ESA Science & Technology - Publication Archive

The aim of this work is to deduce physical characteristics of the aerosols that compose the thin detached haze layer of Titan, for which the composition and origin are currently unknown. We have used four images at three different phase angles and three different wavelengths where the detached haze clearly appears. The low optical path in the layer permits assumptions that greatly simplify the treatment and allow us to consider the scattered intensity as the product of the scattering cross section, the phase function at the considered phase angle, and the number of aerosols. Physical arguments are given to support fractal aerosols in the detached haze, and we investigate a wide range of monomer radii, monomer numbers, and imaginary refractive indexes. This study yields information about the spectral variation of the refractive index for the detached haze tholins that are clearly different from those for the main haze. On the other hand, no firm values are given for the monomer radius and number, but these two parameters are strongly linked through a simple relation

We estimate the wind speeds in Titan's thermosphere by considering the various terms of the wind equation, without actually solving it, with a view to anticipating what might be observed by the Cassini spacecraft in 2004. The winds, which are driven by horizontal pressure gradients produced by solar heating, are controlled in the Earth's thermosphere by ion-drag and coriolis force, but in Titan's thermosphere they are mainly controlled by the nonlinear advection and curvature forces.

Understanding the power balance at the surface of the nucleus is essential to study the chemical and physical evolution of a comet. Therefore, we present a detailed energy budget analysis for the surface of a model comet in the orbit of 46P/Wirtanen, target comet of the European space craft mission Rosetta, for a variety of parameters and assumptions. We will show that for a fast spinning Jupiter-family comet such as 46P/Wirtanen with a rotation period of about 6 h, a fast rotator approximation underestimates the effective energy input. This yields lower gas fluxes from the surface. For an 100% active, non-dust covered surface we obtain a water gas flux on the order of about 1.5×10²⁸ molecules s^-1 at perihelion, assuming a radius of 600 m. The calculated gas flux of water is within the order of measured values for comet 46P/Wirtanen. But our calculated values are maximum gas fluxes at noon—not averaged over one cometary day or taking the lesser insolation at the polar areas into account. Therefore, we conclude that either the radius of comet 46P/Wirtanen may be much larger than the accepted value of 600 m. A radius in the order of 2 km seems more likely to explain the measurements. Or, an other possibility could be that water-ice particles are blown off from the surface like dust particles. This may also increase the effective surface area of sublimation.

The very similar D/H ratio in the water of Comets Halley, Hyakutake, and Hale-Bopp (~3 × 10^-4) is 10-20 times larger then the D/H ratio in the solar nebula. Therefore the cometary water had to originate in a giant molecular cloud, where the HDO is enriched by ion-molecule reactions.

Several instruments have been described at the annual meetings of the EGS (European Geophysical Society) and the DPS (Division for Planetary Sciences). The numerous abstracts which were published can be found in:

Geophysical Research Abstracts, Vol. 1, No. 3 (EGS)

An improved magnetohydrodynamic (MHD) model with chemistry is presented. The analysis of the source and sink terms for H₂O⁺ shows that for small comets up to 11% of water molecules are finally ionized. For large comets (such as Halley) this fraction decreases to less than 3%. From the MHD scaling laws a similarity law for the individual ion densities is deduced which takes into account that the mother molecules are depleted by dissociation. This is applied to H₂O⁺ ions. Radial density profiles from model calculations, observations by Giotto near comet Halley, and ground based observations of three comets confirm this scaling law for H₂O⁺ ions. From the similarity law for the density a scaling law for the column density is derived which is more convenient to apply for ground based observations. From these scaling laws methods are derived which allow the determination of the water production rate from the ground based images of the H₂O⁺ ions. Finally, the two dimensional images of model column densities are compared with observations.

We investigated electron dissociative recombination of N⁺₂ ions, electron impact dissociation of N₂ molecules, nonthermal exothermic ionosphere-related photochemical reactions, atmospheric sputtering via solar wind and magnetospheric particles, solar wind pick-up and the loss of ¹⁴N to ¹⁴C via cosmic rays as possible sources of nitrogen isotope fractionation in Titan's atmosphere where this molecule is the principal constituent.

The recent passage of the Hale-Bopp (C/1995 O1) comet has provided the first opportunity to analyse the infrared spectral properties of a bright comet both from the ground and by the ISO space observatory. Previous works have already been dedicated to study the potential candidates to reproduce the cometary feature at 10 micron observed for different comets. We have applied a similar approach to compare the Hale-Bopp (C/1995 O1) spectra with laboratory data. The best fit has been obtained by using a mixture of crystalline Mg-rich olivine (forsterite), amorphous olivine and amorphous carbon grains. Some constraints on the possible cometary grain types derive from our simulation. Aggregates of submicron particles, composed of amorphous and crystalline olivine and amorphous carbon materials seem to be compatible with the cometary emission. Moreover, the possibility of fitting observational data on a wide IR spectra range, offered by ISO, provides interesting hints about the size distribution of grains responsible for the detected features

I compute speed of sound profiles for the four giant planets using simple atmosphere models, and explore the effect on the profiles of ortho : para hydrogen conversion, and the drop in mixing ratio of condensible species at the tropopause.

III. First Modeling of a CO-Dominated Coma, with Application to Comets 46 P/Wirtanen and 29 P/Schwassmann-Wachmann I

We present the first gasdynamic simulations of the coma formed by the diffusion from a comet nucleus interior of a volatile molecule at large heliocentic distance. The method used is a generalization of that described in J. F. Crifo et al. (1995, Icarus 116, 77-112). The molecule is assumed to be CO.

The laboratory investigation of the atmospheric photochemistry of planets and satellites is mainly carried out in static systems. These studies are often poor models of chemical processes in atmospheres because: (1) much higher mixing ratios of minor constituents must be used to accurately determine the amount of reactant consumed and to obtain sufficient products for analysis, (2) secondary photolysis of the initial photoproducts often occurs, (3) wall reactions occur, and (4) most of the starting material is converted to products to obtain enough for spectroscopic analysis. The use of a photochemical flow reactor either circumvents or minimizes these problems by using gas mixtures and photolysis conditions more representative of a planetary atmosphere. A gas mixture, composed of a small amount of a reactant gas diluted in a much larger amount of carrier gas, is flowed past a UV lamp for an extended period of time. Unconsumed reactants and products are collected in traps downstream until amounts sufficient for spectral analysis are collected. FTIR and NMR analysis provides structural information and quantitative data on their rates of formation.

Imaging of comet 46P/Wirtanen was performed in the standard VRI filters on 10 and 11 November 1996 with the 1.0 m telescope of the South African Astronomical Observatory (SAAO). After proper processing and calibration, the images have been used to derive information about the isophote distribution, magnitude and colour indices. We observed a coma extending to at most ~5×10³ km. We derive absolute magnitudes of the coma for the different filters and the colours V-R = 0.18±0.17 and R-I = 0.39±0.16.

Recent observations with the Wide-Field Planetary Camera (WFPC-2) on the Hubble Space Telescope (HST) show an unexpectedly rapid change in the atmospheric albedo contrast between the north and south hemispheres. In 1994 at blue wavelengths, the north was around 15% brighter than the south, and was expected to fall to about 12% in 1997, but has dropped to only 6% brighter.

From diffraction-limited images produced by the near-IR camera NICMOS aboard the Hubble Space Telescope we derived maps of Titan's surface in the methane windows near 1.1, 1.6, and 2.0 micron. Each part of Titan's atmosphere and surface was imaged at least twice over a time period of 24 h to look for moving clouds.

Eds. Schmidt, R. and Guyenne, T.D.