Hydrogen Dimers in Giant-planet Infrared Spectra Fletcher, Leigh N.; Gustafsson, Magnus; Orton, Glenn S.
The Astrophysical journal. Supplement series,
03/2018, Letnik:
235, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Despite being one of the weakest dimers in nature, low-spectral-resolution Voyager/IRIS observations revealed the presence of (H2)2 dimers on Jupiter and Saturn in the 1980s. However, the ...collision-induced H2-H2 opacity databases widely used in planetary science have thus far only included free-to-free transitions and have neglected the contributions of dimers. Dimer spectra have both fine-scale structure near the S(0) and S(1) quadrupole lines (354 and 587 cm−1, respectively), and broad continuum absorption contributions up to 50 cm−1 from the line centers. We develop a new ab initio model for the free-to-bound, bound-to-free, and bound-to-bound transitions of the hydrogen dimer for a range of temperatures (40-400 K) and para-hydrogen fractions (0.25-1.0). The model is validated against low-temperature laboratory experiments, and used to simulate the spectra of the giant planets. The new collision-induced opacity database permits high-resolution (0.5-1.0 cm−1) spectral modeling of dimer spectra near S(0) and S(1) in both Cassini Composite Infrared Spectrometer observations of Jupiter and Saturn, and in Spitzer Infrared Spectrometer (IRS) observations of Uranus and Neptune for the first time. Furthermore, the model reproduces the dimer signatures observed in Voyager/IRIS data near S(0) on Jupiter and Saturn, and generally lowers the amount of para-H2 (and the extent of disequilibrium) required to reproduce IRIS observations.
The atmospheres of the four giant planets of our Solar System share a common and well-observed characteristic: they each display patterns of planetary banding, with regions of different temperatures, ...composition, aerosol properties and dynamics separated by strong meridional and vertical gradients in the zonal (i.e., east-west) winds. Remote sensing observations, from both visiting spacecraft and Earth-based astronomical facilities, have revealed the significant variation in environmental conditions from one band to the next. On Jupiter, the reflective white bands of low temperatures, elevated aerosol opacities, and enhancements of quasi-conserved chemical tracers are referred to as ‘zones.’ Conversely, the darker bands of warmer temperatures, depleted aerosols, and reductions of chemical tracers are known as ‘belts.’ On Saturn, we define cyclonic belts and anticyclonic zones via their temperature and wind characteristics, although their relation to Saturn’s albedo is not as clear as on Jupiter. On distant Uranus and Neptune, the exact relationships between the banded albedo contrasts and the environmental properties is a topic of active study. This review is an attempt to reconcile the observed properties of belts and zones with (i) the meridional overturning inferred from the convergence of eddy angular momentum into the eastward zonal jets at the cloud level on Jupiter and Saturn and the prevalence of moist convective activity in belts; and (ii) the opposing meridional motions inferred from the upper tropospheric temperature structure, which implies decay and dissipation of the zonal jets with altitude above the clouds. These two scenarios suggest meridional circulations in opposing directions, the former suggesting upwelling in belts, the latter suggesting upwelling in zones. Numerical simulations successfully reproduce the former, whereas there is a wealth of observational evidence in support of the latter. This presents an unresolved paradox for our current understanding of the banded structure of giant planet atmospheres, that could be addressed via a multi-tiered vertical structure of “stacked circulation cells,” with a natural transition from zonal jet pumping to dissipation as we move from the convectively-unstable mid-troposphere into the stably-stratified upper troposphere.
Observations from the Juno and Cassini missions provide essential constraints on the internal structures and compositions of Jupiter and Saturn, resulting in profound revisions of our understanding ...of the interior and atmospheres of Gas Giant planets. The next step to understand planetary origins in our Solar System requires a mission to their Ice Giant siblings, Uranus and Neptune.
Ideas for Citizen Science in Astronomy Marshall, Philip J; Lintott, Chris J; Fletcher, Leigh N
Annual review of astronomy and astrophysics,
08/2015, Letnik:
53, Številka:
1
Journal Article
Recenzirano
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We review the expanding, internet-enabled, and rapidly evolving field of citizen astronomy, focusing on research projects in stellar, extragalactic, and planetary science that have benefited from the ...participation of members of the public. These volunteers contribute in various ways: making and analyzing new observations, visually classifying features in images and light curves, exploring models constrained by astronomical data sets, and initiating new scientific enquiries. The most productive citizen astronomy projects involve close collaboration between the professionals and amateurs involved and occupy scientific niches not easily filled by great observatories or machine learning methods: Citizen astronomers are motivated by being of service to science, as well as by their interest in the subject. We expect participation and productivity in citizen astronomy to increase, as data sets get larger and citizen science platforms become more efficient. Opportunities include engaging citizens in ever-more advanced analyses and facilitating citizen-led enquiry through professional tools designed with citizens in mind.
We present maps of Ganymede's surface composition with almost complete longitude coverage, acquired using high spatial resolution near‐infrared (0.95–1.65 μm) observations from the ground‐based ...VLT/SPHERE instrument. Observed reflectance spectra were modeled using a Markov Chain Monte Carlo method to estimate abundances and associated uncertainties of water ices, acids, salts and a spectrally flat darkening agent. Results confirm Ganymede's surface is dominated by water ice in young bright terrain (impact craters, sulci), and low‐albedo spectrally flat material in older dark terrain (e.g., Galileo Regio). Ice grain size has strong latitudinal and longitudinal gradients, with larger grains at the equator and on the trailing hemisphere. These trends are consistent with the effects of the latitudinal thermal gradient and global variations in radiation driven sputtering. Sulfuric acid has a low abundance and appears potentially spatially correlated with plasma bombardment, where Ganymede's poles are exposed to the external Jovian magnetic field. Best‐estimate abundances suggest a mixture of salts could be present, although their low abundances, spectral degeneracies and associated uncertainties mean individual salt species cannot be detected with confidence. If present, sodium magnesium sulfate and magnesium chlorate appear tentatively correlated with exogenic plasma bombardment, while magnesium chloride and sulfate appear tentatively correlated with younger terrain, implying a possible endogenic origin. MCMC modeling was also performed on Galileo/NIMS data, showing comparable distributions. The high spatial resolution of SPHERE allows the precise mapping of small scale (<150 km) surface features, which could be used along with higher spectral resolution observations to jointly confirm the presence and distribution of potential species.
Plain Language Summary
We have observed Jupiter's icy moon Ganymede, the solar system's largest moon, with the Very Large Telescope in Chile. These observations recorded the amount sunlight reflected from Ganymede's surface at different infrared wavelengths, producing a “reflectance spectrum.” We used these reflectance spectra to understand the surface composition of Ganymede by using a computer model that compares each observed spectrum to spectra of different substances that have been measured in laboratories. Ultimately, this model allows us to calculate the amount of each material at each location on Ganymede's surface. Our results show how Ganymede's surface is made up to two main types of terrain: young areas have large amounts of water ice, whereas ancient areas mainly consist of a dark gray material which we were unable to identify. We detected sulfuric acid near Ganymede's poles, which is likely to originate from the gasses which surround Jupiter. A range of different salts were detected, some of which may originate from within Ganymede itself. These surface composition maps will be useful for understanding the processes happening on, and under, Ganymede's surface. They will also help to plan for the robotic space missions which are due to explore Ganymede up‐close in the coming decades.
Key Points
VLT/SPHERE maps show Ganymede's surface is mainly water ice in young terrain and a dark, spectrally flat material in old terrain
Water ice grain size varies on global scales, controlled by radiation driven sputtering and temperature gradients
Hydrated salts and sulfuric acid have lower abundances, with uncertainties making it difficult to make confident detections
•Uranus and Neptune experience strong seasonal variations in solar insolation.•These seasonal variations affect the abundance of stratospheric constituents.•A photochemical model is used to track ...this time-variable chemistry.•Neptune exhibits more latitudinal/seasonal changes in hydrocarbons than Uranus.•Model predictions are used to simulate future JWST observations.
A time-variable 1D photochemical model is used to study the distribution of stratospheric hydrocarbons as a function of altitude, latitude, and season on Uranus and Neptune. The results for Neptune indicate that in the absence of stratospheric circulation or other meridional transport processes, the hydrocarbon abundances exhibit strong seasonal and meridional variations in the upper stratosphere, but that these variations become increasingly damped with depth due to increasing dynamical and chemical time scales. At high altitudes, hydrocarbon mixing ratios are typically largest where the solar insolation is the greatest, leading to strong hemispheric dichotomies between the summer-to-fall hemisphere and winter-to-spring hemisphere. At mbar pressures and deeper, slower chemistry and diffusion lead to latitude variations that become more symmetric about the equator. On Uranus, the stagnant, poorly mixed stratosphere confines methane and its photochemical products to higher pressures, where chemistry and diffusion time scales remain large. Seasonal variations in hydrocarbons are therefore predicted to be more muted on Uranus, despite the planet’s very large obliquity. Radiative-transfer simulations demonstrate that latitude variations in hydrocarbons on both planets are potentially observable with future JWST mid-infrared spectral imaging. Our seasonal model predictions for Neptune compare well with retrieved C2H2 and C2H6 abundances from spatially resolved ground-based observations (no such observations currently exist for Uranus), suggesting that stratospheric circulation — which was not included in these models — may have little influence on the large-scale meridional hydrocarbon distributions on Neptune, unlike the situation on Jupiter and Saturn.
Rotational modulations are observed on brown dwarfs and directly imaged exoplanets, but the underlying mechanism is not well understood. Here we analyze Jupiter's rotational light curves at 12 ...wavelengths from the ultraviolet (UV) to the mid-infrared (mid-IR). The peak-to-peak amplitudes of Jupiter's light curves range from subpercent to 4% at most wavelengths, but the amplitude exceeds 20% at 5 m, a wavelength sensing Jupiter's deep troposphere. Jupiter's rotational modulations are primarily caused by discrete patterns in the cloudless belts instead of the cloudy zones. The light-curve amplitude is controlled by the sizes and brightness contrasts of the Great Red Spot (GRS), expansions of the North Equatorial Belt (NEB), patchy clouds in the North Temperate Belt (NTB), and a train of hot spots in the NEB. In reflection, the contrast is controlled by upper tropospheric and stratospheric hazes, clouds, and chromophores in the clouds. In thermal emission, the small rotational variability is caused by the spatial distribution of temperature and opacities of gas and aerosols; the large variation is caused by the NH3 cloud holes and thin-thick clouds. The methane-band light curves exhibit opposite-shape behavior compared with the UV and visible wavelengths, caused by a wavelength-dependent brightness change of the GRS. Light-curve evolution is induced by periodic events in the belts and longitudinal drifting of the GRS and patchy clouds in the NTB. This study suggests several interesting mechanisms related to distributions of temperature, gas, hazes, and clouds for understanding the observed rotational modulations on brown dwarfs and exoplanets.
•Global spectral maps of Jupiter’ s 5–20 µm thermal emission are provided by IRTF/TEXES.•Quality of retrieved maps of temperature, composition and aerosols surpass previous spacecraft ...results.•Jupiter’ s NEB hotspots are identifiable throughout the thermal-IR and tilt westward with height.•Ammonia plumes are observed at the equator, southeast of desiccated NEB hotspots.•Long-term asymmetries in phosphine, hydrocarbons and 2D wind field are detected.
Global maps of Jupiter’s atmospheric temperatures, gaseous composition and aerosol opacity are derived from a programme of 5–20 µm mid-infrared spectroscopic observations using the Texas Echelon Cross Echelle Spectrograph (TEXES) on NASA’s Infrared Telescope Facility (IRTF). Image cubes from December 2014 in eight spectral channels, with spectral resolutions of R ∼2000−12,000 and spatial resolutions of 2–4° latitude, are inverted to generate 3D maps of tropospheric and stratospheric temperatures, 2D maps of upper tropospheric aerosols, phosphine and ammonia, and 2D maps of stratospheric ethane and acetylene. The results are compared to a re-analysis of Cassini Composite Infrared Spectrometer (CIRS) observations acquired during Cassini’s closest approach to Jupiter in December 2000, demonstrating that this new archive of ground-based mapping spectroscopy can match and surpass the quality of previous investigations, and will permit future studies of Jupiter’s evolving atmosphere. The visibility of cool zones and warm belts varies from channel to channel, suggesting complex vertical variations from the radiatively-controlled upper troposphere to the convective mid-troposphere. We identify mid-infrared signatures of Jupiter’s 5-µm hotspots via simultaneous M, N and Q-band observations, which are interpreted as temperature and ammonia variations in the northern Equatorial Zone and on the edge of the North Equatorial Belt (NEB). Equatorial plumes enriched in NH3 gas are located south-east of NH3-desiccated ‘hotspots’ on the edge of the NEB. Comparison of the hotspot locations in several channels across the 5–20 µm range indicate that these anomalous regions tilt westward with altitude. Aerosols and PH3 are both enriched at the equator but are not co-located with the NH3 plumes. The equatorial temperature minimum and PH3/aerosol maxima have varied in amplitude over time, possibly as a result of periodic equatorial brightenings and the fresh updrafts of disequilibrium material. Temperate mid-latitudes display a correlation between mid-IR aerosol opacity and the white albedo features in visible light (i.e., zones). We find hemispheric asymmetries in the distribution of tropospheric PH3, stratospheric hydrocarbons and the 2D wind field (estimated via the thermal-wind equation) that suggest a differing efficiency of mechanical forcing (e.g., vertical mixing and wave propagation) between the two hemispheres that we argue is driven by dynamics rather than Jupiter’s small seasonal cycle. Jupiter’s stratosphere is notably warmer at northern mid-latitudes than in the south in both 2000 and 2014, although the latter can be largely attributed to strong thermal wave activity near 30°N that dominates the 2014 stratospheric maps and may be responsible for elevated C2H2 in the northern hemisphere. A vertically-variable pattern of temperature and windshear minima and maxima associated with Jupiter’s Quasi Quadrennial Oscillation (QQO) is observed at the equator in both datasets, although the contrasts were more subdued in 2014. Large-scale equator-to-pole gradients in ethane and acetylene are superimposed on top of the mid-latitude mechanically-driven maxima, with C2H2 decreasing from equator to pole and C2H6 showing a polar enhancement, consistent with a radiatively-controlled circulation from low to high latitudes. Cold polar vortices beyond ∼60° latitude can be identified in the upper tropospheric and lower stratospheric temperature maps, suggesting enhanced radiative cooling from polar aerosols. Finally, compositional mapping of the Great Red Spot confirms the local enhancements in PH3 and aerosols, the north–south asymmetry in NH3 gas and the presence of a warm southern periphery that have been noted by previous authors.
•5–37μm spectra of Uranus from the Spitzer IRS were analyzed for composition.•Abundances of CH4, C2H2, C2H6, CH3C2H, C4H2 and CO2 were derived.•Our results imply an extremely sluggish atmosphere ...compared with other outer planets.•The upper atmospheric composition is dominated by oxygen-bearing constituents.•The CH3D/CH4 ratio is 3.0±0.2×10–4, consistent with independent measurements.
Mid-infrared spectral observations Uranus acquired with the Infrared Spectrometer (IRS) on the Spitzer Space Telescope are used to determine the abundances of C2H2, C2H6, CH3C2H, C4H2, CO2, and tentatively CH3 on Uranus at the time of the 2007 equinox. For vertically uniform eddy diffusion coefficients in the range 2200–2600cm2s−1, photochemical models that reproduce the observed methane emission also predict C2H6 profiles that compare well with emission in the 11.6–12.5μm wavelength region, where the υ9 band of C2H6 is prominent. Our nominal model with a uniform eddy diffusion coefficient Kzz=2430cm2s−1 and a CH4 tropopause mole fraction of 1.6×10−5 provides a good fit to other hydrocarbon emission features, such as those of C2H2 and C4H2, but the model profile for CH3C2H must be scaled by a factor of 0.43, suggesting that improvements are needed in the chemical reaction mechanism for C3Hx species. The nominal model is consistent with a CH3D/CH4 ratio of 3.0±0.2×10−4. From the best-fit scaling of these photochemical-model profiles, we derive column abundances above the 10-mbar level of 4.5+01.1/−0.8×1019molecule-cm−2 for CH4, 6.2±1.0×1016molecule-cm−2 for C2H2 (with a value 24% higher from a different longitudinal sampling), 3.1±0.3×1016molecule-cm−2 for C2H6, 8.6±2.6×1013molecule-cm−2 for CH3C2H, 1.8±0.3×1013molecule-cm−2 for C4H2, and 1.7±0.4×1013molecule-cm−2 for CO2 on Uranus. A model with Kzz increasing with altitude fits the observed spectrum and requires CH4 and C2H6 column abundances that are 54% and 45% higher than their respective values in the nominal model, but the other hydrocarbons and CO2 are within 14% of their values in the nominal model. Systematic uncertainties arising from errors in the temperature profile are estimated very conservatively by assuming an unrealistic “alternative” temperature profile that is nonetheless consistent with the observations; for this profile the column abundance of CH4 is over four times higher than in the nominal model, but the column abundances of the hydrocarbons and CO2 differ from their value in the nominal model by less than 22%. The CH3D/CH4 ratio is the same in both the nominal model with its uniform Kzz as in the vertically variable Kzz model, and it is 10% lower with the “alternative” temperature profile than the nominal model. There is no compelling evidence for temporal variations in global-average hydrocarbon abundances over the decade between Infrared Space Observatory and Spitzer observations, but we cannot preclude a possible large increase in the C2H2 abundance since the Voyager era. Our results have implications with respect to the influx rate of exogenic oxygen species and the production rate of stratospheric hazes on Uranus, as well as the C4H2 vapor pressure over C4H2 ice at low temperatures.
•Air rises above midlatitudes, and descends over the poles and equator.•Circulation extends from <0.1mbar down to 40bar.•South polar vortex at a relative humidity of 5%, extends from 66°S down to ...90°S.•At midlatitudes there are 2 cloud layers, one above and one below the tropopause.•Stratospheric clouds are tops of anticyclones rising up through tropopause.
We observed Neptune between June and October 2003 at near- and mid-infrared wavelengths with the 10-m W.M. Keck II and I telescopes, respectively; and at radio wavelengths with the Very Large Array. Images were obtained at near-infrared wavelengths with NIRC2 coupled to the adaptive optics system in both broad- and narrow-band filters between 1.2 and 2.2μm. In the mid-infrared we imaged Neptune at wavelengths between 8 and 22μm, and obtained slit-resolved spectra at 8–13μm and 18–22μm. At radio wavelengths we mapped the planet in discrete filters between 0.7 and 6cm.
We analyzed each dataset separately with a radiative-transfer program that is optimized for that particular wavelength regime. At southern midlatitudes the atmosphere appears to be cooler at mid-infrared wavelengths than anywhere else on the planet. We interpret this to be caused by adiabatic cooling due to air rising at midlatitudes at all longitudes from the upper troposphere up to ≲0.1mbar levels. At near-infrared wavelengths we find two distinct cloud layers at these latitudes: a relatively deep layer of clouds (presumably methane) in the troposphere at pressure levels P∼300–≳600mbar, which we suggest to be caused by the large-scale upwelling and its accompanying adiabatic cooling and condensation of methane; and a higher, spatially intermittent, layer of clouds in the stratosphere at 20–30mbar. The latitudes of these high clouds encompass an anticyclonic band of zonal flow, which suggests that they may be due to strong, but localized, vertical upwellings associated with local anticyclones, rather than plumes in convective (i.e., cyclonic) storms. Clouds at northern midlatitudes are located at the highest altitudes in the atmosphere, near 10mbar.
Neptune’s south pole is considerably enhanced in brightness at both mid-infrared and radio wavelengths, i.e., from ∼0.1mbar levels in the stratosphere down to tens of bars in the troposphere. We interpret this to be due to subsiding motions from the stratosphere all the way down to the deep troposphere. The enhanced brightness observed at mid-infrared wavelengths is interpreted to be due to adiabatic heating by compression in the stratosphere, and the enhanced brightness temperature at radio wavelengths reveals that the subsiding air over the pole is very dry; the relative humidity of H2S over the pole is only 5% at altitudes above the NH4SH cloud at ∼40bar. The low humidity region extends from the south pole down to latitudes of 66°S. This is near the same latitudes as the south polar prograde jet signifying the boundary of the polar vortex. We suggest that the South Polar Features (SPFs) at latitudes of 60–70° are convective storms, produced by baroclinic instabilities expected to be produced at latitudes near the south polar prograde jet.
Taken together, our data suggest a global circulation pattern where air is rising above southern and northern midlatitudes, from the troposphere up well into the stratosphere, and subsidence of dry air over the pole and equator from the stratosphere down into the troposphere. We suggest that this pattern extends all the way from ≲0.1mbar down to pressures of ≳40bar.
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