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Rosetta Results: Comets "Did Not Bring Water To Earth"
An anonymous reader writes with findings from the Rosetta mission which suggests water on Earth probably came from asteroids, and not comets."Scientists have dealt a blow to the theory that most water on Earth came from comets. Results from Europe's Rosetta mission, which made history by landing on Comet 67P in November, shows the water on the icy mass is unlike that on our planet. The results are published in the journal Science. The authors conclude it is more likely that the water came from asteroids, but other scientists say more data is needed before comets can be ruled out."
Why does the water have to have come from comet/asteroid/whatever impacts? Maybe it just kind of seeped out of rocks or something. Hydrogen and oxygen are pretty common.
surely the Earth has hit by so many objects with varying D/H that the D/H of each object is not important to tell us the source of water, but rather tells us of the formation environment of the object itself. I don't see how anyone could claim that such D/H comets could not seed earth. i just see a larger D/H range of the seeders.
The press release should have said: The comet is dry as bone, Earth water must have come from somewhere else.
So far no water ice has been found and the pictures shows a completely dry hard rock. That they keep calling it an "Ice mountain" is just crazy. There is no proof that there is ice on a comet. Only that there is hydroxyl in the coma. Saying "We know of no other way for there to be hydroxyl in the coma without there being ice in the comet" is just bad science. Especially when there have been lab experiments showing how hydroxyl can be created from silica rock hit by protons (solar wind)
Deuterium/Hydrogen (D/H) isotope ratio is significantly higher (more than three times, in fact) than that of water found on Earth.
Q: How do you separate heavy water from light water?
A: Distillation. Light water boils off / evaporates more easily, because the molecules are lighter, and leaves the heavier water behind.
Why shouldn't this be true of vacuum sublimation as well?
Leave a chunk of dirty ice orbiting the sun in a hard vaccuum for a few million years, with the water quietly sublimating away. Seems to me the result would be that last remaining chunk of dirty ice would have a substantially larger fraction of heavy water molecules than the water on the planet where the deep gravity well hangs on to the lighter molecules.
Is it enough to explain a 3:1 enrichment? No clue. But I'd like to see that the analysis was done and what the scientists' estimates were.
(Not to say they ignored it. The last time I raised a similar question about a scientific paper reported here it turned out that the scientists HAD examined the issue.)
Bantam Dominique roosters crow a four-note song. Once you've heard it as "Happy BIRTHday" you can't NOT hear it that way
There seems to be a lot of confusion and conjecture in the comments about the grandiosity of the claim. This does not necessarily rule out all comets. Maybe an attempt at a better summary of the article would be helpful:
Background:
- Not all water is the same. Some water is heavier due to a presence of a certain amount of deuterium.
The general consensus is:
- When the solar system formed, the components for water were created.
- These components eventually formed with the early Earth and a water cycle was created.
- Yes, the early Earth was hot, but heat and elements were plentiful and Earth managed to hold onto some of these elements and would have had water evaporating and raining back down again.
- The planet Theia *collided* into the Earth. A certain amount of the debris coalesced into the moon. Imagine Pluto smashing into your house.
- The heat from the collision would have evaporated/released all elements lighter than X, which includes water. (ed: perhaps water on the moon is more closely related to early earth water coalesced and re-condensed?)
- Sometime later, the Earth received much more water than would have been sustained from such an impact.
- The weight (deuterium ppm) of this "new" water is different (much lighter) than the weight of "old" water, and generally any other water in the solar system.
So where did this "new" water come from?
This article suggests:
"We have light water in some comets and very heavy water in other comets. We have to assume the mixture of all these comets is something that is heavier than what we have on Earth, so this probably rules out Kuiper Belt comets as the source of terrestrial water."
And I believe this means:
It would have taken many of these Kuiper Belt comets to contribute a great deal of water to the Earth. If we use probe measurements to confirm other measurements and calculate the *average* weight of water on a number of Kuiper Belt comets (along the order of magnitude necessary be a main source of "new" water for the Earth), then we see that the amount of deuterium in Earth's water would have been much greater -- i.e. the water would contain an average weight of all impacts needed to saturate.
Thus this rules out Kuiper Belt comets being the main source of "new" water for Earth. Their water in general is simply too heavy on average. As soon as enough Kuiper Belt comets impact the Earth to come close to the amount of water needed, the calculations show that the level of deuterium would be much, much higher than what we see.
And the article itself turns to conjecture with:
So where do we look for lighter water? Maybe asteroids?