Yeah, That's a Freakin' Lava Lake on a Moon
56 minutes ago
Saturday, October 10, 2020
Roko's Basilisk in the Form of a Metal Song
When Metallica writes a song about the singularity, it's probably close. "Stop breathing, and dedicate to me - stop dreaming, and terminate for me."
Friday, October 2, 2020
Prediction: Venusian Phosphine is a Metabolic Product of Living Cells Already Detected As Unknown Absorbers
The last two years have provided us with the strongest evidence ever assembled of extraterrestrial life:
Here I propose that Venus had an iron-sulfur ecosystem with a chlorophyll-equivalent that absorbs closer to the UV spectrum rather than visible light - essentially, "UV-synthetic" Venusian cyanobacteria. The oceans boiled away and Venus became hotter and more acidic from volcanism and possibly, their own Great Sulfuration (or Sulfur Oxidation, equivalent to Earth's Great Oxygenation.) The only survivors were the UV-synthetic Venusian archaebacteria that now live in the upper atmosphere. Today these have a life cycle like that described by Seager et al (2020), powered by UV and producing phosphine - Unknown Absorber Phosphine Producers (UAPPs.) They are likely related at great time depths to life on Earth. Initial research question is to see if areas of unknown absorbers correlates with phosphine, which can be done from Earth. Probes that collect material in the upper atmosphere could fairly straightforwardly check for aspects of biochemistry using an onboard instrument, and a sample return mission could be extremely productive.
Phosphine Production in the Clouds of Venus
If you're reading this you likely know that phosphine (PH3) was detected in the atmosphere of Venus - Vox explainer here; original paper by Greaves et al here. The measured concentrations are at biology-consistent levels, at an elevation where the pressure and temperature are similar to Earth's. This is by far the strongest evidence of extraterrestrial life yet discovered, with evidence from multiple sources.
Phosphine has been advanced as a possible seed compound delivered to Earth on comets or asteroids early in its history. But the chemistry of its formation in space (or on gas giants) is not mysterious. It's in the Venusian atmosphere where so far we can't explain its presence without some process that continuously replenishes it. One criticism of speculation about possible Venusian biochemistry is that just because we don't know how to make phosphine under Venusian conditions, doesn't mean we're looking at alien biology. True; but among these criticisms have not been any suggestions so far about what it might be. (Either way, we're about to learn something.) It's suggestive that this data is not completely unexpected - it can be fitted to prior hypotheses. We've been speculating more and more concretely for decades about how life might survive in the atmosphere of Venus for decades (see Morowitz and Sagan 1967.) A fairly elaborated model of microbial life in the atmosphere of Venus was advanced recently by Seager et al, consistent with observations so far. This should also increase our confidence in the Venusian-cloud-life hypothesis, that even before phosphine was detected, Sousa-Silva et al suggested phosphine as a biosignature molecule, independent of finding it on Venus.
A Related Mystery? The Unknown Absorbers
In visible light and false-color UV absorption. It's unusual to have such contrast in absorption at different wavelengths. Image credit syfy.com
For decades we have known that there are partciles about 10^-6 meters (the size of bacteria) in the Venusian atmosphere at a similar altitude (at 47 to 64km) as the phosphine detection above (at 57km and above). The dark bands we can see with the naked eye in the Venusian atmosphere contain more of them, but as you can see above in the UV image, they are much higher contrast (more absorbant). As with the origin of Venusian phosphine, the identity of the absorbers remains controversial, and Venusian biology had been advanced previously as a candidate explanation (Limaye et al 2018). The phosophine paper points out that there is more phosphine at mid-latitudes than the equator or poles, which by naked-eye examination of images of Venus, seems also to be where the absorbers are. It seems a relatively straightforward study to correlate the two, but as the absorbers move on a scale from minutes to days, data would have to be collected simultaneously. The stronger the correlation (especially within the same latitude) the more our confidence in the UAPP hypothesis of Venus cloud life would be increased.
What About Bacterial Life in Earth's Cloud Decks?
Earth's clouds do indeed contain lots of bacteria, and not just incidentally - some of them clearly evolved to take advantage of the precipitation cycle and indeed to deliberately cause ice to enucleate around it, like Pseudomonas syringae (this is actually economically relevant as the water ice-enucleation proteins produced by this species is used in the water fed into snow guns at ski resorts.) Bacteria have been found all the way up to 28 miles above the surface, where the pressure and temperature are both much lower and considerably less hospitable even to Earth's own life than the cloud decks on Venus. While we can't say there is an actual bacterial ecosystem in Earth's clouds (one which persists without interacting with the surface), we haven't really looked for one either; most of our interest in these organisms thusfar comes from studying plant pathogens that spread through weather events. It's worth pointing out that there is phosphine in Earth's upper atmosphere as well, with no clear mechanism for how it forms there. It should be noted that there is less in Earth's upper atmosphere by about 3 orders of magnitude; the levels in Venus's atmosphere are more similar to that found immediately around actively metabolizing bacteria on Earth's surface.
Toward an Evolutionary History of Venus
Why would life exist on the most hellish world in the solar system? The answer is that for at least 75% of its lifespan, Venus was a much more Earth-like planet with cooler temperatures and oceans.
There are two possible, not mutually exclusive stories that explain how this planet came to be the Venus we know today.
The first is that Venus was a little too close to the Sun, which caused its oceans to evaporate, plate tectonics to cease, and subsequent cataclysmic volcanism. As the oceans evaporated, the water vapor trapped the heat and accelerated the process. The deuterium/hydrogen ratio on Venus is about 150 times higher than Earth, where comets have at most a 3 times higher ratio than Earth, suggesting a very gradual loss to space of hydrogen from water and preferential retention of the heavier nucleus. Water lubricates plate tectonics, per Solomatov 2001. Climate modeling suggests that Venus may have had a habitable climate with liquid water at the surface until 715 MA ago (Way et al 2016.) The subsequenct evaporation of the oceans resulted in a planet where plate tectonics ground to a halt, and with no crustal mechanism to dissipate heat, and finally between 700 and 500 MA ago, Venus erupted in planet-wide massive flows that resurfaced the planet, utterly dwarfing any similar events on Earth (like the Siberian Traps.) This released the massive amounts of sulfur that we see today. This is the received wisdom and could entirely explain the modern state of Venus, and may alone be enough to explain all the sulfur.
There is another version of the story which reverses the causality - eruption causing evaporation, advanced by Way and Del Genio in 2019. It's worth noting that Venus has a thicker crust than Earth, owing to its lack of a large moon; therefore we should expect that the flows, when they do finally cause the crust to fail, are much stronger than in the parallel situation on an evaporated Earth.
The second possibility is obviously more speculative, a parallel to the Great Oxygenation in the history of life on Earth. In Earth's history, anaerobic cyanobacteria produced so much oxygen that they effectively poisoned themselves, but also set the stage for aerobic life. This could have been a great coincidence - there may just have happened to be genes close enough in design space to assemble oxyidation defenses and an aerobic metabolic pathway, and without such a coincidence, that may have been the end of life on Earth, or it may have settled into a simple bloom-and-bust oscillation as our bacterial mats may have for hundreds of millions of years evidenced by banded iron formations found in ancient rocks where they persist at the surface. (See discussion of endogenous extinctions here, which this section partly recapitulates.)
While an interesting idea, by Occam's razor we should spend no further time considering a possible Great Sulfuration, as we can explain the death of the Venusian surface ecosystem entirely based on abiotic meteorological and geological processes as above. It's also the case that the presence of increased CO2 relative to Earth can be easily explained by abiotic processes as well. Using ingenious reasoning about the necessary atmospheric pressure for flying dinosaurs' wings to function as well as the known rates of deposition of CO2 as carbon in continents and the ocean, we can arrive a figure of the equivalent of 85-100 bars' worth of CO2 trapped in the Earth's crust, similar to what is currently in the Venusian atmosphere. Presumably the atmospheric pressure of Venus was lower during its oceanic period owing to the same process, and rose subsequent to the evaporation, but I am not aware of any modeling retrodicting from oceanic evaporation 500-700 MA ago to the current pressure and mass of CO2 on Venus.
All this is to say that life on Venus may have gone a different way, but started quite similarly. We're now fairly confident the first metabolism on Earth was sea vent iron sulfur organisms, using sulfur in what is now oxygen's chemical role. The Great Oxygenation may have only happened when it did, a full 1.5 billion years after the first life and at least 800 million years after photosynthesis appeared, because an asteroid delivered molybdenum, allowing nitrogen fixation and more efficient anaerobic metabolism. Whatever the reason, had this happened prior to photosynthesis, we may have ended up with an Earth poisoned with sulfur or at least with a massive amount of oxidized sulfur.
Two Obvious Problems for the "UAPP Cells" Hypothesis for Life on Venus
There are two major hurdles to overcome in any argument that there is life in the cloudtops of Venus. The first is the question of how life operates without water, or with very little water; this would actually be a more stunning find than merely life which can tolerate high acidity! The second is the failure thusfar to detect any organics in the atmosphere. Without water and organic molecules, it's very hard to see how this won't end up being an interesting abiotic route to phosphine production along with some crystal we weren't anticipating at that altitude. That said, organic compounds on Venus may not be as unlikely as one might think - there was a Venusian equivalent of the Miller-Urey experiment performed, where under conditions of the Venusian atmosphere, organic compounds including amino acids were produced.
Furthermore, there remain arguments for an abiotic explanation for the unknown absorbers, specifically ferric chloride (Petrova 2018). Interestingly, this is partly advanced to explain another mystery which is the presence of rainbows ("Venus glory"), first observed in 2014 in the Venusian atmosphere.
Implications for Evolution in General and the Future of Life of Earth
It is more likely than not that life on Venus will be distantly related to life on Earth. A massive amount of material has been transferred between bodies in the solar system, with actual numbers calculated here; at that same link you will see reference to the survival of uncontrolled re-entry during the Columbia crash by not just bacteria, but animals (C. elegans worms, found alive on the ground weeks after the crash.) This is actually the more boring possibility, because we would learn much more about the basic principles of evolution and the possibilities of biochemistry beyond Earth's provincial commitments, if we really had a novel origin. Either way, if there is life on Venus, the likelihood of life on Mars, Europa, Enceladus and even Titan jumps dramatically, even if it's "just" a long-lost relative. I expect that ultimately the impact of finding life on Venus will be some neat new biochemistry (the old extremophiles will seem quaint) and a bit more information about how evolution can proceed.
It is unclear how we should feel about Venusian cloud UV-cyanobacteria in terms of the Great Filter, which suggests that the more life we find in the universe and the closer in terms of evolutionary stage to humans, the more concerned we should be - because the more likely our own extinction is before we can colonize planets beyond our own. If further exploration of Venus yields trilobites or vertebrates and these cells are all that are left, we should worry much more. In contrast, if Venus never got past vast floating bacterial mats (either in its clouds or ancient oceans). that's a bit more comfortable for us.
REFERENCES
Bains W, Petkowski J, Sousa-Silva C, Seager S. Trivalent phosphorus and phosphines as components of biochemistry in anoxic environments. Astrobiology 19, 7 (July 2019): p. 885-902 doi 10.1089/AST.2018.1958
Glindemann D, Edward M, Kuschk P. Phosphine gas in the upper troposphere. Atmospheric Environment Volume 37, Issue 18, June 2003, Pages 2429-2433
Greaves JS, Richards AMS, Bains W, Rimmer PB, Sagawa H, Clements DL, Seager S, Petkowski JJ, Sousa-Silva C, Ranjan S, Drabek-Maunder E, Fraser HJ, Cartwright A, Mueller-Wodarg I, Zhan Z, Friberg P, Coulson I, Lee E, Hoge J. Phosphine gas in the cloud decks of Venus. Published: 14 September 2020. Nature Astronomy (2020)
Levenspiel O, Fitzgerald TJ, Pettit D. Was the Atmospheric Pressure Different at the Time of Dinosaurs? Chemical Innovation, December 2000 Vol 30, No.12, 50 – 55
Limaye SS, Mogul R, Smith DJ, Ansari AH, Słowik GP, Vaishampayan P. Venus' Spectral Signatures and the Potential for Life in the Clouds. Astrobiology. 2018 Sep 1; 18(9): 1181–1198. Published online 2018 Sep 12. doi: 10.1089/ast.2017.1783
Morowitz H & Sagan C. Life in the Clouds of Venus? Nature volume 215, pages1259–1260(1967). 16 September 1967.
Otroshchenko V.A., Surkov Y.A. (1974) The Possibility of Organic Molecule Formation in the Venus Atmosphere. In: Oró J., Miller S.L., Ponnamperuma C., Young R.S. (eds) Cosmochemical Evolution and the Origins of Life. Springer, Dordrecht. https://doi.org/10.1007/978-94-010-2239-2_40
Petrova EV. Glory on Venus and selection among the unknown UV absorbers. Icarus Volume 306, 15 May 2018, Pages 163-170
Seager S, Petkowski JJ, Gao P, Bains W, Bryan NC, Ranjan S, Greaves J. The Venusian Lower Atmosphere Haze as a Depot for Desiccated Microbial Life: A Proposed Life Cycle for Persistence of the Venusian Aerial Biosphere. Astrobiology. Published Online:13 Aug 2020. https://doi.org/10.1089/ast.2020.2244
Sousa-Silva C, Seager S, Ranjan S, Petkowski JJ, Zhan Z, Hu R, Bains W. Phosphine as a Biosignature Gas in Exoplanet Atmospheres. AstrobiologyVol. 20, No. 2. Published Online:31 Jan 2020 https://doi.org/10.1089/ast.2018.1954
Way MJ, Del Genio AD, Kiang NY, Sohl LE, Grinspoon DH, Aleinov I, Kelley M, Clune T. Was Venus the First Habitable World of our Solar System? Geophysical Research Letters. First published: 11 August 2016 https://doi.org/10.1002/2016GL069790
- Prior theories about relic ecosystems surviving in the more Earth-like parts of Venus's atmosphere.
- Detection of UV absorbers the size of bacteria in Venus's atmosphere, with no explanation as to their identity.
- Prior, independent advancement of phosphine as a biosignature gas.
- Detection of phosphine in the Venusian cloud decks with no explanation for its persistence.
Here I propose that Venus had an iron-sulfur ecosystem with a chlorophyll-equivalent that absorbs closer to the UV spectrum rather than visible light - essentially, "UV-synthetic" Venusian cyanobacteria. The oceans boiled away and Venus became hotter and more acidic from volcanism and possibly, their own Great Sulfuration (or Sulfur Oxidation, equivalent to Earth's Great Oxygenation.) The only survivors were the UV-synthetic Venusian archaebacteria that now live in the upper atmosphere. Today these have a life cycle like that described by Seager et al (2020), powered by UV and producing phosphine - Unknown Absorber Phosphine Producers (UAPPs.) They are likely related at great time depths to life on Earth. Initial research question is to see if areas of unknown absorbers correlates with phosphine, which can be done from Earth. Probes that collect material in the upper atmosphere could fairly straightforwardly check for aspects of biochemistry using an onboard instrument, and a sample return mission could be extremely productive.
Phosphine Production in the Clouds of Venus
If you're reading this you likely know that phosphine (PH3) was detected in the atmosphere of Venus - Vox explainer here; original paper by Greaves et al here. The measured concentrations are at biology-consistent levels, at an elevation where the pressure and temperature are similar to Earth's. This is by far the strongest evidence of extraterrestrial life yet discovered, with evidence from multiple sources.
Phosphine has been advanced as a possible seed compound delivered to Earth on comets or asteroids early in its history. But the chemistry of its formation in space (or on gas giants) is not mysterious. It's in the Venusian atmosphere where so far we can't explain its presence without some process that continuously replenishes it. One criticism of speculation about possible Venusian biochemistry is that just because we don't know how to make phosphine under Venusian conditions, doesn't mean we're looking at alien biology. True; but among these criticisms have not been any suggestions so far about what it might be. (Either way, we're about to learn something.) It's suggestive that this data is not completely unexpected - it can be fitted to prior hypotheses. We've been speculating more and more concretely for decades about how life might survive in the atmosphere of Venus for decades (see Morowitz and Sagan 1967.) A fairly elaborated model of microbial life in the atmosphere of Venus was advanced recently by Seager et al, consistent with observations so far. This should also increase our confidence in the Venusian-cloud-life hypothesis, that even before phosphine was detected, Sousa-Silva et al suggested phosphine as a biosignature molecule, independent of finding it on Venus.
A Related Mystery? The Unknown Absorbers
In visible light and false-color UV absorption. It's unusual to have such contrast in absorption at different wavelengths. Image credit syfy.com
For decades we have known that there are partciles about 10^-6 meters (the size of bacteria) in the Venusian atmosphere at a similar altitude (at 47 to 64km) as the phosphine detection above (at 57km and above). The dark bands we can see with the naked eye in the Venusian atmosphere contain more of them, but as you can see above in the UV image, they are much higher contrast (more absorbant). As with the origin of Venusian phosphine, the identity of the absorbers remains controversial, and Venusian biology had been advanced previously as a candidate explanation (Limaye et al 2018). The phosophine paper points out that there is more phosphine at mid-latitudes than the equator or poles, which by naked-eye examination of images of Venus, seems also to be where the absorbers are. It seems a relatively straightforward study to correlate the two, but as the absorbers move on a scale from minutes to days, data would have to be collected simultaneously. The stronger the correlation (especially within the same latitude) the more our confidence in the UAPP hypothesis of Venus cloud life would be increased.
What About Bacterial Life in Earth's Cloud Decks?
Earth's clouds do indeed contain lots of bacteria, and not just incidentally - some of them clearly evolved to take advantage of the precipitation cycle and indeed to deliberately cause ice to enucleate around it, like Pseudomonas syringae (this is actually economically relevant as the water ice-enucleation proteins produced by this species is used in the water fed into snow guns at ski resorts.) Bacteria have been found all the way up to 28 miles above the surface, where the pressure and temperature are both much lower and considerably less hospitable even to Earth's own life than the cloud decks on Venus. While we can't say there is an actual bacterial ecosystem in Earth's clouds (one which persists without interacting with the surface), we haven't really looked for one either; most of our interest in these organisms thusfar comes from studying plant pathogens that spread through weather events. It's worth pointing out that there is phosphine in Earth's upper atmosphere as well, with no clear mechanism for how it forms there. It should be noted that there is less in Earth's upper atmosphere by about 3 orders of magnitude; the levels in Venus's atmosphere are more similar to that found immediately around actively metabolizing bacteria on Earth's surface.
Toward an Evolutionary History of Venus
Why would life exist on the most hellish world in the solar system? The answer is that for at least 75% of its lifespan, Venus was a much more Earth-like planet with cooler temperatures and oceans.
There are two possible, not mutually exclusive stories that explain how this planet came to be the Venus we know today.
The first is that Venus was a little too close to the Sun, which caused its oceans to evaporate, plate tectonics to cease, and subsequent cataclysmic volcanism. As the oceans evaporated, the water vapor trapped the heat and accelerated the process. The deuterium/hydrogen ratio on Venus is about 150 times higher than Earth, where comets have at most a 3 times higher ratio than Earth, suggesting a very gradual loss to space of hydrogen from water and preferential retention of the heavier nucleus. Water lubricates plate tectonics, per Solomatov 2001. Climate modeling suggests that Venus may have had a habitable climate with liquid water at the surface until 715 MA ago (Way et al 2016.) The subsequenct evaporation of the oceans resulted in a planet where plate tectonics ground to a halt, and with no crustal mechanism to dissipate heat, and finally between 700 and 500 MA ago, Venus erupted in planet-wide massive flows that resurfaced the planet, utterly dwarfing any similar events on Earth (like the Siberian Traps.) This released the massive amounts of sulfur that we see today. This is the received wisdom and could entirely explain the modern state of Venus, and may alone be enough to explain all the sulfur.
There is another version of the story which reverses the causality - eruption causing evaporation, advanced by Way and Del Genio in 2019. It's worth noting that Venus has a thicker crust than Earth, owing to its lack of a large moon; therefore we should expect that the flows, when they do finally cause the crust to fail, are much stronger than in the parallel situation on an evaporated Earth.
The second possibility is obviously more speculative, a parallel to the Great Oxygenation in the history of life on Earth. In Earth's history, anaerobic cyanobacteria produced so much oxygen that they effectively poisoned themselves, but also set the stage for aerobic life. This could have been a great coincidence - there may just have happened to be genes close enough in design space to assemble oxyidation defenses and an aerobic metabolic pathway, and without such a coincidence, that may have been the end of life on Earth, or it may have settled into a simple bloom-and-bust oscillation as our bacterial mats may have for hundreds of millions of years evidenced by banded iron formations found in ancient rocks where they persist at the surface. (See discussion of endogenous extinctions here, which this section partly recapitulates.)
While an interesting idea, by Occam's razor we should spend no further time considering a possible Great Sulfuration, as we can explain the death of the Venusian surface ecosystem entirely based on abiotic meteorological and geological processes as above. It's also the case that the presence of increased CO2 relative to Earth can be easily explained by abiotic processes as well. Using ingenious reasoning about the necessary atmospheric pressure for flying dinosaurs' wings to function as well as the known rates of deposition of CO2 as carbon in continents and the ocean, we can arrive a figure of the equivalent of 85-100 bars' worth of CO2 trapped in the Earth's crust, similar to what is currently in the Venusian atmosphere. Presumably the atmospheric pressure of Venus was lower during its oceanic period owing to the same process, and rose subsequent to the evaporation, but I am not aware of any modeling retrodicting from oceanic evaporation 500-700 MA ago to the current pressure and mass of CO2 on Venus.
All this is to say that life on Venus may have gone a different way, but started quite similarly. We're now fairly confident the first metabolism on Earth was sea vent iron sulfur organisms, using sulfur in what is now oxygen's chemical role. The Great Oxygenation may have only happened when it did, a full 1.5 billion years after the first life and at least 800 million years after photosynthesis appeared, because an asteroid delivered molybdenum, allowing nitrogen fixation and more efficient anaerobic metabolism. Whatever the reason, had this happened prior to photosynthesis, we may have ended up with an Earth poisoned with sulfur or at least with a massive amount of oxidized sulfur.
Two Obvious Problems for the "UAPP Cells" Hypothesis for Life on Venus
There are two major hurdles to overcome in any argument that there is life in the cloudtops of Venus. The first is the question of how life operates without water, or with very little water; this would actually be a more stunning find than merely life which can tolerate high acidity! The second is the failure thusfar to detect any organics in the atmosphere. Without water and organic molecules, it's very hard to see how this won't end up being an interesting abiotic route to phosphine production along with some crystal we weren't anticipating at that altitude. That said, organic compounds on Venus may not be as unlikely as one might think - there was a Venusian equivalent of the Miller-Urey experiment performed, where under conditions of the Venusian atmosphere, organic compounds including amino acids were produced.
Furthermore, there remain arguments for an abiotic explanation for the unknown absorbers, specifically ferric chloride (Petrova 2018). Interestingly, this is partly advanced to explain another mystery which is the presence of rainbows ("Venus glory"), first observed in 2014 in the Venusian atmosphere.
Implications for Evolution in General and the Future of Life of Earth
It is more likely than not that life on Venus will be distantly related to life on Earth. A massive amount of material has been transferred between bodies in the solar system, with actual numbers calculated here; at that same link you will see reference to the survival of uncontrolled re-entry during the Columbia crash by not just bacteria, but animals (C. elegans worms, found alive on the ground weeks after the crash.) This is actually the more boring possibility, because we would learn much more about the basic principles of evolution and the possibilities of biochemistry beyond Earth's provincial commitments, if we really had a novel origin. Either way, if there is life on Venus, the likelihood of life on Mars, Europa, Enceladus and even Titan jumps dramatically, even if it's "just" a long-lost relative. I expect that ultimately the impact of finding life on Venus will be some neat new biochemistry (the old extremophiles will seem quaint) and a bit more information about how evolution can proceed.
It is unclear how we should feel about Venusian cloud UV-cyanobacteria in terms of the Great Filter, which suggests that the more life we find in the universe and the closer in terms of evolutionary stage to humans, the more concerned we should be - because the more likely our own extinction is before we can colonize planets beyond our own. If further exploration of Venus yields trilobites or vertebrates and these cells are all that are left, we should worry much more. In contrast, if Venus never got past vast floating bacterial mats (either in its clouds or ancient oceans). that's a bit more comfortable for us.
REFERENCES
Bains W, Petkowski J, Sousa-Silva C, Seager S. Trivalent phosphorus and phosphines as components of biochemistry in anoxic environments. Astrobiology 19, 7 (July 2019): p. 885-902 doi 10.1089/AST.2018.1958
Glindemann D, Edward M, Kuschk P. Phosphine gas in the upper troposphere. Atmospheric Environment Volume 37, Issue 18, June 2003, Pages 2429-2433
Greaves JS, Richards AMS, Bains W, Rimmer PB, Sagawa H, Clements DL, Seager S, Petkowski JJ, Sousa-Silva C, Ranjan S, Drabek-Maunder E, Fraser HJ, Cartwright A, Mueller-Wodarg I, Zhan Z, Friberg P, Coulson I, Lee E, Hoge J. Phosphine gas in the cloud decks of Venus. Published: 14 September 2020. Nature Astronomy (2020)
Levenspiel O, Fitzgerald TJ, Pettit D. Was the Atmospheric Pressure Different at the Time of Dinosaurs? Chemical Innovation, December 2000 Vol 30, No.12, 50 – 55
Limaye SS, Mogul R, Smith DJ, Ansari AH, Słowik GP, Vaishampayan P. Venus' Spectral Signatures and the Potential for Life in the Clouds. Astrobiology. 2018 Sep 1; 18(9): 1181–1198. Published online 2018 Sep 12. doi: 10.1089/ast.2017.1783
Morowitz H & Sagan C. Life in the Clouds of Venus? Nature volume 215, pages1259–1260(1967). 16 September 1967.
Otroshchenko V.A., Surkov Y.A. (1974) The Possibility of Organic Molecule Formation in the Venus Atmosphere. In: Oró J., Miller S.L., Ponnamperuma C., Young R.S. (eds) Cosmochemical Evolution and the Origins of Life. Springer, Dordrecht. https://doi.org/10.1007/978-94-010-2239-2_40
Petrova EV. Glory on Venus and selection among the unknown UV absorbers. Icarus Volume 306, 15 May 2018, Pages 163-170
Seager S, Petkowski JJ, Gao P, Bains W, Bryan NC, Ranjan S, Greaves J. The Venusian Lower Atmosphere Haze as a Depot for Desiccated Microbial Life: A Proposed Life Cycle for Persistence of the Venusian Aerial Biosphere. Astrobiology. Published Online:13 Aug 2020. https://doi.org/10.1089/ast.2020.2244
Sousa-Silva C, Seager S, Ranjan S, Petkowski JJ, Zhan Z, Hu R, Bains W. Phosphine as a Biosignature Gas in Exoplanet Atmospheres. AstrobiologyVol. 20, No. 2. Published Online:31 Jan 2020 https://doi.org/10.1089/ast.2018.1954
Way MJ, Del Genio AD, Kiang NY, Sohl LE, Grinspoon DH, Aleinov I, Kelley M, Clune T. Was Venus the First Habitable World of our Solar System? Geophysical Research Letters. First published: 11 August 2016 https://doi.org/10.1002/2016GL069790
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