ESA Science & Technology - Publication Archive

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.

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.

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.

Saturn's large moon Titan is unique among planetary satellites in that it possesses a thick atmosphere and a haze layer that is opaque to visible light. This haze is believed to be composed of organic compounds produced by the photolysis of methane. It has been suggested that the photochemical products of methane photolysis, primarily ethane, would "rain out" over time and may produce reservoirs of liquid hydrocarbons on Titan's surface. Such material would appear very dark, with an albedo =<0.02 (Khare et al. 1990, Bull. Am. Astron. Soc.22, 1033). Such low-albedo regions have not been previously detected on Titan's surface. Here we report observations of Titan at a resolution of 0.04 arcsec (0.02 arcsec/pixel) using the technique of speckle imaging from the 10-m Keck I Telescope. By observing Titan at specific infrared wavelengths which are windows through its atmosphere, we have made both an albedo map of Titan's surface at 1.6 and 2.1 µm and an estimate of Titan's haze optical depth at these wavelengths. We clearly distinguish low-albedo features (reflectance <0.05) on Titan's surface.

Presented at International Conference On Environmental Systems, July 1998, Danvers, MA, USA, Session: Satellite, Payload, Instrument and Launch Vehicle Thermal Control. Abstract: A study of the first in-orbit temperatures of Huygens shows that the probe will very likely survive thermally all vacuum cruise and coast phases. Calculated heat fluxes, mass flows of Titan's atmosphere into and out of the probe and temperatures give confidence also for the mission phase proper in 2004, i.e., the 2.5 h descent into Titan's -200 °C atmosphere. Basotect foam bags insulate the probe from this atmosphere. These bags and their fixation had to be drastically modified between Titan test on STPM (May 95) and on FM (June 96). The mission phases, thermal requirements, thermal design, tests with the probe, special tests for the foam bag development and their modification are presented.