The C1XS X-ray Spectrometer on Chandrayaan-1

The Chandrayaan-1 X-ray Spectrometer (C1XS) is a compact X-ray spectrometer for the Indian Space Research Organisation (ISRO) Chandrayaan-1 lunar mission. It exploits heritage from the D-CIXS instrument on ESA's SMART-1 mission. As a result of detailed developments to all aspects of the design, its performance as measured in the laboratory greatly surpasses that of D-CIXS. In comparison with SMART-1, Chandrayaan-1 is a science-oriented rather than a technology mission, leading to far more favourable conditions for science measurements. C1XS is designed to measure absolute and relative abundances of major rock-forming elements (principally Mg, Al, Si, Ca and Fe) in the lunar crust with spatial resolution ?25 FWHM km, and to achieve relative elemental abundances of better than 10%.     

Diamonds from placers in western and central Africa: A problem of primary sources

This paper discusses diamonds and their accessory minerals from the Koidu kimberlites, placers on the Bafit River in Sierra Leone, and Chikapa placers in the Democratic Republic of Congo. Minerals from the placers are characterized by fine preservation and bear no evidence of long-term transportation. In placers, garnet, ilmenite and zircon have been found, but the specific features of their chemical composition do not coincide with those of minerals from kimberlites. Diamonds from the placers are of a mantle origin, but they have no direct analogs in the known kimberlite and lamproite pipes.     

Confocal laser scanning microscopy and Raman imagery of ancient microscopic fossils

Among all problems confronting the study of ancient permineralized (petrified) microscopic fossils, two stand out, the need for (1) accurate documentation of their three-dimensional morphology, and (2) direct analysis of their chemical composition and that of their surrounding mineral matrices. To address these problems we demonstrate the use of two techniques that we have recently introduced to Precambrian paleobiology: confocal laser scanning microscopy and Raman imagery. These techniques, both of which are non-intrusive and non-destructive, can provide data by which to characterize, in situ and at micron-scale resolution, the cellular and organismal morphology of thin section-embedded organic-walled fossils. In addition, Raman imagery provides direct analyses of the molecular–structural composition of the kerogenous components of such fossils and of their surrounding matrices, and a means to assess quantitatively the geochemical maturity of the preserved organics. Use of these techniques for studies of ancient microscopic fossils can provide information in three dimensions at high spatial resolution about their morphology and cellular anatomy, taphonomy and fidelity of preservation, composition and mode of preservation, and their biogenicity and syngenetic origin with the rocks in which they occur.     

X-ray Raman scattering for structural investigation of silica/silicate minerals

We have performed X-ray Raman scattering (XRS) measurements on the oxygen K and silicon L absorption edges of four silica minerals: α-quartz, α-cristobalite, coesite, and stishovite. We have also calculated the partial electron densities of states (DOSs) and compared these with the XRS spectra. This study demonstrates that the short-range structure around the atom of interest strongly influences the XRS spectral features. Importantly, the oxygen K-edge XRS spectra are found to reflect the p-orbital DOS while the silicon L-edge spectra reflect the s- and d-orbital DOSs, even when a product of a momentum transfer and a mean radius of a electron orbit (1s for oxygen and 2p for silicon), Qr, is close to or larger than unity. Building on this, calculations of the partial DOSs for other silica phases are presented, including ultra-high-pressure phases, which provide a good reference for further XRS study of silica and silicate minerals. XRS measurements should be performed on not only either of oxygen or silicon but also on many kinds of constituent elements to reveal the structural change of glasses/melts of silicates under extreme conditions.     

Breakdown of hydrous ringwoodite to pyroxene and spinelloid at high P and T and oxidizing conditions

To get deeper insight into the phase relations in the end-member system Fe₂SiO₄ and in the system (Fe, Mg)₂SiO₄ experiments were performed in a multi-anvil apparatus at 7 and 13 GPa and 1,000–1,200°C as a function of oxygen fugacity. The oxygen fugacity was varied using the solid oxygen buffer systems Fe/FeO, quartz–fayalite–magnetite, MtW and Ni/NiO. The run products were characterized by electron microprobe, Raman- and FTIR-spectroscopy, X-ray powder diffraction and transmission electron microscopy. At fO₂ corresponding to Ni/NiO Fe-ringwoodite transforms to ferrosilite and spinelloid according to the reaction: 9 Fe₂SiO₄ + O₂ = 6 FeSiO₃ + 5 Fe_2.40Si_0.60O₄. Refinement of site occupancies in combination with stoichiometric Fe³⁺ calculations show that 32% of the total Fe is incorporated as Fe³⁺ according to From the Rietveld refinement we identified spl as spinelloid III (isostructural with wadsleyite) and/or spinelloid V. As we used water in excess in the experiments the run products were also analyzed for structural water incorporation. Adding Mg to the system increases the stability field of ringwoodite to higher oxygen fugacity and the spinel structure seems to accept higher Fe³⁺ but also water concentrations that may be linked. At oxygen fugacity corresponding to MtW conditions similar phase relations in respect to the breakdown reaction in the Fe-end-member system were observed but with a strong fractionation of Fe into spl and Mg into coexisting cpx. Thus, through this strong fractionation it is possible to stabilize very Fe-rich wadsleyite with considerable Fe³⁺ concentrations even at an intermediate Fe–Mg bulk composition: assuming constant K _D independent on composition and a bulk composition of x _Fe = 0.44 this fractionation would stabilize spl with x _Fe = 0.72. Thus, spl could be a potential Fe³⁺ bearing phase at P–T conditions of the transition zone but because of the oxidizing conditions and the Fe-rich bulk composition needed one would expect it more in subduction zone environments than in the transition zone in senso stricto.

Ultraviolet observations of Phoebe from the Cassini UVIS

Observations of Saturn's distant moon Phoebe were made at far-ultraviolet (FUV) (1100-1900 Å) and extreme-ultraviolet (EUV) (600-1100 Å) wavelengths by the Cassini Ultraviolet Imaging Spectrograph (UVIS) during the Cassini spacecraft flyby on June 11, 2004. These are the first UV spectra of Phoebe and the first detection of water ice on a Solar System surface using FUV wavelengths. The characteristics of water ice in the FUV are presented, and Hapke models are used to interpret the spectra in terms of composition and grain size; the use of both areal and intimate mixing models is explored. Non-ice species used in these models include carbon, ice tholin, Triton tholin, poly-HCN and kerogen. Satisfactory disk-integrated fits are obtained for intimate mixtures of ∼10% H₂O plus a non-ice species. Spatially resolved regions of higher (∼20%) and lower (∼5%) H₂O ice concentrations are also detected. Phoebe does not display any evidence of volatile activity. Upper limits on atomic oxygen and carbon are 5×¹⁰¹¹ and 2×¹⁰¹² atoms/cm², respectively, for solar photon scattering. The UVIS detection of water ice on Phoebe, and the ice amounts detected, are consistent with IR measurements and contribute to the evidence for a Phoebe origin in the outer Solar System rather than in the main asteroid belt.     

The albedo dichotomy of Iapetus measured at UV wavelengths

The dramatic hemispheric dichotomy in albedo displayed by Saturn's moon Iapetus has intrigued astronomers for centuries. Here we report on far-ultraviolet observations of Iapetus' bright and dark terrains from Cassini. We compare the reflectance spectra of Iapetus's dark terrain, Hyperion and Phoebe and find that both Phoebe and Hyperion are richer in water ice than Iapetus' dark terrain. Spectra of the lowest latitudes of the dark terrain display the diagnostic water ice absorption feature; water ice amounts increase within the dark material away from the apex (at 90° W longitude, the center of the dark leading hemisphere), consistent with thermal segregation of water ice. The water ice in the darkest, warmest low latitude regions is not expected to be stable and may be a sign of ongoing or recent emplacement of the dark material from an exogenic source.     

Carbon dioxide segregation in 1:4 and 1:9 CO2:H2O ices

The mid-infrared spectra of mixed vapor deposited ices of CO₂ and H₂O were studied as a function of both deposition temperature and warming from 15 to 100 K. The spectra of ices deposited at 15 K show marked changes on warming beginning at 60 K. These changes are consistent with CO₂ segregating within the ice matrix into pure CO₂ domains. Ices deposited at 60 and 70 K show a greater degree of segregation, as high as 90% for 1:4 CO₂:H₂O ice mixtures deposited at 70 K. As the ice is warmed above 80 K, preferential sublimation of the segregated CO₂ is observed. The kinetics of the segregation process is also examined. The segregation of the CO₂ as the ice is warmed corresponds to temperatures at which the structure of the water ice matrix changes from the high density amorphous phase to the low density amorphous phase. We show how these microstructural changes in the ice have a profound effect on the photochemistry induced by ultraviolet irradiation. These experimental results provide a framework in which observations of CO₂ on the icy bodies of the outer Solar System can be considered.     

Venus upper atmospheric CO, temperature, and winds across the afternoon/evening terminator from June 2007 JCMT sub-millimeter line observations

The Venus mesosphere constitutes a highly variable transition region between the zonal rotation of the lower atmosphere and the diurnal circulation of the upper atmosphere. It further serves as the primary photochemical region of the Venus atmosphere. We obtained James Clerk Maxwell Telescope (JCMT, Mauna Kea Hawaii) sub-millimeter line observations of mesospheric ¹²CO and ¹³CO during coordinated space (MESSENGER and Venus Express) and ground-based observations of Venus in June of 2007. Such CO spectra line measurements support temperature, CO mixing ratio, and wind retrievals over the 80-110 km altitude range, encompassing the upper mesosphere and lower thermosphere of Venus. Five-point beam integrations were obtained across the observed Venus disk, allowing distinction of afternoon (noon to 6 p.m.) versus evening (6 p.m. to midnight) local times and northern (0-60N) versus southern (0-60S) latitudes. Distinctive diurnal variations (noon to midnight) are retrieved for both temperatures above 95 km and CO mixing ratios above 85 km altitudes. Separate CO line maps obtained on (UT) June 2, 3, 6, and 11 indicate moderate daily variability in afternoon and evening CO mixing ratios (20-50%) and temperatures (5-10 K). Average Venus mesospheric temperatures over this period were 10 K warmer than returned from 1978 to 1979 Pioneer Venus or 2000-01 sub-millimeter measurements, without evidence for the very large temperature inversions indicated by Venus Express SPICAV measurements at 90-100 km altitudes (Bertaux, J.L., Vandaele, A.-C., Korablev, O., Villard, E., Fedorova, A., Fussen, D., Quémerais, E., Belyaev, D., Mahieux, A., Montmessin, F., Muller, C., Neefs, E., Nevejans, D., Wilquet, V., Dubois, J.P. Hauchecorne, A., Stepanov, A., Vinogradov, I., Rodin, A., Bertaux, J.-L., Nevejans, D., Korablev, O., Montmessin, F., Vandaele, A.-C., Fedorova, A., Cabane, M., Chassefière, E., Chaufray, J.Y., Dimarellis, E., Dubois, J.P., Hauchecorne, A., Leblanc, F., Lefèvre, F., Rannou, P., Quémerais, E., Villard, E., Fussen, D., Muller, C., Neefs, E., Van Ransbeeck, E., Wilquet, V., Rodin, A., Stepanov, A., Vinogradov, I., Zasova, L., Forget, F., Lebonnois, S., Titov, D., Rafkin, S., Durry, G., Gérard, J.C., Sandel, B., 2007. A warm layer in Venus’ cryosphere and high-altitude measurements of HF, HCl, H₂O and HDO. Nature 450, 646-649). Measured Doppler shifts associated with June 2 and 11 ¹²CO line center absorptions indicate nearly supersonic (200 m/s, Mach 1) afternoon-to-evening (retrograde) circulation; composed of additive subsolar-to-antisolar (SSAS) and zonal retrograde wind components, which are not separable due to the particular observational geometry.