Today, 21 percent of the air we breathe is made up of molecular oxygen. But this gas was not always in such ample, life-sustaining supply, and in fact was largely absent from the atmosphere for the first 2 billion years of Earth's history. When, then, did oxygen first accumulate in the atmosphere? MIT scientists now have an answer. In a paper appearing today in Science Advances, the team reports that the Earth's atmosphere experienced the first significant, irreversible influx of oxygen as early as 2.33 billion years ago. This period marks the start of the Great Oxygenation Event, which was followed by further increases later in Earth's history. The scientists have also determined that this initial rise in atmospheric oxygen, although small, took place within just 1 to 10 million years and set off a cascade of events that would ultimately lead to the advent of multicellular life. http://phys.org/news/2016-05-oxygen-earth-atmosphere.html Study: http://advances.sciencemag.org/content/2/5/e1600134
The dating of the Great Oxygenation Event to 2.3bn years ago, has been known for a while, though: https://en.wikipedia.org/wiki/Great_Oxygenation_Event What this research does is give a more accurate date of 2.33bn years ago and, more interestingly, an estimate of how it took for an atmosphere with free oxygen to get established, viz. a mere 10m years. So it seems to have been, geologically speaking, quite sudden. What is not clear from this however is what level of oxygen became established. The model described in Wiki suggests it rose to a few % and stayed at that level until about 0.85bn years ago, at which point it underwent a second major rise to something like today's level.
"create" Oxygen seems to be he wrong phrase Has the total oxygen content of the earth changed due to creation of oxygen on earth? What percentage or current amount do we get/have we gotten from extra terrestrial sources?
Who said "create"? If anyone did, then I think they implicitly meant creation of O₂(g). Though I think I may have read somewhere that Earth may have acquired some extra water due to the impact of comets.........
It now seems most likely that most if not all of earth's water has come from/ is coming from from asteroids.
Really? I find that very odd, considering the abundance of hydrogen and oxygen and the presence of so many hydrated forms of minerals in the Earth's crust, and mantle too, if I am not mistaken. I know I have read about a lot of water being exhaled by volcanoes, at any rate. Is there thought to have been some process in the early Earth's history that sent all the light molecules off into space, then? If so, where did all the nitrogen come from? And if not, given that nitrogen stayed around in the atmosphere, why could not water vapour have done so too?
"Really" ? Really, this is just a tad outside my main fields of interest----------however, we are of this earth, this shared co-evolutionary biome, and what effects our home effects us. So, the interests remains, but attestations as to veracity ain't quite mine to make. The idea/hypothesis/claim/theory------ Seems to hinge on the concept that it was too damned hot for water/hydrogen/oxygen/nitrogen/...etc to have remained on or in our core rock. Then came the late heavy bombardment and the wanderings/orbital meanderings of Jupiter ,which brought us water and nitrogen among other goodies. It seems most likely that we are still receiving extraterrestrial water and organic molecules from our friends and neighbors in the asteroid belt and ort cloud. Here are 3 links both pro and con: http://www.space.com/27969-earth-water-from-asteroids-not-comets.html http://www.sciencemag.org/news/2015/11/earth-may-have-kept-its-own-water-rather-getting-it-asteroids http://arstechnica.com/science/2015/01/where-did-earths-nitrogen-come-from/ .......... From my biases, logic, and perspective I am leaning toward the extraterrestrial models. ergo: "It now seems most likely"...(ceveat---what I do not know far exceeds what I do know)
This (the source of Earth's water) is an area of active research and each new publication tends to contradict the previous one, while - as is often the case with evolving hypotheses - there seem to be fashion trends as to which "theory" is favoured. Consequently I generally refrain from making comments that could easily be wrong three weeks before I make them. That said, here are a handful of observations: 1. Most of the water is thought to have come from asteroids, or comets, or comets and asteroids, or certain kinds of asteroids, or certain kinds of comets, or certain kinds of asteroids and comets. Interstellar ducks are not thought to be involved. 2. Determining which of these sources is "the one" will be made by ever more precise measurements of isotope ratios in the candidates. 3. There is a substantial mass of water locked up within minerals in the mantle. 4. The bulk of the water emitted by volcanoes is ocean water recycled via subducting plates.
Thanks to you both for these contributions, which I have now digested. Let me attempt a summary to see if I've now understood it properly:- It seems from these articles that the currently dominant hypothesis is that the inner part of the collapsing nebula that formed the solar system (i.e. the part from which the rocky planets coalesced) would have become too hot for light gases (nitrogen, water vapour, noble gases) to remain gravitationally bound to the Earth as it formed. In other words, the speed of the molecules of these gases would have been > escape velocity. I suppose in effect there was a sort of "distillation" process, whereby these gases accumulated in the colder parts of the solar system. So according to that hypothesis, the water and nitrogen would have had to come back to the Earth after it had cooled, from these colder regions, transported by comets and asteroids. But the second article (the one about isotope ratios in Baffin Island lavas) challenges this by suggesting there was bound water of some kind that withstood the heat. However that would not account for where the nitrogen came from, seeing as it is harder to envisage a way in which nitrogen could become chemically or physically bound to minerals. The hypothesised method of transport of the nitrogen intrigues me, actually. While it is easy to see how water could be carried on small bodies such as comets and asteroids in the form of ice, nitrogen would remain a gas if present as N2, which would not stay gravitationally bound to small objects. I suppose if it were present as ammonia, NH3, then that would form an ice.........
Another theory that's been posted here previously is that Earth formed in a manner similar to the outer planets. That is, it condensed out from a ball of hot gaseous material, mostly H2/He with some H2O, NH3, etc., forming a gas-ball giant. Then, over time, the H2/He eroded, leaving behind the rocky materials (which cooled/rained-out to form our liquid-rock earth) surrounded by an ocean of H2O and NH3, etc. We see this in exoplanets, in which large gas-ball giants are close to their suns. In our solar system, the ones farther out only partially eroded during our star's early high-UV/blue-star stage, leaving behind the rocky cores of the inner planets, with the rocky cores still surrounded by liquid H/He for the outer planets.
