Jacques Blamont

Extreme Ultraviolet Observations from Voyager 1 Encounter with Jupiter

Observations of the optical extreme ultraviolet spectrum of the Jupiter planetary system during the Voyager 1 encounter have revealed previously undetected physical processes of significant proportions. Bright emission lines of S III, S IV, and O III indicating an electron temperature of 105 K have been identified in preliminary analyses of the Io plasma torus spectrum. Strong auroral atomic and molecular hydrogen emissions have been observed in the polar regions of Jupiter near magnetic field lines that map the torus into the atmosphere of Jupiter. The observed resonance scattering of solar hydrogen Lyman α by the atmosphere of Jupiter and the solar occultation experiment suggest a hot thermosphere (≥ 1000 K) wvith a large atomic hydrogen abundance. A stellar occultation by Ganymede indicates that its atmosphere is at most an exosphere.

Ultraviolet Spectrometer Observations of Neptune and Triton

Results from the occultation of the sun by Neptune imply a temperature of 750 � 150 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane, acetylene, and ethane at lower levels. The ultraviolet spectrum of the sunlit atmosphere of Neptune resembles the spectra of the Jupiter, Saturn, and Uranus atmospheres in that it is dominated by the emissions of H Lyman α (340 � 20 rayleighs) and molecular hydrogen. The extreme ultraviolet emissions in the range from 800 to 1100 angstroms at the four planets visited by Voyager scale approximately as the inverse square of their heliocentric distances. Weak auroral emissions have been tentatively identified on the night side of Neptune. Airglow and occultation observations of Tritons atmosphere show that it is composed mainly of molecular nitrogen, with a trace of methane near the surface. The temperature of Tritons upper atmosphere is 95 � 5 kelvins, and the surface pressure is roughly 14 microbars.

ULTRAVIOLET SPECTROMETER OBSERVATIONS OF URANUS.

Data from solar and stellar occultations of Uranus indicate a temperature of about 750 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane and acetylene in the lower levels. The ultraviolet spectrum of the sunlit hemisphere is dominated by emissions from atomic and molecular hydrogen, which are kmown as electroglow emissions. The energy source for these emissions is unknown, but the spectrum implies excitation by low-energy electrons (modeled with a 3-electron-volt Maxwellian energy distribution). The major energy sink for the electrons is dissociation of molecular hydrogen, producing hydrogen atoms at a rate of 1029 per second. Approximately half the atoms have energies higher than the escape energy. The high temperature of the atmosphere, the small size of Uranus, and the number density of hydrogen atoms in the thermosphere imply an extensive thermal hydrogen corona that reduces the orbital lifetime of ring particles and biases the size distribution toward larger particles. This corona is augmented by the nonthermal hydrogen atoms associated with the electroglow. An aurora near the magnetic pole in the dark hemisphere arises from excitation of molecular hydrogen at the level where its vertical column abundance is about 1020 per square centimeter with input power comparable to that of the sunlit electroglow (approximately 2x1011 watts). An initial estimate of the acetylene volume mixing ratio, as judged from measurements of the far ultraviolet albedo, is about 2 x 10-7 at a vertical column abundance of molecular hydrogen of 1023 per square centimeter (pressure, approximately 0.3 millibar). Carbon emissions from the Uranian atmosphere were also detected.

Simultaneous nighttime Lidar measurements of atmospheric sodium and potassium

Abstract Simultaneous measurements of the nighttime atmospheric sodium and potassium layer have been performed over a period of one year, using a lidar facility set up at the Haute Provence Observatory. A detailed description of the calibration method is given together with the estimated accuracy of the experiment. The similarity observed in the behaviour of the spatial parameters of both layers indicates that the same processes are responsible for their day-to-day variations. However, the long term behaviour of the sodium and potassium total column abundances is very different: whereas the sodium one shows the usual seasonal variation already observed, no marked increase is seen in the potassium abundance during the year. The abundance ratio of these two elements varies then between a low summer value (∼10) and a high winter value (∼50). Two different origins can then be assumed for the alkalis in the upper atmosphere: a meteoritic one constant over the year and a terrestrial source due to the vertical transport of particles at high latitudes which only works in winter. Confirmation of this hypothesis can be found in the lack of sodium seasonal variation as observed at low latitudes.

Vertical Structure and Size Distributions of Martian Aerosols from Solar Occultation Measurements

Solar occultations performed with a spectrometer on board the Soviet spacecraft Phobos 2 (Blamont et al. 1991) provided data on the vertical structure of the Martian aerosols in the equatorial region (0°–20° N latitude) near the northern spring equinox (LS = 0°–20°). All measurements were made close to the evening terminator. Five clouds were detected above 45 km altitude and their vertical structure recorded at six wavelengths between 0.28 and 3.7 μm. They have a small vertical extent (3–6 km) and a vertical optical depth less than 0.03. The thermal structure, as derived from saturated profiles of water vapor observed by our instrument in the infrared, does not allow the CO2 frost point to be reached at cloud altitude, strongly suggesting that cloud particles are formed of H2O ice. Under the assumption of spherical particles, a precise determination of their effective radius, which varies from cloud to cloud and with altitude, is obtained and ranges from 0.15 to 0.85 μm; an estimate of the effective variance of the particle size distribution is ∼ 0.2. The number density of cloud particles at the peak extinction level is ∼1 cm−3. Dust was also observed and monitored at two wavelengths, 1.9 and 3.7 μm, on nine different occasions. The top of the dust opaque layer, defined as the level above which the atmosphere becomes nearly transparent at the wavelengths of observation, is located near 25 km altitude, with variations smaller than ±3 km from place to place. The scale height of dust at this altitude is 3–4 km. The effective radius of dust particles near the top of the opaque layer is 0.95 ± 0.25 μm and increases below with a vertical gradient of ∼0.05 μm km−1. Assuming that particles are levitated by eddy mixing, the eddy diffusion coefficient, K, is found to be ∼106 cm2 sec−1 at 25 km and 105−106 cm2 sec−1 at 50 km using, respectively, dust and cloud observations. An effective variance of 0.25 (±50%) for the dust size distribution is obtained on the basis of a simple theoretical model for the observed vertical gradient of the effective radius of dust particles. Three clouds observed by Viking at midlatitude during the northern summer are reanalyzed. The analysis gives K ≈ 106 cm2 sec−1 below 50 km altitude and at least 107 cm2 sec−1 above. Since the clouds seen from Phobos 2 are observed at twilight, which coincides with the diurnal maximum of the ambient temperature, they can be assumed to be in a steady state. If their thermodynamic state were to vary quickly during the day, our optical thickness at twilight would correspond to unrealistic values in earlier hours when the temperature is lower. Clouds are well fitted by theoretical profiles obtained assuming the steady state. An atmospheric temperature of 165–170 K at ∼50 km is inferred. The negative temperature gradient above the cloud is large (1.5–2 K km−1). A parallel is established between these thin clouds and the polar mesospheric clouds observed on Earth. It is shown that upwelling in equatorial regions at equinox could be a significant factor in levitating cloud particles.

Preliminary Results of the Pioneer Venus Nephelometer Experiment

Preliminary results of the nephelometer experiments conducted aboard the large sounder, day, north, and night probes of the Pioneer Venus mission are presented. The vertical structures of the Venus clouds observed simultaneously at each of the four locations from altitudes of from 63 kilometers to the surface are compared, and similarities and differences are noted. Tentative results from attempting to use the data from the nephelometer and cloud particle size spectrometer on the sounder probe to identify the indices of refraction of cloud particles in various regions of the Venus clouds are reported. Finally the nephelometer readings for the day probe during impact on the surface of Venus are presented.

Large, solid particles in the clouds of Venus: Do they exist?

Abstract The discovery of large, solid particles in the clouds of Venus is one of the most significant findings of Pioneer Venus because it means that a substantial mass of the clouds is composed of a material other than sulfuric acid. The evidence which suggests that solid particles form a distinctive size mode is reexamined. The mode is defined by a discontinuity between two size ranges of the Pioneer Venus particle size spectrometer. This discontinuity could represent a real size mode. However, it could also be an artifact of the measurement technique. R. G. Knollenberg (1984) discusses several possible instrumental effects which might have caused this discontinuity. It is hypothesized herein that such effects did occur and that the large particles are really the tail of the mode 2 sulfuric acid particle size distribution and are not a separate mode of solid particles. Using such a revised size distribution, it is shown that all of the Pioneer Venus and Venera optical data from the lower clouds can be explained with sulfuric acid droplets without introducing any solid particles. As a by-product of this analysis, it is also found that the upper clouds of Venus must contain a material with a higher refractive index than sulfuric acid. A small quantity of sulfur could account for this observation.

Nature of the ultraviolet absorber in the venus clouds: inferences based on pioneer venus data.

Several photometric measurements of Venus made from the Pioneer Venus orbiter and probes indicate that solar near-ultraviolet radiation is being absorbed throughout much of the main cloud region, but little above the clouds or within the first one or two optical depths. Radiative transfer calculations were carried out to simulate both Pioneer Venus and ground-based data for a number of proposed cloud compositions. This comparison rules out models invoking nitrogen dioxide, meteoritic material, and volatile metals as the source of the ultraviolet absorption. Models involving either small (∼1 micrometer) or large (∼10 micrometers) sulfur particles have some serious difficulties, while ones invoking sulfur dioxide gas appear to be promising.

Further results of the Pioneer Venus nephelometer experiment

Backscattering data for the nephelometer experiments conducted aboard the Pioneer Venus mission probes, including data up to the highest altitudes measured by the probes, are presented. A few small signals were detected below the main cloud deck. Ambient radiation was measured at near-ultraviolet and visible wavelengths; the variation of extinction of near-ultraviolet with altitude is inferred. Ambient radiance decreased more rapidly at 530 than at 745 nanometers in the lower atmosphere.

Vertical profiles of dust and ozone in the Martian atmosphere deduced from solar occultation measurements

Abstract The high atmosphere of Mars (z > 30 km) has been observed from Phobos 2, using the solar occultation technique. Due to a major error in the pointing system, resulting from a wrong orientation of the spacecraft in the pointing software, the lower atmosphere is never observed in ultraviolet (220–330 nm) and high resolution near-infrared (760 nm, 936 nm) channels. Pointing data sometimes provide useful information on the visible opacity below 30 km. showing that the scale height of dust decreases from ≅ 8 km below 20 km to ≅ 4 km above. Among 32 occultations, four show the presence of a water ice cloud high in the atmosphere (z = 50 km), with a maximum tangential optical thickness varying from ≅ 0.05 to ≅ 2 and a vertical extent in the range from 5 to 15 km. Indications on the particles size (r ≫ 0.01 μm) are obtained using spectral information. From a simple cloud model, where eddy diffusion and sedimentation processes are taken into account, an upper limit of 107 cm2 s−1 is derived for the eddy diffusion coefficient. Similarly, the absence of ozone in detectable amounts above 30 km is interpreted as the signature of a rather weak value of K. Using a simple stationary photochemical model, a value of K as low as 106 cm2 s−1 seems to be required.