Resurfacing history of Europa from pole-to-pole geological mapping     

Patricio H. Figueredo  Ronald Greeley 《Icarus》2004,167(2):287-312

We produced geologic maps from two regional mosaics of Galileo images across the leading and trailing hemispheres of Europa in order to investigate the temporal distribution of units in the visible geologic record. Five principal terrain types were identified (plains, bands, ridges, chaos, and crater materials), which are interpreted to result from (1) tectonic fracturing and lineament building, (2) cryovolcanic reworking of surface units, with possible emplacement of sub-surface materials, and (3) impact cratering. The geologic histories of both mapped areas are essentially similar and reflect some common trends: Tectonic resurfacing dominates the early geologic record with the formation of background plains by intricate superposition of lineaments, the opening of wide bands with infilling of inter-plate gaps, and the buildup of ridges and ridge complexes along prominent fractures in the ice. It also appears that lineaments are narrower and more widely spaced with time. The lack of impact craters overprinted by lineaments indicate that the degree of tectonic resurfacing decreased rapidly after ridged plains formation. In contrast, the degree of cryovolcanic resurfacing appears to increase with time, as chaos formation dominates the later parts of the geologic record. These trends, and the transition from tectonic- to cryovolcanic-dominated resurfacing could be attributed to the gradual thickening of Europa's cryosphere during the visible geologic history, that comprises the last 2% or 30-80 Myr of Europa's history: An originally thin, brittle ice shell could be pervasively fractured or melted through by tidal and endogenic processes; the degree of fracturing and plate displacements decreased with time in a thickening shell, and lineaments became narrower and more widely spaced; formation of chaos regions could have occurred where the thickness threshold for solid-state convection was exceeded, and can be aided by preferential tidal heating of more ductile ice. In a long-term context it is not clear at this point whether this inferred thickening trend would reflect a drastic change in the thermal evolution of the satellite, or cyclic or irregular episodes of tectonic and cryovolcanic activity.  相似文献   

Layering stratigraphy of eastern Coprates and northern Capri Chasmata, Mars     

Ross A. Beyer  Alfred S. McEwen 《Icarus》2005,179(1):1-23

Distinct competent layers are observed in the slopes of eastern Coprates Chasma, part of the Valles Marineris system on Mars. Our observations indicate that the stratigraphy of Coprates Chasma consists of alternating thin strong layers and thicker sequences of relatively weak layers. The strong, competent layers maintain steeper slopes and play a major role in controlling the overall shape and geomorphology of the chasmata slopes. The topmost competent layer in this area is well preserved and easy to identify in outcrops on the northern rim of Coprates Chasma less than 100 m below the southern Ophir Planum surface. The volume of the topmost emplaced layer is at least 70 km³ and may be greater than 2100 km³ if the unit underlies most of Ophir Planum. The broad extent of this layer allows us to measure elevation offsets within the north rim of the chasma and in a freestanding massif within Coprates Chasma where the layer is also observed. Rim outcrop morphology and elevation differences between Ophir and Aurorae Plana may be indicative of the easternmost extent of the topmost competent layer. These observations allow an insight into the depositional processes that formed the stratigraphic stack into which this portion of the Valles Marineris is carved, and they present a picture of some of the last volcanic activity in this area. Furthermore, the elevation offsets within the layer are evidence of significant subsidence of the massif and surrounding material.  相似文献   

Ground-based photometric observations of Jupiter's inner satellites Thebe, Amalthea, and Metis at small phase angles     

I. Kulyk  K. Jockers 《Icarus》2004,170(1):24-34

We present the results of photometric measurements of the inner jovian satellites Thebe, Amalthea and Metis based on extensive optical observations taken from October 1999 to January 2002. The observations were made in the phase angle range from 8.1° to 0.3°. The Two-Channel Focal Reducer of the Max-Planck Institute for Aeronomy attached to the 2-m RCC telescope at Terskol Observatory (Pik Terskol, Northern Caucasus) was used in coronagraph mode. The observations were performed at a wavelength of 0.887 μm. Mean observational uncertainties corresponding to 1σ rms errors were 3% for the leading and trailing sides of Amalthea, 7 and 9% for the leading and trailing sides of Thebe and 9% for the leading side of Metis after taking into account the longitude brightness variations. Photometric data calibrated on an absolute scale were used to evaluate the near-opposition behavior of satellite brightness. All three satellites exhibit significant opposition brightening, but the strength of this effect, measured as the ratios of intensities at α₁=1.6° and α₂=6.7° does not vary significantly among these satellites. In order to measure the opposition surge parameters the empirical law proposed by Karkoschka and Hapke's model were used. The parameters of the satellite opposition effects are presented and discussed. The values of geometric albedos calculated with best-fit Hapke parameters are 0.096, 0.157, and 0.24 for Thebe, Amalthea, and Metis respectively. We found that the average leading/trailing ratios of surface reflectance at the measured phase angles are 1.53±0.05, 1.25±0.04, 1.04±0.08 for Amalthea, Thebe, and Metis.  相似文献   

A phase-resolved XMM–Newton campaign on the colliding-wind binary HD 152248     

H. Sana  I. R. Stevens  E. Gosset  G. Rauw  J.-M. Vreux 《Monthly notices of the Royal Astronomical Society》2004,350(3):809-828

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Composition of jovian dust stream particles     

Frank Postberg  Sascha Kempf  Simon F. Green  Neil McBride 《Icarus》2006,183(1):122-134

The Cassini spacecraft encountered Jupiter in late 2000. Within more than 1 AU of the gas giant the Cosmic Dust Analyser onboard the spacecraft recorded the first ever mass spectra of jovian stream particles. To determine the chemical composition of particles, a comprehensive statistical analysis of the dataset was performed. Our results imply that the vast majority (>95%) of the observed stream particles originate from the volcanic active jovian satellite Io from where they are sprinkled out far into the Solar System. Sodium chloride (NaCl) was identified as the major particle constituent, accompanied by sulphurous as well as potassium bearing components. This is in contrast to observations of gas in the ionian atmosphere, its co-rotating plasma torus, and the neutral cloud, where sulphur species are dominant while alkali and chlorine species are only minor components. Io has the largest active volcanoes of the Solar System with plumes reaching heights of more than 400 km above the moons surface. Our in situ measurements indicate that alkaline salt condensation of volcanic gases inside those plumes could be the dominant formation process for particles reaching the ionian exosphere.  相似文献   

A revised model for cycloid growth mechanics on Europa: Evidence from surface morphologies and geometries     

Scott T. Marshall 《Icarus》2005,177(2):341-366

Although a single model currently exists to explain the development of curved Europan cycloids, there have been no systematic studies of the range of morphologies and quantifiable geometric parameters of cycloidal features. We address variations in geometry along individual cycloid segments, characterizing differences in cusp styles and angles, and addressing the morphologic aspects of cycloid segments and cusps. In so doing, we illustrate how geometric and morphologic evidence imply a formation mechanism that differs from the existing model in several aspects. The current model states that cycloids are initiated as tensile fractures that grow in a curved path in response to rotating diurnal tidal stresses on Europa. However, the geometry of a cycloid cusp necessitates that shear stress was resolved onto the existing cycloid segment by the rotating diurnal stresses at the instant of cusp formation. Furthermore, we observe that cycloid cusps have a strikingly similar geometry to tailcracks that developed at the tips of many ridge-like strike-slip faults on Europa in response to shearing at the fault tip. We suggest that this similarity in geometries can be attributed to an identical formation mechanism whereby cycloid cusps form by a tailcracking process. We therefore present a revised, mechanically-based model for cycloid formation that retains the basic premise that crack growth is governed by diurnal stresses, but describes the development of cycloid cusps in response to resolved shear stresses at the tips of existing cycloid segments. The ratio of normal to shear stress at the time of tailcrack formation dictates the cusp angle and, over longer time periods, influences the morphologic evolution of the cycloid segment as it is repeatedly reworked by tidal stresses.  相似文献   

Dark matter in elliptical galaxies – II. Estimating the mass within the virial radius     

Gary A. Mamon  Ewa L. &#;okas 《Monthly notices of the Royal Astronomical Society》2005,363(3):705-722

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Erratum: A high-velocity ionized outflow and XUV photosphere in the narrow emission line quasar PG1211+143     

《Monthly notices of the Royal Astronomical Society》2005,356(4):1599-1599

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Erratum: The runaway instability of thick discs around black holes – II. Non-constant angular momentum discs     

Frédéric Daigne  José A. Font 《Monthly notices of the Royal Astronomical Society》2004,351(3):1120-1120

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A non-main-sequence secondary in SY Cancri     

Robert Connon Smith  Otto Mehes  Dave Vande Putte  Nigel A. Hawkins 《Monthly notices of the Royal Astronomical Society》2005,360(1):364-374

Simultaneous spectroscopic and photometric observations of the Z Cam type dwarf nova SY Cancri were used to obtain absolute flux calibrations. A comparison of the photometric calibration with a wide-slit spectrophotometric calibration showed that either method is equally satisfactory. A radial velocity study of the secondary star, made using the far-red Na  i doublet, yielded a semi-amplitude of   K ₂= 127 ± 23 km s⁻¹  . Taking the published value of  86 ± 9 km s⁻¹  for K ₁ gives a mass ratio of   q = M ₂/ M ₁= 0.68 ± 0.14  ; this is very different from the value of  1.13 ± 0.35  quoted in the literature. Using the new lower mass ratio, and constraining the mass of the white dwarf to be within reasonable limits, then leads to a mass for the secondary star that is substantially less than would be expected for its orbital period if it satisfied a main-sequence mass–radius relationship. We find a spectral type of M0 that is consistent with that expected for a main-sequence star of the low mass we have found. However, in order to fill its Roche lobe, the secondary must be significantly larger than a main-sequence star of that mass and spectral type. The secondary is definitely not a normal main-sequence star.  相似文献