Spaceborne visible and thermal infrared lithologic mapping of impact‐exposed subsurface lithologies at the Haughton impact structure,Devon Island,Canadian High Arctic: Applications to Mars

Abstract— This study serves as a proof‐of‐concept for the technique of using visible‐near infrared (VNIR), short‐wavelength infrared (SWIR), and thermal infrared (TIR) spectroscopic observations to map impact‐exposed subsurface lithologies and stratigraphy on Earth or Mars. The topmost layer, three subsurface layers and undisturbed outcrops of the target sequence exposed just 10 km to the northeast of the 23 km diameter Haughton impact structure (Devon Island, Nunavut, Canada) were mapped as distinct spectral units using Landsat 7 ETM+ (VNIR/SWIR) and ASTER (VNIR/SWIR/TIR) multispectral images. Spectral mapping was accomplished by using standard image contrast‐stretching algorithms. Both spectral matching and deconvolution algorithms were applied to image‐derived ASTER TIR emissivity spectra using spectra from a library of laboratory‐measured spectra of minerals (Arizona State University) and whole‐rocks (Ward's). These identifications were made without the use of a priori knowledge from the field (i.e., a “blind” analysis). The results from this analysis suggest a sequence of dolomitic rock (in the crater rim), limestone (wall), gypsum‐rich carbonate (floor), and limestone again (central uplift). These matched compositions agree with the lithologic units and the pre‐impact stratigraphic sequence as mapped during recent field studies of the Haughton impact structure by Osinski et al. (2005a). Further conformation of the identity of image‐derived spectra was confirmed by matching these spectra with laboratory‐measured spectra of samples collected from Haughton. The results from the “blind” remote sensing methods used here suggest that these techniques can also be used to understand subsurface lithologies on Mars, where ground truth knowledge may not be generally available.     

Comparison of Five Numerical Codes for Automated Tracing of Coronal Loops

The three-dimensional (3D) modeling of coronal loops and filaments requires algorithms that automatically trace curvilinear features in solar EUV or soft X-ray images. We compare five existing algorithms that have been developed and customized to trace curvilinear features in solar images: i) the oriented-connectivity method (OCM), which is an extension of the Strous pixel-labeling algorithm (developed by Lee, Newman, and Gary); ii) the dynamic aperture-based loop-segmentation method (developed by Lee, Newman, and Gary); iii) unbiased detection of curvilinear structures (developed by Steger, Raghupathy, and Smith); iv) the oriented-direction method (developed by Aschwanden); and v) ridge detection by automated scaling (developed by Inhester). We test the five existing numerical codes with a TRACE image that shows a bipolar active region and contains over 100 discernable loops. We evaluate the performance of the five codes by comparing the cumulative distribution of loop lengths, the median and maximum loop length, the completeness or detection efficiency, the accuracy, and flux sensitivity. These algorithms are useful for the reconstruction of the 3D geometry of coronal loops from stereoscopic observations with the STEREO spacecraft, or for quantitative comparisons of observed EUV loop geometries with (nonlinear force-free) magnetic field extrapolation models.     

The roughness of the dark side of Iapetus from the 2004 to 2005 flyby

The roughness of a planetary surface offers clues to its past geologic history. We apply a surface roughness model developed by Buratti and Veverka (Buratti, B.J., Veverka, J. [1985]. Icarus 64, 320-328) to Cassini ISS data from the January 1st, 2005 flyby of Iapetus. This model uses the observed scattering behavior to provide a depth to radius factor q quantifying the size of idealized craters on the surface. Our findings indicate that the surface on the dark side is significantly smoother than the surfaces of other icy low-albedo saturnian satellites. We have found that the average depth to radius on the leading (dark) side is 0.084, corresponding to a Hapke mean slope angle of 6°. As compared to the 13-33° Hapke mean slope angle of other icy satellites (Buratti, B.J., and 10 colleagues [2008]. Icarus 193, 309-322), our results present a clearly different picture for the leading surface of Iapetus, suggesting that the dark deposit contributes to the decrease in macroscopic surface roughness of the leading side. Attempts were made to obtain an average depth to radius value for the trailing (bright) side; however the scans of the bright side from this flyby exhibited large variations in albedo, resulting in results that were physically unrealistic.     

Mass density of the upper atmosphere derived from Starlette’s Precise Orbit Determination with Satellite Laser Ranging

The atmospheric mass density of the upper atmosphere from the spherical Starlette satellite’s Precise Orbit Determination is first derived with Satellite Laser Ranging measurements at 815 to 1115 km during strong solar and geomagnetic activities. Starlette’s orbit is determined using the improved orbit determination techniques combining optimum parameters with a precise empirical drag application to a gravity field. MSIS-86 and NRLMSISE-00 atmospheric density models are compared with the Starlette drag-derived atmospheric density of the upper atmosphere. It is found that the variation in the Starlette’s drag coefficient above 800 km corresponds well with the level of geomagnetic activity. This represents that the satellite orbit is mainly perturbed by the Joule heating from geomagnetic activity at the upper atmosphere. This result concludes that MSIS empirical models strongly underestimate the mass density of the upper atmosphere as compared to the Starlette drag-derived atmospheric density during the geomagnetic storms. We suggest that the atmospheric density models should be analyzed with higher altitude acceleration data for a better understanding of long-term solar and geomagnetic effects.     

Volcanic hazards from Bezymianny- and Bandai-type eruptions

Major slope failures are a significant degradational process at volcanoes. Slope failures and associated explosive eruptions have resulted in more than 20 000 fatalities in the past 400 years; the historic record provides evidence for at least six of these events in the past century. Several historic debris avalanches exceed 1 km³ in volume. Holocene avalanches an order of magnitude larger have traveled 50–100 km from the source volcano and affected areas of 500–1500 km². Historic eruptions associated with major slope failures include those with a magmatic component (Bezymianny type) and those solely phreatic (Bandai type). The associated gravitational failures remove major segments of the volcanoes, creating massive horseshoe-shaped depressions commonly of caldera size. The paroxysmal phase of a Bezymianny-type eruption may include powerful lateral explosions and pumiceous pyroclastic flows; it is often followed by construction of lava dome or pyroclastic cone in the new crater. Bandai-type eruptions begin and end with the paroxysmal phase, during which slope failure removes a portion of the edifice. Massive volcanic landslides can also occur without related explosive eruptions, as at the Unzen volcano in 1792.The main potential hazards from these events derive from lateral blasts, the debris avalanche itself, and avalanche-induced tsunamis. Lateral blasts produced by sudden decompression of hydrothermal and/or magmatic systems can devastate areas in excess of 500km² at velocities exceeding 100 m s^–1. The ratio of area covered to distance traveled for the Mount St. Helens and Bezymianny lateral blasts exceeds that of many pyroclastic flows or surges of comparable volume. The potential for large-scale lateral blasts is likely related to the location of magma at the time of slope failure and appears highest when magma has intruded into the upper edifice, as at Mount St. Helens and Bezymianny.Debris avalanches can move faster than 100 ms^–1 and travel tens of kilometers. When not confined by valley walls, avalanches can affect wide areas beyond the volcano's flanks. Tsunamis from debris avalanches at coastal volcanoes have caused more fatalities than have the landslides themselves or associated eruptions. The probable travel distance (L) of avalanches can be estimated by considering the potential vertical drop (H). Data from a catalog of around 200 debris avalanches indicates that the H/L rations for avalanches with volumes of 0.1–1 km³ average 0.13 and range 0.09–0.18; for avalanches exceeding 1 km³, H/L ratios average 0.09 and range 0.5–0.13.Large-scale deformation of the volcanic edefice and intense local seismicity precede many slope failures and can indicate the likely failure direction and orientation of potential lateral blasts. The nature and duration of precursory activity vary widely, and the timing of slope faliure greatly affects the type of associated eruption. Bandai-type eruptions are particularly difficult to anticipate because they typically climax suddenly without precursory eruptions and may be preceded by only short periods of seismicity.     

Spheroidal pyroxenite aggregates in the Bushveld Complex — a special case of silicate liquid immiscibility

Spherical aggregates of orthopyroxene are reported from some parts of the Bushveld Complex in a variety of host rocks.Detailed mapping has shown that these spherical aggregates, comprising pyroxenite spheroids in a quartz-norite matrix, are contact phenomena and not stratigraphic markers. Orthopyroxene, biotite and amphibole are enriched in spheroids relative to matrix; their mineral chemistry showing a fairly constant orthopyroxene and plagioclase composition through the spheroids and into the matrix, indicating in-situ formation.Bulk chemistry shows spheroid to matrix tie-lines orthogonal to those generally accepted for silicate liquid immiscibility, but other chemical information is consistent with the occurrence of immiscibility.The formation of the aggregates may be related to the industrial process of spherical agglomeration, by which spheroids are formed by the introduction of an immiscible “bridging liquid” to the melt — probably derived from the floor rocks in this case. The mechanism accounts for the field relationships, petrography and chemistry of the aggregate-matrix system. The petrology of the process equates with a special case of silicate liquid immiscibility induced by local contamination and ageing of the original magma.A similar “bridging liquid” mechanism could also account for the formation of the so-called “boulder bed” beneath the Merensky Reef.     

The 1786 earthquake-triggered landslide dam and subsequent dam-break flood on the Dadu River, southwestern China — Reply

A short reply to discussion by Chanson.     

The granite problem as exposed in the southern Snake Range,Nevada

A geochemically and mineralogically diverse group of granitoids is present within an area of 900 km² in the southern Snake Range of eastern Nevada. The granitoids exposed range in age from Jurassic through Cretaceous to Oligocene and include two calcic intrusions, two different types of two-mica granites, and aplites. The younger intrusions appear to have been emplaced at progressively more shallow depths. All of these granitoid types are represented elsewhere in the eastern Great Basin, but the southern Snake Range is distinguished by the grouping of all these types within a relatively small area. The Jurassic calcic pluton of the Snake Creek-Williams Canyon area displays large and systematic chemical and mineralogical zonation over a horizontal distance of five km. Although major element variations in the pluton compare closely with Daly's average andesite-dacite-rhyolite over an SiO₂ range of 63 to 76 percent, trace element (Rb, Sr, Ba) variations show that the zonation is the result of in situ fractional crystallization, with the formation of relatively mafic cumulates on at least one wall of the magma chamber. Models of trace element and isotopic data indicate that relatively little assimilation took place at the level of crystallization. Nonetheless, an initial ⁸⁷Sr/⁸⁶Sr value of 0.7071 and δ ¹⁸O values of 10.2 to 12.2 permil suggest a lower crustal magma that was contaminated by upper crustal clastic sedimentary rocks before crystallization. The involvement of mantle-derived magmas in its genesis is difficult to rule out. Two other Jurassic plutons show isotopic and chemical similarities to the Snake Creek-Williams Canyon pluton. Cretaceous granites from eastern Nevada that contain phenocrystic muscovite are strongly peraluminous, and have high initial Sr-isotope ratios and other features characteristic of S-type granitoids. They were probably derived from Proterozoic metasediments and granite gneisses that comprise the middle crust of this region. Another group of granitoids (including the Tertiary aplites) show chemical, mineralogic, and isotopic characteristics intermediate between the first two groups and may have been derived by contamination of magmas from the lower crust by the midcrustal metasediments.