Life Around Cooling Stars

by Paul Gilster on November 27, 2012

Red dwarfs offer fascinating astrobiological speculation, allowing us to ponder whether flare activity or tidal lock could be the game-changer that prevents life from developing around them. We have much to learn on that score, but new work from Rory Barnes (University of Washington) and René Heller (Leibniz Institute for Astrophysics, Potsdam) looks beyond red dwarfs to brown and white dwarfs and their own prospects for life. The prognosis: Poor. Planets around these objects, the researchers say, would have an early history that could remove surface water.

The problem is nuclear burning and the lack thereof. Yes, both brown and white dwarfs could support a habitable zone, but what sets them apart from red dwarfs is that they cool slowly and continuously, meaning their habitable zones shrink inward toward the star. Imagine, Barnes and Heller say, a planet that starts out as a Venus-like world beset with a runaway greenhouse effect. Eventually the habitable zone contracts enough to create the needed temperatures for liquid water to exist, but by now the planet’s surface water is gone and so is the chance for life.

“These planets, if we find them today in a current habitable zone, previously had to have gone through a phase which sterilized them forever,” Barnes said. Heller added, “So, even if they are located in the habitable zone today, they are dead.”

The paper speaks in terms of the ‘insolation habitable zone,’ that region for which radiation flux from the star equals what we find in the habitable zone around main-sequence stars. But simply being in that zone is not sufficient. Here’s the case in a nutshell, as described in the paper:

The identification of this so-called insolation habitable zone (IHZ) is an important first step in constraining the possibility of habitable planets orbiting WDs and BDs, but one cannot neglect how a planet behaves prior to the arrival of the IHZ at its orbit. The inner edge of the HZ is defined to be the orbits at which a desiccating greenhouse, either moist, runaway or tidal, is just possible (Kasting et al. 1993; Barnes et al. 2012). These phenomena may ultimately remove all surface water and leave an uninhabitable planet behind. Thus, planets initially interior to the HZ may not actually be habitable after the primary has cooled and/or tidal heating has subsided sufficiently for the planet to reside in it. Note that we will refer to the HZ as resulting from both irradiation and tidal heating, but to the IHZ as the classic habitability model of e.g. Kasting et al. (1993).

The reference to tidal heating is inserted because the close proximity of the IHZ to the star raises concern that tidal heating effects could be in themselves strong enough to trigger the runaway greenhouse scenario, even for planets with extremely low eccentricities. But what of planets that enter the IHZ from a more distant orbit, jarred by gravitational instabilities in the system? Large eccentricities could still dish up enough tidal heating to sterilize the surface, but Barnes and Heller add that we would not be able to recognize such a history for a planet whose orbit had gradually been circularized, so there is still some possibility of life in such places.

Image: An artist’s impression of a massive asteroid belt in orbit around a star. Recent work with SDSS data shows that similar rubble around many white dwarfs contaminates these stars with rocky material and water. Could planets support life in the habitable zone around such stars? Credit: NASA-JPL / Caltech / T. Pyle (SSC).

Is this category of star likely to have planets in the first place? The researchers note that no terrestrial planets have yet been found around either white or brown dwarfs, but they may well exist. We do have observations suggesting gas giants around both classes of star — the paper on this work goes into the details — and there is evidence of protoplanetary disks around brown dwarfs that could form into terrestrial worlds, while some white dwarfs are known to have metal-rich disks. And we have the interesting example of KOI 55, which has two terrestrial planet candidates orbiting a ‘horizontal branch’ star that seems well on its way to white dwarf status.

More work is needed to determine whether planets can become habitable after long periods in a runaway greenhouse phase, and whether migration scenarios exist for white and brown dwarf planets that could result in habitable outcomes. The authors, noting the complexity of the analysis, add that they cannot rule out habitability but call the journey to it ‘a difficult path.’ The paper is Barnes and Heller, “Habitable Planets Around White and Brown Dwarfs: The Perils of a Cooling Primary.” I don’t see the preprint up on the arXiv site as yet but will post the link (and the full citation in Astrobiology) as soon as I have them. More in this University of Washington news release.

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lurscher November 27, 2012 at 14:27

Does this suggest that terraforming Venus would be pointless? is water being evaporated into space out of the Venus atmosphere?

Tarmen November 27, 2012 at 14:31

White dwarf systems. Not the best place to look for viral or bacterial life. Maybe good for robo sentinels or alien archeologists. Perhaps some artifacts of times gone by.

FrankH November 27, 2012 at 16:17

@lurscher – Venus lost all (or almost all) of it’s water billions of years ago. If you want water on Venus, you’ll have to bring it with you – from a glass to an ocean.

I think Jupiter can be a good substitute for a cooling brown dwarf system. The Galilean moons all show a progression, from waterless Io through icy, dead Callisto with one (Europa) and possibly two (+Ganymede) moons in between with sub surface liquid water oceans.

While most of that evolution (especially Io’s) is due to tidal interactions, is it possible that the Jupiter was putting out enough heat as it contracted to produce some of the gradient that we see in its largest moons?

Michael November 27, 2012 at 16:49

Asteriods and dust have been found to orbit White Dwarfs so why not comets coming in from afar distrupted by the Red giant phase to bring water to parched planets that survived? Our Solar system is surrounded by icy objects theirs should be no different.

Daniel Suggs November 27, 2012 at 17:08

lurscher, it seems that, if we had the technology to terraform Venus, we would be able to transport a small ice moon or asteroid in to build an ocean.

Wojciech J November 27, 2012 at 21:32

Daniel, if we had the ability to transport small ice moons, there would be no point in terraform Venus. You could build artificial disks, orbitals and stations with controlled biospheres and with much more living space(if you would need one at this point, which is doubtful)

Alex Tolley November 27, 2012 at 22:15

Isn’t there also the very small, but finite probability that life could arrive at an opportune time via panspermia?

Eniac November 27, 2012 at 23:40

@lurscher – Venus lost all (or almost all) of it’s water billions of years ago. If you want water on Venus, you’ll have to bring it with you – from a glass to an ocean.

Interestingly, the partial pressure of water in the atmosphere of Venus is larger than that on Earth: Wikipedia says 20 ppm water vapour. Venus’s atmosphere is 92 times denser than Earth’s, meaning if we found a way to get rid of the CO2, the sulfur dioxide, and some of the nitrogen, the air would be moist enough: About 2%, compared with Earth’s average of 1%. No oceans, though. Those contain 98% of the water on Earth, and presumably do not exist on Venus. There would only be enough water for the occasional lake.

Let’s find some photosynthetic critter that can live in the clouds, make do with the atmospheric water vapor, and will be all too happy to fix that pesky carbon, nitrogen and sulfur thickening up the air.

James Jason Wentworth November 28, 2012 at 2:09

A Jovian planet orbiting even a black dwarf cinder (a white dwarf star that has cooled to darkness) might still harbor life–although probably not intelligent life–in warm and wet levels of its atmosphere.

Michael November 28, 2012 at 13:07

@James Jason Wentworth,
There has not been enough time for Black Dwarfs to have formed.

andy November 28, 2012 at 19:07

White dwarfs may be a lost cause since the initial state is an extremely nasty environment in terms of luminosity and high-energy radiation. To me it seems that the only possibility for habitable planets there is a result of second-generation planet formation: produce the planets after the WD has cooled down to less destructive temperatures in a disc accreted from a mass-losing companion star. Mira B might be an example of this kind of process. I suspect that producing habitable planets this way would still require some very finely-tuned binary star properties though…

I’m more optimistic about brown dwarfs, they are presumably less abundant in high-energy radiation: unfortunately as is noted in the paper (pdf) the evolution of XUV/UV emission from brown dwarfs is pretty much unknown.

Eniac November 28, 2012 at 23:07

Oops, looks like I missed a decimal point there. The Venus “air” would be 0.2% water vapor, vs. 1% on Earth. Never mind….

James Jason Wentworth November 29, 2012 at 3:10

Michael wrote:

[@James Jason Wentworth,
There has not been enough time for Black Dwarfs to have formed.]

I’m not sure. Between spectral classes O and G, I can envision stars of just the right mass that might have “burned” their nuclear fuel quickly in the universe’s early days, expanded and blown off their outer layers without exploding, and left small enough white dwarfs that they could have cooled by now. Such objects, by definition (being dark and cold), would be hard to find. :-)

jkittle November 29, 2012 at 11:04

woj:
I agree. lets think about moving moderately sized asteriods and bagging them in a ” tent like” structure that then self inflates as volitiles evaporate . These would then also heat up in the sun and also provide some very minimal radiation shielding. Put the habitats a meter or so under the surface…. plenty of energy fr0m the sun ( solar cells on the surface of the tent/ bag) . Plenty of mass to play with and, depending on the choice of asteriod… plenty of materials to build and breath and grow food. Near enough to earth to share resources and communicate easily. Not in earth orbit so not in crowded space to worried about hazardous debris. easy to launch from with low delta vee. easy to land .. Sounds like home…

Rob Henry December 2, 2012 at 16:50

Eniac, Those figures still look interesting to me. The Cytherean atmosphere is a 1000 tons per square metre and that represents a challenge that could be too much for the terraforming option. 20ppm is still twenty litres per square metre, so if plant life was optimised to scavenge it, up to one or two hundred of tons of floating plant life per hectare is possible – in other words this is not a real limiting factor. My main worry is that there is too much sulphur, but the high cost of water might also make the (presumably) hydrogen filled flotation bladders expensive to continually refill.

Obviously, allowing collisions with a few small comets would make such schemes easier.

Eniac December 3, 2012 at 23:52

Rob, you are right, it is 20 liters per square meter, or 2 centimeters. If lakes cover 1% of the surface, they could be 2 m deep, on average. Of course, this is a very simplistic model, but I do think it demonstrates that the water situation on Venus, while dire, is not necessarily fatal. Maybe huge and expansive oceans like we have on Earth are overrated, and not at all necessary for life.

Add to that the fact that a few billion years ago the fainter sun might have made for much more temperate conditions on the surface, and I think that Venus should have a place on the expanding list of potential places for life.

Rob Henry December 5, 2012 at 4:43

Eniac, your further speculation is also interesting. One problem I have though, is that life does not evolve to an aerobiological habitat because other niches close, rather it evolves there because this niche is open. This is the case for Earth, so where are Earth’s whole-life-cycle examples?

Sure the, the air is fill with great numbers of fungal and bacterial spores, but have we really looked closely for those specialists. Some tropospheric “spores“ have no known counterpart on Earth‘s surface, and it seems that that shortfall is very high with stratospheric forms. Still what I really would expect if any such life had really taken hold, is for surface dwellers to occasionally be drenched in them when a cloud of them met a water cloud.

However, I favour a shockingly unorthodox answer to this mystery of this non-detection: this niche might already be filled. More shockingly, I suggest that it may turn out to me the major habitat of our hitherto undiscovered shadow biosphere. I present as evidence the red rain of Kerala. A couple of groups have claimed evidence for this growing at much higher temperature than any other known life, but I wonder if this is due to the high mineral scavenging potential (of even dead) spores, that each cell seems loaded with. Readable DNA has never been able to be extracted from these spores, so it seems most have lost interest. The unorthodox groups investigating hoped they had found alien life seeded from a comet that had air bursted nearby the first investigated fall, but this possibility seemed quashed when this particular event recurred in the same region several years later.

Sure, seven (and only 7) amino acids that are common with other terrestrial life have been found in them, but I can’t see how this proves them Earthly as some claim.

Eniac December 6, 2012 at 1:26

Rob,

If we can speculate that life arose at hydrothermal deep-sea vents on Earth, it is not out of the question that it could also evolve on the surface of Venus. Especially if that surface was once cooler than it is now.

Rob Henry December 7, 2012 at 0:15

A very good point Eniac, but it also emphasises the problem. Take another look at life on Earth, and then think of the following. Transitions metal ions are common in those hydrothermal environments, so has ribosomal life ever lost its extreme dependence upon them. Hint our mid ocean surfaces have about as much chlorophyll as deserts, despite plenty of carbon, nitrogen water, and sunlight. Perhaps a red herring, but I wonder?

Eniac December 7, 2012 at 23:52

Rob, You are speaking in riddles, sometimes. I assume you are referring to essential elements that are rare and limit the density of life? So what? Another reason life on Venus might have originated at the surface, and then spread to the atmosphere. If there were as much life in the Venusian atmosphere as there are Prochlorococci in the open ocean (I assume you mean that with “mid ocean surfaces”?), that would be phenomenal, but much less will also do.

The biggest problem would be the scarcity of water. These organisms would have to procure water from a very low partial pressure, similar to how Earth organisms procure CO2. It seems that , because water is the main constituent of cells and evaporates easily, the minimum density of surrounding water needed for a cell to survive may be much larger than that of CO2. But, maybe not…

In my view, the strongest evidence for the absence of photosynthetic life on Venus, apart from the aridity, is the presence of so much CO2. It seems that should have been used up long ago, like on Earth, if there was such life.

Rob Henry December 9, 2012 at 2:53

Eniac, if I have been speaking in riddles you must have solved them well. My only quibble is that you may have lost sight of those rare essential elements hardly being randomly distributed according to elemental abundance in any common modern surface habitat.

In our known ribosomal life much absorption is done over wet surfaces, and every system that transfers water internally uses osmosis. Perhaps the internal fluid would have to be something as incredibly hydroscopic as sulphuric acid for biology to work here.

Anyhow, my back of the envelope calculations indicate that a bacteria sized particle would only fall about 3m per day at 9g with one atmospheres pressure so perhaps no gas filled float is required after all, if those rare non-volatile elements can be scavenged from the infall of cometary dust. Perhaps the surprising longevity of those dark areas after Shoemaker-Levy 9 hit Jupiter are due to algal blooms after a massive infusion of rare essential elements?

Rob Henry December 9, 2012 at 5:30

Talking or riddles, what’s this life using up all Venus’ CO2 business?

Would photosynthetic carbon fixation that is rate limited by mineral deficiency be sufficient to equate to the atmospheric dose even in four billion years?
Would organics that fell towards the surface carbonise rather than disproportionate into simpler volatile compounds?
If they did O2 would also build up fast, so how would further falling organics be protected from burning in a few hundred degrees of heat?
If C reached the surface how could it be permanently sequestered without aid of plate tectonics?

In short, I can’t quite put you question in any context where I begin to answer it.

Rob Henry December 9, 2012 at 18:02

Eniac, I should do follow up calculations to illustrate the size of the problem that I see. I now give that for one of my above factors

Take that infall rate of non-volatile essential elements problem. If Venus has the same rate of collecting cosmic dust as the Earth, it gains 40,000 tons per annum. That is just 9*10-^8 kg/yr/square m. Thus if we allow 4 billion years of life, we must be confidant that 3000kg of carbon dioxide is permanently sequestered for every kg of cosmic dust fertiliser added to the system before we start worrying about your problem.

Eniac December 10, 2012 at 23:35

Rob

Talking or riddles, what’s this life using up all Venus’ CO2 business?

I hear you loud and clear. Maybe I am being naive, but to me CO2 is the natural atmosphere of rocky planets, and its fixation by photosynthesis is what makes it so rare on Earth. Call it a daring working hypothesis supported statistically by the three examples of such planets we have….

Let me attempt to hand-wave around a few of your excellent points:

Would organics that fell towards the surface carbonise rather than disproportionate into simpler volatile compounds?

I suppose they would. After you take away the volatiles, some carbon remains, as with coal and charcoal.

If they did O2 would also build up fast, so how would further falling organics be protected from burning in a few hundred degrees of heat?

O2 will oxidize all sorts of stuff other than carbon. It is not hard to imagine that it has better things to do than accumulate in the atmosphere and burn things.

If C reached the surface how could it be permanently sequestered without aid of plate tectonics?

It could just form deposits, as on Earth. I am not convinced plate tectonics is needed to bury things. Sedimentation will do, with the help of wind or water (the latter not on Venus, I suppose, at least not lately), as well as volcanic activity to turn the surface over.

Thus if we allow 4 billion years of life, we must be confidant that 3000kg of carbon dioxide is permanently sequestered for every kg of cosmic dust fertiliser added to the system before we start worrying about your problem.

Very few elements are more common in biomass than that (~ 1000 ppm). And cosmic dust is hardly the only possible source. Volcanic eruptions and dust storms being among possible mechanisms that can loft material from the surface. Besides that, very few elements are actually essential in the sense that they cannot be replaced by more available elements. Witness the alternative roles of iron and copper in the blood of vertebrates and insects, for example. Sulfur is very versatile, as is nitrogen. In a pinch, life might be able to make do completely without metals.

I like your idea about the utility of sulfuric acid for retaining water. Maybe this is why it is so common in the Venusian atmosphere? :-))

Rob Henry December 12, 2012 at 0:39

ENIAC You have far more confidence than me in the potential of life to substitute one transition metal for another once an ecosystem has first been robustly established in its original environment. Take nitrogenase. It is essential to the whole biosphere, yet only ever evolved once, despite our form being ultra sensitive to O2. It has an iron-sulphur-molybdenum centre. Now, it is true that vanadium has been swapped for molybdenum in some forms of nitrogenase, but the activity is so much less specific that this variant enzyme is retained beside the original only expressed if the cell sense very low Mo levels. Iron can also be swapped, but this is so much worse than either Mo or V that the only organisms that I use to know of that used it, retained all three forms.

Molybdenum is very rare, being about a ppm in the crust, and its comic molar abundance is half a million times less than silicon, yet it is frequently used in many essential enzymes with primordial roots. At a quick look through literature, I found a 0.5ppm dry weight given as the minimum for growing plants. If life on Earth was transferred to Venus (very similar to your scenario of transfer from the Cytherean surface), was perfect at scavenging minerals and Mo was the limiting one, I would thus not expect each kg of zodiacal dust to give no more than one or two kg dry weight. So now it becomes very important if volcanism on Venus can thrust particulate matter above the lower 30 km of atmosphere. If it does not (and I have a feeling that the vertical air flows on Venus are not extensive enough to allow it)

Rob Henry December 12, 2012 at 1:21

Eniac, you see signs that Venus is dead, but there something that jars with me much more in this statement than anything discussed so far. James Lovelock figured Mars was dead, because a ecosystem had a strong all-or-nothing character to him, and the disequilibrium it its atmosphere was so much less than Earth’s. However on Venus, this is not the case.
The combination of SO2, H2S is difficult to explain, and its carbonyl sulphide even harder (without invoking life)

Eniac December 13, 2012 at 0:04

Rob, according to Wikipedia: “All nitrogenases have an iron- and sulfur-containing cofactor that includes a heterometal complex in the active site (e.g., FeMoCo). In most, this heterometal complex has a central molybdenum atom, though in some species it is replaced by a vanadium [3] or iron atom.”

Which, I submit, supports my view of considerable flexibility in the use of metals, even in this case where there is only a single known family of related proteins supporting the function. For most other functions, there is more than one family, and the alternatives are often radically different in terms of both structure and required cofactors. Take photosynthesis, which in its modern form requires a magnesium complex. There is an alternative, presumably more ancient photosynthetic system still found in archaea: bacteriorhodopsin, which relies on retinal as a cofactor and uses no metals at all.

I still feel justified in my belief that most, if not all, cofactors are used in biochemistry because they are available and nice to have, not because of some unique property that makes them essential and irreplaceable.

Rob Henry December 14, 2012 at 1:26

Eniac, the central plateau of the North Island of New Zealand if referred to locally as “the desert” despite it receiving plenty of rain. Cobalt deficiency means that very few plants survive there, and what does grows slowly. Sure – it is not fair of me to equate the biochemical evolutionary potential of tracheophytes with bacteria but it sure makes me think (and the problem is JUST Co, since if you add tiny amounts of it to the soil without any other fertiliser the results can be spectacular).

Rob Henry December 14, 2012 at 18:44

Oh, I have just realised something that I did not make clear. This “a few times 40,000 tons per annum” is for my postulated maximum organic deposition rate, not a limit for biomass or the power of the ecosystem.

Even if the bacteria there are free falling, it should take about ten years for them to pass through the growing zone, so we could multiply that figure by ten. If it contains floating plants, that multiplier could be hundreds.

And, sure, if all Cytherean life is the producer sort, then they should allocate about half their energy to reproduction. This would limit the ecosystem to the 0.01 to 0.1 W/sqkm range. But if there were also consumer and decomposer types, activity would be much higher. My guess would be hundreds of times higher.

Actually, while I am writing this, I’m stating to question my use of Earth norms. Bacteria in fast growth phase can use 10,000 W/kg, but here I postulate that life still allocates 50% of its energy to reproduction. A rate of once per decade should generate about 0.05 W/kg. Considering how plentiful photosynthetic energy is, the amount necessary to create a lifting gas bladder starts to seem trivial.

Eniac December 14, 2012 at 23:18

Rob, what you say about cobalt is not entirely consistent with what it says here:

http://www.fao.org/ag/AGP/AGPC/doc/publicat/FAOBUL4/FAOBUL4/B402.htm

According to which cobalt is only indirectly needed by plants, and only be legumes, with the rhizobium (nitrogen fixing bacteria) actually wielding the cobalt for nitrogen fixation. So, if there is nitrogen nutrient, and for any plants other than legumes, cobalt is not needed. Perhaps this desert is lacking nitrates along with cobalt?

Rob Henry December 15, 2012 at 15:39

That’s interesting Eniac, but you should give cobalt its due. In all known life it is one of only 5 elements that form a direct covalent bond with carbon. In that narrow respect it exceeds even phosphorus. Cobalamin rules!

Rob Henry December 16, 2012 at 17:47

Earlier I postulated that the path to Cytherean life evolving a gas bladder is open and energetically favourable, but I have struck a problem. In CO2 at standard temperature and pressures 10x mean free path length of gas molecules is around bacterial size. At 10x mean free path length calculations cross from straight Brownian motion considerations predominating to Navier-Stokes dominating. Thus it may well be possible that a half inflated bladder might make bacteria actually fall faster due to its promotion of bulk flow! This is such a numerically tricky problem, that I will just ask if anyone has solved it.

Rob Henry December 16, 2012 at 18:53

I should give figures that illustrate the magnitude of my problem. A bacteria sized object reaching terminal velocity solely by molecular collision considerations falls 3m/day in Earths atmosphere near ground level. If calculated by normal drag considerations scaled down to bacterial size, we get 30km/day. I am not then just quibbling over trivia – this transition has massive effect.

Eniac December 17, 2012 at 0:11

I think we do not have to fret overly much about gravity in the bacterial realm. We see cloud droplets suspended in the sky nearly every day, and those are larger than most bacteria. Theoretical calculations be damned! (kidding, theorist myself …)

In my opinion the key problem is not suspension, but humidity. Can airborne bacteria exist that keep their innards together in the face of the low water vapor pressure that we find on Venus? I suspect the answer is not known, and the study of Earthly airborne bacteria might be extremely helpful in this respect. Do they exist at all? Is there a minimum humidity below which they are not found? Do they have a means of propulsion, like their cousins that live in the water?

Perhaps bulk water is overrated, and with the proper evolutionary pressure organisms can evolve that are more a conglomeration of proteins with some water for lubrication than the soup-like aqueous solution that we are used to dealing with. Or, perhaps sulfuric acid can substitute for water.

It is amazing and fascinating how much we do not know about life as we don’t know it!

Eniac December 17, 2012 at 0:14

Here is a pertinent quote from http://en.wikipedia.org/wiki/Bioaerosol:

Air often contains tiny organisms such as fungi, bacteria, mycotoxins and viruses. It is currently thought that the majority of these airborne microorganisms are not in a viable state while in the atmosphere. However, current research has shown that certain groups of bacteria are capable of performing basic metabolic activity within cloud water.[2] Groups of the small organisms clump up and enhance survival while airborne. Due to evaporation of water, bacterial cells usually die when they become airborne but under high humidity conditions bioaerosol levels are increased. Fungal cells such as spores, molds and yeast can be active at low humidity levels and high or low temperatures.

Rob Henry December 17, 2012 at 16:59

Eniac, note this bit in that article
“These particles are very small and range in size from less than one micrometer (0.00004″) to one hundred micrometers”
It then acknowledges much larger particles are present, but then posits that they drop out in “hours”

What you really want though aerobiota, as the term bioaerosol rather assumes that all life found in air is transient – and there has been way too much of that thinking already.

I suggest, that in order to explain life that has persistently been found by sampling our stratosphere, the true answer might be, as you suggested, that these microbes need to find a means of locomotion. I suggest that if several Earthly forms have evolved to spend there entire life in our air, that the energy considerations I mentioned above (ie growth limited by nutrient considerations, but have very high available energy) that even mechanisms that actively charge these bacteria are not out of the question if they enable them to dwell in their preferred zone for longer. To me, it seems natural that many would overshoot and give us those puzzlingly high stratospheric counts (that never seem fully explainable through volcanism).

Rob Henry December 17, 2012 at 20:28

Actually Eniac, the more I look at it the better your motility aspect to aerobiota looks. To cancel a fall of 3m/day requires just 0.0003 W/Kg. It must be difficult for an organism not much more than a wavelength across to sense the direction of sunlight, but if it does and propels itself towards the sun slightly more often than not, the problem could be solved even with huge inefficiencies.

If say it fires nitrogen molecules that has previously been absorbed from its dark side, with the power of one ATP, that would give an exhaust velocity of 1500m/s ( On Venus this would CO2 and 1200m/s), and if it has an excess over random in the upwards direction equivalent to 1% of its overall thrust, then it would only take about 0.1W/kg to do the trick.

Protozoa-sized organisms would have far greater ease finding “up”, so that even with falling four orders of magnitude faster, we might only be talking of 10W/kg (this may sound a lot to we humans with a 1W/kg metabolism, but some bacteria can have a metabolic rate of 10,000W/kg).

Rob Henry December 17, 2012 at 22:55

I have just twigged to something. By far the easiest way to counter this freefall problem is for a largely transparent organism to absorb most of its light on its underside, and to have a low heat conductivity across it. Now this mechanism is much easier for larger organisms, in fact so much better that it could, in principle, cope with their propensity to fall much faster…

Now if Earth has a major and highly evolve aerobiota system, then we might EXPECT many forms about 100 microns across, with a powerful ability to orient themselves. Here’s the kick: Clumps of bacteria and fungi have persistently been found in our stratosphere, where their rapid expected fall rate presents such a puzzle that it has been repeatedly claimed by some that this is prima facie evidence of a space-borne origin!

Eniac December 17, 2012 at 23:37

What you really want though aerobiota, as the term bioaerosol rather assumes that all life found in air is transient

Precisely. I just could not find anything on that. Perhaps because I did not think of the term?

In any case, either we are both completely off the rocker and it is obvious to anyone with any biological credentials that microbial life in the air is impossible, or there is a gaping hole in the body of biological research.

Eniac December 17, 2012 at 23:44

“These particles are very small and range in size from less than one micrometer (0.00004″) to one hundred micrometers”
It then acknowledges much larger particles are present, but then posits that they drop out in “hours”

As I recall, there is a transition from frictional to Brownian/diffusive motion as you decrease particle size (at around a few micrometers, I believe), and the particles at the lower end of the range are, for all practical purposes, not sinking at all. Their gravitational tendency to settle is counteracted by their entropic tendency to diffuse apart.

Rob Henry December 18, 2012 at 15:56

“As I recall, there is a transition from frictional to Brownian/diffusive motion as you decrease particle size (at around a few micrometers, I believe)…”

From what I have read it should be very close to 10x mean free path length.
http://web.mit.edu/ngh/www/DFD05paper.pdf
So, a few microns in the stratosphere, and about 0.5-1 microns in the troposphere.

“…and the particles at the lower end of the range are, for all practical purposes, not sinking at all”

And that sounds more like a way of thinking than reality. Sure if the surface of these particles has no adhesive properties, and bounces of the ground, it will distribute to infinity as if a gas. The problem is that the scale height of about 5 microns. Likewise, the averaging effect of having so much momentum in one spot compared to other particles, means that its random velocity as a particle is just 1m/day at any one instant, where as its directed fall rate due to average pressure differences on its surfaces is 3m/day.

Rob Henry December 18, 2012 at 16:30

Oops, I meant to write that that 3m/day downward velocity to which the Boltzmann distribution velocity rate of 1m/day (read as 10^-5 m/s)should be added, was LIMITED by pressure differences, not caused by them.

Stephen December 19, 2012 at 13:54

It seems to me that we have the physics and chemistry to fix Venus now. We currently have the physics required to move the orbit of Venus. Transfer orbital energy from Jupiter to Venus via big asteroid passes. Move big asteroids with little asteroids. Get Venus to spin faster with an impact or several. Create a planetary scale magnetic field with what, an electromagnet? Use a hydrogen blimp as a platform. Solar cells or a nuke plant to provide power to split CO2 or whatever down to atoms and do whatever we want with the results. Cool it down. Add water with impacts, maybe using icy moons of Saturn. We could save Venus from the Sun’s red giant phase. Earth and Venus could be comfy around the Sun’s white dwarf phase, and for quite awhile. There are probably other ways that will work for each of these tasks, and many would be cheaper. How about a relatively large moon around each planet to prevent tidal locking? I hear that Earth has one already. The biggest unsolved problem IMO is politics.

Rob Henry December 19, 2012 at 22:10

Oh, I should also have added a third and fourth factor to that, ‘permanent’ suspension being more just a suitable way of thinking than a reality.

In our troposphere, typical vertical air movements are much more rapid than that drop rate.

And, also in our troposphere, sub-micron particles rain out of the sky far more rapidly than gravity clears them. To me, this factor is a red herring here since, while some bacterial surfaces are exceptionally good at nucleating ice (then, presumably rain drops), others are bad at it.

Also note that neither factor applies to the presumptive life zone of Venus.

Eniac December 20, 2012 at 13:54

Also note that neither factor applies to the presumptive life zone of Venus

I am afraid I disagree.

I would submit that there surely are vertical winds on Venus, and that they are much more rapid than the drop rate for some suitably sized and shaped organisms. I would also submit that since there are clouds, nucleation would be just as much, if not more, a factor as here on Earth.

I am thinking that encouraging and discouraging nucleation could be one of the means by which floating organisms could regulate their altitude efficiently.

Rob Henry December 20, 2012 at 23:46

Well Eniac, when I look for figures it seems that the evidence is in your favour. I see that typical vertical air motions in the troposphere are about 1m/s.
http://journals.ametsoc.org/doi/abs/10.1175/2008JTECHA1240.1
That is four and a half orders of magnitude more than necessary to counter that drop rate. Such great currents are driven by the cycling of water and its refrigerant properties.

Thus even given that cloud formation on Venus would provide much less power, it still looks easy to make it work. Actually, this look better than any mechanism given by me so far.

So cloud formation and light absorption from high density bacterial communities might be the signs to look for.

Eniac December 21, 2012 at 14:39

I agree. We would expect organism populations to be most viable where there is a steady updraft. Such areas almost certainly exist, as planetary atmospheres tend to have fairly regular circulation patterns. If the populations were visible due to excess light absorption, we would expect dark patches against a white background, changing a bit according to weather patterns. It is doubtful that that would look any different from nonbiological color patterns caused weather, such as those observed on the gas giants. Still, definitely something to look for, especially if accurate absorption spectra could be taken and identified as coming from complex molecules.

Rob Henry December 22, 2012 at 0:13

“It is doubtful that that would look any different from nonbiological color patterns caused weather”

I keep thinking of Saturn’s weirdly stable hexagon.

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