Exoplanet atmospheres

Abstract:

© 2019 Elsevier Inc. We present an analysis of Voyager-1-IRIS and Cassini-CIRS spectra of Jupiter's high latitudes acquired during the spacecrafts' respective flybys in November 1979 and January 2001. We performed a forward-model analysis in order to derive the abundances of ethylene (C 2 H 4 ), methylacetylene (CH 3 C 2 H), diacetylene (C 4 H 2 ) and benzene (C 6 H 6 ) in Jupiter's northern and southern auroral regions. We also compared these abundances to: 1) lower-latitude abundances predicted by the Moses et al. (2005) ‘Model A’ photochemical model, henceforth ‘Moses 2005A’, and 2) abundances derived at non-auroral longitudes in the same latitude band. This paper serves as an extension of Sinclair et al. (2017b), where we retrieved the vertical profiles of temperature, C 2 H 2 and C 2 H 6 from similar datasets. We find that an enrichment of C 2 H 4 , CH 3 C 2 H and C 6 H 6 with respect to lower-latitude abundances is required to fit the spectra of Jupiter's northern and southern auroral regions. For example, for CIRS 0.5 cm −1 spectra of Jupiter's southern auroral region, scale factor enrichments of 6.40 −1.15+1.30 and 9.60 −3.67+3.98 are required with respect to the Moses 2005A vertical profiles of C 2 H 4 and C 6 H 6 , respectively, in order to fit the spectral emission features of these species at ∼950 and ∼674 cm −1 . Similarly, in order to fit the CIRS 2.5 cm −1 spectra of Jupiter's northern auroral region, scale factor enrichments of 1.60 −0.21+0.37 , 3.40 −1.69+1.89 and 15.00 −4.02+4.01 with respect to the Moses 2005A vertical profiles of C 2 H 4 , CH 3 C 2 H and C 6 H 6 were required, respectively. Outside of Jupiter's auroral region in the same latitude bands, only upper-limit abundances of C 2 H 4 , CH 3 C 2 H and C 6 H 6 could be determined due to the limited sensitivity of the measurements, the weaker emission features combined with cooler stratospheric temperatures (and therefore decreased thermal emission) of these regions. Nevertheless, for a subset of the observations, derived abundances of C 2 H 4 and C 6 H 6 in Jupiter's auroral regions were higher (by 1 σ) with respect to upper-limit abundances derived outside the auroral region in the same latitude band. This is suggestive that the influx of energetic ions and electrons from the Jovian magnetosphere and external solar-wind environment into the neutral atmosphere in Jupiter's auroral regions drives enhanced ion-related chemistry, as has also been inferred from Cassini observations of Saturn's high latitudes (Fletcher et al., 2018; Guerlet et al., 2015; Koskinen et al., 2016). We were not able to constrain the abundance of C 4 H 2 in either Jupiter's auroral regions or non-auroral regions due to its lower (predicted) abundance and weaker emission feature. Thus, only upper-limit abundances were derived in both locations. From CIRS 2.5 cm −1 spectra, the upper limit abundance of C 4 H 2 corresponds to a scale factor enhancement of 45.6 and 23.8 with respect to the Moses 2005A vertical profile in Jupiter's non-auroral and auroral regions.

Abstract:

Hydrogen cyanide (HCN) is a key feedstock molecule for the production of life's building blocks. The formation of HCN in an N2-rich atmospheres requires first that the triple bond between N≡N be severed, and then that the atomic nitrogen find a carbon atom. These two tasks can be accomplished via photochemistry, lightning, impacts, or volcanism. The key requirements for producing appreciable amounts of HCN are the free availability of N2 and a local carbon to oxygen ratio of C/O ≥ 1. We discuss the chemical mechanisms by which HCN can be formed and destroyed on rocky exoplanets with Earth-like N2 content and surface water inventories, varying the oxidation state of the dominant carbon-containing atmospheric species. HCN is most readily produced in an atmosphere rich in methane (CH4) or acetylene (C2H2), but can also be produced in significant amounts (>1 ppm) within CO-dominated atmospheres. Methane is not necessary for the production of HCN. We show how destruction of HCN in a CO2-rich atmosphere depends critically on the poorly-constrained energetic barrier for the reaction of HCN with atomic oxygen. We discuss the implications of our results for detecting photochemically produced HCN, for concentrating HCN on the planet's surface, and its importance for prebiotic chemistry.