New visible spectra and mineralogical assessment of (21) Lutetia, a target of the Rosetta mission

The Rosetta spacecraft, launched on March 2nd 2004, in the course of its journey to the comet 67P/Churyumov-Gerasimenko (encounter foreseen in 2014), will fly past two asteroids: (2867) Steins and (21) Lutetia. On September 5th 2008 (2867) Steins was encountered. In this paper, we present two visible spectra of (21) Lutetia of different spectral resolutions covering the spectral ranges where possible absorption bands were previously revealed by Lazzarin and collaborators. We confirm detection of a broad complex feature between 0.45 and 0.55 μm and two narrower features around 0.47 and 0.52 μm. We discuss possible assignments of these bands and suggest that they might originate from electronic transitions in pyroxenes, although unambiguous identification is difficult and the published thermal infrared (TIR) spectrum of (21) Lutetia suggests that pyroxene cannot be the dominant silicate component at its surface. Furthermore, we discuss the published spectra of (21) Lutetia in the range from near-UV to thermal infrared. We conclude that carbonaceous meteorites (chondrites and achondrites) appear to be the closest meteorite analogues of (21) Lutetia, based on the observed spectral features. Among these meteorites, metal-rich carbonaceous chondrites seem to be the most plausible analogue materials

The composition of the atmosphere of Venus below 100 km altitude: An overview

International audienceWe review the progress in our understanding of the composition of the Venus atmosphere since the publication of the COSPAR Venus International Reference Atmosphere volume in 1985. Results presented there were derived from data compiled in 1982-1983. More recent progress has resulted in large part from Earth-based studies of the near-infrared radiation from the nightside of the planet. These observations allow us to probe the atmosphere between the cloud tops and the surface. Additional insight has been gained through: (i) the analysis of ultraviolet radiation by satellites and rockets; (ii) data collected by the Vega 1 and 2 landers; (iii) complementary analyses of Venera 15 and 16 data; (iv) ground-based and Magellan radio occultation measurements, and (v) re-analyses of some spacecraft measurements made before 1983, in particular the Pioneer Venus and Venera 11, 13 and 14 data. These new data, and re-interpretations of older data, provide a much better knowledge of the vertical profile of water vapor, and more information on sulfur species above and below the clouds, including firm detections of OCS and SO. In addition, some spatial and/or temporal variations have been observed for CO, H 2O, H 2SO 4, SO 2, and OCS. New values of the D/H ratio have also been obtained

Space weathering in the main asteroid belt: The big picture

The optical properties of silicate-rich asteroidal surfaces (namely, S-complex asteroids) evolve over time under the influence of several processes, known as "space weathering"(SW), and are amply analyzed in laboratory experiments. A first estimate of the spectral reddening rate, due to SW, on family S-type main-belt asteroids (MBAs; Jedicke et al.) has been confirmed and generalized, using a different approach, which also provided us with clues about the relative relevance of the various SW processes (Marchi et al.). However, in the main belt, the S complex accounts for about 40% of the listed bodies, and even less in terms of mass. Most of the remaining main-belt population is composed of spectrally featureless asteroids (i.e., with nonsilicate-dominated spectra) and can be divided into two main groups: the C complex and the X complex; the three complexes comprise up to more than 90% of the MBAs. The question as to whether SW has general observable consequences for the whole main belt is still open. In this work we show, on the basis of a wide statistical analysis, that the spectral trends due to SW are similar for the three major complexes. We have also been able to identify some underlying relevant physical processes. Our observational findings, and the related clues, are also supported by new experimental results, which are also summarized here. The resulting scenario has several important implications for the formation of evolutionary models of the main asteroid belt as well as for the existing taxonomical classification

Ion irradiation of asphaltite: Optical effects and implications for Trans-Neptunian objects and Centaurs.

Trans-Neptunian Objects (TNOs) and Centaurs show remarkable colour variations in the visual and near-infrared spectral regions. Surface alteration processes such as space weathering (e.g., bombardment with ions) and impact resurfacing may play an important role in the colour diversity of such bodies. Ion irradiation of hydrocarbon ices and their mixtures with water ice transforms neutral (grey) surface colours of ices to red and further to grey. Along with the ices, TNOs and Centaurs probably contain complex carbonaceous compounds, in particular, complex hydrocarbons. Unlike ices, such refractory organic materials have originally low visual albedos and red colours in the visible and near-infrared ranges. Here we present the first results of ion irradiation experiments on asphaltite. Asphaltite is a natural complex hydrocarbon material. The reflectance spectra of asphaltite in the 0.4-0.8 microns range have been recorded before irradiation and after each irradiation step. We demonstrate that irradiation of this red dark material with 30 keV H+ and 15 keV N+ ions gradually transforms its colour from red to grey as a result of carbonization. A moderate increase in the visual albedo has been observed. These results may imply that the surfaces of primitive red objects optically dominated by complex refractory organics may show a similar space weathering trend. Our laboratory results were compared with published colours of TNOs and Centaurs. A broad variety of spectral colours observed for TNOs and Centaurs may be reproduced by various spectra of irradiated organics corresponding to different ion fluences. However, such objects probably also contain ices and silicate components which show different space weathering trends. This fact, together with a lack of information about albedos, may explain difficulties to reveal correlations between surface colours within TNO and Centaur populations and their other properties, such as absolute magnitudes and orbital parameters