Low-velocity structure beneath Africa from forward modeling

Seismic waveforms observed in South Africa containing the first arrival crossover of S to SKS (70° to 110°) are analyzed. The data consist of analog records from the World Wide Seismographic Station Network (WWSSN) of deep events beneath South America. The S-waves arrive 2 to 3 s early relative to PREM at ranges from 70° to 95° and then become increasingly delayed, becoming 5 to 6 s late at 110°. The SKS phase is late by 3 to 5 s over the entire range. This pushes crossover between S and SKS, normally observed at about 81°, out about 2° to 3°, which is the most anomalous shift ever reported. To model such features, we modified Grand's tomography model [Grand et al., GSA Today 7 (1997) 1–7], and generated 2D synthetics to match the data. The overall shape and position of the lower mantle low-velocity anomaly proposed by Grand predicts good results if lower mantle anomalies are enhanced to a level of about 4%. This results in a complex tabular structure extending upward from the core–mantle boundary about 1500 km into the mantle. These features appear to be consistent with a large young plume which is erupting off the CMB.     

Stone run (block stream) formation in the Falkland Islands over several cold stages,deduced from cosmogenic isotope (10Be and 26Al) surface exposure dating

Cosmogenic isotope (¹⁰Be and ²⁶Al) surface exposure dating has been applied to valley‐axis and hillslope stone runs (relict periglacial block streams) and their source outcrops in the Falkland Islands, South Atlantic. The data indicate that stone runs are considerably older landforms than previously envisaged and afford no evidence that they are a product of the Last Glacial Maximum; the samples range in apparent ¹⁰Be age from 42k to 731k yr BP, but some of these are minima. The results indicate that valley‐axis stone runs may be up to 700–800k yr old, have simple exposure histories and are composite landforms that developed over several cold stages. Analyses of some hillslope and outcrop samples also demonstrate simple exposure histories with ¹⁰Be ages from 42k to 658k yr BP. In contrast, isotopic ratios from other hillslope and outcrop samples reveal they have had a complex exposure history involving periods of burial or shielding; the samples range in ¹⁰Be age from 59k to 569k yr BP and these are regarded as minimum age estimates. Larger stone runs may be older than smaller runs and there is a possibility that stone runs older than 800k yr exist in other parts of the Falklands. The assertion that glaciation in the Falklands was restricted to the highest uplands is supported by the data, and the potential for age determination of other boulder‐strewn and bedrock landforms, using cosmogenic isotope analysis, in order to extend the geochronology of Quaternary events and processes is noted. Copyright © 2008 John Wiley & Sons, Ltd.     

The organic composition of C/2001 A2 (LINEAR): II. Search for heterogeneity within a comet nucleus

The nucleus of Comet C/2001 A2 (LINEAR) split several times during its recent apparition, presenting an unusual opportunity to search for chemical differences in freshly exposed material. We conducted this search using NIRSPEC at the W.M. Keck Observatory on four dates in 2001: 9.5 and 10.5 July and 4.4 and 10.5 August. We detected the R0 and R1 lines of the ν₃ vibrational band of CH₄ near 3.3 μm on all dates. The R2 line was detected on 4.4 and 10.5 August. When we compare production rates of CH₄ to H₂O, we find evidence of a significant enhancement in August relative to that found in July. H₂CO was securely detected via its ν₁ and ν₅ bands on 9.5 July. On 10.5 July, H₂CO emission was much weaker, and its mixing ratio had dropped by a factor of about four. The mixing ratios for other detected volatile species did not change significantly over the course of the observations. We discuss the implications of this evidence for chemical heterogeneity in the nucleus of Comet C/2001 A2.     

Structural evidence from shock metamorphism in simple and complex impact craters: Linking observations to theory

Abstract— The structure of Canadian impact craters formed in crystalline rocks is analyzed using shock metamorphism and evidence for movement along shear zones. The analysis is based on an interpretation that, beyond the near field region, shock pressure attenuates down axis as P ? R^?2, in agreement with nuclear test and computed results, and as P ? R^?3 near the surface. In both simple and complex craters, the transient cavity is defined by the limit of fragmentation due to direct and reflected shock waves. The intersection of the transient cavity with hemispheric shock isobars indicates that the transient cavity has a parabolic form. Weakening by dilation during early uplift allows late stage slumping of the walls of simple craters. This is controlled by a spheroidal primary shear of radius r_s ～ 2d_t, where d_t is the depth of the transient crater due to excavation and initial compression. With increasing crater diameter, the size of the transient cavity decreases relative to the shock imprint, suggesting that fragmentation and excavation is limited by progressively earlier collapse of the margins under gravity. Central peak formation in complex craters may be initiated by relaxation of the shock‐compressed central parautochthone, so the primary shear, lubricated by friction melting, meets below the crater floor and drives the continuing upward motion.     

Effects of soil heterogeneity on martian ground-ice stability and orbital estimates of ice table depth

Data from the Mars Odyssey Gamma-Ray Spectrometer (GRS) instrument suite and results from numerical simulations of subsurface ground-ice stability have been used to estimate the depth of martian ground-ice. Geographic correlation between these estimates is remarkable; the relative ice table depth distributions also agree well. However, GRS-based estimates of ice table depth are generally deeper than predictions based on ground-ice stability simulations. This discrepancy may be related to heterogeneities in the martian surface such as rocks, dust, and albedo variations. We develop a multi-dimensional numerical model of ground-ice stability in a heterogeneous martian subsurface and use it to place the first quantitative constraints on the response of the ice table to meter-scale heterogeneities. We find that heterogeneities produce significant undulations/topography in the ice table at horizontal length scales of a few meters. Decimeter scale rocks create localized areas of deep ice, producing a vertical depression of 10-60 cm in the ice table over a horizontal range of 1-2 rock radii. Decimeter scale dust lenses produce locally shallow ice; however the magnitude of the vertical deflection of the ice table is small (1-4 cm). The effects of decimeter scale albedo variations on the ice table are nearly negligible, although albedo very weakly enhances the effects of dark rocks and bright dust on the ice table. Additionally, we investigate the role played by rocks in estimates of ice table depth based on orbital data. Surface rocks can account for more than half of the discrepancy between ice table depths inferred from GRS data and those predicted by theoretical ice-stability simulations that utilize thermophysical observations. Our results have considerable relevance to the up-coming Mars Scout Mission, Phoenix, because they indicate that the uncertainty in the ice table depth of a given region is greater than differences between current depth estimates. Likewise, small-scale depth variability due to heterogeneities at the eventual landing site is potentially greater than differences between current depth estimates.     

Ground-based near infrared spectroscopy of Jupiter's ring and moons

The backscattered reflectivity of Jupiter's ring has been previously measured over distinct visible and near infrared wavelength bands by a number of ground-based and spaceborne instruments. We present spectra of Jupiter's main ring from 2.21-2.46 μm taken with the NIRSPEC spectrometer at the W.M. Keck observatory. At these wavelengths, scattered light from Jupiter is minimal due to the strong absorption of methane in the planet's atmosphere. We find an overall flat spectral slope over this wavelength interval, except for a possible red slope shortward of 2.25 μm. We extended the spectral coverage of the ring to shorter wavelengths by adding a narrow-band image at 1.64 μm, and show results from 2.27-μm images over phase angles of 1.2°-11.0°. Our images at 1.64 and 2.27 μm reveal that the halo contribution is stronger at the shorter wavelength, possibly due to the redder spectrum of the ring parent bodies as compared with the halo dust component. We find no variation in main ring reflectivity over the 1.2°-11.0° phase angle range at 2.27 μm. We use adaptive optics imaging at the longer wavelength L′ band (3.4-4.1 μm) to determine a 2-σ upper limit of 22 m of vertically-integrated I/F. Our observing campaign also produced an L′ image of Callisto, showing a darker leading hemisphere, and a spectrum of Amalthea over the 2.2-2.5 and 2.85-3.03 μm ranges, showing deep 3-μm absorption.     

Raman spectroscopic analysis of cyanobacterial colonization of hydromagnesite, a putative martian extremophile

Raman spectra of an extremophile cyanobacterial colony in hydromagnesite from Lake Salda in Turkey have revealed a biogeological modification which is manifest as aragonite in the stratum associated with the colony. The presence of key spectral biomarkers of organic protectant molecules such as β-carotene and scytonemin indicate that the survival strategy of the cyanobacteria is significantly one of UV-radiation protection. The terrestrial location of this extremophile is worthy of consideration further because of its possible putative link with the “White Rock” formations in Sabaea Terra and Juventae Chasma on Mars.