Spectral observations of 19 weathered and 23 fresh NEAs and their correlations with orbital parameters

Results of our visible to near-infrared spectrophotometric observations of 41 near-Earth asteroids (NEAs) are reported. These moderate-resolution spectra, along with 14 previously published spectra from our earlier survey [Hicks, M.D., Fink, U., Grundy, W.M., 1998. Icarus 133, 69-78] show a preponderance of spectra consistent with ordinary chondrites (23 NEAs with this type of spectrum, along with 19 S-types and 13 in other taxonomic groups). There exists statistically significant evidence for orbit-dependent trends in our data. While S-type NEAs from our survey reside primarily in (1) Amor orbits or (2) Aten or Apollo orbits which do not cross the asteroid main-belt, the majority of objects with spectra consistent with ordinary chondrites in our survey are in highly eccentric Apollo orbits which enter the asteroid main-belt. This trend toward fresh, relatively unweathered NEAs with ordinary chondrite type spectra in highly eccentric Apollo orbits is attributed to one or a combination of three possible causes: (1) the chaotic nature of NEA orbits can easily result in high eccentricity orbits/large aphelion distances so that they can enter the collisionally enhanced environment in the main-belt, exposing fresh surfaces, (2) they have recently been injected into such orbits after a collision in the main-belt, or (3) such objects cross the orbits of several terrestrial planets, causing tidal disruption events that expose fresh surfaces.     

Woodleigh impact structure,Australia: Shock petrography and geochemical studies

Abstract— The large, complex Woodleigh structure in the Carnarvon basin of Western Australia has recently been added to the terrestrial impact crater record. Many aspects of this structure are, however, still uncertain. This work provides a detailed petrographic assessment of a suite of representative drill core samples from the borehole Woodleigh 1 that penetrated uplifted basement rocks of the central part of this structure. Fundamental rock and mineral deformation data and high‐precision chemical data, including results of PGE and oxygen isotopic analysis, are presented. The sampled interval displays likely impact‐produced macrodeformation in the form of fracturing and breccia veining at the microscopic scale. Contrary to earlier reports that these breccias represent pseudotachylite (friction melt) or even shock/shear‐produced pseudotachylitic melt breccia cannot be confirmed due to pervasive post‐impact alteration. Abundant planar deformation features (PDFs) in quartz, in addition to diaplectic glass and partial isotropization, are the main shock deformation effects observed, confirming that Woodleigh is of impact origin. Over the investigated depth interval, the statistics of quartz grains with a variable number of sets of PDFs does not change significantly, and the patterns of crystallographic orientations of PDFs in randomly selected quartz grains does not indicate a change in absolute shock pressure with depth either. The value of oxygen isotopes for the recognition of meteoritic contamination, as proposed by earlier Woodleigh workers, is critically assessed. Neither INA nor PGE analyses of our samples support the presence of a meteoritic component within this basement section, as had been claimed in earlier work.     

Electron acceleration in solar flares

For the period September 1978 to December 1982 we have identified 55 solar flare particle events for which our instruments on board the ISEE-3 (ICE) spacecraft detected electrons above 10 MeV. Combining our data with those from the ULEWAT spectrometer (MPI Garching and University of Maryland) electron spectra in the range from 0.1 to 100 MeV were obtained. The observed spectral shapes can be divided into two classes. The spectra of the one class can be fit by a single power law in rigidity over the entire observed range. The spectra of the other class deviate from a power law, instead exhibiting a steepening at low rigidities and a flattening at high rigidities. Events with power-law spectra are associated with impulsive (<1 hr duration) soft X-ray emission, whereas events with hardening spectra are associated with long-duration (<1 hr) soft X-ray emission. The characteristics of long-duration events are consistent with diffusive shock acceleration taking place high in the corona. Electron spectra of short-duration flares are well reproduced by the distribution functions derived from a model assuming simultaneous second-order Fermi acceleration and Coulomb losses operating in closed flare loops.     

A taxonomic survey of comet composition 1985-2004 using CCD spectroscopy

A summary is presented of our spectroscopic survey of comets extending for roughly 19 years from 1985 to 2004 comprising data for 92 comets of which 50 showed good emissions. All data were re-analyzed using consistent reduction techniques. Our observations of comets over several apparitions and comets observed over an extended period indicate no major changes in compositional classification. To our regret, no major unidentified cometary features were found in our surveyed spectral region of 5200-10400 Å. Absolute production rates for the dominant parent molecule H₂O and the daughter species C₂, NH₂ and CN are determined within the limits of the Haser model as are values for the dust continuum, Afρ. From these data, production rate ratios are calculated for C₂/H₂O, NH₂/H₂O, CN/H₂O and Afρ/H₂O. Excluding the odd Comets Yanaka (1988r), 43P/Wolf-Harrington and 19P/Borrelly, with unusual spectra, our set of comets exhibited relatively uniform composition. Detailed analyses of our data resulted in four taxonomic classes:

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Comets of typical composition (∼70%); exhibiting typical ratios with respect to water of C₂, NH₂, and CN.

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Tempel 1 type (∼22%); having a deficiency in C₂ but normal NH₂ abundance.

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G-Z type (∼6%); having both low C₂ and NH₂ ratios.

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The unusual object Yanaka (1988r) (∼2%?); no detectable C₂ or CN emission but normal NH₂.

It is uncertain whether there is a clear separation between the comets of typical composition and those with C₂ depletion, or whether the latter consists of a group showing a continuum of decreasing C₂/CN ratios. Our spectroscopic investigations result in a visual record of the various compositional classes, which are illustrated in a number of figures. Production rate comparisons with the comet photometry program of Schleicher and A'Hearn [A'Hearn, M.F., and 4 colleagues, 1995. Icarus 118, 223-270] for 13 comets in common yielded good agreement once the different scale lengths are taken into account. An investigation into the possible origin of our compositional groups with respect to dynamical families of comets shows that the Halley family exhibits essentially no C₂ depletion. These objects were presumably formed in the region of Saturn and Uranus and scattered into the Oort cloud. Comets formed in the space near Neptune, responsible for the scattered Kuiper Belt show a mixture of “typical” and C₂ depleted objects, while we associate comets formed in-situ in the classical Kuiper belt with our C₂ depleted group.     

Role of electrochemical reactions in pressure solution

Using a Surface Forces Apparatus we have measured changes in the electrical potential difference between quartz and mica surfaces that correlate with the changing quartz dissolution rate when surfaces are pressed together at relatively low pressures (2-3 atm) in aqueous electrolyte solutions of 30 mM CaCl₂ at 25 °C. No detectable dissolution or voltage potential difference is measured in symmetrical systems (e.g. mica-mica or quartz-quartz) or between dry surfaces subjected to similar pressures, indicating that the dissolution can not be attributed to a simple pressure effect, slow aging (creep), or plastic deformation of the quartz surface. In quartz-mica systems brought together under pressure or to close proximity in electrolyte solution, the onset of quartz dissolution is marked by a sudden, rapid decrease in the quartz thickness at initial rates in the range from 1 to 4 nm/min, which after several hours settles into a constant rate of approximately 0.01 nm/min (∼5 μm/yr). Concomitantly, the potential drops to a constant value once the dissolution rate has stabilized. The decrease in the decay rate is interpreted as being due to saturation of the confined aqueous film and/or to the buildup of a Stern layer on the quartz surface, and the constant rate as being due to the steady-state chemical dissolution and diffusion of the dissolving silica into the surrounding reservoir. The dissolution is ‘non-uniform’: the surfaces become rough as dissolution proceeds, with the appearance of pits in a manner analogous to corrosion. On occasions, the process of rapid dissolution followed by a gradual transition to steady dissolution repeats itself, suggesting that the pit structure and Stern layer are fragile and subject to collapse and/or expulsion from the gap. Preliminary experiments on the dissolution of multi-faceted milled quartz particles (∼1.0 μm diameter) compressed between two muscovite surfaces suggest an asymmetry in the dissolution rates at different crystallographic planes. The origin of the electrical potential is interpreted as arising from the overlapping of the electric double-layers of two dissimilar surfaces when they are forced into close proximity. This electrical potential difference, for as yet unknown reasons, appears to be the driving force for the dissolution, rather than pressure.     

The variation and controls of mean transit times in Australian headwater catchments

Determining mean transit times in headwater catchments is critical for understanding catchment functioning and understanding their responses to changes in landuse or climate. Determining whether mean transit times (MTTs) correlate with drainage density, slope angle, area, or land cover permits a better understanding of the controls on water flow through catchments and allows first-order predictions of MTTs in other catchments to be made. This study assesses whether there are identifiable controls on MTTs determined using ³H in headwater catchments of southeast Australia. Despite MTTs at baseflow varying from a few years to >100 years, it was difficult to predict MTTs using single or groups of readily-measured catchment attributes. The lack of readily-identifiable correlations hampers the prediction of MTTs in adjacent catchments even where these have similar geology, land use, and topography. The long MTTs of the Australian headwater catchments are probably in part due to the catchments having high storage volumes in deeply-weathered regolith, combined with low recharge rates due to high evapotranspiration. However, the difficulty in estimating storage volumes at the catchment scale hampers the use of this parameter to estimate MTTs. The runoff coefficient (the fraction of rainfall exported via the stream) is probably also controlled by evapotranspiration and recharge rates. Correlations between the runoff coefficient and MTTs in individual catchments allow predictions of MTTs in nearby catchments to be made. MTTs are shorter in high rainfall periods as the catchments wet up and shallow water stores are mobilized. Despite the contribution of younger water, the major ion geochemistry in individual catchments commonly does not correlate with MTTs, probably reflecting heterogeneous reactions and varying degrees of evapotranspiration. Documenting MTTs in catchments with high storage volumes and/or low recharge rates elsewhere is important for understanding MTTs in diverse environments.