10th Planet Discovered

[cheesehead]

Originally Posted by retina

But perhaps other life doesn't need such an analogous system organisation.

Show me a viable alternative.

Perhaps not, but also parhaps a myriad of possibilities is not actually required, maybe just a few choice molecules can do the trick.

Show me a realistic example.

I hope you are not expecting too much detail,

I've been arguing in the context of our having limited resources, too limited to squander on zero-probability searches, so what I want to see is enough detail to persuade me that diverting resources has a reasonable chance of succeeding.

Once we have an abundance of resources for life-hunting, so that we can easily afford to spend some on more speculative alternatives, I say fine, look for it in enviroments inimicable to Earth-type lifeforms (and I mean more inimicable than Mars or Europa, e.g.).

else one might think it requires actually demonstating the alternative life for real in a lab.

It requires demonstrating enough evidence for the alternative life to justify the expenditure of resources on the search for it.

Actually that already happened here on Earth, the original environment 1BY ago was modified by earlier life and new life came along to replace it.

Nope, doesn't count. It was the original environment in which life developed from non-life, and that's the development I'm talking about.

Of course, any ET life we find may also currently live in a similarly altered environment, so of course our searches have to take that into account.

One can argue that the new life was created by the previous life,

< snip >

Perhaps they've already seen it happening in lots of places.

Perhaps. But I'm talking about the conditions necessary for evolution of life from non-life, not life from life.

[xilman]

Originally Posted by retina

Actually that already happened here on Earth, the original environment 1BY ago was modified by earlier life and new life came along to replace it. One can argue that the new life was created by the previous life, so perhaps we and the non-natural life.

That's a very acute observation, IMO.

The Earth suffered a massive runaway ecological catastrophe a gigayear or so ago. Some organisms started excreting a viciously reactive metabolic by-product. Almost everything else went extinct, though a few pathetic remnants still manage to cling on in especially protective environments. If those remnants had gone extinct, everything around us would be in the position of hypothetical robots that wiped out their biological precursors.

I refer, of course, to the invention of photosynthesis.
Paul

[xilman]

Originally Posted by cheesehead

Well, that depends on just how far the environment is not like Earth. +-80% gravity -- fine. +30-10 degrees K -- fine. Zero atmosphere -- I don't think so

It's not at all clear to me why gravity can't be much stronger than 1.8g without precluding life. It certainly can't have anything to do with the structural strength of organisms as there are many here on Earth that can't withstand even 0.2g without collapsing under their own weight. Consider jellyfish ...

Perhaps the argument is that very massive planets attract dense atmospheres, which is true in our solar system. Even assuming that dense atmospheres preclude the development of Iife, I see no reason why massive planets necessarily have dense atmospheres. Atmosphere-stripping mechanisms have been proposed by planetary geologists. For instance, a massive planet close in to a young star would have quite a lot of its initial envelope blown off when the star went through its T Tauri phase. Planetary collisions can also remove volatiles very effectively. The moon, for example, is very deficient in hydrogen, nitrogen and halogens compared with the Earth and Mars, though it has much the same fraction of oxygen as its neighbours. It's believed that these elements were boiled off when the moon was formed from the debris of the collision which formed it. The Earth subsequently collected its hydrogen from comets; it was massive enough to hold on to the water and ammonia so provided but the moon was not.

I accept that planets with <0.2g may have difficulty maintaining a dense atmosphere unless it is very cold. On the other hand, see my comments about atmospheric pressure below.

As for temperature ranges, I suggest you look up the phase diagram of water --- it is liquid at temperatures much higher than we see here on the Earth's surface (which is just as well, if you think about the conditions around thermal vents on the ocean floor). When performing that research, you should check the phase diagram of ammonia/water mixtures. They are liquid at temperatures much lower than zero Celsius.

I see no particular need for a dense atmosphere. An ocean kept liquid under an ice crust has nothing obviously wrong with it as a habitat. Think Europa if you want a concrete example.

Originally Posted by cheesehead

]100% pure water without contaminants won't do much to form those carbon compounds in the first place, of course ...

My comment was intended to suggest substantial contamination, of the order of several per cent or more of water in ammonia.

Originally Posted by cheesehead

No, I specified silicon-based analogs to the carbon stuff, not exact replacements. My "... can join together to form proteins, lipids, amino acids, RNA and so on up the chain" phrase refers to what carbon compounds can do, not (necessarily) to the silicon compounds.

Fine, so the explanation will have to show what differences are feasible.

I think that feasible alternatives will require at least as much complexity as the carbon system.

Can the range of possibilities in the non-carbon system match that of the carbon system? Not that I know of. Carbon is way close to the front of the periodic table. Bigger atoms are more cumbersome to combine.

Pure silicon chains are notoriously fragile, in much the way that pure carbon chains are not. Silicone chains, on the other hand, are robust and permit a very wide range of structures to be built up. For the non-chemists reading, a silicone chain is -Si-O-Si-O-Si-... with two free bonds at each Si where side-groups can be attached. The side-groups can be other silicone chains or most anything that we're familiar with from organic chemistry.

There are very serious proposals that life started in a silicone-like environment. This is the theory that clays had the correct chemistry and microstructure to bring together the first complex organic molecules where they could react to form more complex molecules. "Silicone-like" becauses clays are alumino-silicates with complex AlSiO structures.

Some other chemical systems also show complexity which appears to be comparable with that of organic chemistry. Only "appears" because much of the research goes back only a few decades and some of the compounds are extremely oxygen and/or water sensitve so are difficult to work with, whereas organic chemistry has had several gigayears to show us some of its capabilities. One such system of which I'm aware is provided by boron-nitrogen compounds. Unfortunately, the low cosmic abundance of boron makes it unlikely, IMO, that BN-based life is very common.

Originally Posted by cheesehead

Until someone demonstrates (in serious detail, not just science fiction hand-waving) that the feasibility of a non-carbon alternative evolution is nonzero, I prefer placing my bets on places that can be possible winners.

The above, especially that about silicone chemistry and the clay biogenesis hypthothesis may start you investigating what feasible alternatives are already being considered by chemists and biologists.

Paul

[retina]

Originally Posted by retina

One can argue that the new life was created by the previous life, so perhaps we and the non-natural life.

There was a typo there, the "and" should be "are".

One can argue that the new life was created by the previous life, so perhaps we are the non-natural life.

[S485122]

One thing about water, is that, because of its electrically polarised molecule all its characteristics are very different from other possible solvents (melting and boiling temperatures, the temperature range of its liquid phase, specific heat, energy needed for phase changes, the fact that the liquid phase increases in volume when cooled after reaching a peak at 277 K or 4 C...) Just compare it with molecules formed with other elements in the neighbourhood of oxygen be it vertically or horizontally in the periodic table : water remains a "loner".

Other life supporting universal solvents may be possible but are not very probable because of the small range of conditions appropriate, this would not allow for seasons for instance, meaning that a planet with a non circular orbit or with a tilt of its axis could not harbour the “non water” chemistry.

[xilman]

Originally Posted by Jacob Visser

One thing about water, is that, because of its electrically polarised molecule all its characteristics are very different from other possible solvents (melting and boiling temperatures, the temperature range of its liquid phase, specific heat, energy needed for phase changes, the fact that the liquid phase increases in volume when cooled after reaching a peak at 277 K or 4 C...) Just compare it with molecules formed with other elements in the neighbourhood of oxygen be it vertically or horizontally in the periodic table : water remains a "loner".

Some of this statement is almost trivially true --- all solvents are different from all other solvents in some respects. For example propan-2-ol has a different liquid range from propan-1-ol. I'll therefore concentrate on the other claims made in the statement quoted. In particular, I'll compare it with a couple of other solvents formed by neighbouring elements as requested. To keep the posting from becoming too large, I'll restrict myself to horizontal neighbours. I could very easily include phosphorus and sulphur as well.


First consider ammonia. It is an excellent polar solvent with a wide liquid range. It is liquid between 195K (-78C) and 405K (+132C), those being its triple point and critical point respectively. Do not make the mistake of quoting its melting and boiling points measured at 1 standard atmosphere. There is nothing magic about that particular pressure!

Those figures are for the pure liquid. Dissolved salts lower the melting point (as they do for water); solutes such as water raise the boiling point at anygiven pressure.

The specifc heat capacity at the triple point is about 75 kJ/kg/K, very similar to that of water which has a value of around 81.

The solid is always denser than the liquuid with which it is in equilibrium, so the solid sinks. So what? A convecting ocean will transport heat from warm surface areas to colder deeper areas.

Ammonia is very widely used in chemistry labs where water is too hot and/or too acidic to be used as a solvent. For instance, sodium dissolves nicely in NH_3, (producing Na+ and a solvated electron) and reacting only very slowly, but the solvated electron has a very short lifetime in water and the reaction is vigorous, to put it mildly. Some substances don't react particularly rapidly with either H2O or NH3 but are temperature sensitive; ammonia is much the better choice for these.

The dielectric constant of ammonia at its triple point is about 25. For comparison, that of water is about 88. Ammonia is less polar than water, but it's still a fine polar solvent!


Ok, now for another solvent, one from the more acidic side this time. I'll choose nitric acid, HNO3. Again, a superb polar solvent. Its dielectric constant is 50 at 14C. Unfortunately, I haven't been able to find a value at the triple point.

Its liquid range is 231K to 520K (-42C to +247C). It actually has a wider liquid range at 1 atmosphere than water, -42C to +83C.

The specifc heat capacity at the triple point is close to 50 kJ/kg/K, again comparable with that of water

It is much more acidic than water and although it's occasionally used as a solvent for substances where water is too basic, its major use in the lab and in industry is as a powerful oxidising agent.


Returning to weak acids similar to water and ammonia, consider hydrogen cyanide, HCN, another excellent polar solvent. Its dieletric constant is 95.4 at 25C, even higher than that of water!

Again, it has a wide liquid range from 260K to 457K (-13C to +184C).

The specifc heat capacity at the triple point is 88 kJ/kg/K, almost the same as that of water.

HCN isn't much used as a solvent in chemstry labs, largely because of its extreme toxicity to organisms that have evolved in its absence. That reason, of course, makes sense only to human health & safety concerns. The compound itself works just fine as a solvent.


In summary: water is indeed a remarkable substance, but there are several other compounds which are also remarkable in their own way and which are thoroughly capable of being used as very versatile solvents.


Paul

(Hmm, it's been several decades since I last wrote a chemistry essay on this scale. It probably shows!)

[S485122]

Originally Posted by xilman

The solid is always denser than the liquuid with which it is in equilibrium, so the solid sinks. So what? A convecting ocean will transport heat from warm surface areas to colder deeper areas.

Convection would certainly not be enough : imagine a big lake (or sea or ocean) of fluid, it freezes bit by bit, the solid part sinks, this is the case with almost all fluids except water. Once the solid is in the bottom, it is isolated from heat by all the fluid above it. When the temperature rises again only the upper part of the lake will melt. The main body of the lake will be frozen for ever, except in cases where the temperature changes are extreme. Water near the freezing point and ice of course "floats" and isolates the rest of the water from the cold, meaning that most "lakes" do not freeze all over.

When comparing water with other elements, I was comparing it with FH, NH3, CH4... or with SH2, SeH2...

Also water is abundant in in space (a lot of meteroids are basically dirty water for instance.) As far as I know that it is not the case with the alternate compounds you named, execpt NH3 perhaps.

I do not claim that another life supporting chemistry is impossible, just that it is not very likely. On the other hand the possible places where life could have started is so huge, that other life supporting chemistries are a definite possibility. To go back to the origine of this discussion, in my opinion it is easier to limit the search for life to the most probable places first.

[xilman]

Originally Posted by Jacob Visser

Convection would certainly not be enough : imagine a big lake (or sea or ocean) of fluid, it freezes bit by bit, the solid part sinks, this is the case with almost all fluids except water. Once the solid is in the bottom, it is isolated from heat by all the fluid above it. When the temperature rises again only the upper part of the lake will melt. The main body of the lake will be frozen for ever, except in cases where the temperature changes are extreme. Water near the freezing point and ice of course "floats" and isolates the rest of the water from the cold, meaning that most "lakes" do not freeze all over.

I suggest that you carry out an experiment. Try using a low-melting fat for your ocean (goose fat is particularly good as it melts at only slightly above room temperature) for your ocean and a bright light bulb to mimic the Sun. Bring the bulb in closer during "summer" and further away during "winter". Switch it on during the day and off at night. The requirement on the fat is that the day time temperature, or summer temperature if you prefer, is well above the melting point but the night time or winter temperature is not.

You will find that during day time, and especially during the summer, substantial amounts of the ocean will be liquid. Thermal inertia will very likely result in some remaining liquid even during winter nights though, clearly, this will depend on the precise details of the temperature change regime and the mass of your ocean. Convection will ensure that even reasonably deep locations are liquid throughout the summer So what if the bottom remains frozen all year round? Life here on earth has adapted quite well to locations where water is a hard rock for much of the year. As long as it is available as a liquid somewhere in reasonable amounts, it gets on just fine.

If you want to make your model more elaborate and more realistic, work out how to mimic volcanic vents at the base of your ocean and see what effect that has.

Originally Posted by Jacob Visser

When comparing water with other elements, I was comparing it with FH, NH3, CH4... or with SH2, SeH2...

I know you were. However, that comparson is irrelevant and misleading when the subject under discussion was the suitability of compounds as solvents for supporting life. If you wish to ignore perfectly good solvents, feel free, but don't be surprised if that blinds you to the possibilities. Why the concentration on hydrides anyway?

Originally Posted by Jacob Visser

Also water is abundant in in space (a lot of meteroids are basically dirty water for instance.) As far as I know that it is not the case with the alternate compounds you named, execpt NH3 perhaps.

I suggest you read up more on the chemistry of solar system objects. For a start, it's comets that are largely dirty ice, meteoroids tend to be stony. However, comets in the inner solar system release significant quantities of cyanogen (C2N2) and cyanide (CN-). Pristine comets, those which have not been baked dry by solar heating, also have significant amounts of NH3. I really don't know whether HCN is common in comets but, given the evidence from C2N2 and CN- emission, I would expect there to be quite a lot --- again until it's been baked out.

Originally Posted by Jacob Visser

I do not claim that another life supporting chemistry is impossible, just that it is not very likely. On the other hand the possible places where life could have started is so huge, that other life supporting chemistries are a definite possibility. To go back to the origine of this discussion, in my opinion it is easier to limit the search for life to the most probable places first.

Given that we know so little about the nature of life, having only one example to study, it's rather hard to tell whether reasonably pure liquid water is the most likely life-supporting solvent or not. We can surely say that life is possible in liquid water but that's about all. Given that there are other solvents with useful properties, as I've indicated, and that those solvents (possibly not HNO3, I'll accept) appear to be common in our solar system, it seems to me that we should investigate their possibilities and the locations elsewhere in the galaxy where they may support life.

Paul

[xilman]

Originally Posted by xilman

comets in the inner solar system release significant quantities of cyanogen (C2N2) and cyanide (CN-). Pristine comets, those which have not been baked dry by solar heating, also have significant amounts of NH3. I really don't know whether HCN is common in comets but, given the evidence from C2N2 and CN- emission, I would expect there to be quite a lot --- again until it's been baked out.

Sorry. Although I intended to write "cyanide", I did not mean to write "CN-". The spectrum of neutral cyanide, CN, is what's observed in cometary comae.

CN dimerizes to cyanogen, C2N2, at moderate pressures; that's the chemical generally found in earthly labs, though it's easy enough to produce CN from C2N2 by photolysis or pyrolysis at low pressures.

I'm so used to thinking of cyanide as an anion...


Paul

[ewmayer]

Originally Posted by xilman

I'm so used to thinking of cyanide as an anion...

Since my college chemistry classes, I've always wondered who the poor soul was who was the first to discover that colorless HCN gas "smells like bitter almonds." Unless they were fortunate enough to catch only a slight non-fatal whiff, I keep envisioning a Pythonesque "well, he wouldn't have *written*, 'Smells ... like ... bitter ... almonds ... aaaaaaaaaaaaaaarghhhhh ...' before he died, would he?" scenario.

[cheesehead]

Originally Posted by xilman

It's not at all clear to me why gravity can't be much stronger than 1.8g without precluding life.

Nor is it to me. I didn't say +.81 or -.81 was not fine. I just picked .8 as a figure to help illustrate what I meant by "just how far the environment is not like Earth".

Atmosphere-stripping mechanisms have been proposed by planetary geologists. For instance, a massive planet close in to a young star would have quite a lot of its initial envelope blown off when the star went through its T Tauri phase.

... or by the UV blast from a massive star born nearby after the first star's planets had time to gather themselves.

As for temperature ranges,

My figures were simply casual, and I had averages in mind. But thanks for the details anyway!

The above, especially that about silicone chemistry and the clay biogenesis hypthothesis may start you investigating what feasible alternatives are already being considered by chemists and biologists.

This isn't high enough of a priority to me to put much work into research. I'll wait for a Scientific American-type article or a book laying it out like Rare Earth (especially if it refutes the latter's thesis).