No abstract

Aerosols in Titan's atmosphere play an important role in determining its thermal structure. They also serve as sinks for organic vapours and can act as condensation nuclei for the formation of clouds, where the condensation efficiency will depend on the chemical composition of the aerosols. So far, however, no direct information has been available on the chemical composition of these particles. Here we report an in situ chemical analysis of Titan's aerosols by pyrolysis at 600 degrees C. Ammonia (NH3) and hydrogen cyanide (HCN) have been identified as the main pyrolysis products. This clearly shows that the aerosol particles include a solid organic refractory core. NH3 and HCN are gaseous chemical fingerprints of the complex organics that constitute this core, and their presence demonstrates that carbon and nitrogen are in the aerosols.

Abstract. Electrostatic charging of a spacecraft modifies the distribution of electrons and ions before the particles enter the sensors mounted on the spacecraft body. The floating potential of magnetospheric satellites in sunlight very often reaches several tens of volts, making measurements of the cold (several eV) component of the ambient ions impossible. The plasma electron data become contaminated by large fluxes of photoelectrons attracted back into the sensors.The Cluster spacecraft are equipped with emitters of the liquid metal ion source type, producing indium ions at 5 to 9 keV energy at currents of some tens of microampere. This current shifts the equilibrium potential of the spacecraft to moderately positive values. The design and principles of the operation of the instrument for active spacecraft potential control (ASPOC) are presented in detail.Experience with spacecraft potential control from the commissioning phase and the first two months of the operational phase are now available. The instrument is operated with constant ion current for most of the time, but tests have been carried out with varying currents and a "feedback" mode with the instrument EFW, which measures the spacecraft potential . That has been reduced to values according to expectations. In addition, the low energy electron measurements show substantially reduced fluxes of photoelectrons as expected. The flux decrease in photoelectrons returning to the spacecraft, however, occurs at the expense of an enCorrespondence to: K. Torkar (klaus.torkar@oeaw.ac.at) larged sheath around the spacecraft which causes problems for boom-mounted probes.

A series of scientific spacecraft either in orbit or to be launched in the very near future carries ion emitters for active control of the electrostatic surface potential. The ion beam emission changes, via the associated reduction of the positive surface potential, the photoelectron sheath around the spacecraft which may result in the formation of a potential barrier. In this study, a spherically symmetric model of the photoelectron sheath around a spacecraft (with conductive surface) is analyzed. An algebraic equation is set up to describe the current equilibrium between the escaping photoelectrons, incoming ambient plasma electrons, and the artificially emitted charged particles at the surface of the spacecraft. The height of the potential barrier can be given by solving the algebraic equation, if the electrostatic surface potential of the spacecraft, the density and temperature of the ambient plasma electrons are known. The position of the potential barrier is estimated with the assumption that the electrostatic potential distribution is of a Debye form/The method has been applied to the data gained from the Geotail spacecraft at times when the ion emitter was operational. From the calculation, it is shown that the height of the potential barrier grows from 0.27(Tph/e) to 0.91(Tph/e), as the ion emitter current increases from 9.2(μA) to 38.3(μA). The maximum barrier height appears to be about 2 V, when the ion emitter operates at its maximum current about 50 μA.