4 ms·

You also need plate tectonics. And exactly the right level of stellar radiation for the carbonate-silicate cycle to work, if you want photosynthesis, and withou

You also need plate tectonics. And exactly the right level of stellar radiation for the carbonate-silicate cycle to work, if you want photosynthesis, and without that you likely won't have anywhere near enough energy available for complex life. Get any of it wrong and life either won't develop, or will wreck its own biosphere in an unrecoverable way very early on and get wiped out (if there was ever life on Mars, that's my bet for why traces of it are elusive—it wasn't around very long, wasn't very complex, and it was a very long time ago).

lordnacho 4y ago
Can you elaborate? What's the reason it wouldn't work if all the plates were frozen?
- corrral 4y ago
  Maybe not strictly necessary for any life, but the broad principal is that any process that recycles material is going to be really helpful to life, with big bonus points for it working in a dynamic fashion so it tends to self-stabilize—plate tectonics don't have that quality, but other processes do, like any chemical process where the concentration affects the rate at which the process occurs, or how our climate tends to find stable points rather than having runaway freezing or runaway heating. In the Earth's case, vulcanism is critical to balancing atmospheric CO2 levels. Over time (not on human timescales) carbon tends to get trapped in ways that make it unavailable to life, so you need something to undo that, and that's a major mechanism for it. Life may be (probably is) possible without some elements of what makes life work on Earth, but with the wrong environment may destroy itself relatively quickly (as life on Earth nearly did, during the oxygenation crisis) or fail to capture enough energy for the development of complex life (think: a planet where the only energy available to life comes from a few undersea vents, photosynthesis being impossible for any of several reasons). For this reason, a planet being in the "Goldilocks zone" around its host star is just the bare minimum required for widespread, complex life. The planet itself must also be more-or-less the right size, and must have the right composition of surface and atmosphere and various processes occurring (geological and otherwise) to keep those in balance, and so on. It's possible there are certain very hardy chemistries possible for life that we simply don't see on Earth because they're easily outcompeted by what we've already got, but those pretty much by definition won't provide as much energy to the host body's biosphere, so complex life is less likely. It's also possible there are planets far more amenable to life than Earth (though perhaps wholly incompatible with Earth-like life), with unknown-to-us chemistries that are only viable in that kind of even-better environment.
  - saiya-jin 4y ago
    The list of prerequisites is much longer. Right amount of radiation, rotating molten metallic core (that probably came from collision with another planet!), jupiter-like vacuum cleaner to not have massive asteroid impacts too often - but sometimes they may help, like for us. Stable, boring star. The list is probably much longer, ie chemistry part mixed with geology, astronomy and other topics.
  - lumost 4y ago
    A good way to understand this is through an ocean world thought experiment. If a hypothetical planet existed which was covered in a 100km deep ocean, all nutrients would steadily fall to the bottom of the ocean - where there would be minimal to no energy to make use of the nutrients. You'd eventually be left with a surface layer containing only the minimal amounts of nutrients which diffusion pulls back up to the surface. In order for this hypothetical water world to host a thriving ecosystem, you would need a large moon or heavy vulcanism to ensure that the oceans remain mixed.
- bbojan 4y ago
  Plate tectonics helps with carbon recycling and regulating the temperature of the planet. Temperature drops, rainfall drops (less evaporation), rock weathering slows down (rock weathering normally removes CO2 from the atmosphere). With plate tectonics, volcanism continues to spew CO2 in the atmosphere, temperature eventually rises. Negative feedback. No plate tectonics, no volcanism, albedo increases, temperature drops further. Positive feedback.

lordnacho 4y ago

Can you elaborate? What's the reason it wouldn't work if all the plates were frozen?

corrral 4y ago
Maybe not strictly necessary for any life, but the broad principal is that any process that recycles material is going to be really helpful to life, with big bonus points for it working in a dynamic fashion so it tends to self-stabilize—plate tectonics don't have that quality, but other processes do, like any chemical process where the concentration affects the rate at which the process occurs, or how our climate tends to find stable points rather than having runaway freezing or runaway heating. In the Earth's case, vulcanism is critical to balancing atmospheric CO2 levels. Over time (not on human timescales) carbon tends to get trapped in ways that make it unavailable to life, so you need something to undo that, and that's a major mechanism for it. Life may be (probably is) possible without some elements of what makes life work on Earth, but with the wrong environment may destroy itself relatively quickly (as life on Earth nearly did, during the oxygenation crisis) or fail to capture enough energy for the development of complex life (think: a planet where the only energy available to life comes from a few undersea vents, photosynthesis being impossible for any of several reasons). For this reason, a planet being in the "Goldilocks zone" around its host star is just the bare minimum required for widespread, complex life. The planet itself must also be more-or-less the right size, and must have the right composition of surface and atmosphere and various processes occurring (geological and otherwise) to keep those in balance, and so on. It's possible there are certain very hardy chemistries possible for life that we simply don't see on Earth because they're easily outcompeted by what we've already got, but those pretty much by definition won't provide as much energy to the host body's biosphere, so complex life is less likely. It's also possible there are planets far more amenable to life than Earth (though perhaps wholly incompatible with Earth-like life), with unknown-to-us chemistries that are only viable in that kind of even-better environment.
- saiya-jin 4y ago
  The list of prerequisites is much longer. Right amount of radiation, rotating molten metallic core (that probably came from collision with another planet!), jupiter-like vacuum cleaner to not have massive asteroid impacts too often - but sometimes they may help, like for us. Stable, boring star. The list is probably much longer, ie chemistry part mixed with geology, astronomy and other topics.
- lumost 4y ago
  A good way to understand this is through an ocean world thought experiment. If a hypothetical planet existed which was covered in a 100km deep ocean, all nutrients would steadily fall to the bottom of the ocean - where there would be minimal to no energy to make use of the nutrients. You'd eventually be left with a surface layer containing only the minimal amounts of nutrients which diffusion pulls back up to the surface. In order for this hypothetical water world to host a thriving ecosystem, you would need a large moon or heavy vulcanism to ensure that the oceans remain mixed.
bbojan 4y ago
Plate tectonics helps with carbon recycling and regulating the temperature of the planet. Temperature drops, rainfall drops (less evaporation), rock weathering slows down (rock weathering normally removes CO2 from the atmosphere). With plate tectonics, volcanism continues to spew CO2 in the atmosphere, temperature eventually rises. Negative feedback. No plate tectonics, no volcanism, albedo increases, temperature drops further. Positive feedback.