Jupiter's auroral-related stratospheric heating and chemistry III: Abundances of C2H4, CH3C2H, C4H2 and C6H6 from Voyager-IRIS and Cassini-CIRS

Abstract

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 '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. (2017a), 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 +1.30 −1.15 and 9.60 +3.98 −3.67 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 +0.37 −0.21 , 3.40 +1.89 −1.69 and 15.00 +4.01 −4.02 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 greater than 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 (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.

Ultraviolet photochemistry of ethane: implications for the atmospheric chemistry of the gas giants

“…Most of the imaged CH 3 fragments appear with much lower TKER, however. Table 1 shows several potential sources of slow CH 3 radicals, including three-body dissociations (6), (12) and (14) and the production of an electronically excited CH 3 partner (channel (16)), the relative likelihoods of which are discussed below. .…”

“…The distributions for each photolysis wavelength are offset vertically for display purposes and the dotted lines in plots (A) and (C) show the square root of P(TKER)normalised to the same maximum valuein order to accentuate the high TKER tails. The TKER max values associated with the two-body fragmentation channels (4), (5) and (8) as well as the most exoergic three body fragmentations yielding CH 2 and/or CH 3 fragments (channels (12) and (6)) are also indicated by vertical arrows in panels (A) and (C). TKER max values for other many-body fragmentation channels can be derived from Table 1 but, as noted in the text, given the assumed TOF to TKER conversion scheme it is likely that the products from such many-body fragmentations would appear at TKER values well below TKER max .…”

“…Sci., 2020, 11,5089 All publication charges for this article have been paid for by the Royal Society of Chemistry coupled to methane photolysis. 3,9,10 Neglect of ion-molecule chemistry has been suggested as one possible explanation for this discrepancy, 3,11,12 but it is also appropriate to question the inputs to the commonly used photochemical schemes. These draw on data 8 from a range of (oen indirect) sources, including predictions, wherein chemical pathways have been included or removed on the basis of how well the model ts the measurements.…”

Monitoring of the evolution of H2O vapor in the stratosphere of Jupiter over an 18-yr period with the Odin space telescope

“…A possible explanation is that these species still vary locally more sharply as a function of latitude than the factor of two to three indicated by the low spatial resolution observations of Cavalié et al 2013; we note that these variations cannot be studied with 1D models, by definition. Sinclair et al (2018Sinclair et al ( , 2019 already demonstrated that the auroral regions of Jupiter harbor chemistry influencing the hydrocarbons that is not seen elsewhere on the planet. The same may be also be true for H 2 O, but diskresolved observations with more resolution than with Herschel (2009-2010) would be required to test this hypothesis, possibly with the James Webb Space Telescope Norwood et al (2016).…”

“…They found that C 2 H 2 and C 2 H 4 were significantly enhanced at millibar and submillibar pressures under the aurora, while C 2 H 6 remained fairly constant. In addition, Sinclair et al (2019) found that heavier hydrocarbons like C 6 H 6 were also enhanced under the aurorae. This points to a richer chemistry than that seen at lower latitudes, increasing the production of several hydrocarbons and ultimately producing Jupiter's aerosols (Zhang et al 2013(Zhang et al , 2016Giles et al 2019).…”