Solar system

Abstract:

© Copyright 2017 by the International Astronautical Federation (IAF). All rights reserved. Measurements in the atmosphere and at the surface of Venus are required to understand fundamental processes of how terrestrial planets evolve and how they work today. While the European Venus community is unified in its support of the EnVision orbiter proposal as the next step in European Venus exploration, many scientific questions also require in situ Venus exploration. We suggest a long-duration lander at Venus, which would be capable of undertaking a seismometry mission, operating in the 460°C surface conditions of Venus. Radar maps have shown Venus to be covered with volcanic and tectonic features, and mounting evidence, including observations from Venus Express, suggests that some of these volcanoes are active today. Assessing Venus' current seismicity, and measuring its interior structure, is essential if we are to establish the geological history of our twin planet, for example to establish whether it ever had a habitable phase with liquid water oceans. Although some constraints on seismic activity can be obtained from orbit, using radar or ionospheric observation, the most productive way to study planetary interiors is through seismometry. Seismometry requires a mission duration of months or (preferably) years. Previous landers have used passive cooling, relying on thermal insulation and the lander's thermal inertia to provide a brief window of time in which to conduct science operations - but this allows mission durations of hours, not months. Proposals relying on silicon electronics require an electronics enclosure cooled to < 200 °C; the insulation, cooling and power system requirements escalate rapidly to require a > 1 ton, > €1bn class mission, such as those studied in the context of NASA flagship missions. However, there are alternatives to silicon electronics: in particular, there have been promising advances in silicon carbide (SiC) electronics capable of operating at temperatures of 500°C. Within the coming decade it will be possible to assemble at least simple circuits using SiC components, sufficient to run a seismometry lander. We are proposing a Venus Long-Lived Surface Package (VL2SP) consisting of power source (RTG), science payload (seismometer and meteorology sensors), and ambient temperature electronics including a telecommunications system weighing < 100 kg. We do not specify how this VL2SP gets to the surface of Venus, but we estimate that an orbiter providing data relay would be essential. This presentation is based on a response sumitted to ESA's Call for New Scientific Ideas in September 2016.

Abstract:

We describe Castalia, a proposed mission to rendezvous with a Main Belt Comet (MBC), 133P/Elst-Pizarro. MBCs are a recently discovered population of apparently icy bodies within the main asteroid belt between Mars and Jupiter, which may represent the remnants of the population which supplied the early Earth with water. Castalia will perform the first exploration of this population by characterising 133P in detail, solving the puzzle of the MBC’s activity, and making the first in situ measurements of water in the asteroid belt. In many ways a successor to ESA’s highly successful Rosetta mission, Castalia will allow direct comparison between very different classes of comet, including measuring critical isotope ratios, plasma and dust properties. It will also feature the first radar system to visit a minor body, mapping the ice in the interior. Castalia was proposed, in slightly different versions, to the ESA M4 and M5 calls within the Cosmic Vision programme. We describe the science motivation for the mission, the measurements required to achieve the scientific goals, and the proposed instrument payload and spacecraft to achieve these.

Abstract:

During three low-altitude (99, 66, 66 km) flybys through the Enceladus plume in 2010 and 2011, Cassini's ion neutral mass spectrometer (INMS) made its first high spatial resolution measurements of the plume's gas density and distribution, detecting in situ the individual gas jets within the broad plume. Since those flybys, more detailed Imaging Science Subsystem (ISS) imaging observations of the plume's icy component have been reported, which constrain the locations and orientations of the numerous gas/grain jets. In the present study, we used these ISS imaging results, together with ultraviolet imaging spectrograph stellar and solar occultation measurements and modeling of the three-dimensional structure of the vapor cloud, to constrain the magnitudes, velocities, and time variability of the plume gas sources from the INMS data. Our results confirm a mixture of both low and high Mach gas emission from Enceladus' surface tiger stripes, with gas accelerated as fast as Mach 10 before escaping the surface. The vapor source fluxes and jet intensities/densities vary dramatically and stochastically, up to a factor 10, both spatially along the tiger stripes and over time between flyby observations. This complex spatial variability and dynamics may result from time-variable tidal stress fields interacting with subsurface fissure geometry and tortuosity beyond detectability, including changing gas pathways to the surface, and fluid flow and boiling in response evolving lithostatic stress conditions. The total plume gas source has 30% uncertainty depending on the contributions assumed for adiabatic and nonadiabatic gas expansion/acceleration to the high Mach emission. The overall vapor plume source rate exhibits stochastic time variability up to a factor ∼5 between observations, reflecting that found in the individual gas sources/jets.