2022-08-18
August Science readings
| Topic 01 Last July, Ammonia considered as a biosignature |
| Topic 2010 asteroids may be result of recent breakup |
| Topic Topic 03 |
| End of document |
Not the most productive of months, nor Sept either, with me closing this page 21 Sept.
Last July - Ammonia, considered as a biosignature
https://arxiv.org/pdf/2107.12424.pdfAssessment of Ammonia as a Biosignature Gas in Exoplanet Atmospheres, Huang, Seager et al, ArXiv posting
Abstract -Ammonia (NH3) in a terrestrial planet atmosphere is generally a good biosignature gas, primarily because terrestrial planets have no significant known abiotic NH3 source. The conditions required for NH3 to accumulate in the atmosphere are, however, stringent. NH3's high water solubility and high bio-useability likely prevent NH3 from accumulating in the atmosphere to detectable levels unless life is a net source of NH3 and produces enough NH3 to saturate the surface sinks. Only then can NH3 accumulate in the atmosphere with a reasonable surface production flux.
For the highly favorable planetary scenario of terrestrial planets with H2-dominated atmospheres orbiting M dwarf stars (M5V), we find a minimum of about 5 ppm column-averaged mixing ratio is needed for NH3 to be detectable with JWST, considering a 10 ppm JWST systematic noise floor. When the surface is saturated with NH3 (i.e., there are no NH3-removal reactions on the surface), the required biological surface flux to reach 5 ppm is on the order of 1010 molecules cm-2 s-1, comparable to the terrestrial biological production of CH4. However, when the surface is unsaturated with NH3, due to additional sinks present on the surface, life would have to produce NH3 at surface flux levels on the order of 1015 molecules cm-2 s-1 (~4.5×106 Tg year-1). This value is roughly 20,000 times greater than the biological production of NH3 on Earth and about 10,000 times greater than Earth’s CH4 biological production.
Volatile amines have similar solubilities and reactivities to NH3 and hence share NH3's weaknesses and strengths as a biosignature. Finally, to establish NH3 as a biosignature gas, we must rule out mini-Neptunes with deep atmospheres, where temperatures and pressures are high enough for NH3’s atmospheric production.
This was brought to my attention by Slashdot user "burtosis" when I mentioned my interest in the multiple eutectics in the ammonia-water-CO2 system in the ~240 - 270 K temperature range, at modest pressures achievable with a less than Venusian atmosphere on a terrestrial planet. A number of people have pointed out the potential importance of the low density of ice compared to liquid water, possibly helping persistence of pre-biotic chemistry in water solution below a (seasonal) skim of ice ; another strand of thought is that, in such a "cold-Darwinian" "icy little pool" could to a degree bypass the "concentration problem" for (proto-)biomolecules by the zone-refining effect where minor contaminants are excluded on a growing crystal surface, and concentrate in the region ahead of the growing crystal. This industrially useful process could concentrate (proto-)biomolecules into small volumes of interstitial water between ice crystals, where reactions might take place at higher concentrations in the bulk. If you were to combine this scenario with, say, a hydrothermal spring's outlet pool on an early Earth under a faint young Sun, you could easily get a diurnal (possibly as little as 12 hours) cycle of chemical concentration, then re-heating and mixing, then re-concentration under ice ... which is a very good situation for processing lots of similar chemical reactions.
When I've thought about possible (not necessarily Earth-like) conditions in which a chemical life-like system could develop, I've often considered this NH3 - H2O - CO2 as a useful pounterpoint to the general assumption of a water-based chemistry. (This does not mean that I think this happened on Earth ; I'm using it as a theoretical counterpoint to terrestrial water-based chemistry). I admit that I hadn't particularly considered the abundance of ammonia in planetary atmospheres - it's well known in the atmospheres of Jupiter, Saturn, Uranus ("George, don't do that!" ; I get fed up with the juvenuile wittering of people incapable of pronouncing that planet's name without adding a scatological joke, so I'll counter with Madam Joyce's cut-glass admonitions to her little star, George. Yes, it's a Herschel joke.), so I just assumed it is there. Maybe a mistake to just assume that. Ammonia is also reported as a (small) component of volcanic out-gassing.
The authors of this paper point out that around Sun-like stars ammonia is subject to UV photolysis, so my putative NH3 - H2O - CO2 atmosphere would probably need to be restricted to "red dwarf" (K, M, N spectral classes) stars. But since the interesting atmospheres would also need to be somewhat colder than Earth's (present) atmosphere, I don't really see that as a terrible constraint. Also, K, M, N dwarfs are the commonest types of stars, by a considerable margin.
These authors also point out that today there are a number of geochemical sinks for ammonia, largely involving photochemically-mediated oxidation by atmospheric oxygen. Well, it's certainly a point - but since oxygen itself is generally also considered a biomarker, and I've already "turned down" the UV dose by restricting myself to K,M,N dwarfs, I don't think it's a disabling point for a "atmosphere for discussion.
"Burtosis" did provoke me to think more closely about possible places for such an environment to exist. To get the right range of temperatures and pressures, without being on an "ice giant", I thought about the "ice balls" (minor bodies like Europa, Callisto, Ganymede, and possibly Pluto or Charon) which have substantial ice shells above (probable) watery oceans. If (and it's an "if" ; I'm world-building here, to look for non-aqueous solvent life-formation environments here, not necessarily being serious) the planet (minor body) produced or released significant ammonia by (tidal friction-driven) vulcanicity, then you might get the relevant mix of compounds to produce this "environment for argument" on the underside of the ice shell.
All in all, I think I'll continue using this as a counter-argument to arrertions that "origin of life needs liquid water". I'm perfectly happy that the example of life which we have (Earth's) originated using water as a dominant solvent. But that's an argument about "Life Jim, but as we know it!" ; but trying to extend the discussion to life generally, we also need to try to consider systems of "Life, Jim, but NOT as we know it". And this is an environment for argument about that. I'll continue to use it, but bear in mind the (purported) realtive paucity of ammonia as a nitrogen-containing species.
Several of the papers authors also made some discussion of a putative planet type with a deep nitrogen-hydrogen (H2O -N2) atmosphere, where putative life could "burn" hydrogen to ammonia using whatever biochemical counterpart of photosynthesis it needs to invent. That's an interesting world - they describe it as a "Cold Haber" world - and I might pilfer that idea too. (They cite it as "Seager, S., Bains, W. & Hu, R. Biosignature gases in H2-Dominated atmospheres on rocky exoplanets. Astrophys. J. 777, 95 (2013)" ; I'll have to get a copy one of these days.)
Extremely young asteroid pair (458271) 2010 UM26 and 2010 RN221
arxiv.org pdf 2208.06207.pdfAbstract - Aims. Extremely similar heliocentric orbital elements of the main-belt objects (458271) 2010 UM26 and 2010 RN221 make them the tightest known pair and promise its very young age. We analyzed the conditions of its origin and determined its age.
Methods. We conducted dedicated observations of (458271) 2010 UM26 and 2010 RN221 in summer 2022 that resulted in a high- accuracy astrometric set of data. Joining them with the previously available observations, we improved the precision of the orbit determination of both asteroids. We used numerical simulations backward in time to constrain the origin of this new pair by observing orbital convergence in the Cartesian space.
Results. Using a large number of possible clone variants of (458271) 2010 UM26 and 2010 RN221 we find they all converge in a narrow time interval around March 2003 having extremely tight minimum distances (≤ 1000 km) and minimum relative velocities (≤ 3 cm s−1). These conditions require to include mutual gravitational attraction of the asteroids constituting the pair for its age determination. Extending our model by this effect even improves the convergence results. We find there is more than 55% probability that the pair formed after the year 2000. However, quasi-satellite captures make the possible age uncertainty of this pair prolonged possibly to the 1960s. Still, this is by far the youngest known asteroid pair, a prime target for future astronomical observations.
Well, there's not a lot to say that isn't in the Abstract. That being what abstracts are for, after all.
The authors do grant that the precision of orbital calculations (and calculations of perturbations from all the other planets and asteroids) isn't enough to distinguish cleanly between an origin of the pair by fission of a progenitor body in 2003, versus fission of the progenitor as early as the 1960s followed by a period of unstable mutual orbit before reaching the point of orbital separation.
It's not as if we haven't seen this sort of thing before - a significant number of comets fragemnt into multiple parts, often but not always as the pass close to the Sun ; there's no particular reason to think it can't happen to asteroids too, as they are spun-up by effects like the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect. Then there's the close-approach effect, as exemplified by Comet Shoemaker-Levy-9.
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