Titan is "64%" Earthlike?

[Ann]

The top news item reported online right now by Sweden's most prestigious daily, Dagens Nyheter, is that our chances of finding life in space are good. Please note that the newspaper claims that our chances of finding life in space are good, not just that there is a good chance that life exists somewhere in space.

But if our chances of actually finding life in space are good, then this life must be nearby. And that is so, according to Dagens Nyheter, or at least according to the source they quote, a publication called Astrobiology. I know nothing about that publication. Where does it get its money and backing? Who is its editor? What is its general reputation?

Actually, Dagens Nyheter makes claims that can't be corroborated by the link they refer to. I was unable to read more than the abstract there. Perhaps someone at Dagens Nyheter is a subscriber and has access to the full article.

According to Dagens Nyheter, however, Gliese 581g at 20 light-years away from us has an "Earth-like index" of 0.89. Still more interesting to me, however, Titan, the largest moon of Saturn, got an "Earth-like index" of 0.64. A claim made by Dagens Nyheter referring to the article in Astrobiology is that Titan is the most Earth-like body in our Solar system away from the Earth, and therefore, Dagens Nyheter seems to suggest, our chances of finding like in our solar system away from the Earth are greatest on Titan. Titan is more life-friendly than Mars, Europa or Enceladus, the way I read Dagens Nyheter.

Would you agree? Personally, I instinctively think of Titan as impossible for life, due to its extremely low temperatures. Also, there are few if any signs of volcanoes or other forms of geological activity on Titan, which reduces the chances of life on Titan, in my opinion. If there is no geological activity on Titan, then any life there must exist on the super-cold surface or in the super-cold lakes of that moon.

Undoubtedly Titan got its fantastic rating because of its precipitation cycle with methane rain and methane lakes. That's impressive, I agree. However, what reason do we have to believe that liquid methane at close to absolute zero is a good solvent for living organisms? Should we immediately assume that liquid methane is better for life than possibly liquid water at close to Earth-like temperatures under the surface of solar system bodies like Mars, Europa and Enceladus?

I would appreciate a comment here. Thank you!

Ann

[Beyond]

According to the link you provided, Ann, they are not exactly looking for life like US. That's why they think there's such a good chance of finding it. Why they want to expend so much energy trying to find a different life-form, i don't know. But for myself, i don't think anyone could find much of anything that resembles US, in this physical realm, in this solar system. Unless they're just passing through and have installed Romulan cloaking devices. But who knows? Strange things happen all the time.

[Ann]

Thanks for your input, Beyond! I think I'm even more pessimistic than you are when it comes to any sort of life being able to function on super-cold Titan. But of course, when our Sun becomes a red giant, Titan might just possibly become all warm and cosy and full of strange little critters.

Meanwhile, I think you are right: there is no life similar to us anywhere else in our Solar system, unless the Romulans are passing by in a ship with a cloaking device. Who knows, they may even have stopped by on Titan to fill 'er up with methane.


Ann

[Beyond]

Actually, according to the TV series, they used what is called a 'singularity' power source, which, under certain rare conditions, could produce some strange effects.

[Chris Peterson]

I think I'm even more pessimistic than you are when it comes to any sort of life being able to function on super-cold Titan.

Why? Titan seems every bit as hospitable to life as Earth. What does the temperature matter, as long as you have energy sources (which Titan has in abundance), and liquid solvents (which Titan also has)?

Titan seems very Earth-like in many biological contexts, what with lots of light, liquid oceans and lakes, dynamic weather and the sorts of tidal zones that were probably important to the development of life in Earth. It is only "super-cold" in the sense that it would likely be inhospitable to water-based life. But water is just one of several likely solvents that could be used by life broadly similar to what we have on Earth.

[Ann]

Thank you very much for your opinion, Chris. I value it very highly.

However, as long as we have an example of one of life-bearing planets in the universe, and as long as the only life we know is water-based, I'm going to regard it as an opinion, although a very informed opinion, that liquid methane would be an equally good solvent for life as liquid water. The problem is that we don't know how life arose in the first place. If we knew that, we could make models and run them through a super computer to try to find out if methane works as well for life as water does.

We know that water can be a good solvent for life. You believe that methane would be an equally good solvent, and you may very well be right, but it remains to be seen.

Ann

[Chris Peterson]

However, as long as we have an example of one of life-bearing planets in the universe, and as long as the only life we know is water-based, I'm going to regard it as an opinion, although a very informed opinion, that liquid methane would be an equally good solvent for life as liquid water. The problem is that we don't know how life arose in the first place. If we knew that, we could make models and run them through a super computer to try to find out if methane works as well for life as water does.

We know that water can be a good solvent for life. You believe that methane would be an equally good solvent, and you may very well be right, but it remains to be seen.

Ann- please don't read more into my words than I intended. I don't believe that methane would be an equally good solvent to water (nor do I believe otherwise). Water has some very unique properties, and I've read excellent discussions of how some of those properties make it particularly suitable as a solvent in biological systems as we understand them. But I've also read credible discussions about the biological potential of other solvents, such as methane and ammonia. In fact, nobody really knows just what is required (other than the sample-of-one example we have here, as you point out). I was really taking exception to your apparent characterization of Titan as inhospitable primarily because of its temperature. I think there are better arguments to be made (such as the relative merits of methane versus water) than temperature. Because Titan is cold only by human terms. What is important (and what is considered in this approach of classifying bodies as "earth-like") is that there is plenty of energy available, and the environment is dynamic. The temperatures are high enough for liquids to exist, and for complex chemical reactions to occur.

While I suspect, based on my understanding of biology and the development of life on Earth, that life is extremely common in the Universe, and that we will ultimately find it in most places where complex chemistry can occur, we just don't know. Happily, people are figuring out ways that we can look, and I think this approach to classification makes good sense, and is a step in the right direction in telling us the most profitable places to focus our attention.

[Ann]

Thanks for your clarification, Chris. One more question, though. Let's assume, for the sake of the argument, that Titan has a non-negligible amount of methane-based life forms. Let us try to specify the amount by assuming that, if we define the biomass of the Earth divided by the total mass of planet Earth as 1, then let's assume that the amount of biomass on Titan divided by the total mass of Titan is no less than 0.001. These life forms on Titan would need to "eat" nutrients and to get rid of waste products. Would this amount of biomass change the composition of Titan, for example of the atmosphere of Titan, in a way that would be in any way measurable? Could we say that there is probably life on Titan because its atmosphere has a composition that could not be maintained without the constant enrichment or depletion by life forms? And can we say anything at all about what kind of waste products that methane-based life forms would leave behind?

Ann

[Chris Peterson]

One more question, though. Let's assume, for the sake of the argument, that Titan has a non-negligible amount of methane-based life forms. Let us try to specify the amount by assuming that, if we define the biomass of the Earth divided by the total mass of planet Earth as 1, then let's assume that the amount of biomass on Titan divided by the total mass of Titan is no less than 0.001. These life forms on Titan would need to "eat" nutrients and to get rid of waste products. Would this amount of biomass change the composition of Titan, for example of the atmosphere of Titan, in a way that would be in any way measurable? Could we say that there is probably life on Titan because its atmosphere has a composition that could not be maintained without the constant enrichment or depletion by life forms?

I think that is a fair assumption, and one that most astrobiologists would agree with. Indeed, the current focus on detecting extrasolar life is largely dependent upon looking for just that sort of atmospheric signature- something that presumably couldn't be explained by non-biological processes.

And can we say anything at all about what kind of waste products that methane-based life forms would leave behind?

I can't. I'm sure that there are researchers who have looked at how methane-based or ammonia-based life forms might operate, but that has to be hugely speculative. I think the reality is that as we start acquiring the ability to look at extrasolar planetary atmospheres, it won't be for the chemicals that are indicative of life so much as finding things we can't explain on the basis of geology alone. ("Methane-based" or "ammonia-based" are probably not the best terms, although I think we both know what we mean in using them. Life on Earth isn't so much "water-based" as it is carbon-based, with water simply acting as the primary solvent for our organic chemistry. Switching from water to methane or ammonia wouldn't seem to require a move away from carbon-based life, meaning that in many fundamental ways such life could be quite similar to ours. It isn't until the conditions change enough that you have to start looking at something like silicon as the fundamental building block, rather than carbon, that you might get into the realm of wondering if you're even looking at "life" as we understand it.)

[neufer]

And can we say anything at all about what kind of waste products that methane-based life forms would leave behind?

Methane

Water based life form waste product
(Ditto for beer based life forms)

A Look at Methane-Based Life
by Paul Gilster on November 10, 2011

<<Could life exist on a world with a methane rather than a water cycle? The nitrogen-rich atmosphere of Titan, laden with hydrocarbon smog, is a constant reminder of the question. Cassini has shown us the results of ultraviolet radiation from the Sun interacting with atmospheric methane, and we’ve had radar glimpses of lakes as well as the haunting imagery from the descending Huygens probe. Our notion of a habitable zone depends upon water, but adding methane into the mix would extend the region where life could exist much further out from a star. Chris McKay and Ashley Gilliam (NASA Ames) have been actively speculating on the possibilities around red dwarfs and have published a recent paper on the subject.

It’s intriguing, of course, that with methane we get the ‘triple point’ that allows a material to exist in liquid, solid or gaseous form at a particular temperature and pressure. That makes Titan ‘Earthlike’ in the sense that our initial view showed a landscape with the clear signs of running liquid, but this is a world where temperatures dip to 94 K (-179 Celsius) and water is the local analog of rock. In a fine essay on McKay’s work in Astrobiology Magazine, Keith Cooper notes an earlier McKay paper that suggested a potential life mechanism in this kind of environment: Local methanogens would consume hydrogen, acetylene and ethane while exhaling methane. That’s a mechanism useful for astrobiologists because it would show a particular signature in the depletion of hydrogen, acetylene and ethane at the surface.

But the fact that Titan does show signs of such depletion isn’t necessarily indicative of life, for these signs are themselves dependent on atmospheric models that are still in play, and in any case we know little about other processes that could mimic the same characteristics without implications for life. Exo-Titans may be relatively common, for all we know, but to find them we are going to have to first establish that life can exist in this environment and then work out an atmospheric signature we can search for. Cooper quotes Lisa Kaltenegger (Max Planck Institute) on the issue:

“We just don’t know what the tell-tale signs for life would be in such an atmosphere because it is so vastly different from ours. That said, it will change in a flash if Chris [McKay] finds life on Titan and can tell us what it produces and what we could look for remotely with a telescope.”

That makes future probes of Titan all the more interesting, and adds to the desirability of a long-term presence on the moon, either through a surface rover or an aerostat that could range high over the surface and give us a highly-focused look. As for those red dwarfs McKay studied in his recent paper, a methane habitable zone should exist between 0.63 and 1.66 astronomical units (99 million and 248 million kilometers) around the star Gliese 581, that frequently invoked site of habitable planet speculation. Unfortunately, while we do have four planets confirmed in the system (with two others considered controversial), none exists in the methane sweet spot.

While Gliese 581 is an M2.5V dwarf, the authors also calculate the numbers for an M4 dwarf, finding a closer habitable zone between 0.084 AU and 0.23 AU (12.6 million kilometers to 34.4 million kilometers) in methane terms. The beauty of studying habitable environments — water or methane — around M-dwarfs is that these are systems where the orbital distances involved will be small and detection of planets through radial velocity and planetary transits somewhat easier. But what happens on the surface of such a planet is another matter. Much depends on how the atmosphere is affected by stellar conditions, as Cooper notes:

Titan’s atmosphere is opaque to blue and ultraviolet light, but transparent to red and infrared light, and red dwarfs produce more of the latter than the former. If Titan orbited a red dwarf, more red light would seep through to its surface, warming the planet and extending the range of the liquid methane habitable zone. (Interestingly, a red giant, which is close to the endpoint in the life cycle of a Sun-like star, produces light of similar red wavelengths. When our Sun expands into a bloated red giant in about five billion years, engulfing all the planets up to Earth and possibly Mars, Titan may well reap the benefits – for a short while at least before the red giant puffs away to leave behind a white dwarf star.)

Countering this warming is the effect of large stellar flares on evolving life, frequent on younger red dwarfs. McKay’s work suggests that such active M-dwarfs would dissociate atmospheric molecules on a Titan-like world, making the place more and more smoggy and reducing the surface temperature. The net effect would be to move the methane habitable zone closer to the star. Clearly we have a long way to go to be able to actively search for methane-based life outside our own Solar System, and probably decades to go before we get back to Titan.

For the time being, then, a methane habitable zone is sheer speculation, but it’s interesting to ponder the life that might appear on such worlds. One thing seems sure: The temperatures at which liquid methane exists would produce creatures with slow metabolisms. Will a future Titan probe find life? Given our relatively greater understanding of life’s relation to liquid water, we’re obviously going to keep the focus there, but a ‘second genesis’ on Titan would change the equation considerably as we ponder how frequently life can form and with what constituents.

The paper is McKay and Gilliam, “Titan under a red dwarf star and as a rogue planet: requirements for liquid methane,” Planetary and Space Science Volume 59, Issue 9, pp. 835-839 (July 2011). Abstract available.>>

[neufer]

And can we say anything at all about what kind of waste products that methane-based life forms would leave behind?

Methane

Click to play embedded YouTube video.

A Horta: a silicon-based life-form in the Star Trek universe.

Editor’s note: Bruce Dorminey, science journalist and author of “Distant Wanderers: The Search for Planets Beyond the Solar System,” interviews NASA astrochemist Max Bernstein for Universe Today about the possibility of Silicon-based life.

<<Conventional wisdom has long had it that Carbon-based life, so common here on earth, must surely be abundant elsewhere; both in our galaxy and the universe as a whole.

This line of reasoning is founded on two major assumptions; the first being that complex carbon chain molecules, the building blocks of life as we know it, have been detected throughout the interstellar medium. Carbon’s abundance appears to stretch across much of cosmic time, since its production is thought to have peaked some 7 billion years ago, when the universe was roughly half its current age.

The other major assumption is that life needs an elixir, a solvent on which it can advance its unique complex chemistry. Water and Carbon go hand in hand in making this happen.

While the world as we know it runs on Carbon, science fiction’s long flirtation with Silicon-based life — “It’s life, but not as we know it” — has become a familiar catchphrase. But life of any sort should evolve, eat, excrete, reproduce, and respond to stimulus.

And although non-Carbon based life is a very long shot, we thought we’d broach the issue with one of the country’s top astrochemists — Max Bernstein, the Research Lead of the Science Mission Directorate at NASA headquarters in Washington,D.C.

Max Bernstein — It’s important for us to keep an open mind about alien life, lest we come across it and miss it. On the other hand, Carbon is much better than any other element in forming the main structures of living things. Carbon can form many stable complex structures of great diversity. When Carbon forms molecules containing Oxygen and Nitrogen, the Carbon bonds to Nitrogen and Oxygen are stable. But not so much so that they can’t be fairly easily undone, unlike Silicon-Oxygen bonds, for example.

Dorminey — DOES THE RECENT NASA-FUNDED RESEARCH AT MONO LAKE, CALIFORNIA WHICH TOUTED THE DISCOVERY OF BACTERIA WITH DNA THAT USES ARSENIC INSTEAD OF PHOSPHORUS RATTLE THE CURRENT PARADIGM?

Bernstein — That was a really cool result, but the basic structure was still Carbon. The Arsenic was said to have replaced Phosphorus, not Carbon. The discovery of this putative Arsenic organism may prove to be incorrect, but it’s a hypothesis with science behind it, and not just someone tossing out an idea and leaving it at the level of what if you replaced Carbon with Silicon?

Dorminey — SILICON SEEMS TO BE THE MOST POPULAR NON-CARBON BASED CANDIDATE, ARE THERE OTHERS THAT ALSO MIGHT BE FEASIBLE?

Bernstein — It’s hard to imagine anything that would be more likely that Silicon because there is nothing closer to Carbon than Silicon in terms of its chemistry. It’s in the right place on the periodic table, just below Carbon. On the face of it, [Silicon-based life] doesn’t seem too absurd since Silicon, like Carbon, forms four bonds. CH4 is Methane and SiH4 is Silane. They are analogous molecules so the basic idea is that perhaps Silicon could form an entire parallel chemistry, and even life. But there are tons of problems with this idea. We don’t see a complex stable chemistry [solely] of Silicon and Hydrogen, as we see with Carbon and Hydrogen. We use HydroCarbon chains in our lipids (molecules that make up membranes), but the analogous Silane chains would not be stable. Whereas Carbon-Oxygen can be made and unmade — this goes on in our bodies all the time — this is not true for Silicon. This would severely limit Silicon’s life-like chemistry. Maybe you could have something Silicon-based that’s sort of alive, but only in the sense that it passes on information.

Dorminey — IF SILICON-BASED LIFE IS OUT THERE, HOW COULD WE EVER DETECT IT REMOTELY?

Bernstein — We are seriously arguing about how we would remotely detect life just like us, so I really couldn’t say. Presumably technology-using organisms, whatever their biochemistry, will produce technology, so the Search for Extraterrestrial Intelligence (SETI) may be our best shot.

Dorminey— HOW WOULD YOU LOOK FOR SILICON-BASED LIFE HERE ON EARTH?

Bernstein — When seeking an alien organism its really tough because you just don’t know what molecules to look for. One would have to be satisfied by something a bit more ambiguous, like sets of molecules that should not be there. For example, if you were an alien Silicon organism, you might not be looking for our biochemistry, but the fact that you kept seeing exactly the same chain lengths over and over again might tip you off to the fact that those darn Carbon chains might actually be the basis of an organism’s membranes.

Dorminey — WHERE ARE THE LARGEST CONCENTRATIONS OF SILICON HERE?
IN SAND?

Bernstein — In sand or rock. There are literally megatons of silicate minerals on Earth.

Dorminey — HAS ANYONE EVER CLAIMED DETECTION OF SELF-REPLICATING EXAMPLES OF SILICON HERE ON EARTH?

Bernstein — There have been ideas about minerals holding information just as DNA holds information. DNA holds information in a chain that is read from one end to the other. In contrast, a mineral could hold information in two dimensions [on its surface]. A crystal grows when new atoms arrive on the surface, building layer upon layer. So, if a crystal sheet cleaved off and then started to grow that would be like the birth of a new organism and would carry information from generation to generation. But is a replicating crystal alive? To date, I don’t think that there is actually any evidence that minerals pass information like this.

Dorminey — IS THE CRUX OF THE PROBLEM THAT SILICON-BASED LIFE WOULD BE SO SLOWLY REPLICATING THAT IT COULD NEVER MAKE IT IN A DYNAMIC UNIVERSE?

Bernstein — I don’t think that any Silicon life form could be a biological threat to us. If they were high tech, they might eat our buildings or shoot guns at us but I don’t see how they could infect us. We run hot and move fast. If we don’t, things will catch us and eat us.

If they are also tougher than we are and whatever feeds on them is also slow and Silicon based maybe being slow doesn’t matter.

Dorminey — WHAT WOULD BE THE SIGNATURES OF SILICON-BASED LIFE?

Bernstein — If they are not technological, they would be very tough to detect. We could look for unstable, unexpected Silicon molecules; some high energy molecule that should not be there, or molecular chains of all the same length.

Dorminey — DO YOU THINK THAT SILICON-BASED LIFE MIGHT EXIST SOMEWHERE OUT THERE?

Bernstein — Maybe deep below the surface of a planet in some very hot hydrogen-rich, Oxygen-poor environment, you would have this complex Silane chemistry. There, maybe Silanes would form reversible Silicon bonds with Selenium or Tellurium.

Dorminey — IF SUCH SILICON-BASED LIFE DID CROP UP, WHAT WOULD BE ITS EVOLUTIONARY ENDGAME?

Bernstein — If it could evolve past the protist [microorganism] stage, then I think it could evolve intelligence. I have no idea how likely it is for intelligence to evolve, but I can believe in Silicon crystals passing information from layer to layer or in Silicon artificial intelligence, but I don’t expect to see Silicon apes playing their equivalent of “Angry Birds” on their Silicon-Phones.

Dorminey — IF SILICON-LIFE DID EVOLVE, WOULD ITS LIFESPAN BE MUCH LONGER THAN ITS CARBON-BASED ANALOGUES?

Bernstein — The replicating mineral that I described earlier would be living very, very slowly on Earth’s surface. But maybe somewhere very much hotter, its lifespan would be shorter. That’s because presumably lifespan is connected to the pace of your chemistry, which depends on temperature.

Dorminey — FINALLY, WHAT WOULD ENDANGER NON-CARBON-BASED LIFE?

Bernstein — Physical harm for sure. Presumably you could take a jackhammer to it?

But our biochemistry would not be pathogens to it; we could not “infect” them as was the case in “War of the Worlds.”>>

[BMAONE23]

Neufer, I believe that image of Spock is from when he was studying
Horta Culture

[Flase]

I am an amateur sci fi cartoonist so my interest in these subjects is as much imagination as actual science, but I think I'm quite sensible about it.
My personal bet is that there are no extra-terrestrial lifeforms in this solar system, but exactly how many systems have them is anyone's guess.

- Life on Earth needed absolutely perfect conditions for the first faltering steps of evolution to happen, regardless of what temperature extremes some bacteria have been shown to withstand today. In a more hostile environment, it is less likely to begin.

- No other biochemistry besides what we know is as adaptable and able to form so many useful chemical compounds. They would all be less likely. If you examine the question for example of "why not just replace carbon with silicon," you can start by looking at respiration. We breathe in O₂ and breathe out CO₂. If we were to breathe out SiO₂, we would be coughing up sand. This is only gaseous at about 2000°C, much hotter than the surface of Mercury or Venus, so it gets complicated...

- The fact that no lifeforms involving alternative biochemstries have been found on Earth is an indication that it is unlikely. But hey there are so many planets out there in the universe that it would be highly probable that something improbable exists somewhere...

My personal interest is imagining human colonisation of the solar system, maybe 500 years in the future when interplanetary (though not interstellar) space travel is commonplace. How could we make use of a planet with a similar atmospheric pressure to our own? How would you like to get up in the morning, open the curtains and see the landscape of Titan, with a slowly-flowing river of oily hydrocarbons, strangely familiar and yet thoroughly alien?

[Flase]

Also in a way it is good that no lifeforms have been found in the Solar System. It means that we don't have to worry about protecting any precious ecosystems on Mars or anywhere. Environmental concerns are less relevant when considering human activity on these rocks.

[Chris Peterson]

- Life on Earth needed absolutely perfect conditions for the first faltering steps of evolution to happen, regardless of what temperature extremes some bacteria have been shown to withstand today.

That's possible, but I don't think there's any evidence to support that. It seems just as likely that life forms very easily, under almost any reasonable conditions.

In a more hostile environment, it is less likely to begin.

What's "hostile"?

- No other biochemistry besides what we know is as adaptable and able to form so many useful chemical compounds. They would all be less likely. If you examine the question for example of "why not just replace carbon with silicon," you can start by looking at respiration. We breathe in O₂ and breathe out CO₂. If we were to breathe out SiO₂, we would be coughing up sand.

But we don't really know much about other possible biochemistries. And we're not talking here about something as radical as replacing carbon with silicon. Life in a methane environment could still be carbon based- the main difference is the need for a different solvent than water. That's a much more plausible scenario, and one that doesn't require such an extreme change in biochemistry.

- The fact that no lifeforms involving alternative biochemstries have been found on Earth is an indication that it is unlikely.

Maybe. Or it might just show that a given environment will strongly favor a given biochemistry. Or, it might show that different chemistries would be competitive, so the first one to take hold would win. There are many possibilities.

[TNT]

In a more hostile environment, it is less likely to begin.

What's "hostile"?

Harsh or difficult, so as to say a harsh or difficult environment. But there is a special type of bacteria that can withstand extreme conditions (can't remember the name of it, though) and life could have eventually evolved from that.

[Chris Peterson]

In a more hostile environment, it is less likely to begin.

What's "hostile"?

Harsh or difficult, so as to say a harsh or difficult environment. I would have to agree with Flase on this statement.

Sorry, I still don't know how to interpret "hostile" here. It sounds like you're just saying someplace that would be difficult for Earth life. That's not a reasonable definition, since we're not talking about Earth life. I see nothing about Titan that makes it more intrinsically hostile to life than Earth. Indeed, Earth could be considered pretty hostile- anaerobic life forms have a much simpler chemistry- oxygen is highly toxic, and after it killed off the most successful life forms ever to exist on the Earth, what was left had to develop all sorts of extraordinary measures to deal with it.

[TNT]

Chris, I just edited my earlier post (deleted that last sentence and changed it) so maybe you should read that.

But geez again! While I was editing my post you typed up that pretty long message! What is going on

Huh. And now that last message that I accidentally posted can't be deleted. Great. Just great.

[Flase]

In a more hostile environment, it is less likely to begin.

What's "hostile"?

I suppose a definition in this context would have to be along the lines of a seriously decreased likelihood of viable chemical building blocks of life interacting within a temperature range that could allow them to evolve.

[Jim Franklin]

Whilst the idea of finding a form of life on Titan is certainly appealing, and whilst I am certainly not ruling out either Methane or Ammonia being capable of sustaining life in the way water does here, I am with Ann on the idea of life on Titan being a "busted flush" for the temperature reasons.

I understand that within the Nitrogen rich atmosphere of this intriguing moon we have condition that look like those of Earth, albeit with different chemicals, but the shear low temperatures are critical, because most other chemical processes at those temperatures stop, metals become brittle and simply doing any form of complex chemical reaction is near impossible. It is easy to see the environment of Titan and see analogies with Earth, but the analogy is flawed unless we look at the whole chemical picture, and the chemical picture has some very large and serious gaps in it.

I think if the conditions on a planet or moon are right that methane or Ammonia can exist in all three states but the surface temperature is more around the -50 to -100°C mark, where complex chemical reactions can still occur, at an all be it slower rate, then it is possible, so perhaps we should look for environments on Titan that have higher temperatures than the background environment as this would stand a higher chance of perhaps being an environment where we may find life on Titan, although personally I am unconvinced that we will find life there. We have a greater chance on Europa, Callisto and perhaps Ganymede, but as with Titan, I remain to be convinced, although truth is often stranger than fiction and nature has a habit of proving us wrong, as such we should continue to look in all environments because we learn regardless.

[BMAONE23]

64% realy isn't all that similar. The differing 36% makes a vast difference.
Much like DNA, the protein chains that make Us Human, differs from that of the Chimpanzee by only 1.6%.
98.4% similarity for what makes life what is.
Our DNA is about 75% similar to that of a nematode, which is basically a small soil-dwelling worm.
That 25% difference is really huge.
The 36% difference is likely as vast on a planetary scale also

[Chris Peterson]

64% realy isn't all that similar. The differing 36% makes a vast difference.

Does it? This is a question of environment, not of life itself. I think that by the criteria used, there are places on Earth, harboring life, that are less than 64% Earthlike.

Much like DNA, the protein chains that make Us Human, differs from that of the Chimpanzee by only 1.6%.
98.4% similarity for what makes life what is.
Our DNA is about 75% similar to that of a nematode, which is basically a small soil-dwelling worm.

That's because, when considering the nature of life on Earth, there's scarcely any difference between a nematode and a human being. Or between any other living things on this planet.

We don't know if life on another Earthlike planet would have the same fundamental protein chemistry, or something radically different. We certainly don't know what form life would take on a planet without liquid water, but with other liquid solvents available. Any speculation on that matter is just that- there isn't even enough scientific knowledge in this area to make what could be considered an educated guess.

[Ann]

But here is a problem I have with your reasoning, Chris.

When I first became interested in astronomy, in the late sixties, there were still some astronomy popularizers around who seemed to believe that there could be some moderately advanced life forms on Mars. Okay, maybe not in the late sixties, but certainly in the early sixties and late fifties. I vividly remember a Reader's Digest coffee table book about the world, including the world's dinosaur past and its spacefaring future. The chapter about Earth's space future described how a spaceship full of tourists would take off for Mars in the year 2000. The chapter brought us all the way to Mars and described what we would see there: no little green men, alas, but natural rivers and a lot of vegetation.

So my first lesson about Mars was that it was a planet with liquid water and vegetation. Then, in the very early seventies, the science reporter in my local newspaper wrote a lament about the fact that planet Venus had been found to have a surface temperature of about 400 degrees Celsius. The reporter seemed to want to take this horrible truth to a scientific court of law to have it overturned. He insisted that Venus "should" have been a balmy 90 degrees Celsius instead, cool enough to allow liquid water to exist.

I was brought up being told that there was, or should have been, liquid surface water on Mars and Venus.

Then I saw how astronomers fervently sought for liquid water elsewhere, in our own solar system and away from it. The message they seemed to bring home was that if there was liquid water on a planet, then there was probably also life there.

The astronomers began scrutinizing Mars, and they found signs of minuscule amounts of liquid water there. The conclusion seemed to be that we can be hopeful about finding life on Mars.

However, it became obvious that more than negligible amounts of liquid surface water was a rarity among planets, and that most planets probably lacked it. Then the message given by astronomers was that the living beings exist underground on the other planets.

On some planets, it seemed likely that there might not even be any liquid water underground. Then the message was that the living beings there must have metabolisms totally different from our own, so that they didn't need liquid water in the first place.

I can't know that the astronomers are wrong about this. I note, however, that we have an example of one when it comes to biological life on a planet. From this, astronomers have concluded that there is life on most planets that have liquid water, whether this liquid water is on the surface or underground. And if there is no liquid water at all to be had on a planet that contains other gases and liquids, then the life there must make use of these other gases and liquids.

To summarize, we have two observations and four conclusions.

Observation 1: There is a lot of life on the Earth, there is a lot of liquid surface water on the Earth, and all earthly life forms are dependent on water.

Observation 2: We have not detected life on any other planet than the Earth.

Conclusion 1: There is life on all planets that have liquid surface water.

Conclusion 2: If there is no liquid surface water on a planet, then the life forms of that planet live in the liquid water underground.

Conclusion 3: If there is no liquid water anywhere on a planet, but there are other gases and liquids, then there is probably life in those other gases and liquids.

Conclusion four: Life is ubiquitous.


I, however, remain unconvinced that the observation of one example warrants the conclusion that life is (almost) everywhere.

Ann

[Chris Peterson]

But here is a problem I have with your reasoning, Chris.

Which reasoning? Because I don't see anything in your observations that conflicts with anything I said.

I think it's unlikely there is life elsewhere in the Solar System. My point was only that we simply lack the knowledge to make much of a case one way or the other whether some form of life can develop in a non-water environment.

To summarize, we have two observations and four conclusions.

Observation 1: There is a lot of life on the Earth, there is a lot of liquid surface water on the Earth, and all earthly life forms are dependent on water.

I would say there is fundamentally one form of life on Earth, and no evidence there has ever been another. There is a large mass of life on Earth. And yes, the one kind of life found on Earth depends on water. Which tells us with absolute certainly that water-based life is possible. And it tells us absolutely nothing about the possibility of non-water-based life.

Observation 2: We have not detected life on any other planet than the Earth.

Certainly true.

Conclusion 1: There is life on all planets that have liquid surface water.

I'm not sure I understand. Do you mean that in our Solar System, there is life on all the planets known to have liquid water (i.e., one planet, Earth)?

Conclusion 2: If there is no liquid surface water on a planet, then the life forms of that planet live in the liquid water underground.

Conclusion 3: If there is no liquid water anywhere on a planet, but there are other gases and liquids, then there is probably life in those other gases and liquids.

Conclusion four: Life is ubiquitous.

I don't see any of these "conclusions" as anything other than possibilities. And they are certainly not the only ones.

I, however, remain unconvinced that the observation of one example warrants the conclusion that life is (almost) everywhere.

I certainly hold to no such conclusion, either!

[Ann]

Okay, Chris. Thanks.

NASA, however, pursues its "Follow the water" strategy, and it's hard not to think, from how they present their strategy to the general public, that if an extrasolar planet is firmly inside the "habitable zone" of its star and has a rocky composition and an atmosphere, then it "almost certainly" has life, too.

And there are definitely astronomers who say - or seem to say - that if planets definitely lack liquid water, then life may very well find other liquids to sustain itself on these planets.

Ann