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Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Observations of 433 Eros from 1.25 to 3.35 microns
A. S. Rivkin* and B. E. Clark
*Correspondence author's address: Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA; e-mail address: asrivkin@MIT.edu
Abstract–We have spectrophotometrically observed 433 Eros, the
target of the NEAR Shoemaker spacecraft, on 4 December 1995 from 1.25 to 3.35 um. As expected, Eros shows no
evidence of an absorption feature greater than 5% in the
3 um region, and is interpreted to have an anhydrous surface within observational uncertainties. Our observations
in the JHK region agree with previous work by Chapman
and Morrison (1976) and Murchie and Pieters (1996), but
differ from the NIS spectra reported by Clark et al. (2001).
Our calculations indicate that thermal flux from Eros is not
responsible for this mismatch.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
The Scale Size of Chondrule Formation Regions: Constraints Imposed by Chondrule Cooling Rates
Lon L. Hood* and Fred J. Ciesla
*Correspondence author's address: Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721-0092, USA; e-mail address: lon@lpl.arizona.edu
Abstract–Meteoritic data strongly suggest that most chondrules reached maximum temperatures in a range of 1650 - 2000 K and cooled at relatively slow rates of 100 - 1000 K/hour, implying a persistence of external energy supply. The presence of fine-grained rims around chondrules in most unequilibrated chondrites also indicates that a significant quantity of micron-sized dust was present inchondrule formation regions. Here, we assume that the persistent external energy source needed to explain chondrule cooling rates consists primarily of radiation from surrounding heated chondrules, fine dust, and gas after the formation event. Using an approximate one-dimensional numerical model for the outward diffusion of thermal radiation from such a system, the scale sizes of formation regions required to yield acceptable cooling rates are determined for a range of possible chondrule, dust, and gas parameters. Results show that the inferred scale sizes depend sensitively on the number densities of micron-sized dust and on their adopted optical properties. In the absence of dust, scale sizes >1000 km are required for plausible maximum chondrule number densities and heated gas parameters. In the presence of dust with mass densities comparable to those of the chondrules and with absorptivities and emissivities of ~0.01 calculated for Mie spheres with a pure mineral composition, scale sizes as small as ~100 km are possible. If dust absorptivities and emissivities approach unity (as may occur for particles with more realistic shapes and compositions), then scale sizes as small as ~10 km are possible. Considering all uncertainties in model parameters, it is concluded that small scale sizes (10-100 km) for chondrule formation regions are allowed by the experimentally inferred cooling rates.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
The composition of 433 Eros: A mineralogical-chemical synthesis
T.J. McCoy*, T.H. Burbine, L.A. McFadden, R.D. Starr, M.J. Gaffey, L.R. Nittler, L.G. Evans, N. Izenberg, P. Lucey, J.I. Trombka, J.F. Bell III, B.E. Clark, P.E. Clark, S.W. Squyres, C.R. Chapman, W.V. Boynton and J. Veverka
*Correspondence author's address: Dept. of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560-0119 USA ; e-mail address: mccoy.tim@nmh.si.edu
Abstract–The Near Earth Asteroid Rendezvous (NEAR) mission carried X-ray/gamma-ray spectrometers and multi-spectral imager/near-infrared spectrometer instrument packages which give complementary information on the chemistry and mineralogy, respectively, of the target asteroid 433 Eros. Synthesis of these two data sets provides information not available from either alone, including the abundance of non-mafic silicates, metal and sulfide minerals. We have utilized four techniques to synthesize these data sets. Venn diagrams, which examine overlapping features in two data sets, suggest that the best match for 433 Eros is an ordinary chondrite, altered at the surface of the asteroid, or perhaps a primitive achondrite derived from material mineralogically similar to these chondrites. Normalized element distributions preclude FeO-rich pyroxenes and suggest that the X-ray and gamma-ray data can be reconciled with a common silicate mineralogy by inclusion of varying amounts of metal. Normative mineralogy cannot be applied to these data sets owing to uncertainties in oxygen abundance and lack of any constraints on the abundance of sodium. Matrix inversion for simultaneous solution of mineral abundances yields reasonable results for the X-ray-derived bulk composition, but seems to confirm the inconsistency between mineral compositions and opx:cpx ratios. A unique solution does not seem possible in synthesizing these multiple data sets. Future missions including a lander to fully characterize regolith distribution and sample return would resolve the types of problems faced in synthesizing the NEAR data.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Importance of Space Weathering Simulation Products in Compositional Modeling of Asteroids: 349 Dembowska and 446 Aeternitas as Examples
Takahiro Hiroi and Sho Sasaki
*Correspondence author's address: Department of Geological Sciences, Brown University, Providence, Rhode Island, USA; e-mail address: takahiro_hiroi@brown.edu
Abstract–Based on recent progress in simulating space weathering on asteroids using pulse-laser irradiation onto olivine and orthopyroxene samples, detailed analyses of two of the A and R type asteroid reflectance spectra have been performed using reflectance spectra of laser-treated samples. The visible-NIR spectrum of olivine is more altered than that of pyroxene at the same pulse-laser energy, suggesting that olivine weathers more rapidly than orthopyroxene in space. The same trend can be detected from reflectance spectra of the asteroids, where the more olivine an asteroid has, the redder its 1-um band continuum can become. Comparison of the 1-um band continuum slope and the 2-um / 1-um band area ratio between the asteroids and olivine and pyroxene samples (including the laser-treated ones) suggests that asteroids may be limited in the degree of space weathering they can exhibit, possibly due to the short life of their surface regolith. Their pyroxenes may also have a limited chemical composition range. Fitting the visible continuum shape and other parts of the spectra (especially the 2-um part) has been impossible with any combination of common rock-forming minerals such as silicates and metallic irons. However, this study shows, for the first time, excellent fits of reflectance spectra of an A asteroid (Aeternitas) and an R asteroid (Dembowska), including their visible spectral curves, band depths and shapes, and overall continuum shapes. Our results provide estimates that Aeternitas consists of 2% fresh olivine, 93% space-weathered olivine, 1% space-weathered orthopyroxene, and 4% chromite, and that Dembowska consists of 1% fresh olivine, 55% space-weathered olivine, and 44% space-weathered orthopyroxene. These results suggest that space weathering effects may be important to the interpretation of asteroid reflectance spectra, even those with deep silicate absorption bands. Modified Gaussian model deconvolutions of the laser-irradiated olivine samples show that their identity as olivine remained. The most recent submicroscopic mineralogical analyses have revealed that the laser-irradiated olivine samples contain nanophase iron particles similar to those in space-weathered lunar samples.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Space Weathering on Eros: Constraints from Albedo and Spectral Measurements of Psyche Crater
Beth E. Clark*, Paul Lucey, P. Helfenstein, J.F. Bell III, C. Peterson, J. Veverka, T. McConnochie, M.S. Robinson, B. Bussey, S.L. Murchie, N.I. Izenberg and C.R. Chapman
*Correspondence author's address: Beth Ellen Clark, 308 Space Sciences, Cornell University, Center for Radiophysics and Space Research, Ithaca, NY 14853, USA; e-mail address: beth@astrosun.tn.cornell.edu
Abstract–We present combined Multi-Spectral Imager (MSI) (0.95 um) and Near-Infrared Spectrometer (NIS) (0.8 to 2.4 um) observations of Psyche crater on S-type asteroid 433 Eros obtained by the Near-Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft. At 5.3 km in diameter, Psyche is one of the largest craters on Eros which exhibit distinctive brightness patterns consistent with downslope motion of dark regolith material overlying a substrate of brighter material. At spatial scales of 620 m/spectrum, Psyche crater wall materials exhibit albedo contrasts of 32-40% at 0.946 um. Associated spectral variations occur at a much lower level of 4-8% (+2-4%). We report results of scattering model and lunar analogy investigations into several possible causes for these albedo and spectral trends: grain size differences, olivine, pyroxene, and troilite variations, and optical surface maturation. We find that the albedo contrasts in Psyche crater are not consistent with a cause due solely to variations in grain size, olivine, pyroxene or lunar-like optical maturation. A grain size change sufficient to explain the observed albedo contrasts would result in strong color variations that are not observed. Olivine and pyroxene variations would produce strong band-correlated variations that are not observed. A simple lunar-like optical maturation effect would produce strong reddening that is not observed. The contrasts and associated spectral variation trends are most consistent with a combination of enhanced troilite (a dark spectrally neutral component simulating optical effects of shock) and lunar-like optical maturation. These results suggest that space weathering processes may affect the spectral properties of Eros materials, causing surface exposures to differ optically from subsurface bedrock. However, there are significant spectral differences between Eros' proposed analog meteorites (ordinary chondrites and/or primitive achondrites), and Eros' freshest exposures of subsurface bright materials. After accounting for all differences in the measurement units of our reflectance comparisons, we have found that the bright materials on Eros have reflectance values at 0.946 um consistent with meteorites, but spectral continua that are much redder than meteorites from 1.5 to 2.4 um. Most importantly, we calculate that average Eros surface materials are 30-40% darker than meteorites.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Leonid fluxes from 1994-1998 activity patterns
Josep M. Trigo-Rodriguez*, Juan Fabregat and Jordi Llorca
*Correspondence author's address: Depto. Ciències Experimentals, Universitat Jaume I., 12071 Castello, Spain; e-mail address: trigo@exp.uji.es
Abstract–The Leonid shower was observed in November 1998 worldwide in an intensive campaign without precedent. During this international effort near 35,500 meteors were reported by members and collaborators of the International Meteor Organization (IMO) using a standard methodology. Despite the absence of a meteor storm in 1998, the rich observational data allows to obtain a detailed unprecedented knowledge of the stream structure between 1994-1998.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Elemental composition from gamma-ray spectroscopy of the NEAR-Shoemaker landing site on 433 Eros
Larry G. Evans*, Richard D. Starr, Johannes Brückner, Robert C. Reedy, William. V. Boynton, Jacob I. Trombka, John O. Goldsten, Jozef Masarik, Larry R. Nittler, and Timothy J. McCoy
*Correspondence author's address: Science Programs, Computer Sciences Corporation, Lanham-Seabrook, Maryland 20706 USA; e-mail address: larry.evans@gsfc.nasa.gov
Abstract–Elemental composition and composition ratios derived from gamma-ray measurements collected by the NEAR-Shoemaker spacecraft while on the surface of 433 Eros are reported. Performance of the gamma-ray spectrometer (GRS) during cruise and orbit is reviewed. The best gamma-ray data were collected on the surface of Eros after the spacecraft's controlled descent on 12 February 2001. Methods used in spectral analysis, to convert peak areas to incident photons, and photons to elemental composition are described in some detail. The elemental abundance of K and the Mg/Si, Fe/Si, Si/O and Fe/O abundance ratios were determined. The Mg/Si and Si/O ratios and the K abundance are roughly chondritic, but the Fe/Si and Fe/O ratios are low compared to expected chondritic values. Three possible explanations for the apparent Fe depletion are considered.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
X-ray fluorescence measurements of the surface elemental composition of asteroid 433 Eros
Larry R. Nittler*, Richard D. Starr, Lucy Lim, Timothy J. McCoy, Thomas H. Burbine, Robert C. Reedy, Jacob I. Trombka, Paul Gorenstein, Steven W. Squyres, William V. Boynton, Timothy P. McClanahan, Jesse S. Bhangoo, Pamela E. Clark, Mary Ellen Murphy and Rosemary Killen
*Correspondence author's address: Goddard Space Flight Center, Code 691, Greenbelt, Maryland 20771, USA; e-mail address: lrn@dtm.ciw.edu
Abstract–We report major-element ratios determined for the S-class asteroid 433 Eros using remote-sensing x-ray fluorescence spectroscopy with the Near Earth Asteroid Rendezvous Shoemaker x-ray spectrometer. Data analysis techniques and systematic errors are described in detail. Data acquired during five solar flares and during two extended "quiet Sun" periods are presented; these results sample a representative portion of the asteroid's surface. Although systematic uncertainties are potentially large, the most internally consistent and plausible interpretation of the data is that Eros has primitive Mg/Si, Al/Si, Ca/Si and Fe/Si ratios, closely similar to H or R chondrites. Global differentiation of the asteroid is ruled out. The S/Si ratio is much lower than that of chondrites, probably reflecting impact-induced volatilization and/or photo- or ion-induced sputtering of sulfur at the surface of the asteroid. An alternative explanation for the low S/Si ratio is that it reflects a limited degree of melting with loss of an FeS-rich partial melt. Size-sorting processes could lead to segregation of Fe-Ni metal from silicates within the regolith of Eros; this could indicate that the Fe/Si ratios determined by the XRS are not representative of the bulk Eros composition.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Mineralogical Interpretation of Reflectance Spectra of Eros from NEAR NIS Low Phase Flyby
Lucy A. McFadden*, Dennis D. Wellnitz, Matthew Schnaubelt, Michael J. Gaffey, J. F. Bell, III, Noam Izenberg, Scott Murchie and Clark R. Chapman
*Correspondence author's address: University of Maryland, Astronomy Department, College Park, Maryland 20742, USA; e-mail address: mcfadden@astro.umd.edu
Abstract–High Signal-to-Noise Near-Infrared Spectrometer (NIS) spectra acquired during the Low Phase Flyby of the NEAR mission to 433 Eros are analyzed to determine mineral chemistry and proportions of mafic silicates across the asteroid's surface at 2.68 x 5.50 km spatial resolution. Spectral band parameters are derived, and compared with those of laboratory samples of known mineral composition, grain size distribution and terrestrial, meteoritic and lunar pyroxene spectral properties. The NIS derived band parameters are consistent with ordinary chondrite meteorites. We invoke the presence of a clinopyroxene component in the spectra which is consistent with ordinary chondrite mineralogy and/or some degree of partial melting of ordinary chondritic material. Spectra measured across the surface of Eros can reveal small but real spectral variations. Most relative spectra are uniform to within 1-2%. Some areas suggest compositional variations of a few percent Spectral slope variations of a few percent are seen indicating a non-uniform distribution of materials affecting the slope parameter but with no resolved absorption bands. We find no correlation of slope with viewing geometry or compositional variation . The band parameter values do not consistently indicate a specific ordinary chondrite class but Eros is definitely undifferentiated with possible compositional variations of no more than 1-2%.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
The Near-Shoemaker XGRS Experiment: Overview And Lessons Learned
J. I. Trombka*, L. R. Nittler, R. D. Starr, L. G. Evans, T. J. McCoy, W. V. Boynton, T. H. Burbine, J. Brückner, P. Gorenstein, S. W. Squyres, R. C. Reedy, J. O. Goldsten, L. Lim, K. Hurley, P. E. Clark, S. R. Floyd, T. P. McClanahan, E. McCartney, J. Branscomb, J. S. Bhangoo, I. Mikheeva, and M. E. Murphy
*Correspondence author's address: Goddard Space Flight Center, Laboratory for Extraterrestrial Physics, Code 691, Greenbelt, Maryland 20771, USA; e-mail address: u1jit@lepvx3.gsfc.nasa.gov
Abstract–The Near Earth Asteroid Rendezvous (NEAR) Shoemaker remote sensing X-Ray/Gamma-Ray Spectrometer (XGRS) completed more than a year of operation in orbit and on the surface of 433 Eros. Elemental compositions for a number of regions on the surface of Eros have been derived from analyses of the characteristic x-ray and gamma-ray emission spectra. The NEAR XGRS detection system was included as part of the Inter-Planetary Network (IPN) for the detection and localization of gamma-ray bursts (GRBs). Preliminary results for both the elemental composition of the surface of Eros and the detection of GRBs have been obtained. In addition to the science results, the design and operation of the NEAR XGRS is considered. Significant information important for the design of future remote sensing XGRS systems has been obtained and evaluated. We focus on four factors that became particularly critical during NEAR: (1) overall spacecraft design, (2) selection of materials, (3) increase of the signal-to-noise ratio and (4) knowledge of the incident solar x-ray spectrum.
Meteoritics & Planetary Science 36 (2001)
© Meteoritical Society, 2001. Printed in USA.
Radar constraints on asteroid regolith properties using 433 Eros as ground truth
Christopher Magri*, Guy J. Consolmagno, S. J., Steven J. Ostro, Lance A. M. Benner and Brett R. Beeney
*Correspondence author's address: University of Maine at Farmington, 173 High Street, Preble Hall, Farmington, Maine 04938, USA; e-mail address: magri@maine.edu
Abstract–Radar data enable us to estimate an asteroid's near-surface bulk density, thus providing a joint constraint on near-surface porosity and solid density. We investigate two different approaches to simplifying this joint constraint: estimating solid densities by assuming uniform porosities for all asteroids; and estimating porosities by assuming uniform mineralogy within each taxonomic class. Methods used to estimate asteroids' near-surface solid densities from radar data have not previously been calibrated via independent estimates. Recent spacecraft results on the chondritic nature of 433 Eros now permit such a check, and also support porosity estimation for S-class objects.
We use radar albedos and polarization ratios estimated for 36 main-belt asteroids and 9 near-Earth asteroids to estimate near-surface solid densities using two methods, one of which is similar to the uncalibrated algorithms used in previous studies, the other of which treats Eros as a calibrator. We also derive porosities for the same sample by assigning solid densities for each taxonomic class in advance. Density-estimation results obtained for Eros itself are consistent with the uncalibrated method being valid in the mean; those derived for the full sample imply that uncalibrated solid densities are, at most, a few tens of percent too large on average. However, some derived densities are extremely low, whereas most porosity estimates are physically plausible. We discuss the relative merits of these two approaches.
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