Reference : arXiv:2404.05356v1 [astro-ph.EP]. Published on 8 Apr 2024
https://arxiv.org/pdf/2404.05356This is obviously a riposte to the claim, several years ago now, of phosphine in the radio spectrum of Venus' atmosphere. Which has been disputed, the instrument readings disputed, the noise profile challenged ... all the usual suspects.
Just from the title, it sounds like a discussion of the (theoretical) requirements, and potentially their observability, rather than actual new observations.
Now, here's weird - in some of the Blogger Preview modes, ":hover" doesn't work. Or is there something else going on? It's a problem with their preview system. I hadn't noticed that before.
Sections
| Abstract |
| I. Introduction |
| II. LIFE CYCLE FOR VENUSIAN AERIAL MICROBES |
| III. REPLICATION RATES AND FALLOUT TIMES |
| IV. COSMIC RAY EFFECTS ON MICROBIAL LIFE |
| V. CONCLUSIONS |
| End of document |
Abstract
Millimeter-waveband spectra of Venus from both the James Clerk Maxwell Telescope (JCMT) and the Atacama Large Millimeter/submillimeter Array (ALMA) provide conclusive evidence (signal-to-noise ratio of about 15σ) of a phosphine absorption-line profile against the thermal background from deeper, hotter layers of the atmosphere. Phosphine is an important biomarker; e.g., the trace of phosphine in the Earth’s atmosphere is uniquivocally associated with anthropogenic activity and microbial life (which produces this highly reducing gas even in an overall oxidizing environment). Motivated by the JCMT and ALMA tantalizing observations we reexamine whether Venus could accommodate Earthly life. More concretly, we hypothesize that the microorganisms populating the venusian atmosphere are not free floating but confined to the liquid environment inside cloud aerosols or droplets. Armed with this hypothesis, we generalize a study of airborne germ transmission to constrain the maximum size of droplets that could be floating in the venusian atmosphere and estimate whether their Stokes fallout times to reach moderately high temperatures are pronouncedly larger than the microbe’s replication time. We also comment on the effect of cosmic ray showers on the evolution of aerial microbial life.
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So, one useful point - that airborne life is more likely in droplets, rather than actual free-floating microbes. Fair point. From which, settling velocities are an approachable topic, while the supply of minerals from the ground isn't so approachable - needs considerably more assumptions. The question of vertical mixing in the atmosphere should make an appearance too. Note: the Wiki atmosphere composition given below asserts significant ferric chloride as a component, which would be an important mineral.
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I. Introduction
Their citations for early discussion of Venus-life as cloud-life starts with "H. Morowitz and C. Sagan, Life in the clouds of Venus?, Nature 215, 1259 (1967) doi:10.1038/2151259a0" - which shouldn't really be a surprise. Sagan gets everywhere.
And they go straight into modelling a "sHigo" (spherical Hydrogen isopyenic gasbag organism) and, with reasonably conservative assumptions get a minimum buoyant size of ~4cm (diameter). That's not insane for a multicellular organism, but a bit much from terrestrial experience of microbes. It also sort of (to me) implies an origin on the ground, getting lofted (evolving into buoyancy) as the environment went from Hadean era (with solar illumination ~20% down on today) to triggering the runaway greenhouse and boiling the oceans. But I'll leave that aside for the time being.
RETURN TO THIS
Refereences [4] through [12] cover the controversy about the original detection claim, and raised concerns about the calibration and interpretation of the signals. Clearly these authors feel that the issues raised have bene answered, and the phosphine detection can be treated as valid.
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II. LIFE CYCLE FOR VENUSIAN AERIAL MICROBES
This seems to be a re-hash and expansion of :
[14] S.Seager, J.J.Petkowski, P.Gao, W.Bains, N.C.Bryan, S.Ranjan, and J.Greaves, "The venusian lower atmosphere haze as a depot for desiccated microbial life: A proposed cycle persistence of the venusian aerial biosphere." Astrobiology 21, 1206 (2021) doi:10.1089/ast.2020.2244 [arXiv: 2009.06474]
This modle life cycle is predicated on the atmosphere of Venus. Only two references are given, but the structure of the atmosphere has been probed by multiple landers and tested by various combinations of radar from Earth and from (Venus) orbit, so it's not much in dispute.
I'm in the habit of collecting such bits of data on the expectation that I'll need them again. So ... I'll do exactly that, and put the information below. Wait - what's this - from Wiki : Additionally, the clouds consist of approximately 1% ferric chloride. Other possible constituents of the cloud particles are ferric sulfate, aluminium chloride and
Well, that's a thing I hadn't considered when making my "supply of minerals" comment above. Hmmm.
phosphoric anhydride. Altitude (km) Temperature (K) Pressure (Pa) 0 735.16 9332032.5 5 697.16 6753311.25 10 658.16 4801791.75 15 621.16 3347778 20 579.16 2281839 25 537.16 1512782.25 30 495.16 998152.575 35 453.16 599540.025 40 416.16 354738.825 45 383.16 200522.175 50 348.16 108012.45 55 300.16 53844.105 60 263.16 23882.3025 65 243.16 9894.38625 70 230.16 3738.8925 80 197.16 482.307 90 169.16 37.85502 100 161.16 2.695245
Well, let's find some pressure-temperature-altitude data. Ah, good, Wiki has done the searching for me. But ... it seems difficult to generate a chart (in LibreOffice) with multiple ranges for the X-axis, and a common factor for the Y axis. So - subterfuge. Including "tweaking" the drawing in LO.Draw. Not perfect, but it'll do for the moment.
Clearly, I've forgotten the details of Blogger's floating of elements. I'm trying to put the data table beside the graph, but that's not working. I thought I'd figured that out a while ago, but I'll have to work on it again.
Updating on 2024-05-19.
The figures in the "Temperature" column have spurious accuracy. Originally they were temperatures in °ree; Celsius at , but I've conmverted them to Kelvin, which produces about 3 seemingly significant digits.
Maybe I'll do better plotting this with GnuPlot? A project.
I need to add a line for "Earth sea level" pressure (100.0e+03 Pa) and temperature (288.16 K, like I said, "spurious accuracy"). I also need to annotate my diagram with the cloud levels and temperatures for the cycling. Or should I just copy the paper's Fig 1?
I seem to have borked the blog style. That approach at setting up "previous" and next" post-links. I knew I'd regret it.
Lower limit ~45km altitude / pressure ~1.2 bar (~120kPa) / 100°ree;C (~370 K) ; upper limit 60km / 0.2 bar (20 kPa) / 0°ree;C (~270 K). Temperatures and presures well within the range of biological activity on Earth. The visible cloud top moves through this region at different levels at different latitudes. Aerosol droplet sizes vary between around 0.1µm near the lower boundary to up to 4µm towards the upper parts of that range - reasonably realistic for bacteria.
Low in the atmosphere (within the range of the topography) is a low level "haze", with temperatures and pressures rising rapidly out of the (terrestrial) biological window.
A 5-stage life cycle is envisaged :
| Stage | Description |
|---|---|
| 1 | The cycle begins with dormant desiccated spores (black blobs in Fig. 1) which partially populate the lower haze layer of the atmosphere. |
| 2 | Updraft of spores transports them up to the habitable layer. |
| 3 | Shortly after reaching the (middle and lower cloud) habitable layer, the spores act as cloud condensation nuclei, and once surrounded by liquid (with necessary chemicals dissolved) germinate and become metabolically active. |
| 4 | Metabolically active microbes (dashed blobs in Fig. 1) grow and divide within liquid droplets (shown as solid circles in the figure). The liquid droplets grow by coagulation. |
| 5 | The droplets reach a size large enough to gravitationally settle down out of the atmosphere; higher temperatures and droplet evaporation trigger cell division and sporulation. The spores are small enough to withstand further downward sedimentation, remaining suspended in the lower haze layer (a depot of hibernating microbial life) to restart the cycle. |
The authors are using "spore" to designate a resistant, inactive life stage specifically light enough to be suspended in the atmosphere in the upper parts of the lower atmosphere. They're also using "hibernate" where "estivate" (resting through the "summer" might be more appropriate.
One thing I don't see is consideration of suunlight intensity. With about twice terrestrial illumination intensity at the cloud tops (Venus orbits at about 0.723 AU, so has an irradience of 1.912 that of Earth) that should be good, but 15km depth of cloud around terrestrial troposphere pressure will make for a lot of light absorbtion by the time you get to the lower clouds.
The Venera probes took photos. Someone knows what the illumination at the surface level is. From Wiki, although Venus is closer than Earth to the Sun, it receives less sunlight on the ground, with only 10% of the received sunlight reaching the surface, resulting in average daytime levels of illumination at the surface of 14,000 lux, comparable to that on Earth "in the daytime with overcast clouds".[68] although Venus is closer than Earth to the Sun, it receives less sunlight on the ground, with only 10% of the received sunlight reaching the surface,[67] resulting in average daytime levels of illumination at the surface of 14,000 lux, comparable to that on Earth "in the daytime with overcast clouds".
OK, that will have an effect on photosynthetic yields, but not a show-stopper.
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III. REPLICATION RATES AND FALLOUT TIMES
Wrapped up in some entropic justification, and a list of quite variable "generation times" for various bacteria - they choose the old standby of a 20 minute generation time. Yeah, that might be true in aerobic, comfotable temperature, well-fed nutrient broth. It seems optimistic to me for something floating in a gas, depending on meeting aerosol droplets for most of it's chemical inventroy. They go for a 12 hour settling time, giving a potential 2(3×12) growth factor in that settling time. I distrust that 20min fuigure as being representative. Achievable, certainly, but as an average?
OK - I did the calculations, The mean of their cited dataset is 163 minutes. But the harmonic mean is 25-3 minutes. Maybe it's not such a bad extimate after all - but it's throwing away a lot of variability.
Various calculations ensue about the terminal velocity of these drops. The come to the conclusion that it's not a big problem - which we knew already because there is a cloud layer.
My thoughts
I think they're being a bit optimistic about their growth rates. The rates they're assuming are some of the fastest - not just that I've heard of, but also of their own examples. In reality, only a short part of each cycle will be spent near "optimum conditions" (whatever they are), so the average growth rates will be considerably lower.
Their calculation of fallout speeds takes no account of the "gasbag" part of their organism's name. Even a fairly modest "gasbag", supporting only half the organism's mass , would halve the settling force, while approximately doubling the drag forces. The buoyancy would be unbalanced though - sinking slightly below optimum pressure level would compress the "gasbag", accelerating the drift down, while rising above the optimum level would increase the lifting forces and accelerate the drift from optimum. The situation will be familiar to anyone who has tried scuba diving with a buoyancy compensator or "ABLJ" - maintaining your level in the water column - particularly at a decompression stop depth - can be really tricky. But it can be done. For example, fish (teleosts, those with a swim bladder) use two buoyancy systems - a major one using liquids of different density to the water, and a smaller one using gases with a much greater pressure- volume feedback as described above. That works for fishes in water - it's harder for "gasbags in gas". But not being a balloonist, I can't talk to that from experience.
I'm pretty dubious of their estimated "dry" density for E.coli (and by extension, other organic materials) 300 kg/cu.m is quite low. But I haven't read ahead yet to see where they're going with that.
They come out concluding that settling velocities are usefully long for droplets/ gasbags below about 0.1mm ... which we knew already from the overved persistence of water droplet clouds.
So what was the point of that digression?
IV. COSMIC RAY EFFECTS ON MICROBIAL LIFE
Again, I struggle to see the importance of this section. By definition, we're talking about organisms spending most of their life cycle at pressures similar to on the Earth's surfce. So, above the putative organism is a similar mass of atmosphere ("shielding") to an organism on Earth. Yes, radiation is important, but also, yes terrestrial organisms exhibit over a thousand-fold difference in radiation resistance (e.g. Deinococcus radiodurans) … so, again, wile it's important to state your assumptions, we already know the cosmic ray conditions at the appropriate levels on Venus aren't going to be wildly exclusionary.
They do make an interesting point about the conductive (UV-ionised) ionosphere providing a local magnetic field via generation of eddy currents, mitigating to a degree the absence of a geomagnetic (veneromagnetic?) field. But only up to a few hundred keV particle energy - and only (obviously) for charged particles. Which isn't as much protection as Earth's magnetic field provides, but helps to reduce the aparrent differences between Earth's surface and the Venusian cloud layers.
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Adding these effects together,
numerical simulations show that cosmic radiation would not have had any hazardous effect on putative microorganisms within the potentially temperate zone (51 to 62 km)
Which is where I started in my consideration of this section. Good to see that we agree, by rather different route.
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V. CONCLUSIONS
we can conclude that if updraughts exist, a stable population of microorganisms that in the early history of Venus emigrated from the surface to the atmospheric clouds and now remain confined to aerosols may be possible.
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The caveat about "if updraughts exist" is important. The surface topography of Venus varies by 5-8 km, while the Earth-comparable zones are 30~50km above this agitation, so it's not clear from first principles that there would be much topographically-induced vertical updraughts.
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