Origin of Life Molecules in the Atmosphere After Big Impacts on the Early Earth

Brief

Wogan et al. (2023) model three sequential post-impact atmospheric phases using a time-dependent, coupled 0-D kinetics-climate box model and a 1-D photochemical-climate code (Photochem). Iron delivered by large asteroids reacts with impact-vaporized steam to produce H2, which drives CH4 and NH3 synthesis as the atmosphere cools over ~4,200 years; subsequent N2-CH4 photochemistry then generates HCN and HCCCN for roughly one million years. The critical threshold is a CH4/CO2 mole ratio above 0.1, which requires impactors of 4×10^20 to 5×10^21 kg (570–1330 km diameter) depending on iron reactivity and nickel catalysis assumptions. Peak HCN and HCCCN rainout reaches 10^9 molecules cm^-2 s^-1 in hazy atmospheres, but nitrile-rich conditions also imply surface temperatures above 360 K, raising questions about RNA longevity during the productive window.

Metadata

Category

Search

Venue

The Planetary Science Journal

Type

Peer-reviewed

Year

2023

Authors

Nicholas F. Wogan, David C. Catling, Kevin J. Zahnle, Roxana Lupu

arXiv

2307.09761

Access

Open access

Length

1.5 M

Programs

Virtual Planetary Laboratory

Instruments

Photochem (1-D photochemical-climate model), 0-D kinetics-climate box model, CVODE BDF integrator, radiative transfer code

Data sources

NIST thermodynamic data (96 gas-phase species), SPH simulations from Citron & Stewart 2022, Schmider et al. 2021 nickel-surface reaction network

Tags

astrobiology, origin of life, prebiotic chemistry, exoplanet habitability, biosignature, impact cratering, atmospheric chemistry

Key points

• Peak HCN and HCCCN rainout reaches 10^9 molecules cm^-2 s^-1 in hazy atmospheres when CH4/CO2 > 0.1; below that threshold, HCN rainout drops below 10^5 molecules cm^-2 s^-1 and HCCCN is negligible.p.1
• The minimum impactor mass needed to achieve CH4/CO2 > 0.1 is 4×10^20 to 5×10^21 kg (570–1330 km diameter), spanning a factor of ~12 depending on iron availability and nickel catalysis.p.1
• Gas-phase kinetics alone require impactors larger than 1.6×10^21 kg (~900 km diameter) to convert most atmospheric CO2 to CH4; a 10^22 kg impactor produces a 'dry' H2 partial pressure of 23.8 bars.p.7
• Nickel surface catalysis from impactor-delivered spherules can substantially lower the minimum impactor threshold: a Vesta-sized impactor (2.6×10^20 kg, 500 km) with sufficient nickel area (>1000 cm^2 Ni / cm^2 Earth) could convert most pre-impact CO2 to CH4.p.7
• The post-impact steam atmosphere condenses to an ocean in ~4,200 years (for a 1.58×10^21 kg impactor), leaving an H2-dominated atmosphere that then persists for millions of years until hydrogen escapes to space.p.5
• N2-CH4 photochemistry sustains HCN production for approximately one million years in the post-impact hazy atmosphere; the dominant production pathway is N + 3CH2 → HCN + H, with atomic N from photolyzed N2 and hydrocarbon radicals from photolyzed CH4 at pressures below 10^-5 bar.p.8
• Nitrile-rich post-impact atmospheres carry surface temperatures above 360 K, potentially incompatible with RNA longevity, though the authors note that stockpiled cyanide remains usable after hydrogen escape cools the planet.p.1
• If only 15–30% of impactor iron reaches the atmosphere (SPH-based Model 1B), the required impactor mass shifts upward by a factor of ~5 relative to the optimistic 100%-iron scenario.p.8

Verbatim

• “These paradoxes are resolved by iron-rich asteroid impacts that transiently reduced the entire atmosphere, allowing nitriles to form in subsequent photochemistry.”

  p.1

• “HCN and HCCCN production and rainout to the surface can reach 10 9 molecules cm − 2 s − 1 in hazy atmospheres with a mole ratio of CH 4 / CO 2 > 0 . 1.”

  p.1

• “We find that most of the CO 2 in the atmosphere is converted to CH 4 for impactors larger than 1 . 6 × 10 21 kg ( ∼ 900 km diameter), and that bigger impacts generate more NH 3 , e.g., a 10 22 kg impactor makes 0.013 "dry" bars of NH 3 .”

  p.7

• “We find that N 2 and CH 4 photochemistry generates HCN in a hazy Titan-like atmosphere for about one million years until it is halted by hydrogen escape to space.”

  p.8

• “In this model, the dominant channel producing HCN is N + 3 CH 2 → HCN + H where 3 CH 2 is ground (triplet) state of the methylene radical derived form methane photolysis.”

  p.8

Most interesting

• The same reaction network used to model the post-impact Hadean atmosphere is validated against Titan's present-day atmosphere, which makes HCN, HCCCN, and NCCN from an analogous N2-CH4 chemistry.
• Ultra-fine nickel particles below 300 nm, experimentally observed in ocean-impact wake experiments, would provide six orders of magnitude more catalytic surface area than 1 mm spherules, potentially making even Vesta-scale impactors prebiotically effective.
• NH3 quenches at ~1200 K and CH4 at ~950 K during atmospheric cooling, meaning the two key nitrogen-bearing precursors are locked in before steam fully condenses, a kinetic freeze rather than equilibrium outcome.
• The paper explicitly asks how 'lucky' early life was: post-impact nitrile windows must produce viable chemistry before subsequent impacts annihilate any nascent organisms, framing the calculation as a survivorship problem.
• Even if RNA cannot survive the >360 K surface temperatures during peak nitrile production, cyanide can be geochemically stockpiled and mobilized for prebiotic synthesis only after hydrogen escape has cooled the planet, decoupling synthesis from utilization by millions of years.
• A 10^22 kg impactor, roughly the mass of Mars's moon Phobos scaled up, would generate an H2 atmosphere with a 'dry' partial pressure of 23.8 bars, briefly giving Earth a hydrogen envelope comparable in scale to a mini-Neptune.