Gregory's ServerThere are a number of ideas on the #OriginOfLife π§¬π±π , but a thread of recent #experimental results indicate high yields of interesting #biomolecules in #surface settings on the early #Hadean #Earth. See attached figure from Sasselov+ (2020, SciAdv)
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13 comments
Tim Lichtenberg
This is what motivated the famous Miller-Urey experiment, but Miller-Urey atmospheres actually produce a lot of "goo" that makes the resulting soup unusable and unproductive. Currently suggested alternatives to produce chemically reduced feedstocks like HCN include lightning and meteoritic impact events. The idea for the impacts is that the iron in the impact vaporizes an already present ocean, which makes CH4, which then reacts with UV light to produce HCN. See attached figure from Benner+2020.
Tim Lichtenberg
The problem is we have no way to test this, since >4.5 Gyr of tectonic activity on Earth have erased essentially all evidence from this time. #Exoplanets to the rescue! π Hydrogen-rich atmosphere have large scale heights, so if such reduced post-impact atmospheres would be common, we should see it with exoplanet telescopes like #JWST, and possibly with #Ariel. Attached a figure from Rimmer+2019, showing potential ethylene and acetylene features in a post-impact atmosphere.
Tim Lichtenberg
If we want to do this, we need to look at M star exoplanets, because there are only a handful of G star exoplanets around, but they are too old. We want to look at *young* planets/systems. Figure from Timothy Gebhard (TΓΌbingen/ETHZ).
Tim Lichtenberg
We set up numerical N-body simulations to statistically analyse the composition and timing of impactors onto young M-dwarf exoplanets. Composition regions of planetesimals and embryos are sub-divided into three regions, modelling different initial states and geophysical scenarios for protoplanets.
Tim Lichtenberg
The evolution of the protoplanets and impacting planetesimals are motivated by two mechanisms: internal evolution from radioactive decay (being heated drives water and volatiles off); and dehydration of protoplanets during their "magma ocean" phases, which is caused by heating from the central star,
Tim Lichtenberg
Our findings point to two major conclusions. (1) Impacts are too late on M dwarf exoplanets to trigger prebiotic synthesis. When impacts fall, M dwarf stars are still extremely luminous and the planets are hot and molten (in the magma ocean state). No ocean, no surface, just magma. The plot shows how all impacts fall before the M stars transition onto their main sequence (cool down). Only Sun-like stars experience late-impacting bombardment.
Tim Lichtenberg
Finding (2) is that essentially all nominal model scenarios end up *way* too wet. Rocky planets in the "liquid water habitable zone" of their M stars all receive 1-2 orders of magnitude more water than the Earth does. This is not good. This is too much oxidation, way too much. These planets are big giant comet, iceballs, water worlds, whatever you want to call them. Our simulations only produce compositionally "Earth-like" planets (water/rock ratio) if planetesimals dehydrate substantially.
Tim Lichtenberg
Magma ocean losses are limited to a few tens of Earth oceans. This is not enough to balance the massive accretion of water just from late impact bombardment alone. This suggests that M dwarf rocky planets should have no problem creating an "atmosphere", but actually they are not rocky, they are massive comets. Only outgassing from radioactive decay in early-formed planetesimals can dehydrate planetesimals strong enough that the resulting planets are in an Earth-like water regime.
Tim Lichtenberg replied to Tim
Sounds wild? Theoretically predicted >20 years ago already, we have now increasing evidence for water worlds, specifically around M dwarfs. See attached figure from Luque & PallΓ© (2022, Science), providing distinct evidence for planets with ~50 wt% water around M dwarfs. Our simulations here (and previous papers from me and collaborators) suggest an alternative to the migration theory: *timing* of planet formation. If planets form early, they receive balanced volatile abundances.
Tim Lichtenberg replied to Tim
(Hint: stay tuned for next week, another paper upcoming on this topic. π€ )
Tim Lichtenberg replied to Tim
Coming back to the original question we posed: are M dwarf exoplanets good testbeds for surficial origin of life theories, and scenarios of impact triggering of transient reduced atmospheres? Probably not. Are they still interesting? Absolutely. M dwarf exoplanets are crucial in helping us to refine our understanding of planetary physics and chemistry and test Solar System-derived theories in a grand context.
Tim Lichtenberg replied to Tim
Finally, one more link to the 20 min video of the talk, since I had to massively cut out material and explanation given the threaded nature: https://youtu.be/M2CopK35Jrs. Happy to answer any questions, thanks for dropping by. π |
There's a catch though: "subaerial" scenarios require feedstock molecules like HCN or CH2O at or close to the surface (at best from the atmosphere) to initiate prebiotic reactions. Unfortunately, these molecules are unstable in the modern, oxidised atmosphere of the Earth. See attached plot from Lichtenberg+ (2022, PPVII), showing the major atmospheric gases released from volcanoes for different interior compositions/chemical oxidation states. The Earth is too far right for stable HCN and CH2O.