[QUOTE=ewmayer;88512]Venus is bright-white, but usually much brighter than any nearby stars. Mercury has a distinct metallic yellow-orange tint. Both of the above are always seen as distinct crescents from earth, and that gives them a somewhat non-stellar aspect, even to the unaided eye, which can't clearly make the crescent out.[/QUOTE]
Your first two sentences are true. The third, unfortunately, is not. The paragraphs below describe what really happens.

Venus and Mercury each go through all the phases shown by the moon, from new (only the night hemisphere turned towards us) to full. Draw a diagram to see why.

The new phase is extremely hard to see, except when the planet passes in front of the Sun; both planets do this somewhat infrequently but by chance each has done it in recent years.

The full phase is also hard to see, not only because the planet appears extremely close to the Sun in the sky (or behind it) but because it is also the furthest away from us in its orbit and so smaller and fainter.

The brightness of the planet depends on their distance and the amount of the illuminated hemisphere we can see. The two quantities are anti-correlated (again, refer to the diagram you drew) and so the phase at maximum brightness is a fat crescent which is the compromise between distance and illumination. Venus is still easily visible to the naked eye when it's at half phase, strongly gibbous or a narrow crescent.

Some people, my wife included, can resolve the crescent of Venus with the naked eye when the planet is showing the thin crescent phase. I can see it only as distinctly non-stellar, perhaps elongated.


Paul

[QUOTE="ewmayer"]I find it shocking that anyone would *not* be interested in knowing whether there is other intelligent life in the universe.[/QUOTE]I would be disappointed to learn that someone is *not* interested in knowing whether there is other intelligent life in the universe. I [b]am[/b] very much interested to know. But looking for an Earth equivalent might be too narrow a search for life. With so much variety of planets and environments I expect there are more exotic forms of life aside from the carbon based DNA things like ourselves. I would not be too surprised if it is discovered that there is some type of tiny living life form on a moon around Jupiter. I would also be delighted by such a discovery. But I fear that a narrow search for things like water, 0-30 degrees C temperature, oxygen etc. would miss other possibilities for life of a different kind.

The versatility of carbon-based compounds surpasses that of compounds based on any other element. I've never seen any sincere serious attempt to show how a non-carbon-based system of living organisms could arise naturally. Yes, I've seen expositions of how, say, silicon-based life could operate, but never accompanied by an explanation of the silicon analog of how simple carbon compounds with H, O, N, S, P ... can join together to form proteins, lipids, amino acids, RNA and so on up the chain.

(IMHO the problem with silicon is that it's just too big to have as flexible a geometry in its bonds as carbon -- the angles just don't work as well in as many cases.

Anyone who thinks non-carbon-based possibilities are being unfarly ignored should try showing how hydrosilicons and silicohydrates are as versatile as hydrocarbons and carbohydrates. And the silicon analog of DNA is ...?)

[QUOTE=cheesehead;88604]'ve never seen any sincere serious attempt to show how a non-carbon-based system of living organisms could arise naturally.[/QUOTE]
I suggest you read [i]Dragon's Egg[/i] by Robert Forward.

A silicon-based lifeform seems to be arising here on Earth, though that may not meet your "naturally" criterion.

See, also, [i]The Black Cloud[/i] by Fred Hoyle. Though other authors have treated the theme better, IMO, that's the earliest source of which I'm aware.

Even if we limit our discussion to carbon-based life forms, we're not restricted to Earth-like environments. I remember a statement (I wish I could also remember by whom it was stated) that to an outsider Jupiter is the most likely place in the Solar system to contain life --- [i]including the Earth in the list of candidates[/i].

It's certainly not the case that almost pure water is the only usable solvent for the complex reactions that occur in carbon-based life forms. Impure ammonia, especially if the impurity is a few percent of water, will do very nicely. Cyanogen should work, though its use as a solvent in labs here on earth has been limited because of its toxicity to our kind of life. Personally, I seen no particular reason why impure sulphuric or nitric acids couldn't also serve. Some of our existing lifeforms are perfectly happy living at very low pH and any objections against the oxidizing powers and reactivity of those acids will be met by pointing out that free oxygen is viciously reactive. Come to that, water is also a very reactive chemical and is much more acidic than, say, ammonia.

Paul

[QUOTE="cheesehead"]Yes, I've seen expositions of how, say, silicon-based life could operate, but never accompanied by an explanation of the silicon analog of how simple carbon compounds with H, O, N, S, P ... can join together to form proteins, lipids, amino acids, RNA and so on up the chain.[/QUOTE]But that supposes that other life will be organised the same as us, perhaps things like proteins, RNA etc. are unique to Earth based life. Maybe the life can be of much simpler form using different available atoms and molecules. Or maybe the opposite with more complexity, I see a range of possibilities and arrangements.

For sure Earth type plants may allow us to hope for other carbon DNA water based life, but let's not be too narrow in the search and keep watching for other possibilities.

If there are all these other forms of life out there that are truly so vastly different from humans, how would we ever know they were alive? It's likely that such beings, or whatever you want to call them, would live in environments that humans could not enter and even if we could, we wouldn't be able to communicate as far as I understand it.

[QUOTE="Jwb52z"]... forms of life out there that are truly so vastly different from humans, how would we ever know they were alive?[/QUOTE]By their effect on the surroundings, and they would most likely be big enough to see.[QUOTE="Jwb52z"]... would live in environments that humans could not enter ...[/QUOTE]but we can use our eyes (telescopes, microscopes, cameras, probes etc.)[QUOTE="Jwb52z"]... we wouldn't be able to communicate as far as I understand it.[/QUOTE]sure we could, sound or EM waves (laser, radio etc.), depending on their actual level of sophistication. For microbe sized beings/things perhaps they would not be intelligent enough to communicate with [b]anything[/b], but larger aliens might be capable of some form of sound or sight.

If they are at the level of, say, a dog then we can do simple tests for intelligence and communication abilities with a probe, if they are at our level or higher then radio would probably be the communication channel of choice. If we ever get to a point of actually [b]meeting[/b] them then suitable arrangements could be made.

[QUOTE=retina;88798]For microbe sized beings/things perhaps they would not be intelligent enough to communicate with [b]anything[/b], but larger aliens might be capable of some form of sound or sight.[/QUOTE]I've already recommended [i]Dragon's Egg[/i] as a source of ideas about the form non-human intelligence may take.

An organism with a mass of 100kg living on the surface of a neutron star would be about the size of a grain of rice. Bigger than a microbe, certainly, but not a lot bigger.

A grain of rice is a few millimeters across. A microbe is a about 1% of that in each dimension, so something of that size at the density of neutron star crust would have a mass of around a tenth of a gram, or about that of an insect. Terrestial insects are not overly bright (not individually, though a bee, ant or termite colony appears to be about as intelligent as a good many individual birds and mammals) but they certainly show quite complex behaviour when interacting with each other and their environment.

Paul

[quote=xilman;88631]I suggest you read [I]Dragon's Egg[/I] by Robert Forward.[/quote]Does he specify how a non-carbon-based system of living organisms could arise naturally?

I've read lots of SF about alien lifeforms unlike our own, and that's fine. But what I'm asking for here is a sincere attempt to explain how the non-carbon-based life arises naturally, and I've never seen that.

[quote]A silicon-based lifeform seems to be arising here on Earth, though that may not meet your "naturally" criterion.[/quote]No, in that case the non-carbon-based form is created by a carbon-based lifeform, so doesn't count because that has the same prerequisite conditions (liquid water, etc.). Neither does it count if we were to find, e.g., a planet inhabited only by silcon-based life that was originated by carbon-based life that subsequently went extinct (perhaps exterminated by the silicon-based life, or perhaps not able to survive a change in environment).

[quote]Even if we limit our discussion to carbon-based life forms, we're not restricted to Earth-like environments.[/quote]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.

[quote]It's certainly not the case that almost pure water is the only usable solvent for the complex reactions that occur in carbon-based life forms.[/quote]100% pure water without contaminants won't do much to form those carbon compounds in the first place, of course ...

[quote]Some of our existing lifeforms are perfectly happy living at very low pH and any objections against the oxidizing powers and reactivity of those acids will be met by pointing out that free oxygen is viciously reactive.[/quote]Early Earth apparently had no free oxygen for at least a billion years, until photosynthetic organisms manufactured enough to more than saturate all ocean/land oxidation. But the question is: could those existing extremophiles have evolved without going through earlier stages of lifeforms not so happy at low pH?

- - - - - - -

[quote=retina]But that supposes that other life will be organised the same as us,[/quote]No, I specified silicon-based [I]analogs[/I] 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.

[quote]perhaps things like proteins, RNA etc. are unique to Earth based life.[/quote]Fine, so the explanation will have to show what differences are feasible.

[quote]Maybe the life can be of much simpler form using different available atoms and molecules.[/quote]I think that feasible alternatives will require at least as much complexity as the carbon system.

[quote]Or maybe the opposite with more complexity, I see a range of possibilities and arrangements.[/quote]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.

[quote]For sure Earth type plants may allow us to hope for other carbon DNA water based life, but let's not be too narrow in the search and keep watching for other possibilities.[/quote]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.

[QUOTE="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.[/QUOTE]But perhaps other life doesn't need such an analogous system organisation.[QUOTE="cheesehead"]Can the range of possibilities in the non-carbon system match that of the carbon system?[/QUOTE]Perhaps not, but also parhaps a myriad of possibilities is not actually required, maybe just a few choice molecules can do the trick.[QUOTE="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.[/QUOTE]I hope you are not expecting too much detail, else one might think it requires actually demonstating the alternative life for real in a lab.[QUOTE="cheesehead"][QUOTE]A silicon-based lifeform seems to be arising here on Earth, though that may not meet your "naturally" criterion.[/QUOTE] No, in that case the non-carbon-based form is created by a carbon-based lifeform, so doesn't count because that has the same prerequisite conditions (liquid water, etc.). Neither does it count if we were to find, e.g., a planet inhabited only by silcon-based life that was originated by carbon-based life that subsequently went extinct (perhaps exterminated by the silicon-based life, or perhaps not able to survive a change in environment).[/QUOTE]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. So why not also we (humans) create life (loosely speaking), say, a robot, then the environment is modified and all that remains is the robots. Then aliens come along and visit Earth, see the robots and conclude that, through natural processes, robots can come into existence. If the aliens make a thorough survey and discover the extinct humans and then further the extinct life from 4BY ago they assume it is all just the expected progression of things. Perhaps they've already seen it happening in lots of places.

Great... Guess it's time to update my [URL="http://www.robotcombat.com/video_oldglory_hi.html"]robot insurance[/URL]. :alien: