An exotic planetary environment right here in the Solar System may be a useful test for answering the key question of how common life is in the universe. So argues Jonathan Lunine (University of Arizona) in an upcoming paper. Lunine believes there is a plausible case for life to form on Titan, and that if we were to find it there, its very dissimilarity from Earth would make it a test-case for life in other extreme environments of the sort that may be common in the cosmos.
We’d like to answer this question locally because it may be some time before we can answer it around other stars. After all, the best spectral signatures we can hope to get from the atmospheres of Earth-analogues elsewhere are quite possibly going to be ambiguous. Molecular oxygen can be a sign of photosynthesis but also of the abiotic escape of water from the upper atmosphere. Methane in the same atmosphere makes biology more likely but may be, Lunine thinks, difficult to detect from Earth.
Image: One way to search for life on Titan. A Montgolfière, or hot air balloon, floats high above a methane-ethane lake on the distant world. A power source provides heat for buoyancy, which in turn is regulated by computer-controlled opening and closing of flaps on the balloon. Credit: Tibor Balint, JPL/Caltech, J. Lunine (UA).
Thus the utility of finding a second origin of life right here in the Solar System. And it’s interesting that Europa may not be as viable a candidate for this as Titan. Although Richard Greenberg in his book Unmasking Europa (Springer, 2008) has made a strong case for finding biological materials in the crustal ice and even on the surface through upwellings of oceanic water as the surface ice shifts, Lunine is skeptical:
…this assumes that organisms would be preserved in a recoverable fashion, and that the ice layers where such organisms exist would be cycled near enough to the surface to be sampled without being so close that the prodigious particle radiation would destroy the organic remains. Even so, delivery and operation of a vehicle designed for the surface of Europa, given the intense particle radiation flux from the Jovian magnetosphere, would be a challenge.
Enceladus, then? As with Europa, Lunine worries about the danger, however faint, of cross-contamination from Earth, even though radiation presents much less of a problem. But why take even the smallest chance of contamination via impacts from space debris (possibly carrying life from Earth, or even Mars) when Titan is available? There water is frozen out, to the point that even if there is a layer of liquid water beneath Titan’s ice crust, it is probably more than fifty kilometers deep.
Like Europa, Titan doubtless receives debris from hypervelocity impacts on Earth, but any surviving terrestrial life form should quickly perish in the cryogenic conditions there. But Titan’s lakes and seas of liquid ethane and methane could provide an interesting context for the development of an altogether different kind of life:
Methane and ethane have their own problems, including their non-polarity,
which means that as liquids they provide no support for molecular structures that depend on interaction with the liquid for their stability. But small amounts of polar molecules might exist in the Titan seas. Furthermore, an interesting “bio”chemistry might be built around the dominance of hydrogen bonding between organic molecules immersed in the non-hydrogen-bonding ethane and methane…, and in such a biochemistry, the low temperatures and consequent slow reaction rates are not necessarily a disadvantage…
Here Lunine is drawing on work by Steven Benner (University of Florida) and colleagues and goes on to summarize their conclusions:
While nothing like a complete, theoretical biochemistry in liquid methane and ethane has been constructed, there is no particular property or set of properties of liquid methane and ethane that could lead one to a priori rule out in such a medium a kind of self-sustaining, replicating, catalytic organic chemistry that might be called life.
Finding life of independent origin in our Solar System would radically revise our view of life elsewhere. Ponder this: While a planet around an M-dwarf at a distance that would allow liquid water would face problems like solar flares and tidal lock in developing its own forms of life, a planet 1 AU from such a star would be much like Titan, at a distance where liquid methane and ethane could survive on the surface. Were we to find life on Titan, we would extend the concept of a habitable zone to include that kind of environment, and because M-dwarfs outnumber stars like the Sun by a huge amount, we would have found the prospects for life that much more likely.
It could be argued, of course, that the planets around an M-dwarf might easily mimic Mars or Europa as much as Titan, but Lunine’s point is that in our system, only Titan offers both (relatively) easy access to the habitable environment of interest and an assurance that any life found there would by necessity be of an origin independent of Earth life. He calls Titan ‘a fishing license to broaden the search for planets around other stars’ from Earth-like worlds to those like Saturn’s huge moon.
The paper is Lunine, “Saturn’s Titan: A Strict Test for Life’s Cosmic Ubiquity,” accepted for publication in Proceedings of the American Philosophical Society and available online.
Lunine is right about both the intrinsic difficulty of recovering Europan samples and the possibility of contamination. However, methane and ethane are very problematic as solvents: being neither polar nor chiral, they don’t have a fraction of the capacity of water, or even ammonia. They are satisfactory solvents only for hydrocarbons, and hydrocarbons alone cannot act as the basis for complex organic chemistry. Add to this the slowness of reactions at Titanian temperatures, and you have complex life at a disadvantage — whereas both Europa and Enceladus have an friction heat engine, as evidenced by the upswellings and geysers, respectively, which might give them the equivalent of geothermal vents that can support life.
Hi Paul;
This is a really great topic.
The detection of life on Titan would be one heck of an event for which to throw a party.
Given that Europa, and Enceladus, and if my memory serves me correctly, Io might also have liquid water below their surfaces, we have perhaps 4 test beds for our search for extraterrestrial life in our solar system. Throw in Mars and the number grows to 5.
Even as we start to develop mission plans to other star systems, assuming such plans could be realized by later this century, something I remain very hopeful about, we could concurrently look for life in our solar system.
If ET fish do exist in the underwater aquatic environments of the 4 moons I mentioned above , or perhaps some form of extremophile tube worms or other hardy life, we might have the option to study the actual life forms of 4 or 5 extraterrestrial locations right here in our back yard. If we see some biochemical, and/or tissue and body structure shape similarities in any existent life forms hopefully to be discovered, this would help let us known how nature tends to produce aquatic lifeforms in similar environments throughout the universe.
Note that just as there exist the thousands of species of; fish, mammals, insects, fungi, plants, and micro-organisms on Earth (well over a milion species in total that have been discovered), the opportuinity for a wide range of ET life in our own back yard, and more inclusively throughout our Galaxy, my intuition tells me from a statistical stand point, is very large.
If Titan has life, it will be very simple life. Chemical reaction rates would be much slower on Titan than on earth, even in any hydrothermal vents, if they exist. We do cryo-preservation at temperatures similar to that of Titan so that the reactions are so slow such as to be non-existent over a period of 1-2 centuries. If life is there, it will be very SLOW life. So, not much evolution is going to happen over a 5 billion year time period. Also, as Athena points out, Methane and Ethane are not good solvents for complete carbohydrate reactions.
I think Titan is a rather poor choice for life. Nonetheless, it is an interesting planet and is worthy of more research. I’ve heard that the critical issue for life on Europa is the thickness of the ice that covers its oceans. If this is too thick, there is not enough energy for life to exist there as well. Mars may have had life a few billion years ago, but is likely dead today. Again, not enough energy to sustain a bio-system.
I think Grinspoon’s suggestion that the clouds of Venus could have bacterial life. There is certainly enough energy on Venus to power life and the cloud environments at certain altitudes are supposed to be quite benign. Also, there are bacterias that can take the acidic environment.
The relative rates of chemical reactions on planets like Titan and Europa as compared to Earth can be quantized by the Arrhenius equation:
k = A exp(-E/RT)
http://www.alcor.org/Library/html/HowColdIsColdEnough.html
An article on chemical reaction rates at various temperatures. Note the table a little more than half-way down the article.
A chemical reaction that takes one second on Earth will take about 11,000 years on Titan assuming an average surface temperature of 94K. If there are vents there, it may be warmer, but likely to still be in the cryogenic range. Even if the vents warm things up to 200K, a one second reaction will still take 5 minutes, a 300 times slower rate. 5 billion years of evolution on Earth would be equal to around 15 million years on Titan. That’s at the vents. Forget about it anywhere else on Titan. I think its safe to say that life in unlikely to have evolved on Titan.
Europa’s a different story. This paper,
http://www.lpl.arizona.edu/~showman/publications/melosh-etal-2004.pdf
says Europa’s ocean could actually be quite warm due to tidal stresses with Jupiter. Also, all that radiation hitting the ice surface should also impart some heating effect as well.
More info on the proposed Titan probe here:
http://www.planetary.org/blog/article/00000568/
On a related topic, a link to an actual Russian balloon probe of Venus:
http://articles.adsabs.harvard.edu/full/1986SvAL…12….7K
Ref. Kurt9 August 10, 2009 at 18:09, last paragraph:
Seeding the clouds of Venus with earthly cyano-bacteria as a way of terraforming Venus was, if I remember well, a plan suggested by Sagan and others back in the ’70s.
However, the real problem on Venus appeared to be, not the abundance of CO2 or even sulphur-acid, but the lack of water. Venus almost completely lacks H2O, both in its atmosphere and soil, which would inhibit the proliferation of organic life, either indigenous or introduced, and any terraforming (Zubrin proposed bombarding Venus with Kuiper belt or Oort cloud cometary objects, both to introduce water and to speed up its rotation, a LOT of those objects).
I see no way around this fundamental problem. Introducing life and terraforming of Mars is a walk in the park compared with doing the same with Venus.
Kurt9 said:
“I think Titan is a rather poor choice for life. Nonetheless, it is an interesting planet and is worthy of more research. I’ve heard that the critical issue for life on Europa is the thickness of the ice that covers its oceans. If this is too thick, there is not enough energy for life to exist there as well. Mars may have had life a few billion years ago, but is likely dead today. Again, not enough energy to sustain a bio-system.”
You probably already know this, but just in case: Richard Greenberg’s 2008
book Unmasking Europa gives plenty of evidence that the Europan ice crust
is relatively thin, so that we may not even have to do more than land on that
moon’s surface to get ocean material, including possible life.
I believe it was Freeman Dyson who even went so far as to claim that Europan
“fish” might be found in orbit around Jupiter, being blasted into space by
planetoid and comet impacts. In any case, we probably won’t have to drill
for miles to get to that alien ocean.
More on Unmasking Europa in this Centauri Dreams thread here:
https://centauri-dreams.org/?p=6351
August 10, 2009
Sun, Earth Are Unlikely Pair to Support Life
Written by Anne Minard
We don’t know how lucky we are — really.
We know the interaction between Earth and the Sun is a rarity in that it allowed life to form. But scientists working to understand the possibility that it could have happened elsewhere in the Universe are still far from drawing conclusions.
What is becoming clearer is that life probably shouldn’t have formed here; the Earth and Sun are unlikely hosts.
A series of presentations at this year’s meeting of the International Astronomical Union meeting, in Brazil last week, focused on the role of the Sun and Sun-like stars in the formation of life on planets like Earth.
Edward Guinan, a professor of astronomy and astrophysics at Villanova University in Pennsylvania, and his collaegues have been studying Sun-like stars as windows into the origin of life on Earth, and as indicators of how likely life is elsewhere in the cosmos.
The work has revealed that the Sun rotated more than ten times faster in its youth (over four billion years ago) than today. The faster a star rotates, the harder the magnetic dynamo at its core works, generating a stronger magnetic field, so the young Sun emitted X-rays and ultraviolet radiation up to several hundred times stronger than it does today.
A team led by Jean-Mathias Grießmeier from ASTRON in the Netherlands looked at another type of magnetic fields – that around planets. They found that the presence of planetary magnetic fields plays a major role in determining the potential for life on other planets as they can protect against the effects of both stellar particle onslaughts.
“Planetary magnetic fields are important for two reasons: they protect the planet against the incoming charged particles, thus preventing the planetary atmosphere from being blown away, and also act as a shield against high energy cosmic rays,” Grießmeier said. “The lack of an intrinsic magnetic field may be the reason why today Mars does not have an atmosphere.”
All things considered, the Sun does not seem like the perfect star for a system where life might arise, added Guinan.
“Although it is hard to argue with the Sun’s ‘success’ as it so far is the only star known to host a planet with life, our studies indicate that the ideal stars to support planets suitable for life for tens of billions of years may be a smaller slower burning ‘orange dwarf’ with a longer lifetime than the Sun — about 20-40 billion years,” he said.
Full article here:
http://www.universetoday.com/2009/08/10/sun-earth-are-unlikely-pair-to-support-life/
To quote:
Such stars, also called K stars, “are stable stars with a habitable zone that remains in the same place for tens of billions of years,” he added. “They are 10 times more numerous than the Sun, and may provide the best potential habitat for life in the long run.”
and…
The scientists agree that we do yet know how ubiquitous or how fragile life is, but as Guinan concludes:
“The Earth’s period of habitability is nearly over — on a cosmological timescale. In a half to one billion years the Sun will start to be too luminous and warm for water to exist in liquid form on Earth, leading to a runaway greenhouse effect in less than 2 billion years.”
I agree that K-stars are nice. They are more stable than G-stars like our sun and have projected main sequence lifetimes of 40-50 billion years. They are big enough that their habitable zones are not so close that the planet gets rotationally locked with one side facing the star as would be the case with an M-star. K-stars are more numerous than G-stars as well.
They say that the Earth is already past its biological peak, with regards to the total bio-mass. I’ve heard this actually peaked during the early mesozoic and that the total biomass has been slowly declining since then. We do know that the Earth is relatively cool and dry compared to the Mesozoic, when it was hotter and humid, with a tropical or sub-tropical climate extending to the poles. The Earth was a lot more “chtorr” like during the mesozoic period (in reference to the Chtorran ecology in David Gerrold’s novels).
I talked with Chris MacKay (NASA Aimes) about this subject, and he reported that they tried dissolving Thiolins in liquid Methane and got nothing. It was just too cold. However, interesting things start to happen if you take a larger version of Titan (so it retains its atmosphere) and increase its insolation.
As you warm the planet/moon, the methane disassociates. Ethane is formed. Your ocean works its way up the chain of Hydrocarbons, depending on the temperature. Amines, which are formed by UV in the upper atmosphere of a Titanian type planet, form micelles around and extract polar molecules from the surrounding hydrocarbon ocean with great efficiency. (This process is used to completely dehydrate benzene.) Once the temperature gets above minus 80 deg. C , Ammonia liquifies. Any Ammonia (from cryo-vulcanism) and Cyanide (formed by UV in the upper atmosphere) found in the oceans would be concentrated together in a small cell-like body. Cyanide reacts with Ammonia and polymerizes to form a variety of organic molecules. I also suspect that the Ammonia would carry with it some water molecules, which would react with the Cyanide, further increasing the complexity of the chemistry.
I doubt Titan harbors life, but there are possibilities for this type of planet.
Ref. ljk: I agree with Guinan and Kurt 9 (August 11, 2009 at 14:41). I have argued before that later G stars (from about G5) to early K (about K2) may be more ideal candidate stars for living planets, because of their much longer habitable lifespan and stable habitable zones, and that our sun may actually be sub-optimal.
This is confirmed by a recent publication, ‘Age and mass of solar twins constrained by lithium abundance’ (do Nascimento Jr et al., May 2009), which shows that stars only a bit more massive (maybe 5 – 10% greater mass) than our sun may be the upper limit for ‘habitable stars’ (habstars ref. Margaret Turnbull).
One reason, well-known by now, is of course the exponentially shorter (habitable) lifespan of larger mass stars. A 5% more massive star (i.e. about 15% brighter) may have about 90% of our sun’s lifespan, a 10% more massive star (i.e. about one third brighter) may have only about 80% of our sun’s lifespan. The habitable lifespans may even be relatively shorter, and this might simply not allow for enough time for complex life to evolve.
However, an important additional reason, as elaborated in the mentioned publication, is the fact that younger sunlike stars as well as (slightly) more massive sunlike stars both show an accumulation of lithium in their upper layers, leading to more turbulent behaviour and more aggressive radiation, the former because they have not had the time yet to ‘burn up’ the lithium (lithium depletion), the latter because more massive stars have less convective mixing flows into their interiors (where the lithium destruction can take place).
Sunlike stars up to some 3 – 4 gy may still have much too abundant lithium, as well as stars beyond a few % more than solar mass.
Somewhat smaller mass sunlike stars do not have this problem, first of all because of more complete convective mixing, secondly because of their (much) longer lifespans.
Although I do not know exactly how detrimental lithium is for a star (or rather its terrestrial planets), I wonder, btw, whether this may have been a reason for complex life to arise so relatively late on earth.
It also leaves solar mass stars a relatively short window of opportunity for (complex) life to arise and develop: before 3 – 4 gy the star is too turbulent for complex biological life, after 5 – 6 gy it already begins moving off the main sequence, becoming too hot. As the stellar mass increases, this time window becomes (exponentially) shorter, being squeezed from both ends. My rough guesstimate is, that somewhere between 1.05 and 1.1 solar mass, the lower time limit (sufficient lithium depletion) and the upper (habitable lifespan) meet, reducing the window to virtually zero.
“after 5 – 6 gy it already begins moving off the main sequence, becoming too hot. ”
The Sun will be on the main sequence for another ~6.5 gy /from today/. Its total main sequence lifespan will be just under 11 billion years.
Doug M.
I just read the Nascimiento article you mention. It’s interesting, but it doesnt’ mention lithium abundance in a stellar envelope as a constraint on habitability.
I’ve been unable to find a cite for “an accumulation of lithium… leading to more turbulent behaviour and more aggressive radiation”. Do you have one?
Doug M.
Yes!, In praise of K-Dwarfs!
Many comments here point out the likely superior suitability of these stars to support living planets long term.(For life as we understand it.)
We have dedicated red dwarf surveys like MEarth and we got Kepler looking
at sun- like stars. Have K-dwarfs been overlooked? How well can Kepler detect earth mass planets in the HZ of K-dwarfs?
I hope the K-dwarf population is not overlooked in exoplanet surveys.
I’m not aware of any dedicated surveys directed at these stars.
K-dwarfs are actually pretty much ideal targets for radial velocity surveys… certainly they have not been overlooked: indeed such fascinating systems as HD 69830 (three Neptunes) and HD 40307 (three super-Earths) have been detected around K-dwarfs. K-dwarfs can actually be extremely intrinsically stable, do not suffer from the extremely low luminosities that make M-dwarfs hard to observe and have lower masses than G-dwarfs (so the reflex velocities will be larger).
@Doug August 12, 2009 at 10:30: “The Sun will be on the main sequence for another ~6.5 gy /from today/. Its total main sequence lifespan will be just under 11 billion years”.
True, you are right about this and I should have formulated it more specifically: as the article quoted by ljk above also mentions, the sun will become too hot for higher life as we know it in about 0.5 gy and too hot for any life in about 1 gy. All this due to helium accumulation in the core.
(Schröder, K.-P.; Smith, R.C. (2008). “Distant future of the Sun and Earth revisited”. Monthly Notices of the Royal Astronomical Society 386 (1): 155; and research by James Kasting, at Pennsylvania State University).
But indeed this is all still part of the main sequence.
@Doug August 12, 2009 at 10:50:
the Nascimiento article itself is indeed limited to lithium abundance and depletion in solar twins. I understood it is a common understanding among astronomers that high stellar lithium leads to unpleasant stellar behavior with regard to planetary habitability, which is also why astronomers like Meléndez, J., & Ramírez, I. (in “HIP 56948: A Solar Twin with a Low Lithium Abundance”. The Astrophysical Journal 669 (2), 2007) nowadays call solar twins with high lithium abundance ‘quasi-solar twins.’
I will try to find literature with regard to lithium and stellar behavior.
Any others here know what exactly lithium does to stars? Paul, Adam, others?
“the sun will become too hot for higher life as we know it in about 0.5 gy and too hot for any life in about 1 gy.”
One, those numbers are still being fiercely debated.
Two, the /Earth/ will become too hot for higher life as we know it. But the Sun’s habitable zone will simply creep outwards. By the time the Sun leaves the main sequence, Mars will be getting almost as much insolation as the Earth does today. And if Earth were just a bit further out, it would gain hundreds of millions of years of additional time. Earth may not occupy the very best possible orbit around the Sun for the long-term development of complex life.
Doug M.
@ Doug M. August 12, 2009 at 15:28:
“Two, the /Earth/ will become too hot for higher life as we know it. But the Sun’s habitable zone will simply creep outwards. By the time the Sun leaves the main sequence, Mars will be getting almost as much insolation as the Earth does today. ”
True, again I should have formulated it better, it is the earth’s habitability, which is at stake, not the sun’s.
“those numbers are still being fiercely debated.”
Really, that would be new to me, I thought this was well established. Source(s)?
Mike Brown has a very interesting article on observing Titan clouds from Caltech:
http://www.mikebrownsplanets.com/2009/08/long-road-to-titan-storm.html
Why would it be well established? The results depend sensitively on various assumptions, from weathering rates (which depend on things like how the continents are arranged) to whether plants can evolve something better than C4 photosynthesis.
That last one is a particular bitch. Basically, up until the late Cretaceous all plants used C3 photosynthesis. Then some flowering plants evolved CAM photosynthesis as an alternative. Then in the middle Cenozoic, some other plants evolved C4 photosynthesis. All three now coexist, closely related but optimized for different levels of temperature, humidity, and CO2. And their existence affects the models noticeably — they give us plants that can tolerate higher temperatures, make do with different levels of CO2, etc., which in turn affects things like when the biosphere will start to die.
Has evolution exhausted its creativity or can it come up with a fourth, fifth or more? And if so, what are their limits? To answer that we have to drill deep down into the quantum chemistry of the chlorophyll molecule and its pointy tip, the Rubisco enzyme. And that’s work in progress. still ongoing.
Here’s an example: for years it was a mystery how Rubisco kept working over 40 C, since the models suggested it should shut down at higher temperatures. In the 1980s it was shown that if plants have access to plenty of water, they can cool their leaves quite significantly by transpiration, thus allowing Rubisco to work, and photosynthesis to proceed, even when the general temperature is well over 40 degrees. But that only accounted for part of the difference. Then in the early 2000s it was discovered that plants have adapted heat shock proteins to keep Rubisco going for a few degrees more. Neat, yeah?
But it turns out that both those processes have limits; even with plenty of water, photosynthesis in land plants stops dead around 50 C.
But will that always be the case? After all, temperatures of 50 C are very rare right now, so there’s not much evolutionary incentive. On a hotter Earth, could plants find a way to keep Rubisco going further into the red? Or have they already hit the wall? Depending on how you answer that, your model may give you very different outcomes.
Anyway. The basic model that most people have been working from was set forth in this paper:
http://cat.inist.fr/?aModele=afficheN&cpsidt=1253696
— but results for the death of the biosphere have ranged widely, from <500 my to around 1.5 gy. Furthermore, in the last few years some people have pointed out that the model in this paper has some problems. I can get into details if you like, but the short version is that it's basically a geophysical model with a biological cycle tacked on, and it's increasingly unclear if that's the way to go.
There's also a recent paper that points out that it's possible to gain up to an additional 1.3 gy by reducing atmospheric pressure generally. How this is to be accomplished is not given in detail, but if you're interested it's Li et al. "Atmospheric pressure as a natural climate regulator for a terrestrial planet with a biosphere." Proceedings of the National Academy of Sciences, 2009; DOI: 10.1073/pnas.0809436106
So, no, it's not really a settled question.
Doug M.
I would like to settle the ‘Lithium and aggressive stellar behavior’ issue that I started in August 12, 2009 at 9:31 with ref. to the mentioned do Nascimento paper and others.
I have not been able to find references clearly and unambiguously indicating that lithium may itself be responsible for causing such undesirable stellar behavior. Indeed, I think now that I may have confused cause and effect, or rather, cause and indicator (see below). I usually try to be accurate in my quotes, but in this case mea culpa.
However, at the same time I think that the essential part of my argument still stands, namely: that abundant lithium is a good *indicator* of a young (solar type) star and/or a (solar type) star with greater mass. And it is these stars that are also known for emitting (much) more aggressive radiation, particularly UV-C and X-ray. See, besides the mentioned article, also the next thread ‘In Praise of K-Class’ Stars (https://centauri-dreams.org/?p=9032#comments) and in particular Guinan’s refenced article therein.
This may be the reason that astronomers nowadays attach so much value to lithium when identifying solar twins (see mentioned article ‘A Solar Twin with a Low Lithium Abundance’ above), and why they call solar twins with high lithium abundance ‘quasi-solar twins’.
High lithium may *bode* trouble for a solar type star, though not causing it.
http://www.technologyreview.com/blog/arxiv/24066/
Tuesday, September 01, 2009
Fog Discovered at Titan’s South Pole
The discovery is the first direct evidence of an Earth-like hydrological cycle.
We’ve suspected for many years that something remarkable is going on on Titan and yet the evidence to nail this conjecture has been strangely difficult to find.
The idea in question is whether Titan’s atmosphere actively shapes its surface, as occurs on Earth. There’s no shortage of evidence that hints at a complex, vibrant climate with rains that have carved streams and rivers into the surface, creating lakes and shorelines in the process.
But all this could be misleading, say Mike Brown at Caltech and a few pals. They put it like this:
“It is possible that the identified lake-features could be filled with ethane, an involatile long term residue of atmospheric photolysis; the apparent stream and channel features could be ancient from a previous climate; and the tropospheric methane clouds, while frequent, could cause no rain to reach the surface.”
The drizzle that astronomers have seen appears at such a high altitude that it probably evaporates before it hits the ground.
In this context, the importance of the discovery of fog at Titan’s south pole cannot be underestimated (coming courtesy of images from the Cassini spacecraft).
On Earth, fog can form in a number of ways but most of these mechanisms cannot work on Titan. “Fog on Titan can only be caused by evaporation of liquid methane,” say the team. “The detection of fog provides the first direct link between surface and atmospheric methane.”
And that’s important why? First because it’s evidence of a hydrological cycle in which an evaporating liquid on the surface enters the atmosphere. And second because it finally confirms Titan as an active meterological body in its own right.
Ref: arxiv.org/abs/0908.4087: Discovery of Fog at the South Pole of Titan
Titan’s Prolific Propane: The Cassini CIRS Perspective
Authors: C. A. Nixon, D. E. Jennings, J.-M. Flaud, B. Bezard, N. A. Teanby, P. G. J. Irwin, T. M. Ansty, A. Coustenis, S. Vinatier, F. M. Flasar
(Submitted on 9 Sep 2009)
Abstract: In this paper we select large spectral averages of data from the Cassini Composite Infrared Spectrometer (CIRS) obtained in limb-viewing mode at low latitudes (30S–30N), greatly increasing the path length and hence signal-to-noise ratio for optically thin trace species such as propane.
By modeling and subtracting the emissions of other gas species, we demonstrate that at least six infrared bands of propane are detected by CIRS, including two not previously identified in Titan spectra.
Using a new line list for the range 1300-1400cm -1, along with an existing GEISA list, we retrieve propane abundances from two bands at 748 and 1376 cm-1. At 748 cm-1 we retrieve 4.2 +/- 0.5 x 10(-7) (1-sigma error) at 2 mbar, in good agreement with previous studies, although lack of hotbands in the present spectral atlas remains a problem. We also determine 5.7 +/- 0.8 x 10(-7) at 2 mbar from the 1376 cm-1 band – a value that is probably affected by systematic errors including continuum gradients due to haze and also an imperfect model of the n6 band of ethane.
This study clearly shows for the first time the ubiquity of propane’s emission bands across the thermal infrared spectrum of Titan, and points to an urgent need for further laboratory spectroscopy work, both to provide the line positions and intensities needed to model these bands, and also to further characterize haze spectral opacity.
The present lack of accurate modeling capability for propane is an impediment not only for the measurement of propane itself, but also for the search for the emissions of new molecules in many spectral regions.
Comments: 7 Figures, 3 Tables. Typeset in Latex with elsart.cls. In press for Planetary and Space Science
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
DOI: 10.1016/j.pss.2009.06.021
Cite as: arXiv:0909.1794v1 [astro-ph.EP]
Submission history
From: Conor Nixon [view email]
[v1] Wed, 9 Sep 2009 18:43:05 GMT (313kb)
http://arxiv.org/abs/0909.1794
September 15, 2009
Titan’s Haze Acts as Ozone Layer
Written by Nancy Atkinson
Titan appears to be more like Earth all the time, and a new understanding of Titan’s hazy atmosphere could provide clues to the evolution of Earth’s early atmospheric environment and the development of life on our home planet.
Researchers have discovered a series of chemical reactions on Saturn’s largest moon that may shield the moon’s surface from ultraviolet radiation, similar to how Earth’s ozone layer works. The reactions may also be responsible for forming the large organic molecules that compose the moon’s thick and hazy orange atmosphere.
Scientists have long understood that high in Titan’s atmosphere, sunlight breaks apart methane into carbon and hydrogen. These elements react with nitrogen and other ingredients to form a thick haze of complex hydrocarbons which completely enshrouds the moon.
But recently, the role of polyynes in the chemical evolution of Titan’s atmosphere has been vigorously researched and debated. Polyynes are a group of organic compounds with alternating single and triple bonds, such as diacetylene (HCCCCH) and triacetylene (HCCCCCCH). These polyynes are thought to serve as an UV radiation shield in planetary environments, and could act as prebiotic ozone. This would be important for any life attempting to form on Titan.
Full article here:
http://www.universetoday.com/2009/09/15/titans-haze-acts-as-ozone-layer/