Engineering topics
Publications and source records attributed to Encrenaz, T..
Using Venus nightside data obtained by the Galileo Near Infrared Mapping spectrometer, we hav studied the correclation of 1.74 and 2.30 (sub m) radiation which is transmitted through the clouds.
A systematic study made of the observability of minor species in Titan's atmosphere is reported. This was done by modeling its thermal spectrum at 0.5/cm resolution under the conditions of the Infrared Space Observatory (ISO) and Composite Infrared Radiometer Spectrometer (CIRS), and by exploring the capabilities of the 0.03/cm resolution mode of ISO.
The 225.9 GHz line of HDO has been detected in absorption in the atmosphere of Venus, with the 30-m IRAM antenna. This measurement gives information regarding the vertical distributions of HDO, and thus H2O, in this planet. The observations are consistent with an H2O vertical distribution strongly depleted by saturation at an altitude of 95 km, and, assuming a D/H enrichment of 120 with respect to the terrestrial value, with a mixing ratio of 3.5 (+ or - 2.0) ppm in the range 65-95 km.
The Comet Halley observations showed that carbon is a major component of the comet nucleus, with mass spectroscopic data giving near-cosmic C/O ratios. Gaseous and solid compounds were also observed with infrared spectroscopy, which gave detections of CO and CO2, probable detections or upper limits of H2CO and CH4, and a tentative detection of OCS. The CH4/CO ratio of less than unity in Comet Halley points to a CO-rich solar nebula; however, the ratio is higher than in interstellar clouds. A broad, complicated emission feature near 3.4 microns is evidence for carbonaceous compounds containing C-H groups in gas or solid phases. Analysis of radiation mechanisms and abundance constraints suggests that thermal emission or transient heating by single photons can account for the 3.4-micron emission feature. The band resembles (but is not identical to) bands of carbonaceous chondrite organic material, synthetic materials, and interstellar carbonaceous bands.
The high-resolution records obtained from mm-wave observations of the Mars J = 1-0 C-12O line and the radiodetection of C-13O in September-October 1986 and January 1987 have demonstrated mm-wave observations' utility in the study of planetary atmospheres. The results obtained allow the derivation of the Martian thermal profile at the time of dust-storms, and well as the monitoring of possible short-term changes in CO abundance. It is concluded that the observed C-12O/C-13O ratio is terrestrial, and that strong indications emerge of temporal variations in the thermal profile and CO abundance of Mars.
Infrared photometry of Comet P/Halley is presented over a period ranging from September 1985 to June 1986, prehelion and posthelion. Short-term variations are observed in the intervals Dec. 25, 1985 - Jan. 3, 1986 and Feb.17 - March 3, 1986. When the comet is in a quiescent state, the infrared flux increases linearly with the diaphragm size, which implies a dust expansion at constant velocity up to a distance to the nucleus of at least 10,000 km. The temperature measured in April 1986 ranges between 250 and 300 K, in reasonable agreement with the expected equilibrium temperature of the grains. Between September 1985 and June 1986, no evidence of variation in the dust nature is observed.
The infrared instrument IKS flown on board the VEGA space probes was designed for the detection of emission bands of parent molecules, and for a measurement of the size and temperature of the thermal emitting nuclear region. The instrument had three channels with cooled detectors: an "imaging channel" designed to modulate the signal of the nucleus and two spectroscopic channels operating at 2.5-5 and 6-12 micrometers, respectively, equipped with circular variable filters of resolving power approximately 50. This paper presents and discusses the results from the spectral channels. On VEGA 1, usable spectra were obtained at distances D from the comet nucleus ranging from 250,000 to 40,000 km corresponding to fields of view 4000 and 700 km in diameter, respectively. The important internal background signal caused by the instrument itself, which could not be cooled, had to be eliminated. Since no sky chopping was performed, we obtain difference spectra between the current spectrum and a reference spectrum with little or no cometary signal taken at the beginning of the observing sequence (D approximately 200,000 km). Final discrimination between cometary signal and instrumental background is achieved using their different time evolution, since the instrumental background is proportional to the slow temperature drift of the instrument, and the cometary signal due to parent molecules or dust grains is expected to vary in first order as D-1. The 2.5-5 micrometers IKS spectra definitely show strong narrow signals at 2.7 and 4.25 micrometers, attributed to the nu 3 vibrational bands of H2O and CO2, respectively, and a broader signal in the region 3.2-3.5 micrometers, which may be attributed to CH-bearing molecules. All these signals present the expected D-1 intensity variation. Weaker emission features at 3.6 and 4.7 micrometers could correspond to the nu 1 and nu 5 bands of H2CO and the (1 - 0) band of CO, respectively. Molecular production rates are derived from the observed emissions, assuming that they are due to resonance fluorescence excited by the Sun's infrared radiation. For the strong bands of H2O and CO2, the rovibrational lines are optically thick, and radiative transfer is taken into account. We derive production rates, at the moment of the VEGA 1 flyby, of approximately 10(30) sec-1 for H2O, approximately 2.7 x 10(28) sec-1 for CO2, approximately 5 x 10(28) sec-1 for CO, and 4 x 10(28) sec-1 for H2CO, if attributions to CO and H2CO are correct. The production rate of carbon atoms in CH-bearing molecules is approximately 9 x 10(29) sec-1 assuming fluorescence of molecules in the gas phase, but could be much less if the 3.2-3.5 micrometers emission is attributed to C-H stretch in polycyclic aromatic hydrocarbons or small organic grains. In addition, marginal features are present at 4.85 and 4.45 micrometers, tentatively attributed to OCS and molecules with the CN group, respectively. Broad absorption at 2.8-3.0 micrometers, as well as a narrow emission at 3.15 micrometers, which follow well the D-1 intensity variation, might be due to water ice. Emission at 2.8 micrometers is also possibly present, and might be due to OH created in vibrationally excited states after water photodissociation. The 6-12 micrometers spectrum does not show any molecular emission, nor emission in the 7.5-micrometers region. The spectrum is dominated by silicate emission showing a double structure with maxima at 9.0 and 11.2 micrometers, which suggests the presence of olivine.
The Infrared Net has been active in all areas of comet science in its domain. The more than 100 members of the net and 30 participating observatories are still engaged in observations, and only a preliminary discussion of the highlights of the program is possible at this time.
IUE data on the 1600-2000 A spectral features for Uranus during 1980 are discussed in comparison with equivalent data for Saturn and Jupiter. Attention was focused on detecting possible temporal changes in the C2H2 abundance at 1700 A. The C2H2 abundance in 1980 is estimated to have been a column density of 3.4-10.2 x 10 to the 16th/sq cm. The presence of C2H2 was attributed to photochemical reactions in a layer below 100 km altitude, and was not controlled by saturation, which took place at lower altitudes. A K value of 100,000-1 million/sq cm per sec for Uranus was comparable to Jupiter's and 10-100 times lower than Saturn's.
Models of the spectral reflectivity at the center of the disk of Jupiter from 1450 to 3150 angstroms are presented. The reflectivity was computed from 30 low-dispersion IUE spectra taken during the 1978-1980 solar maximum. A vertically inhomogeneous radiative transfer program was used to compute model reflectivities of various stratospheric compositions for comparison. Ammonia and acetylene are well determined because they show narrow absorption bands in the ultraviolet. Possible compositions to improve the fit to the data below 1800 angstroms are suggested. The data are too noisy to detect possible CO Cameron band absorption near 2000 angstroms.
From 2000 A to 3000 A, both Uranus and Neptune have albedos that are about two times higher than Jupiter or Saturn's, implying that the outer giants have stratospheres that are relatively free of aerosol absorption. Uncertainties in the absolute calibration procedure allow discrepancies of order 15% between conservative models and the observations. A small amount of aerosol absorption is therefore possible. Below 2000 A, the derived albedo is highly dependent on the solar spectrum source used in the data reduction. The most recent result for Uranus is consistent with a secular change in C2H2 mixing ratio from approximately 3 x (10 to the -8 power) in 1980 to or = 10 to the -9 power in 1983. These values are approximately 2 orders of magnitude less than the mixing ratios of this gas on Saturn, and comparable to the amount on Jupiter.
From 2000 to 3000 A, both Uranus and Neptune have albedos that are about two times higher than Jupiter or Saturn's, implying that the outer giants have stratospheres that are relatively free of aerosol absorption. Uncertainties in the absolute calibration procedure allow discrepancies of order 15% between conservative models and the observations. A small amount of aerosol absorption is therefore possible. Below 2000 A the derived albedo is highly dependent on the solar spectrum source used in the data reduction. The most recent result for Uranus, first reported here, is consistent with a secular change in C2H2 mixing ratio from approximately three times ten to the minus eight in 1980 to less than or equal to ten to the minus ninth in 1983. These values are approximately 2 orders of magnitude less than the mixing ratios of this gas on Saturn, and comparable to the amount on Jupiter.
No evidence is found for large scale phosphine abundance variations over Jovian latitudes between -30 and +30 deg, in PH3, CH3D, and GeH4 abundances derived from the 2100-2250/cm region of the Voyager 1 IRIS spectra. The PH3/H2 value of (4.5 + or - 1.5) X 10 to the -7th derived from atmospheric regions corresponding to 170-200 K is 0.75 + or - 0.25 times the solar value, and suggests that the PH3/H2 ratio on Jupiter decreases with atmospheric pressure upon comparison with other PH3 determinations at 10 microns. In the 200-250 K region, CH3D/H2 and GeH4/H2 ratios of 2.0 X 10 to the -7th and 1.0 X 10 to the -9th, respectively, are derived within a factor of 2.0. Assuming a C/H value of 0.001, as derived from Voyager, the CH3D/H2 ratio obtained in this study implies a D/H ratio of 0.000018. This is in agreement with the interstellar medium value.
During its emergence from the Saturnian shadow on June 28, 1980, Titan was observed simultaneously in the visible and IR ranges (6000-9000 A, and 11.8 and 20 microns). The eclipse observation indicates that thin-atmosphere models with a modest greenhouse effect are likely to be incorrect. A thin atmosphere of pure methane in equilibrium with an ice surface of methane is in marginal agreement with the present data, but improved VLA temperature measurements would be a crucial test of this model. Both the eclipse observation and VLA measurements agree with a nitrogen-dominant atmospheric model, as indicated by preliminary Voyager results.
The ratio of deuterium to hydrogen in the Jovian atmosphere has been calculated using a new approach. D/C is obtained from weak lines of HD and CH4 in the visible region of the spectrum while C/H is derived from stronger methane and hydrogen absorptions near 1 micron. This technique permits minimization of the varying effects on line formation caused by scattering in the Jovian atmosphere while relying on absorption bands whose strengths have been measured in the laboratory. The result is a deuterium-to-hydrogen ratio of 2.3 + or - 1.1 times 10 to the -5th power, within the range of local interstellar values
On the basis of recently calculated models of the Jovian atmosphere, a value of 5.0 was derived for the H2/He mixing ratio from the Pioneer 10 infrared radiometer data. A far-infrared spectrum corresponding to the thermal profile obtained in the Pioneer S-band occultation experiment was also computed. The spectrum strongly suggests a misinterpretation of the data obtained in that experiment.
From recent spectra of Jupiter, Saturn and Titan in the 6450-A band of NH3, new values of the abundance of ammonia are derived for Jupiter and Saturn. NH3 abundances of 13 plus or minus 3 m-Am on Jupiter and 2 plus or minus 1 m-Am on Saturn were found. High resolution profiles of NH3 lines on Jupiter are used to estimate a pressure of 2.1 atm at the line formation level in the Jovian atmosphere. In the case of Titan, an NH3 abundance of less than 2 m-Am was found. The implications of these results are discussed.
We have re-examined the visible and near-IR regions of the spectra of Uranus and Neptune to provide additional data for constructing atmospheric models. We find that a true continuum exists only at wavelengths below 4700 A, that the 6420 A absorption previously attributed to hydrogen is probably caused by methane, that there is no evidence for ammonia, ethylene or ethane absorptions in our spectra, and that the abundance of methane is probably much higher than previous estimates suggest. This last finding implies that the value of H/C in the atmospheres of both planets is much less than 1/10 the solar (or Jovian) value. Clear, Rayleigh-scattering model atmospheres are not compatible with the observations, but more work is needed to establish viable alternatives.