Planetary atmosphere observation analysis

Abstract:

The exploration of the asteroid main belt is of the utmost importance to address many of the fundamental questions in modern planetary science (e.g., Solar System formation and evolution theories). This paper investigates potential opportunities for medium-class asteroid belt survey missions in the timeframe of 2029+. The launch as assumed here corresponds to that of the forthcoming ESA call for medium class mission proposals. The study has been developed in support of the CASTAway Asteroid Spectroscopic Survey mission proposal, which is to be submitted to the aforementioned call. CASTAway envisages the launch of a small telescope with relatively straightforward (i.e. high TRL) remote sensing instrumentation to detect asteroids at a range of 0.1-0.5 AU. The spacecraft would then head towards the main belt with the following objectives: The mission should test Solar System evolution theories by; 1) performing a statistical survey of small asteroid belt objects previously unsampled (<1 to a few tens of metre-sized); 2) providing compositional information for 1000s of objects by obtaining spectral data over a wide range of wavelengths, including key regions not observable through the Earth's atmosphere; 3) studying the morphology and geological history from close flybys of a targeted sub-set of objects, at least doubling the number of currently visited main belt asteroids within one single mission (>9 fly-bys). This paper presents a challenging multi-objective optimization problem and discusses the feasibility of such a mission concept. Firstly, a set of competing performance indices are defined that consider the cost of the mission, the quality of the survey (i.e. number of new detections and spectral data) and the number of asteroid fly-bys. The fly-by combinatorial problem is then tackled using the Minimum Intersection Orbital Distance as an heuristic filter to prune out unfeasible targets. Genetic and evolutionary algorithms are used to globaly optimize impulsive transfers, considering also planetary fly-bys, deep space and Δv-leveraging manoeuvres. Low-thrust trajectories are considered, although long thrusting periods impact negatively in the available operational time for remote sensing operations. Shape-based methods are used to globally optimize the low thrust controls, while the GPOPS-II and IPOPT transcribe the continuous-time optimal control problem and solve the subsequent nonlinear programming problem, respectively.

Abstract:

The planned launch of the James Webb Space Telescope (JWST) in 2018 will herald a new era of exoplanet spectroscopy. JWST will be the first telescope sensitive enough to potentially characterize terrestrial planets from their transmission spectra. In this work, we explore the possibility that terrestrial planets with Venus-type and Earth-type atmospheres could be distinguished from each other using spectra obtained by JWST. If we find a terrestrial planet close to the liquid water habitable zone of an M5 star within a distance of 10 parsec, it would be possible to detect atmospheric ozone if present in large enough quantities, which would enable an oxygen-rich atmosphere to be identified. However, the cloudiness of a Venus-type atmosphere would inhibit our ability to draw firm conclusions about the atmospheric composition, making any result ambiguous. Observing small, temperate planets with JWST requires significant investment of resources, with single targets requiring of the order of 100 transits to achieve sufficient signal to noise. The possibility of detecting a crucial feature such as the ozone signature would need to be carefully weighed against the likelihood of clouds obscuring gas absorption in the spectrum.

Abstract:

Although water vapour is the main species observed in the coma of comet 67P/Churyumov-Gerasimenko and water is the major constituent of cometary nuclei, limited evidence for exposed water-ice regions on the surface of the nucleus has been found so far. The absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far. The nucleus of 67P/Churyumov-Gerasimenko appears to be fairly uniformly coated with dark, dehydrated, refractory and organic-rich material. Here we report the identification at infrared wavelengths of water ice on two debris falls in the Imhotep region of the nucleus. The ice has been exposed on the walls of elevated structures and at the base of the walls. A quantitative derivation of the abundance of ice in these regions indicates the presence of millimetre-sized pure water-ice grains, considerably larger than in all previous observations. Although micrometre-sized water-ice grains are the usual result of vapour recondensation in ice-free layers, the occurrence of millimetre-sized grains of pure ice as observed in the Imhotep debris falls is best explained by grain growth by vapour diffusion in ice-rich layers, or by sintering. As a consequence of these processes, the nucleus can develop an extended and complex coating in which the outer dehydrated crust is superimposed on layers enriched in water ice. The stratigraphy observed on 67P/Churyumov-Gerasimenko is therefore the result of evolutionary processes affecting the uppermost metres of the nucleus and does not necessarily require a global layering to have occurred at the time of the comet's formation.