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Lellouch, E.

Publications and source records attributed to Lellouch, E..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2018

Retrieval of H2O Abundance in Titan's Stratosphere: A (Re)Analysis of CIRS/Cassini and PACS/Herschel Observations

Since its first measurement 20 years ago by the Infrared Space Observatory (ISO), the water (H2O) mole fraction in Titan’s stratosphere remains uncertain due to large differences between the determinations from available measurements. More particularly, the recent measurements made from the Herschel observatory (PACS (Photodetector Array Camera and Spectrometer) and HIFI (Heterodyne Instrument for the Far Infrared)) estimated the H2O mole fraction to be 0.023 parts per billion at 12.1 millibars. A mixing ratio of 0.14 parts per billion at 10.7 millibars was, however, retrieved from nadir spatially-resolved observations of Cassini/CIRS (Composite Infrared Spectrometer). At the same pressure level (10.7 millibars), this makes a difference of a factor of 5.5 between PACS and CIRS measurements, and this has notably prevented current models from fully constraining the oxygen flux flowing into Titan’s atmosphere. In this work, we try to understand the differences between the H2O mole fractions estimated from Herschel/PACS and Cassini/CIRS observations. The strategy for this is to 1) analyse recent disc-averaged observations of CIRS to investigate if the observation geometry could explain the previous observed differences, and 2) (re)analyse the three types of observation with the same retrieval scheme to assess if previous differences in retrieval codes/methodology could be responsible for the previous discrepancies. With this analysis, we show that using the same retrieval method better reconcile the previous measurements of these instruments. However, the addition of the disc-averaged CIRS observations, instead of confirming the consistency between the different datasets, reveals discrepancies between one of the CIRS disc-averaged set of observations and PACS measurements. This raises new questions regarding the possibility of latitudinal variations of H2O, which could be triggered by seasonal changes of the meridional circulation. As it has already been shown for nitriles and hydrocarbons, this circulation could potentially impact the latitudinal distribution of H2O through the subsidence or up- welling of air rich in H2O. The possible influence of spatial/time variations of the OH/H2O input flux in Titan’s atmosphere is also discussed. The analysis of more observations will be needed in future work to address the questions arising from this work and to improve the understanding of the sources of H2O in Titan’s atmosphere.

Bauduin, S.↗
NASA NTRS2017

D/H Ratios on Saturn and Jupiter from Cassini CIRS

We present new measurements of the deuterium abundance on Jupiter and Saturn, showing evidence that Saturn's atmosphere contains less deuterium than Jupiter's. We analyzed far-infrared spectra from the Cassini Composite Infrared Spectrometer to measure the abundance of HD on both giant planets. Our estimate of the Jovian D/H = (2.95 +/- 0.55) x 10(exp -5) is in agreement with previous measurements by ISO/SWS: (2.25 +/- 0.35) x 10(exp -5), and the Galileo probe: (2.6 +/- 0.7) x 10(exp -5). In contrast, our estimate of the Saturn value of (2.10 +/- 0.13) x 10(exp -5) is somewhat lower than on Jupiter (by a factor of 0.71(sub -0.15, sup +0..22)), contrary to model predictions of a higher ratio: Saturn/ Jupiter = 1.05-1.20. The Saturn D/H value is consistent with estimates for hydrogen in the protosolar nebula (2.1 +/- 0.5) x 10(exp -5), but its apparent divergence from the Jovian value suggests that our understanding of planetary formation and evolution is incomplete, which is in agreement with previous work.

Pierel, J. D. R.↗
NASA NTRS2016

James Webb Space Telescope Observations of Stellar Occultations by Solar System Bodies and Rings

In this paper, we investigate the opportunities provided by the James Webb Space Telescope (JWST) for significant scientific advances in the study of Solar System bodies and rings using stellar occultations. The strengths and weaknesses of the stellar occultation technique are evaluated in light of JWST's unique capabilities. We identify several possible JWST occultation events by minor bodies and rings and evaluate their potential scientific value. These predictions depend critically on accurate a priori knowledge of the orbit of JWST near the Sun–Earth Lagrange point 2 (L2). We also explore the possibility of serendipitous stellar occultations by very small minor bodies as a byproduct of other JWST observing programs. Finally, to optimize the potential scientific return of stellar occultation observations, we identify several characteristics of JWST's orbit and instrumentation that should be taken into account during JWST's development.

Santos-Sanz, P.↗
NASA NTRS2014

Scientific Rationale for Saturn's in Situ Exploration

Remote sensing observations meet some limitations when used to study the bulk atmospheric composition of the giant planets of our solar system. A remarkable example of the superiority of in situ probe measurements is illustrated by the exploration of Jupiter, where key measurements such as the determination of the noble gases׳ abundances and the precise measurement of the helium mixing ratio have only been made available through in situ measurements by the Galileo probe. This paper describes the main scientific goals to be addressed by the future in situ exploration of Saturn placing the Galileo probe exploration of Jupiter in a broader context and before the future probe exploration of the more remote ice giants. In situ exploration of Saturn's atmosphere addresses two broad themes that are discussed throughout this paper: first, the formation history of our solar system and second, the processes at play in planetary atmospheres. In this context, we detail the reasons why measurements of Saturn's bulk elemental and isotopic composition would place important constraints on the volatile reservoirs in the protosolar nebula. We also show that the in situ measurement of CO (or any other disequilibrium species that is depleted by reaction with water) in Saturn's upper troposphere may help constraining its bulk O/H ratio. We compare predictions of Jupiter and Saturn's bulk compositions from different formation scenarios, and highlight the key measurements required to distinguish competing theories to shed light on giant planet formation as a common process in planetary systems with potential applications to most extrasolar systems. In situ measurements of Saturn's stratospheric and tropospheric dynamics, chemistry and cloud-forming processes will provide access to phenomena unreachable to remote sensing studies. Different mission architectures are envisaged, which would benefit from strong international collaborations, all based on an entry probe that would descend through Saturn's stratosphere and troposphere under parachute down to a minimum of 10 bar of atmospheric pressure. We finally discuss the science payload required on a Saturn probe to match the measurement requirements.

Galileo↗
NASA NTRS2013

The Distribution of Methane in Titan's Stratosphere from Cassini/CIRS Observations

Cassini/CIRS spectra in the far- and mid-infrared region are used to determine the abundance of methanein Titans lower stratosphere and investigate its distribution with latitude. The CIRS spectra include emissionfrom both the CH4 v4 band at 7.7 micron and pure rotational lines longwards of 50 micron, which show differential sensitivities to thermal profile and methane mole fraction. We analyze nadir and limb data taken over the first part of the Cassini mission (August 2005 to June 2010), including a selection of 12 latitudes that provides a reasonably complete and regular sampling of both hemispheres. Unexpectedly, but in a consistent manner for limb and nadir geometries, large variations of the methane mole fraction near 15 mbar (approx. 85 km) are found, with values ranging from approx. 1.0% (at low latitudes and near +/-50-55 deg) to approx. 1.5% (at +/-30-35 deg and polar latitudes). Error bars on the retrieved methane mole fraction are 0.07-0.12% at low latitudes in the Southern hemisphere and 0.14-0.21% northward of 40 deg N. A 1.0% methanemole fraction at low latitudes permits us to reconcile the HASI-measured temperatures below 147 kmaltitude (2.7 mbar) with inferences from CIRS. The roughly hemispherically-symmetric distribution ofmethane gas is reminiscent of that observed or predicted for the tropospheric methane clouds, whichon a yearly-averaged basis, show preferential occurrences at tropical and polar latitudes. We speculatethat convective events at these latitudes result into local stratospheric methane enrichment, whichmay persist year-round due to dynamical mixing times in the lower stratosphere only moderately shorterthan a Titan year.

Methane↗
NASA NTRS2012

Water Vapor in Titan's Stratosphere from Cassini CIRS Far-Infrared Spectra

Here we report the measurement of water vapor in Titan's stratosphere using the Cassini Composite Infrared Spectrometer (CIRS). CIRS senses water emissions in the far infrared spectral region near 50 micron, which we have modeled using two independent radiative transfer codes. From the analysis of nadir spectra we have derived a mixing ratio of 0.14 +/- 0.05 ppb at an altitude of 97 km, which corresponds to an integrated (from 0 to 600 km) surface normalized column abundance of 3.7 +/- 1.3 1014 molecules/cm2. In the latitude range 80S to 30N we see no evidence for latitudinal variations in these abundances within the error bars. Using limb observations, we obtained mixing ratios of 0.13 +/- 0.04 ppb at an altitude of 115 km and 0.45 +/- 0.15 ppb at an altitude of 230 km, confirming that the water abundance has a positive vertical gradient as predicted by photochemical models. We have also fitted our data using scaling factors of 0.1-0.6 to these photochemical model profiles, indicating that the models over-predict the water abundance in Titan's lower stratosphere.

composition↗
NASA NTRS2012

Water Vapor on Titan: The Stratospheric Vertical Profile from Cassini/CIRS Infrared Spectra

Water vapor in Titan's middle atmosphere has previously been detected only by disk-average observations from the Infrared Space Observatory (Coustenis et al., 1998). We report here the successful detection of stratospheric water vapor using the Cassini Composite Infrared Spectrometer (CIRS, Flasar et al., 2004) following an earlier null result (de Kok et al., 2007a). CIRS senses water emissions in the far-infrared spectral region near 50 microns, which we have modeled using two independent radiative transfer and inversion codes (NEMESIS, Irwin et al 2008 and ART, Coustenis et al., 2010). From the analysis of nadir spectra we have derived a mixing ratio of (0.14 plus or minus 0.05) ppb at 100 km, corresponding to a column abundance of approximately (3.7 plus or minus 1.3) x 10(exp 14) moles per square centimeter. Using limb observations, we obtained mixing ratios of (0.13 plus or minus 0.04) ppb at 125 km and (0.45 plus or minus 0.15) ppb at 225 km of altitude, confirming that the water abundance has a positive vertical gradient as predicted by photochemical models. In the latitude range (80 deg. S - 30 deg. N) we see no evidence for latitudinal variations in these abundances within the error bars.

Cottini, V.↗
NASA NTRS2012

Water Vapor in Titan's Stratosphere from Cassini/CIRS Far-infrared Spectra

Since the first detection of water vapor in Titan's stratosphere by disk-average observations from the Infrared Space Observatory (Coustenis et al. 1998) we report here the successful detection of stratospheric water vapor using the Cassini Composite Infrared Spectrometer (CIRS, Flasar et al. 2004). CIRS senses water emissions in the far infrared spectral region near 50 microns, which we have modeled using two independent radiative transfer codes (NEMESIS, Irwin et al 2008 and ART, Coustenis et al. 2007, 2010). From the analysis of nadir spectra we have derived a mixing ratio of (0.14 0.05) ppb at an altitude of 97 kilometers, which corresponds to an integrated (from 0 to 600 kilometers) surface normalized column abundance of (3.7 plus or minus 1.3) x 10(exp 14) molecules per square centimeter. In the latitude range 80 S to 30 N we see no evidence for latitudinal variations in these abundances within the error bars. Using limb observations, we obtained mixing ratios of (0.13 plus or minus 0.04) ppb at an altitude of 115 kilometers and (0.45 plus or minus 0.15) ppb at an altitude of 230 kilometers, confirming that the water abundance has a positive vertical gradient as predicted by photochemical models (e.g. Lara et al. 1996, Wilson and Atreya 2004, Horst et al. 2008); retrieved scaling factors (from approximately 0.1 to approximately 0.6) to the water profile suggested by these models show that water vapor is present in Titan stratosphere with less abundance than predicted.

Cottini, V.↗
NASA NTRS1999

Detection of C2H4 Neptune from ISO/PHT-S Observations

The 6-12 micrometer spectrum of Neptune has been recorded with the PHT-S instrument of the Infrared Space Observatory (ISO) at a resolution of 0.095 micrometer. In addition to the emissions of CH4, CH3D and C2H6 previously identified, the spectrum shows the first firm identification of ethylene C2H4. The inferred column density above the 0.2-mbar level is in the range (1.1 - 3) x 10(exp 14) molecules/cm. To produce this low amount, previous photochemical models invoked rapid mixing between the source and sink regions of C2H4. We show that this requirement can be relaxed if recent laboratory measurements of CH4 photolysis branching ratios at Lyman alpha are used.

Schulz, B.↗
NASA NTRS1999

Photochemistry of Oxygen Compounds in Neptune's Stratosphere

ISO SWS observations of H2O and CO2 on Neptune (Feuchtgruber et al. 1997, Nature 389, 159) coupled with ground based millimeter observations of CO (Marten et al., 1993, Ap. J. 406 285) provide strong constraints on the photochemistry of these compounds in Neptune's stratosphere. Additional constraints on the photochemistry and vertical mixing come from the rich suite of hydrocarbons observed by ISO: CH4, C2H2, C2H6 (Bezard, 1998, Ann. Geophysicae 16, Sup. III C1037), CH3 (Bezard, et al. Ap. J. 515, 868), and C2H4 (Encrenaz et al. this DPS). We will use a one-dimensional global average photochemical model to analyze these compounds. In particular, we will better constrain the estimated external flux of H2O to Neptune's stratosphere given in Feuchtgruber et al., which is important in understanding the source of the observed stratospheric CO2. Kingdoms will come and go, the relentless march of time will continue, all will be explained.

Romani, P. N.↗
NASA NTRS1997

The infrared spectrum of comet Hale-Bopp

The infrared spectroscopic observations of the comet C/1995 O1 (Hale-Bopp), in April and September-October 1996, are summarized. High resolution spectra were obtained with the long and short wavelength spectrometers of the Infrared Space Observatory (ISO). The results showed that the dust in this comet contains crystalline silicates. The dust of Hale-Bopp is rather similar to that observed in the circumstellar disks of Vega-type stars.

Crovisier, J.↗
NASA NTRS1997

ISO observations of Titan with SWS/grating

The observations of Titan performed by the Infrared Space Observatory (ISO) short wavelength spectrometer (SWS), in the 2 micrometer to 45 micrometer region using the grating mode, are reported on. Special attention is given to data from Titan concerning 7 micrometer to 45 micrometer spectral resolution. Future work for improving Titan's spectra investigation is suggested.

Coustenis, A.↗
NASA NTRS1997

Galileo Infrared Observations of Jupiter

Galileo infrared observaitons of Jupiter have been perfomed with two instruments, the Near Infrared Mapping Spectrometer (NIMS) and the Photo-Polarimeter Radiometer (PPR).

Galileo↗
NASA NTRS1995

The thermal structure and dynamics of the atmosphere of Venus between 70 and 90 km from the Galileo-NIMS spectra

An analysis of thermal profiles and dynamics over a wide range of latitudes for the venusian atmosphere between 70 and 90 km is presented based on high spatial resolution infrared spectra of the night side obtained by the near infrared mapping spectrometer (NIMS) experiment during the Galileo-Venus encounter in February 1990. Using the 4.3-micrometer CO2 absorption band, the temperature profile is retrieved in the 75- to 91-km altitude region over a latitudinal range of -59 deg to +64 deg. Compared to earlier observations from the Pioneer Venus mission, the temperature at 91 km is about 10 K higher and between 74 and 83 km about 3.6 K colder. An equator to pole warming at constant pressure levels is found and implications for the zonal wind profiles are drawn under the assumption that the atmosphere is in cyclostrophic balance in the region of 70 to 90 km. The results are in correspondence with direct wind measurements from ground-based observations at 95 km and 105 km altitude.

Roos-Serote, M.↗