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Thread: Bringing Comets to Earth

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    Bringing Comets to Earth

    For over a thousand generations, before the dark times, before the Empire, there was, occasionally, this idea bandied about in science fiction that one of the things people would bring to Earth from other places in the solar system (or the galaxy) would be oil. The implication was that we'd still need fossil fuels for certain things. This was, of course, before global warming became part of public consciousness and we realized that burning the stuff probably wasn't a good idea. (We'll ignore the fact that any civilization capable of economically delivering large volumes of material from space would probably have no need for fossil fuels, not even to make plastics, no matter what Alan Dean Foster might have put in the novelization of Alien.)

    Fast forward to the beginning of the 21st Century when Futurama does their Crimes of the Hot episode which talks about using Halley's Comet (at least until it's all used up) to combat global warming. Now, the ol' random neurons got to firing in my brain, and I started to wonder what could be the unintended consequences of using comets to fight global warming. Assuming, of course, we could find a way to safely capture one and then gently lower it to one of the poles. (That way there's no messy business about them exploding in the atmosphere and flattening large parts of the landscape.) Obviously, there's the possibility of introducing alien microbes which might be contained in the comet, but what would happen with sea levels if we suddenly added three times the ice to them? (I'm assuming, of course, that we'd make sure that the comets were mostly water ice and not dry ice, or something else conducive to global warming.) How cold would the core of the comet be when it was lowered into the ocean (or stuck on one of the few remaining bits of the polar ice caps)? Would there be issues from the outside being just above freezing, while the core was dramatically colder? If we dropped it into water that was a few degrees above freezing would this create a problem? If so, what?

    I don't know, and I'm not entirely sure of what questions to ask, but I'm reasonably certain some of the folks here have some good ideas.

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    Sorry, how exactly does dropping a comet on Earth help with global warming?

    Surely you don't mean literally trying to cool it down with a cold comet?

    Global warming does not work this way.
    Last edited by DaveC426913; 2019-May-11 at 07:23 PM.

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    Quote Originally Posted by DaveC426913 View Post
    Sorry, how exactly does dropping a comet on Earth help with global warming?

    Surely you don't mean literally trying to cool it down with a cold comet?

    Global warming does not work this way.
    Obviously, it wouldn't get rid of the greenhouse gases in the atmosphere. It would, however, increase the albedo effect (depending on how many you dropped on Earth), and it could (depending upon the make up of the comet) provide sources of fresh water that don't involve us doing things like depleting the sources here on Earth. After all, one of the large issues that's going to happen (and to some extent is already happening) are rain patterns shifting, leading to increased droughts in parts of the world. Back in the 70s people had the idea of carving up the polar caps and sailing them to places that were experiencing droughts. Setting aside the practicality of it, this isn't a good idea. But a comet? Well, that's a little different, since you're not depleting the resources of one part of the planet to aid another part, you're adding to the whole.

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    My recollection is that it was just too expensive and impractical to move icebergs. Even desalination was cheaper. Extracting water from comets to safely land it on Earth would be orders of magnitude more expensive than moving icebergs.

    And sure, you could stop greenhouse gas production by crashing comets on earth and destroying civilization, but that seems a bit drastic to me.

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    Quote Originally Posted by Tuckerfan View Post
    Assuming, of course, we could find a way to safely capture one and then gently lower it to one of the poles. (That way there's no messy business about them exploding in the atmosphere and flattening large parts of the landscape.)
    I can't imagine how that would be possible. Comets aren't icebergs, and wouldn't survive intact in Earth gravity. Either you crash them into the Earth (assuming you can move them in the first place, which presents its own challenges) or leave them in space. Sure, you can bring back small samples for scientific purposes, but that's another matter.

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    Quote Originally Posted by Tuckerfan View Post
    Obviously, it wouldn't get rid of the greenhouse gases in the atmosphere. It would, however, increase the albedo effect (depending on how many you dropped on Earth),
    Well, until it melted of course. A very short-term solution.

    Quote Originally Posted by Tuckerfan View Post
    and it could (depending upon the make up of the comet) provide sources of fresh water that don't involve us doing things like depleting the sources here on Earth.
    I'm not sure we're actually running out of fresh water. I think we're running short of cheaply available fresh water.


    Quote Originally Posted by Tuckerfan View Post
    After all, one of the large issues that's going to happen (and to some extent is already happening) are rain patterns shifting, leading to increased droughts in parts of the world. Back in the 70s people had the idea of carving up the polar caps and sailing them to places that were experiencing droughts. Setting aside the practicality of it, this isn't a good idea. But a comet? Well, that's a little different, since you're not depleting the resources of one part of the planet to aid another part, you're adding to the whole.
    Right. Which is why a comet wouldn't solve the problem - you'd still have the problem of getting the fresh water to the locations of drought-ridden areas.

    And dropping the comet(s) in arbitrary locations where the fresh water might be needed could get problematic, as increased flooding might strt to change the local biosphere n unpredictable ways.

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    Something like a billion tons of water falls out of the sky each day, so we’re certainly not running out of fresh water. Plus, the amount of polar ice that’s melting is on the order of thousands of tons per second, so bringing down comets would not really make a dent.


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    Quote Originally Posted by DaveC426913 View Post
    And dropping the comet(s) in arbitrary locations where the fresh water might be needed could get problematic, as increased flooding might strt to change the local biosphere n unpredictable ways.
    Aside from the disastrous effects of even a small comet (think Tunguska) dropped on Earth, the composition is a bit of an issue as well - they can contain various hydrocarbons, carbon dioxide, carbon monoxide, hydrogen cyanide, etc. - lots of volatiles along with dust, a number of them greenhouse gases or toxic. It's a mistake to think of them as snowballs. Also, they appear to be quite fragile structurally, so accelerating them (in space) runs the risk of breaking them up, tidal effects near earth could do the same. Landing one intact is out of the question unless maybe we were to posit new physics (anti-gravity or something similar).

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    A comet coming in from Solar orbit picks up a lot of momentum as it slides further down the gravity well. All that energy has to be dealt with somehow. Bringing the comet into the Earth's atmosphere guarantees that the energy will be dispersed... as friction heat.

    Or impact shock.
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    Do comets really consist of "fresh" water? Seems that they would be on the salty side.

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    Quote Originally Posted by Spacedude View Post
    Do comets really consist of "fresh" water? Seems that they would be on the salty side.
    Salt in the Earth’s oceans results from acidic rainwater washing over the land and, eventually, producing chlorine and sodium ions. So what mechanism would create a salty comet?

    https://oceanservice.noaa.gov/facts/whysalty.html

    Salt in the ocean comes from rocks on land.

    The rain that falls on the land contains some dissolved carbon dioxide from the surrounding air. This causes the rainwater to be slightly acidic due to carbonic acid (which forms from carbon dioxide and water).

    As the rain erodes the rock, acids in the rainwater break down the rock. This process creates ions, or electrically charged atomic particles. These ions are carried away in runoff to streams and rivers and, ultimately, to the ocean. Many of the dissolved ions are used by organisms in the ocean and are removed from the water. Others are not used up and are left for long periods of time where their concentrations increase over time.

    Two of the most prevalant ions in seawater are chloride and sodium. Together, they make up over 90 percent of all dissolved ions in the ocean. Sodium and Chloride are 'salty.'

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    Other icy bodies in the solar system such as Europa, Enchiladas, Ceres, etc don't have rain but are considered to be "salty"

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    Quote Originally Posted by schlaugh View Post
    Salt in the Earth’s oceans results from acidic rainwater washing over the land and, eventually, producing chlorine and sodium ions. So what mechanism would create a salty comet?

    https://oceanservice.noaa.gov/facts/whysalty.html
    Comet volatiles never evaporated, and thus they never got fresh/rain water to begin with.
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    Well, hang on. How are we defining “salty”? As a sodium chloride molecule? Or more broadly such as dissolved minerals (hydrated magnesium) found on Ceres?

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    Quote Originally Posted by Noclevername View Post
    Comet volatiles never evaporated, and thus they never got fresh/rain water to begin with.
    Comet volatiles condensed from vapor to form the particles making up the comet. They've never existed for any amount of time in the form of liquid in contact with rocky materials to leach minerals from. They're going to contain a lot more than "fresh water", with ammonia, methane, carbon dioxide, etc, but they're not going to be "salty".

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    Quote Originally Posted by cjameshuff View Post
    Comet volatiles condensed from vapor to form the particles making up the comet. They've never existed for any amount of time in the form of liquid in contact with rocky materials to leach minerals from. They're going to contain a lot more than "fresh water", with ammonia, methane, carbon dioxide, etc, but they're not going to be "salty".
    Aren't they in contact with cosmic dust while condensing?
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    Quote Originally Posted by Noclevername View Post
    Aren't they in contact with cosmic dust while condensing?
    Do you consider rain and snow on Earth to be "salty"?

    Oceans, dry lakes, and saline seas have high salinity because liquid water has been draining into them for billions of years carrying soluble minerals and then evaporating away while leaving the minerals behind, with erosion, tectonics, and volcanism constantly refreshing the surface. Salts don't tend to get incorporated into ice in the first place, even when forming from saline water. Ice crystals nucleating on dust particles and films of ice forming on cold grains are not going to get "salty" by any reasonable definition of the term.

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    Quote Originally Posted by cjameshuff View Post
    Do you consider rain and snow on Earth to be "salty"?

    Oceans, dry lakes, and saline seas have high salinity because liquid water has been draining into them for billions of years carrying soluble minerals and then evaporating away while leaving the minerals behind, with erosion, tectonics, and volcanism constantly refreshing the surface. Salts don't tend to get incorporated into ice in the first place, even when forming from saline water. Ice crystals nucleating on dust particles and films of ice forming on cold grains are not going to get "salty" by any reasonable definition of the term.
    But rain water is only fresh because it has evaporated and re-condensed. How does volatile vapor in a dusty cloud stay pure?
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    Quote Originally Posted by Noclevername View Post
    But rain water is only fresh because it has evaporated and re-condensed. How does volatile vapor in a dusty cloud stay pure?
    Vapor doesn't dissolve salts.

    I'm honestly a bit baffled about what exactly is confusing about all this. Water doesn't start out salty and then get purified by evaporation. Saltwater on Earth is the product of many, many years of water leaching soluble minerals out of rock and concentrating it in basins where the water only leaves by evaporation. This process doesn't happen in volatile rich clouds condensing out beyond the frost line. To get saline water, you need to pile material up into something like Ceres or the larger gas giant moons, big enough to have internal environments where water is stable in liquid form for geological periods of time. Even then, the frozen portion is mainly going to have volatile impurities, not salts. It's not going to be pure any more than rainwater or snow or sea ice on Earth, but it's not going to be salty.

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    On Earth it does. Salt doesn't need geological periods of time to dissolve, though. Ices in contact with salty dust particles would still pick up salts; sun-crossing comets regularly get dosed with heat, so that ice gets warm enough for some molecular activity, at least.
    Last edited by Noclevername; 2019-May-12 at 06:24 PM.
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    Quote Originally Posted by Noclevername View Post
    On Earth it does. Salt doesn't need geological periods of time to dissolve, though. Ices in contact with salty dust particles would still pick up salts; sun-crossing comets regularly get dosed with heat, so that ice gets warm enough for some molecular activity, at least.
    We're talking about salts embedded in rock, not grains of pure soluble salts. It absolutely does need geological periods of time (good news for freshwater lakes), and liquid water. Diffusion processes would tend to separate water ice from any salts that did get embedded in it, for the same reason that the two separate as saltwater freezes: it's not energetically favorable for them to share a crystal structure.

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    Quote Originally Posted by cjameshuff View Post
    We're talking about salts embedded in rock, not grains of pure soluble salts. It absolutely does need geological periods of time (good news for freshwater lakes), and liquid water. Diffusion processes would tend to separate water ice from any salts that did get embedded in it, for the same reason that the two separate as saltwater freezes: it's not energetically favorable for them to share a crystal structure.
    OK.
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    Quote Originally Posted by cjameshuff View Post
    We're talking about salts embedded in rock, not grains of pure soluble salts.
    Salts on Earth have, as you said, been carried away from minerals for billions of years. But, under conditions of the early Solar System, would the minerals in space have formed that way? Would all salts originally exist only as part of rock particles?
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    Maybe comets that have been in cold storage for 4.6 billion years are still fresh in water/ice content, but wouldn't some leeching still occur over that amount of time? Would it really be a rock and ice mix or rather microscopic particles of both which came together as a solid? Comets that have been periodic could also be "cooked" enough to better blend things. It could be just a matter of a wide range of fresh to salty comets.
    Oh, and yes schlaugh, I was referring to salts in general not just NaCl

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    Quote Originally Posted by Noclevername View Post
    Salts on Earth have, as you said, been carried away from minerals for billions of years. But, under conditions of the early Solar System, would the minerals in space have formed that way? Would all salts originally exist only as part of rock particles?
    How else? Rocks are aggregates of mineral particles. Salts are minerals. They occur as part of rocks, practically by definition. There are very few salts that are still gaseous at the same temperature and pressure range as water, ammonia, methane, nitrogen, etc, and those are likely to react with something to form less-volatile compounds, or get dissolved in rocky material as it condenses and solidifies. What sort of water-soluble minerals would be concentrating and condensing out of the solar nebula in the same regions and conditions as water?

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    Quote Originally Posted by Spacedude View Post
    Maybe comets that have been in cold storage for 4.6 billion years are still fresh in water/ice content, but wouldn't some leeching still occur over that amount of time? Would it really be a rock and ice mix or rather microscopic particles of both which came together as a solid? Comets that have been periodic could also be "cooked" enough to better blend things. It could be just a matter of a wide range of fresh to salty comets.
    Oh, and yes schlaugh, I was referring to salts in general not just NaCl
    Again, diffusion would tend to do the opposite. Bringing salty ice near its freezing point will tend to help it separate into salt and ice crystals. And there's not that much in the way of salts in the first place. Is crushed rock salty? Again, oceans are the product of concentration...on Earth, tectonic action has been refreshing surface minerals and water has been transporting them into the oceans for billions of years, and now they contain about 35 g of minerals per 1 kg of liquid. For objects like gas giants and Ceres, there's been a long history of liquid water in contact with rocky particles settling toward the core, and most of the water has formed solid ice, concentrating the soluble salts in the remaining liquid portion. Evaporation in cryovolcanic activity would cause further concentration.

    Again, it's not going to be pure water. Melt a particularly rocky comet and you likely get a bag of wet muddy gravel and perhaps a bunch of methane, CO2, nitrogen, etc. Filter the water out and it'll probably be fairly mineral-rich, and possibly full of ammonia. But nobody is going to describe it as salty.

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    Quote Originally Posted by cjameshuff View Post
    How else? Rocks are aggregates of mineral particles. Salts are minerals. They occur as part of rocks, practically by definition.

    On Earth they do. On a planet where water has been washing over those rocks for billions of years, as you yourself note. Any external salts, if they ever existed, are long gone here.

    What sort of water-soluble minerals would be concentrating and condensing out of the solar nebula in the same regions and conditions as water?
    I don't know, maybe the ones that form in comets?
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    How much do we actually know about material separation in early Solar System small body formation?
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    In post 13, you said this, which makes it seem like you had great confidence in what you were saying.

    Quote Originally Posted by Noclevername View Post
    Comet volatiles never evaporated, and thus they never got fresh/rain water to begin with.
    But then you write this:

    Quote Originally Posted by Noclevername View Post
    How much do we actually know about material separation in early Solar System small body formation?
    I don't quite understand why you expressed yourself so confidently but then seem to say that you're not sure about it.

    But in any case, this is a bit of a digression. The whole question of whether comets are "salty" or not might be interesting as a debate, but I think we've already established that there are lots of other unwanted things, some toxic, in comets, so it wouldn't see a very good idea to bring them down. So I'm not sure how important the question of whether they're salty or not is...
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    Quote Originally Posted by Jens View Post
    In post 13, you said this, which makes it seem like you had great confidence in what you were saying.



    But then you write this:



    I don't quite understand why you expressed yourself so confidently but then seem to say that you're not sure about it.
    My first statement had no qualifiers, what do you mean by I wrote "confidently"? Serious question. I have communication issues.
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