arXiv subjects

James A. Sinclair

Publications and source records attributed to James A. Sinclair.

15 recordsLinked to original sources

arXiv2026

Detection of propadiene (CH$_2$CCH$_2$), propene (C$_3$H$_6$) and non-detection of propane (C$_3$H$_8$) in Jupiter's northern polar stratosphere

We report the first detection of stratospheric propadiene (CH$_2$CCH$_2$) and propene (C$_3$H$_6$) at Jupiter's mid-to-high northern latitudes using IRTF-TEXES measurements recorded on March 5-6, 2025. Using radiative transfer software to quantitatively test for the presence of propadiene and propene, we report a $>$12-$\sigma$ detection of propadiene and a $>$17-$\sigma$ detection of propene inside Jupiter's northern auroral region (henceforth 'NAR'), where the species are most concentrated. For example, at 62$^\circ$N inside Jupiter's NAR, we derive a 1-mbar propadiene abundance of 2.0 $\pm$ 0.2 ppbv, which is 40 $\pm$ 3 higher than abundances predicted by the Moses & Poppe (2017) photochemical model (henceforth 'MP17'), and significantly higher than the 1.2-ppbv upper limit abundance derived at 42$^\circ$N (the lowest latitude sampled by the observations). Similarly, we derive a 1-mbar propene abundance of 8.1 $\pm$ 0.5 ppbv at 62$^\circ$N inside Jupiter's NAR, which is 28 $\pm$ 2 higher than the MP17 predicted abundance and higher than the 6-ppbv 1-mbar upper limit abundance derived at 42$^\circ$N. The fact that propadiene and propene are most enriched inside Jupiter's NAR strongly suggests that perturbations to the chemistry by auroral-related heating and exogenous ions/electrons are responsible for their significant enrichment. Spectral features of propane (C$_3$H$_8$) were not detected at any of the locations sampled by the data: 3-$\sigma$ upper limits of 10 ppbv were derived at the 10-mbar level at 62$^\circ$N inside Jupiter's NAR. The non-detection of propane could, in part, be explained by the vertical sensitivity of its spectral features to deeper pressures, where there is negligible auroral-related heating. The results of this work advocate for development of ion-neutral chemistry models of Jupiter's polar stratosphere.

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arXiv2025

Improved H2-He and H2-H2 Collision-Induced Absorption Models and Application to Outer-Planet Atmospheres

Using state-of-the-art ab initio interaction-induced dipole and potential-energy surfaces for hydrogen-helium (H2-He) pairs, we compute the rototranslational collision-induced absorption coefficient at 40-400 K for frequencies covering 0-4000 cm-1. The quantum mechanical scattering calculations account for the full anisotropic interaction potential, replacing the isotropic approximation. The absorption data are expected to be accurate with an uncertainty of 2% or better up to 2500 cm-1. The uncertainty is slightly higher at the highest frequencies where the rototranslational absorption is largely obscured by the rovibrational band. Our improved agreement with measurements at 200-800 cm-1 results from the improvement of the potential energy surface. The previously available rototranslational data set for H2-H2 pairs (Fletcher et al., Astrophys. J. Supp. 235, 24 (2018)) is also extended up to 4000 cm-1. In the rovibrational band previous isotropic potential calculations for H2-He (Gustafsson et al. J. Chem. Physics. 113, 3641 (2000)) and H2-H2 (Borysow, Icarus 92, 273 (1992)) have been extended to complement the rototranslational data set. The absorption coefficients are tabulated for ortho-to-para ratios from normal-H2 to pure para-H2, as well as equilibrium-H2, over 40-400 K. The effect of these updates are simulated for the cold atmosphere of Uranus and warmer atmosphere of Jupiter. They are equivalent to a brightness temperature difference of a fraction of a degree in the rototranslational region but up to 4 degrees in the rovibrational region. Our state-of-the-art modifications correct an otherwise +2% error in determining the He/H2 ratio in Uranus from its spectrum alone.

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arXiv2024

The Polar Stratosphere of Jupiter

Observations of the Jovian upper atmosphere at high latitudes in the UV, IR and mm/sub-mm all indicate that the chemical distributions and thermal structure are broadly influenced by auroral particle precipitations. Mid-IR and UV observations have shown that several light hydrocarbons (up to 6 carbon atoms) have altered abundances near Jupiter's main auroral ovals. Ion-neutral reactions influence the hydrocarbon chemistry, with light hydrocarbons produced in the upper stratosphere, and heavier hydrocarbons as well as aerosols produced in the lower stratosphere. One consequence of the magnetosphere-ionosphere coupling is the existence of ionospheric jets that propagate into the neutral middle stratosphere, likely acting as a dynamical barrier to the aurora-produced species. As the ionospheric jets and the background atmosphere do not co-rotate at the same rate, this creates a complex system where chemistry and dynamics are intertwined. The ion-neutral reactions produce species with a spatial distribution following the SIII longitude system in the upper stratosphere. As these species sediment down to the lower stratosphere, and because of the progressive dynamical decoupling between the ionospheric flows and the background atmosphere, the spatial distribution of the auroral-related species progressively follows a zonal distribution with increasing pressures that ultimately produces a system of polar and subpolar hazes that extends down to the bottom of the stratosphere. This paper reviews the most recent work addressing different aspects of this environment.

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arXiv2024

Evidence for auroral influence on Jupiter's nitrogen and oxygen chemistry revealed by ALMA

The localized delivery of new long-lived species to Jupiter's stratosphere by comet Shoemaker-Levy 9 in 1994 opened a window to constrain Jovian chemistry and dynamics by monitoring the evolution of their vertical and horizontal distributions. However, the spatial distributions of CO and HCN, two of these long-lived species, had never been jointly observed at high latitudinal resolution. Atacama large millimeter/submillimeter array observations of HCN and CO in March 2017 show that CO was meridionally uniform and restricted to pressures lower than 3 $\pm$ 1 mbar. HCN shared a similar vertical distribution in the low- to mid-latitudes, but was depleted at pressures between 2$^{+2}_ {-1}$ and 0.04$^{+0.07}_{-0.03}$ mbar in the aurora and surrounding regions, resulting in a drop by two orders of magnitude in column density. We propose that heterogeneous chemistry bonds HCN on large aurora-produced aerosols at these pressures in the Jovian auroral regions causing the observed depletion.

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arXiv2024

Temperature and composition disturbances in the southern auroral region of Jupiter revealed by JWST/MIRI

Jupiters south polar region was observed by JWST Mid Infrared Instrument in December 2022. We used the Medium Resolution Spectrometer mode to provide new information about Jupiters South Polar stratosphere. The southern auroral region was visible and influenced the atmosphere in several ways. 1: In the interior of the southern auroral oval, we retrieved peak temperatures at two distinct pressure levels near 0.01 and 1 mbar, with warmer temperatures with respect to non auroral regions of 12 pm 2 K and 37 pm 4 K respectively. A cold polar vortex is centered at 65S at 10 mbar. 2: We found that the homopause is elevated to 590+25-118 km above the 1-bar pressure level inside the auroral oval compared to 460+60-50 km at neighboring latitudes and with an upper altitude of 350 km in regions not affected by auroral precipitation. 3: The retrieved abundance of C2H2 shows an increase within the auroral oval, and it exhibits high abundances throughout the polar region. The retrieved abundance of C2H6 increases towards the pole, without being localized in the auroral oval, in contrast with previous analysis. We determined that the warming at 0.01 mbar and the elevated homopause might be caused by the flux of charged particles depositing their energy in the South Polar Region. The 1 mbar hotspot may arise from adiabatic heating resulting from auroral driven downwelling. The cold region at 10 mbar may be caused by radiative cooling by stratospheric aerosols. The differences in spatial distribution seem to indicate that the hydrocarbons analyzed are affected differently by auroral precipitation.

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arXiv2024

Investigating Thermal Contrasts Between Jupiter's Belts, Zones, and Polar Vortices with VLT/VISIR

Using images at multiple mid-infrared wavelengths, acquired in May 2018 using the VISIR instrument on ESO's Very Large Telescope (VLT), we study Jupiter's pole-to-pole thermal, chemical and aerosol structure in the troposphere and stratosphere. We confirm that the pattern of cool and cloudy anticyclonic zones and warm cloud-free cyclonic belts persists throughout the mid-latitudes, up to the polar boundaries, and evidence a strong correlation with the vertical maximum windshear and the locations of Jupiter's zonal jets. At high latitudes, VISIR images reveal a large region of mid-infrared cooling poleward $\sim$64$^{\circ}$N and $\sim$67$^{\circ}$S extending from the upper troposphere to the stratosphere, co-located with the reflective aerosols observed by JunoCam, and suggesting that aerosols play a key role in the radiative cooling at the poles. Comparison of zonal-mean thermal properties and high-resolution visible imaging from Juno allows us to study the variability of atmospheric properties as a function of altitude and jet boundaries, particularly in the cold southern polar vortex. However, the southern stratospheric polar vortex is partly masked by a warm mid-infrared signature of the aurora. Co-located with the southern main auroral oval, this warming results from the auroral precipitation and/or joule heating which heat the atmosphere and thus cause a significant stratospheric emission. This high emission results from a large enhancement of both ethane and acetylene in the polar region, reinforcing the evidence of enhanced ion-related chemistry in Jupiter's auroral regions.

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arXiv2023

Long term variability of Jupiter's northern auroral 8-micron CH4 emissions

We present a study of the long term variability of Jupiter's mid-infrared auroral CH4 emissions. 7.7 - 7.9 micron images of Jupiter recorded by Earth-based telescopes over the last three decades were collated in order to quantify the magnitude and timescales over which the northern auroral hotspot's CH4 emissions varies. We find that the ratio of the radiance of the poleward northern auroral emissions to a lower-latitude zonal mean, henceforth 'Relative Poleward Radiance' or RPR, exhibits a 37% variability over a range of timescales. We searched for patterns of variability in order to test whether seasonally-varying solar insolation, the 11-year solar cycle, or short-term solar wind variability at Jupiter's magnetopause could explain the observed evolution. The variability of the RPR exhibits a weak (r < 0.2) correlation with the solar insolation received at Jupiter's high-northern latitudes. This rules out the hypothesis suggested in previous work (e.g. Sinclair et al., 2017a) that shortwave solar heating of aurorally-produced haze particles is the dominant heating mechanism in the lower stratosphere. We also find the variability exhibits negligible (r < 0.18) correlation with the monthly-mean sunspot number, which rules out variability associated with the solar cycle. On shorter timescales, we find moderate correlations of the RPR with solar wind conditions at Jupiter in the preceding days before images were recorded. For example, we find correlations of r = 0.45 and r = 0.51 of the RPR with the mean and standard deviation on the solar wind dynamical pressure in the preceding 7 days. The moderate correlation suggests that either: 1) only a subset of solar wind compressions lead to brighter, poleward, CH4 emissions and/or 2) a subset of CH4 emission brightening events are driven by internal magnetospheric and independent of the solar wind.

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arXiv2023

Enhanced C$_2$H$_2$ absorption within Jupiter's southern auroral oval from Juno UVS observations

Reflected sunlight observations from the Ultraviolet Spectrograph (UVS) on the Juno spacecraft were used to study the distribution of acetylene (C$_2$H$_2$) at Jupiter's south pole. We find that the shape of the C$_2$H$_2$ absorption feature varies significantly across the polar region, and this can be used to infer spatial variability in the C$_2$H$_2$ abundance. There is a localized region of enhanced C$_2$H$_2$ absorption which coincides with the location of Jupiter's southern polar aurora; the C$_2$H$_2$ abundance poleward of the auroral oval is a factor of 3 higher than adjacent quiescent, non-auroral longitudes. This builds on previous infrared studies which found enhanced C$_2$H$_2$ abundances within the northern auroral oval. This suggests that Jupiter's upper-atmosphere chemistry is being strongly influenced by the influx of charged auroral particles and demonstrates the necessity of developing ion-neutral photochemical models of Jupiter's polar regions.

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arXiv2023

Unexpected Long-Term Variability in Jupiter's Tropospheric Temperatures

An essential component of planetary climatology is knowledge of the tropospheric temperature field and its variability. Previous studies of Jupiter hinted at periodic behavior that was non-seasonal, as well as dynamical relationships between tropospheric and stratospheric temperatures. However, these observations were made over time frames shorter than Jupiter's orbit or they used sparse sampling. We derived upper-tropospheric (300-mbar) temperatures over 40 years, extending those studies to cover several orbits of Jupiter, revealing unexpected results. Periodicities of 4, 7 8-9 and 10-14 years were discovered that involved different latitude bands and seem disconnected from seasonal changes in solar heating. Anti-correlations of variability in opposite hemispheres were particularly striking at 16, 22 and 30 degrees from the equator. Equatorial temperature variations are also anticorrelated with those 60-70 km above. Such behavior suggests a top-down control of equatorial tropospheric temperatures from stratospheric dynamics. Realistic future global climate models must address the origins of these variations in preparation for their extension to a wider array of gas-giant exoplanets.

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arXiv2023

A retrospective analysis of mid-infrared observations of the Comet D/Shoemaker-Levy 9 and Wesley impacts on Jupiter

We present a retrospective analysis of Earth-based mid-infrared observations of Jupiter capturing the aftermath of the impacts by Comet D/Shoemaker-Levy 9 (henceforth SL9) in July 1994 and the Wesley impactor in July 2009. While the atmospheric effects of both impacts have been reported previously, we were motivated to re-examine both events using consistent methods to enable robust, quantitative comparisons. We analyzed spectrophotometry and spectroscopy capturing both impacts using two independent analyses: 1) a least-squares search over a grid of candidate mineral species to determine the composition of impact residue and 2) a radiative transfer analysis to derive atmospheric information. We observe that the SL9 impact sites are enhanced in stratospheric CH4 emissions at 7.9 um, due to shock heating and adiabatic compression from plume re-entry, and from 8.5 - 11.5 um due to stratospheric NH3 emission and non-gaseous cometary material. We derive NH3 concentrations of 5.7 ppmv at 30 mbar. In new findings, we find that the SL9 impact sites also exhibit a non-gaseous emission feature at 18 - 19 um. The non-gaseous emission at 8.5 - 11.5 and 18 - 19 um emission is best reproduced by predominantly amorphous olivine and obsidian at similar abundances. The Wesley impact site exhibits enhanced emissions from 8.8 - 11.5 and 18 - 19 um. We found this could be reproduced by predominantly amorphous olivine and stratospheric NH3 at concentrations of 150 ppbv at 30 mbar. Stratospheric NH3 abundances are a factor of 40 higher in the SL9 impacts compared to the Wesley impact, which confirms the former reached deeper, NH3-richer altitudes of the atmosphere. The absence of silicas in the Wesley impact would place an upper limit of 10 km/s on the incident velocity and 9 degree on the entry angle of the impactor such that temperatures were insufficient to convert silicates.

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arXiv2022

Saturn's Seasonal Variability from Four Decades of Ground-Based Mid-Infrared Observations

A multi-decade record of ground-based mid-infrared (7-25 $μ$m) images of Saturn is used to explore seasonal and non-seasonal variability in thermal emission over more than a Saturnian year (1984-2022). Thermal emission measured by 3-m and 8-m-class observatories compares favourably with synthetic images based on both Cassini-derived temperature records and the predictions of radiative climate models. 8-m class facilities are capable of resolving thermal contrasts on the scale of Saturn's belts, zones, polar hexagon, and polar cyclones, superimposed onto large-scale seasonal asymmetries. Seasonal changes in brightness temperatures of $\sim30$ K in the stratosphere and $\sim10$ K in the upper troposphere are observed, as the northern and southern polar stratospheric vortices (NPSV and SPSV) form in spring and dissipate in autumn. The timings of the first appearance of the warm polar vortices is successfully reproduced by radiative climate models, confirming them to be radiative phenomena, albeit entrained within sharp boundaries influenced by dynamics. Axisymmetric thermal bands (4-5 per hemisphere) display temperature gradients that are strongly correlated with Saturn's zonal winds, indicating winds that decay in strength with altitude, and implying meridional circulation cells forming the system of cool zones and warm belts. Saturn's thermal structure is largely repeatable from year to year (via comparison of infrared images in 1989 and 2018), with the exception of low-latitudes. Here we find evidence of inter-annual variations because the equatorial banding at 7.9 $μ$m is inconsistent with a $\sim15$-year period for Saturn's equatorial stratospheric oscillation, i.e., it is not strictly semi-annual. Finally, observations between 2017-2022 extend the legacy of the Cassini mission, revealing the continued warming of the NPSV during northern summer. [Abr.]

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arXiv2022

Saturn's atmospheric response to the large influx of ring material inferred from Cassini INMS measurements

During the Grand Finale stage of the Cassini mission, organic-rich ring material was discovered to be flowing into Saturn's equatorial upper atmosphere at a surprisingly large rate. Through a series of photochemical models, we have examined the consequences of this ring material on the chemistry of Saturn's neutral and ionized atmosphere. We find that if a substantial fraction of this material enters the atmosphere as vapor or becomes vaporized as the solid ring particles ablate upon atmospheric entry, then the ring-derived vapor would strongly affect the composition of Saturn's ionosphere and neutral stratosphere. Our surveys of Cassini infrared and ultraviolet remote-sensing data from the final few years of the mission, however, reveal none of these predicted chemical consequences. We therefore conclude that either (1) the inferred ring influx represents an anomalous, transient situation that was triggered by some recent dynamical event in the ring system that occurred a few months to a few tens of years before the 2017 end of the Cassini mission, or (2) a large fraction of the incoming material must have been entering the atmosphere as small dust particles less than ~100 nm in radius, rather than as vapor or as large particles that are likely to ablate. Future observations or upper limits for stratospheric neutral species such as HC$_3$N, HCN, and CO$_2$ at infrared wavelengths could shed light on the origin, timing, magnitude, and nature of a possible vapor-rich ring-inflow event.

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arXiv2020

Vertical Structure and Color of Jovian Latitudinal Cloud Bands during the Juno Era

The identity of the coloring agent(s) in Jupiter's atmosphere and the exact structure of Jupiter's uppermost cloud deck are yet to be conclusively understood. The Crème Brûlée model of Jupiter's tropospheric clouds, originally proposed by Baines et al. (2014) and expanded upon by Sromovsky et al. (2017) and Baines et al. (2019), presumes that the chromophore measured by Carlson et al. (2016) is the singular coloring agent in Jupiter's troposphere. In this work, we test the validity of the Crème Brûlée model of Jupiter's uppermost cloud deck using spectra measured during the Juno spacecraft's 5$^{\mathrm{th}}$ perijove pass in March 2017. These data were obtained as part of an international ground-based observing campaign in support of the Juno mission using the NMSU Acousto-optic Imaging Camera (NAIC) at the 3.5-m telescope at Apache Point Observatory in Sunspot, NM. We find that the Crème Brûlée model cloud layering scheme can reproduce Jupiter's visible spectrum both with the Carlson et al. (2016) chromophore and with modifications to its imaginary index of refraction spectrum. While the Crème Brûlée model provides reasonable results for regions of Jupiter's cloud bands such as the North Equatorial Belt and Equatorial Zone, we find that it is not a safe assumption for unique weather events, such as the 2016-2017 Southern Equatorial Belt outbreak that was captured by our measurements.

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arXiv2017

Ammonia in Jupiter's troposphere from high-resolution 5-\textmu m spectroscopy

Jupiter's tropospheric ammonia (NH\textsubscript{3}) abundance is studied using spatially-resolved 5-\textmu m observations from CRIRES, a high resolution spectrometer at the European Southern Observatory's Very Large Telescope. The high resolving power (R=96,000) allows the line shapes of three NH\textsubscript{3} absorption features to be resolved. We find that within the 1--4 bar pressure range, the NH\textsubscript{3} abundance decreases with altitude. The instrument slit was aligned north-south along Jupiter's central meridian, allowing us to search for latitudinal variability. There is considerable uncertainty in the large-scale latitudinal variability, as the increase in cloud opacity in zones compared to belts can mask absorption features. However, we do find evidence for a strong NH\textsubscript{3} enhancement at 4--6$^{\circ}$N, consistent with a localised `ammonia plume' on the southern edge of Jupiter's North Equatorial Belt.

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arXiv2014

The Origin of Nitrogen on Jupiter and Saturn from the $^{15}$N/$^{14}$N 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 ($^{15}$N/$^{14}$N) can reveal insights into the molecular carrier (e.g., as N$_2$ or NH$_3$) of nitrogen to the forming protoplanets, and hence the source reservoirs from which these worlds accreted. We targeted two spectral intervals (900 and 960 cm$^{-1}$) that were relatively clear of terrestrial atmospheric contamination and contained close features of $^{14}$NH$_3$ and $^{15}$NH$_3$, 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 $^{15}$N/$^{14}$N ratio (in the range from $1.4\times10^{-3}$ to $2.5\times10^{-3}$), 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 $^{15}$N/$^{14}$N ratio of no larger than $2.0\times10^{-3}$ for the 900-cm$^{-1}$ channel and a less stringent requirement that the ratio be no larger than $2.8\times10^{-3}$ for the 960-cm$^{-1}$ channel ($1σ$ confidence). Specifically, the data rule out strong $^{15}$N-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 $^{15}$N/$^{14}$N ratios appear indistinguishable, implying that $^{15}$N-enriched ammonia ices could not have been a substantial contributor to the bulk nitrogen inventory of either planet, favouring the accretion of primordial N$_2$ from the gas phase or as low-temperature ices.

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