How the Earth Got its Water William H. Waller Ph.D. Waller William H. Ph.D. 6 5 2015

 Water has been identified in the most uncanny of places – as vapors in the nebulae that roam our Milky Way Galaxy, as ices in the protoplanetary disks that surround many protostars, and as liquids below the icy crusts of the Jovian moons Europa, Ganymede, and Callisto. In 2005, the Cassini spacecraft imaged geysers of liquid water erupting from the surface of Saturn’s moon Enceladus. The liquid form of water is especially important to biotic processes, as it provides an essential solvent for making the sundry hookups and energy transfers that are necessary to life.

AstrobiologyAtmospherecometsEarthmilky wayOxygen
<p>Despite compelling evidence for water throughout our Milky Way Galaxy and within our local Solar System, the specific origin of water on Earth remains controversial. We know that the Earth formed some 4.6 billion years ago, as the primordial Solar System gravitationally congealed from the pre-planetary disk of debris that surrounded the proto-Sun. Once the Sun “turned on” its thermonuclear fires, things began to change in a big way.</p> <p/> <p> <fig> <caption> <p> <italic>The primitive Solar System was a busy chaotic place. Did Earth’s cargo of precious water come home-grown 4.6 billion years ago or was it delivered via comets during the Late Heavy Bombardment that ended 3.9 billion years ago? (Caltech-JPL/NASA)</italic> </p> </caption> <media xmlns:xlink="" xlink:href=""><alt-text>waller1</alt-text> <long-desc/><uri xlink:href=""/><permissions><copyright-statement/> <copyright-holder/><license license-type="creative-commons"><license-p> </license-p></license></permissions></media> </fig> </p> <p/> <p>Some scientists contend that the inner Solar System was too hot for any water to remain in the rocky bits that ultimately came together to form the Earth. They look to wayward comets and asteroids that formed farther from the scalding Sun as the key provisioners of Earth’s oceans. However, others find important discrepancies in the relative amounts of regular water and “heavy” (deuterated) water in their comparisons of the Earth, comets, and meteorites. The few comets that have been chemically probed appear to have far more deuterated water than Earth’s oceans – indicating (to these scientists at least) that comets could not have delivered the bulk of Earth’s water.</p> <p/> <p>Meteorites are interplanetary rocks of various kinds that have fallen to Earth. They are thought to represent pieces of much larger asteroids that formed between the orbits of Mars and Jupiter. Some meteorites are found to be rich in water. Others are bone dry. Again, chemical and isotopic analyses seem to rule out asteroidal meteorites delivering most of Earth’s water, as they find that the early Earth was likely the beneficiary of mostly dry meteorites. The only way to wiggle around this situation is to have a single unusually wet and very large (Moon-size) body to hit Earth shortly after its birth. Something like the Jovian moon Europa could have done the trick.</p> <p/> <p>Perhaps surprisingly, several scientists have gone back to square one – working out scenarios which retain the water in the inner Solar System despite the intense solar heating and intense bombardment by other rocky bodies. Here, the water was in the form of a warm vapor that stuck to tiny grains of rock which then aggregated to build up pebbles – and ultimately – a wet Earth. Once the Earth’s crust began to solidify, it would have belched out huge quantities of water during a period of rampant volcanic eruptions. As the saturated atmosphere cooled, it began to rain – and rain – and rain, filling the Earth’s basins with the water that we inherit to this day.</p> <p/> <p>Recent support for this revived scenario of home-grown water has come from analyses of ancient zircons on Earth and, perhaps surprisingly, in lunar soil and rocks that were gathered by astronauts during the Apollo 15 and 17 missions to the Moon. The zircons found in Australia represent the oldest unaltered mineral crystallizations to be found on Earth. Radioactive dating</p> <p>of the uranium that forms part of the zircon crystals indicate ages of 4.4 billion years for the oldest zircons – corresponding to the time when the cooling Earth had just solidified from a molten state. The balance of oxygen isotopes within these minerals suggests a watery origin. Some scientists have used the zircon data to imagine a primordial Earth that was neither scalding nor blanketed by an arid stifling atmosphere of carbon dioxide. Instead, they suggest a rather clement world, whose tectonic activity (also indicated by the zircon properties) would have sequestered the carbon dioxide into carbonate rocks – leaving behind a moist temperate atmosphere and perhaps oceans that would have been hospitable to life.</p> <p/> <p>On the Moon, the Apollo astronauts collected hundreds of pounds of lunar soil (regolith) and rocks. Subsequent analyses of the fine-grained regolith have revealed microscopic glass spherules that have incorporated small but significant amounts of water. These spherules are thought to have been created during a primordial period of intense lunar volcanism. The so-called “fire fountaining” produced mists of molten lava which then cooled into solid spheroidal droplets. Falling back down to the Moon’s surface, the tiny glass beads were collected intact by the Apollo astronauts approximately 3.7 billion years after their formation. Similar amounts of water (of order 1 part per 10,000) have been found inside lunar rocks that contain magnesium-rich olivine. These rocks are also thought to have originated deep inside the Moon in the form of magma which then rose toward the surface and crystallized.</p> <p/> <p> <inline-graphic xmlns:xlink="" xlink:href=""/> </p> <p/> <p> <italic>Tiny green glass beads have been found in samples of lunar regolith that were collected during the Apollo 15 and 17 missions to the Moon. These spherules contain water as part of their mineral composition. The water appears to be identical in its isotopic properties to the water found on Earth, thus suggesting identical origins. (NASA)</italic> </p> <p/> <p>Analyses of the lunar glass beads and magnesium-rich rocks have yielded ratios of deuterated vs. regular water that resemble those found on Earth and in carbonaceous chondrites (a type of meteorite that contains primitive chondrules – the first major crystallizations in the inner Solar System). These sorts of isotopic measurements are tricky, as the so-called D/H ratio can change as the sample undergoes weathering from cosmic rays, solar wind implantation, and degassing of the magma. Only after compensating for such effects did the investigators find the strong resemblance among the waters of the Earth, Moon, and carbonaceous chondrites – and a significant disparity with respect to the D/H ratios that have been measured in a handful of comets.</p> <p/> <p>If these results hold up, Earth’s water was likely archived in place, when the inner planets and carbonaceous asteroids condensed from the proto-planetary disk some 4.6 billion years ago. That the Earth and Moon share similar waters indicate that the archiving took place before Earth experienced the big impact that led to the formation of the Moon 4.5 billion years ago. The early caching of water within the Earth, Moon, and carbonaceous chondrites would also suggest that water is ubiquitous throughout the inner Solar System. Moreover, water is likely to be present in the inner rocky parts of other planetary systems. And where there is water co-mingling with rocky surfaces, there is a much greater chance for life to emerge.</p> <p/> <p>Further research on Earth’s watery past will likely benefit from future robotic probes and landers, whereby the isotopic composition of water across the Solar System can be assayed. That will help to pin down the status of oceanic water on Earth relative to the larger context of planetary, asteroidal, and cometary sources. Meanwhile, sub-millimeter spectroscopic observations of icy objects in the Kuiper Belt may help to determine the isotopic composition of these pristine Solar System objects beyond the orbit of Neptune. Closer to home, deep-sea probes will help to determine the isotopic composition of water emerging from the Earth’s upper mantle. So, the next time you drink a glass of water, think about those trillion-trillions of H2O molecules and their long strange journey to your lips. You may be tasting the first rains that ever fell on Earth.</p> <p/> <p>References:</p> <p>Chang, Kenneth, 2008, "A New Picture of the Early Earth". The New York Times, 1, December 2008.</p> <p/> <p>Rebecca Jacobson, 2013, “Moon and Earth May Share a Water Past,” PBS NewsHour, 15, May 2013,</p> <p/> <p>Alberto E. Saal et al. 2013, “Hydrogen Isotopes in Lunar Volcanic Glasses and Melt Inclusions Reveal a Carbonaceous Chondrite Heritage,” Science, Vol. 340, no. 6138, pp. 1317-1320,</p> <p/> <p>Kevin Stacy, 2013, “Moon and Earth have common water source,” Brown University press release, 9 May 2013,</p> <p/> <p/> <p/> <p> <fig> <caption> <p> <italic>William H. Waller earned his Ph.D. in astronomy at the University of Massachusetts. He has since carried out diverse programs in scientific research and education at the University of Washington, NASA’s Goddard Space Flight Center, Tufts University, and the Museum of Science – Boston. He is author of The Milky Way -- An Insider's Guide (see and co-editor of The Galactic Inquirer – an e-journal and forum on the topics of galactic and extragalactic astronomy, cosmochemistry, astrobiology, and the prospects for interstellar communication (see Bill currently teaches physical sciences at his hometown high school in Rockport, MA, and serves as a Zooniverse Teacher Ambassador (see He can be reached at</italic> </p> </caption> <media xmlns:xlink="" xlink:href=""><alt-text>waller3</alt-text> <long-desc/><uri xlink:href=""/><permissions><copyright-statement/> <copyright-holder/><license license-type="creative-commons"><license-p> </license-p></license></permissions></media> </fig> </p> <p/> <p/> <p> <inline-graphic xmlns:xlink="" xlink:href=""/> </p> <p/> <p/> <p/> <p/> <p/> <p/> <p/> </sec> </body> <response response-type="reply"> <front-stub><contrib-group> <contrib> </contrib> </contrib-group> <pub-date pub-type="epub"> <day>11</day> <month>11</month> <year>2016</year> </pub-date> </front-stub> <body> <p>And here is a video profile of Steve Jacobsen (now at Northwestern University) who has found the deep mantle to be more hydrated than previously expected. </p> </body> </response> <response response-type="reply"> <front-stub><contrib-group> <contrib> </contrib> </contrib-group> <pub-date pub-type="epub"> <day>11</day> <month>11</month> <year>2016</year> </pub-date> </front-stub> <body> <p>Deep reservoirs of water have been found below volcanoes and elsewhere beneath the Earth's crust. These wet pockets in Earth's mantle further suggest that Earth cached much of its water as part of its formation rather than as part of later bombardments by comets and asteroids. </p> </body> </response> <response response-type="reply"> <front-stub><contrib-group> <contrib> </contrib> </contrib-group> <pub-date pub-type="epub"> <day>7</day> <month>10</month> <year>2016</year> </pub-date> </front-stub> <body> <p>Here is a fine talk by Elizabeth Tasker on the exciting Japanese Hayubasa mission to an asteroid which will include a sample return. By assaying the water and organic molecular content of this primordial object, scientists hope to understand the origins of water and life on Earth. </p> </body> </response> <response response-type="reply"> <front-stub><contrib-group> <contrib> </contrib> </contrib-group> <pub-date pub-type="epub"> <day>4</day> <month>11</month> <year>2015</year> </pub-date> </front-stub> <body> <p>The Earths past and how it obtained its water is very interesting, our waters history is still a mystery and is a mystery that when figured out will help the understanding of other planets in our universe. This journey to understanding where we came from also helps with where we are going. our missions of exploration to places in our solar system and beyond are fueled by this mystery of where we came from and new and exciting places to go. Water is around out solar system but in many forms, although non of these icy moons or flying commits contain life it is a signal of hope for researchers in search for life that water is out there and its only a matter of time until a planet with all the right conditions is found. We may never find the exact circumstances of which our water came to us but finding how water travels around the universe is something that is important to find. When the mystery if water can be solved, our understanding of the universe will be expanded and an understanding of our past will be unveiled. </p> </body> </response> <response response-type="reply"> <front-stub><contrib-group> <contrib> </contrib> </contrib-group> <pub-date pub-type="epub"> <day>2</day> <month>9</month> <year>2015</year> </pub-date> </front-stub> <body> <p>I feel that this is a very easily understood article that broaches a topic fairly by viewing all sides. The language used throughout the piece is not too dense or obtuse, making it possible for students or individuals who lack extreme knowledge of physics to understand. Overall though I think that the most appealing part about this article is that it discusses a topic by systematically combing through each separate argument over the origins of our planet's water. In general it was a very well-written, elegant piece that was not so lofty as to fly straight over one's head. </p> </body> </response> <response response-type="reply"> <front-stub><contrib-group> <contrib> </contrib> </contrib-group> <pub-date pub-type="epub"> <day>28</day> <month>6</month> <year>2015</year> </pub-date> </front-stub> <body> <p>I'm increasingly optimistic that scientists will close in on the origins of Earth's waters within the next decade. They will do this by constraining the respective roles of cometary impacts, asteroid/meteorite impacts, solar wind chemistry on Earth's surface, high-pressure chemistry in Earth's mantel, and primeval hydrated rock that was archived during Earth's formation. Meanwhile, here is a wonderful science meets science fiction video celebrating the Rosetta mission that "captured" a comet. The "making of" video is equally compelling. </p> </body> </response> <response response-type="reply"> <front-stub><contrib-group> <contrib> </contrib> </contrib-group> <pub-date pub-type="epub"> <day>6</day> <month>5</month> <year>2015</year> </pub-date> </front-stub> <body> <p>Recent analyses of in-situ measurements by the Rosetta mission to Comet 67P/Churyumov–Gerasimenko have once-again shown a significant discrepancy between the isotopic composition of the comet's water with respect to Earth's waters (see and This time, the comet has about 3 times more deuterated water than found in terrestrial water. If these findings characterize the comets near Jupiter's orbit, then they pretty much rule out impacting comets as provisioners of Earth's water. What's left are impacting asteroids and asteroid fragments (meteorites) having filled Earth's reservoirs along with whatever water was retained from Earth's initial formation. Either way, scientists now appear to agree that Earth's water was archived almost from the very beginning of Earth's history -- well before the late heavy bombardment. </p> </body> </response> </article> <!-- Dynamic page generated in 0.155 seconds. --> <!-- Cached page generated by WP-Super-Cache on 2017-03-28 06:19:53 --> <!-- super cache -->