Identifiable Acetylene Features Predicted for Young Earth-like Exoplanets with Reducing Atmospheres undergoing Heavy Bombardment

TL;DR

This study predicts detectable acetylene features in the atmospheres of young Earth-like exoplanets with reducing atmospheres affected by heavy bombardment, based on experimental simulations and atmospheric modeling.

Contribution

It combines impact simulation experiments with atmospheric models to identify observable acetylene features on young, impact-heavy exoplanets with reducing atmospheres.

Findings

Impacts can produce significant concentrations of acetylene and other molecules.

Simulated spectra show acetylene features at 3.05 μm and 10.5 μm are detectable.

Impacts may have contributed to prebiotic chemistry on early Earth-like planets.

Abstract

The chemical environments of young planets are assumed to be largely influenced by impacts of bodies lingering on unstable trajectories after the dissolution of the protoplanetary disk. We explore the chemical consequences of impacts within the context of reducing planetary atmospheres dominated by carbon monoxide, methane and molecular nitrogen. A terawatt high-power laser was selected in order to simulate the airglow plasma and blast wave surrounding the impactor. The chemical results of these experiments are then applied to a theoretical atmospheric model. The impact simulation results in substantial volume mixing ratios within the reactor of 5% hydrogen cyanide (HCN), 8% acetylene (C$_2$H$_2$), 5% cyanoacetylene (HC$_3$N) and 1% ammonia (NH$_3$). These yields are combined with estimated impact rates for the Early Earth to predict surface boundary conditions for an atmospheric model. We show that impacts might have served as sources of energy that would have led to steady-state surface quantities of 0.4% C$_2$H$_2$, 400 ppm HCN and 40 ppm NH$_3$. We provide simulated transit spectra for an Earth-like exoplanet with this reducing atmosphere during and shortly after eras of intense impacts. We predict that acetylene is as observable as other molecular features on exoplanets with reducing atmospheres that have recently gone through their own `Heavy Bombardments', with prominent features at 3.05 $\mu$m and 10.5 $\mu$m.