Prove a planet’s Dead.
In my lifetime, the great quest of humanity has changed, from putting a man on the moon to finding one there. Even a hint of a modest bacterium on Mars would set off a global mindquake–would leap toward an answer for Carl Sagan’s great question, are we alone?
But suppose the alternative to finding life is to ask a different question. Suppose the evidence already supports life all over the place. Consider:
- Cockroach theory says that when you find one (bad) event out there, there must be more. If we find intelligent life on one planet (Earth) for good or bad, statistically speaking, that argues for others.
- Life arose so early on Earth, it must have been extinguished (by meteor bombardment) and restarted many times. In other words, origin of life is too easy. It happens automatically if you’ve got the right soup.
- Meteors all carry left-handed amino acids, the seeds of life. Even if you boil up a planet to sterilize it, it will get seeded again by interstellar weeds.
Today the world of physics hints at yet another question: Is life inevitable, throughout our universe, because this happens to be a universe where life is possible? The anthropic principle says that the reason the physical constants appear to support life is because we’re here to observe them–out of an infinite number of other universes that are barren.
Suppose our universe is one where life is inevitable, at least subregolithic microbial life, in any solar system with planets.
Prove to me a planet is dead.
Ultraphyte 
Aren’t you asking for proof of a negative – which is not possible? The burden of proof of life is still the one I would demand, whatever the logic.
I would only accept your question if we could show life was extremely common in the universe, rather than conjectured to be so.
What an exciting premise! Is life part and parcel of the naturally occurring constants of physics? Then the question becomes–what are the other universes for? Closet Space? I think your question is paradoxical.
It’s not that I agree with Alex T.–lack of life is not a negative, it is a state–without any specific moral or practical values that would render it a negative–the absence of something we know to ‘sometimes be present’ is not a negative, but a mere quality.
No, I take exception to the question being framed as a state of one particular planet when, by your previous comments, you make the argument that it would be universes, not planets, that would be of a life-existing quality or not.
Plus, the most vague part of the question is the generic term ‘life’ which would need to be defined in more specific terms for the question to have a specific answer. Perhaps we should be asking if life could exist without humans being able to observe it, to recognize it as life? But I guess there’s no answer to that one either–since it would involve things unknowable to human scientists! O well…. Xmas gone for only forty minutes and I miss it already. Happy New Year!
Suppose I asked you to prove that object X was contaminated with bacteria. That would be extremely easy as it would need just one successful experiment to prove the case. The experiment might fail, but just one successful run with a different growth medium would be sufficient to prove the case.
But how do you prove the reverse? For any experiment that shows a -ve, it could be argued that the growth medium or culture conditions were insufficient, that you did not check every possible interior cavity, that the procedure killed delicate bugs,,,on and on and on.
SETI is going through a similar “crisis” since it’s inception. First it was expected that there would be a “radio club” that we could easily tap into if we looked. Nada. As technology improved and not results have materialized, we rationalized that signals would be narrow cast, compressed, possibly optical, and that there probably are no beacons, but if there were, they would be lasers, with very short pulses, and only receivable by looking over long periods of time. Jill Tarter still makes the claim that we “haven’t really looked” because the targets and frequencies looked at are a tiny fraction of the possible ones. [Looking for black cats in a dark room which may or may not be there is an excellent make work program].
I would be delighted if we find life on Mars (and we should seriously look for it, rather than proxies) or even advanced pre-biotic chemistry elsewhere. It is a very worthwhile effort, because the scientific and technological rewards could be great. But to assume that life is a priori abundant is a mistake IMO. We should assume the reverse as the null hypothesis.
Actually we should probably hold off on formulating a null hypothesis, because we don’t know nearly enough yet about the origins and development of early life even in the context of our own planet — nevermind a generalizable context.
We are still dealing with significant uncertainty as to how often life arose and was extinguished on Earth, before the current cladogram radiating off the last universal common ancestor managed to take hold. Are we and everything from bacteria to elm trees products of the first biogenerative event on Earth, or just the ones who survived? Consensus has yet to emerge.
As to SETI, the Fermi paradox is a paradox for a reason: *something* in the initial set of assumptions is clearly mistaken, but is the error in our thinking found closer to the left or the right of the Drake equation — that is, is it a matter of astrophysical parameters, biological ones, or a mistake about the nature and general trajectory of technological civilizations?
If you posit a universe where biogenesis is vanishingly rare and technology is an inexorable juggernaut that drives upward exponentially unless it encounters an existential crisis, it still generates the same results as a universe where biogenesis is common but high-growth civilizations like the global mercantile-industrial one are a late-stage effect of a short, irreproducible spike in available energy brought about by the temporary availability of fossil fuels — either way, it’s dark and quiet out there.
Our Bayesian prior for the abundance of life could stand to be iterated a bit, by way of increasing understanding of how life forms and likely scenarios for its genesis here, before we feel too confident in applying the suggested model to the universe at large.
Is life “extremely common” in the universe? The three points I made offer evidence that it is.
“Is life part and parcel of the naturally occurring constants of physics?”
Yes, I think so.
As a molecular biologist, I think of life as a seamless extension of chemistry. The molecular Lego blocks of life form spontaneously out of carbon, ammonia and water boiled up with a spark. Once you have adenine, you’re half-way to DNA.
Isn’t is amazing that so complex a chemical as adenine forms out of spontaneous chemistry? That says to me that this universe is far more fit for life than even the physicists conclude.
Statistically, it makes sense: If we exist in a life-prone universe, it’s more likely that we exist in a universe extremely prone to life than in one where life just barely gets by.
If life formed and reformed many times on Earth, then surely it formed many times on other planets as well; wherever there’s enough water and carbon to get along. So yes, I think it would be wise to assume that any planet we land on has at least microbial life.
As for intelligent life, that’s probably several orders of magnitude rarer. But still: Where life exists, of a certain level, is “intelligence” an inevitable extension of chemistry?
There is no “evidence” for life in the universe outside of earth. None. The claims you are making are based on appeals to logic. Let me pick apart your claims:
Cockroach theory says that when you find one (bad) event out there, there must be more. If we find intelligent life on one planet (Earth) for good or bad, statistically speaking, that argues for others.
The cockroach theory works for known, abundant organisms..like cockroaches. It
doesn’t work for sparsely distributed megafauna, except in so far as there must be a viable breeding population. Statistics only works when you don’t abuse it. Assuming a population or distribution from the presence of a single instance is incorrect. For example, from the knowledge that there is one instance of Joan Slonczewski, can I infer that there must be a large population of Joan Slonczewski’s? Obviously not, because we have data showing that individuals are very rarely duplicated, and then only from the same parents from a single fertilization.
Life arose so early on Earth, it must have been extinguished (by meteor bombardment) and restarted many times. In other words, origin of life is too easy. It happens automatically if you’ve got the right soup.
There is not one shred of evidence that I am aware of that supports this idea. It is pure speculation based on the evidence that life does appear very early after the formation of the earliest rocks we have found. That evidence does not mean life formed several times and was extinguished. Conditions may not have been conducive to formation until after the last major bombardment.
OTOH, one could take your first “cockroach” argument and suggest that life couldn’t be extinguished and that we should expect to find life based on different patterns. Shouldn’t some early life have survived the bombardment and repopulated the planet? Where are they? Surely signs of this early life should be evident somewhere in the biosphere? (c.f. Davies’ “Shadow Biosphere” argument).
Meteors all carry left-handed amino acids, the seeds of life. Even if you boil up a planet to sterilize it, it will get seeded again by interstellar weeds.
Since aa’s are chiral, any abiotic source of aa’s will contain both L and D forms. A quick check of Google scholar suggests that the non-racemic mixtures of aa’s has been found only on/in meteorites, i.e. subject to contamination. What is needed is evidence of non-racemic aa’s to be found in asteroidal/cometary material. But even with this evidence, it doesn’t prove anything. This material is readily produced by simple chemistry, as shown by Miller over 60 years ago. There is a huge jump from aa’s to proteins, or nucleotides to RNA/DNA and from both to a functioning primitive cell that we might ascribe the term “life”.
We certainly don’t make this leap of logic – “There are lots of leaves on the ground which contain proteins. Animals are made of protein. Therefore animals must spring from leaves.”
Why would you do the same substituting L-aa’s for leaves and life for animals?
If life were abundant, why isn’t it obvious that Mars has life? Shouldn’t life have evolved to meet the conditions of a cooling and drying Mars? We certainly find organisms on earth under similar conditions, why not Mars, with or without cross contamination by meteorites?
If life were abundant, so might we expect intelligent life, somewhere in the universe of 100’s billions of galaxies each with 100’s billions of stars. Yet the Fermi Paradox still stands.
The reason we haven’t found life on Mars is that we haven’t dug far enough beneath the surface to avoid cosmic rays which destroy macromolecules. When we do find microbial life, it will be tough because the cells will be scarce; may not use DNA, perhaps some variant thereof; and will be metabolizing slowly. Still, we should eventually find them.
The issues you raise about “life” depend on many levels.
The first level of life is self-replicating macromolecules that propagate and evolve, like viruses independent of cells. This level I argue should arise anywhere there is enough water plus the six basic elements of life. Already Jack Szostak and others have come close to a Darwinian polymer. I argue there is a direct line from basic physics through chemistry through Darwinian polymers.
The next level, to cell and multicellular organism, is tougher. Probably a much smaller proportion of planets gets this far, but we don’t know enough to speculate.
A higher level question is when does “intelligence” arise, and is there a condition under which intelligence is “inevitable”? I don’t know, but I submit that “intelligence” at the level of a cat, bird, or octopus evolved independently at least three times on Earth. Something about life on Earth makes intelligence “inevitable” here.
Still farther, intelligence to the point where culture and technology overtake DNA-based evolution (human intelligence) is yet another question that I don’t know as much about.
I feel the most comfortable with the first level. I think it will be surprising if there are no self-replicating entities deep within Mars.
The reason we haven’t found life on Mars is that we haven’t dug far enough beneath the surface to avoid cosmic rays which destroy macromolecules. When we do find microbial life, it will be tough because the cells will be scarce; may not use DNA, perhaps some variant thereof; and will be metabolizing slowly. Still, we should eventually find them.
That is a bold assertion. Given your knowledge of the MSL on it’s way to Mars, do you expect it will find supporting evidence of the sort of life you propose? If so, you will have confirmation within a few years at most. (Chemical anylsis only, but it is penetrating the surface for samples ~ 10 cm depth).
Unfortunately we do not have temperature profiles of the Martian crust, otherwise we might have some sense as to whether there is liquid water at depth, and if so, then metabolism could be quite high. If so, then the microbe density could be as high as that in the Earth’s lithosphere.
The first level of life is self-replicating macromolecules that propagate and evolve, like viruses independent of cells.
This assumes the RNA or similar world to the genesis of life. Others would argue the metabolic route is more likely. I’m agnostic, although I do think the RNA world has much to commend it. I’m not sure, but I think I have read that it is energetically difficult to get to an RNA world, making this a less likely scenario for the evolution of life.
I think that a dead planet would not possess any magnetic field. In my first high school chem lab I was surprised to learn that although water is made up of hydrogen and oxygen, these two elements will not form water without a catalyst (electricity). So a form of electricity/electromagnetism is essential for what is commonly considered the most crucial chemical reaction. Also, I believe that a magnetic field is necessary for a planet to hold onto its atmosphere – regardless of that atmosphere’s composition. Based on this, Mars is not a good candidate for terraforming/colonization, but several other planets (and possibly some of their satellites) in our solar system are. Of course if any life – that is, anything that is currently alive, not fossils – is discovered on Mars, then electromagnetism is no longer a deal-breaker. There may be work-arounds though,i.e., ‘imported’ or man-made electromagnetic fields.
http://nssdc.gsfc.nasa.gov/planetary/factsheet/
I’ve wondered whether the presence of certain forms/types of energies is behind ’emergence’, the apparently non-linear progression from subatomic particles to macromolecules, and from simple to complex life forms. This could probably be determined by a side-by-side comparison of Earth’s electromagnetic and biological histories.
these two elements will not form water without a catalyst (electricity). So a form of electricity/electromagnetism is essential for what is commonly considered the most crucial chemical reaction.
Yes and no. There is plenty of existing water already formed, e.g. comets. The issue is the formation of the O-H bonds, which are, or course, electrons. The bond is energetically favorable, but there is an energy “hill” to climb first, hence the need for some form of energy input to start the reaction. But the amount is very small, hence a small spark or other energy input can initiate the reaction which will then continue with the free energy liberated. Hence free oxygen is a rarity.
Also, I believe that a magnetic field is necessary for a planet to hold onto its atmosphere – regardless of that atmosphere’s composition.
A magnetic field is useful to prevent solar flares from potentially stripping an atmosphere. However, this is probably not needed for high gravity worlds. Also, bear in mind that atmospheric gases can be replenished. Temporary/permanent[?] loss of an atmosphere is probably not a problem for organisms living in rocks.
The extremely weak magnetic field is possibly a good indication that Mars has a solid core and that there is little heat flow to the surface. I am encouraged that the Moon has a temperature profile at depth that is probably > 1000K which suggests to me that Mars may be similar and thus there could be a liquid water zone below teh surface capable of supporting life.
Thanks Alex!
How about atmospheric pressure? – Is there a minimal amount of atmospheric pressure needed to ensure that certain types of chemical reactions occur in a particular direction? And, is there such a thing as too much atmospheric pressure?
Same for gravity … to what degree does gravity matter in directing/enabling (bio)chemical reactions?
AFAIK pressure should make no difference to the direction of a reaction. For example, the pressure needs to be high enough for a gasoline internal combustion engine to run properly (gasket/piston ring leaks cause incorrect combustion), or for a wood fire to burn well (low pressure on high mountains make fires difficult to start and maintain). Pressure keeps the reacting molecules together (increases the rate of collision).
Gravity should have no effect, other than the indirect one of modifying the local gas pressures.
Pressure participates in chemical reactions, by its effect on concentration:
http://www.chemguide.co.uk/physical/basicrates/pressure.html
For example, bacteria that consume H2 will do better the higher the pressure of H2.
Also, microbes that grow at the bottom of the sea, or below miles of rock, experience different pressures. Humans descending in the ocean experience “the bends” if they go too fast, because they need to equalized the pressures of gasses in our circulation.
Certain microbes, and animals that live in symbiosis with them, are “barophiles,” that is, adapted to grow at extreme pressures. But it’s clear that life can evolve for a range of pressures.
Gravity does affect microbial growth and development of plants and animals. For example, frog embryo development directly depends on gravity. The effects of gravity on biochemistry are subtle, but interesting enough to put biological experiments on spacecraft.
I still favor Lovelock’s definition: life is a meta-stable condition where the planet is held far from chemical equilibrium by continuous consumption of energy. The nice thing about this definition is that it’s detectible spectroscopically. If you can get a spectrum from a planet and detect molecular oxygen (or something similarly bizarre), then the planet is alive. The bad thing about it is that it’s good for detection of biospheres such as ours, but bad for detection isolated bacteria in a small pocket, which may be what’s happening on Mars with those methane emissions.
If a planet’s close to chemical equilibrium (as with Mars) it’s hard to say the planet is alive, since it lacks a biosphere. There may be life on the planet, but if we’re considering whether a single cell in anhydrobiosis on a planet makes the planet alive, then we’re off near cryptobiology territory. For example, I don’t consider the Moon alive, even though it’s quite possible that some bacteria on an Apollo relic might be revivable if we could get them back into culture here on Earth.
I still favor Lovelock’s definition: life is a condition where a planet’s chemistry is held far from chemical equilibrium through the continuous consumption of energy. The planet has a functioning biosphere, in other words. The nice thing about such a definition is that it’s fairly easy to detect from space. The presence of O2 in an atmosphere will do it, although many other biospheres are possible.
Personally, I don’t think that a few cells on a planet make a biosphere. For example, I don’t consider the Moon to be alive, on the off chance that there are a few dormant, revivable bacteria sitting on the remnants of Apollo or a moon probe. Mars appears closer to the Moon than to Earth, but given detections of methane emissions, it’s possible that there are still colonies of indigenous life on the surface.
Hope you enjoyed good holidays!
The definition based on chemistry is tough because with simple inorganic molecules, there is always the possibility that some kind of mix of conditions can produe O2 or N2. However, a related possibility is the detection of circularly polarized light reflected by chiral photopigments (for photosynthesis). So for example, satellites detect polarized light reflected by Earth’s marine algae. In principle we might detect the same from a planet in outer space.
http://www.nist.gov/manuscript-publication-search.cfm?pub_id=842489
With respect to a microbes-only biosphere, I personally would agree with Stan Robinson–I’d consider that inhabited. And where there’s a few, you’d certainly find a whole planet teeming with them, although it might be a hundred meters below, protected from cosmic rays.
But you’re right, no one else would care about a microbial planet. Look what they do with fracking–who cares about the microbial ecosystems down there that get despoiled. ):