The discovery of volcanic activity on Enceladus stands out amongst the long list of findings by the Cassini mission to Saturn. In particular the compositional analysis of Enceladus ice particles by Cassini's Cosmic Dust Analyser (CDA) (Srama et al., 2004) has proven to be a powerful technique for obtaining information about processes below the moon's ice crust. Small amounts of sodium salts embedded in the particles' ice matrices provide direct evidence for a subsurface liquid water reservoir, which is, or has been, in contact with the moon's rocky core (Postberg et al., 2009, 2011b).
Jupiter's Galilean satellites Ganymede, Europa, and Callisto are also believed to have subsurface oceans and are therefore prime targets for future NASA and ESA outer Solar System missions. The Galilean moons are engulfed in tenuous dust clouds consisting of tiny pieces of the moons' surfaces (Kruger et al., 1999), released by hypervelocity impacts of micrometeoroids, which steadily bombard the surfaces of the moons. In situ chemical analysis of these grains by a high resolution dust spectrometer will provide spatially resolved mapping of the surface composition of Europa. Ganymede, and Callisto, meeting key scientific objectives of the planned missions. However, novel high-resolution reflectron-type dust mass spectrometers (Sternovsky et al., 2007; Srama et al., 2007) developed for dust astronomy missions (Gran et al., 2009) are probably not robust enough to be operated in the energetic radiation environment of the inner Jovian system. In contrast, CDA's linear spectrometer is much less affected by harsh radiation conditions because its ion detector is not directly facing out into space. The instrument has been continuously operated on Cassini for 11 years. In this paper we investigate the possibility of operating a CDA-like instrument as a high resolution impact mass spectrometer. We show that such an instrument is capable of reliably identifying traces of organic and inorganic materials in the ice matrix of ejecta expected to be generated from the surfaces of the Galilean moons. These measurements are complementary, and in some cases superior, compared to other traditional techniques such as infrared remote sensing or in situ ion or neutral mass spectrometers.
Classical methods to analyze the surface composition of atmosphereless planetary objects from an orbiter are IR and gamma ray spectroscopy and neutron backscatter measurements. The idea to analyze surface properties with an in-situ instrument has been proposed by Johnson et al. (1998). There, it was suggested to analyze Europa's thin atmosphere with an ion and neutral gas spectrometer. Since the atmospheric components are released by sputtering of the moon's surface, they provide a link to surface composition. Here we present an improved, complementary method to analyze rocky or icy dust particles as samples of planetary objects from which they were ejected. Such particles, generated by the ambient meteoroid bombardment that erodes the surface, are naturally present on all atmosphereless moons and planets. The planetary bodies are enshrouded in clouds of ballistic dust particles, which are characteristic samples of their surfaces. In situ mass spectroscopic analysis of these dust particles impacting onto a detector of an orbiting spacecraft reveals their composition. Recent instrumental developments and tests allow the chemical characterization of ice and dust particles encountered at speeds as low as 1 km/s and an accurate reconstruction of their trajectories. Depending on the sampling altitude, a dust trajectory sensor can trace back the origin of each analyzed grain with about 10 km accuracy at the surface. Since the detection rates are of the order of thousand per orbit, a spatially resolved mapping of the surface composition can be achieved. Certain bodies (e.g., Europa) with particularly dense dust clouds, could provide impact statistics that allow for compositional mapping even on single flybys. Dust impact velocities are in general sufficiently high at orbiters about planetary objects with a radius > 1000 km and with only a thin or no atmosphere. In this work we focus on the scientific benefit of a dust spectrometer on a spacecraft orbiting Earth's Moon as well as Jupiter's Galilean satellites. This 'dust spectrometer' approach provides key chemical and isotopic constraints for varying provinces or geological formations on the surfaces, leading to better understanding of the body's geological evolution.
My essay attends to a number of passages in Alexander von Humboldt’s Personal Narrative in which the Prussian explorer expresses anxiety about the apparent dangers posed by the overwhelmingly productive tropical landscapes he observes. In these passages, the excesses of an “exotic nature” threaten European identity and modes of civilization—and they trouble the accuracy of Humboldt’s own observational project. I also explore Humboldt’s related worry that South American vegetable (and visual) overload will exert a destabilizing effect on his aesthetic sensibility, disrupting his ability to represent the “New Continent” accurately in writing. Finally, I sketch the influence of Humboldt’s representations of tropical excess on nineteenth-century British cultural thought and literary practice. Studying the instabilities experienced by Personal Narrative’s expatriates and colonists promises to draw out important tensions latent in Humboldt’s treatment of tropical landscape and to illuminate broader epistemological and aesthetic shifts being worked out during the period.
Der Beitrag “Humboldteanisierung des Westens? Die Bedeutung der Reise Humboldts für Europa und Lateinamerika” behandelt der preußischen emprischen Universalgelehrten, in Übereinstimmung mit dem von Ottmar Ette entwickelten Konzept, als einen, wenn nicht den wichtigsten Begründer der westlichen Moderne. Im Gegensatz zu den bekannten Gründern der Moderne (Descartes, Kant, Buffon, Schiller, Hegel, Marx ... etc.) war Humboldt Empiriker und Globaldenker und er tat dies auf transkulturelle Weise, manchmal auch schon auf interkulturelle Weise; allerdings meist auf Basis der römisch-klassizistischen Denktradition und Ästhetik Europas. Der Beitrag arbeitet drei Wissenstypen Humboldts heraus und versucht zu zeigen, dass der Universalismus Humboldt wichtige interkulturelle Grundlagen in der Kommunikation mit amerikanischen Gelehrten und im amerikanischen Empirismus (“lokales Wissen”) hat. Die Bedeutung einer solchen Konzeption der Humboldt-Reise zeigt sich – in der Negierung – in den gegenwärtigen Debatten um Diasporas und “Race” im atlantischen Westen (“Black Atlantic”). Humboldt Kenntnisse der amerikanischen Sklavereien und seine absolute Verurteilung der Sklaverei ist dabei nicht präsent.
Im zweiten Teil zeigt der Beitrag mögliche Lesarten der Humboldt-Tagebücher, indem er “imaginäre Humboldt-Essays” konstruiert.