D. O. Kataria

The loss of ions from Venus through the plasma wake

Venus, unlike Earth, is an extremely dry planet although both began with similar masses, distances from the Sun, and presumably water inventories. The high deuterium-to-hydrogen ratio in the venusian atmosphere relative to Earth’s also indicates that the atmosphere has undergone significantly different evolution over the age of the Solar System. Present-day thermal escape is low for all atmospheric species. However, hydrogen can escape by means of collisions with hot atoms from ionospheric photochemistry, and although the bulk of O and O2 are gravitationally bound, heavy ions have been observed to escape through interaction with the solar wind. Nevertheless, their relative rates of escape, spatial distribution, and composition could not be determined from these previous measurements. Here we report Venus Express measurements showing that the dominant escaping ions are O+, He+ and H+. The escaping ions leave Venus through the plasma sheet (a central portion of the plasma wake) and in a boundary layer of the induced magnetosphere. The escape rate ratios are Q(H+)/Q(O+) = 1.9; Q(He+)/Q(O+) = 0.07. The first of these implies that the escape of H+ and O+, together with the estimated escape of neutral hydrogen and oxygen, currently takes place near the stoichometric ratio corresponding to water.

Electron optical study of the Venus Express ASPERA-4 Electron Spectrometer (ELS) top-hat electrostatic analyser

The performance of the Venus Express (VEX) ASPERA-4 Electron Spectrometer (ELS) is different from the nominal response shown by the ASPERA-3 ELS aboard Mars Express due to machining tolerance. Up to now, the precise mechanism for this was unknown and, therefore, the results of the experimental calibration could not be supported with a theoretical understanding of the fundamental instrument science behind the device. In this study, we show that the difference is due to a misalignment of the inner hemisphere and a widening of the entrance aperture of the instrument. The response of the VEX ELS can be approximated by a combination of a vertical offset of the inner hemisphere of ≈0.6 mm and a lateral offset of less than 0.125 mm, combined with an aperture that is ≈0.54 mm wider than nominal. The resulting K-factor, geometric factor, energy resolution and peak elevation are in good agreement with those observed experimentally. Therefore, we now have a good agreement between both laboratory calibration data and computer simulation, giving a firm foundation for future scientific data analysis.

Carbon Chain Anions and the Growth of Complex Organic Molecules in Titan’s Ionosphere

Cassini discovered a plethora of neutral and ionized molecules in Titans ionosphere including, surprisingly, anions and negatively charged molecules extending up to 13,800 u q−1. In this Letter, we forward model the Cassini electron spectrometer response function to this unexpected ionospheric component to achieve an increased mass resolving capability for negatively charged species observed at Titan altitudes of 950–1300 km. We report on detections consistently centered between 25.8 and 26.0 u q−1 and between 49.0–50.1 u q−1 which are identified as belonging to the carbon chain anions, CN−/C3N− and/or C2H−/C4H−, in agreement with chemical model predictions. At higher ionospheric altitudes, detections at 73–74 u q−1 could be attributed to the further carbon chain anions C5N−/C6H− but at lower altitudes and during further encounters extend over a higher mass/charge range. This, as well as further intermediary anions detected at >100 u, provide the first evidence for efficient anion chemistry in space involving structures other than linear chains. Furthermore, at altitudes below <1100 km, the low-mass anions (<150 u q−1) were found to deplete at a rate proportional to the growth of the larger molecules, a correlation that indicates the anions are tightly coupled to the growth process. This study adds Titan to an increasing list of astrophysical environments where chain anions have been observed and shows that anion chemistry plays a role in the formation of complex organics within a planetary atmosphere as well as in the interstellar medium.

Cluster-PEACE In-flight Calibration Status

We briefly summarise key aspects of our on-going in-flight calibration work for the Cluster Plasma Electron And Current Experiment (PEACE) instruments, and demonstrate the quality of moments which may be achieved, by comparisons with measurements from other Cluster instruments. As improved calibrations are generated, data in scientific units which have been produced for the Cluster Active Archive will be systematically updated. This article is not intended as a detailed description of our calibration studies, but rather as a snapshot of the calibration status at the time of writing, which will give the researcher using the CAA an indication of the levels of accuracy that can be achieved at this time.