I was watching a BBC programme about comets last night, which left me with a couple of questions:
The programme seemed to suggest that comets/meteors crashing into Earth in its early days supplied the water so essential to life. Is this generally accepted as likely? I know there is a related theory that life itself was brought by comets, but I’ve never heard of the water itself arriving in this fashion. Sounds like a hellish number of impacts to get that much water…
Studying tree rings preserved in peat bogs, researchers had found a stunting in growth of trees across Europe and Asia around 500 AD, which they inferred to be evidence of an impact, and the resulting dust cloud, hampering trees’ growth. Is there anything about this in historical writings? Could this event have stunted human development during the Dark Ages? Or is it another ill-documented bunch of cobblers?
There is substantial evidence from tree ring analysis all around the world for some major Event around that time. Given the early Dark Age date I think about the ‘dying land’ at the tragic climax of the Arthurian legends, when the sun was weak in the sky and crops blighted.
Given the millions of years of earth evolution it is quite conceivable that all the water came from comets.
Despite being an English graduate, I’m ignorant of Arthurian legend, so thank you. Quite literally a dark age by the sounds of it.
I suppose so - these incomprehensively large stretches of time can explain a great deal.
Both quite feasible theories then - I think the crumby CGI reconstructions and overblown action music made me think the programme more flimsy than it actually was…
But the Event wasn’t necessarily a comet impact. I’ve also seen a theory that a large eruption of Krakatoa (or some other major volcano) might have thrown large amounts of dust into the atmosphere, with similar results.
The ‘experts’ on that programme suggested that because there was no evidence of sulphur (I think) being ejected into the atmosphere, volcano-style, that a comet impact was more likely…
If comets, particularly asteroid-belt rather than way-out-there comets, filled the oceans, why isn’t there more water on Mars? Did it all evaporate into space?
I’m fuzzy on the details of the lesser (compared w/Earth) Martian atmosphere and how much effect that has on (not) holding in water.
From what I gather, Mars has no magnetic field to speak of, so the atmosphere has been mostly stripped away by the solar wind. If Mars had a magnetosphere, then that would help deflect the solar wind and an atmosphere could persist. There’s a lot of erosion on Mars that almost certainly was caused by a lot of water, long ago.
Mars is also much smaller than Earth, so has less gravity to hang onto molecules of water vapor. The molecules are going fast enough (the speed being determined by the temperature) that a significant proportion of them get to escape velocity. You can do a calculation (I did it in a planetary science class I took) that shows that Mars has trouble hanging onto water vapor in the long term because its gravity is too low and it’s too warm (if it were colder, the molecules would be moving more slowly).
The same calculation can be done to explain why Venus, despite being almost as big as Earth, doesn’t have much water vapor either- it’s just too hot.
I’ve also seen an assertion that, because of Earth’s higher gravity, a comet or asteroid that crosses the orbits of both Earth and Mars is more likely to hit Earth than Mars. I’m not sure if the shielding effect of the Moon (the Moon intercepts some comets that would have hit Earth if the Moon weren’t there) cancels that out, though.
The Earth’s oceans consist of approximately 320,000,000 cubic miles of water. That’s a mass of 1.35×10^18 metric tons. We got pelted by a lot of dirty snowballs.
Not to drag this on unnecessarily, but would it be fair to say that comet water added to the total global reservoir, but that a proportion of it simply condensed with the rest of Earth’s ingredients when the solar system was formed? Wikipedia’s article on Earth suggests that “Condensing water vapor, augmented by ice delivered by comets, produced the oceans.”. Or would Earth in its infancy have been too hot to hold onto water, while the outer frosty comets could keep it stored as ice?
Hanging onto ice when you’re not too much above absolute zero is considerably easier than hanging onto water when you, well, are.
Also, I believe that the consensus is not that the comet theory is the most likely, but that it is as plausible as any other.*
*-IANAA. Consult with doctor before using. Do not take if you are pregnant, about to become pregnant, or a comet. Void where prohibited. All rights reserved. Copyright MCCCXXXVII. Hi, Opal!
There is also a theory that Earth formed via “wet accretion”- that the water existed in and on grains that later came together to make the planet. (Pressure can be much higher between grains than in the general environment, allowing water to exist where it may not seem likely.) The cometary source of water vs. asteroidal vs. wet accretion is often assessed from the point of view of the D/H ratio of the water- that is, the amount of water containing the heavy isotope of hydrogen (deuterium) vs. the amount containing the normal isotope. Comets, meteorites, and the Earth have distinctly different water D/H ratios. By modeling the effect of processes which alter the D/H ratio, an estimate can be made of the original Earth D/H. Currently those calculations point towards water that formed around the feeder zone of the Earth (the ring of debris around the sun from which the Earth was made.)
That’s not to say that water couldn’t have been delivered via cometary impact- there were all kinds of impacts during the early Solar System history (the Late Heavy Bombardment), but there is not really a final consensus at this point as to where the bulk of the water on Earth came from.
Since this is GQ, I’ll try to go dig up a cite- but for now, I’m off to school!
They don’t, when they’re as near the Sun as Mars is- they lose water and other stuff, which we see as a coma and tail.
Mars spends all its time at about the same distance from the Sun, since its orbit is pretty close to circular, so its temperature stays pretty much constant (with local seasonal variations). Many comets don’t- they spend a lot of their time much further from the Sun. A famous example is Halley’s Comet (more properly Comet Halley). It is in a 76-year orbit. It spends a few months of that time close enough to the Sun to have a coma and tail, but spends most of it far from the Sun as just a lump of ice.
Comets that come near the Sun often enough, or that get into orbits that keep them near the Sun, do eventually lose their water (and other ices). There are some burned-out comets like that around in the Solar System, but nobody but astronomers really take notice of them- you can’t see them with the naked eye, since they don’t form a significant coma or tail.
