solar wind implantation into fine-grained silicates may have been a particularly important process in the early Solar System, potentially providing a means to recreate Earth’s current water isotope ratios.
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- The discovery may mean something for future astronauts, too. As humanity continues to explore space, the presence or lack of water will enable and constrain our space-faring activities. But if this process of creating water by the solar wind is widespread, it means that asteroid and lunar regolith contains water, there for the taking.
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- The same space weathering process which created water on Itokawa likely occurred on other airless planets, meaning astronauts may be able to process fresh supplies of water straight from the dust on a planet’s surface, such as the Moon.
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- The Man in the Moon is smiling.
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December 18, 2021 EARTH - where did the water come from? 3375
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