Panspermia, anyone?

[McCoy]

Re: Panspermia, anyone?

 

Disagree' date=' the universe is about 99.a google of 9's per cent "somewhere else," so I would hardly call that even odds.[/quote']

 

A better response to this' date=' actually: yes, it is. Which is [i']exactly[/i] why it's so vanishingly unlikely that life from anywhere else would happen to end up here, instead of in the other 99.99~% of the universe.

UNLESS it is the nature of life to spread. Then whether abiogenesis occured only once or every other Thursday, life will eventually occupy all nitches it possibly can. There will be a phase change from a lifeless universe to one teeming with life. Has it happened yet? Unknown. But human action has sent microbes to the Moon, Mars, Titan and into interstellar space. Will any of them find a place where they can be be fruitful and multiply? Unlikely, not impossible.

[McCoy]

Re: Panspermia, anyone?

 

oops' date=' more later, time to go lecture on exactly this subject! [/quote']

Eagerly looking forward to the "more later."

 

Just what was the temperature of deep space 6 Gybp? Warmer than the current 2.7 K I'm sure, but I don't remember how much.

[Kristopher]

Re: Panspermia, anyone?

 

And surviving a million years inside a tiny pocket of water inside a salt block several meters accross buried underground differs from surviving a million years inside a tiny pocket of water inside a salt block several meters accross drifting through deep space exactly how?

 

We know a lifeform from Earth survived exposuer to deep space virtually unprotected, we know some Earth lifeforms survived being cut off from any contact with the outside enviroment for a very, very long time. I think we have established the possibility that a natural "suspended animation chamber" could transport life-as-we-know-it across at least interplanetary distances.

 

It differs in that in space, especially beyond the heliopause, it's exposed to millions of years worth of radiation. The meteorites that we've found on Earth that appear to be Martian in origin took millions of years to happen to land on Earth, IIRC, and Mars is right next door, not out in the Kuiper Belt or in some other far-off star system.

 

Millions of years worth of particle radiation, slowly chipping away at the inactive chemistry of tiny organisms, chemistry that can't repair itself (if it could otherwise) because it's frozen solid -- after even a few years in space in that rock, there's no way for the water to still be liquid. So you're asking for a lifeform that can survive being cooked, frozen, irradiated for millions of years, flash-reheated, and lithobreaking, and being introduced into a new environment.

 

And so far, we've found 34 rocks that are probably from right next door, and only one of them shows something that might have been a kind of life we've never seen, or could just be nonbiological crystals -- and in either case, there was definately nothing alive in that rock when it left Mars, let alone when it finally got here.

 

Again' date=' I do not insist on the warm nebula hypothesis, just point out that your dismissing it as impossible requiresassumptions not based on the currently available data. You may be right, you are probably right, but to state as you do as fact that it is impossible requires more knowledge of the nature of life than any Earth-human possesses at this time.[/quote']

 

My position doesn't require any assumptions, just the knowledge we already have. Your position requires assuming that things exist that we've never observed.

 

So many assumptions!

 

No, again, only basing my position on what we've already observed and determined.

 

We've seen life on Earth adapt to temperatures of 761° F (678° K) and pressures of 283 atmospheres. (Or contrarywise' date=' if black smokers were the site of abiogenesis, as one current hypothesis suggest, it has adapted from 761° F and 283 atmospheres down to 32 ° F (273° K) and 1 atmosphere.)[/quote']

 

You'll note that neither of those conditions are like interplanetary or interstellar space, and that the high temps go along with the high pressures, which offset -- have we found life adapted to 678° K @ 1 atmosphere? Not that I know of, because of the little problem of the boiling point of water, which goes up as the pressure goes up (which I think you know).

 

Are current Earth-surface conditions closer to deep sea or deep space? Is 273° K closer to 678° K or 3° K? (Keep in mind also that the "temperature" of the cosmic microwave background radiation is a function of the age of the universe' date=' which was some half to two thirds as old as it is now when the Sun and Earth formed. Someone ask Cancer what the temperature of deep space was 6 billion ybp, I've misplaced the formula.)[/quote']

 

Based on the latest estimate, the universe would have been about 9 billion years old when the earth was forming into a ball of molten metal and rock about 4.5 billion years ago.

 

Anyway, you're focusing on simple temperature scale as opposed to what those temperatures actually mean for life that depends on liquid water, and ignoring the effects of pressure, atmosphere, presence of liquid water, protection from radiation, etc.

 

For example, life on Earth probably didn't leave the water until long after there was ozone to protect it from UV radiation.

 

Conditions on Earth now, and billions of years ago, are more alike than either is like the conditions in interplanetary or interstellar space. One example - the magnetic field and the presence of atmosphere and water and rock combine to screen out a great deal of the EM and particle radiation that's going to slow-cook even the heartiest organism travelling for millions of years in space.

 

Yes, some organisms survived on the moon for a few years. A few years, shielded to some degree by the mass of the moon and the Earth, shielded to some degree by the Earth's magnetic field (and I know it doesn't extend to the moon, but it is something in the way of some of the incoming radiation). That's a far stretch from the millions or even billions of years with cosmic rays zipping through its molecules day after day after day...

 

 

Unless you can show us something we've never seen, unless you can show us something new and different, there's nothing to support the idea of panspermia, and it's just an unnecessary complication to the real question. That's not an assumption, that's not dismissing the improbable as the impossible, it's just based on what we've observed up until this moment.

[Cancer]

Re: Panspermia, anyone?

 

Just what was the temperature of deep space 6 Gybp? Warmer than the current 2.7 K I'm sure' date=' but I don't remember how much.[/quote']

 

Not importantly different. The temperature evolution is strongly concentrated toward early times. I would very surprised if it was as high as a factor of ten higher at 6 Gya than now, and even that would just bring it up to about 30 K ... which, though at least it's not liquid helium temperatures, is still far too cold for relevant chemistry.

 

The real problem is that we have only one instance of life, and that instance is very far removed from its origin. Trying to generalize from a non-chosen sample of one is just about pointless.

 

Relevant analog from history: Twelve years ago, immediately before the first real extrasolar planets were discovered, no one had predicted the existence of "hot Jupiters", jovian-mass planets within a quarter of an AU of the central star. Because of the selection bias inherent in the radial velocity method, those planets make a majority of the ~250 systems we now know about. We think we understand how such things are made now, but it's a low-level nagging concern that there are so few systems known that look sort of like ours (within our ability to detect things): the jovian planets out at 3.5 AU or beyond, with nearly circular orbits. (Presently we cannot detect terrestrial planets orbiting other stars.)

 

With that kind of context, I don't think one can rule out panspermia-esque hypotheses (though that still leaves the initial abiotic origin question), though a number of the reasons why those were originally put forward some decades ago are clearly invalid. Yes, it seems improbable, but there's some things that seem to be required in the initial abiotic origin scenario that seem improbable too. But arguments about improbability are very unreliable when it comes to a discussion of unique events, and it's probably better simply to leave things like that in the bin of untestable ideas, perhaps with a casual preference ranking, but no more energy or importance attached than that.

 

Most astrophysicists are not believers in panspermia, but I think most are like me, unwilling to rule it out, either, purely because of the paucity of knowledge.

[McCoy]

Re: Panspermia, anyone?

 

Millions of years worth of particle radiation' date=' [/quote']

*deep sigh* And several meters of salt provide no proection from radiation because?

 

Don't know, and am too lazy to look it up, how opaque sodium cloride is to UV or gamma, but I do know it will stop a lot of charged and neutral particles.

 

My position doesn't require any assumptions' date=' just the knowledge we already have. [i']Your[/i] position requires assuming that things exist that we've never observed.

No, your position requires lots of assumptions not justified by existing data. My position requires not calling something impossible because we have yet to observe it.

 

You use the word impossible a lot, like five days ago when you knew it was impossible for there to be lquid water on Titan. You may be right in some, or even most, cases, but the current data does not justify that level of confidence.

 

Based on the latest estimate' date=' the universe would have been about 9 billion years old when the earth was forming into a ball of molten metal and rock about 4.5 billion years ago. [/quote']

Think you're underestimating the current consensus of both the age of the universe and of Earth.

 

Cancer?

[McCoy]

Re: Panspermia, anyone?

 

Yes' date=' it seems improbable, but there's some things that seem to be required in the initial abiotic origin scenario that seem improbable too. [/quote']

Thank you. Rep you when I can.

 

Neither hypothesis ruled out by current available data.

 

End of discussion.

[Kristopher]

Re: Panspermia, anyone?

 

End of discussion.

 

[Kristopher]

Re: Panspermia, anyone?

 

*deep sigh* And several meters of salt provide no proection from radiation because?

 

Don't know, and am too lazy to look it up, how opaque sodium cloride is to UV or gamma, but I do know it will stop a lot of charged and neutral particles.

 

If several meters of a material will stop 99.9% of the cosmic rays from penetrating, a lethal does still gets through on the time scale of millions or billions of years.

 

And really, where is a chunk of salt that large going to come from, with enough velocity to get from there to here and not just be caught up in whatever gravity well it started out in? And it's just going to happen to have life in it? Really?

 

No' date=' your position requires lots of assumptions not justified by existing data. My position requires not calling something impossible because we have yet to observe it.[/quote']

 

If the probability of life originating in a particular spot is 1/X, then the probability of life originating in a particular spot and then getting to another world somewhere across interstellar space is 1/X * 1/A * 1/B * 1/C * 1/D * 1/E. At that point, it just strikes me as pointless to look for the origin of life out there where there's no sign of it and the odds are so much lower just based on simple geometry and physics.

 

You use the word impossible a lot' date=' like five days ago when you knew it was impossible for there to be lquid water on Titan. You may be right in some, or even most, cases, but the current data does not justify that level of confidence.[/quote']

 

There is a difference between "last I checked, based on what I was able to find and read, it would be impossible", and "it's impossible".

 

Think you're underestimating the current consensus of both the age of the universe and of Earth.

 

Looking around, I'm finding figures on various sources of just over 13.5 billion years for the universe, and around 4.5 billion (or a bit more) for Earth. 13.5 - 4.5 = 9, doesn't it?

[Kristopher]

Re: Panspermia, anyone?

 

UNLESS it is the nature of life to spread. Then whether abiogenesis occured only once or every other Thursday' date=' life will eventually occupy all nitches it possibly can. There will be a phase change from a lifeless universe to one teeming with life. Has it happened yet? Unknown. But human action has sent microbes to the Moon, Mars, Titan and into interstellar space. Will any of them find a place where they can be be fruitful and multiply? Unlikely, not impossible.[/quote']

 

I was under the impression that the Mars probes, at least, were quite thoroughly cleaned and sealed to prevent exactly that.

 

Anyway, I'm not sure how it can be "the nature of life to spread" on an interplanetary level. Really, at that point, the statement gets into the "panspermia is quasireligion" thing that I've run into more than once. The spread of life between planets, let alone between solar systems, would rely so heavily on a series of unlikely accidents, that it's an accident. It can't be "the nature of life" to rely on multiple "happy accidents" to do something over and over and over again. The phrase "seven consecutive miracles" comes to mind.

[Clonus]

Re: Panspermia, anyone?

 

I was under the impression that the Mars probes, at least, were quite thoroughly cleaned and sealed to prevent exactly that.

 

Anyway, I'm not sure how it can be "the nature of life to spread" on an interplanetary level. Really, at that point, the statement gets into the "panspermia is quasireligion" thing that I've run into more than once. The spread of life between planets, let alone between solar systems, would rely so heavily on a series of unlikely accidents, that it's an accident. It can't be "the nature of life" to rely on multiple "happy accidents" to do something over and over and over again. The phrase "seven consecutive miracles" comes to mind.

 

Of course if your campaign actually includes species who existed billions of years ago and were technologically capable of interstellar flight, then there's no conceptual problem with panspermia except that an explanation for where they went could be an issue. It can be handled using the glowy ball of light approach or the lovecraftian "monsters ate 'em" version.

[McCoy]

Re: Panspermia, anyone?

 

Really think Cancer said everything that needs to be said. Just to tie up a few loose ends:

 

And really' date=' where is a chunk of [i']salt[/i] that large going to come from, with enough velocity to get from there to here and not just be caught up in whatever gravity well it started out in?

What are the odds that something would hit Earth hard enough to knock off a Moon-sized chunk?

 

Looking around' date=' I'm finding figures on various sources of just over 13.5 billion years for the universe, and around 4.5 billion (or a bit more) for Earth. 13.5 - 4.5 = 9, doesn't it?[/quote']

OK, let's go with your figure, Earth formed 4.5 Gybp. That means life appeared even earlier than using my figure of 6 Gybp.

 

I was under the impression that the Mars probes' date=' at least, were quite thoroughly cleaned and sealed to prevent exactly that. [/quote']

Some of the Ranger Lunar probes were sterilized, some failed, and the sterilization process was thought to be at fault. All American probes since then have been built using the same Clean Room technology as the Surveyor. The air in there is cleaner and more germ free than any operating room, but nothing built by humans is perfect. Statistically probable approching certainty that some few bacteria have hitchhiked on all the American probes.

 

Do not know the conditions under which the Russian probes or Huygens were assembled,

 

Anyway' date=' I'm not sure how it can be "the nature of life to spread" on an interplanetary level. Really, at that point, the statement gets into the "panspermia is quasireligion" thing that I've run into more than once. The spread of life between planets, let alone between solar systems, would rely so heavily on a series of unlikely accidents, that it's an accident. It can't be "the nature of life" to rely on multiple "happy accidents" to do something over and over and over again. The phrase "seven consecutive miracles" comes to mind.[/quote']

Again, I do not insist on the hypothesis.

 

But think about it. What are the odds that pondscum could evolve to the point that it could throw other pondscum to other planets?

 

Our species will either live and die on this one planet, or spread life to other planets. Logically no third alternative. If we, or our evolutionary sucessors, leave this solar system, we will be spreading life star system to star system.

 

So even if we are the only technological species in the universe (seems unlikely, not impossible), unless we do something spectacularly stupid as a species (far more likely, unfortunately), life will spread beyond this planet.

 

Contrarywise, have we found any enviroment where life could exist but doesn't? Again, look at how much effort we have to go through to keep it out of the Clean Rooms.

[McCoy]

Re: Panspermia, anyone?

 

Some of the Ranger Lunar probes were sterilized' date=' some failed, and the sterilization process was thought to be at fault. All American probes since then have been built using the same Clean Room technology as the [i']Surveyor[/i]. The air in there is cleaner and more germ free than any operating room, but nothing built by humans is perfect. Statistically probable approching certainty that some few bacteria have hitchhiked on all the American probes.

Should also mention that any Earth bacteria arriving on Mars may have found the ozone and peroxides in the Martian atmosphere more deadly than deep space. That oxygen is dangerous stuff!

[Kristopher]

Re: Panspermia, anyone?

 

What are the odds that something would hit Earth hard enough to knock off a Moon-sized chunk?

 

The object had to be about the right size, travelling at about the right relative velocity, and impacting at about the right angle -- and so the odds aren't very good. Still better, however, than odds of certain other things.

 

OK' date=' let's go with your figure, Earth formed 4.5 Gybp. That means life appeared even earlier than using my figure of 6 Gybp.[/quote']

 

4.5 to 4.6 Gybp has been the figure I've read in books and online and seen on documentaries -- never 6 Gybp.

 

http://en.wikipedia.org/wiki/Age_of_the_Earth

http://en.wikipedia.org/wiki/Age_of_the_universe

 

The shorter the timespan between the origin of the Earth and the origin of life, the less likely panspermia as an origin of life on Earth becomes -- because the time window for the random arrival of a random life-bearing interstellar object on this minescule random target becomes gets smaller.

 

Again, I do not insist on the hypothesis.

 

But think about it. What are the odds that pondscum could evolve to the point that it could throw other pondscum to other planets?

 

Our species will either live and die on this one planet, or spread life to other planets. Logically no third alternative. If we, or our evolutionary sucessors, leave this solar system, we will be spreading life star system to star system.

 

So even if we are the only technological species in the universe (seems unlikely, not impossible), unless we do something spectacularly stupid as a species (far more likely, unfortunately), life will spread beyond this planet.

 

Contrarywise, have we found any enviroment where life could exist but doesn't? Again, look at how much effort we have to go through to keep it out of the Clean Rooms.

 

None of which makes it the nature of life to spread from planet to planet, from star to star.

[Vestnik]

Re: Panspermia, anyone?

 

I hereby postulate a sort of interstellar Wilt Chamberlain, travelling from world to world with the collected hits of Barry White reverberating through the cosmic aether as he zooms about on his task.

[Clonus]

Re: Panspermia, anyone?

 

Really think Cancer said everything that needs to be said. Just to tie up a few loose ends:

 

 

What are the odds that something would hit Earth hard enough to knock off a Moon-sized chunk?

 

 

OK, let's go with your figure, Earth formed 4.5 Gybp. That means life appeared even earlier than using my figure of 6 Gybp.

 

 

Some of the Ranger Lunar probes were sterilized, some failed, and the sterilization process was thought to be at fault. All American probes since then have been built using the same Clean Room technology as the Surveyor. The air in there is cleaner and more germ free than any operating room, but nothing built by humans is perfect. Statistically probable approching certainty that some few bacteria have hitchhiked on all the American probes.

 

Do not know the conditions under which the Russian probes or Huygens were assembled,

 

 

Again, I do not insist on the hypothesis.

 

But think about it. What are the odds that pondscum could evolve to the point that it could throw other pondscum to other planets?

 

Our species will either live and die on this one planet, or spread life to other planets. Logically no third alternative. If we, or our evolutionary sucessors, leave this solar system, we will be spreading life star system to star system.

 

So even if we are the only technological species in the universe (seems unlikely, not impossible), unless we do something spectacularly stupid as a species (far more likely, unfortunately), life will spread beyond this planet.

 

Contrarywise, have we found any enviroment where life could exist but doesn't?

 

The only way we know life could exist in an environment is by observing life existing in it. And of course if we leave this solar system we'll preclude the evolutionary development of any other advanced tool-using species. Which would indicate that our existence indicates that there's no billions of years old species responsible for the origin of life on Earth.

[Cancer]

Re: Panspermia, anyone?

 

The shorter the timespan between the origin of the Earth and the origin of life' date=' the [i']less[/i] likely panspermia as an origin of life on Earth becomes -- because the time window for the random arrival of a random life-bearing interstellar object on this minescule random target becomes gets smaller.

[...]

None of which makes it the nature of life to spread from planet to planet, from star to star.

 

Actually, this made me sit back and think about the problem from a different perspective.

 

Solid objects, presumably the life-carriers that "infect" new star systems, are going to be moving at roughly a km/sec in interstellar space. That's the random motion scale for things floating free in the Galaxy.

 

1 km/sec is within a factor of order unity of 1 pc/Myr.

 

Going at this in the same sort of direction as the arguments leading to the Fermi paradox, that gives you a pretty short timescale for "infecting" the whole Galaxy. The galactic disk is about 30,000 parsecs in diameter, so that means the timescale for crossing the Galaxy is (VERY crudely) 3 Gyr. That's comfortably less than the age of the disk (about 10 Gyr).

 

There's some extra "latency times" that probably need to be included. If there's a maximum exposure time that an infecting agent can survive in interstellar space, then the process is slowed down. It has to infect a star system before starting the next hop. Presumably it takes a while to get an ecosystem going strongly enough that the random collisions can knock infected-enough pieces (that is, viable infection carriers) off the infected planet. That "incubation period", given the state of the fossil record, seems likely to be less than 1 Gyr, but it's hard to say how much less.

 

Anyway, pick your maximum survival time in space; divide by a million to get the number of parsecs in the longest possible infection hop. Divide the size of the Galactic Disk (30 kpc) by that to get the number of hops needed to span the Disk. Multiply that number of hops by (maximum survival time + incubation period), and that's the minimum time needed to infect the whole disk.

 

I gotta run, so I haven't pushed any numbers around yet, but I will this evening just for amusement when I get fed up with grading papers.

[McCoy]

Re: Panspermia, anyone?

 

The shorter the timespan between the origin of the Earth and the origin of life' date=' the [i']less[/i] likely panspermia as an origin of life on Earth becomes -- because the time window for the random arrival of a random life-bearing interstellar object on this minescule random target becomes gets smaller.

Also, ironically, lessens the likelyhood of spontaneous abiogenesis -- because the time window for the fortunate arrangement of random life-generating factors gets smaller.

 

Obviously, we were delt a royal flush on the first hand. What we can't tell is if all the cards were on this planet (spontaneous abiogenesis), arriving at this planet (panspermia), or if the deal was crooked (ID).

 

Most astrophysicists are not believers in panspermia' date=' but I think most are like me, unwilling to rule it out, either, purely because of the paucity of knowledge.[/quote']

[McCoy]

Re: Panspermia, anyone?

 

Anyway' date=' pick your maximum survival time in space; divide by a million to get the number of parsecs in the longest possible infection hop. Divide the size of the Galactic Disk (30 kpc) by that to get the number of hops needed to span the Disk. Multiply that number of hops by (maximum survival time + incubation period), and that's the minimum time needed to infect the whole disk. [/quote']

You're the Pro from Dover, but shouldn't there be a factor in there to show that each "infected" site can "infect" more than one other site, coreward, rimward, spinward and antispinward? The "infection" doesn't have to start at the rim and move to the opposite rim, maybe we should be using the radius rather than the diameter of the disk?

[Kristopher]

Re: Panspermia, anyone?

 

Also' date=' ironically, lessens the likelyhood of spontaneous abiogenesis -- because the time window for the fortunate arrangement of random life-generating factors gets smaller.[/quote']

 

Only if you presume that it takes a long time for it to happen. If it's simply the natural result of a certain set of environmental conditions and complex chemistry, then it could happen in a very short time (in a geological and astronomical sense) once the conditions are right.

 

Obviously' date=' we were delt a royal flush on the first hand. What we can't tell is if all the cards were on this planet (spontaneous abiogenesis), arriving at this planet (panspermia), or if the deal was crooked (ID).[/quote']

 

Come up with a hypothesis for panspermia that is workable and fits the observed facts, and isn't just adding multiple extra layers of complexity and longshot odds to the more important question of "how did life arise?", and I'll reconsider it as a plausible origin of life on Earth.

[McCoy]

Re: Panspermia, anyone?

 

Only if you presume that it takes a long time for it to happen.

The fact that we have been unable to by design to recreate what happened spontaneously indicates a large number of factors, some of which we do not yet understand, must be "just right." The addition of each new factor decreases the odds of a random event occuring exponentionally.

 

Either we were very luck then, or we are missing something that should be obvious now.

 

If it's simply the natural result of a certain set of environmental conditions and complex chemistry' date=' then it could happen in a very short time (in a geological and astronomical sense) once the conditions are right. [/quote']

Yep. If. Not excluded by the observed data.

 

But using that "if" to rule out other hypotheses is relying too much on assumption.

 

And again, "If it's simply the natural result of a certain set of environmental conditions and complex chemistry, then it could happen in a very short time (in a geological and astronomical sense) once the conditions are right," then life in the universe is common, and panspermia becomes more probable.

[Lucius]

Re: capitulating

 

The Argument:

 

Let X = The odds of life arising independently in any place it conceivably COULD arise (such as archaic Earth.)

 

 

For our purposes, the exact figure does not matter; it could be as high as 1 (certainty) or ridiculously low.

 

Let Y = The odds of life, having arisen, being transported to Earth.

 

For our purposes, we don't have to enumerate the various obstacles or estimate the odds of each or even the total odds of Y. We only have to postulate that Y is something less than certainty.

 

The odds of the terragenic hypothesis would be simple: X

 

The odds of the panspermian hypothesis require that life not only arise, but somehow travel from elsewhere to here. Therefore, the odds are X times Y.

 

It is obvious that whatever values X and Y have, within the stated parameters, X MUST BE greater than X times Y. Therefore, the terragenic hypothesis is both simpler, and more likely - how much more likely being a function of just how low a value we assign to Y.

 

The Counter-Argument:

 

The more I think of it, the less I see any effective counter-argument. One could for example posit that there is another factor, Z, namely the number of places in the galaxy on which life could arise; that is, to argue that there are far more places that aren't Earth than places that are Earth, therefore the chances of life arising elsewhere are greater than the chances of life arising here. But it seems to me (intuitively, I haven't done heavy calculations on this) that for that to be a significant factor, we either need a high value for X - leading to the conclusion that life arose independently in several places, and there's STILL a high probability that Earth life started on Earth - or a very high value for Y.

 

I think the best that can be said is that panspermia cannot be "ruled out." But unless there's some reason to think that life is somehow MORE LIKELY to have arisen elsewhere than in the one place we know it exists in abundance, I can't see a justification for saying more than "possible, but less likely than the alternative."

 

Lucius Alexander

 

The palindromedary has a Y chromosome and a Y-Not chromosome.

[McCoy]

Re: capitulating

 

The Argument:

 

Let X = The odds of life arising independently in any place it conceivably COULD arise (such as archaic Earth.)

 

 

For our purposes, the exact figure does not matter; it could be as high as 1 (certainty) or ridiculously low.

 

Let Y = The odds of life, having arisen, being transported to Earth.

 

For our purposes, we don't have to enumerate the various obstacles or estimate the odds of each or even the total odds of Y. We only have to postulate that Y is something less than certainty.

 

The odds of the terragenic hypothesis would be simple: X

 

The odds of the panspermian hypothesis require that life not only arise, but somehow travel from elsewhere to here. Therefore, the odds are X times Y.

 

It is obvious that whatever values X and Y have, within the stated parameters, X MUST BE greater than X times Y. Therefore, the terragenic hypothesis is both simpler, and more likely - how much more likely being a function of just how low a value we assign to Y.

 

The Counter-Argument:

One counter-arguement is that you have assumed that archaic Earth is a place with the right conditions for spontaneous abiogenesis. Presumes facts not in evidence.

 

So let x represent the probability of spontaneous abiogenesis anywhere in the galaxy, x_e the probability of spontaneous abiogenesis on Earth, and y = the probability of exteresterial life being transported to Earth.

 

Now, if x =/= 0, and x_e = 0 is x_e greater than, less than, or equal to xy?

[Kristopher]

Re: capitulating

 

One counter-arguement is that you have assumed that archaic Earth is a place with the right conditions for spontaneous abiogenesis. Presumes facts not in evidence.

 

Life is here. Panspermia is vanishingly unlikely.

 

The simplest solution to that "quandry"? Life originated on Earth.

[Kristopher]

Re: capitulating

 

Let X = The odds of life arising independently in any place it conceivably COULD arise (such as archaic Earth.)

 

It's more like 1/A * 1/B * 1/C *1/D *1/E * 1/F = the odds of life making it from one world to another by natural means.

 

 

EDIT:

 

Let's be extremely generous, and use a figure for each of 1/10; 0.1^6 = .000001

 

One chance in a million that life from anywhere made it to Earth, IF we use extremely generous odds for each step in the hypothetical process of panspermia.

 

Let's be generous, again, and assume that this hypothetical travelling life can survive for 1 million years in space (extreme cold, cosmic rays, etc), which gives us a range of 1 parsec, per the figures Cancer gave us earlier. How many life-bearing worlds are within 1 parsec?

 

We don't really know yet. However, keep in mind that while we can't find earthlike worlds yet, what we are seeing is a lot of stars that couldn't have a world with earthlike life, because the sizes and orbits of the worlds we are finding around those stars all but completely preclude a world with the right conditions for earthlike life.

 

For that matter, how many stars within 1 parsec of earth? Actually, there are no stars within 1 parsec. The closest star is about 1.22 parsecs away.

 

So let's be generous, again, and give our hypothetical lifebearing rock fifteen million years to get here. That's about 275 stars, it seems from what I'm reading. Total. Only a fraction of those will have a planet capable of giving rise to life at all.

 

Let's be generous again, and say that life arose on a planet orbitting 1 in 10 of those stars. Give it even odds that it arose before it could have on Earth, so there's half of them. So, 1 in 20, for a total of 14 (we'll round up).

 

Using our generous generous figure from earlier, of one in a million, that would give us about a 1 in 71500 chance of life getting here from some other star system.

 

However... one of the factors at the beginning is whether it's "aimed" at Earth to begin with. Does anyone want to calculate the exact percentage of the sky that Earth makes up from a hypothetical planet 15 parsecs away? Because it's a lot less than 10%, many orders of magnitude less in fact, and right there the odds go way down. If it doesn't hit that pinpoint, it goes right on by, or off in some other direction entirely.

 

Also, we know that life first appeared on Earth in a time span of about 500 million years, from 4 billion to 3.5 billion years ago, give or take -- any sooner, and Earth hadn't cooled off enough yet and was still undergoing too many large impacts; any later and you're past the date of the first fossil microorganisms. This cuts another of the figures down below 10%.

 

So many orders of magnitude less than that 1 in 71500 rough estimate we came up with. More like 1 in 715000000000.

[Kristopher]

Re: Panspermia, anyone?

 

And again' date=' "If it's simply the natural result of a certain set of environmental conditions and complex chemistry, then it could happen in a very short time (in a geological and astronomical sense) once the conditions are right," then life in the universe is common, and panspermia becomes more probable.[/quote']

 

Life could be all over the place, and it wouldn't change the Rube Goldberg nature of getting it from one star system to another.

 

 

Quick question about orbital mechanics -- would a life-bearing rock knocked off a planet ever actually leave the gravity well of the star that planet was orbiting, or would it just end up in orbit around that star? Hmmm...it would seem to me that the only way to increase the odds of it making it out into intestellar space would be to increase the energy of the impact event, and that would decrease the odds of the life surviving the event because the heating would be more intense.

 

And right there, you have another fraction to multiply by -- what fraction of impacts are just right to send the lifebearing rock into interstellar space without cooking the life to death?