43 results on '"Helfenstein P"'
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2. Enceladus’s measured physical libration requires a global subsurface ocean
- Author
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Thomas, P.C., Tajeddine, R., Tiscareno, M.S., Burns, J.A., Joseph, J., Loredo, T.J., Helfenstein, P., and Porco, C.
- Published
- 2016
- Full Text
- View/download PDF
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3. The inner small satellites of Saturn: A variety of worlds
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Thomas, P.C., Burns, J.A., Hedman, M., Helfenstein, P., Morrison, S., Tiscareno, M.S., and Veverka, J.
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- 2013
- Full Text
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4. Tidal control of jet eruptions on Enceladus as observed by Cassini ISS between 2005 and 2007
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Hurford, T.A., Helfenstein, P., and Spitale, J.N.
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- 2012
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- View/download PDF
5. An experimental study of Hapke’s modeling of natural granular surface samples
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Souchon, A.L., Pinet, P.C., Chevrel, S.D., Daydou, Y.H., Baratoux, D., Kurita, K., Shepard, M.K., and Helfenstein, P.
- Published
- 2011
- Full Text
- View/download PDF
6. Comparing Phoebe’s 2005 opposition surge in four visible light filters
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Miller, C., Verbiscer, A.J., Chanover, N.J., Holtzman, J.A., and Helfenstein, P.
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- 2011
- Full Text
- View/download PDF
7. Geological implications of a physical libration on Enceladus
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Hurford, T.A., Bills, B.G., Helfenstein, P., Greenberg, R., Hoppa, G.V., and Hamilton, D.P.
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- 2009
- Full Text
- View/download PDF
8. Shapes of the saturnian icy satellites and their significance
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Thomas, P.C., Burns, J.A., Helfenstein, P., Squyres, S., Veverka, J., Porco, C., Turtle, E.P., McEwen, A., Denk, T., Giese, B., Roatsch, T., Johnson, T.V., and Jacobson, R.A.
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- 2007
- Full Text
- View/download PDF
9. The opposition effect and negative polarization of structural analogs for planetary regoliths
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Shkuratov, Yu, Ovcharenko, A., Zubko, E., Miloslavskaya, O., Muinonen, K., Piironen, J., Nelson, R., Smythe, W., Rosenbush, V., and Helfenstein, P.
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Polarization (Light) ,Astronomy ,Earth sciences - Abstract
To better understand the negative polarization and brightness opposition effects observed on airless celestial bodies, we carried out simultaneous photometric and polarimetric measurements of laboratory samples that simulate the structure of planetary regoliths. Computer modeling of shadow-hiding and coherent backscatter in regolith-like media are also presented. The laboratory investigations were carried out with a photometer/polarimeter at phase angles covering 0.2[degrees]-4[degrees] and wavelengths of 0.63 and 0.45 [micro]m. We studied samples that characterize a variety of microscopic structures and albedos. A particle-size dependence of the negative branch of polarization for powdered dielectric surfaces was found. Colored samples such as a powder [Fe.sub.2][O.sub.3] exhibit a very prominent wavelength dependence of the photometric and polarimetric opposition phenomena. Metallic powders usually exhibit a wide branch of the negative polarization independent of the size of particles. For fine dielectric powders, both opposition phenomena become more prominent when the samples were compressed. Our computer modeling based on ray tracing in particulate media shows that shadow-hiding affects the negative polarization only in combination with the coherent backscatter enhancement. Modeling reveals that scattering orders higher than second contribute to negative polarization even in dark particulate surfaces. Our model qualitatively reproduces the effects of varying sample-compression that we observed in the laboratory. Our experimental and computer modeling studies mutually confirm that the degree of polarization for highly reflective dielectric surfaces depends not only on phase angle but also on surface tilt. Even at exactly zero phase the degree of polarization for tilted surfaces can be nonzero. A tilt of the surface normal to the scattering plane gives a parallel shift of the negative polarization branch to large values of [absolute value of P]. The tilt in the perpendicular plane gives the same shift in the direction of positive polarization. At exactly zero phase angle, a celestial body of irregular shape can exhibit nonzero polarization even in integral polarimetric observations. Key Words: regolith; polarimetry, photometry; radiative transfer. more...
- Published
- 2002
10. NEAR infrared spectrometer photometry of asteroid 433 Eros
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Clark, Beth E., Helfenstein, P., Bell, J.F., III, Peterson, C., Veverka, J., Izenberg, N.I., Domingue D., Wellnitz, D., and McFadden, Lucy
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Asteroids -- Observations ,Eros (Asteroid) -- Observations ,Near infrared spectroscopy -- Usage ,Astronomy ,Earth sciences - Abstract
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. more...
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- 2002
11. Galileo observations of Europa's opposition effect
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Helfenstein, P., Currier, N., Clark, B.E., Veverka, J., Bell, M., Sullivan, R., Klemaszewski, J., Greeley, R., Pappalardo, R.T., Head, James W., III, Jones, T., Klaasen, K., Magee, K., Geissler, P., Greenberg, R., McEwen, A., Phillips, C., Colvin, T., Davies, M., Denk, T., Neukum, g., and Belton, M.J.S. more...
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Galileo (Space probe) -- Usage ,Europa (Satellite) -- Observations ,Satellites -- Jupiter ,Astronomy ,Earth sciences - Abstract
During Galileo's G7 orbit, the Solid State Imaging (SSI) camera acquired pictures of the spacecraft shadow point on Europa's surface as well as a comparison set of images showing the same geographic region at phase angle [Alpha] = 5 [degrees]. Coverage, obtained at three spectral bandpasses (VLT, 0.41 [[micro]meter], GRN, 0.56 [[micro]meter]; and 1MC, 0.99 [[micro]meter]) at a spatial resolution of 404 m/pixel, shows a 162 x 220-km region of Europa's surface located at 30 [degrees] N, 162 [degrees] W. We have used these images to measure the near-opposition spectrophotometric behavior of four primary europan terrain materials: IR-bright icy material, IR-dark icy material, dark lineament material, and dark spot material. The high spatial resolution of the G7 images reveal low-albedo materials in dark spots that are among the darkest features (17% albedo at 0.56 [[micro]meter] and 5 [degrees] phase) yet found on icy Galilean satellites. While material of comparable albedo is found on Ganymede and Callisto, low-albedo europan materials are much redder. All europan surface materials exhibit an opposition effect; however, the strength of the effect, as measured by the total increase in reflectance as phase angle decreases from [Alpha] = 5 [degrees] to [Alpha] = 0 [degrees], varies among terrains. The opposition effects of IR-bright icy and IR-dark icy materials which dominate Europa's surface are about 1.5 times larger than predicted from pre-Galileo studies. Low-albedo materials in dark spots exhibit unusually intense opposition effects (up to four times larger than bright icy europan terrains), consistent with the presence of a strong shadow-hiding opposition surge. The strengths of the opposition surges among average europan terrains systematically vary with terrain albedo and can be explained in terms of the simultaneous contributions of shadow-hiding and coherent-backscatter to the total opposition effect. Coherent backscatter introduces a narrow angular contribution ( Key Words: Europa; photometry; regolith; spectrophotometry; ices; albedo; spectra; geological processes; Galileo; Voyager. more...
- Published
- 1998
12. Evolution of lineaments on Europa: clues form Galileo multispectral imaging observations
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Geissler, P.E., Greenberg, R., Hoppa, G., McEwen, A., Tufts, R., Phillips, C., Clark, B., Ockert-Bell, M., Helfenstein, P., Burns, J., Veverka, J., Sullivan, R., Greeley, R., Pappalardo, R.T., Head, J.W., III, Belton, M.J.S., and Denk, T. more...
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Galileo (Space probe) -- Usage ,Europa (Satellite) -- Observations ,Satellites -- Jupiter ,Astronomy ,Earth sciences - Abstract
Four distinct classes of lineaments can be described on the basis of Galileo's improved spectral and spatial coverage of Europa: (1) incipient cracks are narrow (
- Published
- 1998
13. Latitude variations of the polar caps on Ganymede
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Hillier, J., Helfenstein, P., and Veverka, J.
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Ganymede (Satellite) -- Observations ,Latitude -- Observations ,Polar regions -- Observations ,Astronomy ,Earth sciences - Abstract
We have modeled Ganymede's polar regions as a frost layer overlying typical non-polar materials using a two-layer photometric model. The optical depth of the overlying layer is found to increase from a few tenths at the cap boundary to more than four at the poles. Such an increase is consistent with the predictions of Johnson's (1985) sputtering model but not with those of thermal migration models of the formation of Ganymede's polar caps. If the frost forms a distinct layer, it must be physically thin - on the order of millimeters or less - particularly near the cap boundary. Therefore, the mechanism responsible for its formation must be ongoing since impact gardening would destroy such a thin layer on short time scales. Interestingly, the photometric properties of the layer are similar to those of the surface of Europa. more...
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- 1996
14. The surface of Deimos: contribution of materials and processes to its unique appearance
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Thomas, P.C., Adinolfi, D., Helfenstein, P., Simonelli, D., and Veverka, J.
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Deimos (Satellite) -- Analysis ,Astronomy ,Earth sciences - Abstract
Among the well-imaged small satellites and asteroids, Deimos displays a unique surface: very smooth with global-scale albedo features. We have examined the disk-resolved photometry of Deimos using Viking Orbiter images for clues to its distinctive appearance. Hapke parameters were fit to characterize the phase behavior and to compute normal reflectance. The opposition surge amplitude ([B.sub.0]) is smaller for Deimos than for Phobos. Outside the range of the opposition effect the two martian satellites have similarly shaped phase curves, but Deimos is about 20-30% brighter than Phobos from 10 [degrees]-80 [degrees] phase. The calculated mean normal reflectance of Deimos ([[Lambda].sub.eff] = 0.54 [[micro]meter]) is 0.068 [+ or -] 0.007. The brighter and darker areas on Deimos exhibit constant contrast between 0.6 [degrees] and 81 [degrees] phase; this characteristic allows a calculation of the range of normal reflectances over most of its surface, nearly all of which values are between 0.06 and 0.09. The trailing side of Deimos has a larger relative distribution of the brighter material, and is on average about 10% brighter than the leading side. The mean normal reflectance cannot be formally distinguished from that of Phobos (0.071 [+ or -] 0.012; Simonelli, D. P., M. Wisz, A. Switala, D. Adinolfi, J. Veverka, P. C. Thomas, and P. Helfenstein 1996. Submitted to Icarus). Although the statistical distribution of normal reflectances on the two satellites is similar, the geography of the albedo variations is very different. Deimos has gradational changes in albedo downslope from ridge crests, primarily manifested in long albedo 'streamers.' On Phobos there is a more patchy global distribution of albedos, apparently related to ejecta from the large crater Stickney. Because of the similarity of mean density, spectral properties, mean normal reflectance, the range of normal reflectance, and phase function outside the opposition effect, Deimos appears to be made of materials with compositions very similar to those on Phobos, although the apparent wider distribution of ejecta on Deimos has been cited as indicating a greater role for strength scaling in cratering on Deimos (Lee, S. W., P. Thomas, and J. Veverka 1986. Icarus 68, 77-86). Simple modeling of the formation of the albedo patterns by gardening, creep, and 'weathering' of bright material from crater rims suggests that impact gardening contributes very little to the motion of the material downslope, and that vertical mixing and/or 'weathering' must be important in addition to an unspecified creep process. The distinction of Deimos is primarily in the smooth surface that allows a particularly large scale of downslope movement of regolith on very gentle slopes. This smoothness is most easily explained by the effects from impact formation of a 10-km concavity at high southern latitudes in the latter half of Deimos' surface history. This impact scar is relatively much larger than is the crater Stickney on Phobos. The effects of this large impact probably include blanketing by an average of nearly 200 m of ejecta, but also may include seismic erasing of craters similar to that proposed for Ida by Asphaug et al. (Asphaug, E., J. M. Moore, D. Morrison, W. Benz, and R. A. Sullivan 1996. Icarus 120, 158-184). more...
- Published
- 1996
15. Dactyl: Galileo observations of Ida's satellite
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Veverka, J., Thomas, P.C., Helfenstein, P., Lee, P., Harch, A., Calvo, S., Chapman, C., Belton, M.J.S., Klaasen, K., Johnson, T.V., and Davies, M.
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Galileo (Space probe) -- Usage ,Asteroids -- Photographic measurements ,Satellites -- Analysis ,Astronomy ,Earth sciences - Abstract
Galileo's flyby of 243 Ida in August 1993 led to the discovery of a small satellite, Dactyl, some 85 km from the asteroid's center. From Earth at mean opposition, the satellite is a V = +20.3 mag object (some 6.7 magnitudes fainter than Ida). Forty-seven images of the satellite at 18 different observing times were played back, including one multicolor sequence in which the satellite is resolved adequately to distinguish surface markings ([approximately]105 m/pxl) and three higher resolution single-color views (89, 39, and 24 m/pxl). The satellite, mean radius = 0.7 km, is an elongated, but not angular body with principal diameters of 1.6 x 1.4 x 1.2 km. In the highest resolution view, the longest axis points approximately in the direction of Ida, and its shortest axis is perpendicular to the orbital plane. The spin period is slow (> 8 hr?) and may be synchronous. The satellite shows no conspicuous sharp edges and is much less irregular in shape than Ida. Limb profiles are remarkably smooth over distances of 200-300 m. The geometric albedos of the two objects are similar (0.20 vs 0.21), as are the 0.4-1.0-[[micro]meter] colors. Like Ida, Dactyl is an S-asteroid, but has a slightly deeper 1-[[micro]meter] band than Ida (by 5-8%). While no identical regions (in color) are seen on Ida, the color difference is consistent with color variations reported within the Koronis family and may be due to a slightly higher pyroxene/olivine ratio on the satellite. More than a dozen craters ranging from [less than or approximately]90 to 280 m diameter are visible in the best image (39 m/pxl at 47 [degrees] phase). The largest contains an off-centered, positive relief feature some 75 m across. The image includes an intriguing crater chain, but no grooves, ridges, or sharp edges are evident. In terms of limb roughness, Dactyl is much smoother than Ida, but comparable to the two satellites of Mars, Phobos and Deimos. While the satellite's origin is uncertain, a likely scenario would have the satellite date from the breakup of the Koronis family. It is interesting that crater densities on the satellite are similar to those on Ida itself. more...
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- 1996
16. Ida and Dactyl: spectral reflectance and color variations
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Veverka, J., Helfenstein, P., Lee, P., Thomas, P., McEwen, A., Belton, M., Klaasen, K., Johnson, T.V., Granahan, J., and Fanale, F.
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Asteroids -- Analysis ,Spectrum analysis -- Research ,Color -- Analysis ,Astronomy ,Earth sciences - Abstract
Galileo SSI color data between 0.4 and 1.0 [[micro]meter] demonstrate that both Ida and Dactyl are S-type asteroids with similar, but distinct spectra. Small but definite color variations are also observed on Ida itself and involve both the blue part of the spectrum and the depth of the 1-[[micro]meter] pyroxene-olivine band. Ida's surface can be classified into two color terrains: Terrain A has a shallower 1-[[micro]meter] absorption and a steeper visible red slope than does Terrain B. Qualitatively, the color-albedo systematics of these two terrains follow those noted for color units on Gaspra and the variations in 1-[[micro]meter] band depth with weathering described by Gaffey et al. (Gaffey, M. J., J. F. Bell, R. H. Brown, T. H. Burbine, J. Piatek, K. L. Reed, and D. A. Chaky 1993. Icarus 106, 573-602). Terrain A, with its slightly lower albedo, its shallower 1-[[micro]meter] band, and its slightly steeper visible red slope relative to Terrain B could be interpreted as the 'more processed,' 'more mature,' or the 'more weathered' of the two terrains. Consistent with this interpretation is that Terrain A appears to be the ubiquitous background on most of Ida, while Terrain B is correlated with some small craters as well as with possible ejecta from the 10-km Azzurra impact structure. Because of these trends, it is less likely that differences between Terrains A and B are caused by an original compositional inhomogeneity within the body of Ida, although they do fall within the range known to occur within the Koronis family. The spectrum of Dactyl is similar to, but definitely different from, that of Terrain B on Ida. It does not conform to the pattern that obtains between the colors and albedos of Terrains A and B: the satellite's 1-[[micro]meter] band is deeper than that of Terrain B, but its albedo is lower, rather than higher. By itself, the deeper band depth could be interpreted, following Gaffey et al., to mean that Dactyl is a less weathered version of Terrain B on Ida, but such an interpretation is at odds with Dactyl's redder spectral slope. Thus, the explanation for the color difference between Dactyl and Ida is likely to be different from that which accounts for the differences between the two terrains on Ida. Given that Dactyl and Ida have very similar photometric properties (Helfenstein, P., J. Veverka, P. C. Thomas, D. P. Simonelli, K. Klassen, T. V. Johnson, F. Fanale, J. Granahan, A. S. McEwen, M. J. S. Belton, and C. R. Chapman 1996 Icarus 120, 48-65), thus ruling out any dramatic texture differences between the two surfaces, the most likely explanation is that the satellite has a slightly different composition (more pyroxene?) than Ida. The spectral difference is within the range reported by Binzel et al. (Binzel, R. P., S. Xu, and S. J. Bus 1993. Icarus 106, 608-611.) for members of the Koronis family, and could be caused by compositional inhomogeneities of the Koronis parent body rather than by post-breakup regolith processes. more...
- Published
- 1996
17. Galileo photometry of asteroid 243 Ida
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Helfenstein, P., Veverka, J., Thomas, P.C., Simonelli, D.P., Klaasen, K., Johnson, T.V., Fanale, F., Granahan, J., McEwen, A.S., and Belton, M.
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Galileo (Space probe) -- Usage ,Asteroids -- Photographic measurements ,Astronomical photometry -- Analysis ,Astronautics ,Astronomy ,Earth sciences - Abstract
Galileo imaging observations over phase angles 19.5 [degrees] to 109.8 [degrees] are combined with near-opposition Earth-based data to derive the photometric properties of Ida. To first order these properties are uniform over the surface and well modeled at [Lambda] = 0.55 [[micro]meter] by Hapke parameters [Mathematical Expression Omitted], h = 0.020, [B.sub.0] = 1.5, g = -0.33, and [Theta] = 18 [degrees] with corresponding geometric albedo [Mathematical Expression Omitted] and Bond albedo [Mathematical Expression Omitted]. Ida's photometric properties are more similar to those of 'average S-asteroids' (P. Helfenstein and J. Veverka 1989, Asteroids II, Univ. of Arizona Press, Tucson) than are those of 951 Gaspra. Two primary color units are identified on Ida: Terrain A exhibits a spectrum with relatively shallower 1-[[micro]meter] absorption and a relatively steeper red spectral slope than average Ida, while Terrain B has a deeper 1-[[micro]meter] absorption and a less steep red slope. The average photometric properties of Ida and Terrain A are similar while those of Terrain B differ mostly in having a slightly higher value of [Mathematical Expression Omitted] (0.22 versus 0.21), suggesting that Terrain B consists of slightly brighter, more transparent regolith particles. Galileo observations of Ida's satellite Dactyl over phase angles 19.5 [degrees] to 47.6 [degrees] suggest photometric characteristics similar to those of Ida, the major difference being Dactyl's slightly lower albedo (0.20 compared to 0.21). more...
- Published
- 1996
18. Photometric diversity of terrains on Triton
- Author
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Hillier, J., Veverka, J., Helfenstein, P., and Lee, P.
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Triton (Satellite) -- Observations ,Astronomy ,Earth sciences - Abstract
Voyager disk-resolved observations of Triton indicate that photometric properties of the different Triton terrains vary and cause changes in the phase angle of the terrain. Hapke's macroscopic roughness parameter theta has a value of 14 degrees, indicating the smoothness of Triton. Estimates of hemispherical albedos derived from the photometric features show that nitrogen frost accumulates on reddish northern terrains near the south of the equator. more...
- Published
- 1994
19. The shape of Gaspra
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Thomas, P.C., Veverka, J., Simonelli, D., Helfenstein, P., Carcich, B., Belton, M.J.S., Davies, M.E., and Chapman, C.
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Gaspra (Asteroid) -- Research ,Asteroids -- Photographic measurements ,Astronomy ,Earth sciences - Abstract
The images of asteroid 951 Gaspra, taken by the Galileo space craft during October 1991, yield the shape model of the asteroid. The shape model reveals that Gaspra is an irregular astronomical body with an average radius of 6.1 kilometers. The rotation axis is aligned with the asteroid's moment of inertia as its mass distribution is uniform. Several flat regions, large and shallow concave crater-like areas characterize the surface of the asteroid. more...
- Published
- 1994
20. Galileo photometry of asteroid 951 Gaspra
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Helfenstein, P., Veverka, J., Thomas, P.C., Simonelli, D.P., Lee, P., Klaasen, K., Johnson, T. V., Breneman, H., Head, J.W., Murchie S., Fanale, F., Robinson, M., Clark, B., Granahan, J., Garbeil, H., McEwen, A.S., Kirk, R.L., Davies, M., Neukum, G., Mottola, S., Wagner, R., Belton, M., Chapman, C., and Pilcher, Carl more...
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Astronomical photometry -- Usage ,Gaspra (Asteroid) -- Research ,Astronomy ,Earth sciences - Abstract
Photometric studies of the images of asteroid 951 Gaspra, taken by the Galileo space craft during the October 1991 flyby, reveal the particle-phase function characteristics and microscopic texture of the asteroid. The texture and phase function characteristics are similar to those of other asteroids and the moon. The particle scattering albedo is 0.36 and the macroscopic surface roughness parameter is 29 degrees. more...
- Published
- 1994
21. Analysis of Gaspra lightcurves using Galileo shape and photometric models
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Simonelli, Damon P, Veverka, J, Thomas, P. C, Helfenstein, P, and Belton, M. J. S
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Astronomy - Abstract
Galileo-based models for the shape of 951 Gaspra and the global-average photometric behavior of its surface have been used to model a representative subset of the asteroid's telescopic lightcurves. Fitting the synthetic lightcurves to the observed timing of lightcurve extrema, and knowing the orientation of Gaspra's axes at the time of the Galileo flyby, leads to a sidereal rotation period for the asteroid of 7.042024 +/- 0.000020 hr, a slight change from the period reported by Magnusson et al. (1992). Initially, the shapes, amplitudes, and absolute photometry of the synthetic and observed lightcurves agree with each other to within 0.05-0.1 mag. Small modifications to the Gaspra shape model on sides of the asteroid poorly imaged by Galileo (changes of 700 m or less in the southern hemisphere at longitudes 90 deg-270 deg W) reduce the typical discrepancies to approximately 0.05 mag in lightcurve shape and less than 0.03 mag in absolute photometry. The result demonstrates that Earth-based lightcurves can be used to refine the shape of a spacecraft-imaged irregular object in areas that are poorly constrained by the spacecraft observations. The consistency and phase-angle dependence of the Galileo-based model for Gaspra photometry, supports the accuracy of the absolute calibration of the Galileo SSI camera, and confirms the Earth-based determination of the V-filter geometric albedo of the asteroid (0.22 +/- 0.03; Tholen et al., submitted for publication). Remaining discrepancies between the synthetic and observed lightcurves show no indication of systematic latitudinal variations in albedo and also cannot be explained entirely by isolated albedo spots. These discrepancies are most likely caused by (1) small, remaining, hard-to-constrain errors in the Gaspra shape model and/or (2) moderate variations in macroscopic roughness across the asteroid's surface, in particular making longitudes 130 deg to 300 deg W moderately rougher than the opposite hemisphere. more...
- Published
- 1995
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22. The photometric roughness of Ariel is not unusual
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Skypeck, A, Veverka, J, Helfenstein, P, and Baker, L
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Astronomy - Abstract
Voyager 2 images of the Uranian satellite Ariel are reanalyzed, with a focus on the large-scale roughness parameter theta-bar. The techniques used to reduce the data and fit the Hapke parameters are described, and the results are presented in tables and graphs. Using the whole-disk phase curve, theta-bar of 27 + or - 1 deg is obtained, somewhat higher than but in good general agreement with the values for the other Uranian satellites. The anomalously high value (42 deg) found by Helfenstein et al. (1988) is attributed to miscalibration of the clear-filter image for phase angle 144.7 deg. more...
- Published
- 1991
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23. NEAR Photometry of Asteroid 253 Mathilde
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Clark, Beth E., Veverka, J., Helfenstein, P., Thomas, P.C., Bell, J.F., III, Harch, A., Robinson, M.S., Murchie, S.L., McFadden, L.A., and Chapman, C.R.
- Published
- 1999
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24. Miranda - Color and albedo variations from Voyager photometry
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Hillier, J, Helfenstein, P, and Veverka, J
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Lunar And Planetary Exploration - Abstract
Voyager 0.48-micron images of Miranda covering phase angles of 13-39 deg, together with 0.35-micron 0.41-micron, and 0.56-micron data at 16-deg phase angle, are the bases of the Minnaert parameters from which the present albedo/color maps are derived. Color ratios are found to be essentially constant over all terrain types and albedo ranges on the satellite; spectrally, Miranda is gray. The color data are noted to conform with a model in which varying amounts of materials spectrally resembling the asteroidal F-material are mixed with an icy component. more...
- Published
- 1989
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25. Minnaert photometric parameters for the satellites of Uranus
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Veverka, J, Helfenstein, P, Skypeck, A, and Thomas, P
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Lunar And Planetary Exploration - Abstract
After deriving the Minnaert (1961) parameters for the five largest satellites of Uranus as functions of both wavelength and phase angle, the well-documented case of Titania is invoked to assess the usefulness of the Minnaert description vs the more rigorous approach of Hapke (1981). While the limb-darkening Minnaert parameter k possesses only a weak wavelength dependence, it exhibits a strong dependence on phase angle that varies to some extent from satellite to satellite; in the case of Titania, this dependence is similar to that for the earth's moon. more...
- Published
- 1989
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26. Patterns of fracture and tidal stresses due to nonsynchronous rotation - Implications for fracturing on Europa
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Helfenstein, P and Parmentier, E. M
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Lunar And Planetary Exploration - Abstract
This study considers the global patterns of fracture that would result from nonsynchronous rotation of a tidally distorted planetary body. The incremental horizontal stresses in a thin elastic or viscous shell due to a small displacement of the axis of maximum tidal elongation are derived, and the resulting stress distributions are applied to interpret the observed pattern of fracture lineaments on Europa. The observed pattern of lineaments can be explained by nonsynchronous rotation if these features formed by tension fracturing and dike emplacement. Tension fracturing can occur for a small displacement of the tidal axis, so that the resulting lineaments may be consistent with other evidence suggesting a young age for the surface. more...
- Published
- 1985
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27. Patterns of fracture and tidal stresses on Europa
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Helfenstein, P and Parmentier, E. M
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Lunar And Planetary Exploration - Abstract
A comparison of dark band, triple band, and cuspate ridge orientations with the fracture patterns predicted for tidal distortion due to orbital recession and eccentricity is undertaken, to test the hypothesized identification of Europa's lineaments as tidal distortion and planetary volume change fractures. Short, reticule dark bands near the anti-Jove point could be tension cracks caused by orbital eccentricity. Long, arcuate dark bands and triple bands peripheral to the anti-Jove point may be strike-slip faults due to orbital recession. The orientation and distribution of cuspate ridges, if they are compressional, suggests their formation in response to a combination of orbital recession and planetary volume decrease. If surface fracturing is due to tidal deformation, important constraints are exerted by it on Europa's orbital evolution. more...
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- 1983
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28. NEAR Lightcurves of Asteroid 433 Eros
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Clark, Beth E., Thomas, P.C., Veverka, J., Helfenstein, P., Robinson, M.S., and Murchie, S.L.
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- 2000
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29. The Opposition Effect and the Quasi-fractal Structure of Regolith: I. Theory
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Shkuratov, Yurij G. and Helfenstein, Paul
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We present a new heuristic model combining the coherent-backscatter and shadow-hiding opposition effects. The model considers the aggregate structure of regolith in a realistic way and accounts for the fractal architecture of planetary surfaces both on microscopic particulate and macroscopic, texturally faceted size scales. It describes how the shadow-hiding opposition effect is manifested in a fractally arranged surface, the important contribution of incoherent, multiply scattered light among small scatterers within a surface composed of aggregate grains and its amplification by backscatter interference. We provide a general theoretical model as well as a preliminary, simple analytical approximation that is suitable for investigating the opposition behavior of planet, satellite, and asteroid surfaces. The analytical approximation utilizes only four model parameters; ω0, the average, single-scattering albedo of elemental scatterers that are the building blocks of aggregates, h, Hapke's (1986, Icarus67, 264–280) angular-width parameter for the shadow-hiding opposition effect, L, the diffusion scale length of light through the regolith, and q, a structural parameter that characterizes the relative size and complexity of aggregate particles. Our model explains the simultaneous occurrence of the coherent-backscatter enhancement and shadow-hiding in moderate to low albedo soils in addition to the dominance of coherent-backscatter in bright soils. It predicts that coherent backscatter and shadow-hiding can occur at a variety of size scales within the regolith. Individual regolith particles can exhibit their own opposition effects which are convolved with the contribution that arises from their packing within the planetarysurface. The size domains over which coherent-backscatter and shadow-hiding, respectively, each have their most pronounced effect is strongly controlled by the albedo of internal scatterers and the number of hierarchical generations of aggregates required to characterize the particulate-scale behavior of the regolith surface. more...
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- 2001
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30. Submillimeter-Scale Topography of the Lunar Regolith
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Helfenstein, Paul and Shepard, Michael K.
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We have applied computer stereophotogrammetry to Apollo Lunar Surface Closeup Camera (ALSCC) pictures of the lunar surface to construct the first-ever digital topographic relief maps of undisturbed lunar soil over spatial scales from 85 μm to 8.5 cm. Using elevation histograms, fractal analysis, and Hapke's photometric roughness model we show that Apollo 14(Fra Mauro) Imbrium ejecta is rougher than average Apollo 11(Mare Tranquilitatis) and Apollo 12(Oceanus Procellarum) mare surfaces at submillimeter to decimeter size-scales. We confirm the early result of K. Lumme et al.(1985, Earth Moon Planets33, 19–29) that the cumulativedistribution of elevations for lunar soil is typically well described by Gaussian statistics. However, cumulative distributions are insensitive to asymmetries in the shapes of elevation histograms: Of 11 discrete elevation histograms we measured, about half exhibit significant deviations from Gaussian behavior. We also confirm Lumme et al.'s finding that the roughnesses of all lunar surfaces increase with decreasing size-scale. We further show that the scale dependence of roughness is well represented by fractal statistics. The rates of change of roughness with size scale, represented by fractal dimension D, are remarkably similar among terrians. After correcting for the contribution of large-scale roughness, our average value of D=2.31±0.06 falls within the range 2.0≤D≤2.4 reported from lunar radar studies. The amplitude of roughness, which we characterize with the rms slope angle at 1-mm scale, varies significantly among terrains. For lunar mare, the average rms slope angle is 16°±4°3and that for Fra Mauro regolith is 25°±1°. By comparison to radar data, we suggest that the roughness of Fra Mauro (Imbrium ejecta) regolith is similar to that of lunar highland terrains. We find that the Gaussian slope distribution assumed in B. W. Hapke's model (1984, Icarus59, 41–59) adequately describes typical lunar regolith surfaces. A revised form of Hapke's equation that models realistic particle phase functions and the coherent backscatter opposition effect was fitted to disk-resolved lunar photometric observations and yields estimates of θ=27±1° for highland and θ=24±1° for mare regolith. These values of θas well as the implied relative highland:mare photometric roughness ratio are best matched in our elevation data by the cummulative contributions of surface topography covering all scales greater than 0.1 mm. Less than 5% of the photometrically detected roughness of lunar regolith is contributed by surface relief at scales larger than 8 cm. This conclusion implies that values of θderived from whole-disk and disk-resolved photometry, respectively, may be taken to represent the same physical quantity. In addition, particulate samples used in goniophotometric measurements should not be assumed to be photometrically smooth (i.e., θ=0°), as is often done in laboratory applications of Hapke's photometric model. The predicted photometric roughness at size scales of 0.1 mm and less significantly exceed photometric estimates and suggests that there exists a measurable size scale below which topographic relief either is not photometrically detectable or is not represented in the Hapke model as macroscopic roughness. more...
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- 1999
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31. Phoebe: Albedo Map and Photometric Properties
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Simonelli, Damon P., Kay, Jennifer, Adinolfi, Daniel, Veverka, Joseph, Thomas, Peter C., and Helfenstein, Paul
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Using clear-filter images from Voyager 2 (effective wavelength 0.48 μm), we have constructed the first-ever digital albedo map of Saturn's moon Phoebe. Most normal reflectances in this new map are between 0.07 and 0.11; the albedo histogram is largely bimodal, suggesting that the satellite is covered predominantly by two different types of surface materials. The highest albedos are confined to isolated, quasi-circular spots 40 to 100 km across, including three spots of varying albedo in a band immediately south of the equator and one especially bright spot at latitude 60°N (normal reflectance as high as 0.13, ≈50% brighter than the average surface). The bright northern spot and the brightest of the southern spots occur at approximately the same longitude, an alignment that gives Phoebe its significant rotational lightcurve. The low resolution of the Voyager images does not permit interpretation of the bright spots' origin. more...
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- 1999
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32. Galileo's Multiinstrument Spectral View of Europa's Surface Composition
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Fanale, Fraser P., Granahan, James C., McCord, Thomas B., Hansen, Gary, Hibbitts, Charles A., Carlson, Robert, Matson, Dennis, Ocampo, Adriana, Kamp, Lucas, Smythe, William, Leader, Frank, Mehlman, Robert, Greeley, Ronald, Sullivan, Robert, Geissler, Paul, Barth, Charles, Hendrix, Amanda, Clark, Beth, Helfenstein, Paul, Veverka, Joseph, Belton, Michael J.S., Becker, Kris, Becker, Tammy, NIMS, the Galileo, SSI, and teams, UVS instrument more...
- Abstract
We have combined spectral reflectance data from the Solid State Imaging (SSI) experiment, the Near-Infrared Mapping Spectrometer (NIMS), and the Ultraviolet Spectrometer (UVS) in an attempt to determine the composition and implied genesis of non-H2O components in the optical surface of Europa. We have considered four terrains: (1) the “dark terrains” on the trailing hemisphere, (2) the “mottled terrain,” (3) the linea on the leading hemisphere, and (4) the linea embedded in the dark terrain on the trailing hemisphere. The darker materials in these terrains exhibit remarkably similar spectra in both the visible and near infrared. In the visible, a downturn toward shorter wavelengths has been attributed to sulfur. The broad concentrations of dark material on the trailing hemisphere was originally thought to be indicative of exogenic sulfur implantation. While an exogenic cause is still probable, more recent observations by the UVS team at higher spatial resolution have led to their suggestions that the role of the bombardment may have primarily been to sputter away overlying ice and to reveal underlying endogenic non-H2O contaminants. If so, this might explain why the spectra in all these terrains are so similar despite the fact that the contaminants in the linea are clearly endogenic and those in the mottled terrain are almost certainly so. more...
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- 1999
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33. Mass Movement and Landform Degradation on the Icy Galilean Satellites: Results of the Galileo Nominal Mission
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Moore, Jeffrey M., Asphaug, Erik, Morrison, David, Spencer, John R., Chapman, Clark R., Bierhaus, Beau, Sullivan, Robert J., Chuang, Frank C., Klemaszewski, James E., Greeley, Ronald, Bender, Kelly C., Geissler, Paul E., Helfenstein, Paul, and Pilcher, Carl B. more...
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The Galileo mission has revealed remarkable evidence of mass movement and landform degradation on the icy Galilean satellites of Jupiter. Weakening of surface materials coupled with mass movement reduces the topographic relief of landforms by moving surface materials down-slope. Throughout the Galileoorbiter nominal mission we have studied all known forms of mass movement and landform degradation of the icy galilean satellites, of which Callisto, by far, displays the most degraded surface. Callisto exhibits discrete mass movements that are larger and apparently more common than seen elsewhere. Most degradation on Ganymede appears consistent with sliding or slumping, impact erosion, and regolith evolution. Sliding or slumping is also observed at very small (100 m) scale on Europa. Sputter ablation, while probably playing some role in the evolution of Ganymede's and Callisto's debris layers, appears to be less important than other processes. Sputter ablation might play a significant role on Europa only if that satellite's surface is significantly older than 108years, far older than crater statistics indicate. Impact erosion and regolith formation on Europa are probably minimal, as implied by the low density of small craters there. Impact erosion and regolith formation may be important on the dark terrains of Ganymede, though some surfaces on this satellite may be modified by sublimation–degradation. While impact erosion and regolith formation are expected to operate with the same vigor on Callisto as on Ganymede, most of the areas examined at high resolution on Callisto have an appearance that implies that some additional process is at work, most likely sublimation-driven landform modification and mass wasting. The extent of surface degradation ascribed to sublimation on the outer two Galilean satellites implies that an ice more volatile than H2O is probably involved. more...
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- 1999
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34. Inflight Calibration of the NEAR Multispectral Imager
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Murchie, Scott, Robinson, Mark, Hawkins, S.Edward, Harch, Ann, Helfenstein, Paul, Thomas, Peter, Peacock, Keith, Owen, William, Heyler, Gene, Murphy, Patricia, Darlington, E.H., Keeney, Allen, Gold, Robert, Clark, Beth, Izenberg, Noam, Bell, James F., Merline, William, and Veverka, Joseph more...
- Abstract
The multispectral imager on the Near-Earth Asteroid Rendezvous spacecraft has been subjected to a comprehensive series of inflight tests to validate its radiometric characteristics measured onground and to characterize instrument stability, pointing, geometric distortion, coalignment with other instruments, and light-scattering characteristics under flight conditions. The results of these tests, described herein, support the conversion of images of 253 Mathilde and 433 Eros into scientifically valid products with known geometric and radiometric characteristics. Key results include stability of dark current during cruise to within 1 data number; stability of the flat field to within the limits of inflight detectability; absolute radiometric accuracy of ∼5%, with no evident systematic change with time; validation of the focal length with an inflight measurement of 166.85 mm, compared to 167.0±0.2 mm derived onground; measurement of coalignment with the near-infrared spectrometer under flight conditions; and quantification of the intensity and distribution of scattered light. more...
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- 1999
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35. The Distribution of Bright and Dark Material on Ganymede in Relationship to Surface Elevation and Slopes
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Oberst, Jürgen, Schreiner, Björn, Giese, Bernd, Neukum, Gerhard, Head, James W., Pappalardo, Robert T., and Helfenstein, Paul
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The unique high-resolution digital terrain data set available for Ganymede's Uruk Sulcus area and Galileo Regio is analyzed by quantitative image analysis techniques to identify relationships between surface brightness, elevation, and slopes. Broad and narrow ridges and grooves of the Uruk Sulcus terrain model are found to be aligned with bright and dark lineament patterns visible in images. These prominent lineaments represent areas covered by bright and dark albedo material, not effects of photometric shading. There is a strong correlation between material locations and elevation: bright material is found on topographic highs, whereas dark material is located in depressions; the relative abundance of the dark material increases with lower elevation. In Galileo Regio, we find that bright material is also located on slopes facing north. We suggest that the bright material in Uruk Sulcus and Galileo Regio was emplaced or exposed during or after formation of the terrain in its present location, whereas the dark material was eroded and redistributed down-slope. Effects of net solar illumination may play a role in the peculiar azimuthal distribution of bright and dark material on slopes. more...
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- 1999
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36. Search for Temperature-Related Albedo Changes in Nightside and Posteclipse Images of Io
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Simonelli, Damon P., Boucher, Jennifer, Helfenstein, Paul, Veverka, Joseph, and O'Shaughnessy, Megan
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Using an image-summing process that increases the visibility of Jupiterlit surface features in Voyager images, we have produced the best-ever violet-filter image of the nightside of Io and the best-ever nightside/dayside brightness ratio map of this jovian moon. The ratio map shows no convincing evidence, on either global or local scales, of diurnal temperature-dependent albedo variations. We have also taken an image-ratioing technique developed by O'Shaughnessy et al. (1989, Lunar Planet. Sci. 20, 812-813), which those authors applied to Voyager violet-filter observations of one Io eclipse reappearance, and extended it to two other, higher-resolution Voyager posteclipse imaging sequences. In none of the three imaging sequences do we find any isolated surface regions that convincingly exhibit posteclipse temperature-related albedo variations. These negative results suggest that on Io, pure cyclo-octasulfur (S8), and transient nighttime or in-eclipse deposits of SO2 frost, are at best limited to isolated areas smaller than the resolution of the images in use (i.e., smaller than a few tens of kilometers in size). Such limits are consistent with (1) the negative results reported by the majority of telescopic observers who have searched for posteclipse brightening of Io, (2) indications that physical processes in the ionian surface environment will change any S8 into other allotropes of sulfur, and (3) suggestions that Io's atmosphere is too thin to allow the deposition of transient, optically thick SO2 frost layers at nighttime or during eclipse. Copyright 1994, 1999 Academic Press more...
- Published
- 1994
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37. Multispectral Terrain Analysis of Europa from Galileo Images
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Clark, Beth E., Helfenstein, Paul, Veverka, Joseph, Ockert-Bell, Maureen, Sullivan, R.J., Geissler, P.E., Phillips, C.B., McEwen, A.S., Greeley, R., Neukum, G., Denk, T., and Klaasen, K.
- Abstract
Galileo's Solid State Imaging camera recorded six images at wavelengths from 0.41 to 0.99 μm of Europa's trailing hemisphere (∼1.6 km/pixel resolution) during the G1 orbit (1st orbit—target Ganymede) of the nominal mission. We have photometrically corrected these data and extracted spectra representing Europa's diverse geologic terrains. The goals of the analysis of these spectra are (1) to determine whether Europa's geologic units differ spectrally from one another, (2) to determine the number of color components necessary to explain Europa's dark material spectral behavior, and (3) to examine how europan dark materials may change in color with time. Our data indicate that europan dark spots, lineaments and triple band side materials represent a single “dark” and reddish endmember component. We see no evidence for more than one dark endmember; however the dark endmember seen in these G1 data is probably not a “pure” exposure of europan dark material. Mottled terrains, brighter lineaments, and aging triple bands can be modeled by a mixture of bright plains materials and the dark component. Exposures of europan dark materials may thus be brightening with time, eventually blending into the surrounding bright plains. more...
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- 1998
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38. Dark Terrain on Ganymede: Geological Mapping and Interpretation of Galileo Regio at High Resolution
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Prockter, Louise M., Head, James W., Pappalardo, Robert T., Senske, David A., Neukum, Gerhard, Wagner, Roland, Wolf, Ursula, Oberst, Jürgen Oberst, Giese, Bernd, Moore, Jeffrey M., Chapman, Clark R., Helfenstein, Paul, Greeley, Ronald, Breneman, H.Herbert, and Belton, Michael J.S. more...
- Abstract
During its first two encounters with Ganymede, the Galileo spacecraft obtained images of a 16,500 km2portion of Galileo Regio, a large expanse of dark terrain, at high resolution (76–86 m/pixel). Through mapping of the G1 and G2 target sites within Galileo Regio, we are able to characterize geological units based on their morphology and relative albedo. We find three generally low albedo units: an intermediate albedo plains unit, a lower albedo plains unit, and the lowest albedo unit which is found on furrow and crater floors. We also find high albedo units which include crater rims, furrow rims, and isolated knobs and massifs. Other features include an intermediate albedo lobate feature interpreted to be a palimpsest and a hummocky unit interpreted to be impact ejecta. Several processes are interpreted to have occurred within Galileo Regio. These include tectonic deformation, mass wasting, sublimation, resurfacing by impact ejecta, and possibly cryovolcanism and isostatic adjustment. We observe that the NW–SE trending furrows (Lakhmu Fossae) in Galileo Regio are degraded and are crosscut by the younger N–S trending furrows (Zu Fossae). We also find several other tectonic features which may be minor faults or fractures related to one or other of these systems. Through mapping and crater size–frequency distributions, we are able to propose a stratigraphy for the Galileo Regio target site. The oldest features in the area are high albedo knobs and massifs, which are interpreted to be remnants of early impact-related features and furrow rims. These may have formed at approximately the same time as the intermediate and low albedo plains units and the furrow systems. The lowest albedo unit of furrow floors probably subsequently evolved through sublimation and mass wasting. Much of the northeast portion of the target area was subsequently obscured by one of the youngest units, ejecta from an impact just to the north. We use our mapping of the high-resolution images of Galileo Regio to evaluate three end-member models for the formation of dark terrain: (1) the crust is dark throughout, (2) material on the surface is the result of a low albedo cryovolcanic layer over a higher albedo crust, and (3) dark material is distributed in small quantities throughout the crust, and geological processes have acted to concentrate low albedo material on the surface. Although it is possible that elements of more than one of these models are present within the dark terrain, we find that the third model, that of a thin veneer of low albedo material, best fits observations of Galileo Regio more...
- Published
- 1998
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39. The Lunar Opposition Effect: A Test of Alternative Models
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Helfenstein, Paul, Veverka, Joseph, and Hillier, John
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We test the hypotheses that the lunar opposition effect is due to shadow-hiding, coherent backscatter, or some combination of the two phenomena. Hapke's photometric model is extended to include975726description of the coherent backscatter opposition effect. The model was fit simultaneously toV-filter (λ = 0.55 μm) whole-disk photometry of the Moon over many phase angles (2° ≤ α ≤ 143°) and to disk-resolved observations of the lunar surface over a broad range of incidence, emission, and phase angles. The shape of the Moon's opposition surge is accurately represented by the combination of a narrow coherent-backscatter peak whose effect is most strongly defined at α < 2° and a very broad shadow-hiding peak, best defined over α < 20°. The relative angular widths of the shadow-hiding and coherent backscatter contributions can be used to estimate the grain sizes most important for scattering at small phase angles. Scatterers with sizes comparable to the wavelength of light are required for coherent backscattering and are present in the smallest size fraction of lunar soil, but are predicted only for a model in which coherent backscatter causes the narrow component and shadow-hiding the wide one. The amplitude of the shadow-hiding opposition effect measures grain transparency and our solution implies that the lunar regolith particles responsible for the shadow-hiding contribution behave like opaque grains, consistent with complex structure of lunar regolith particles at much larger scales than those which control the coherent backscatter contribution. Most other airless planetary surfaces exhibit opposition surges whose broad components also appear to be caused by shadow-hiding. We propose that the submicron sized grains that control coherent backscatter do not contribute to the shadow-hiding opposition surge because the minute shadows cast by these grains are illuminated by their own forward-scattered light. We suggest that coherent backscatter cannot be responsible for the broad component of the Moon's opposition surge because such forward scattered light would illuminate all particle shadows and thereby eliminate the shadow-hiding contribution. more...
- Published
- 1997
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40. Grooved Terrain on Ganymede: First Results from Galileo High-Resolution Imaging
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Pappalardo, Robert T., Head, James W., Collins, Geoffrey C., Kirk, Randolph L., Neukum, Gerhard, Oberst, Juergen, Giese, Bernd, Greeley, Ronald, Chapman, Clark R., Helfenstein, Paul, Moore, Jeffrey M., McEwen, Alfred, Tufts, B.Randy, Senske, David A., Breneman, H.Herbert, and Klaasen, Ken more...
- Abstract
High-resolution Galileo imaging has provided important insight into the origin and evolution of grooved terrain on Ganymede. The Uruk Sulcus target site was the first imaged at high resolution, and considerations of resolution, viewing geometry, low image compression, and complementary stereo imaging make this region extremely informative. Contrast variations in these low-incidence angle images are extreme and give the visual impression of topographic shading. However, photometric analysis shows that the scene must owe its character to albedo variations. A close correlation of albedo variations to topography is demonstrated by limited stereo coverage, allowing extrapolation of the observed brightness and topographic relationships to the rest of the imaged area. Distinct geological units are apparent across the region, and ridges and grooves are ubiquitous within these units. The stratigraphically lowest and most heavily cratered units (“lineated grooved terrain”) generally show morphologies indicative of horst-and-graben-style normal faulting. The stratigraphically highest groove lanes (“parallel ridged terrain”) exhibit ridges of roughly triangular cross section, suggesting that tilt-block-style normal faulting has shaped them. These extensional-tectonic models are supported by crosscutting relationships at the margins of groove lanes. Thus, a change in tectonic style with time is suggested in the Uruk Sulcus region, varying from horst and graben faulting for the oldest grooved terrain units to tilt block normal faulting for the latest units. The morphologies and geometries of some stratigraphically high units indicate that a strike-slip component of deformation has played an important role in shaping this region of grooved terrain. The most recent tectonic episode is interpreted as right-lateral transtension, with its tectonic pattern of two contemporaneous structural orientations superimposed on older units of grooved terrain. There is little direct evidence for cryovolcanic resurfacing in the Uruk Sulcus region; instead tectonism appears to be the dominant geological process that has shaped the terrain. A broad wavelength of deformation is indicated, corresponding to the Voyager-observed topography, and may be the result of ductile necking of the lithosphere, while a finer scale of deformation probably reflects faulting of the brittle near surface. The results here form a basis against which other Galileo grooved terrain observations can be compared. more...
- Published
- 1998
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41. Photometric Properties of Phobos Surface Materials From Viking Images
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Simonelli, Damon P., Wisz, Michael, Switala, Andrew, Adinolfi, Daniel, Veverka, Joseph, Thomas, Peter C., and Helfenstein, Paul
- Abstract
Clear-filter Viking images, and an accurate numerical model of the shape of Phobos, have been used to determine this satellite's photometric properties. A global-average Hapke function derived from disk-resolved data confirms previous indications that Phobos has a strong opposition surge. Photometrically corrected images were mosaicked into an albedo map; most of the resulting normal reflectances are in the range 0.06–0.10, and the brightest region on Phobos is the northeast rim of the crater Stickney, the portion of that rim with the highest concentration of grooves. Globally, there are three albedo classes, reasonably separated geographically: (1) Bright material is to the east and south of Stickney, corresponding approximately to the locations on Phobos having the highest, “bluest” visible/near-IR ratio (Murchieet al. 1991). (2) The darkest material is to Stickney's west, correlating with material having an intermediate visible/NIR ratio (Murchieet al.'s “bluish gray” unit). (3) Intermediate-albedo material dominates the anti-Stickney hemisphere, corresponding to material with a lower visible/NIR ratio (Murchieet al.'s “reddish gray” unit). more...
- Published
- 1998
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42. Ida Lightcurves: Consistency with Galileo Shape and Photometric Models
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Simonelli, Damon P., Veverka, Joseph, Thomas, Peter C., Helfenstein, Paul, Carcich, Brian T., and Belton, Michael J.S.
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Models for the shape and global photometric behavior of 243 Ida derived from Galileo images have been used to model the asteroid's telescopic lightcurves. We find excellent agreement between the calculated and observed times of lightcurve extrema, confirming the sidereal rotation period of 4.633632 ± 0.000007 hr determined by Binzelet al. (1993.Icarus105, 310–325.). The shapes and amplitudes of the synthetic and observed lightcurves agree with each other to within ≈0.05 mag, and the average offset in absolute photometry between model and observation is only 0.01 mag. This consistency confirms the accuracy of the shape model derived for Ida by Thomaset al. (1996.Icarus120, 20–32.) and supports the model of the asteroid's photometric behavior developed by Helfensteinet al. (1996.Icarus120, 48–65.). more...
- Published
- 1996
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43. Ejecta Blocks on 243 Ida and on Other Asteroids
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Lee, Pascal, Veverka, Joseph, Thomas, Peter C., Helfenstein, Paul, Belton, Michael J.S., Chapman, Clark R., Greeley, Ronald, Pappalardo, Robert T., Sullivan, Robert, and Head, James W.
- Abstract
Seventeen positive relief features ∼45–150m across are identified as probable blocks in Galileo high-resolution images of Ida. Their presence provides direct evidence for regolith retention on asteroids. The spatial distribution, maximum size, and integrated volume of the blocks are consistent with those of blocks associated with craters on the Earth, the Moon, Phobos, and Deimos. The concordance suggests that the features are impact ejecta fragments and that cratering mechanics on Ida, an object of average diameter ∼32 km, are similar to those applying on previously studied rocky bodies. The blocks that lie within or near the rims of craters Lascaux and Mammoth were likely mobilized in the low-velocity tail portion of the excavation flow that formed those craters. A few blocks located near smaller craters may have migrated some distance away from their source, possibly by impact-induced spallation, hopping, rolling, and/or sliding. Some blocks on Ida could be surviving fragments from the Koronis parent body, accreted after its breakup. The lifetime of 102-m sized boulders against collisional disruption is estimated to be in the 108–109year range, consistent with ages considered for the largest and oldest craters on Ida. Extrapolation of successful ejecta scaling laws to other asteroids suggests that blocks ∼15 and 70m across could be present on Dactyl and Gaspra, respectively (in both cases too small to be identified in available Galileo images). Blocks 100m in size could be present on 433 Eros, and km-sized megablocks on 4 Vesta. more...
- Published
- 1996
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