Comment by 8bitsrule
6 days ago
The major flaw in Panspermia is that it all had to start somewhere without Panspermia. If it did that there, why not here?
6 days ago
The major flaw in Panspermia is that it all had to start somewhere without Panspermia. If it did that there, why not here?
We know 2 things that are apparently incoherent:
1 - Abiogenesis is incredibly rare. We don't know how much exactly, but it's a lot.
2 - Abiogenesis happened on Earth about as soon as it became possible. Where "as soon as" means within half a billion years, but it's still way quicker than its rarity implies.
A lot of people think panspermia is what made those two happen. Life had about a full billion years to appear in meteors before they could appear here.
There are some problems, e.g. that each meteor only stayed chemically active for less than that half-a-billion years Earth had. Or that all the meteors that fell on Earth had only a fraction of the material that was later available here. But IMO, the largest issue is that just doubling the time is absolutely unsatisfying.
Life cannot appear in any of the small bodies that become meteors, because there is no source of energy for it.
Life can appear only on big planets or on big satellites, like the big satellites of Jupiter and Saturn, if they have a hot interior and volcanism.
Volcanism brings at the surface substances that are in chemical equilibrium at the high temperatures of the interior, but which are no longer at chemical equilibrium at the low temperatures of the surface, providing chemical energy that can be used to synthesize macromolecules.
Solar energy cannot be used for the appearance of life. Capturing light requires very complex structures that can be developed only after a very long evolution and which cannot form spontaneously in the absence of already existing living beings.
The only theory of panspermia that is somewhat plausible is that life could have appeared on Mars, which had habitable conditions earlier than Earth. Then, some impacts on Mars have ejected fragments that have fallen as meteorites on Earth and some remote ancestors of bacteria have survived this interplanetary trip.
There are many meteorites on Earth that have their origin in impacts from Mars, so at least this part is known as being possible.
>Solar energy cannot be used for the appearance of life. Capturing light requires very complex structures that can be developed only after a very long evolution and which cannot form spontaneously in the absence of already existing living beings.
I think that is putting the cart well before the horse. Earliest "life" I would say looks something like a short sequence of random RNA, in some structure (as in secondary), in some solution, among some nucleotides, where brownian motion lead to collision with nucleotides in the chain that grow the chain and/or template off the chain and make a copy. The energy requirements for this sort of pre cell life are far less than cell based life which has to spend energy on cell membrane or wall building. Energy could be quite low, it would just reduce the number of interactions over time. Likely also that this pre cell "life" would not die either so long as it is protected somewhat by cosmic radiation bombarding the chain (although to an extent this is also a ripe source of mutagenic potential).
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> no source of energy
Asteroids early in the solar system would be very radioactive due to short lived isotopes formed during the formation of the solar system. The existence of these isotopes is known from the patterns of decay products found in asteroids. The current guess is the early protoplanetary disk was bombarded by intense GeV-scale protons accelerated in the shock of a nearby supernova, causing large scale transmutation in the disk.
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> because there is no source of energy for it.
Couldn't it have started in the accretion disk?
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People make the inference that "early occurence of life" implies "life must be easy to start". But that inference requires the assumption that the chance of OoL (origin of life) remains mostly constant with time. An alternative would be that the conditions under which life could arise are transient, so life either starts early or not at all. We don't know enough about OoL to rule this out. Some chemicals that might be needed for OoL, like ammonia, are not stable for long. And if life originated in small asteroids, this might have only a few million years for it to occur while they are still warm enough from early short lived radioisotopes like Al-26.
It is indeed possible that in the early Solar System, before the short-lived radioactive elements completely decayed, there were some asteroids with good conditions for the appearance of life.
However, I would not describe those as small. The majority of the interplanetary bodies that orbit the Sun and which fall from time to time on Earth as meteorites are far too small to have ever had conditions for the appearance of life.
Even an asteroid like that which has wiped out the dinosaurs, with a diameter of a few km could not ever have suitable conditions.
Only relatively large asteroids, presumably with diameters from tens of km to hundreds of km, might have had warm interiors and volcanism for enough time to allow the appearance of life.
Such asteroids must also have been among those distant from the Sun, in order to contain enough water and volatile chemical elements.
The fact that the most volatile chemical elements are those most important for life is not due to chance, but due to the necessity. The volatile elements are those prone to forming covalent chemical bonds. Unlike the metallic or the ionic chemical bonds, the covalent bonds are strictly required for forming the complex molecular structures that may lead to living beings.
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One way to think about that is time required:
If earth is about 4 billion years old, but it takes say 400 trillion years for natural processes to produce this chemistry, then it happened out there not here.
This was a key reason why Hoyle preferred a steady state model of the universe — the part of the universe we inhabit needs to be very, very old for this stuff to work out, according to his thinking. A minority opinion, for sure, his rejection of the Big Bang model and timelines lost him a lot of respect among his peers. And his ideas could be wrong, I’m just pointing out that historically panspermia proponents have taken this position as to “why not here”.
One theory is that the early plasma shortly after big bang had the right conditions to set the life building block chemistry.
Imagine the entire universe contains those buildings block.
It had to start somewhere which is favourable to preserve the necessary molecules. Early Earth was not such place.