Global photometric properties of Asteroid (4) Vesta observed with Dawn Framing Camera

Dawn spacecraft orbited Vesta for more than one year and collected a huge volume of multispectral, high-resolution data in the visible wavelengths with the Framing Camera. We present a detailed disk-integrated and disk-resolved photometric analysis using the Framing Camera images with the Minnaert model and the Hapke model, and report our results about the global photometric properties of Vesta. The photometric properties of Vesta show weak or no dependence on wavelengths, except for the albedo. At 554 nm, the global average geometric albedo of Vesta is 0.38 ± 0.04, and the Bond albedo range is 0.20 ± 0.02. The bolometric Bond albedo is 0.18 ± 0.01. The phase function of Vesta is similar to those of S-type asteroids. Vesta’s surface shows a single-peaked albedo distribution with a full-width-half-max ∼17% relative to the global average. This width is much smaller than the full range of albedos (from ∼0.55× to >2× global average) in localized bright and dark areas of a few tens of km in sizes, and is probably a consequence of significant regolith mixing on the global scale. Rheasilvia basin is ∼10% brighter than the global average. The phase reddening of Vesta measured from Dawn Framing Camera images is comparable or slightly stronger than that of Eros as measured by the Near Earth Asteroid Rendezvous mission, but weaker than previous measurements based on ground-based observations of Vesta and laboratory measurements of HED meteorites. The photometric behaviors of Vesta are best described by the Hapke model and the Akimov disk-function, when compared with the Minnaert model, Lommel–Seeliger model, and Lommel–Seeliger–Lambertian model. The traditional approach for photometric correction is validated for Vesta for >99% of its surface where reflectance is within ±30% of global average.     

Mapping of Io's thermal radiation by the Galileo photopolarimeter-radiometer (PPR) instrument

Between 1999 and 2002, the Galileo spacecraft made 6 close flybys of Io during which many observations of Io's thermal radiation were made with the photopolarimeter-radiometer (PPR). While the NIMS instrument could measure thermal emission from hot spots with T>200 K, PPR was the only Galileo instrument capable of mapping the lower temperatures of older, cooling lava flows, and the passive background. We tabulate all data taken by PPR of Io during these flybys and describe some scientific highlights revealed by the data. The data include almost complete coverage of Io at better than 250 km resolution, with extensive regional coverage at higher resolutions. We found a modest poleward drop in nighttime background temperatures and evidence of thermal inertia variations across the surface. Comparison of high spatial resolution temperature measurements with observed daytime SO₂ gas pressures on Io provides evidence for local cold trapping of SO₂ frost on scales smaller than the 60 km resolution of the PPR data. We also calculated the power output from several hot spots and estimated total global heat flow to be about 2.0-2.6 W m⁻². The low-latitude diurnal temperature variations for the regions between obvious hot spots are well matched by a laterally-inhomogeneous thermal model with less than 1 W m⁻² endogenic heat flow.     

Magmatic inclusions and felsic clasts in the Dar al Gani 319 polymict ureilite

Abstract— Magmatic inclusions occur in type II ureilite clasts (olivine‐orthopyroxene‐augite assemblages with essentially no carbon) and in a large isolated plagioclase clast in the Dar al Gani (DaG) 319 polymict ureilite. Type I ureilite clasts (olivine‐pigeonite assemblages with carbon), as well as other lithic and mineral clasts in this meteorite, are described in Ikeda et al.(2000). The magmatic inclusions in the type II ureilite clasts consist mainly of magnesian augite and glass. They metastably crystallized euhedral pyroxenes, resulting in feldspar component‐enriched glass. On the other hand, the magmatic inclusions in the large plagioclase clast consist mainly of pyroxene and plagioclase, with a mesostasis. They crystallized with a composition along the cotectic line between the pyroxene and plagioclase liquidus fields. DaG 319 also contains felsic lithic clasts that represent various types of igneous lithologies. These are the rare components not found in the common monomict ureilites. Porphyritic felsic clasts, the main type, contain phenocrysts of plagioclase and pyroxene, and their groundmass consists mainly of plagioclase, pyroxene, and minor phosphate, ilmenite, chromite, and/or glass. Crystallization of these porphyritic clasts took place along the cotectic line between the pyroxene and plagioclase fields. Pilotaxitic felsic clasts crystallized plagioclase laths and minor interstitial pyroxene under metastable conditions, and the mesostasis is extremely enriched in plagioclase component in spite of the ubiquitous crystallization of plagioclase laths in the clasts. We suggest that there are two crystallization trends, pyroxene‐metal and pyroxene‐plagioclase trends, for the magmatic inclusions and felsic lithic clasts in DaG 319. The pyroxene‐metal crystallization trend corresponds to the magmatic inclusions in the type II ureilite clasts and the pilotaxitic felsic clasts, where crystallization took place under reducing and metastable conditions, suppressing precipitation of plagioclase. The pyroxene‐plagioclase crystallization trend corresponds to the magmatic inclusions in the isolated plagioclase clast and the porphyritic felsic clasts. This trend developed under oxidizing conditions in magma chambers within the ureilite parent body. The felsic clasts may have formed mainly from albite component‐rich silicate melts produced by fractional partial melting of chondritic precursors. The common monomict ureilites, type I ureilites, may have formed by the fractional partial melting of alkali‐bearing chondritic precursors. However, type II ureilites may have formed as cumulates from a basaltic melt.     

Intrinsic spectra of the AXPs

Using direct measurements of photo-electric absorption edges, I derive the intrinsic spectra for the Anomalous X-ray Pulsars. In the past, the hydrogen column density, N _H had been found by fitting the X-ray spectra with a variety of simple continuum models. Since different models fit equally well, with different values of N _H, little is learned about the true column density or intrinsic spectra. Here, I measure the column densities in a model-independent way, and thus derive intrinsic spectra without first assuming what those spectra ought to look like. Particularly for the brightest source, 4U 0142+61, the column density can be determined accurately, and is a factor of 1.4 smaller that the typically N _H quoted. With this new value, the new intrinsic spectrum can be investigated anew, and shows a slight hint of a feature around 13 Å. To be emphasised, this is an empirical method with the minimum of assumptions, as is appropriate for these beguiling sources, the behaviour of which has mystifies astronomers for over a decade. This paper summarises the methods and conclusions to be published in the Astrophysical Journal by Durant and Van Kerkwijk (2006; see astro-ph/0606604).     

Solar–Stellar Irradiance Comparison Experiment II (SOLSTICE II): Pre-Launch and On-Orbit Calibrations

The Solar–Stellar Irradiance Comparison Experiment {II (SOLSTICE {II), aboard the Solar Radiation and Climate Experiment (SORCE) spacecraft, consists of a pair of identical scanning grating monochromators, which have the capability to observe both solar spectral irradiance and stellar spectral irradiance using a single optical system. The SOLSTICE science objectives are to measure solar spectral irradiance from 115 to 320 nm with a spectral resolution of 1 nm, a cadence of 6 h, and an accuracy of 5%, to determine its variability with a long-term relative accuracy of 0.5% per year during a 5-year nominal mission, and to determine the ratio of solar irradiance to that of an ensemble of bright B and A stars to an accuracy of 2%. Those objectives are met by calibrating instrument radiometric sensitivity before launch using the Synchrotron Ultraviolet Radiation Facility at the National Institute for Standards and Technology in Gaithersburg, Maryland. During orbital operations irradiance measurements from an ensemble of bright, stable, main-sequence B and A stars are used to track instrument sensitivity. SORCE was launched on 25 January 2003. After spacecraft and instrument check out, SOLSTICE {II first observed a series of three stars to establish an on-orbit performance baseline. Since 6 March 2003, both instruments have been making daily measurements of both the Sun and stars. This paper describes the pre-flight and in-flight calibration and characterization measurements that are required to achieve the SOLSTICE science objectives and compares early SOLSTICE{II measurements of both solar and stellar irradiance with those obtained by SOLSTICE {I on the Upper Atmosphere Research Satellite.     

Further finds of the Derrick Peak meteorite,Transantarctic Mountains,and implications for terrestrial age

Abstract— Nine additional iron meteorite fragments weighing a total of 72 kg were recovered from the Derrick Peak area by a Canterbury Museum geological party in late 1988. One iron was located in the Onnum Valley, 6 km south of the previous finds. Geochemical analysis indicates that all irons belong to a single meteorite shower, greatly increasing the known extent of the fall zone. Kamp and Lowe (1982) have previously estimated the terrestrial age of the meteorite from glacial geological evidence. The location of the 1988 finds supports Kamp and Lowe's interpretation that the meteorites lie in situ, but recent revisions of the chronology of Cenozoic glacial history of the region reduce the interpreted terrestrial age. An age of between Oxygen Isotope stages 6 and 2 is probable (190–125 to 35–12 ka BP). This conflicts with a terrestrial age estimate of 1.0 ± 0.1 Ma BP from cosmogenic radionuclides.     

Monte Carlo modeling of neutral gas and dust in the coma of Comet 1P/Halley

The neutral gas environment of a comet is largely influenced by dissociation of parent molecules created at the surface of the comet and collisions of all the involved species. We compare the results from a kinetic model of the neutral cometary environment with measurements from the Neutral Mass Spectrometer and the Dust Impact Detection System onboard the Giotto spacecraft taken during the fly-by at Comet 1P/Halley in 1986. We also show that our model is in good agreement with contemporaneous measurements obtained by the International Ultraviolet Explorer, sounding rocket experiments, and various ground based observations.The model solves the Boltzmann equation with a Direct Simulation Monte Carlo technique (Tenishev, V., Combi, M., Davidsson, B. [2008]. Astrophys. J. 685, 659-677) by tracking trajectories of gas molecules and dust grains under the influence of the comet’s weak gravity field with momentum exchange among particles modeled in a probabilistic manner. The cometary nucleus is considered to be the source of dust and the parent species (in our model: H₂O, CO, H₂CO, CO₂, CH₃OH, C₂H₆, C₂H₄, C₂H₂, HCN, NH₃, and CH₄) in the coma. Subsequently our model also tracks the corresponding dissociation products (H, H₂, O, OH, C, CH, CH₂, CH₃, N, NH, NH₂, C₂, C₂H, C₂H₅, CN, and HCO) from the comet’s surface all the way out to 10⁶ km.As a result we are able to further constrain cometary the gas production rates of CO (13%), CO₂ (2.5%), and H₂CO (1.5%) relative to water without invoking unknown extended sources.     

Climate changes detected through the world's longest sea level series

The sea level series of Stockholm in the Baltic Sea, commencing already in 1774, is analysed in various ways together with contemporary climate data, in order to investigate long-term sea level changes and their relations to climate changes.First, a study of the eustatic rise of sea level, based on annual mean sea levels, is peformed, and compared with other sea level and climate studies. It is concluded that the general climatic rise of sea level has increased significantly (99.9%) from about 0.0 mm/year during the end of the Little Ice Age, to about 1.0 mm/year during the past century, characterized by melting of glaciers. Such sea level changes due to northern hemisphere climate variations since 800 A.D. have (hitherto) probably always kept within −1.5 and +1.5 mm/year, with an average fairly close to zero.Second, an investigation of the sea level variability, also based on annual mean sea levels, is performed together with temperature and wind variabilities. It is found that the interannual sea level variability of the Baltic Sea has decreased significantly (98%) from the end of the 1700s to the beginning of the 1900s; after that it has increased significantly (95%) again. Precisely the same is found to apply to winter climate or, more specifically, to the interannual winter temperature variability and the interannual winter wind variability. The common origin of all these long-term changes turn out to be two consecutive winter wind processes over the North and Baltic Seas, especially the Baltic entrance. From the end of the 1700s to the beginning of the 1900s, there has been a rapidly decreasing number of dominating winter winds from northeast, and after that there has been an increasing number of dominating winter winds from southwest. This may indicate corresponding long-term changes in the North Atlantic Oscillation.Third, using monthly mean sea levels together with corresponding wind data, seasonal variations are investigated. The seasonal sea level variation in the Baltic Sea has increased significantly (99%) since the early 1800s, together with a shift of the maximum from late summer to early winter. It is found that the main origin is a secular change of the winter wind conditions over the Baltic entrance, with increasing southwesterly winds in early winter. This might also be related to a long-term change in the North Atlantic Oscillation.     

140kaBP以来南海西南部上升流影响区沉积记录及其古海洋环境变化

对南海西南部现代上升流区沉积物柱样BIS-187-61孔有机碳、生物蛋白石、碳酸盐等各沉积组分进行了详细的分析,试图探讨末次间冰期(MIS 5)以来南海西南部夏季上升流影响区沉积过程及其所反映的海洋环境变化过程。研究结果发现,140kaBP以来该孔沉积记录期总体表现为:碳酸盐百分含量冰期低间冰期高,主要受周边陆源物质输入稀释的控制,为典型的"大西洋型碳酸盐旋回",有机碳、生物蛋白石、碳酸钙以及陆源沉积物的堆积速率均在MIS 2期、MIS 4期和MIS 5e期出现高值。其中,MIS 2期和MIS 4期生源和陆源沉积堆积速率增加可能与冰期时冬季风增强及海平面下降导致陆源营养物质输入增加对初级生产力的刺激有关,也可能与陆源物质输入增加对海洋生源颗粒物输出的压载作用增强有关。而MIS 5e期南海海平面高度与现代相仿,间冰期较强的夏季风引起的越南沿岸上升流增强是导致该时段本研究区域表层初级生产力增强,沉积物总堆积速率及各组分堆积速率都相应增加的主要原因。