Starship Asterisk*

Ann wrote: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.

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

Methane

http://www.centauri-dreams.org/?p=20551 wrote:

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 wrote:

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

Methane

http://www.universetoday.com/91449/why-silicon-based-aliens-would-rather-eat-our-cities-than-us-thoughts-on-non-carbon-astrobiology/#more-91449 wrote:

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.”>>

Flase wrote:- 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.

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

Chris Peterson wrote:

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

What's "hostile"?

TNT wrote:

Chris Peterson wrote:

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.

Chris Peterson wrote:

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

What's "hostile"?

BMAONE23 wrote: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.

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

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.