Amplitude Of Opposition Effect Research Articles

Lunar phase function at 1064 nm from Lunar Orbiter Laser Altimeter passive and active radiometry

We present initial calibration and results of passive radiometry collected by the Lunar Orbiter Laser Altimeter onboard the Lunar Reconnaissance Orbiter over the course of 12 months. After correcting for time- and temperature-dependent dark noise and detector responsivity variations, the LOLA passive radiometry measurements are brought onto the absolute radiance scale of the SELENE Spectral Profiler. The resulting photometric precision is estimated to be ∼5%. We leverage the unique ability of LOLA to measure normal albedo to explore the 1064 nm phase function’s dependence on various geologic parameters. On a global scale, we find that iron abundance and optical maturity (quantified by FeO and OMAT) are the dominant controlling parameters. Titanium abundance (TiO2), surface roughness on decimeter to decameter scales, and soil thermophysical properties have a smaller effect, but the latter two are correlated with OMAT, indicating that exposure age is the driving force behind their effects in a globally-averaged sense. The phase function also exhibits a dependence on surface slope at ∼300 m baselines, possibly the result of mass wasting exposing immature material and/or less space weathering due to reduced sky visibility. Modeling the photometric function in the Hapke framework, we find that, relative to the highlands, the maria exhibit decreased backscattering, a smaller opposition effect (OE) width, and a smaller OE amplitude. Immature highlands regolith has a higher backscattering fraction and a larger OE width compared to mature highlands regolith. Within the maria, the backscattering fraction and OE width show little dependence on TiO2 and OMAT. Variations in the phase function shape at large phase angles are observed in and around the Copernican-aged Jackson crater, including its dark halo, a putative impact melt deposit. Finally, the phase function of the Reiner Gamma Formation behaves more optically immature than is typical for its composition and OMAT, suggesting the visible-to-near-infrared spectrum and phase function respond differently to the unusual regolith evolution and properties at this location.

Asteroid (21) Lutetia: Disk-resolved photometric analysis of Baetica region

(21) Lutetia has been visited by Rosetta mission on July 2010 and observed with a phase angle ranging from 0.15° to 156.8°. The Baetica region, located at the north pole has been extensively observed by OSIRIS cameras system. Baetica encompass a region called North Pole Crater Cluster (NPCC), shows a cluster of superposed craters which presents signs of variegation at the small phase angle images. For studying the location, we used 187 images distributed throughout 14 filter recorded by the NAC (Narrow Angle Camera) and WAC (Wide Angle Camera) of the OSIRIS system on-board Rosetta taken during the fly-by. Then, we photometrically modeled the region using Minnaert disk-function and Akimov phase function to obtain a resolved spectral slope map at phase angles of 5° and 20°. We observed a dichotomy between Gallicum and Danuvius-Sarnus Labes in the NPCC, but no significant phase reddening (-0.04±0.045%μm-1deg-1). In the next step, we applied the Hapke (Hapke, B. [2008]. Icarus 195, 918–926; Hapke, B. [2012]. Theory of Reflectance and Emittance Spectroscopy, second ed. Cambridge University Press) model for the NAC F82+F22 (649.2nm), WAC F13 (375nm) and WAC F17 (631.6nm) and we obtained normal albedo maps and Hapke parameter maps for NAC F82+F22. On Baetica, at 649.2nm, the geometric albedo is 0.205±0.005, the average single-scattering albedo is 0.181±0.005, the average asymmetric factor is -0.342±0.003, the average shadow-hiding opposition effect amplitude and width are 0.824±0.002 and 0.040±0.0007, the average roughness slope is 11.45°±3° and the average porosity is 0.85±0.002. We are unable to confirm the presence of coherent-backscattering mechanism. In the NPCC, the normal albedo variegation among the craters walls reach 8% brighter for Gallicum Labes and 2% fainter for Danuvius Labes. The Hapke parameter maps also show a dichotomy at the opposition effect coefficients, single-scattering albedo and asymmetric factor, that may be attributed to the maturation degree of the regolith or to compositonal variation. In addition, we compared the Hapke (Hapke, B. [2008]. Icarus 195, 918–926; Hapke, B. [2012]. Theory of Reflectance and Emittance Spectroscopy, second ed. Cambridge University Press) and Hapke (Hapke, B. [1993]. Theory of Reflectance and Emittance Spectroscopy) parameters with laboratory samples and other small Solar System bodies visited by space missions.

OPTICAL PROPERTIES OF (162173) 1999 JU3: IN PREPARATION FOR THE JAXAHAYABUSA 2SAMPLE RETURN MISSION

We investigated the magnitude-phase relation of (162173) 1999 JU3, a target asteroid for the JAXA Hayabusa 2 sample return mission. We initially employed the international Astronomical Union's H-G formalism but found that it fits less well using a single set of parameters. To improve the inadequate fit, we employed two photometric functions, the Shevchenko and Hapke functions. With the Shevchenko function, we found that the magnitude-phase relation exhibits linear behavior in a wide phase angle range (alpha = 5-75 deg) and shows weak nonlinear opposition brightening at alpha< 5 deg, providing a more reliable absolute magnitude of Hv = 19.25 +- 0.03. The phase slope (0.039 +- 0.001 mag/deg) and opposition effect amplitude (parameterized by the ratio of intensity at alpha=0.3 deg to that at alpha=5 deg, I(0.3)/I(5)=1.31+-0.05) are consistent with those of typical C-type asteroids. We also attempted to determine the parameters for the Hapke model, which are applicable for constructing the surface reflectance map with the Hayabusa 2 onboard cameras. Although we could not constrain the full set of Hapke parameters, we obtained possible values, w=0.041, g=-0.38, B0=1.43, and h=0.050, assuming a surface roughness parameter theta=20 deg. By combining our photometric study with a thermal model of the asteroid (Mueller et al. in preparation), we obtained a geometric albedo of pv = 0.047 +- 0.003, phase integral q = 0.32 +- 0.03, and Bond albedo AB = 0.014 +- 0.002, which are commensurate with the values for common C-type asteroids.

Laboratory photometry of planetary regolith analogs

Laboratory phase curves of planetary regolith analog materials are presented. A study is made of the effect of compaction of the material on its backscattering properties. Further study is also made of the contribution of material surface roughness on its light scattering. First photometric light backscattering measurements in a microgravity environment are introduced as well as some improvements of the laboratory experiment techniques. The measurements show a strong increase in both reflectance and opposition effect amplitude under compaction. Also a broadening of the opposition effect width is observed. These results are in contrast with some of the previous studies on the subject. The surface roughness of a sample is found to be an important factor in measurements of samples with the same packing density. This should be taken into account in further studies.

Opposition Effect of Asteroids

The opposition effect behavior is analyzed for 33 asteroids with well-measured magnitude-phase curves. It is shown that surface albedo is the main factor influencing the amplitude and width of the asteroid opposition effect. The amplitude of opposition effect depends on asteroid albedo in a non-monotonic way with the maximum for moderate albedo asteroids decreasing both for dark and for high albedo asteroids. This can be explained by the combined influence of both shadow hiding and coherent-backscatter mechanisms. The relative contribution of the coherent-backscatter effect to the asteroid magnitude-phase dependence close to opposition (α=0.3°) is estimated to be about 20–60% for low albedo asteroids and 80–90% for high and moderate albedo asteroids. At α>5° the shadow-hiding effect has the dominant influence. A tendency was found for asteroids with larger U-B colors to show larger values of the opposition effect. We also compare the opposition effects of asteroids with those of some atmosphereless satellites and meteorites.

Europa's phase curve: Implications for surface structure

Ground-based photoelectric measurements taken by two of us (W.L. and D.T.) with narrow band filters at 0.47 and 0.55 μm at small phase angles clearly show the existence of a narrow opposition peak. We have combined these data with Voyager images to obtain a range of phase angle coverage from 0.3° to 119°. The integral phase data were analyzed using Hapke's photometric function. The photometric parameters are remarkably similar for both wavelengths and hemispheres. The following values at λ = 0.55 μm for the leading side are typical: single-scattering albedo w = 0.964, opposition effect amplitude B 0 = 0.5, opposition effect width h = 0.0016, double-lobed Henyey-Greenstein factors b = −0.429, c = 0.113, and mean roughness angle θ = 10° . The photometric roughness is much smaller than on other bodies of the Solar System and implies that low surface relief seen at kilometer scales in the Voyager images extends down to centimeter scales. If the very narrow opposition surge is due to shadow hiding, the porosity must be large, on the order of 96%, which is much higher than that of a regolith generated by meteoritic gardening. Both the small roughness and high porosity imply that endogenic processes dominate surface formation even at centimeter scales. The single particle scattering function is more back scattering on the leading hemisphere, which contradicts the scattering properties expected of a planetary surface that is being preferentially bombarded by ions on its trailing hemisphere.