Stefan E. Schröder

Color and Albedo Heterogeneity of Vesta from Dawn

A New Dawn Since 17 July 2011, NASAs spacecraft Dawn has been orbiting the asteroid Vesta—the second most massive and the third largest asteroid in the solar system (see the cover). Russell et al. (p. 684) use Dawns observations to confirm that Vesta is a small differentiated planetary body with an inner core, and represents a surviving proto-planet from the earliest epoch of solar system formation; Vesta is also confirmed as the source of the howardite-eucrite-diogenite (HED) meteorites. Jaumann et al. (p. 687) report on the asteroids overall geometry and topography, based on global surface mapping. Vestas surface is dominated by numerous impact craters and large troughs around the equatorial region. Marchi et al. (p. 690) report on Vestas complex cratering history and constrain the age of some of its major regions based on crater counts. Schenk et al. (p. 694) describe two giant impact basins located at the asteroids south pole. Both basins are young and excavated enough amounts of material to form the Vestoids—a group of asteroids with a composition similar to that of Vesta—and HED meteorites. De Sanctis et al. (p. 697) present the mineralogical characterization of Vesta, based on data obtained by Dawns visual and infrared spectrometer, revealing that this asteroid underwent a complex magmatic evolution that led to a differentiated crust and mantle. The global color variations detailed by Reddy et al. (p. 700) are unlike those of any other asteroid observed so far and are also indicative of a preserved, differentiated proto-planet. Spacecraft data provide a detailed characterization of the second most massive asteroid in the solar system. Multispectral images (0.44 to 0.98 μm) of asteroid (4) Vesta obtained by the Dawn Framing Cameras reveal global color variations that uncover and help understand the north-south hemispherical dichotomy. The signature of deep lithologies excavated during the formation of the Rheasilvia basin on the south pole has been preserved on the surface. Color variations (band depth, spectral slope, and eucrite-diogenite abundance) clearly correlate with distinct compositional units. Vesta displays the greatest variation of geometric albedo (0.10 to 0.67) of any asteroid yet observed. Four distinct color units are recognized that chronicle processes—including impact excavation, mass wasting, and space weathering—that shaped the asteroid’s surface. Vesta’s color and photometric diversity are indicative of its status as a preserved, differentiated protoplanet.

Dark material on Vesta from the infall of carbonaceous volatile-rich material

Localized dark and bright materials, often with extremely different albedos, were recently found on Vesta’s surface. The range of albedos is among the largest observed on Solar System rocky bodies. These dark materials, often associated with craters, appear in ejecta and crater walls, and their pyroxene absorption strengths are correlated with material brightness. It was tentatively suggested that the dark material on Vesta could be either exogenic, from carbon-rich, low-velocity impactors, or endogenic, from freshly exposed mafic material or impact melt, created or exposed by impacts. Here we report Vesta spectra and images and use them to derive and interpret the properties of the ‘pure’ dark and bright materials. We argue that the dark material is mainly from infall of hydrated carbonaceous material (like that found in a major class of meteorites and some comet surfaces), whereas the bright material is the uncontaminated indigenous Vesta basaltic soil. Dark material from low-albedo impactors is diffused over time through the Vestan regolith by impact mixing, creating broader, diffuse darker regions and finally Vesta’s background surface material. This is consistent with howardite–eucrite–diogenite meteorites coming from Vesta.

E-type Asteroid (2867) Steins as Imaged by OSIRIS on Board Rosetta

Smooth Space Pebble In September 2008, on its way to meet comet 67P/Churyumov-Gerasimenko, the Rosetta spacecraft flew by asteroid Steins, a member of a very rare class of asteroids that had never been observed closely by spacecraft. Keller et al. (p. 190) analyzed the images to generate a reconstruction of the asteroids shape. Steins is oblate with an effective spherical diameter of 5.3 kilometers, and it lacks small craters, which may have been erased by surface reshaping. Indeed, Steinss shape resembles that of a body that was spun-up by the YORP effect—a torque produced by incident sunlight, which can alter the rotation rate of a small body—that causes material to slide toward the equator. This effect may have produced Steinss distinctive diamond-like shape. Incident sunlight probably caused this asteroid to spin, which redistributed its mass and smoothed its surface. The European Space Agency’s Rosetta mission encountered the main-belt asteroid (2867) Steins while on its way to rendezvous with comet 67P/Churyumov-Gerasimenko. Images taken with the OSIRIS (optical, spectroscopic, and infrared remote imaging system) cameras on board Rosetta show that Steins is an oblate body with an effective spherical diameter of 5.3 kilometers. Its surface does not show color variations. The morphology of Steins is dominated by linear faults and a large 2.1-kilometer-diameter crater near its south pole. Crater counts reveal a distinct lack of small craters. Steins is not solid rock but a rubble pile and has a conical appearance that is probably the result of reshaping due to Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) spin-up. The OSIRIS images constitute direct evidence for the YORP effect on a main-belt asteroid.

Insolation, erosion, and morphology of comet 67P/Churyumov-Gerasimenko

The complex shape of comet 67P and its oblique rotation axis cause pronounced seasonal effects. Irradiation and hence activity vary strongly. Aims. We investigate the insolation of the cometary surface in order to predict the sublimation of water ice. The strongly varying erosion levels are correlated with the topography and morphology of the present cometary surface and its evolution. Methods. The insolation as a function of heliocentric distance and diurnal (spin dependent) variation is calculated using >105 facets of a detailed digital terrain model. Shading, but also illumination and thermal radiation by facets in the field of view of a specific facet are iteratively taken into account. We use a two-layer model of a thin porous dust cover above an icy surface to calculate the water sublimation, presuming steady state and a uniform surface. Our second model, which includes the history of warming and cooling due to thermal inertia, is restricted to a much simpler shape model but allows us to test various distributions of active areas. Results. Sublimation from a dirty ice surface yields maximum erosion. A thin dust cover of 50 μm yields similar rates at perihelion. Only about 6% of the surface needs to be active to match the observed water production rates at perihelion. A dust layer of 1 mm thickness suppresses the activity by a factor of 4 to 5. Erosion on the south side can reach more than 10 m per orbit at active spots. The energy input to the concave neck area (Hapi) during northern summer is enhanced by about 50% owing to self-illumination. Here surface temperatures reach maximum values along the foot of the Hathor wall. Integrated over the whole orbit this area receives the least energy input. Based on the detailed shape model, the simulations identify “hot spots” in depressions and larger pits in good correlation with observed dust activity. Three-quarters of the total sublimation is produced while the sub-solar latitude is south, resulting in a distinct dichotomy in activity and morphology. Conclusions. The northern areas display a much rougher morphology than what is seen on Imhotep, an area at the equator that will be fully illuminated when 67P is closer to the Sun. Self-illumination in concave regions enhance the energy input and hence erosion. This explains the early activity observed at Hapi. Cliffs are more prone to erosion than horizontal, often dust covered, areas, which leads to surface planation. Local activity can only persist if the forming cliff walls are eroding. Comet 67P has two lobes and also two distinct sides. Transport of material from the south to the north is probable. The morphology of the Imhotep plain should be typical for the terrains of the yet unseen southern hemisphere.

Spectrophotometric properties of dwarf planet Ceres from the VIR spectrometer on board the Dawn mission

Aims. We present a study of the spectrophotometric properties of dwarf planet Ceres in the visual-to-infrared (VIS-IR) spectral range by means of hyper-spectral images acquired by the VIR imaging spectrometer on board the NASA Dawn mission. Methods. Disk-resolved observations with a phase angle within the 7 ◦ < α < 132 ◦ interval were used to characterize Ceres’ phase curve in the 0.465-4.05 µm spectral range. Hapke’s model was applied to perform the photometric correction of the dataset to standard observation geometry at VIS-IR wavelength, allowing us to produce albedo and color maps of the surface. The V-band magnitude phase function of Ceres as been computed from disk-resolved images and fitted with both the classical linear model and H-G formalism. Results. The single- scattering albedo and the asymmetry parameter at 0.55 µm are w = 0.14±0.02 and ξ = −0.11±0.08, respectively (two-lobe Henyey-Greenstein phase function); at the same wavelength, Ceres’ geometric albedo as derived from our modeling is 0.094±0.007; the roughness parameter is ¯ θ = 29 ◦ ±6 ◦ . Albedo maps indicate small variability on a global scale with an average reflectance at standard geometry of 0.034 ± 0.003. Nonetheless, isolated areas such as the Occator bright spots, Haulani, and Oxo show an albedo much higher than average. We measure a significant spectral phase reddening, and the average spectral slope of Ceres’ surface after photometric correction is 1.1%kA −1 and 0.85%kA −1 at VIS and IR wavelengths, respectively. Broadband color indices are V−R = 0.38±0.01 and R−I = 0.33±0.02. Color maps show that the brightest features typically exhibit smaller slopes. The H-G modeling of the V-band magnitude phase curve for α < 30 ◦ gives H = 3.14±0.04 and G = 0.10±0.04, while the classical linear model provides V(1,1,0 ◦ ) = 3.48±0.03 and β = 0.036±0.002. The comparison of our results with spectrophotometric properties of other minor bodies indicates that Ceres has a less back-scattering phase function and a slightly higher albedo than comets and C-type objects. However, the latter represents the closest match in the usual asteroid taxonomy.

SURFACE ALBEDO AND SPECTRAL VARIABILITY OF CERES

Previous observations suggested that Ceres has active, but possibly sporadic, water outgassing as well as possibly varying spectral characteristics over a timescale of months. We used all available data of Ceres collected in the past three decades from the ground and the Hubble Space Telescope, as well as the newly acquired images by the Dawn Framing Camera, to search for spectral and albedo variability on Ceres, on both a global scale and in local regions, particularly the bright spots inside the Occator crater, over timescales of a few months to decades. Our analysis has placed an upper limit on the possible temporal albedo variation on Ceres. Sporadic water vapor venting, or any possibly ongoing activity on Ceres, is not significant enough to change the albedo or the area of the bright features in the Occator crater by >15%, or the global albedo by >3% over the various timescales that we searched. Recently reported spectral slope variations can be explained by changing Sun–Ceres–Earth geometry. The active area on Ceres is less than 1 km2, too small to cause global albedo and spectral variations detectable in our data. Impact ejecta due to impacting projectiles of tens of meters in size like those known to cause observable changes to the surface albedo on Asteroid Scheila cannot cause detectable albedo change on Ceres due to its relatively large size and strong gravity. The water vapor activity on Ceres is independent of Ceres’ heliocentric distance, ruling out the possibility of the comet-like sublimation process as a possible mechanism driving the activity.

Close-up images of the final Philae landing site on comet 67P/Churyumov-Gerasimenko acquired by the ROLIS camera

After coming to rest on the night side of comet 67P/Churyumov-Gerasimenko, the ROLIS camera on-board Rosetta’s Philae lander acquired five images of the surface below the lander, four of which were with the aid of LED illumination of different colors. The images confirm that Philae was perched on a sloped surface. A local horizon is visible in one corner of the image, beyond which we can see the coma. Having spent a full day on the surface Philae was commanded to lift and rotate, after which a final, sixth, LED image was acquired. The change in perspective allowed us to construct a shape model of the surface. The distance to the foreground was about 80 cm, much larger than the nominal 30 cm. This caused stray light, rather than directly reflected LED light, to dominate the image signal, complicating the analysis. The images show a lumpy surface with a roughness of apparently fractal nature. Its appearance is completely different from that of the first landing site, which was characterized by centimeter to meter-sized debris (Mottola et al., 2015). We recognize neither particles nor pores at the image resolution of 0.8 mm per pixel and large color variations are absent. The surface has a bi-modal brightness distribution that can be interpreted in terms of the degree of consolidation, a hypothesis that we support with experimental evidence. We propose the surface below the lander to consist of smooth, cracked plates with unconsolidated edges, similar to terrain seen in CIVA images.

A conceptual proposal for an expert system to analyze smart policy options for urban CEP transports

Courier, express and parcel (CEP) transports have been growing significantly. CEP is one of the biggest segments in urban freight transport. It does not only contribute to the wealth of our cities, but also comes with negative impacts such as noise, congestion and pollution. Transport policy has developed measures to reduce these negative impacts. This paper proposes a conceptual framework to analyze smart policy options. It focuses on the derivation of CEP demand and supply.

Generierung der Nachfragestrukturen für die mikroskopische Simulation des städtischen Distributionsverkehrs im Lebensmittelhandel

Wirtschaftsverkehrs- und City-Logistik-Modelle ermoeglichen die Simulation des Verkehrsverhaltens sowie die Untersuchung und Bewertung von verschiedenen verkehrspolitischen Massnahmen. Prominente Massnahmen sind beispielsweise Einfahrverbote bestimmter Lkw-Typen, raeumlich und zeitlich differenzierte Mautgebuehren oder die Nutzung von Gueterverkehrszentren. Diese Arbeit ist eingebettet in einen uebergeordneten Ansatz zur mikroskopischen Modellierung des staedtischen Distributionsverkehrs im Lebensmitteleinzelhandel. Neben der Verhaltensmodellierung der Entscheidungstraeger sowie der physischen Simulation der Fahrzeugbewegungen liegt die grosse Herausforderung in der Gewinnung der Modelleingangsdaten zur Beschreibung des umfassenden Entscheidungsproblems in der Distributionslogistik. Die Ableitung dieser Daten sowie die Illustration am Beispiel eines Berlin-Szenarios ist Gegenstand des Aufsatzes. (A) (Wirtschaftsverkehr 2013: Datenerfassung und verkehrstraegeruebergreifende Modellierung des Gueterverkehrs als Entscheidungsgrundlage fuer die Verkehrspolitik.)