Jupiter's auroral-related stratospheric heating and chemistry I: Analysis of Voyager-IRIS and Cassini-CIRS spectra

Auroral processes are evident in Jupiter's polar atmosphere over a large range in wavelength (X-ray to radio). In particular, previous observations in the mid-infrared (5-15 μm) have shown enhanced emission from CH4, C2H2 and C2H4 and further stratospheric hydrocarbon species in spatial regions coincident with auroral processes observed at other wavelengths. These regions, described as auroral-related hotspots, observed at approximately 160°W to 200°W (System III) at high-northern latitudes and 330°W to 80°W at high-southern latitudes, indicate that auroral processes modify the thermal structure and composition of the neutral atmosphere. However, previous studies have struggled to differentiate whether the aforementioned enhanced emission is a result of either temperature changes and/or changes in the concentration of the emitting species. We attempt to address this degeneracy in this work by performing a retrieval analysis of Voyager 1-IRIS spectra (acquired in 1979) and Cassini-CIRS spectra (acquired in 2000/2001) of Jupiter. Retrievals of the vertical temperature profile in Cassini-CIRS spectra covering the auroral-related hotspots indicate the presence of two discrete vertical regions of heating at the 1-mbar level and at pressures of 10-μbar and lower. For example, in Cassini-CIRS 2.5 cm-1 'MIRMAP' spectra at 70°N (planetographic) 180°W (centred on the auroral oval), we find temperatures at the 1-mbar level and 10-μbar levels are enhanced by 15.3 ± 5.2 K and 29.6 ± 15.0 K respectively, in comparison to results at 70°N, 60°W in the same dataset. High temperatures at 10 μbar and lower pressures were considered indicative of joule heating, ion and/or electron precipitation, ion-drag and energy released form exothermic ion-chemistry. However, we conclude that the heating at the 1-mbar level is the result of either a layer of aurorally-produced haze particles, which are heated by incident sunlight and/or adiabatic heating by downwelling within the auroral hot-spot region. The former mechanism would be consistent with the vertical profiles of polycyclic aromatic hydrocarbons (PAHs) and haze particles predicted in auroral-chemistry models (Wong et al., 2000; 2003). Retrievals of C2H2 and C2H6 were also performed and indicate C2H2 is enriched but C2H6 is depleted in auroral regions relative to quiescent regions. For example, using CIRS δν ˜= 2.5 cm-1 spectra, we determined that C2H2 at 0.98 mbar increases by 175.3 ± 89.3 ppbv while C2H6 at 4.7 mbar decreases by 0.86 ± 0.59 ppmv in comparing results at 70°N, 180°W and 70°N, 60°W. These results represent a mean of values retrieved from different initial assumptions and thus we believe they are robust. We believe these contrasts in C2H2 and C2H6 between auroral and quiescent regions can be explained by a coupling of auroral-driven chemistry and horizontal advection. Ion-neutral and electron recombination chemistry in the auroral region enriches all C2 hydrocarbons but in particular, the unsaturated C2H2 and C2H4 hydrocarbons. Once advected outside of the auroral region, the unsaturated C2 hydrocarbons are converted into C2H6 by neutral photochemistry thereby enriching C2H6 in quiescent regions, which gives the impression it is depleted inside the auroral region

From Voyager-IRIS to Cassini-CIRS: Interannual variability in Saturn's stratosphere?

We present an intercomparison of Saturn's stratosphere between Voyager 1-IRIS observations in 1980 and Cassini-CIRS observations in 2009 and 2010. Over a saturnian year (~29.5. years) has now passed since the Voyager flybys of Saturn in 1980/1981. Cassini observations in 2009/2010 capture Saturn in the same season as Voyager observations (just after the vernal equinox) but one year later. Any differences in Saturn's atmospheric properties implied by a comparison of these two datasets could therefore reveal the extent of interannual variability. We retrieve temperature and stratospheric acetylene and ethane concentrations from Voyager 1-IRIS (δν̃=4.3cm-1) observations in 1980 and Cassini-CIRS (δν̃=15.5cm-1) 'FIRMAP' observations in 2009 and 2010. We observe a difference in temperature at the equator of 7.1. ±. 1.2. K at the 2.1-mbar level that implies that the two datasets have captured Saturn's semiannual oscillation (SSAO) in a slightly different phase suggesting that its period is more quasi-semiannual. Elevated concentrations of acetylene at 25°S in 1980 with respect to 2010 imply stronger downwelling at the former date which may also be explained by a difference in the phase of the SSAO and its dynamical forcing at low latitudes. At high-southern and high-northern latitudes, stratospheric temperatures and hydrocarbon concentrations appear elevated in 1980 with respect to 2009/2010. This could be an artefact of the low signal-to-noise ratio of the corresponding observations but might also be explained by increased auroral activity during solar maximum in 1980. © 2014 Elsevier Inc

Jupiter's auroral-related stratospheric heating and chemistry I: Analysis of Voyager-IRIS and Cassini-CIRS spectra

Auroral processes are evident in Jupiter's polar atmosphere over a large range in wavelength (X-ray to radio). In particular, previous observations in the mid-infrared (5-15 μm) have shown enhanced emission from CH4, C2H2 and C2H4 and further stratospheric hydrocarbon species in spatial regions coincident with auroral processes observed at other wavelengths. These regions, described as auroral-related hotspots, observed at approximately 160°W to 200°W (System III) at high-northern latitudes and 330°W to 80°W at high-southern latitudes, indicate that auroral processes modify the thermal structure and composition of the neutral atmosphere. However, previous studies have struggled to differentiate whether the aforementioned enhanced emission is a result of either temperature changes and/or changes in the concentration of the emitting species. We attempt to address this degeneracy in this work by performing a retrieval analysis of Voyager 1-IRIS spectra (acquired in 1979) and Cassini-CIRS spectra (acquired in 2000/2001) of Jupiter. Retrievals of the vertical temperature profile in Cassini-CIRS spectra covering the auroral-related hotspots indicate the presence of two discrete vertical regions of heating at the 1-mbar level and at pressures of 10-μbar and lower. For example, in Cassini-CIRS 2.5 cm-1 'MIRMAP' spectra at 70°N (planetographic) 180°W (centred on the auroral oval), we find temperatures at the 1-mbar level and 10-μbar levels are enhanced by 15.3 ± 5.2 K and 29.6 ± 15.0 K respectively, in comparison to results at 70°N, 60°W in the same dataset. High temperatures at 10 μbar and lower pressures were considered indicative of joule heating, ion and/or electron precipitation, ion-drag and energy released form exothermic ion-chemistry. However, we conclude that the heating at the 1-mbar level is the result of either a layer of aurorally-produced haze particles, which are heated by incident sunlight and/or adiabatic heating by downwelling within the auroral hot-spot region. The former mechanism would be consistent with the vertical profiles of polycyclic aromatic hydrocarbons (PAHs) and haze particles predicted in auroral-chemistry models (Wong et al., 2000; 2003). Retrievals of C2H2 and C2H6 were also performed and indicate C2H2 is enriched but C2H6 is depleted in auroral regions relative to quiescent regions. For example, using CIRS δν ˜= 2.5 cm-1 spectra, we determined that C2H2 at 0.98 mbar increases by 175.3 ± 89.3 ppbv while C2H6 at 4.7 mbar decreases by 0.86 ± 0.59 ppmv in comparing results at 70°N, 180°W and 70°N, 60°W. These results represent a mean of values retrieved from different initial assumptions and thus we believe they are robust. We believe these contrasts in C2H2 and C2H6 between auroral and quiescent regions can be explained by a coupling of auroral-driven chemistry and horizontal advection. Ion-neutral and electron recombination chemistry in the auroral region enriches all C2 hydrocarbons but in particular, the unsaturated C2H2 and C2H4 hydrocarbons. Once advected outside of the auroral region, the unsaturated C2 hydrocarbons are converted into C2H6 by neutral photochemistry thereby enriching C2H6 in quiescent regions, which gives the impression it is depleted inside the auroral region

Jupiter’s auroral-related stratospheric heating and chemistry II: Analysis of IRTF-TEXES spectra measured in December 2014

We present a retrieval analysis of TEXES (Texas Echelon Cross Echelle Spectrograph (Lacy et al., 2002)) spectra of Jupiter's high latitudes obtained on NASA's Infrared Telescope Facility on December 10 and 11th 2014. The vertical temperature profile and vertical profiles of C 2 H 2 , C 2 H 4 and C 2 H 6 were retrieved at both high-northern and high-southern latitudes and results were compared in ‘quiescent’ regions and regions known to be affected by Jupiter's aurora in order to highlight how auroral processes modify the thermal structure and hydrocarbon chemistry of the stratosphere. In qualitative agreement with Sinclair et al. (2017a), we find temperatures in auroral regions to be elevated with respect to quiescent regions at two discrete pressures levels at approximately 1 mbar and 0.01 mbar. For example, in comparing retrieved temperatures at 70°N, 60°W (a representative quiescent region) and 70°N, 180°W (centred on the northern auroral oval), temperatures increase by 19.0 ± 4.2 K at 0.98 mbar, 20.8 ± 3.9 K at 0.01 mbar but only by 8.3 ± 4.9 K at the intermediate level of 0.1 mbar. We conclude that elevated temperatures at 0.01 mbar result from heating by joule resistance of the atmosphere and the energy imparted by electron and ion precipitation. However, temperatures at 1 mbar are considered to result either from heating by shortwave radiation of aurorally-produced haze particulates or precipitation of higher energy population of charged particles. Our former conclusion would be consistent with results of auroral-chemistry models, that predict the highest number densities of aurorally-produced haze particles at this pressure level (Wong et al., 2000, 2003). C 2 H 2 and C 2 H 4 exhibit enrichments but C 2 H 6 remains constant within uncertainty when comparing retrieved concentrations in the northern auroral region with quiescent longitudes in the same latitude band. At 1 mbar, C 2 H 2 increases from 278.4 ± 40.3 ppbv at 70°N, 60°W to 564.4 ± 72.0 ppbv at 70°N, 180°W and at 0.01 mbar, over the same longitude range at 70°N, C 2 H 4 increases from 0.669 ± 0.129 ppmv to 6.509 ± 0.811 ppmv. However, we note that non-LTE (local thermodynamic equilibrium) emission may affect the cores of the strongest C 2 H 2 and C 2 H 4 lines on the northern auroral region, which may be a possible source of error in our derived concentrations. We retrieved concentrations of C 2 H 6 at 1 mbar of 9.03 ± 0.98 ppmv at 70°N, 60°W and 7.66 ± 0.70 ppmv at 70°N, 180°W. Thus, C 2 H 6 's concentration appears constant (within uncertainty) as a function of longitude at 70°N

Seasonal variations of temperature, acetylene and ethane in Saturn's atmosphere from 2005 to 2010, as observed by Cassini-CIRS

Acetylene (C2H2) and ethane (C2H6) are by-products of complex photochemistry in the stratosphere of Saturn. Both hydrocarbons are important to the thermal balance of Saturn's stratosphere and serve as tracers of vertical motion in the lower stratosphere. Earlier studies of Saturn's hydrocarbons using Cassini-CIRS observations have provided only a snapshot of their behaviour. Following the vernal equinox in August 2009, Saturn's northern and southern hemispheres have entered spring and autumn, respectively, however the response of Saturn's hydrocarbons to this seasonal shift remains to be determined. In this paper, we investigate how the thermal structure and concentrations of acetylene and ethane have evolved with the changing season on Saturn. We retrieve the vertical temperature profiles and acetylene and ethane volume mixing ratios from δν̃=15.5cm-1 Cassini-CIRS observations. In comparing 2005 (solar longitude, Ls~308°), 2009 (Ls~3°) and 2010 (Ls~15°) results, we observe the disappearance of Saturn's warm southern polar hood with cooling of up to 17.1K±0.8K at 1.1mbar at high-southern latitudes. Comparison of the derived temperature trend in this region with a radiative climate model (Section 4 of Fletcher et al., 2010 and Greathouse et al. (2013, in preparation)) indicates that this cooling is radiative although dynamical changes in this region cannot be ruled out. We observe a21±12% enrichment of acetylene and a 29±11% enrichment of ethane at 25°N from 2005 to 2009, suggesting downwelling at this latitude. At 15°S, both acetylene and ethane exhibit a decrease in concentration of 6±11% and 17±9% from 2005 to 2010, respectively, which suggests upwelling at this latitude (though a statistically significant change is only exhibited by ethane). These implied vertical motions at 15°S and 25°N are consistent with a recently-developed global circulation model of Saturn's tropopause and stratosphere(Friedson and Moses, 2012), which predicts this pattern of upwelling and downwelling as a result of a seasonally-reversing Hadley circulation. Ethane exhibits a general enrichment at mid-northern latitudes from 2005 to 2009. As the northern hemisphere approaches summer solstice in 2017, this feature might indicate an onset of a meridional enrichment of ethane, as has been observed in the southern hemisphere during/after southern summer solstice. © 2013 Elsevier Inc

Assessing the long-term variability of acetylene and ethane in the stratosphere of Jupiter

Acetylene (C 2 H 2 ) and ethane (C 2 H 6 ) are both produced in the stratosphere of Jupiter via photolysis of methane (CH 4 ). Despite this common source, the latitudinal distribution of the two species is radically different, with acetylene decreasing in abundance towards the pole, and ethane increasing towards the pole. We present six years of NASA IRTF TEXES mid-infrared observations of the zonally-averaged emission of methane, acetylene and ethane. We confirm that the latitudinal distributions of ethane and acetylene are decoupled, and that this is a persistent feature over multiple years. The acetylene distribution falls off towards the pole, peaking at ~30°N with a volume mixing ratio (VMR) of ~0.8 parts per million (ppm) at 1 mbar and still falling off at ± 70° with a VMR of ~0.3 ppm. The acetylene distributions are asymmetric on average, but as we move from 2013 to 2017, the zonally-averaged abundance becomes more symmetric about the equator. We suggest that both the short term changes in acetylene and its latitudinal asymmetry is driven by changes to the vertical stratospheric mixing, potentially related to propagating wave phenomena. Unlike acetylene, ethane has a symmetric distribution about the equator that increases toward the pole, with a peak mole fraction of ~18 ppm at about ± 50° latitude, with a minimum at the equator of ~10 ppm at 1 mbar. The ethane distribution does not appear to respond to mid-latitude stratospheric mixing in the same way as acetylene, potentially as a result of the vertical gradient of ethane being much shallower than that of acetylene. The equator-to-pole distributions of acetylene and ethane are consistent with acetylene having a shorter lifetime than ethane that is not sensitive to longer advective timescales, but is augmented by short-term dynamics, such as vertical mixing. Conversely, the long lifetime of ethane allows it to be transported to higher latitudes faster than it can be chemically depleted

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

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

The origin of nitrogen on Jupiter and Saturn from the N-15/N-14 ratio

The Texas Echelon cross Echelle Spectrograph (TEXES), mounted on NASA’s Infrared Telescope Facility (IRTF), was used to map mid-infrared ammonia absorption features on both Jupiter and Saturn in February 2013. Ammonia is the principle reservoir of nitrogen on the giant planets, and the ratio of isotopologues (15N/14N) can reveal insights into the molecular carrier (e.g., as N2 or NH3) of nitrogen to the forming protoplanets, and hence the source reservoirs from which these worlds accreted. We targeted two spectral intervals (900 and 960 cm1) that were relatively clear of terrestrial atmospheric contamination and contained close features of 14NH3 and 15NH3, allowing us to derive the ratio from a single spectrum without ambiguity due to radiometric calibration (the primary source of uncertainty in this study). We present the first ground-based determination of Jupiter’s 15N/14N ratio (in the range from 1:4 103 to 2:5 103), which is consistent with both previous space-based studies and with the primordial value of the protosolar nebula. On Saturn, we present the first upper limit on the 15N/14N ratio of no larger than 2:0 103 for the 900-cm1 channel and a less stringent requirement that the ratio be no larger than 2:8 103 for the 960-cm1 channel (1r confidence). Specifically, the data rule out strong 15N-enrichments such as those observed in Titan’s atmosphere and in cometary nitrogen compounds. To the extent possible with ground-based radiometric uncertainties, the saturnian and jovian 15N/14N ratios appear indistinguishable, implying that 15N-enriched ammonia ices could not have been a substantial contributor to the bulk nitrogen inventory of either planet. This result favours accretion of primordial N2 on both planets, either in the gas phase from the solar nebula, or as ices formed at very low temperatures. Finally, spatially-resolved TEXES observations are used to derive zonal contrasts in tropospheric temperatures, phosphine and 14NH3 on both planets, allowing us to relate thermal conditions and chemical compositions to phenomena observed at visible wavelengths in 2013 (e.g., Jupiter’s faint equatorial red colouration event and wave activity in the equatorial belts, plus the remnant warm band on Saturn following the 2010–11 springtime storm)