Where did all this water come from?

Water is the second most common molecule in the universe. It's all over the place. It's so common that it's basically impossible to avoid - wherever there is a concentration of matter, water will almost always be there in some amount. The moon has water ice. Mars used to be wet, and now still has water vapor and ice at the very least. Europa is a moon that's basically made entirely of water - and there are others like it right here in our solar system.The outer solar system is riddled with comets. Giant balls of dirt and ice in the Kuiper belt - and as we know, some of them are on orbits visible from Earth. Not too many years ago we witnessed, directly, comet Shoemaker-Levy become trapped by Jupiter's gravity well and fall into the gas giant. Imagine how many more comet impacts must have happened all over the solar system in the billions of years it's been around, compared to the scant few hundreds of years that we've been watching the sky with telescopes!Why would you ever be surprised that Earth has water?

Not all of it necessarily, but likely most of it. Notice that the inner planets (Mercury, Venus, Earth and Mars) are rocky, while the outer solar system mostly consists of gas giants. Outside of Neptune's orbit, we have the Kuiper belt, which is a massive cloud of dwarf planets like Pluto, and lots and lots of comets. Most never wind up entering the inner solar system - but all it takes is a massive object (another comet, a large asteroid, etc) to knock a comet or three or twenty into a new trajectory that may bring them into the inner solar system.Remember also that we're talking about billions of years, a length of time that's difficult for the human mind to really grasp. The solar system today looks much different from the solar system of a billion or two years ago - the massive bodies of Jupiter and the other gas giants do a great job of capturing any smaller stray objects. Those icy moons I spoke about were very likely Kuiper belt objects themselves, which became trapped by the gravitational pull of Jupiter or Saturn, etc, and happened to arrive in a stable orbit (this is the case with most of the moons of gas giants).Imagine, then, if a Kuiper object the size of Europa collided with Earth. It's more than that: "goddidit" is not a useful explanation. It doesn't actually explain anything; it proposes no mechanism whatsoever for how the water arrived, it simply assigns responsibility to the deity of your choice. That's not scientific. Science attempts to describe the mechanisms by which the universe works, as observable by our senses. Instead of saying "magic," scientists say "we don't know - yet" and proceed to try to find an actual, plausible mechanism supported by observable evidence to explain the unknown phenomenon.In this case, Kuiper belt objects are observably real, we have evidence that comets do impact planets, and evidence suggests that impacts were far more common in the past (geologically and atmospherically inert bodies like the the moon show us how common impacts used to be, since their craters never disappear - and even Earth bears the signs of some very large impacts despite the fact that our geological and atmospheric activity mean that any impacts we can see on Earth today are relatively young). It's a plausible mechanism supported by evidence that actually offers an explanation rather than an assignment of credit. That makes it a far more reasonable solution than "goddidit." Take a look at the Wiki entry on Europa:

quote:

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it is predicted that the outer crust of solid ice is approximately 10—30 km (6—19 mi) thick, including a ductile "warm ice" layer, which could mean that the liquid ocean underneath may be about 100 km (60 mi) deep.[25] This leads to a volume of Europa's oceans of 3 1018 m3, slightly more than two times the volume of Earth's oceans.

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Compared to this moon, we don't have much water at all. "A lot of comets" is a relative judgment - "a lot" compared to what? "Massive" compared to what? Remember, Earth itself isn't all that large compared to most of the planets in the solar system, and it's absolutely tiny compared to most of the planets we've discovered in other solar systems. Our human sense of scale becomes meaningless when we talk about astronomy. This is why "it seems to me" and other subjective personal feelings are rather meaningless in science - this case is a perfect example of something that normal human experience (and thus "common sense" reactions) simply doesn't cover. If a single object with the entire mass of the oceans crashed into Earth, yes - the kinetic energy would be equivalent to (insert obscenely large number of nuclear warheads here). The same amount of energy would be released with multiple, smaller objects as well - it would just be spread out over the multiple impacts.But the Earth has suffered such massive collisions before. The moon is thought to have been the result of a massive body impacting Earth and fragmenting the planet; gravitational accretion eventually resulted in a much changed Earth with the moon orbiting it. There was a time when the entire Earth was a molten ball of slag. Accretion and collision is how the planet formed in the first place, after all. Rather than saying "that's how it happened," I'd rather say "that's the explanation that best fits the evidence we have currently." Remember, science doesn't tend to make claims of absolutes. Rather, we use a sliding scale of sorts where our degree of certainty in the accuracy of a given theory increases as additional evidence supports it; theories supported by lots of evidence with very accurately verified predictions are counted as extremely accurate (ie, General Relativity, the Theory of Evolution, etc), but absolute proof is the realm of mathematics.Also, I'd lose the specifics regarding the size of the objects colliding with Earth. Yes, the current model states that Earth's water is the result of watery objects impacting the planet, but we can't say exactly how large or how many - any evidence of those sorts of specifics have long since been destroyed by ongoing geological processes. We know that there was plenty of time over the formation of the solar system for many, many impacts to have occurred, and we know that icy bodies are pretty common. That's really all we need to know.

I think you're misunderstanding taz. he's not dismissing your question - he's providing an answer. It's the same answer I gave, simply much less verbose.Water on Earth arrived the same way the rest of the matter on Earth arrived: the process of collision and accretion of matter left over in a stellar nebula after the star forms. The matter coalesces towards areas of increased density, and slowly those objects in stable orbits "clean up" everything else with their gravitational fields. Anything in an unstable orbit either impacts with another body (including the Sun itself) or flung out of the solar system.It really is the same process.

I never claimed you were either of those things. Another (former?) WoW player, eh? My girlfriend has a resto shaman. I have a holy paladin for my healing character. I'm still not following. At no point did I say you were an idiot, or a retard. I haven't made any comments regarding your intellectual abilities at all. If I've come across as condescending, I apologize. There are very few "idiots" here. I haven't seen any reason to count you as one. It simply looked like you were dismissing Taz's response as a sarcastic rebuke, but in actuality there was a real answer contained in his post. If I was mistaken, again I apologize.