Speaker
Description
The landing of the ROSETTA module Philae in 2014 kicked up a lot of dust, figuratively as well as literally. The not-quite-as-planned landing displaced about 0.4 m3 of dust[1], some of which ended up in the COSAC instrument[2], where it warmed to around 20°C, releasing its volatiles. Although the COSAC instrument could never play out its full potential, it did acquire a few “sniff-mode” mass spectra of the gas in and around the lander. The mass spectrum acquired a few minutes after the first touchdown showed a large number of signals, which were attributed to 16 organic molecules, some of which had never before been detected on a comet[2].
Due to the nature of the instrument, which was primarily a gas chromatograph, deconvolution of mass spectra containing multiple compounds is difficult, and will yield non-unique solutions. The COSAC interpretation of the results was disputed on the grounds that there are other possible solutions, which yield equal or better fits to the data[3]. These suggestions however failed to address the problem of chemical cohesiveness, i.e. a mixture of compounds that makes sense from a chemical point of view.
Here, a chemical reaction network is proposed, that can explain why certain molecules were found on the comet, while others were not. Starting from the simple molecules water (H2O), ammonia (NH3), carbon monoxide (CO), and methane (CH4), the presence of which on a comet is undisputed, a reaction network with just two basic types of reaction can explain the presence of all the molecules reported by COSAC, as well as a few molecules which were later detected by other instruments.
The reaction network is based on electron-induced chemistry, triggered by the interaction of molecules with secondary electrons. These low energy electrons typically have very large cross sections for the activation of molecules[4], as well as being produced in vast numbers. These activated molecules or fragments can undergo subsequent chemical reactions, forming a wide variety of chemical products. These reactions can be studied in the lab, enabling the elucidation of specific reaction mechanisms, where up until recently the statistical recombination of radicals[5] was the most often cited “mechanism” for the formation of organics in condensed phase.
References
[1] J.-P. Bibring et al. (2015) Science 349(6247):aac5116.
[2] F. Goesmann et al. (2015) Science 349(6247):aab0689.
[3] K. Altwegg et al. (2017) MNRAS 467(Suppl.2):S130-S141.
[4] E. Böhler et al. (2013) Chem. Soc. Rev. 42(24):9219-9231.
[5] C.J. Shen et al. (2004) Astron. Astrophys. 415(1):203–215.
