Bruno Bézard

Deuterium on Venus: Observations From Earth

Absorption lines of HDO and H2O have been detected in a 0.23-wave number resolution spectrum of the dark side of Venus in the interval 2.34 to 2.43 micrometers, where the atmosphere is sounded in the altitude range from 32 to 42 kilometers (8 to 3 bars). The resulting value of the deuterium-to-hydrogen ratio (D/H) is 120 � 40 times the telluric ratio, providing unequivocal confirmation of in situ Pioneer Venus mass spectrometer measurements that were in apparent conflict with an upper limit set from International Ultraviolet Explorer spectra. The 100-fold enrichment of the D/H ratio on Venus compared to Earth is thus a fundamental constraint on models for its atmospheric evolution.

The C/H ratio in Jupiter from the Voyager infrared investigation

From a selection of Voyager IRIS spectra corresponding to cloud-free areas of Jupiter, we have determined the CH/sub 4//H/sub 2/ volume ratio in the atmosphere of this planet as equal to (1.95 +- 0.22)10/sup -3/ which corresponds to 2.07 +- 0.24 times the solar value of Lambert (C/H = 4.7 x 10/sup -4/). Estimate of errors includes both instrument noise and systematic uncertainties. Implications of this result on the formation and evolution of Jupiter are discussed.

An equatorial oscillation in Saturn's middle atmosphere

The middle atmospheres of planets are driven by a combination of radiative heating and cooling, mean meridional motions, and vertically propagating waves (which originate in the deep troposphere). It is very difficult to model these effects and, therefore, observations are essential to advancing our understanding of atmospheres. The equatorial stratospheres of Earth and Jupiter oscillate quasi-periodically on timescales of about two and four years, respectively, driven by wave-induced momentum transport. On Venus and Titan, waves originating from surface–atmosphere interaction and inertial instability are thought to drive the atmosphere to rotate more rapidly than the surface (superrotation). However, the relevant wave modes have not yet been precisely identified. Here we report infrared observations showing that Saturn has an equatorial oscillation like those found on Earth and Jupiter, as well as a mid-latitude subsidence that may be associated with the equatorial motion. The latitudinal extent of Saturn’s oscillation shows that it obeys the same basic physics as do those on Earth and Jupiter. Future highly resolved observations of the temperature profile together with modelling of these three different atmospheres will allow us determine the wave mode, the wavelength and the wave amplitude that lead to middle atmosphere oscillation.

Thermal profiles in the auroral regions of Jupiter

The temperature structure within the northern auroral region of Jupiter is studied by reanalyzing the Voyager 1/infrared interferometer and radiometer spectrometer (IRIS) spectra. The total measured excess infrared auroral zone emission (averaged over the IRIS field of view) in the hydrocarbon bands between 7 and 13 μm is found to be about 208 ergs cm−2 s−1 over an area of about 2 × 1018 cm2 with a resulting power output of 4 × 1013 W. In comparison, the total energy deposition by magnetospheric charged particles has been estimated on the basis of UV observations to range between 1 × 1013 and 4 × 1013 W over a comparable area. The large amount of radiated energy observed in the infrared may imply an additional heat source in the auroral regions (possibly Joule heating). A new set of thermal profiles of Jupiters high-latitude upper atmosphere has also been derived. These profiles have a large temperature enhancement in the upper stratosphere and are constrained to reproduce the CH4 emission at 7.7 μm. The emission in the other hydrocarbon bands (C2H2 and C2H6) is found to depend on the depth to which the temperature enhancement extends, which further constrains the thermal profiles. This study shows that a large temperature enhancement in the upper stratosphere and lower thermosphere can explain the observed excess hydrocarbon emission bands; thus smaller variations in hydrocarbon abundances (between the high latitudes and the equatorial and middle latitudes) are required than has been assumed in previous models.

The abundance of sulfur dioxide below the clouds of Venus

We present a new method for determining the abundance of sulfur dioxide below the clouds of Venus. Absorption by the 3ν3 band of SO2 near 2.45 µm has been detected in high-resolution spectra of the night side of Venus recorded at the Canada-France-Hawaii telescope in 1989 and 1991. The inferred SO2 abundance is 130±40 ppm at all observed locations and pertains to the 35–45 km region. These values are comparable to those measured by the Pioneer Venus and Venera 11/12 entry probes in 1978. This stability stands in contrast to the apparent massive decrease in SO2 observed at the cloud tops since these space missions. These results are consistent with laboratory and modelling studies of the SO2 destruction rates in the lower atmosphere of Venus. The new spectroscopic technique presented here allows a remote monitoring of the SO2 abundance below the clouds, a likely tracer of Venusian volcanism.

Stratospheric profile of HCN on Titan from millimeter observations

Abstract Measurements of the (1−0) line of hydrogen cyanide at 88.6 GHz in the Titan atmosphere are reported. Synthetic spectra were fitted to the observations to derive the vertical distribution of HCN in the stratosphere. The observed line is significantly narrower than that computed for constant stratospheric mixing ratios, implying an increase in the HCN concentration with altitude. From a least-squares analysis taking into account measurement noise and calibration uncertainties, a mean mixing ratio scale height of 47 −11 +36 km is derived for the 100- to 300-km region. The HCN abundance is found to be best constrained around the 170-km level where the inferred mixing ratio is 3.3 −0.8 +0.9 × 10 −7 . The results are consistent with recent analyses of Voyager infrared measurements. The inferred vertical concentration gradient is much steeper and the abundance in the lower stratosphere smaller than predicted by current photochemical models. Theoretical HCN profiles may, however, be brought into agreement with the present results by reducing the magnitude of the vertical eddy mixing assumed in the stratosphere.

Hydrocarbons in Neptune's stratosphere from Voyager infrared observations

Emission from the acetylene and ethane bands at 729 and 822 cm-1detected in the Voyager infrared spectra of Neptune has been analyzed. A large selection of low-spatial resolution spectra was used to derive the disk-averaged abundances of C2H2 and C2H6. Under the assumption of uniform vertical distributions above the saturation region, a C2H2 mixing ratio of 6−4+14 x 10 −8 and a C2H6 mixing ratio of 1.5−0.5+2.5 x 10−6 were inferred. The accuracy of the retrievals is limited by the large uncertainty in the stratospheric temperature structure. The maximum contribution to the observed C2H2 and C2H6emission comes from the 0.2- and 0.7-mbar regions, respectively. Mixing ratio profiles derived from photochemical modeling, which are not constant with height above the saturation region, indicate that the hydrocarbon emission is most sensitive to the assumed eddy diffusion coefficient in the millibar region. Either the C2H2 or the C2H6 emission can be reproduced by the photochemical model to within the accuracy of the retrievals, but not both simultaneously. Best fits to both emission features simultaneously occur with C2H2 mixing ratios a factor of 2 too high and C2H6 mixing ratios a factor of 2 too low. We consider this agreement satisfactory considering the unknowns in the chemical and photolytic processes. A set of Voyager spectra at higher spatial resolution was used to study the latitudinal variation of the C2H2 emission between 30°N and 80°S. Zonal mean radiances at the C2H2 peak show a minimum near 50°–60°S and maxima near the south pole and equator. This behavior is similar to that observed at 350 and 250 cm−1, where the lower stratosphere and troposphere are sounded. The mid-latitude minimum can be explained by a fivefold depletion of acetylene or a temperature decrease of about 15 K (or any combination of the two effects) in the 0.03- to 2-mbar region. The latitude variation in the C2H2emission could result from a circulation pattern forced from deep levels, with upwelling at mid-latitudes and subsidence at low and high latitudes.

Study of the deep cloud structure in the equatorial region of Jupiter from voyager infrared and visible data

Abstract High spatial resolution infrared and visible data obtained by the Voyager 1 spacecraft have been analyzed simultaneously to infer properties of the deep cloud structure of the Jovian troposphere in the 1- to 4-bar pressure range. Influence of the ammonia upper cloud layer, in the 5μm Jovian window, has been investigated through a cloud model derived from far ir Voyager IRIS measurements. The attenuation, computed with an anisotropic scattering formulation, is too weak to explain 5-μm measurements and provides evidence for existence of a cloud structure at deeper levels. The main conclusions derived from the present analysis are summarized below: (1) the deep cloud structure appears to be vertically associated with the NH 3 upper layer; (2) the ammonia cloud is mainly responsible for the visible appearance of the Jovian equatorial region; (3) the deep cloud structure exhibits a grey opacity in the 5-μm window; (4) coldest 5-μm spectra can be interpreted by the existence of a thick cloud layer located at levels in the 180–195°K temperature range. Implications of these results are discussed in conjunction with predictions of dynamical and thermochemical models. NH 4 SH is shown to be a likely candidate for the main deep cloud constituent. An even deeper thick H 2 O cloud may be present too, but should not be responsible for the observed spread in 5-μm brightness temperatures.

Tropospheric carbon monoxide concentrations and variability on Venus from Venus Express/VIRTIS-M observations

We present nightside observations of tropospheric carbon monoxide in the southern hemisphere near the 35 km height level, the first from Venus Express/Visible and Infrared Thermal Imaging Spectrometer (VIRTIS)-M-IR. VIRTIS-M data from 2.18 to 2.50 μm, with a spectral resolution of 10 nm, were used in the analysis. Spectra were binned, with widths ranging from 5 to 30 spatial pixels, to increase the signal-to-noise ratio, while at the same time reducing the total number of retrievals required for complete spatial coverage. We calculate the mean abundance for carbon monoxide at the equator to be 23 ± 2 ppm. The CO concentration increases toward the poles, peaking at a latitude of approximately 60°S, with a mean value of 32 ± 2 ppm. This 40% equator-to-pole increase is consistent with the values found by Collard et al. (1993) from Galileo/NIMS observations. Observations suggest an overturning in this CO gradient past 60°S, declining to abundances seen in the midlatitudes. Zonal variability in this peak value has also been measured, varying on the order of 10% (~3 ppm) at different longitudes on a latitude circle. The zonal variability of the CO abundance has possible implications for the lifetime of CO and its dynamics in the troposphere. This work has definitively established a distribution of tropospheric CO, which is consistent with a Hadley cell circulation, and placed limits on the latitudinal extent of the cell.

A seasonal model of the Saturnian upper troposphere: Comparison with Voyager infrared measurements

An extension of the seasonal climate model of R. D. Cess and J. Caldwell (1979, Icarus, 38, 349–357) to Saturns upper troposphere is presented. The ring-modulated latitudinal dependence of the insolation, the ring thermal emission, the oblateness of the planet, the orbit eccentricity, and the latitudinal variation of the internal heat flux are taken into account. Calculations agree closely with the temperature—latitude profiles retrieved from Voyager IRIS measurements at atmospheric levels located above the 0.2-bar pressure level; they reproduce the observed large-scale hemispheric asymmetry which is then shown to result from the seasonally variable insolation. Aerosol absorption is found to be the dominant source of atmospheric solar heating in the troposphere and the model suggests an aerosol mean unit optical depth around the 0.25-bar level in the equatorial region and around the 0.35-bar level at other latitudes. The model fails to predict the retrieved temperature—latitude profiles below the 0.3-bar level. This discrepancy is attributed to the existence of clouds at these levels which are responsible for an additional far-infrared opacity not taken into account in the temperature retrieval. The cloud-top altitude would be about 0.3 bar except in the 20 to 40°N region where these clouds would be confined below the 0.6-bar level. The poor correlation between infrared measurements and visible images is discussed and a possible model of Saturns cloud structure is proposed.