Origin of Life's Building Blocks in Carbon- and Nitrogen-Rich Surface Hydrothermal Vents

Abstract

There are two dominant and contrasting classes of origin of life scenarios: those predicting that life emerged in submarine hydrothermal systems, where chemical disequilibrium can provide an energy source for nascent life; and those predicting that life emerged within subaerial environments, where UV catalysis of reactions may occur to form the building blocks of life. Here, we describe a prebiotically plausible environment that draws on the strengths of both scenarios: surface hydrothermal vents. We show how key feedstock molecules for prebiotic chemistry can be produced in abundance in shallow and surficial hydrothermal systems. We calculate the chemistry of volcanic gases feeding these vents over a range of pressures and basalt C/N/O contents. If ultra-reducing carbon-rich nitrogen-rich gases interact with subsurface water at a volcanic vent they result in 10 - 3 ⁻ 1 M concentrations of diacetylene (C₄H₂), acetylene (C₂H₂), cyanoacetylene (HC₃N), hydrogen cyanide (HCN), bisulfite (likely in the form of salts containing HSO₃^-), hydrogen sulfide (HS^-) and soluble iron in vent water. One key feedstock molecule, cyanamide (CH₂N₂), is not formed in significant quantities within this scenario, suggesting that it may need to be delivered exogenously, or formed from hydrogen cyanide either via organometallic compounds, or by some as yet-unknown chemical synthesis. Given the likely ubiquity of surface hydrothermal vents on young, hot, terrestrial planets, these results identify a prebiotically plausible local geochemical environment, which is also amenable to future lab-based simulation.

Keywords: hydrothermal vents; origin of life; volcanism on the early earth.

Figure A1

The ratio N₂/³⁶Ar as a function of ⁴⁰Ar/³⁶Ar with a linear fit to the data${}^X$.

Figure A2

The change in Oxygen fugacity, in the form −$log\delta f_{{\mathrm{O}}_2}$, as a function of pressure for various C/O ratios from 0.8 (blue)–10 (black), with the values noted next to the relevant curves.

Figure A3

The mixing ratios of water, CO and CO₂ as a function of pressure for various C/O ratios from 0.8 (blue)–10 (black), with the values noted next to the relevant curves.

Figure A4

The mixing ratios of the nitriles, HCN and HC₃N, as a function of pressure for various C/O ratios from 0.8 (blue)–10 (black), with the values noted next to the relevant curves.

Figure 1

A comparison of the mixing ratios (x-axis) as a function of pressure (bar, y-axis) between STAND and a modified version of MagmARGO (solid) with the D-Compress gas-phase (dashed).

Figure 2

Mixing ratios of species in melt+gas-phase system at 1200 ${}^{\circ }$C versus pressure, p [bar], for three different scenarios (see Section 3 for details), carbon-poor and nitrogen-rich (top), carbon-rich nitrogen-poor (middle), and carbon-rich nitrogen-rich (bottom).

Figure 3

A surface hydrothermal vent fed by a carbon-rich nitrogen-rich magma. The edges of the pool may expand and recede, providing dry-wet cycles. The water of the pool would plausibly be enriched by sulfites (SO₃²⁻), sulfates (SO₄²⁻), and Iron(II). Various amounts of diacetylene (C₄H₂), acetylene (C₂H₂), cyanoacetylene (HC₃N), hydrogen cyanide (HCN), hydrogen (H₂) and methane (CH₄) bubble up out of the pool in ratios determined by the depth at which the gas is quenched. One released into the air above the pool, these species are photolysed and react to produce small amounts of cyanamide. The cyanide in the pool will react with the sulfates to slowly form thiocyanate (CNS⁻), and will react with solvated electrons to produce simple sugars and amino-acid precursors.