The chemical composition of solar-type stars in comparison with that of the Sun     

Bengt Gustafsson  Jorge Meléndez  Martin Asplund  David Yong 《Astrophysics and Space Science》2010,328(1-2):185-191

The question whether the solar chemical composition is typical for solar-type stars is analysed by comparing the Sun with different stellar samples, including a sample of stars with very similar parameters, solar twins. Although typical in terms of overall metallicity for stars of solar age and galactic orbit, the solar atmosphere is found to have abundances, as compared with solar twins, that indicate that its gas has once been affected by dust formation and dust separation. It is concluded that this may be related to the formation of the solar planetary system and its special properties.  相似文献   

Thermal contraction crack polygons on Mars: A synthesis from HiRISE, Phoenix, and terrestrial analog studies     

Joseph S. Levy  David R. Marchant 《Icarus》2010,206(1):229-216

Thermal contraction crack polygons are complex landforms that have begun to be deciphered on Earth and Mars by the combined investigative efforts of geomorphology, environmental monitoring, physical models, paleoclimate reconstruction, and geochemistry. Thermal contraction crack polygons are excellent indicators of the current or past presence of ground ice, ranging in ice content from weakly cemented soils to debris-covered massive ice. Relative to larger topographic features, polygons may form rapidly, and reflect climate conditions at the time of formation—preserving climate information as relict landforms in the geological record. Polygon morphology and internal textural characteristics can be used to distinguish surfaces modified by the seasonal presence of a wet active layer or dry active layer, and to delimit subsurface ice conditions. Analysis of martian polygon morphology and distribution indicates that geologically-recent thermal contraction crack polygons on Mars form predominantly in an ice-rich latitude-dependent mantle, more likely composed of massive ice deposited by precipitation than by cyclical vapor diffusion into regolith. Regional and local heterogeneities in polygon morphology can be used to distinguish variations in ice content, deposition and modification history, and to assess microclimate variation on timescales of ka to Ma. Analyses of martian polygon morphology, guided by investigations of terrestrial analog thermal contraction crack polygons, strongly suggest the importance of excess ice in the formation and development of many martian thermal contraction crack polygons—implying the presence of an ice-rich substrate that was fractured during and subsequent to obliquity-driven depositional periods and continually modified by ongoing vapor equilibration processes.  相似文献   

Search for Phobos and Deimos gas/dust tori using in situ observations from Mars Global Surveyor MAG/ER     

Marit Øieroset  David A. Brain  David L. Mitchell  Jasper S. Halekas  Mario H. Acuña 《Icarus》2010,206(1):189-198

More than 490 elliptical aerobraking and science phasing orbits made by Mars Global Surveyor (MGS) in 1997 and 1998 provide unprecedented coverage of the solar wind in the vicinity of the orbits of the martian moons Phobos and Deimos. We have performed a comprehensive survey of magnetic field perturbations in the solar wind to search for possible signatures of solar wind interaction with dust or gas escaping from the moons. A total of 1246 solar wind disturbance events were identified and their distribution was examined relative to Phobos, the Phobos orbit, and the Deimos orbit. We find that the spatial distribution of solar wind perturbations does not increase near or downstream of Phobos, Phobos’ orbit, or Deimos’ orbit, which would have been expected if there is significant outgassing or dust escape from the martian moons. Of the 1246 magnetic field perturbation events found in the MGS data set, 11 events were found within 2000 km of the Phobos orbit, while three events were found within 2000 km of the Deimos orbit. These events were analyzed in detail and found to likely have other causes than outgassing/dust escape from the martian moons. Thus we conclude that the amount of gas/dust escaping the martian moons is not significant enough to induce detectable magnetic field perturbations in the solar wind. In essence we have not found any clear evidence in the MGS magnetic field data for outgassing or dust escape from the martian moons.  相似文献   

The formation and stability of carbonic acid on outer Solar System bodies     

Z. Peeters  R.L. Hudson  M.H. Moore 《Icarus》2010,210(1):480-487

The radiation chemistry, thermal stability, and vapor pressure of solid-phase carbonic acid (H₂CO₃) have been studied with mid-infrared spectroscopy. A new procedure for measuring this molecule’s radiation stability has been used to obtain intrinsic IR band strengths and half-lives for radiolytic destruction. We report, for the first time, measurements of carbonic acid’s vapor pressure (0.290-2.33 × 10⁻¹¹ bar for 240-255 K) and its enthalpy of sublimation (71 ± 9 kJ mol⁻¹). We also report the first observation of a chemical reaction involving solid-phase carbonic acid. Possible applications of these findings are discussed, with an emphasis on the outer Solar System icy surfaces.  相似文献   

Whole-disk spectrophotometric properties of Mercury: Synthesis of MESSENGER and ground-based observations     

Deborah L. Domingue  Faith Vilas  Johan Warell  Scott L. Murchie  David T. Blewett  Brian J. Anderson 《Icarus》2010,209(1):101-124

Disk-integrated and disk-resolved measurements of Mercury’s surface obtained by both the Mercury Dual Imaging System (MDIS) and the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) onboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft were analyzed and compared with previous ground-based observations of Mercury at 11 wavelengths. The spectra show no definitive absorption features and display a red spectral slope (increasing reflectance with increasing wavelength) typical of space-weathered rocky surfaces. The MDIS spectra show evidence of phase reddening, which is not observed in the MASCS spectra. The MDIS spectra are commensurate with ground-based observations to within 10%, whereas the MASCS spectra display greater discrepancies with ground-based observations at near-infrared wavelengths. The derived photometric calibrations provide corrections within 10% for observations taken at phase angles less than ∼100°. The derived photometric properties are indicative of a more compact regolith than that of the lunar surface or of average S-type asteroids. The photometric roughness of the surface is also much smoother than the Moon’s. The calculated geometric albedo (reflectance at zero phase) is higher than lunar values. The lower reflectance of immature units on Mercury compared with immature units on the Moon, in conjunction with the higher geometric albedo, is indicative of more complicated grain structures within Mercury’s regolith.  相似文献   

The PL calibration for Milky Way Cepheids and its implications for the distance scale     

David G. Turner 《Astrophysics and Space Science》2010,326(2):219-231

The rationale behind recent calibrations of the Cepheid PL relation using the Wesenheit formulation is reviewed and reanalyzed, and it is shown that recent conclusions regarding a possible change in slope of the PL relation for short-period and long-period Cepheids are tied to a pathological distribution of HST calibrators within the instability strip. A recalibration of the period-luminosity relation is obtained using Galactic Cepheids in open clusters and groups, the resulting relationship, described by log L/L _⊙=2.415(±0.035)+1.148(±0.044)log P, exhibiting only the moderate scatter expected from color spread within the instability strip. The relationship is confirmed by Cepheids with HST parallaxes, although without the need for Lutz-Kelker corrections, and in general by Cepheids with revised Hipparcos parallaxes, albeit with concerns about the cited precisions of the latter. A Wesenheit formulation of W _V =−2.259(±0.083)−4.185(±0.103)log P for Galactic Cepheids is tested successfully using Cepheids in the inner regions of the galaxy NGC 4258, confirming the independent geometrical distance established for the galaxy from OH masers. Differences between the extinction properties of interstellar and extragalactic dust may yet play an important role in the further calibration of the Cepheid PL relation and its application to the extragalactic distance scale.  相似文献   

Binary population synthesis and sdBs at different metallicities     

David Brown 《Astrophysics and Space Science》2010,329(1-2):33-39

One of the ways by which subdwarf B stars are thought to form is through binary star interactions. The metallicity of the sdB progenitor stars in such binary systems should not seem to be a major factor in the formation of sdB stars. However, given the different environments in which sdB stars are found, binary population synthesis simulations have been conducted in order to examine how metallicity might be a subtle factor in the formation of sdB stars in such environments. This is then applied to clusters of stars and to the UV Upturn phenomenon.  相似文献   

Flow patterns of lobate debris aprons and lineated valley fill north of Ismeniae Fossae, Mars: Evidence for extensive mid-latitude glaciation in the Late Amazonian     

David M.H. Baker  James W. Head 《Icarus》2010,207(1):186-209

A variety of Late Amazonian landforms on Mars have been attributed to the dynamics of ice-related processes. Evidence for large-scale, mid-latitude glacial episodes existing within the last 100 million to 1 billion years on Mars has been presented from analyses of lobate debris aprons (LDA) and lineated valley fill (LVF) in the northern and southern mid-latitudes. We test the glacial hypothesis for LDA and LVF along the dichotomy boundary in the northern mid-latitudes by examining the morphological characteristics of LDA and LVF surrounding two large plateaus, proximal massifs, and the dichotomy boundary escarpment north of Ismeniae Fossae (centered at 45.3°N and 39.2°E). Lineations and flow directions within LDA and LVF were mapped using images from the Context (CTX) camera, the Thermal Emission Imaging Spectrometer (THEMIS), and the High Resolution Stereo Camera (HRSC). Flow directions were then compared to topographic contours derived from the Mars Orbiter Laser Altimeter (MOLA) to determine the down-gradient components of LDA and LVF flow. Observations indicate that flow patterns emerge from numerous alcoves within the plateau walls, are integrated over distances of up to tens of kilometers, and have down-gradient flow directions. Smaller lobes confined within alcoves and superposed on the main LDA and LVF represent a later, less extensive glacial phase. Crater size-frequency distributions of LDA and LVF suggest a minimum (youngest) age of 100 Ma. The presence of ring-mold crater morphologies is suggestive that LDA and LVF are formed of near-surface ice-rich bodies. From these observations, we interpret LDA and LVF within our study region to result from formerly active debris-covered glacial flow, consistent with similar observations in the northern mid-latitudes of Mars. Glacial flow was likely initiated from the accumulation and compaction of snow and ice on plateaus and in alcoves within the plateau walls as volatiles were mobilized to the mid-latitudes during higher obliquity excursions. Together with similar analyses elsewhere along the dichotomy boundary, these observations suggest that multiple glacial episodes occurred in the Late Amazonian and that LDA and LVF represent significant reservoirs of non-polar ice sequestered below a surface lag for hundreds of millions of years.  相似文献   

Geologic mapping of the Hi’iaka and Shamshu regions of Io     

Melissa K. Bunte  David A. Williams  Windy L. Jaeger 《Icarus》2010,207(2):868-30

We produced regional geologic maps of the Hi’iaka and Shamshu regions of Io’s antijovian hemisphere using Galileo mission data to assess the geologic processes that are involved in the formation of Io’s mountains and volcanic centers. Observations reveal that these regions are characterized by several types of volcanic activity and features whose orientation and texture indicate tectonic activity. Among the volcanic features are multiple hotspots and volcanic vents detected by Galileo, one at each of the major paterae: Hi’iaka, Shamshu, and Tawhaki. We mapped four primary types of geologic units: flows, paterae floors, plains, and mountains. The flows and patera floors are similar, but are subdivided based upon emplacement environments and mechanisms. The floors of Hi’iaka and Shamshu Paterae have been partially resurfaced by dark lava flows, although portions of the paterae floors appear bright and unchanged during the Galileo mission; this suggests that the floors did not undergo complete resurfacing as flooding lava lakes. However, the paterae do contain compound lava flow fields and show the greatest activity near the paterae walls, a characteristic of Pele type lava lakes. Mountain materials are tilted crustal blocks that exhibit varied degrees of degradation. Lineated mountains have characteristic en echelon grooves that likely formed as a result of gravitational sliding. Undivided mountains are partially grooved but exhibit evidence of slumping and are generally lower elevation than the lineated units. Debris lobes and aprons are representative of mottled mountain materials. We have explored the possibility that north and south Hi’iaka Mons were originally one structure. We propose that strike-slip faulting and subsequent rifting separated the mountain units and created a depression which, by further extension during the rifting event, became Hi’iaka Patera. This type of rifting and depression formation is similar to the mechanism of formation of terrestrial pull-apart basins. With comparison to other regional maps of Io and global studies of paterae and mountains, this work provides insight into the general geologic evolution of Io.  相似文献   

Multi-wavelength simulations of atmospheric radiation from Io with a 3-D spherical-shell backward Monte Carlo radiative transfer model     

Sergey L. Gratiy  Andrew C. Walker  David B. Goldstein  Laurence M. Trafton 《Icarus》2010,207(1):394-408

Conflicting observations regarding the dominance of either sublimation or volcanism as the source of the atmosphere on Io and disparate reports on the extent of its spatial distribution and the absolute column abundance invite the development of detailed computational models capable of improving our understanding of Io’s unique atmospheric structure and origin. Improving upon previous models, Walker et al. (Walker, A.C., Gratiy, S.L., Levin, D.A., Goldstein, D.B., Varghese, P.L., Trafton, L.M., Moore, C.H., Stewart, B. [2009]. Icarus) developed a fully 3-D global rarefied gas dynamics model of Io’s atmosphere including both sublimation and volcanic sources of SO₂ gas. The fidelity of the model is tested by simulating remote observations at selected wavelength bands and comparing them to the corresponding astronomical observations of Io’s atmosphere. The simulations are performed with a new 3-D spherical-shell radiative transfer code utilizing a backward Monte Carlo method. We present: (1) simulations of the mid-infrared disk-integrated spectra of Io’s sunlit hemisphere at 19 μm, obtained with TEXES during 2001-2004; (2) simulations of disk-resolved images at Lyman-α obtained with the Hubble Space Telescope (HST), Space Telescope Imaging Spectrograph (STIS) during 1997-2001; and (3) disk-integrated simulations of emission line profiles in the millimeter wavelength range obtained with the IRAM-30 m telescope in October-November 1999. We found that the atmospheric model generally reproduces the longitudinal variation in band depth from the mid-infrared data; however, the best match is obtained when our simulation results are shifted ∼30° toward lower orbital longitudes. The simulations of Lyman-α images do not reproduce the mid-to-high latitude bright patches seen in the observations, suggesting that the model atmosphere sustains columns that are too high at those latitudes. The simulations of emission line profiles in the millimeter spectral region support the hypothesis that the atmospheric dynamics favorably explains the observed line widths, which are too wide to be formed by thermal Doppler broadening alone.  相似文献