•Eleven of 29M-class asteroids have high radar albedos consistent with metal.•More than half of high-radar-albedo asteroids have a 3μm hydration feature.•Many in our sample have bifurcated radar ...echoes indicating a complex structure.
Using the S-band radar at Arecibo Observatory, we observed thirteen X/M-class asteroids; nine were previously undetected and four were re-observed, bringing the total number of Tholen X/M-class asteroids observed with radar to 29. Of these 29M-class asteroids, 13 are also W-class, defined as M-class objects that also display a 3-μm absorption feature which is often interpreted as the signature of hydrated minerals (Jones, T.D., Lebofsky, L.A., Lewis, J.S., Marley, M.S. 1990. Icarus 88, 172–192; Rivkin, A.S., Howell, E.S., Britt, D.T., Lebofsky, L.A., Nolan, M.C., Branston, D.D. 1995. Icarus 117, 90–100; Rivkin, A.S., Howell, E.S., Lebofsky, L.A., Clark, B.E., Britt, D.T. 2000. Icarus 145, 351–368).
Consistent with our previous work (Shepard, M.K. et al. 2008. Icarus 195, 184–205; Shepard, M.K., Harris, A.W., Taylor, P.A., Clark, B.E., Ockert-Bell, M., Nolan, M.C., Howell, E.S., Magri, C., Giorgini, J.D., Benner, L.A.M. 2011. Icarus 215, 547–551), we find that 38% of our sample (11 of 29) have radar albedos consistent with metal-dominated compositions. With the exception of 83 Beatrix and 572 Rebekka, the remaining objects have radar albedos significantly higher than the mean S- or C-class asteroid (Magri, C., Nolan, M.C., Ostro, S.J., Giorgini, J.D. 2007. Icarus 186, 126–151).
Seven of the eleven high-radar-albedo asteroids, or 64%, also display a 3-μm absorption feature (W-class) which is thought to be inconsistent with the formation of a metal dominated asteroid. We suggest that the hydration absorption could be a secondary feature caused by low-velocity collisions with hydrated asteroids, such as CI or CM analogs, and subsequent implantation of the hydrated minerals into the upper regolith. There is recent evidence for this process on Vesta (Reddy, V. et al. 2012. Icarus 221, 544–559; McCord, T.B. et al. 2012. Nature 491, 83–86; Prettyman, T.H. et al. 2012. Science 338, 242–246; Denevi, B.W. et al. 2012. Science 338, 246–249).
Eleven members of our sample show bifurcated radar echoes at some rotation phases; eight of these are high radar albedo targets. One interpretation of a bifurcated echo is a contact binary, like 216 Kleopatra, and several of our sample are contact binary candidates. However, evidence for other targets indicates they are not contact binaries. Instead, we hypothesize that these asteroids may have large-scale variations in surface bulk density, i.e. isolated patches of metal-rich and silicate-rich regions at the near-surface, possibly the result of collisions between metal and silicate-rich asteroids.
Radar observations of 214 near-Earth asteroids (NEAs) reveal a very strong correlation of circular polarization ratio with visible–infrared taxonomic class, establishing distinct differences in the ...centimeter-to-several-decimeter structural complexity of objects in different spectral classes. The correlation may be due to the intrinsic mechanical properties of different mineralogical assemblages but also may reflect very different formation ages and collisional histories. The highest ratios are measured for groups associated with achondritic igneous rocky meteorites: the E class, whose parent body may be 3103 Eger, and the V class, derived from the mainbelt asteroid (and Dawn mission target) 4 Vesta.
► Photometric parameters of soils modeled using new 400–950nm goniometer data. ► Mars analogs backscattering; some lunar analogs very narrowly forward-scattering. ► Spherule soil experiment mimics ...some MER Opportunity soil photometric parameters. ► Phase reddening seen at angles <50°; some phase bluing at >50° (<10° for mare soil). ► No lunar analog soil perfectly matches the Apollo soils’ photometric parameters.
We present new visible/near-infrared multispectral reflectance measurements of seven lunar soil simulants, two Apollo soils, and eight martian analog samples as functions of illumination and emission angles using the Bloomsburg University Goniometer. By modeling these data with Hapke theory, we provide constraints on photometric parameters (single scattering albedo, phase function parameters, macroscopic roughness, and opposition effect parameters) to provide additional “ground truth” photometric properties to assist analyses of spacecraft data. A wide range of modeled photometric properties were variably related to albedo, color, grain size, and surface texture. Finer-grained samples here have high single-scattering albedo values compared to their coarser-grained counterparts, as well as lower macroscopic roughness values. The Mars analog samples and Apollo soils exhibit slightly lower opposition effect width parameter values than the lunar analogs, whereas the opposition effect magnitude is not well constrained by the models. The Mars analog soils are typically relatively backscattering and consistent with fairly rough particles with a moderate density of internal scatterers, similar to the in situ observations of some soils by the Mars Exploration Rover (MER) Spirit. Some lunar analog soil models result in moderately-forward scattering behaviors, as do the two Apollo soils. Other fine-grained and/or glass-rich lunar analog samples populate a narrowly forward-scattering regime similar to model results from observations of some rover tracks observed by the MER Opportunity rover and some dust-poor “gray” rocks by the Spirit rover. An experiment to mimic the spherule-rich soils observed by Opportunity demonstrated a large decrease in single-scattering albedo compared to spherule-free soil surfaces, as well as increased surface roughness, narrow opposition effects, and a significant increase in backscattering, similar to some of the Opportunity soils. Phase reddening effects are documented in many soils as an increase in near-infrared/visible ratios with phase angle. Some samples exhibit falloffs in these ratio phase curves at phase angles beyond 50–80° that are likely related to an increased importance of surface scattering at high phase angles. None of the lunar analog soils perfectly match the modeled photometric parameters of the two Apollo soils. The phase reddening nature of the mare soil included an upturn in ratio values at phase angles <10° that was not observed for the highland sample. It remains to be verified whether this is a consistent observation between mare and highland samples.
A radar survey of M- and X-class asteroids Shepard, Michael K.; Clark, Beth Ellen; Nolan, Michael C. ...
Icarus (New York, N.Y. 1962),
05/2008, Letnik:
195, Številka:
1
Journal Article
Recenzirano
We observed ten M- and X-class main-belt asteroids with the Arecibo Observatory's S-band (12.6 cm) radar. The X-class asteroids were targeted based on their albedos or other properties which ...suggested they might be M-class. This work brings the total number of main-belt M-class asteroids observed with radar to 14. We find that three of these asteroids have rotation rates significantly different from what was previously reported. Based on their high radar albedo, we find that only four of the fourteen—16 Psyche, 216 Kleopatra, 758 Mancunia, and 785 Zwetana—are almost certainly metallic. 129 Antigone has a moderately high radar albedo and we suggest it may be a CH/CB/Bencubbinite parent body. Three other asteroids, 97 Klotho, 224 Oceana, and 796 Sarita have radar albedos significantly higher than the average main belt asteroid and we cannot rule out a significant metal content for them. Five of our target asteroids, 16 Psyche, 129 Antigone, 135 Hertha, 758 Mancunia, and 785 Zwetana, show variations in their radar albedo with rotation. We can rule out shape and composition in most cases, leaving variations in thickness, porosity, or surface roughness of the regolith to be the most likely causes. With the exception of 129 Antigone, we find no hydrated M-class asteroids (W-class; Rivkin, A.S., Howell, E.S., Lebofsky, L.A., Clark, B.E., Britt, D.T., 2000. Icarus 145, 351–368) to have high radar albedos.
Predictions of two widely-used regolith reflectance models, a numerically exact computer code and an approximate analytic equation, based on the equation of radiative transfer were tested against the ...measured reflectance of a medium of close-packed spheres, whose properties supposedly can be well-characterized. Surprisingly, the approximate analytic model was a better match to the experimental data than the numerically exact computer solution. Other approximate regolith models were tested briefly with similar results. Discrepancies between the two models and between models and experiment can be explained if the phase functions and albedos of the spheres are not the same as when the particles are isolated. Differences include the absence of the Fraunhoffer diffraction peak, which is an intrinsic assumption of the approximate analytical model but not the exact numerical model, and increased scattering in the mid-range of phase angles, which the approximate analytic model fortuitously describes more accurately than the exact numerical model. These changes may be caused by the close proximity of surrounding particles. If they are taken into account, models based on the radiative transfer equation appear able to quantitatively predict the reflectances of regoliths and other particulate media. Interparticle perturbations are also predicted to cause a coherent backscatter opposition effect in the backward direction that was observed, but its angular width was found to be much larger than predicted by theories for sparsely-packed media.
During the third flyby of Mercury by the MESSENGER spacecraft, a dedicated disk-integrated photometric sequence was acquired with the wide-angle multispectral camera to observe Mercury's global ...photometric behavior in 11 spectral filters over as broad a range of phase angle as possible within the geometric constraints of the flyby. Extraction of disk-integrated measurements from images acquired during this sequence required careful accounting for scattered light and residual background effects. The photometric model fit to these measurements is shown to fit observed radiances at phase angles below 110°, possibly except where both solar incidence and emission angles are high (>70°). The complexity of the scattered light at wavelengths greater than 828
nm contributes to a less accurate photometric correction at these wavelengths. The model is used to correct the global imaging data set acquired at a variety of geometries to a common geometry of incidence angle=30°, emission angle=0°, and phase angle=30°, yielding a relatively seamless mosaic. The results here will be used to correct image mosaics of Mercury acquired in orbit.
► A dedicated photometric image sequence was acquired during MESSENGER's third Mercury flyby. ► This sequence provided sufficient observations to derive photometric corrections to the imaging data. ► The corrections enable the construction of useful global color mosaics.
Laboratory visible/near‐infrared multispectral observations of Mars Exploration Rover Pancam calibration target materials coated with different thicknesses of Mars spectral analog dust were acquired ...under variable illumination geometries using the Bloomsburg University Goniometer. The data were fit with a two‐layer radiative transfer model that combines a Hapke formulation for the dust with measured values of the substrate interpolated using a He‐Torrance approach. We first determined the single‐scattering albedo, phase function, opposition effect width, and amplitude for the dust using the entire data set (six coating thicknesses, three substrates, four wavelengths, and phase angles 3°–117°). The dust exhibited single‐scattering albedo values similar to other Mars analog soils and to Mars Pathfinder dust and a dominantly forward scattering behavior whose scattering lobe became narrower at longer wavelengths. Opacity values for each dust thickness corresponded well to those predicted from the particles sizes of the Mars analog dust. We then restricted the number of substrates, dust thicknesses, and incidence angles input to the model. The results suggest that the dust properties are best characterized when using substrates whose reflectances are brighter and darker than those of the deposited dust and data that span a wide range of dust thicknesses. The model also determined the dust photometric properties relatively well despite limitations placed on the range of incidence angles. The model presented here will help determine the photometric properties of dust deposited on the MER rovers and to track the multiple episodes of dust deposition and erosion that have occurred at both landing sites.
•Radar confirms asteroid 16 Psyche to be the largest metal asteroid in the main belt.•A Psyche shape model was generated using radar, adaptive optics, and occultation data.•The shape model of Psyche ...shows evidence of large scale features.•Psyche displays significant variations in radar and optical albedo with rotation.
Using the S-band radar at Arecibo Observatory, we observed 16 Psyche, the largest M-class asteroid in the main belt. We obtained 18 radar imaging and 6 continuous wave runs in November and December 2015, and combined these with 16 continuous wave runs from 2005 and 6 recent adaptive-optics (AO) images (Drummond et al., 2016) to generate a three-dimensional shape model of Psyche. Our model is consistent with a previously published AO image (Hanus et al., 2013) and three multi-chord occultations. Our shape model has dimensions 279 × 232 × 189km (± 10%), Deff= 226 ± 23km, and is 6% larger than, but within the uncertainties of, the most recently published size and shape model generated from the inversion of lightcurves (Hanus et al., 2013). Psyche is roughly ellipsoidal but displays a mass-deficit over a region spanning 90° of longitude. There is also evidence for two ∼50–70km wide depressions near its south pole. Our size and published masses lead to an overall bulk density estimate of 4500 ± 1400kgm−3. Psyche's mean radar albedo of 0.37 ± 0.09 is consistent with a near-surface regolith composed largely of iron-nickel and ∼40% porosity. Its radar reflectivity varies by a factor of 1.6 as the asteroid rotates, suggesting global variations in metal abundance or bulk density in the near surface. The variations in radar albedo appear to correlate with large and small-scale shape features. Our size and Psyche's published absolute magnitude lead to an optical albedo of pv= 0.15 ± 0.03, and there is evidence for albedo variegations that correlate with shape features.
We have reanalyzed three sets of Viking Lander 1 and 2 (VL1 and VL2) images of the Martian atmosphere to better evaluate the radiative properties of the atmospheric dust particles. The properties of ...interest are the first two moments of the size distribution, the single-scattering albedo, the dust single-scattering phase function, and the imaginary index of refraction. These properties provide a good definition of the influence that the atmospheric dust has on heating of the atmosphere. Our analysis represents a significant improvement over past analyses (Pollack et al. 1977, 1979) by deriving more accurate brightness closer to the sun, by carrying out more precise analyses of the data to acquire the quantities of interest, and by using a better representation of scattering by nonspherical particles. The improvements allow us to better define the diffraction peak and hence the size distribution of the particles. For a lognormal particle size distribution, the first two moments of the size distribution, weighted by the geometric cross section, are found. The geometric cross-section weighted mean radius r(sub eff) is found to be 1.85 +/- 0.3 micrometers at VL2 during northern summer when dust loading was low and 1.52 +/- 0.3 micrometers at VL1 during the first dust storm. In both cases the best cross-section weighted mean variance nu(sub eff) of the size distribution is equal to 0.5 +/- 0.2 micrometers. The changes in size distribution, and thus radiative properties, do not represent a substantial change in solar energy deposition in the atmosphere over the Pollak et al. (1977, 1979) estimates.
•Comprehensive BUG BRDF measurements of Apollo 11 and 16 lunar soil samples.•Determined solar spectrum averaged hemispheric albedo versus incidence angle.•Compared Orbital Diviner temperatures to ...models based on BRDF measurements.
We have acquired a comprehensive laboratory bidirectional measurements of Apollo 11 and Apollo 16 lunar soil samples and have successfully fit photometric models to the laboratory data and have determined the solar spectrum averaged hemispheric reflectance as a function of incidence angle. The Apollo 11 (sample 10,084) and 16 (sample 68,810) soil samples are two representative end member samples from the Moon, dark lunar maria and bright lunar highlands. We used our solar spectrum averaged albedos in a thermal model and compared our model-calculated normal bolometric infrared emission curves with those measured by the LRO Diviner Lunar Radiometer Experiment. We found excellent agreement at the Apollo 11 site, but at the Apollo 16 site, we found that the albedos we measured in the laboratory were 33% brighter than those required to fit the Diviner infrared data. We attribute this difference at Apollo 16 to increased compaction and decreased maturity of the laboratory sample relative to the natural lunar surface, and to local variability in surface albedos at the Apollo 16 field area that are below the spatial resolution of Diviner.
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