Search

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.06.002 Byline: Jian-Yang Li, Michael F. A'Hearn, Tony L. Farnham, Lucy A. McFadden Keywords: Comets, Nucleus; Photometry; Comet Wild-2 Abstract: The disk-resolved flyby images of the nucleus of Comet 81P/Wild 2 collected by Stardust are used to perform a detailed study of the photometric properties of this cometary nucleus. A disk-integrated phase function from phase angle 11[degrees] to about 100[degrees] is measured and modeled. A phase slope of 0.0513[+ or -]0.0002 mag/deg is found, with a V-band absolute magnitude of 16.29[+ or -]0.02. Hapke's photometric model yields a single-scattering albedo of 0.034, an asymmetry factor of phase function -0.53, a geometric albedo 0.059, and a V-band absolute magnitude of 16.03[+ or -]0.07. Disk-resolved photometric modeling from both the Hapke model and the Minnaert model results in 11% model RMS, indicating small photometric variations. The roughness parameter is modeled to be 27[+ or -]5[degrees] from limb-darkening profile. The modeled single-scattering albedo and asymmetry factor of the phase function are 0.038[+ or -]0.004 and -0.52[+ or -]0.04, respectively, consistent with those from disk-integrated phase function. The bulk photometric properties of the nucleus of Wild 2 are comparable with those of other cometary nuclei. The photometric variations on the surface of the nucleus of Wild 2 are at a level of or smaller than 15%, much smaller than those on the nucleus of Comet 19P/Borrelly and comparable or smaller than those on the nucleus of Comet 9P/Tempel 1. The similar photometric parameters of the nuclei of Wild 2, Tempel 1, and the non-source areas of fan jets on Borrelly may reflect the typical photometric properties of the weakly active surfaces on cometary nuclei. Author Affiliation: Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA Article History: Received 13 March 2009; Revised 29 May 2009; Accepted 8 June 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.11.015 Byline: Jian-Yang Li (a), Michael F. A'Hearn (a), Lucy A. McFadden (a), Michael J.S. Belton (b) Keywords: Comet Borrelly; Photometry Abstract: Images returned from the Deep Space 1 (DS-1) spacecraft during its encounter with Comet 19P/Borrelly are used to study its disk-integrated and disk-resolved photometry and its thermal properties. A disk-integrated phase function was constructed from a combination of DS-1 images and ground-based observations, giving a geometric albedo of 0.072[+ or -]0.020 and a phase slope of 0.043 magadeg.sup.-1. The shape model of Borrelly [Kirk, R.L., Howington-Kraus, E., Soderblom, L.A., Giese, B., Oberst, J., 2004a. Icarus 167, 54-69] and the ephemerides of DS-1 were used to analyze the disk-resolved photometric data with Hapke's theoretical model. It was found that the surface of Borrelly displays large photometric heterogeneities in its photometric parameters. The single-scattering albedo, w, varies by a factor of 2.5 with an average of 0.057[+ or -]0.009; the asymmetry factor, g, ranges from almost isotropic (-0.1) to strongly backscattering (-0.7) with an average of -0.43[+ or -]0.07; the roughness parameter, I[cedilla], is less than 35[degrees] for most parts of surface but ranges up to 55[degrees] in some areas. Its average is 22[degrees][+ or -]5[degrees]. The observed 1-D temperature profile is modeled well by the standard thermal model (STM) for inactive regions and is found to be consistent with a very low thermal inertia. Water sublimation in the source region of the fan jet is observed to decrease the surface temperature from the STM predictions by 20-40 K. The source areas of two collimated jets could not be determined from either photometric model or thermal model. It is evident that the fan jet activity occurring on Borrelly's surface can be correlated to areas of relatively high albedo, weak backscattering, and high roughness. Author Affiliation: (a) Department of Astronomy, University of Maryland, College Park, MD 20742, USA (b) Belton Space Exploration Initiatives, LLC, 430 S. Randolph Way, Tucson, AZ 85716, USA Article History: Received 14 February 2006; Revised 15 September 2006

The highest resolution (pixel scale 30 km) images of Ceres to date have been acquired by the Advanced Camera for Surveys onboard Hubble Space Telescope, through three wide band filters, centered at 535, 335, and 223 nm, covering more than one rotation of Ceres. The lightcurve at 535 nm agrees with earlier observations at V-band [Tedesco, E.F., Taylor, R.C., Drummond, J., Harwood, D., Nickoloff, I., Scaltriti, F., Schober, H. J., Zappala, V., 1983. Icarus 54, 23-29] in terms of magnitude, amplitude, and shape. The 0.04 magnitude lightcurve amplitude cannot be matched by Ceres' rotationally symmetric shape, and is modeled here by albedo patterns. The geometric albedos at the above three wavelengths are measured to be 0.087 [+ or -] 0.003, 0.056 [+ or -] 0.002, and 0.039 [+ or -] 0.003, respectively. V-band geometric albedo is calculated to be 0.090 [+ or -] 0.003, consistent with earlier observations [Tedesco, E.F., 1989. In: Binzel, R.P., Gehrels, T., Matthews, M.S. (Eds.), Asteroids n. Univ. of Arizona Press, Tucson, pp. 1090-1138]. A strong absorption band (30%) centered at about 280 nm is observed, but cannot be identified with either laboratory UV spectra or the spectra of Europa or Ganymede. The single-scattering albedo has been modeled to be 0.070 [+ or -] 0.002, 0.046 [+ or -] 0.002, and 0.032 [+ or -] 0.003, respectively. The photometric roughness of Ceres' surface is found to be about 44[degrees] [+ or -] 5[degrees] from photometric modeling using Hapke's theory, consistent with earlier radar observations [Mitchell, D.L., Ostro, S.J., Hudson, R.S., Rosema, K.D., Campbell, D.B., Velez, R., Chandler, J. E, Shapiro, I.I., Giorgini, J.D., Yeomans, D.K., 1996. Icarus 124, 113-133]. The first spatially resolved surface albedo maps of Ceres at three wavelengths have been constructed from HST observations, as well as the corresponding color maps. Eleven surface albedo features are identified, ranging in scale from 40-350 kM. Overall the range of these albedo and color variations is small compared to other asteroids and some icy satellites. Keywords: Asteroids; Photometry; Surfaces

A photometric model of (433) Eros at wavelengths from 450 to 1050 nm is constructed using the combination of the images from the multispectral imager (MSI) obtained during the one-year long orbital phase of the NEAR mission, ground-based lightcurves from earlier observations, and our theoretical forward modeling simulations coupled with the NEAR shape model. The single scattering albedo is found to be 0.33 [+ or -] 0.03 at 550 nm, which is smaller than past findings by 30%. The amplitude and width of the opposition effect are 1.4 [+ or -] 0.1 and 0.010 [+ or -] 0.004 from ground based lightcurves. It is confirmed that the asymmetry factor of the single-particle phase function and the surface roughness parameter do not depend on wavelength from 450 to 1050 nm, and their values are estimated to be -0.25 [+ or -] 0.02 and 28[degrees] [+ or -] 3[degrees], respectively, comparable with the earlier measurements from the NEAR NIS data. The geometric albedo and the Bond albedo at 550 nm are calculated to be 0.23 and 0.093, respectively, which make Eros less reflective than previous models, but still slightly more reflective than average S-type asteroids. The lower albedos of Eros are more consistent with our forward modeling simulations, as well as with its spectrum. Eros is a typical S-type asteroid like (951) Gaspra and (243) Ida, and has similar surface regolith properties. Combining the single-scattering albedo with the olivine composition of ordinary chondrites, taking into account space weathering darkening, we constrain the grain size of the regolith particles on Eros to a range of 50 to 100 [micro]m. Keywords: Asteroids, 433 Eros; Photometry; NEAR

We present near-infrared spectrometer (NIS) observations (0.8 to 2.4 [micro]m) of the S-type asteroid 433 Eros obtained by the NEAR Shoemaker spacecraft and report results of our Hapke photometric model analysis of data obtained at phase angles ranging from 1.2 [degrees] to 111.0 [degrees] and at spatial resolutions of 1.25 x 2.5 to 2.75 x 5.5 km/spectrum. Our Hapke model fits successfully to the NEAR spectroscopic data for systematic color variations that accompany changing viewing and illumination geometry. Model parameters imply a geometric albedo at 0.946/[micro]m of 0.27 [+ or -] 0.04, which corresponds to a geometric albedo at 0.550 [micro]m of 0.25 4 [+ or -] 0.05. We find that Eros exhibits phase reddening of up to 10% across the phase angle range of 0-100 [degrees]. We observe a 10% increase in the 1-[micro]m band depth at high phase angles. In contrast, we observe only a 5% increase in continuum slope from 1.486 to 2.363 [micro]m and essentially no difference in the 2-[micro]m band depth at higher phase angles. These contrasting phase effects imply that there are phase-dependent differences in the parametric measurements of 1- and 2- [micro]m band areas, and in their ratio. The Hapke model fits suggest that Eros exhibits a weaker opposition surge than either 951 Gaspra or 243 Ida (the only other S-type asteroids for which we possess disk-resolved photometric observations). On average, we find that Eros at 0.946 [micro]m has a higher geometric albedo and a higher single-scatter albedo than Gaspar or Ida at 0.56 [micro]m; however, Eros's single-particle phase function asymmetry and average surface macroscopic roughness parameters are intermediate between Gaspra and Ida. Only two of the five Hapke model parameters exhibit a notable wavelength dependence: (1) The single-scatter albedo mimics the spectrum of Eros, and (2) there is a decrease in angular width of the opposition surge with increasing wavelength from 0.8 to 1.7 [micro]m. Such opposition surge behavior is not adequately modeled with our shadow-hiding Hapke model, consistent with coherent backscattering phenomena near zero phase. Key Words: asteroid; photometry; spectrophotometry; asteroid surfaces; NEAR; 433 Eros.

The Near Earth Asteroid Rendezvous (NEAR) spacecraft will conduct the first photometric imagery of a C-class asteroid when it flies by 253 Mathilde. An absorption feature seen in ground-based disk-integrated spectra of many C-class asteroids has been attributed to oxidized iron in phyllosilicate minerals of the serpento-chlorite groups. We have designed a method of identifying this absorption feature in the NEAR photometry of Mathilde. A linear mixing model was used to combine two sets of synthetic NEAR photometry created from a spectrum of Mathilde that does not show this feature and the average of nine spectra of other C-class asteroids each having the 0.7-[[micro]meter] absorption feature. We find that a spatial unit must contain a minimum of 55% material that has a spectrum showing the 0.7-[[micro]meter] absorption feature for the presence of this material to be spectrally detectable in the NEAR photometric imaging data.

We have analyzed the only two known images taken of Comet 107P = Asteroid (4015) Wilson-Harrington while it was a distinctly cometary object. The images reside on two Palomar Observatory Sky Survey (POSS) photographic plates taken on 19 November 1949 UT and provide a unique way of studying the fading gasps of a dying comet. The comet appears as a streak, a tail is evident, but the coma is indistinguishable. A comparison of the profiles of the streaks and the stellar PSF yielded no coma, implying the coma's scale height is small (upper limit of a few hundred kilometers). Finson-Probstein modeling of the tail demonstrates that it is not a dust tail. If it were, the size of the particles would have to be tens to hundreds of micrometers in size (which contradicts the tail's blue color [vs. the Sun]) and they would have to have been released several weeks before the observations (which contradicts observers' reports that the tail dissipated in a few days). Instead, we are seeing [CO.sup.+] and [[H.sub.2]O.sup.+] fluorescence in a plasma tail. With this composition, the tail's blue hue and short lifetime are explained. The lag angle of the tail on the image is about 15 [degrees], larger than the 'typical' for Type I tails, but the value is not implausible. We show that the deviation of the solar wind from radial need not have been atypical to explain it. We have calibrated the relevant portions of the photographic plates and, from the surface brightness of the tail and an estimate of its age, we have calculated a plausible maximum to the production rate of [H.sub.2]O and CO during Wilson-Harrington's outburst: [Q.sub.[H.sub.2]O] [approximately equal to] [Q.sub.CO] [approximately equal to] 5 x [10.sup.27] molecules [sec.sup.-1]. The measurements indicate that CO is roughly as abundant as [H.sub.2]O on the dormant comet's nucleus.

Spectrophotometric observation-based analysis of emission bands of CN, C2, C3 and two continuum bands of Comet P/Swift-Tuttle reveal that all five components exhibit spatial and temporal variabilities. Emission band profiles and a vectorial model help calculate CN, C2 and C3 production rates. One half of the profiles exhibit correct fitting under steady-state conditions while introduction of time-dependent production rate explains the prominent bumps in the other half. Presence of gaseous and dust jets in the coma suggest two active areas on the nucleus surface.

A pair of nonbrecciated basaltic lithologies connected along a planar igneous contact constitute a shergottite meteorite, EETA 79001 which is revealed by geochemical and petrological studies of the meteorite. The two lithologies, termed A and B, are basaltic types which rarely occur on the surfaces of planetary bodies. A Modified Gaussian Model (MGM) is used in the analysis of the reflectance spectrums of lithologies A and B. Application of MGM identifies the existence of two pyroxene phases in both lithologies. These two pyroxenes are responsible for all observed spectral features which were earlier attributed to plagioclase.

A CCD spectrograph was used to investigate the narrowband reflectance spectra of 35 asteroids to determine the presence of weak features. The results showed that 1 Ceres exhibited weak absorption features at 0.60 and 0.67 micrometers. In addition, weak absorption features were prevalent among asteroids in the Hilda and Cybele groups. However, the Trojan D asteroids did not exhibit any absorption features.

The Near Infrared Spectrometer (NIS) instrument on the Near Earth Asteroid Rendezvous (NEAR) spacecraft performed a comprehensive series of in-flight tests to validate its preflight radiometric characteristics and to characterize instrument stability, pointing, and co-alignment with other instruments under flight conditions. The results of these tests form the basis of the NIS data reduction and calibration procedure and will support the ability of NIS to achieve its science goal of spectrally mapping NEAR's target asteroid, 433 Eros. Key results include the following: (i) Radiometric calibration of NIS has been confirmed to be accurate to within 4-10%, based on NIS measurements of the Earth and Moon and comparisons to radiances derived from other (NEAR and non-NEAR) optical instruments; (ii) The radiometric response has been demonstrated to be stable on the scale of months; (iii) Gradual, expected detector sensitivity decay of 1-3 % over 2 years of operations in space has been characterized; (iv) Temperature dependencies of detector response are identified and characterized; (v) The co-alignment of NIS with the Multi-Spectral Imager (MSI) and with respect to other NEAR instruments has been determined; (vi) A calibration program (NISCAL) that applies appropriate functions and algorithms to convert raw instrument data to analyzable spectra has been developed; (vii) Remaining unresolved instrument calibration and characterization issues have been identified, to be resolved with the help of approach and orbital observations from the main mission. [C] 2000 Academic Press Key Words: instrumentation; asteroids; Eros; infrared observations; NEAR.