Surface Photometric Properties Research Articles

Sample data selection based on the feature subspace for photometric correction of the Chang'E-4 in situ observations

The lunar surface exhibits noticeable variations in photometric properties across different regions, influenced by their specific compositional and physical characteristics. These variations are particularly pronounced at the millimeter to centimeter scale. In this study, we propose a feature subspace-based sample selection method to identify Chang'E-4 Visible Near-infrared Imaging Spectrometer (CE-4 VNIS) in situ observations that share similar photometric properties at the centimeter scale. By constructing a feature subspace through principal component transformation, the CE-4 VNIS observation sites with comparable surface photometric properties are able to be selected from a series of observations. The selection results demonstrate that the majority of observation sites are covered by the lunar regolith, with a few exceptions consisting of rocks, soil bulk, shadows, and rough surfaces. These observations should be excluded from the photometric correction process during phase function fitting. Analyzing the photometric performance of the VNIS data with lunar regolith reveals that reflectance decreases as the phase angle increases below 80°, while above 80°, reflectance increases with increasing phase angle. This phenomenon is likely attributed to stronger forward scattering. Thus, a second-order polynomial is employed to fit the phase function for VNIS data. Consequently, a photometric model is developed for lunar regolith observed by CE-4 VNIS, incorporating the fitted phase function. Experimental results demonstrate reduced differences among observations with varying phase angles, and the corrected VNIS spectra exhibit a phase-reddening effect. These findings validate the effectiveness of the proposed model. A comparison between the corrected data with and without sample selection reveals a 33 % decrease in mean standard deviation for VIS/NIR bands and a 19 % decrease for SWIR bands in the corrected spectra after sample selection. These results indicate the potential applicability of our method for future VNIS observations, enabling the acquisition of photometrically consistent data.

Pre and post-landing atmospheric optical depths at the Zhurong landing site on Mars retrieved using a single-image-based approach

The atmosphere of Mars has complex photometric processes due to multiple scattering and absorption by the suspended aerosols. The atmospheric optical depth is an indicator of the aerosol concentrations and can be used to model the contribution of atmospheric scattering, thereby correcting surface spectra. The atmospheric optical depth is also important in analysing the variations of atmospheric dust and evaluating the risks facing landing and roving missions. Retrieving the atmospheric optical depth from a single image is desirable as simultaneous stereo observations of the Martian surface are not common. However, most of the existing single-image-based methods rely on shadows in the image, which can be challenging to identify when the site is smooth or when the atmosphere becomes turbid. In this paper, we present a method of retrieving the atmospheric optical depth from a single image based solely on non-shadowed surfaces. This method was validated using HiRISE images and measurements acquired by rovers on the Martian surface. The method achieved an RMSE of 2%–7% in most cases, depending on the different surface photometric models used. The results indicated that aerosol scattering parameters have less impact on the retrievals than the surface photometric properties, likely due to the fact that the data is optically thin. The optical depths at the Zhurong landing site before and after landing were estimated using the proposed method. The results show that the optical depths first decreased and then increased, with the turning point being around the landing date, indicating that the Zhurong rover landed at an appropriate time. The proposed method is of significance for the analysis of Martian atmospheric dust and surface spectra with better spatio-temporal resolutions.

Influence of solar incidence angle on single-image photoclinometry for precision lunar topographic mapping

Photoclinometry is an important method of reconstructing the topography of planetary surfaces. The technique relies on the photometric response of the surface under known illumination and viewing conditions. Each image pixel embeds the geometric and radiometric information of the surface, thereby allowing the underlying surface to be inferred and reconstructed in full. Photoclinometry is renowned for its ability to reconstruct pixel-wise three-dimensional (3D) models from a single image. However, it is affected by various error sources, including the radiometric quality of the sensor and variations in the surface photometric properties (e.g., albedo). This paper investigated the optimal solar incidence angle for photoclinometric reconstruction using error propagations and simulation analysis, and validated the findings using up-to-date photoclinometric methods and Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) datasets. The results suggested that variations in albedo have a significant effect on the quality of the reconstructed topography. The findings agreed with previous studies, which implies a potential bottleneck in the development of photoclinometric methods. Moreover, the photoclinometric performance becomes stable when the solar incidence angle exceeds 30° and the optimal incidence angle is approximately 60°, depending on the overall topography. Too large an incidence angle results in severe shadows, which hides topographic details, whereas too small an incidence angle introduces errors. The findings enrich the theoretical understanding of photoclinometry and are useful for optimising data capture strategies for future planetary missions to maximise the ability of photoclinometry and provide a foundation for future development of the technique. The recommendations on solar incidence angle and assessment strategies can be directly applied to other airless planetary bodies, such as Mercury and asteroids, for topographic mapping based on photoclinometry.

Surface reflectance of Mars observed by CRISM/MRO: 2. Estimation of surface photometric properties in Gusev Crater and Meridiani Planum

AbstractThe present article proposes an approach to analyze the photometric properties of the surface materials from multi‐angle observations acquired by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on‐board the Mars Reconnaissance Orbiter. We estimate photometric parameters using Hapke's model in a Bayesian inversion framework. This work also represents a validation of the atmospheric correction provided by the Multi‐angle Approach for Retrieval of Surface Reflectance from CRISM Observations (MARS‐ReCO) proposed in the companion article. The latter algorithm retrieves photometric curves of surface materials in reflectance units after removing the aerosol contribution. This validation is done by comparing the estimated photometric parameters to those obtained from in situ measurements by Panoramic Camera instrument at the Mars Exploration Rover (MER) Spirit and MER Opportunity landing sites. Consistent photometric parameters with those from in situ measurements are found, demonstrating that MARS‐ReCO gives access to accurate surface reflectance. Moreover, the assumption of a non‐Lambertian surface as included in MARS‐ReCO is shown to be significantly more precise to estimate surface photometric properties from space in comparison to methods based on a Lambertian surface assumption. In the future, the presented method will allow us to map from orbit the surface bidirectional reflectance and the related photometric parameters in order to characterize the Martian surface.

Results from the EPOXI and StardustNExT Missions – A Changing View of Comet Volatiles and Activity

AbstractWithin a period of ~3 months there were two extended mission flybys of comets. Both encounters have provided an exciting new view of comet activity and volatile composition that is changing our paradigm of these small early solar system remnants. The EPOXI mission flew past the nucleus of comet 103P/Hartley 2 on 4 Nov. 2010. This small nucleus was known to be exceptionally active prior to the encounter, by virtue of a very large water production rate relative to its surface area. Both the encounter and ground-based data showed that comet Hartley 2fs perihelion activity was dominated by sub-surface CO2 outgassing rather than by water, suggesting our classic comet formation picture is not correct. The gas flow carried large grains (up to >10 cm in diameter) from the nucleus, and the icy grains contributed to the large observed water production. The CO2 abundance relative to water varies with rotation between 10-20% between the two lobes of the nucleus. The bi-lobed nucleus is rotating in an excited state, with a period that varied rapidly from ~16.5 hrs to longer than 18.5 hrs over 3 months. The nucleus morphology was different from that of other nuclei visited by space craft, with some regions of rough topography in which surface ice was visible. On 2011 Feb. 14 the Stardust-NExT spacecraft flew past the nucleus of comet 9P/Tempel 1, the target of the Deep Impact (DI) experiment in July 2005. The mission goal was to look at the nucleus after and intervening perihelion passage, extending the surface area imaged during the DI encounter and also image the 2005 impact site. The layering seen during the DI flyby was exhibited over the areas newly imaged in the NExT flyby, and it was found that 30% of the nucleus was covered by smooth deposits that were likely caused by eruption of subsurface materials. Although it has long been known that comets lose on average ~ a meter of their surface per perihelion passage, it was surprising to see that in the regions imaged by both DI and NExT there was little change in the surface photometric properties and morphology with the exception of the prominent smooth flow edges. As seen from both the spacecraft and ground-based campaign, the comet continued its trend of decreasing activity from previous perihelion passages. We will present highlights from both missions and discuss implications for formation scenarios.

GALEXULTRAVIOLET IMAGING OF DWARF GALAXIES AND STAR FORMATION RATES

We present ultraviolet integrated and azimuthally-averaged surface photometric properties of a sample of 44 dIm, BCD, and Sm galaxies measured from archival NUV and FUV images obtained with GALEX. We compare the UV to Halpha and V-band properties and convert FUV, Halpha, and V-band luminosities into star formation rates (SFRs). We also model the star formation history from colors and compare the integrated SFRs and SFR profiles with radius for these methods. In most galaxies, the UV photometry extends beyond Halpha in radius, providing a better measure of the star formation activity in the outer disks. The Halpha appears to be lacking in the outer disk because of faintness in low density gas. The FUV and V-band profiles are continuous with radius, although they sometimes have a kink from a double exponential disk. There is no obvious difference in star formation properties between the inner and outer disks. No disk edges have been observed, even to stellar surface densities as low as 0.1 Msun/pc2 and star formation rates as low as 10^{-4} Msun/yr/kpc2. Galaxies with low HI to luminosity ratios have relatively low FUV compared to V-band emission in the outer parts, suggesting a cessation of star formation there. Galaxies with relatively high HI apparently have fluctuating star formation with a Gyr timescale.

25143 Itokawa: direct detection of the current decelerating spin state due to YORP effect

Aims. The Hayabusa mission revealed fundamental physical properties of the small near-Earth asteroid 25143 Itokawa, such as shape and mass, during its rendezvous with the asteroid in 2005. Resulting from this, the YORP-induced change in the asteroid’s spin state has been predicted theoretically. The purpose of this study is to investigate the YORP effect for Itokawa directly from an observational perspective. Methods. A long-term campaign of ground-based photometric observations of Itokawa were performed from March 2001 to December 2006. The observed asteroid lightcurves were compared with numerical modeling using the detailed shape model and global surface photometric properties derived from the Hayabusa mission. Results. As a non-linear time evolution of rotational phase lag is shown, we found that Itokawa has been decreasing its spin rate by u ω = −1.2(±0.2) × 10 −17 rad s −2 . The detected deceleration rate is almost consistent with, but slightly smaller than, the theoretically predicted value due to the YORP effect.

The Small Inner Satellites of Jupiter

The images of the four inner small jovian satellites obtained by the Galileo Solid State Imaging (SSI) experiment have much more detailed shape, color, and photometric information than were provided previously by Voyager images. The satellites are in synchronous rotation and show no binary or bifurcated shapes. Thebe and Amalthea have densities of large craters approximately at “empirical equilibrium” levels. The leading sides of Metis, Amalthea, and Thebe are all 25–35% brighter than their trailing sides; the global-average, clear-filter (λ = 0.64 μm) geometric albedos of these three satellites are 0.063, 0.091, and 0.049, respectively. A definite color gradient is observed, with the satellites closer to Jupiter being redder: the mean violet/green ratio (0.42/0.56 μm) decreases from Thebe to Metis. This ratio also is lower for the trailing sides of Thebe and Amalthea than for their leading sides. Bright spots on Amalthea and Thebe are small (<20 km) patches on ridges or near crater rims and have albedo contrasts with their surroundings of more than 2:1. The colors of the spots are slightly different from the average surrounding surfaces. There are suggestions of absorption features near 1 μm on Thebe and Amalthea. While the colors of the small satellites indicate possible contamination by materials lost from Io, the brighter leading sides of all three well-measured objects, spanning a region from inside to outside the synchronous orbit distance, suggest that macroscopic impacts may have a substantial influence on surface photometric properties.

Spectrophotometry of Mars in the KRFM experiment of the Phobos mission : some properties of the particles of atmospheric aerosols and the surface

A new analysis of the data of the KRFM experiment ( Phobos mission) is presented. The five complete photometric profiles (eight wavelengths for each) obtained in this experiment have been processed, and the calibration and the method of modelling calculations have been specified. The optical depth of the constant haze has been estimated to be within the range 0.1 < τ < 0.2. This is essentially smaller than the value predicted for the same season on the basis of the Viking landers' measurements. The imaginary part of the refraction index of the particles in the constant haze changes smoothly from 0.015 at 315 nm to 0.005 at 550 nm. This behaviour corresponds to some basaltic material, containing several percent of iron oxides (limonite, geothite, hematite). Their presence in the Martian soil was recognized a long time ago, but our study gives some new quantitative estimations of Fe 2O 3 content. The clouds above the large volcanoes have an optical depth ∼0.1 and contain about 5 × 10 −6 g cm −2 of ice particles. All the evaluations of the constant haze particles' properties have been made assuming a gamma size distribution of particles with modal radius r m = 0.4 μm and α = γ = 1. For the description of the surface's photometric properties in the whole range 315–550 nm, the roughness factor q = 0.5 is acceptable as a first approximation, although one cannot exclude that q rises with increasing wavelength. The averaged radius of the optically active particles of the surface has been evaluated as 6–7 μm. The method for the calculation of photometric profiles of Mars used in the paper is more reliable than any published version we know, because it takes into account the main optical properties of the Martian atmosphere (the presence of absorption, strong forward scattering) and surface (including a non-Lambertian reflection law).

Surface scattering properties estimated from modeling ASAS multiple emission angle reflectance data over the Lunar Crater Volcanic Field, Nevada

ASAS multispectral data were acquired over Lunar Lake playa at 12 different lighting and viewing geometries. Atmospheric models and field‐based data demonstrate that atmospheric contributions to ASAS data were relatively small (&lt;15%) and were thus ignored in analyses of surface photometric properties. ASAS data of an artificially roughened playa and cobble‐strewn surfaces exhibit trends of increasing reflectance as phase angle decreases. The reflectance of a natural playa surface exhibits Lambertian scattering properties that cannot be replicated with the Hapke [1986] photometric function. The Hapke model does fit the rough and cobble‐strewn playa data with single scattering albedo, asymmetry factor, and roughness terms consistent with known properties of these surfaces. No published photometric model explains the reflectance variations of all three surfaces.

Voyager photometry of Triton: Haze and surface photometric properties

We have analyzed Voyager whole‐disk observations of Triton at violet (0.41 μm), blue (0.48 μm),and green (0.56 μm) filter wavelengths, using a model which combines an improved version of Hapke's photometric equation with a thin atmospheric haze layer in the appropriate spherical geometry. The model accurately describes the phase curves over a wide range of phase angles (10° ≤ α ≤ 159°) and agrees well with disk‐resolved brightness scans along the photometric equator and mirror meridian. An upturn in the phase curves seen at the highest phase angles observed (Smith et al., 1989) can be explained by including scattering in a thin atmospheric haze layer with optical depths ranging from 0.066 in the violet to 0.036 for the green filter data. The haze is forward scattering with g ∼ +0.6, requiring particle sizes of ≳0.2 μm. The haze single‐particle scattering albedo is near unity at all three wavelengths, suggesting the haze particles are relatively neutral ice condensates. The geometric albedo, phase integral, and spherical albedo of Triton in each filter corresponding to our best fit Hapke and atmospheric parameters are derived. The bolometric Bond albedo of Triton calculated from our results is 0.85 ± 0.05. If the 16‐μbar N2 atmosphere detected by the Voyager radio occultation experiment (Tyler et al., 1989) is in vapor equilibrium with the surface (therefore implying a surface temperature of 37.7 K), then our Bond albedo implies a surface emissivity of 0.46 ± 0.16.