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

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 Corresponding author: P. B. Rimmer

Timescales for Prebiotic Photochemistry Under Realistic Surface Ultraviolet Conditions

“…These constraints can also be transformed into conditions for further delineating the abiogenesis zone for the Sun, and for other stars, including for ultracool stars. This is especially relevant given our present knowledge about the UV environments of planetary systems around ultracool host stars such as TRAPPIST-1 (Ducrot et al, 2020) and systems discovered by TESS (Günther et al, 2020), and future discoveries around these systems, as well as around young exoplanet systems (Bottrill et al, 2020;Rimmer et al, 2020). If biosignatures are discovered on a statistically TIMESCALES FOR UV-DRIVEN PREBIOTIC PHOTOCHEMISTRY significant subset of these planets, the distribution of that subset may provide the only real way to test prebiotic chemical scenarios in the future.…”

“…It may have been more reducing, and if so, there would likely have been thick photochemical hazes that can obscure the UV light between 200 and 300 nm (Arney et al , 2016 ). If life's origins were coincident with the tail end of accretion, hazes generated by accretionary impact may obscure the UV light (Rimmer et al , 2020 ). NH 3 would also attenuate UV light below 230 nm (Cheng et al , 2006 ).…”

Ariel – a window to the origin of life on early earth?

“…Shock-wave chemical reprocessing following the delivery of 3750 J of energy in 25 laser shots leads to a 5% yield of HCN, 8% yield of acetylene, 5% yield of cyanoacetylene and 1% yield of ammonia. The authors [76] predict that the amount of acetylene produced in earlystage rocky planetary atmospheres with the studied composition would be observable remotely when subjected to a heavy bombardment similar to what happened on the early Earth. This finding has profound implications for exoplanetary observations.…”

“…A goal of ongoing research is to find out whether this chemistry provides distinct gaseous markers that can be observed with facilities such as JWST and Ariel. As an example, take the impact-induced shock-wave reprocessing of a reducing planetary atmosphere dominated by CO2, CH4 and N2 [76]. Such a shock wave was experimentally simulated in a model planetary atmosphere by dielectric breakdown induced by a terawatt-power laser.…”

Lamb-dip cavity ring-down spectroscopy of acetylene at 1.4 μm

“…In fact, acetylene is expected to be present in the exoplanetary atmospheres [5,6]. Theoretical atmospheric models, supported by chemical laboratory data, predict a relatively large volume mixing-ratio for acetylene in the atmosphere of young Earth-like exoplanets [7]. Clear C 2 H 2 signatures can be found in star forming regions [8], as well as in brown dwarf disks [9].…”