Lithologic Mapping of HED Terrains on Vesta using Dawn Framing Camera Color Data

The surface composition of Vesta, the most massive intact basaltic object in the asteroid belt, is interesting because it provides us with an insight into magmatic differentiation of planetesimals that eventually coalesced to form the terrestrial planets. The distribution of lithologic and compositional units on the surface of Vesta provides important constraints on its petrologic evolution, impact history and its relationship with Vestoids and howardite-eucrite-diogenite (HED) meteorites. Using color parameters (band tilt and band curvature) originally developed for analyzing lunar data, we have identified and mapped HED terrains on Vesta in Dawn Framing Camera (FC) color data. The average color spectrum of Vesta is identical to that of howardite regions, suggesting an extensive mixing of surface regolith due to impact gardening over the course of solar system history. Our results confirm the hemispherical dichotomy (east-west and north-south) in albedo/color/composition that has been observed by earlier studies. The presence of diogenite-rich material in the southern hemisphere suggests that it was excavated during the formation of the Rheasilvia and Veneneia basins. Our lithologic mapping of HED regions provides direct evidence for magmatic evolution of Vesta with diogenite units in Rheasilvia forming the lower crust of a differentiated object.Comment: Accepted for Meteoritics and Planetary Science special issue for Composition of Vesta/Dawn Missio

Olivine-rich exposures at Bellicia and Arruntia craters on (4) Vesta from Dawn FC

We present an analysis of the olivine-rich exposures at Bellicia and Arruntia craters using Dawn Framing Camera (FC) color data. Our results confirm the existence of olivine-rich materials at these localities as described by Ammannito et al. (2013a) using Visual Infrared Spectrometer (VIR) data. Analyzing laboratory spectra of various Howardite-Eucrite-Diogenite meteorites, high-Ca pyroxenes, olivines and olivine-orthopyroxene mixtures, we derive three FC spectral band parameters that are indicators of olivine-rich materials. Combining the three band parameters allows us, for the first time, to reliably identify sites showing modal olivine contents >40%. The olivine-rich exposures at Bellicia and Arruntia are mapped using higher spatial resolution FC data. The exposures are located on the slopes of outer/inner crater walls, on the floor of Arruntia, in the ejecta, as well as in nearby fresh small impact craters. The spatial extent of the exposures ranges from a few hundred meters to few kilometers. The olivine-rich exposures are in accordance with both the magma ocean and the serial magmatism model (e.g., Righter and Drake 1997; Yamaguchi et al. 1997). However, it remains unsolved why the olivine-rich materials are mainly concentrated in the northern hemisphere (~36-42{\deg} N, 46-74{\deg} E) and are almost absent in the Rheasilvia basin.Comment: Accepted for publication in Meteoritics and Planetary Scienc

Mineralogie und Geologie des Asteroiden (4) Vesta von Dawn Framing Camera

Der Asteroid (4) Vesta (~ 520 km Durchmesser) wird als analog zu einem kleinen terrestrischen Planeten angesehen. Seine magmatischen Prozesse kamen höchstwahrscheinlich in den ersten 10 bis 100 Ma nach seiner Akkretion zum Erliegen. Die Untersuchung eines solch einzigartigen Objektes ist essentiell um die frühe geologische Evolution eines Planeten zu verstehen. In dieser Arbeit werden Analysen der Oberflächenzusammensetzung von Vesta mittels hochauflösender Bilder der Dawn Framing Camera präsentiert. Die Framing Camera (FC) besitzt sieben Farbfilter im Wellenlängenbereich von 0,4 bis 1,0 μm und einen ‚clear filter‘. Die Kartierung der lithologischen Variationen auf Vesta’s Oberfläche wurde mittels FC Farbfilter Daten (~ 480 m/Pixel) vorgenommen, welche in der frühen Phase der Mission aufgenommen wurden. Basierend auf diesen Beobachtungen kommt Thangjam et al. (2013) zu dem Schluss, dass der Großteil der Oberfläche eine howarditische Zusammensetzung aufweist. Es wurden erstmals olivinreiche Regionen im Arruntia und Bellicia Krater mittels FC Farbfilter Daten mit einer Auflösung von ~ 60 m/Pixel identifiziert (Thangjam et al., 2014). Die Entdeckung von Olivin von Ammannito et al. (2013b) mittels Datensätzel des VIR Spektrometers im Bereich von 0,4-2,5 μm wird hier bestätigt. Die verwendeten Bandparameter der FC Farbfilter Daten sind‚ ‚Band-Tilt‘ (BT=R0.92 μm/R0.96μm) und ‚Mid Ratio‘ (MR=(R0.75μm/R0.83μm)/(R0.83μm/R0.92μm)), wobei Rλ die Reflektanz in dem entsprechenden Wellenlängenbereich darstellt. Hier wird zum ersten Mal eine dreidimensionale spektrale Herangehensweise vorgestellt, um unterschiedliche Zusammensetzungen zu analysieren und zu kartieren (Thangjam et al., im Press). Die Arruntia-Region wurde als Fallstudie ausgewählt und anhand der Bandparameter BT, MR und Reflektanz bei 0,55 μm (R0,55μm) analysiert. Die Untersuchungen zeigen, dass diese Region überwiegend von Eukrit-dominierten Howarditen bedeckt ist. Olivinreiche Aufschlüsse wurden im Vergleich zu früheren Arbeiten in relativ großem Umfang entdeckt. Um die petrologische Evolution von Vesta nachvollziehen zu können, wurde in dieser Studie eine chondritische Zusammensetzung von 80 % H Chondriten plus 20 % CV Chondriten gewählt. Das Sauerstoffisotopenverhältnis dieser Mischung entspricht jenem der HED Meteorite. Um Aussagen über Vestas interne Geometrie zu machen, wurden thermodynamische Berechnungen mittels MELTS verwendet. Dies ergab einen harzburgitischen Mantel mit einer durchschnittlichen Mächtigkeit von ~137 km und eine Kruste von ~15 km. Wenn ein Absenken der früh gebildeten Olivinphase angenommen wird, sind die Mächtigkeiten eines dunitschen Mantels, gefolgt von einem orhtopyroxenischen Mantel und einer Kruste ~46, ~84 und ~22 km. Diese Resultate können erklären, warum Dawn nicht in der Lage war, signifikante Olivin-Mengen im Rheasilvia Becken zu findeAsteroid (4) Vesta (~ 520 Km diameter) is thought to serve as an analog for a small terrestrial planet. Its magmatic processes probably ceased in the first few 10’s to 100’s of Ma after accretion. The study of such an object is a key in understanding the early geologic evolution of planetary bodies. This thesis presents surface compositional analysis using high spatial resolution images obtained by the Dawn Framing Camera. The Framing Camera (FC) houses seven color filters in the wavelength range between 0.4 and 1.0 μm along with a clear filter. Lithologic variations on Vesta's surface are mapped using FC color data (~480 m/pixel) acquired during the early phase of the mission. Based on these observations, Thangjam et al. (2013) suggested that the majority of the surface is howarditic in composition. Olivine-rich sites are identified for the first time using FC color data at a resolution of ~60m/pixel at Arruntia and Bellicia craters (Thangjam et al., 2014). The discovery of olivine by Ammannito et al. (2013b) by the VIR spectrometer data cubes in the range 0.4-2.5 μm is confirmed. The band parameters employed in FC color data are Band Tilt (BT=R0.92 μm/R0.96μm) and Mid Ratio (MR=(R0.75μm/R0.83μm)/(R0.83μm/R0.92μm)), where Rλ is the reflectance at the given wavelength. A three-dimensional spectral approach is introduced for the first time for analyzing and mapping compositional heterogeneities (Thangjam et al., in press). The Arruntia region is selected for a case study and is analyzed by combining the band parameters BT, MR and reflectance at 0.55 μm (R0.55μm). This study reveals that this region is mostly covered by eucrite-dominated howardite. Olivine-rich exposures are found in rather wide extents when compared to earlier works. In an attempt to understand the petrological evolution of Vesta, a chondritic composition of 80% H chondrite plus 20% CV chondrite is presented as a case study. This mixture meets observed HED oxygen isotope relations. The results of thermodynamic calculation by MELTS are used to predict Vesta’s internal geometry. The thicknesses of a harzburgitic mantle and crust are ~137 and ~15 km, respectively. If segregation of early-crystallized olivine phase is considered, the thicknesses of dunitic mantle followed upward by orthopyroxenitic mantle and crust are ~46, ~84, and ~22 km, respectively. These results may explain why Dawn failed to detect significant amounts of olivine in the Rheasilvia basin and emphasize that olivine on Vesta’s surface is likely exogenic in origin

Effects of viewing geometry, aggregation state, and particle size on reflectance spectra of the Murchison CM2 chondrite deconvolved to Dawn FC band passes

Several current and soon-to-launch missions will investigate ‘dark’ asteroids, whose spectra have few weak or no distinct spectral features. Some carbonaceous chondrites, particularly the CI and CM groups, are reasonable material analogues for many dark asteroid surfaces. In addition to compositional variations, many non-compositional effects, including viewing geometry, surface particle size and particle sorting, can influence reflectance spectra, potentially complicating mineralogical interpretation of such data from remote surfaces. We have carried out an investigation of the effects of phase angle, particle size, aggregation state, and intra-sample heterogeneity on the reflectance spectra (0.4–1.0 μm) of the Murchison CM2 carbonaceous chondrite, deconvolved to Dawn Framing Camera (FC) band passes. This study was motivated by the desire to derive information about the surface of Ceres from Dawn FC data. Key spectral parameters derived from the FC multispectral data include various two-band reflectance ratios as well as three-band ratios that have been derived for mineralogical analysis. Phase angle effects include increased visible slope with increasing phase angle, a trend that may reverse at very high phase angles. Fine-grained particles exert a strong influence on spectral properties relative to their volumetric proportion. Grain size variation effects include a decrease in spectral contrast and increased visible spectral slope with decreasing grain size. Intra-sample heterogeneity, while spectrally detectable, is of relatively limited magnitude

Science Drivers for the Future Exploration of Ceres: From Solar System Evolution to Ocean World Science

International audienceDawn revealed that Ceres is a compelling target whose exploration pertains to many science themes. Ceres is a large ice- and organic-rich body, potentially representative of the population of objects that brought water and organics to the inner solar system, as well as a brine-rich body whose study can contribute to ocean world science. The Dawn observations have led to a renewed focus on planetary brine physics and chemistry based on the detection of many landforms built from brines or suspected to be emplaced via brine effusion. Ceres’ relative proximity to Earth and direct access to its surface of evaporites that evolved from a deep brine reservoir make this dwarf planet an appealing target for follow-up exploration. Future exploration, as described here, would address science questions pertinent to the evolution of ocean worlds and the origin of volatiles and organics in the inner solar system