Molecule formation

## 2020 - Nat Astron - PH3

• J.S. Greaves, A.M.S. Richards, W. Bains, P.B. Rimmer, H. Sagawa, D.L. Clements, S. Seager, J.J. Petkowski, C. Sousa-Silva, S. Ranjan, E. Drabek-Maunder, H.J. Fraser, A. Cartwright, I. Mueller-Wodarg, Z. Zhan, P. Friberg, I. Coulson, E. Lee, and J. Hoge, Nat. Astron. 1 (2020).
• https://www.nature.com/articles/s41550-020-1174-4

PH3 is thought to be a biosignature, since the production of PH3 on Earth is often related with the activities of microorganisms. In addition, PH3 gets easily oxidized under Venus's atmosphere and is unstable. The confirmation of PH3 on Venus's atmosphere means that there must be a stable mechanism to generate PH3.

• It is difficult to detect PH3 outside Earth, because PH3's spectra features are strongly absorbed by Earth's atmosphere.
• The existence of PH3 is not the evidence of life.
• The formation pathway of PH3 is still unknown.

## 2020 - Nat Astron - An experimental study of the surface formation of methane in interstellar molecular clouds

• D. Qasim, G. Fedoseev, K.-J. Chuang, J. He, S. Ioppolo, E.F. van Dishoeck, and H. Linnartz, Nat Astron 1 (2020).
• https://www.nature.com/articles/s41550-020-1054-y

This paper is the first experimental proof of solid-state CH4 formation by atomic C and H under conditions mimicking those of interstellar molecular cloud environments. It starts with the existence of CH4 in space and interstellar environments where CH4 is detected. Experimentalists have tried to reproduce interstellar environments in laboratories to reveal how CH4 is formed in space. The new part of this work is that it solves a technical issue which is the combination between an atomic carbon source and a ultrahigh vacuum (UHV) set-up. This special design is necessary to study atom-induced surface reactions under molecular cloud conditions.

### What is the guess for CH4 formation from observational point of view?

CH4 is expected to be formed by the hydrogenation of C on dust grains, and CH4 ice is strongly correlated with solid H2O.

### What systems are investigated in laboratories and what are found by experiments?

Three layers structure: Species/Surface/Substrate

Species Surface Substrate
atomic C + atomic H amorphous carbon (carbonaceous surface) Inert gold substrate (10 K)
atomic C + atomic H + H2O amorphous carbon (carbonaceous surface) Inert gold substrate (10 K)
• CH4 is the main product from experiments.
• $\text{CH}_n$ radicals and their recombination products such as C2H4, C2H4, C2H6 are not detected.
• The author's explanation is that recombination between $\text{CH}_n$ radicals and atomic H is efficient and the life time of $\text{CH}_n$ radicals is short in their experiment environment
• There are no citations or further description of this statement. Does this mean the recombination between $\text{CH}_n$ radicals and atomic H is easier than the recombination between $\text{CH}_n$ radicals and $\text{CH}_n$ radicals?

• The producing rate of CH4 is higher when H2O is included.

## The Abundance of C2H4 in the Circumstellar Envelope of IRC+10216

• J.P. Fonfría, K.H. Hinkle, J. Cernicharo, M.J. Richter, M. Agúndez, and L. Wallace, ApJ 835, 196 (2017).
• C2H4 relative abundance to H2: ~ 10^-8
• up to 25% of the C2H4 molecules at 20R* could condense onto dust grains