Our definition of the habitable zone is water-based, focusing on planetary surfaces warm enough that liquid water can exist there. New work by Steven Benner (Foundation for Applied Molecular Evolution) and colleagues considers other kinds of habitable zones, specifically those supporting hydrocarbons, which can be liquids, solids or gases depending on the ambient temperature and pressure. Benner’s work focuses on compounds called ethers that can link together to form polyethers, offering life a chance to emerge and adapt in hydrocarbon environments.
Out of this comes the notion of ‘warm Titans,’ moons with hydrocarbon seas that are not made up of methane. We have no such worlds in our Solar System, and they needn’t be moons of gas giants to fit the bill. Think of them, as this Astrobio.net news release does, as being oily Earths drenched in hydrocarbons like propane or octane. Although they do not appear in any genetic molecules on Earth, ethers may be the key to fill the function of DNA and RNA on such worlds.
The nucleobases in the four-letter code of DNA and RNA can mutate even as the molecule’s form is retained, and out of this come the proteins that help life interact and adapt with its environment. Like DNA, ethers show repeating elements, in this case of carbon and oxygen, in their chemical backbones. But unlike DNA and RNA, they have no outward negative charge of the kind that lets them dissolve and float freely so they can interact with other biomolecules. Says Benner:
“This is the central point of the ‘polyelectrolyte theory of gene,’ which holds that any genetic biopolymer able to support Darwinian evolution operating in water must have an ever-repeating backbone charge. The repeating charges so dominate the physical behavior of the genetic molecule that any changes in the nucleobases that influence genetic information have essentially no significant impact on the molecule’s overall physical properties.”
Molecules like DNA and RNA cannot dissolve in a hydrocarbon ocean, making them unable to provide the necessary interactions on worlds like Titan. But ethers, strung together in complex polyethers, while they lack an outward charge, do have internal charge repulsions that allow small parts of the molecule to function in ways similar to DNA and RNA nucleobases.
Image: An artist’s impression of the low-lit surface of Titan under the moon’s thick, orange haze, with liquid hydrocarbons pooling and eroding the surface much like water on Earth. Credit: Steven Hobbs (Brisbane, Queensland, Australia).
Benner’s experiments with ethers show that they are not soluble when we get down to temperatures as low as Titan’s, making Saturn’s largest moon an unlikely venue for such life. But while methane has a narrow liquid range (between -184 and -173 degrees Celsius), we can still put ethers to work in warmer hydrocarbon oceans. Thus the emergence of the ‘warm Titan,’ a world perhaps covered with propane instead of methane oceans that can stay liquid over a broad range (-184 degrees Celsius to -40 degrees). Octane turns out to be even better, not freezing until it reaches -57 degrees Celsius or vaporizing until it hits a temperature of 125 degrees.
Thus hydrocarbon molecules larger than methane come to the rescue. Once again we reconsider the notion of a habitable zone. Certainly in terms of life that we are familiar with, liquid water at the surface is a prerequisite. But as we’ve seen on the icy moons of our system’s gas giants, oceans can provide subsurface environments where life could conceivably emerge. Now we have to consider a hydrocarbon habitable zone where propane or octane can exist in a liquid state. “Virtually every star,” says Benner, “has a habitable zone for every solvent.”
The paper is Christopher et al., “Solubility of Polyethers in Hydrocarbons at Low Temperatures. A Model for Potential Genetic Backbones on Warm Titans,” Astrobiology Vol. 15, Issue 3 (11 March 2015). Thanks to Ivan Vuletich for the pointer to this one.
I am puzzled about the “ethers that can link together to form polyethers.”
Polyethers are formed from dihydroxy organic compounds such as ethylene glycol. Polyethylene glycol (PEG) is commercially produced in a range of molecular weights, e.g., PEG400, PEG 800, etc. and all are highly water soluble and insoluble in hydrocarbons. Maybe alkene ethers could polymerize at the carbon-carbon double bond. It is not clear just what the mentioned ethers are. In a warm hydrocarbon Titan sea, octane would be just one of the hydrocarbons from methane to polycyclic aromatics that would be present as it is in petroleum. n-octane is not a common component in motor fuel. Octane ratings are based on 2,2,4-trimethyl pentane as the 100 rating. Gasoline is a variable mixture commonly adulterated with ethanol.
Harold, let me forward you a copy of the paper as published, once I have it.
We have hydrocarbon lakes on Earth, they are just underground and have to squeeze between rock grains. If polyethers could be the information molecule of such alternative life, might we not expect signs in these reservoirs? (I fully realize that that our DNA life would have out-competed it, but maybe fossil chemistry could be detected?) .
While polyethers might form a backbone for information, on Earth we need both information storage for replication and metabolism. In addition, it is hypothesized that there was an RNA world that did both to some extent, and this is retained by the use of non-coding RNA that is functional. Another possibility is that there may be hydrocarbon layers in Jupiter’s or Saturn’s atmosphere. Could the hypothesized biochemistry appear there? Is it worth adding a detector to a probe that samples the atmosphere?
Benner is a very smart guy, but I would like to see this idea worked out a lot more to show that a life form could emerge, rather than maybe some replication of polyether molecules that handles just one function of living things. (I don’t know how much further they look beyond the solubility of polyethers in hydrocarbons).
Even if life was possible on warm hydrocarbon worlds, don’t we then have a problem for the astronomers analyzing exoplanets? How will we detect living from non-living hydrocarbon worlds? At least we have potential biosignatures for terrestrial type life.
Unlike Titan, won’t the surface of these worlds be too smoggy for photosynthesis? Also won’t photolysis of methane (no effective cold trap as there is for water vapour) result in an extreme outflow of atomic/molecular H unless they have superEarth mass? And won’t an extreme greenhouse effect their mean that seas will be vapourised much closer than the equivalent of Jupiter’s orbit?
This means that hydrocarbon life based on polyethers would be forced to subsist on geothermal processes and the high atmosphere photolysis of methane driven by a narrow band of ultravoilet light. I fear this would place its highest potential surface activity three or four orders of magnitude below that of photosynthetic carbene based hydrocarbon life, should it exist on Titan.
I wonder how complex life could get on such a planet. What the article didn’t cover is what sort of reactions the local life could use to fill its energy requirements and the implications that would have for the types of organisms that would be possible.
Would such a planet be restricted to chemosynthetic microorganisms or could you have the equivalent of plants and animals? One things for sure, photosynthesis as we know it would be downright hazardous :-).
While Titan’s surface doesn’t have the necessary hydrocarbon compounds and ambient conditions that are mentioned in the article, might a microclimate (perhaps even a more-or-less continuous layer) some distance below Titan’s surface possess those parameters? Such a region would be warmer, and octane and propane might form there because of the higher temperatures, with such reactions maybe even being “catalyzed” by other materials that are present in the crust or mantle.
Not sure how a hydrocarbon atmosphere would do under heavy UV exposure, maybe breakdown completely into a tarry surface with Nitrogen atmosphere.
@Alex Tolley
I heard some guy at the Deep Carbon Observatory speculate that proto-life might have survived at depth which are too hot for modern life (+140 C or so). Maybe a bio film without cellular structure. I wish that as much money was invested in such research as on spacecrafts and astronomical telescopes. Astronomers complain about their budgets, but what other field of research gets a 9 billion dollar observatory like JWST?
OT, but a news I think will interest many readers here.
Stone tools have been found which are 3.3 million years old. It is before the first “Homo”, they were manufactured by non-human apes. Tools have been manufactured (note, not just using a natural stone or a stick, but manufacturing tools) beyond historic time scales and across the evolution of species. Once you have sharp stone tools you can hunt much more efficiently and you can do what you want with wood, bones, hides.
Since it is half the time since we diverged from chimpanzee, I suppose that the genetic difference between us and them is about half that between us and chimpanzee. So I wouldn’t trust a guy like that with a driver’s license, but the news is that they seem more clever than previously believed.
I try to understand 3.3 million years by thinking about 3,300 years ago, well before the Roman empire and even Hellenistic Greece, although the pyramids were 1,000 years old already back then. About 150 generations ago. Now take that times a thousand! 150,000 generations. Could a relative high culture have emerged temporarily somewhere, by development of language? I think this is as close to alien civilization we know of. It is non-human.
Nature:
http://www.nature.com/nature/journal/v521/n7552/full/nature14464.html
Accessible news paper article:
http://www.latimes.com/science/sciencenow/la-sci-sn-earliest-stone-tools-20150520-story.html
This calls to mind Thomas Gold’s Deep Hot Biosphere hypothesis of which I include a link to below. Gold suggests that microbial life, as we understand it, may abound in the universe and is not primarily due to surface conditions.
Here is the abstract;
“There are strong indications that microbial life is widespread at depth in the crust of the Earth, just as such life has been identified in numerous ocean vents. This life is not dependent on solar energy and photosynthesis for its primary energy supply, and it is essentially independent of the surface circumstances. Its energy supply comes from chemical sources, due to fluids that migrate upward from deeper levels in the Earth. In mass and volume it may be comparable with all surface life. Such microbial life may account for the presence of biological molecules in all carbonaceous materials in the outer crust, and the inference that these materials must have derived from biological deposits accumulated at the surface is therefore not necessarily valid. Subsurface life may be widespread among the planetary bodies of our solar system, since many of them have equally suitable conditions below, while having totally inhospitable surfaces. One may even speculate that such life may be widely disseminated in the universe, since planetary type bodies with similar subsurface conditions may be common as solitary objects in space, as well as in other solar-type systems.”
Here is the full *paper;
https://www.heartland.org/sites/all/modules/custom/heartland_migration/files/pdfs/7764.pdf
*Proc Natl Acad Sci U S A. 1992 Jul 1; 89(13): 6045–6049.
LocalFluff,
If truly manufactured tools existed that were 3.3 million years old that should be cause to question the established timeline of human evolution. But they won’t. Therefore they must assume that non-human hominids had this capability to fit into the existing paradigm. Over the last century there have been numerous finds indicating anatomically modern humans have existed for at least one and perhaps a few million years but the evidence is basically discounted by modern archeologists because it does not fit and they tend to distrust 19th century scientists as ignorant amateurs. Specimens found by ‘accident’ are also suspect. If you can handle a non ‘mainstream’ book, Cremo and Thompson explore these ideas in their book ‘Forbidden Archeology’.
@Robert
No, I don’t “handle” sci fi. I think it is utterly boring. The only thing interesting is the discipline to respect the real restrictions of nature. Any pothead can make up whatever fantasy (especially along lines of conspiracy, deep sigh) if all criticism is simply denied.
That said, let’s move on. Although 3.3 million year old tools are interesting, we don’t want to get too off-topic. The question is ‘warm Titans’ and ramifications of the paper on same.
@Alex Tolley, petroleum could be loaded with life with that sort of genetic material and we wouldn’t have a clue. If you give that material to a modern microbiologist as say that it might contain organisms that that divide only once every few years, they will never look directly for novel chains of amino acids, but use PCR to amplify its DNA. The problem is so extreme that Xenoturbella was first thought to be a phyla of degenerate mollusks, because DNA from their prey amplified more efficiently than its own DNA. If it had no DNA of its own, we would still be assuming that they are mollusks today and just know it as an organism from which only some labs are able to extract DNA, presumably because of methodological differences. And that is for a eumetazoan!!!
@Michael, yes lots of tar, but if the gravity is high enough lots of H2 also. If life ever gets hold it fixes the problem, deriving its energy by reducing the tar and pumping out methane as a byproduct till all but a trace of ground level H2 or tar is gone.
@LocalFluff, why such extreme measures to look for a shadow biosphere. The Red Rain of Kerala has never been explained. It looks so much like a lifeform, yet no one can extract DNA from it. It also has too much Al and too little P to be the sort of life we know of.
http://en.wikipedia.org/wiki/Red_rain_in_Kerala
If it is life, then its habitat is at high altitude where their is little completion from traditional life, yet we know that traditional life can and does exist there – though with numbers many orders of magnitude less required to occasionally swamp a large proportion of one Indian state in coloured rain.
http://www.climatecentral.org/news/high-flying-bacteria-play-role-in-forming-clouds-and-precipitation-15525
The science of the paper is the measurement of very short-chained ethylene glycol polyethers and paraformaldehyde, a polyacetal: long chain polyethers would not be soluble, thus the speculation of alternate life chemistry is only remotely connected to the lab work. The analogy to CHON based life is doesn’t fit.
The idea of “shadow life” on earth is intriguing, but I haven’t heard of any success in finding any. OTOH, sampling the environment (e.g. oceans) and doing shotgun sequencing has resulted in vast numbers of genes being found, far more than associated with the clearly distinguishable flora and fauna observed. It is a very rich bounty indeed.
It is certainly possible that the universe will allow some very exotic forms of life, but I think we should focus on looking for our sort of life unless we find exotic life in our own solar system as a model. Even then, we need to find clear biosignatures that our telescopes can look for around other stars. Suppose those Jupiter life forms that Clarke wrote about (“A Meeting with Medusa”) existed in Jupiter’s atmosphere, they certainly don’t appear to be having an obvious impact on te atmosphere that we can detect with our space probes, whereas we can detect the impact of life on Earth. It therefore seems more reasonable to search for life “as we know it” rather than life as “we don’t know it”. The latter may result in findings as common as alien intelligence found by SETI.
Alex Tolley writes: “It therefore seems more reasonable to search for life “as we know it” rather than life as “we don’t know it”. The latter may result in findings as common as alien intelligence found by SETI.”
Agreed. But still, the idea of multiple kinds of habitable zones beyond the liquid water flowing on planetary surfaces paradigm is useful. Such as in deep crusts (Thomas Gold) or in the upper atmosphere of gas giants as Carl Sagan speculated, or even cometary orbits or many other situations like interstellar dust grains as Hoyle and Wickramasinghe proposed (and claims evidence for).
I think that life previously established could survive and through mutation adapt to radically changed planetary conditions – say phototropic “lily pads” with hydrogen-filled bladders floating on atmospheric density gradients and excreting oxygen as a waste stream into an anaerobic biosphere. Life as found does not necessarily reflect that life as it first appeared: I am sure the strikingly attractive young lady sunbathing down the street does not resemble in the slightest her distant ancestors in the primordial ooze of a pioneering ocean. So let’s separate conditions necessary for life to form from those that eventually require adaptation to change in order to prevail.
Rob Henry: If you were to read the Wikipedia article you cite, you would find that the “red rain” was red because it contained spores of Trentepohlia annulata and that DNA was indeed found in it. There may be whackier theories still coursing around, but I will take this one as the most plausible and well supported by the evidence.
Thanks for following those references Eniac. Let me explain the controversy as I last left following it. ..
That red rain has so much solid material in it that it can easily be tested directly. When it is, no DNA has ever been found by any of the laboratories around the world that it has ever been given to. The Indian authorities insist on taking the samples and seeing what they can grow from that ground water. That is why they find DNA – and why it only has significance because they claim they repeatedly find the same algae. The chemical composition of this algae bears no resemblance to the unique signature of the red rain.
@Robert, Hoyle and Wickramasinghe didn’t just ‘claim’ evidence for their hypothesis, they had it by any conceivable definition by the close match of Hoyle’s prediction of the infrared part of the trapezium nebula before it was measured. Also Hoyle and Wickramasinghe claim a closer match of their prediction for Halley’s comet tail dust than anyone else, though I don’t know how true that second claim is.
Of cause, all this does not make Hoyle and Wickramasinghe correct, it just means that their work should be followed up with the seriously consideration of other scientists. To date, this look like yet another epic failure of human psychology by that fraternity.
Rob Henry,
You are correct but I was actually referring to experiments collecting samples from the upper atmosphere thought to be from space but I agree with your sentiments. Consider SETI. In spite of the stated mission, it would be career suicide for a SETI scientist to actually assert that a signal was most likely of alien origin. I would be more shocked that they ever made such a claim than I would be that alien life existed.
On reading that Wikipedia article it seems that DNA like material has now been detected but none extracted from the red rain itself. Also intriguing is that they still mention only a third of proteinogenic amino acids detected in the sample. There thus seems no advance on this to date which is strange. Admittedly though, one of them is arginine, which is often thought to be the least likely of these aa to be synthesised in an independent origin of life.
Few have followed through on the reproduction at 300C claims simply because observation under these conditions is too difficult. Some interesting autocatalytic reaction does seem to be occurring even if it is hard to know what. Finally, its fluorescent characteristics seem to be unique among all known life – and that IS a claim by Kumar that could be easily and quickly disproved. None (to my knowledge) have.
http://www.researchgate.net/publication/48410778_Unusual_autofluorescence_characteristic_of_cultured_red-rain_cells
Sure Eniac, this is not a proven claim, but I can’t see how it doesn’t warrant further investigation. To me this episode shows, if nothing else, how inept we are at looking for life without coding DNA.