Planetary atmospheres with ALMA Lellouch, Emmanuel
Astrophysics and space science,
2008/1, Volume:
313, Issue:
1-3
Journal Article
Peer reviewed
Thanks to its sensitivity, spatial resolution and instantaneous
uv
-coverage, ALMA will permit many new studies related to the general topic of the couplings between chemistry and dynamics in ...planetary atmospheres. It will include: (1) three-dimensional mapping of composition, temperatures and winds in the atmospheres of Mars, Venus and Titan; (2) several aspects of Giant Planet composition and dynamics, such as the origin of oxygen, the evolution of Shoemaker–Levy 9 products in Jupiter’s atmosphere, and the deep atmosphere structure and meteorology; (3) the study of tenuous and distant atmospheres (Io, Enceladus, Pluto, Triton and other Kuiper Belt objects).
Pluto and Eris are icy dwarf planets with nearly identical sizes, comparable densities and similar surface compositions as revealed by spectroscopic studies. Pluto possesses an atmosphere whereas ...Eris does not; the difference probably arises from their differing distances from the Sun, and explains their different albedos. Makemake is another icy dwarf planet with a spectrum similar to Eris and Pluto, and is currently at a distance to the Sun intermediate between the two. Although Makemake's size (1,420 ± 60 km) and albedo are roughly known, there has been no constraint on its density and there were expectations that it could have a Pluto-like atmosphere. Here we report the results from a stellar occultation by Makemake on 2011 April 23. Our preferred solution that fits the occultation chords corresponds to a body with projected axes of 1,430 ± 9 km (1σ) and 1,502 ± 45 km, implying a V-band geometric albedo p(V) = 0.77 ± 0.03. This albedo is larger than that of Pluto, but smaller than that of Eris. The disappearances and reappearances of the star were abrupt, showing that Makemake has no global Pluto-like atmosphere at an upper limit of 4-12 nanobar (1σ) for the surface pressure, although a localized atmosphere is possible. A density of 1.7 ± 0.3 g cm(-3) is inferred from the data.
ABSTRACT
Between 2014 December 31 and 2015 March 17, the OSIRIS cameras on Rosetta documented the growth of a $140\, \mathrm{\hbox{-}m}$ wide and $0.5\, \mathrm{\hbox{-}m}$ deep depression in the ...Hapi region on Comet 67P/Churyumov–Gerasimenko. This shallow pit is one of several that later formed elsewhere on the comet, all in smooth terrain that primarily is the result of airfall of coma particles. We have compiled observations of this region in Hapi by the microwave instrument MIRO on Rosetta, acquired during October and November 2014. We use thermophysical and radiative transfer models in order to reproduce the MIRO observations. This allows us to place constraints on the thermal inertia, diffusivity, chemical composition, stratification, extinction coefficients, and scattering properties of the surface material, and how they evolved during the months prior to pit formation. The results are placed in context through long-term comet nucleus evolution modelling. We propose that (1) MIRO observes signatures that are consistent with a solid-state greenhouse effect in airfall material; (2) CO2 ice is sufficiently close to the surface to have a measurable effect on MIRO antenna temperatures, and likely is responsible for the pit formation in Hapi observed by OSIRIS; (3) the pressure at the CO2 sublimation front is sufficiently strong to expel dust and water ice outwards, and to compress comet material inwards, thereby causing the near-surface compaction observed by CONSERT, SESAME, and groundbased radar, manifested as the ‘consolidated terrain’ texture observed by OSIRIS.
Context. The tenuous nitrogen (N2) atmosphere on Pluto undergoes strong seasonal effects due to high obliquity and orbital eccentricity, and has recently (July 2015) been observed by the New Horizons ...spacecraft. Aims. The main goals of this study are (i) to construct a well calibrated record of the seasonal evolution of surface pressure on Pluto and (ii) to constrain the structure of the lower atmosphere using a central flash observed in 2015. Methods. Eleven stellar occultations by Pluto observed between 2002 and 2016 are used to retrieve atmospheric profiles (density, pressure, temperature) between altitude levels of ~5 and ~380 km (i.e. pressures from ~ 10 μbar to 10 nbar). Results. (i) Pressure has suffered a monotonic increase from 1988 to 2016, that is compared to a seasonal volatile transport model, from which tight constraints on a combination of albedo and emissivity of N2 ice are derived. (ii) A central flash observed on 2015 June 29 is consistent with New Horizons REX profiles, provided that (a) large diurnal temperature variations (not expected by current models) occur over Sputnik Planitia; and/or (b) hazes with tangential optical depth of ~0.3 are present at 4–7 km altitude levels; and/or (c) the nominal REX density values are overestimated by an implausibly large factor of ~20%; and/or (d) higher terrains block part of the flash in the Charon facing hemisphere.
•Analysis of the VIMS solar occultations to characterize Titan’s vertical atmosphere.•Extraction of CH4, CO profiles, detection/identification of additional absorptions.•CH4 stratospheric abundance ...lower than GCMS, CO in agreement with previous results.•Gaseous ethane plays an important role on near-IR spectrum of Titan’s atmosphere.•Detection of aliphatic, aromatic and nitriles absorptions linked to aerosols.
We present an analysis of the VIMS solar occultations dataset, which allows us to extract vertically resolved information on the characteristics of Titan’s atmosphere between ∼100 and 700km with a vertical resolution of ∼10km. After a series of data treatment procedures to correct problems in pointing stability and parasitic light, 4 occultations out of 10 are retained. This sample covers different seasons and latitudes of Titan. The transmittances show clearly the evolution of the haze, with the detection of the detached layer at ∼310km in September 2011 at mid-northern latitudes. Through the inversion of the transmission spectra with a line-by-line radiative transfer code we retrieve the vertical distribution of CH4 and CO mixing ratio. For methane inversion we use its 1.4, 1.7 and 2.3μm bands. The first two bands are always in good agreement and yield an average stratospheric abundance of 1.28±0.08%, after correcting for forward-scattering effects, with no significant differences between the occultations. This is significantly less than the value of 1.48% obtained by the GCMS/Huygens instrument. We find that the 2.3μm band cannot be used for the extraction of methane abundance because it is blended with other absorptions, not included in our atmospheric model. The analysis of the residual spectra after the inversion shows that such additional absorptions are present through a great part of the VIMS wavelength range. We attribute many of these bands, including the one at 2.3μm, to gaseous ethane, whose near-infrared spectrum is not well modeled yet. Ethane also contributes significantly to the strong absorption at 3.2–3.5μm that was previously attributed only to C–H stretching bands from aerosols. Ethane bands may affect the surface windows too, especially at 2.7μm. Other residual bands are generated by stretching modes of C–H, C–C and C–N bonds. In addition to the C–H stretch from aliphatic hydrocarbons at 3.4μm, we detect a strong and narrow absorption at 3.28μm which we tentatively attribute to the presence of PAHs in the stratosphere. C–C and C–N stretching bands are possibly present between 4.3 and 4.5μm. Finally, we obtain the CO mixing ratio between 70 and 170km, through the inversion of its 4.7μm band. The average result of 46±16ppm is in good agreement with previous studies.
► Measurements of H2O rotational lines were performed with the Herschel/PACS and the Herschel/HIFI instruments. ► Semi-empirical water vertical profiles are derived over the 100–450km altitude range. ...► Photochemical models of oxygen species in Titan’s atmosphere are reconsidered. ► The constrained OH/H2O influx is of (2.7–3.4)×105molcm−2s−1. ► Both interplanetary dust particles and Enceladus’ activity appear to provide sufficient supply for the current Titan H2O.
Disk-averaged observations of water vapor in Titan’s atmosphere acquired with the Herschel satellite are reported. We use a combination of unresolved measurements of three H2O rotational lines at 66.4, 75.4 and 108.0μm with the PACS instrument, and spectrally-resolved observations of two other transitions at 557GHz (538μm) and 1097GHz (273μm) with the HIFI instrument, to infer the vertical profile of H2O over the 100–450km altitude range. Monitoring of the 66.4μm line indicates no variation between Titan leading and trailing sides, nor variation over a ∼1year interval. Both the narrow (2–4MHz) widths of the HIFI-observed lines, and the relative contrasts of the five H2O lines indicate that the H2O mole fraction strongly increases with altitude, with a best fit mole fraction of q0=(2.3±0.6)×10−11 at a pressure p=12.1mbar, a slope −d(lnq)/d(lnp)=0.49±0.07, and a H2O column density of (1.2+/−0.2)×1014cm−2. This H2O profile also matches the original ISO observations of Titan H2O. Water vertical profiles previously proposed on the basis of 1-D photochemical models are too water-rich, and none of them have the adequate slope; in particular, the water profiles of Lara et al. (Lara, L.M., Lellouch, E., López-Moreno, J.J., Rodrigo, R. 1996. J. Geophys. Res. E 101, 23261–23283) and Hörst et al. (Hörst, S.M., Vuitton, V., Yelle, R.V. 2008. J. Geophys. Res. E 113, E10006) are too steep and too shallow, respectively, in the lower stratosphere. Photochemical models of oxygen species in Titan’s atmosphere are reconsidered, updating the Lara et al. model for oxygen chemistry, and adjusting the eddy diffusion coefficient in order to match both our H2O observations and the C2H6 and C2H2 vertical profiles determined from Cassini/CIRS. We find that the H2O profile can be reproduced by invoking a OH/H2O influx of (2.7–3.4)×105molcm−2s−1, referred to the surface. This is essentially one order of magnitude lower than invoked by previous modellers, and also a factor of ∼10 less than required to match the observed CO2 mole fraction. As H2O has a more shorter atmospheric lifetime than CO2 (∼9years vs ∼450years), we suggest that this reflects a temporal change in the oxygen influx into Titan, that could be currently much smaller than averaged over the past centuries. Both interplanetary dust particles and Enceladus’ activity appear to provide sufficient supply for the current Titan H2O. We tentatively favor the latter source as potentially more prone to time variability.
ESA’s Jupiter Icy Moons Explorer (JUICE) will provide a detailed investigation of the Jovian system in the 2030s, combining a suite of state-of-the-art instruments with an orbital tour tailored to ...maximise observing opportunities. We review the Jupiter science enabled by the JUICE mission, building on the legacy of discoveries from the Galileo, Cassini, and Juno missions, alongside ground- and space-based observatories. We focus on remote sensing of the climate, meteorology, and chemistry of the atmosphere and auroras from the cloud-forming weather layer, through the upper troposphere, into the stratosphere and ionosphere. The Jupiter orbital tour provides a wealth of opportunities for atmospheric and auroral science: global perspectives with its near-equatorial and inclined phases, sampling all phase angles from dayside to nightside, and investigating phenomena evolving on timescales from minutes to months. The remote sensing payload spans far-UV spectroscopy (50-210 nm), visible imaging (340-1080 nm), visible/near-infrared spectroscopy (0.49-5.56 μm), and sub-millimetre sounding (near 530-625 GHz and 1067-1275 GHz). This is coupled to radio, stellar, and solar occultation opportunities to explore the atmosphere at high vertical resolution; and radio and plasma wave measurements of electric discharges in the Jovian atmosphere and auroras. Cross-disciplinary scientific investigations enable JUICE to explore coupling processes in giant planet atmospheres, to show how the atmosphere is connected to (i) the deep circulation and composition of the hydrogen-dominated interior; and (ii) to the currents and charged particle environments of the external magnetosphere. JUICE will provide a comprehensive characterisation of the atmosphere and auroras of this archetypal giant planet.
Context. Triton possesses a thin atmosphere, primarily composed of nitrogen, sustained by the sublimation of surface ices. Aims. We aim at determining the composition of Triton's atmosphere to ...constrain the nature of surface-atmosphere interactions. Methods. We perform high-resolution spectroscopic observations in the 2.32–2.37 μm range, using CRIRES at the VLT. Results. From this first spectroscopic detection of Triton's atmosphere in the infrared, we report (i) the first observation of gaseous methane since its discovery in the ultraviolet by Voyager in 1989; and (ii) the first ever detection of gaseous CO in the satellite. The CO atmospheric abundance is remarkably similar to its surface abundance, and appears to be controlled by a thin, CO-enriched, surface veneer resulting from seasonal transport and/or atmospheric escape. The CH4 partial pressure is several times higher than inferred by Voyager. This confirms that Triton's atmosphere is seasonally variable and is best interpreted by the warming of CH4-rich icy grains as Triton passed southern summer solstice in 2000. The presence of CO in Triton's atmosphere also affects its temperature, photochemistry, and ionospheric composition. An improved upper limit on CO in Pluto's atmosphere is also reported.
The dwarf planet Eris is a trans-Neptunian object with an orbital eccentricity of 0.44, an inclination of 44 degrees and a surface composition very similar to that of Pluto. It resides at present at ...95.7 astronomical units (1 AU is the Earth-Sun distance) from Earth, near its aphelion and more than three times farther than Pluto. Owing to this great distance, measuring its size or detecting a putative atmosphere is difficult. Here we report the observation of a multi-chord stellar occultation by Eris on 6 November 2010 UT. The event is consistent with a spherical shape for Eris, with radius 1,163 ± 6 kilometres, density 2.52 ± 0.05 grams per cm(3) and a high visible geometric albedo, Pv = 0.96(+0.09)(-0.04). No nitrogen, argon or methane atmospheres are detected with surface pressure larger than ∼1 nanobar, about 10,000 times more tenuous than Pluto's present atmosphere. As Pluto's radius is estimated to be between 1,150 and 1,200 kilometres, Eris appears as a Pluto twin, with a bright surface possibly caused by a collapsed atmosphere, owing to its cold environment. We anticipate that this atmosphere may periodically sublimate as Eris approaches its perihelion, at 37.8 astronomical units from the Sun.
We report on the first identification of hydrogen isocyanide (HNC) in Titan’s atmosphere, from observations using the HIFI instrument on the Herschel⋆ Space Observatory. An emission line from the HNC ...J = 6 → 5 rotational transition at 543.897 GHz was measured in Titan on June 14 and December 31, 2010. Radiative transfer modeling indicates that the bulk of HNC is located above 400 km, with a column density in the range (0.6−1.5) × 1013 cm-2, but the observations cannot establish its vertical profile. In particular HNC could be restricted to the upper thermosphere (~1000 km), in which case its local abundance relative to HCN could be as high as ~0.3. HNC is probably formed mostly at ionospheric levels (950–1150 km) from dissociative recombination of HCNH+ and possibly other heavier nitrile ions. Ionospheric loss of HNC occurs by protonation with XH+ ions. Additional formation (e.g. from N(4S) +3CH2) and loss routes (e.g. from isomerization to HCN) in the neutral atmosphere remain to be investigated.
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