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I disagree with this assessment of silicon based life. While it is true that Earth's crust is mostly silicon, it's also at a temperature that would make metabol

I disagree with this assessment of silicon based life. While it is true that Earth's crust is mostly silicon, it's also at a temperature that would make metabolic processes using silicon very difficult until you get down to the mantle. It could be that silicon based life is in fact constrained by temperature and can only arise in planets where the mean temperature is in the thousands. This could in fact mean that we do have a parallel silicon based biosphere underneath our feet. There is obviously no evidence for this whatsoever but then again, there is no reason to assume that the occasional leakage in the form of volcanic eruptions would leave any trace that we could use to deduce that these samples were once living entities.

hackeraccount 5y ago
Doesn't it intuitively seem like the life by definition would evolve out of any particular niche - say 1,000 Kilometers under the surface - to other areas? I'm at best doubtful about life on Mars because I find it hard to imagine some form of life that occupies a small part of Mars i.e. it's only in the parts we haven't seen. I tend to think if there's any life on Mars it'll find a way to deal with the conditions on all of Mars. By the same token if there's any silicon based life then it'll pop up in places that we can find as well as those we can't. Maybe it does best in places we can't but surely there would be some oddball silicon based creature that would beat the odds. The macro example would be the extinction of the South American marsupials when the North and South American land masses merged. All the placental animals in North America went south and all the marsupials in South America went extinct. Except for Opossums which for whatever weird reasons not only survived but went into North America.
- rendall 5y ago
  > Doesn't it intuitively seem like the life by definition would evolve out of any particular niche - say 1,000 Kilometers under the surface - to other areas? Other areas that are similar, sure. But if you're asserting that we can be confident there are no subterranean mantle-creatures because we don't see any wandering about out here, I don't think it's a good argument, for the same reason we can't go down there ourselves and look
  - rendall 5y ago
    If mantle-creatures were to exist, they would have a far, far greater volume of living space to expand into than we surface-dwellers do. Maybe they just haven't gotten around to us yet, for the same reason we don't have any large cities in the antarctic.
- vimacs2 5y ago
  No because if this was true, then we would see carbon based life do the exact same process in reverse. However, there is a limit to adaptability, particularly through the dumb and highly stochastic process of natural selection. We have no evidence of a void ecology in near earth orbit or of life persisting close to the Moho barrier - where the crust meets the mantle. There are hard limitations to the encroachment of life that only technology or radically different chemistry can circumvent. The same hard barriers might also mean that life on Mars is largely constrained to those few underground liquid water lakes that we have recently found evidence for. Keep in mind that even the most barren deserts on Earth have several orders of magnitude more water present in the air than on Mars.
- drdeca 5y ago
  What do you mean by “by definition” in that first sentence? Like, if it failed to do so, it would fail the definition of life? I don’t think anyone defines life that way, and I don’t think it would be a good definition (would I even count as alive under such a definition? It seems like no.)
  - hackeraccount 5y ago
    I think of life as dynamic. There are limits of course - it's taken billions of years for life to operate in the vacuum of space but it really does seem like part of the definition life should be that it spreads. I'm confident that you too move around.

hackeraccount 5y ago

Doesn't it intuitively seem like the life by definition would evolve out of any particular niche - say 1,000 Kilometers under the surface - to other areas? I'm at best doubtful about life on Mars because I find it hard to imagine some form of life that occupies a small part of Mars i.e. it's only in the parts we haven't seen. I tend to think if there's any life on Mars it'll find a way to deal with the conditions on all of Mars. By the same token if there's any silicon based life then it'll pop up in places that we can find as well as those we can't. Maybe it does best in places we can't but surely there would be some oddball silicon based creature that would beat the odds. The macro example would be the extinction of the South American marsupials when the North and South American land masses merged. All the placental animals in North America went south and all the marsupials in South America went extinct. Except for Opossums which for whatever weird reasons not only survived but went into North America.

rendall 5y ago
> Doesn't it intuitively seem like the life by definition would evolve out of any particular niche - say 1,000 Kilometers under the surface - to other areas? Other areas that are similar, sure. But if you're asserting that we can be confident there are no subterranean mantle-creatures because we don't see any wandering about out here, I don't think it's a good argument, for the same reason we can't go down there ourselves and look
- rendall 5y ago
  If mantle-creatures were to exist, they would have a far, far greater volume of living space to expand into than we surface-dwellers do. Maybe they just haven't gotten around to us yet, for the same reason we don't have any large cities in the antarctic.
vimacs2 5y ago
No because if this was true, then we would see carbon based life do the exact same process in reverse. However, there is a limit to adaptability, particularly through the dumb and highly stochastic process of natural selection. We have no evidence of a void ecology in near earth orbit or of life persisting close to the Moho barrier - where the crust meets the mantle. There are hard limitations to the encroachment of life that only technology or radically different chemistry can circumvent. The same hard barriers might also mean that life on Mars is largely constrained to those few underground liquid water lakes that we have recently found evidence for. Keep in mind that even the most barren deserts on Earth have several orders of magnitude more water present in the air than on Mars.
drdeca 5y ago
What do you mean by “by definition” in that first sentence? Like, if it failed to do so, it would fail the definition of life? I don’t think anyone defines life that way, and I don’t think it would be a good definition (would I even count as alive under such a definition? It seems like no.)
- hackeraccount 5y ago
  I think of life as dynamic. There are limits of course - it's taken billions of years for life to operate in the vacuum of space but it really does seem like part of the definition life should be that it spreads. I'm confident that you too move around.