arXiv subjects

Emmanuel Lellouch

Publications and source records attributed to Emmanuel Lellouch.

At least 19 recordsLinked to original sources

arXiv2026

Unveiling the Trans-Neptunian Region with the SKA

We explore the potential of the SKA staged delivery AA4 (SKA-AA4) to detect and characterise the thermal emission of trans-Neptunian objects (TNOs) and Centaurs at centimetre wavelengths. These distant icy bodies preserve important information on the formation and evolution of the outer Solar System and provide a valuable link to planetary systems observed around other stars. The unprecedented sensitivity of SKA-AA4 will enable thermal detections of several of the brightest TNOs and Centaurs, extending radiometric studies beyond the capabilities of current radio facilities. Combined with shorter-wavelength observations and occultation measurements, these data will provide new constraints on spectral emissivity behaviour, thermophysical properties, and subsurface structure. At its highest angular resolutions, SKA-AA4 may partially resolve the largest systems, enabling investigations of surface heterogeneity, extended structures such as rings, and the partial resolution of some wide binary systems. We also assess the feasibility of radio occultation observations, which can provide independent constraints on object sizes, shapes, atmospheres, rings, or satellites. Together with observations from ALMA, JWST, and occultation campaigns, SKA-AA4 will establish centimetre-wavelength studies as a powerful new tool for investigating the thermal and structural properties of outer Solar System bodies.

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arXiv2026

Constraints on the Atmospheric Composition of 2002 XV$_{93}$ from JWST Spectroscopy

The recent detection of an atmosphere surrounding the trans-Neptunian object (TNO) 2002 XV$_{93}$ from stellar occultation measurements has challenged the longstanding view that only the largest TNOs can sustain an atmosphere. Atmospheric refraction modeling of the occultation light curves indicated a surface pressure of 100$-$200 nbar, despite 2002 XV$_{93}$'s relatively small size (~510 km in diameter) and weak surface gravity. Together with the detection of methane fluorescence on Makemake, this result suggests that tenuous atmospheres may be more common among TNOs than previously thought. We report JWST/NIRSpec observations acquired before and after the 2024 stellar occultation measurements, obtained with the PRISM and medium-resolution gratings at resolving powers of ~100 and ~1000, respectively. We detect no statistically significant emission features attributed to methane or carbon monoxide gas. By comparing the higher spectral resolution data with synthetic fluorescence models, we report upper limits for the methane and carbon monoxide surface partial pressures of $(3-10)\times10^{-6}$ and $(50-300)\times10^{-6}$ nbar, respectively, substantially below the atmospheric pressure inferred from the occultation measurements. Additionally, we report no evidence of an extended source of either methane gas or refractory material. Our results indicate that the atmospheric interpretation of the occultation measurements may require either a composition dominated by volatile species other than methane and carbon monoxide, with nitrogen and argon as possible candidates, or a methane-dominated atmosphere confined near the surface with a steeply decreasing vertical density profile.

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arXiv2026

Latitudinal variations in Neptune's temperature profile observed with ALMA

Despite the low solar irradiation it receives, Neptune shows a very active atmosphere with some of the most intense dynamics observed in Solar System atmospheres. Characterizing the atmospheric temperature profiles of the planet is a key to understand these observed processes. In this work, we derived the Neptune pressure-latitude thermal field, using 2016 ALMA measurements of the CO(3-2) spectral line at 345.796 GHz, with a spatial resolution of about 0.37" on Neptune's 2.24" disk. To analyse the data, we developed MCMC retrieval methods to derive both the temperature profiles and the CO abundance profile. We find that our data probes the upper troposphere and the lower stratosphere of the planet, between 2 bar and 0.1 mbar. Although temperature and CO profile are strongly correlated, simultaneous retrievals of both parameters for disk-integrated observations reveal a factor of 2-3 larger CO abundance in the stratosphere than in the troposphere, reinforcing the hypothesis of CO delivery by a recent cometary impact. CO retrievals with fixed temperature profile do not fit the spatially-resolved observations, implying underlying temperature variations. By performing temperature retrievals with spatially constant CO, we find distinct trends of the thermal profile between the southern polar region, mid-latitudes and the equator. At 10--100 mbar, this structure is consistent with that observed by Voyager 2 in 1989, i.e. colder mid-latitudes, with a warmer equator and south polar regions. At 300-600 mbar, however, we find a cold layer of about 45 K at Southern polar regions (-80{\deg}) which disappears towards mid-latitudes and the equator. By probing pressure levels not easily accessible to other observing methods e.g. infrared sounding, submillimeter observations offer a new view of the complex thermal structure in the upper troposphere and lower stratosphere of the Icy Giants.

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arXiv2026

Vertical Temperature Structure in Io's Atmosphere from ALMA SO$_2$ Observations

The structure of Io's atmosphere is controlled by competing processes, from volcanic outgassing and sublimation to radiative cooling and plasma heating. Yet, the lack of an observationally-derived temperature profile has left this balance unconstrained. We used four epochs of Atacama Large Millimeter/submillimeter Array (ALMA) Band 7 (275-373 GHz) and Band 8 (385-500 GHz) SO$_2$ spectroscopy to retrieve Io's vertical atmospheric temperature profiles. To mitigate longstanding degeneracies common in atmospheric retrievals, we performed a simultaneous multi-line analysis combined with line-of-sight disk-resolved Doppler velocity maps and a forward model that included a sub-beam velocity-dispersion term. This modeling approach enabled the separation of thermal and dynamical line-shape contributions. On the leading hemisphere, we retrieved a cold, quasi-isothermal lower atmosphere ($\sim$124-137~K up to $\sim$0.5~nbar), followed by a thermospheric rise reaching hundreds of kelvins by $\sim10^{-2}$~nbar. On the trailing hemisphere, our fits yielded qualitatively similar profiles but consistently retrieved lower SO$_2$ column densities. The lower column densities confined line formation to the first few kilometers, making the trailing hemisphere spectra statistically consistent with an isothermal atmosphere. Across datasets, we retrieved fractional gas coverages of $\sim$35-50$\%$ and sub-beam velocity dispersions of $\sim$25-85$\mathrm{~m~s^{-1}}$, encoding line-of-sight velocity dispersion within a beam element in excess of the disk-resolved Doppler velocity map. Together, these retrievals deliver the first vertically resolved temperature profiles of Io's atmosphere, reveal robust vertical structure on the dayside leading hemisphere, and offer new constraints on Io's thermal energy balance.

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arXiv2026

Surveying exogenous species in Saturn with ALMA I. Detecting and Mapping CO

The origin of carbon monoxide (CO) in Saturn's stratosphere remains uncertain, with proposed sources including internal thermochemical production, cometary impacts, and exogenic material from the rings and icy moons (i.e. Enceladus). We aim to constrain the vertical and meridional distribution of stratospheric CO and assess the relative contributions of these potential sources. Here, we analysed high-spectral-resolution ALMA observations of the CO (J=3-2) line obtained on 25 May 2018, sampling Saturn's limb from 20{\deg}S to 69{\deg}N. CO vertical profiles were retrieved using a line-by-line radiative transfer model combined with spectral inversion techniques, testing multiple prior scenarios representative of different source hypotheses. CO is confined to a narrow layer between 0.1 and 1 mbar, with a robust negative vertical gradient and mean abundances of (3.7+/- 0.8) x 10$^{-8}$ at 0.1 mbar and (7.2 +/- 0.9) x 10$^{-8}$ at 1 mbar. The meridional distribution is statistically homogeneous, with a marginal enhancement near 60{\deg} N plausibly related to Enceladus. No significant equatorial enhancement is detected. The absence of a strong equatorial enhancement rules out a long-lived steady source associated with ring infall. The observations are most consistent with a relatively recent ($\approx$200-year-old or younger) cometary impact whose material has since been horizontally mixed, while any Cassini Grand Finale ring influx was either too recent or inefficient to affect CO abundances at the probed pressure levels.

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arXiv2025

Why the Northern Hemisphere Needs a 30-40 m Telescope and the Science at Stake: Key Targets of Opportunity on Gas and Ice Giants and their satellites

The Extremely Large Telescope (ELT) will transform our knowledge of the outer planets and their satellite systems; however the visibility of unique targets of opportunity with high scientific value will be reduced for northern objects. Uranus' declination favors observations from the Northern Hemisphere until 2055, and Neptune will be favored from the Northern Hemisphere from 2027 for the next 90 years. Jupiter and Saturn experience cycles of better observability from either hemisphere on cycles of 10 and 30 years. These planets and their satellite systems often offer unique opportunities for discovery through time-critical observations. We argue that a 30-m class size telescope in the Northern Hemisphere with complementary scientific instrumentation to that on the ELT will secure the possibility of observing high-impact unpredictable phenomena in these systems.

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arXiv2025

CH$_3$OH and HCN in Interstellar Comet 3I/ATLAS Mapped with the ALMA Atacama Compact Array: Distinct Outgassing Behaviors and a Remarkably High CH$_3$OH/HCN Production Rate Ratio

We report the detection of methanol (CH$_3$OH) toward interstellar comet 3I/ATLAS using the Atacama Compact Array of the Atacama Large Millimeter/Submillimeter Array (ALMA) on UT 2025 August 28, September 18 and 22, and October 1, and of hydrogen cyanide (HCN) on September 12 and 15. These observations spanned pre-perihelion heliocentric distances ($r_H$) of 2.6 -- 1.7 au. The molecules showed outgassing patterns distinct from one another, with HCN production being depleted in the sunward hemisphere of the coma, whereas CH$_3$OH was enhanced in that direction. Statistical analysis of molecular scale lengths in 3I/ATLAS indicated that CH$_3$OH included production from coma sources at $L_p>258$ km at 99% confidence. However low signal-to-noise on long baselines, which sample emission on small spatial scales closest to the nucleus, prevented definitively ruling out CH$_3$OH as purely a parent species. In contrast, HCN production was indistinguishable from direct nucleus sublimation. The CH$_3$OH production rate increased sharply from August through October, including an uptick near the inner edge of the H$_2$O sublimation zone at $r_H$ = 2 au. Compared to comets measured to date at radio wavelengths, the derived CH$_3$OH/HCN ratios in 3I/ATLAS of $124^{+30}_{-34}$ and $79^{+11}_{-14}$ on September 12 and 15, respectively, are among the most enriched values measured in any comet, surpassed only by anomalous solar system comet C/2016 R2 (PanSTARRS).

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arXiv2025

Rotational Dynamics in Pulsational Pair-Instability Supernovae: Implications for Mass-Loss and Transient Events

Pulsational pair-instability supernovae (PPISNe) are transient events occurring in progenitor stars with helium cores of approximately 32-65 solar masses, where rapid electron-positron pair production induces pressure loss, collapse, and pulsations driving episodic mass loss. The number, strength, and duration of these pulses can lead to shell collisions that produce shock-powered transients, potentially explaining some of the most luminous events, such as superluminous supernovae, and other rare transients. Rapid progenitor rotation lowers the PPISN mass threshold and influences the dynamics, energetics, and chemical composition of PPISN-driven pulses. In this study, we computed 1D evolutionary models of massive, rotating PPISN progenitor stars with zero-age main-sequence masses of 85-140 solar masses and solar metallicity and 10% solar metallicity. Our analysis reveals strong correlations between PPISN ejected mass and total energy as well as between ejected mass and peak ejected shell velocity. Additionally, moderate correlations indicate that higher initial PPISN progenitor mass leads to greater mass ejection and energy release, while negative correlations show that rapid rotation appears to reduce mass ejection and kinetic energy of the shells. Subsequent pulses lead to hydrogen-poor, carbon- and oxygen-enriched ejected shells, indicating the effect of rotationally-induced chemical mixing in PPISN-driven episodic mass loss with implications for their transients. We model the light curve and synthetic spectra that arise from the collision of two H-poor shells for one of our models using the radiation transport code SuperLite. We find that shock-heated H-poor PPISN shell collisions from rapidly rotating progenitors can lead to moderately luminous H-poor transients that share some similarities with observed SLSN-I events.

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arXiv2025

The Atmosphere of Titan in Late Northern Summer from JWST and Keck Observations

Saturn's moon Titan undergoes a long annual cycle of 29.45 Earth years. Titan's northern winter and spring were investigated in detail by the Cassini-Huygens spacecraft (2004-2017), but the northern summer season remains sparsely studied. Here we present new observations from the James Webb Space Telescope (JWST) and Keck II telescope made in 2022 and 2023 during Titan's late northern summer. Using JWST's mid-infrared instrument, we spectroscopically detected the methyl radical, the primary product of methane break-up and key to the formation of ethane and heavier molecules. Using the near-infrared spectrograph onboard JWST, we detected several non-local thermodynamic equilibrium CO and CO2 emission bands, which allowed us to measure these species over a wide altitude range. Lastly, using the near-infrared camera onboard JWST and Keck II, we imaged northern hemisphere tropospheric clouds evolving in altitude, which provided new insights and constraints on seasonal convection patterns. These observations pave the way for new observations and modelling of Titan's climate and meteorology as it progresses through the northern fall equinox, when its atmosphere is expected to show notable seasonal changes.

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arXiv2025

Observations of the temporal evolution of Saturn's stratosphere following the Great Storm of 2010-2011 I. Temporal evolution of the water abundance in Saturn's hot vortex of 2011-2013

Water vapour is delivered to Saturn's stratosphere by Enceladus' plumes and subsequent diffusion in the planet system. It is expected to condense into a haze in the middle stratosphere. The hot stratospheric vortex (the `beacon') that formed as an aftermath of Saturn's Great Storm of 2010 significantly altered the temperature, composition, and circulation in Saturn's northern stratosphere. Previous photochemical models suggested haze sublimation and vertical winds as processes likely to increase the water vapour column density in the beacon. We aim to quantify the temporal evolution of stratospheric water vapour in the beacon during the storm. We mapped Saturn at 66.44 and 67.09 $\mu$m on seven occasions from July 2011 to February 2013 with the PACS instrument of the Herschel Space Observatory. The observations probe the millibar levels, at which the water condensation region was altered by the warmer temperatures in the beacon. Using radiative transfer modelling, we tested several empirical and physically based models to constrain the cause of the enhanced water emission found in the beacon. The observations show an increased emission in the beacon that cannot be reproduced only accounting for the warmer temperatures. An additional source of water vapour is thus needed. We find a factor (7.5$\pm$1.6) increase in the water column in the beacon compared to pre-storm conditions using empirical models. Combining our results with a cloud formation model for July 2011, we evaluate the sublimation contribution to 45-85% of the extra column derived from the water emission increase in the beacon. The observations confirm that the storm conditions enhanced the water abundance at the millibar levels because of haze sublimation and vertical winds in the beacon. Future work on the haze temporal evolution during the storm will help to better constrain the sublimation contribution over time.

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arXiv2024

Prominent mid-infrared excess of the dwarf planet (136472) Makemake discovered by JWST/MIRI indicates ongoing activity

We report on the discovery of a very prominent mid-infrared (18-25 {\mu}m) excess associated with the trans-Neptunian dwarf planet (136472) Makemake. The excess, detected by the MIRI instrument of the James Webb Space Telescope, along with previous measurements from the Spitzer and Herschel space telescopes, indicates the occurrence of temperatures of about 150 K, much higher than what solid surfaces at Makemake's heliocentric distance could reach by solar irradiation. We identify two potential explanations: a continuously visible, currently active region, powered by subsurface upwelling and possibly cryovolcanic activity, covering <1% of Makemake's surface, or an as yet undetected ring containing very small carbonaceous dust grains, which have not been seen before in trans-Neptunian or Centaur rings. Both scenarios point to unprecedented phenomena among trans-Neptunian objects and could greatly impact our understanding of these distant worlds.

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arXiv2024

A patchy CO$_2$ exosphere on Ganymede revealed by the James Webb Space Telescope

Jupiter's icy moon Ganymede has a tenuous exosphere produced by sputtering and possibly sublimation of water ice. To date, only atomic hydrogen and oxygen have been directly detected in this exosphere. Here, we present observations of Ganymede's CO$_2$ exosphere obtained with the James Webb Space Telescope. CO$_2$ gas is observed over different terrain types, mainly over those exposed to intense Jovian plasma irradiation, as well as over some bright or dark terrains. Despite warm surface temperatures, the CO$_2$ abundance over equatorial subsolar regions is low. CO$_2$ vapor has the highest abundance over the north polar cap of the leading hemisphere, reaching a surface pressure of 1 pbar. From modeling we show that the local enhancement observed near 12 h local time in this region can be explained by the presence of cold traps enabling CO$_2$ adsorption. However, whether the release mechanism in this high-latitude region is sputtering or sublimation remains unclear. The north polar cap of the leading hemisphere also has unique surface-ice properties, probably linked to the presence of the large atmospheric CO2 excess over this region. These CO2 molecules might have been initially released in the atmosphere after the radiolysis of CO$_2$ precursors, or from the sputtering of CO$_2$ embedded in the H$_2$O ice bedrock. Dark terrains (regiones), more widespread on the north versus south polar regions, possibly harbor CO$_2$ precursors. CO$_2$ molecules would then be redistributed via cold trapping on ice-rich terrains of the polar cap and be diurnally released and redeposited on these terrains. Ganymede's CO$_2$ exosphere highlights the complexity of surface-atmosphere interactions on Jupiter's icy Galilean moons.

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arXiv2024

Storms and convection on Uranus and Neptune: impact of methane abundance revealed by a 3D cloud-resolving model

Uranus and Neptune have atmospheres dominated by molecular hydrogen and helium. In the upper troposphere, methane is the third main molecule and condenses, yielding a vertical gradient in CH4. This condensable species being heavier than H2 and He, the resulting change in mean molecular weight due to condensation comes as a factor countering dry and moist convection. As observations also show latitudinal variations in methane abundance, one can expect different vertical gradients from one latitude to another. In this paper, we investigate the impact of this methane vertical gradient on the atmospheric regimes, especially on the formation and inhibition of moist convective storms in the troposphere of ice giants. We develop a 3D cloud-resolving model to simulate convective processes. Using our simulations, we conclude that typical velocities of dry convection in the deep atmosphere are rather low (of the order of 1 m/s) but sufficient to sustain upward methane transport, and that moist convection at methane condensation level is strongly inhibited. Previous studies derived an analytical criterion on the methane vapor amount above which moist convection should be inhibited. We first validate this analytical criterion numerically. We then show that the critical methane abundance governs the inhibition and formation of moist convective storms, and we conclude that the intensity and intermittency of these storms should depend on the methane abundance and saturation. In ice giants, dry convection is weak, and moist convection is strongly inhibited. However, when enough methane is transported upwards, through dry convection and turbulent diffusion, sporadic moist convective storms can form. These storms should be more frequent on Neptune than on Uranus, because of Neptune's internal heat flow. Our results can explain the observed sporadicity of clouds in ice giants.

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arXiv2024

Twin Worlds, Divergent Fates: How Obliquity has differently shaped Pluto's and Triton's landscapes and climates

Triton and Pluto are believed to share a common origin, both forming initially in the Kuiper Belt but Triton being later captured by Neptune. Both objects display similar sizes, densities, and atmospheric and surface ice composition, with the presence of volatile ices N2, CH4 and CO. Yet their appearance, including their surface albedo and ice distribution strongly differ. What can explain these different appearances? A first disparity is that Triton is experiencing significant tidal heating due to its orbit around Neptune, with subsequent resurfacing and a relatively flat surface, while Pluto is not tidally activated and displays a pronounced topography. Here we present long-term volatile transport simulations of Pluto and Triton, using the same initial conditions and volatile inventory, but with the known orbit and rotation of each object. The model reproduces, to first order, the observed volatile ice surface distribution on Pluto and Triton. Our results unambiguously demonstrate that obliquity is the main driver of the differences in surface appearance and in climate properties on Pluto and Triton, and give further support to the hypothesis that both objects had a common origin followed by a different dynamical history.

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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

Abundances of trace constituents in Jupiter's atmosphere inferred from Herschel/PACS observations

$Context.$ On October 31, 2009, the Photodetector Array Camera and Spectrometer (PACS) on board the Herschel Space Observatory observed far-infrared spectra of Jupiter between 50 and 220$\,\mu$m as part of the program "Water and Related Chemistry in the Solar System". $Aims.$ We investigate the disk-averaged chemical composition of Jupiter's atmosphere as a function of height using these observations. $Methods.$ We used the Planetary Spectrum Generator (PSG) and the least-squares fitting technique to infer the abundances of trace constituents. $Results.$ The PACS data include numerous spectral lines attributable to ammonia (NH$_3$), methane (CH$_4$), phosphine (PH$_3$), water (H$_2$O), and deuterated hydrogen (HD) in the Jovian atmosphere. We infer an ammonia abundance profile that decreases from a mole fraction of $(1.7\pm 0.8)\times 10^{-4}$ at $p\sim 900\,$mbar to $(1.7\pm 0.9)\times 10^{-8}$ at $p\sim 275\,$mbar, following a fractional scale height of about 0.114. For phosphine, we find a mole fraction of $(7.2\pm 1.2)\times 10^{-7}$ at pressures higher than $(550\pm 100)\,$mbar and a decrease of its abundance at lower pressures following a fractional scale height of $(0.09\pm 0.02)$. Our analysis delivers a methane mole fraction of $(1.49\pm 0.09)\times 10^{-3}$. Analyzing the HD $R(0)$ line at $112.1\,\mu$m yields a new measurement of Jupiter's D/H ratio, $\text{D/H}=(1.5\pm 0.6)\times 10^{-5}$. Finally, the PACS data allow us to put the most stringent $3\sigma$ upper limits yet on the mole fractions of hydrogen halides in the Jovian troposphere. These new upper limits are $<1.1\times 10^{-11}$ for hydrogen fluoride (HF), $<6.0\times 10^{-11}$ for hydrogen chloride (HCl), $<2.3\times 10^{-10}$ for hydrogen bromide (HBr) and $<1.2\times 10^{-9}$ for hydrogen iodide (HI) and support the proposed condensation of hydrogen halides into ammonium halide salts in the Jovian troposphere.

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arXiv2024

Seasonal and longitudinal variability in Io's SO2 atmosphere from 22 years of IRTF/TEXES observations

Between 2001 and 2023, we obtained high spectral resolution mid-infrared observations of Io using the TEXES instrument at NASA's Infrared Telescope Facility. These observations were centered at 529.8 cm-1 (18.88 {\mu}m) and include several SO2 absorption lines. By modeling the shapes and strengths of these absorption lines, we are able to determine how Io's SO2 atmospheric density varies over the 22-year time period, covering nearly two Jovian years. Previous analysis has shown that the density of Io's atmosphere on the anti-Jovian hemisphere exhibits clear seasonal temporal variability, which can be modeled as the sum of a seasonally-varying frost sublimation component and a constant component, assumed to be volcanic. The new data show that the seasonal pattern repeats during the second Jovian year, confirming the importance of sublimation support. The considerable longitudinal variability in Io's atmospheric density found in previous work is also stable over the second Jovian year with the SO2 column density on the Jupiter-facing hemisphere being 5--8 times lower than the anti-Jovian hemisphere. For the first time, we detect seasonal variability on the Jupiter-facing hemisphere as well. This can also be modeled as a combination of sublimation and a small constant source. The lower atmospheric density on the Jupiter-facing hemisphere can plausibly be explained by the daily Jupiter eclipses, which decrease the surface temperature and therefore reduce the sublimation-driven component of the atmosphere, combined with a lower level of volcanic activity directly emitting SO2 into the atmosphere.

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