Single-scattering Phase Function Research Articles

Vertical structure and optical properties of Titan’s aerosols from radiance measurements made inside and outside the atmosphere

Prompted by the detection of stratospheric cloud layers by Cassini’s Composite Infrared Spectrometer (CIRS; see Anderson, C.M., Samuelson, R.E. [2011]. Icarus 212, 762–778), we have re-examined the observations made by the Descent Imager/Spectral Radiometer (DISR) in the atmosphere of Titan together with two constraints from measurements made outside the atmosphere. No evidence of thin layers (<1km) in the DISR image data sets is seen beyond the three previously reported layers at 21km, 11km, and 7km by Karkoschka and Tomasko (Karkoschka, E., Tomasko, M.G. [2009]. Icarus 199, 442–448). On the other hand, there is evidence of a thicker layer centered at about 55km. A rise in radiance gradients in the Downward-Looking Visible Spectrometer (DLVS) data below 55km indicates an increase in the volume extinction coefficient near this altitude. To fit the geometric albedo measured from outside the atmosphere the decrease in the single scattering albedo of Titan’s aerosols at high altitudes, noted in earlier studies of DISR data, must continue to much higher altitudes. The altitude of Titan’s limb as a function of wavelength requires that the scale height of the aerosols decrease with altitude from the 65km value seen in the DISR observations below 140km to the 45km value at higher altitudes. We compared the variation of radiance with nadir angle observed in the DISR images to improve our aerosol model. Our new aerosol model fits the altitude and wavelength variations of the observations at small and intermediate nadir angles but not for large nadir angles, indicating an effect that is not reproduced by our radiative transfer model. The volume extinction profiles are modeled by continuous functions except near the enhancement level near 55km altitude. The wavelength dependence of the extinction optical depth is similar to earlier results at wavelengths from 500 to 700nm, but is smaller at shorter wavelengths and larger toward longer wavelengths. A Hapke-like model is used for the ground reflectivity, and the variation of the Hapke single scattering albedo with wavelength is given. Fits to the visible spectrometers looking upward and downward are achieved except in the methane bands longward of 720nm. This is possibly due to uncertainties in extrapolation of laboratory measurements from 1km-am paths to much longer paths at lower pressures. It could also be due to changes in the single scattering phase functions at low altitudes, which strongly affect the path length through methane that the photons travel. We demonstrate the effects on the model fits by varying each model parameter individually in order to illustrate the sensitivity of our determination of each model parameter.

Asteroid lightcurve inversion with Lommel–Seeliger ellipsoids

We derive initial rotation, shape, and scattering properties for asteroids from sparse and dense photometry based on the so-called Lommel–Seeliger ellipsoid (LS ellipsoid). Due to the analytical disk-integrated brightness, the LS ellipsoid allows for fast rotation-period, pole-orientation, and shape analyses, as well as efficient Markov-chain Monte Carlo solutions (MCMC). We apply the methods to simulated sparse Gaia photometry, as well as to ground-based photometry composed of dense lightcurves. For a specific Gaia simulation, we make use of a numerical reflection coefficient developed for particulate surfaces, and utilize the LS ellipsoid in the inversion of the simulated data. We conclude that, in a majority of cases, initial LS ellipsoid retrieval of the parameters is satisfactory. Finally, we formulate a single-scattering phase function that, for a spherical asteroid, results in the H,G1,G2 photometric phase function.

Seasonal variation of Titan’s haze at low and high altitudes from HST-STIS spectroscopy

The Space Telescope Imaging Spectrograph accumulated image cubes of Titan in five years between 1997 and 2004 that we calibrated and analyzed. The observations probe Titan’s early northern fall to early winter. Methane bands between 543 and 990nm wavelength are well resolved spectrally, and Titan’s latitudinal and center-to-limb reflectivity variations are resolved spatially. A principal component analysis revealed two large components and two small components of less significance. The first principal component describes a variation of Titan’s haze below 80±20km altitude. Haze particles change their size, opacity, and/or shape of the single scattering phase function. The largest and smallest opacities occurred both in 1997 at high southern latitudes and northern latitudes, respectively. The hemispherical asymmetry switched sign in 2002 at low latitudes, in 2003 at mid latitudes, and in early 2004 at high latitudes. The seasonal amplitude increased almost linearly with distance from the Equator. Tropical latitudes had slightly lower opacities than the annual and global average if the observed variation is seasonally symmetric and shaped like a sine curve. The cause for the variation may be condensation of gases onto aerosols seasonally driven by atmospheric dynamics. The second principal component describes a variation of haze opacity at altitudes above 150±50km. Largest and smallest opacities both occurred in 2004 at northern and high southern latitudes, respectively. The asymmetry switched in late 2001. Tropical latitudes had significantly higher haze opacity than the annual and global average, opposite to the case at low altitudes. The cause for the high-altitude variation may be aerosols transported at varying speeds driven by atmospheric dynamics. We present a seasonal model that completely describes the haze parameters at each altitude, latitude, and time. It compares fairly well with Cassini results obtained since 2004. The north–south asymmetry may reverse in 2016 at high altitudes and 2017 through 2018 at low altitudes. The observed variations are significant for modeling photometric data of Titan’s surface. They describe several characteristics of Titan’s haze variations that can be compared with results from Global Circulation Models.

Scattering in structured two-layered medium

Multiple scattering in two-layered medium was modeled using Monte Carlo numerical simulation. Special case of group formation in such layers was considered. To estimate the impact of structure factor on the overall results, the rigorous calculation of single scattering phase function was used. Comparison with the Monte Carlo simulations for the respective model phase function shows that there were some differences in the scattered radiance. These differences had no significant energy redistribution in the angular dependence of the scattered radiance, but different elongation in the zero-direction occurred. So it was shown that the group formation and special structure still leaded to some changes in the scattering process, even if the medium had several layers with different parameters and had rather large width.

Martian airfall dust on smooth, inclined surfaces as observed on the Phoenix Mars Lander telltale mirror

The telltale mirror, a smooth inclined surface raised over 1m above the deck of the Phoenix Mars Lander, was observed by the Surface Stereo Imager (SSI) several times per sol during the Phoenix Mars Lander mission. These observations were combined with a radiative transfer model to determine the thickness of dust on the wind telltale mirror as a function of time. 239 telltale sequences were analyzed and dustiness was determined on a diurnal and seasonal basis. The thickness of accumulated dust did not follow any particular diurnal or seasonal trend. The dust thickness on the mirror over the mission was 0.82±0.39µm, which suggests a similar thickness to the modal scattering particle diameter. This suggests that inclining a surface beyond the angle of repose and polishing it to remove surface imperfections is an effective way to mitigate the accumulation of dust to less than a micron over a wide range of meteorological conditions and could be beneficial for surfaces which can tolerate some dust but not thick accumulations, such as solar panels. However, such a surface will not remain completely dust free through this action alone and mechanical or electrical clearing must be employed to remove adhered dust if a pristine surface is required. The single-scattering phase function of the dust on the mirror was consistent with the single-scattering phase function of martian aerosol dust at 450nm, suggesting that this result is inconsistent with models of the atmosphere which require vertically or horizontally separated components or broad size distributions to explain the scattering behavior of these aerosols in the blue. The single-scattering behavior of the dust on the mirror is also consistent with Hapke modeling of spherical particles. The presence of a monolayer of particles would tend to support the spherical conclusion: such particles would be most strongly adhered electrostatically.

Optical properties of the Venus upper clouds from the data obtained by Venus Monitoring Camera on-board the Venus Express

During more than 6 years of the Venus Express (VEx) mission, the Venus Monitoring Camera (VMC) took around 300000 images in four channels covering almost all the latitudes, including night and day sides. Here we give an overview of the VMC data and summarize results of retrievals of the optical properties of the Venus upper clouds. The in-flight characterization and calibration of VMC are also described. We model the phase dependence of brightness (phase range α=0–140°) retrieved from the dayside images obtained in NIR1 VMC channel at various latitudes (30°N–60°S) and local solar times (6–18h). The radiative transfer calculations were performed for the plane-parallel atmospheric layers, and the Mie theory was used for the calculations of the single scattering phase functions of the cloud aerosols. The size distribution of cloud particles and their refractive index were estimated for each of the regions observed. These retrievals show some temporal and spatial variations. In general, the particles at low latitudes are somewhat larger than in the regions closer to the southern pole (Reff=1.2–1.4μmversus0.9–1.05μm). At latitudes 40°S–60°S the refractive index is usually smaller than in the other regions (mr=1.44–1.45versus 1.45–1.47 with sporadic spikes of up to 1.49). The retrievals robustly show presence of particles with a radius of about Reff=0.9μm in the clouds and/or the haze above them in these mid-latitudes. Small submicron (Reff=0.23μm) particles are detected mostly in the morning.

Shadow mechanism and the opposition effect of brightness of atmosphereless celestial bodies

We consider the Irvine-Yanovistkii modification of the shadow model developed by Hapke for the opposition effect of brightness. The relation between the single scattering albedo ω and the transparency coefficient of particles κ is suggested to be used in the form κ = (1 − ω)n, which allows the number of unknowns in the model to be reduced to two parameters (the packing density of particles g and ω) and the single-scattering phase function χ(α). The analysis of spectrophotometric measurements of the moon and Mars showed that the data on the observed opposition effect and the changes in the color index with the phase angle α well agree if the values of n = 0.25 and g = 0.4 (the moon) and 0.6 (Mars) are assumed in calculations. When being applied to asteroids of several types, this method also yielded a satisfactory agreement. For the E-type asteroids, the sets of parameters are [g = 0.6, ω = 0.6, Ag = 0.21, and q = 0.83] or [g = 0.3, ω = 0.4, Ag = 0.15, and q = 0.71] under the Martian single-scattering phase function; for the M-type asteroids, it is [g = 0.4, ω ≤ 0.1, Ag ≤ 0.075, and q ≤ 0.42] under the lunar single-scattering phase function; for the S-type asteroids, it is [g = 0.4, ω = 0.4, Ag = 0.28, and q = 0.49] under the lunar single-scattering phase function; and for the C-type asteroids, it is [g = 0.6, ω ≤ 0.1, Ag ≤ 0.075, and q = 0.43] under the modified lunar single-scattering phase function. The polarization measurements fulfilled by Gehrels et al. (1964) for the bright feature on the lunar surface, Copernicus (L = -20°08′, φ = +10°11′), at a phase angle α = 1.6° revealed the deviations in the position of the polarization plane from that typical for the negative branch. They were 22° and 12° in the G and I filters, respectively. At the same time, the deviation was within the error (±3°) in the U filter and for the dark feature Plato (L = -10°32′, φ = +51°25′), which can be caused by the coherent mechanism of the formation of the polarization peak.

Lateral light scattering in fibrous media

Lateral light scattering in fibrous media is investigated by computing the modulation transfer function (MTF) of 22 paper samples using a Monte Carlo model. The simulation tool uses phase functions from infinitely long homogenous cylinders and the directional inhomogeneity of paper is achieved by aligning the cylinders in the plane. The inverse frequency at half maximum of the MTF is compared to both measurements and previous simulations with isotropic and strongly forward single scattering phase functions. It is found that the conical scattering by cylinders enhances the lateral scattering and therefore predicts a larger extent of lateral light scattering than models using rotationally invariant single scattering phase functions. However, it does not fully reach the levels of lateral scattering observed in measurements. It is argued that the hollow lumen of a wood fiber or dependent scattering effects must be considered for a complete description of lateral light scattering in paper.

Retrieval of jovian cloud structure from the Cassini ISS limb-darkening data: I. Continuum scattering phase functions for cloud and haze in the South Tropical Zone

We have deduced the scattering properties of aerosols in the jovian upper troposphere by analyzing imaging data obtained at a wide variety of solar phase angles (4–140°) by the Cassini Imaging Science Subsystem (ISS) Narrow Angle Camera (NAC) during its flyby of Jupiter. The limb-darkening curves along the South Tropical Zone (STrZ) are extracted from CB2 images (effective wavelength: 750nm) to constrain the single scattering phase functions of aerosols. The best-fit Mie scattering phase function for cloud is obtained with the real part of the refractive index nr,cloud=1.85 and the effective radius reff,cloud=0.3μm. The best-fit combination of nr,cloud and reff,cloud would strongly suggest the idea that the abundant small particle population in the upper troposphere is not composed of pure NH3 ice. Although the optical properties of the stratospheric haze are not well constrained compared with those of cloud, the haze is found to be optically thin (<0.06) and to be strongly forward scattering (effective radius reff,haze=0.5μm).We compare our results with the scattering phase function at red wavelength (640nm) for the STrZ derived by Tomasko et al. (Tomasko, M.G., West, R.A., Castillo, N.D. [1978]. Icarus 33, 558–592), which was deduced from analysis of the Pioneer 10 Imaging Photopolarimeter (IPP) data (12–150° for solar phase angles). Our new Mie scattering phase function can reproduce the Pioneer 10 observations well. In contrast, their scattering phase function described by the double Henyey–Greenstein function does not reproduce the Cassini ISS observations. This is attributed to the fact that their scattering phase function is underconstrained, primarily due to a considerable gap in observations for an intermediate solar phase angle range (34–109°).Our new Mie scattering phase function has advantages over that of Tomasko et al. (Tomasko, M.G., West, R.A., Castillo, N.D. [1978]. Icarus 33, 558–592).(1)Since the Cassini ISS data do not have a large gap in solar phase angle, the new Mie scattering phase function is better constrained.(2)The Mie scattering phase function can be applied easily to different wavelengths.With such characteristics, we now have a reliable baseline scattering phase function that can be used to interpret the ever-changing appearance of jovian clouds as changes of the vertical cloud structure and/or distribution of chromophores in the atmosphere.

Multiple scattering of light by mixtures of two different aerosol types

If particles are mixed with other type of particles, the light scattering characteristics will be changed. In this paper, the light scattering characteristics of water clouds mixed with black carbon are studied. Based on the Mie theory, the single scattering phase function, the single scattering albedo and the asymmetric parameters of water clouds and black carbon aerosols are computed. The Monte Carlo method for simulating the scattering of mixed particles is given, the methods of sampling particle types, sampling free path length, and the sampling scattering directions based on the Mie phase function are given. The changes of the scattering characteristics for water clouds mixed with black carbon aerosols are studied. Reflected light intensity varying with observation angle is calculated with the light normal incidence, and the plane albedo is computed as a function of incidence angle. The influences of mixing ratio and effective radius of black carbon on the scattering characteristic of mixture are discussed. The results show that the black carbon can greatly increase the absorption of the clouds, and the ratio and the size of black aerosols can greatly affect the scattering characteristics of the mixture.

Lateral light scattering in paper - MTF simulation and measurement

The modulation transfer function (MTF) of 22 paper samples is computed using Monte Carlo simulations with isotropic or strongly forward single scattering. The inverse frequency at half maximum of the MTF (kp) is found inappropriate as a single metric for the MTF since it is insensitive to the shape of the modeled and simulated MTF. The single scattering phase function has a significant impact on the shape of the MTF, leading to more lateral scattering. However, anisotropic single scattering cannot explain the larger lateral scattering observed in paper. It is argued that the directional inhomogeneity of paper requires a light scattering model with both the phase function and scattering distances being dependent on the absolute direction.

Do different turbid media with matched bulk optical properties also exhibit similar polarization properties?

We here investigate polarimetric behavior of thick samples of porcine liver, Intralipid, and microsphere-based tissue phantoms whose absorption and scattering properties are matched. Using polarized light we measured reflection mode Mueller matrices and derived linear/circular/total depolarization rates, based on polar decomposition. According to our results, phantoms exhibit greater depolarization rates in the backscattering geometry than the liver sample. The enhanced tissue polarization preservation differs from previous reports of polarimetric transmission studies, with the likely cause of this difference being the angular dependence of the single-scattering phase function. Also, Intralipid approximated polarimetric liver behavior well, whereas the polystyrene phantoms did not.

On Hapke photometric model predictions on reflectance of closely packed particulate surfaces

In a recent paper Hapke et al. (Hapke, B., Shepard, M., Nelson, R., Smythe, W., Piatek, J. [2009]. Icarus 199, 210–218) performed bi-directional reflectance measurements on closely-packed particulate surfaces of micrometer-sized particles and compared these with both the Hapke IMSA photometric model, and a numerical radiative transfer algorithm, the MDYZ (Mishchenko, M., Dlugach, J., Yanovitskij, E., Zakharova, N. [1999a]. J. Quant. Spectrosc. Radiat. Trans. 63, 409–432). To account for the effects of close packing, Hapke et al. applied a diffraction truncation scheme to remove the diffraction spike and supplied the renormalized single scattering phase function to the IMSA. They found that the IMSA prediction is a better match with measurement than that of MDYZ. In this work we demonstrate that the diffraction truncation procedure outlined by Hapke et al. contains an error. By following Hapke et al.’s intended truncation scheme, we have found that the IMSA model is not sufficiently anisotropic to describe the reflectance pattern of measurements on surface reflectance of closely packed large spherical particles.

Effective scattering phase functions for the multiple scattering regime

The propagation of light through turbid media is of fundamental interest in a number of areas of optical science including atmospheric and oceanographic science, astrophysics and medicine amongst many others. The angular distribution of photons after a single scattering event is determined by the scattering phase function of the material the light is passing through. However, in many instances photons experience multiple scattering events and there is currently no equivalent function to describe the resulting angular distribution of photons. Here we present simple analytic formulas that describe the angular distribution of photons after multiple scattering events, based only on knowledge of the single scattering albedo and the single scattering phase function.

Modelling Laboratory Data of Bidirectional Reflectance of a Regolith Surface Containing Alumina

AbstractBidirectional reflectance of a surface is defined as the ratio of the scattered radiation at the detector to the incident irradiance as a function of geometry. Accurate knowledge of the bidirectional reflection function for layers composed of discrete, randomly positioned scattering particles is essential for many remote sensing, engineering, and biophysical applications, as well as for different areas of astrophysics. Computations of bidirectional reflection functions for plane parallel particulate layers are usually reduced to solving the radiative transfer equation by the existing techniques. In this work we present our laboratory data on bidirectional reflectance versus phase angle for two sample sizes of alumina, 0.3 and 1 μm, for the He–Ne laser at wavelengths of 632.8 nm (red) and 543.5 nm (green). The nature of the phase curves of the asteroids depends on the parameters like particle size, composition, porosity, roughness, etc. In the present study we analyze data which are being generated using a single scattering phase function, that is, Mie theory of treating particles as a compact sphere. The well-known Hapke formula, along with different particle phase functions such as Mie and Henyey–Greenstein, will be used to model the laboratory data obtained at the asteroid laboratory of Assam University.

Contrast improvement by selecting ballistic-photons using polarization gating

In this paper a new approach to improve contrast in optical subsurface imaging is presented. The method is based on time-resolved reflectance and selection of ballistic photons using polarization gating. Numerical studies with a statistical Monte Carlo method also reveal that weakly scattered diffuse photons can be eliminated by employing a small aperture and that the contrast improvement strongly depends on the single-scattering phase function. A possible experimental setup is discussed in the conclusions.

Optical properties of aerosols in Titan’s atmosphere: Large fluffy aggregates

Scattering properties of large ( N>2000) aggregates of monomers as found in Titan’s atmosphere during the descent of the Cassini/Huygens probe are numerically calculated. We extend our previous quantitative analysis of small aggregates ( N≤256) to verify the obtained conclusions. To calculate the light scattering of such big non-spherical aggregates we use the DDSCAT code based on the discrete dipole approximation (DDA) technique introduced by Draine and Flatau (1994). The orientation averaged single-scattering phase function as well as the degree of linear polarization is calculated. Clusters with up to 2048 monomers are investigated in the optical wavelength range. We conclude that (a) the applied mathematical aggregation mechanism plays only a minor role, and it is sufficient to test particles formed by diffusion limited aggregation (DLA) with different cluster parameters; (b) the aggregate “fluffiness” can be qualitatively derived from polarization data; (c) for a fixed number of monomers the scattering function of intensity is sensitive to the size parameter of the aggregates (e.g. to the effective cluster density); (d) consequently, characterization of aggregates by their number of monomers only is generally insufficient and more detail cluster descriptions are needed; (e) even when an aggregate is described more explicitly significant intensity variations can be observed for “identical” model particles over the whole range of scattering angles, particular for large fluffy aggregates; (j) random generation averaging should be added to the standard orientation averaging procedure. Finally, we discuss our results in respect of the conclusions of the Tomasko et al. (2008) paper.

Determination of aerosol optical thickness through the derivation of an atmospheric correction for short-wavelength Landsat TM and ASTER image data: an application to areas located in the vicinity of airports at UK and Cyprus

Aerosol optical thickness is considered to be the most important unknown parameter of every atmospheric correction approach for removing atmospheric effects from satellite remotely sensed images. This study presents a description of the basics of the proposed atmospheric correction procedure, which combines the darkest object subtraction principle and the radiative transfer equations. The method considers the true reflectance values of the selected dark targets acquired in situ and the atmospheric parameters such as the aerosol single scattering phase function, single scattering albedo and water vapour absorption, which are also found from ground measurements. The proposed procedure is applicable to short wavelengths such as Landsat TM band 1, 2 and ASTER band 1 in which water vapour absorption is negligible. The proposed image processing method has been tested successfully to determine the aerosol optical thickness on Landsat-5/TM images of the Lower Thames Valley area located to West London (UK) in the vicinity of Heathrow Airport and to Landsat TM/ETM+ and ASTER images of an area located in the vicinity of Paphos International Airport (Cyprus). The determined aerosol optical thicknesses for the Heathrow Airport area were 0.60, 013 and 0.75 for the Landsat TM images (0.45–0.52 µm) acquired on 17th of May 1985, 2nd of June 1985 and 4th of July 1985. The determined aerosol optical thicknesses for the ASTER (0.52–0.60 µm) images acquired on the 4th of February 2008, 26th of February 2008, 17th of December and 24th of December 2007 were 0.18, 0.39, 0.49 and 0.90, respectively. The accuracy assessment applied using the in situ spectroradiometric and sun-photometer data during the satellite overpass acquired on July–August 2008 for the Paphos area in Cyprus shows satisfactory results both for removing the atmospheric effects and for determining the aerosol optical thickness. Indeed, the high correlation between the determined aerosol optical thickness and those extracted from the visibility values increases the potential of the proposed method.

A model of Titan's aerosols based on measurements made inside the atmosphere

The descent imager/spectral radiometer (DISR) instrument aboard the Huygens probe into the atmosphere of Titan measured the brightness of sunlight using a complement of spectrometers, photometers, and cameras that covered the spectral range from 350 to 1600 nm, looked both upward and downward, and made measurements at altitudes from 150 km to the surface. Measurements from the upward-looking visible and infrared spectrometers are described in Tomasko et al. [2008a. Measurements of methane absorption by the descent imager/spectral radiometer (DISR) during its descent through Titan's atmosphere. Planet. Space Sci., this volume]. Here, we very briefly review the measurements by the violet photometers, the downward-looking visible and infrared spectrometers, and the upward-looking solar aureole (SA) camera. Taken together, the DISR measurements constrain the vertical distribution and wavelength dependence of opacity, single-scattering albedo, and phase function of the aerosols in Titan's atmosphere. Comparison of the inferred aerosol properties with computations of scattering from fractal aggregate particles indicates the size and shape of the aerosols. We find that the aggregates require monomers of radius 0.05 μm or smaller and that the number of monomers in the loose aggregates is roughly 3000 above 60 km. The single-scattering albedo of the aerosols above 140 km altitude is similar to that predicted for some tholins measured in laboratory experiments, although we find that the single-scattering albedo of the aerosols increases with depth into the atmosphere between 140 and 80 km altitude, possibly due to condensation of other gases on the haze particles. The number density of aerosols is about 5/cm 3 at 80 km altitude, and decreases with a scale height of 65 km to higher altitudes. The aerosol opacity above 80 km varies as the wavelength to the −2.34 power between 350 and 1600 nm. Between 80 and 30 km the cumulative aerosol opacity increases linearly with increasing depth in the atmosphere. The total aerosol opacity in this altitude range varies as the wavelength to the −1.41 power. The single-scattering phase function of the aerosols in this region is also consistent with the fractal particles found above 60 km. In the lower 30 km of the atmosphere, the wavelength dependence of the aerosol opacity varies as the wavelength to the −0.97 power, much less than at higher altitudes. This suggests that the aerosols here grow to still larger sizes, possibly by incorporation of methane into the aerosols. Here the cumulative opacity also increases linearly with depth, but at some wavelengths the rate is slightly different than above 30 km altitude. For purely fractal particles in the lowest few km, the intensity looking upward opposite to the azimuth of the sun decreases with increasing zenith angle faster than the observations in red light if the single-scattering albedo is assumed constant with altitude at these low altitudes. This discrepancy can be decreased if the single-scattering albedo decreases with altitude in this region. A possible explanation is that the brightest aerosols near 30 km altitude contain significant amounts of methane, and that the decreasing albedo at lower altitudes may reflect the evaporation of some of the methane as the aerosols fall into dryer layers of the atmosphere. An alternative explanation is that there may be spherical particles in the bottom few kilometers of the atmosphere.

Titan: Preliminary results on surface properties and photometry from VIMS observations of the early flybys

Cassini observations of the surface of Titan offer unprecedented views of its surface through atmospheric windows in the 1–5 μm region. Images obtained in windows for which the haze opacity is low can be used to derive quantitative photometric parameters such as albedo and albedo distribution, and physical properties such as roughness and particle characteristics. Images from the early Titan flybys, particularly T0, Ta, and T5 have been analyzed to create albedo maps in the 2.01 and 2.73 μm windows. We find the average normal reflectance at these two wavelengths to be 0.15±0.02 and 0.035±0.003, respectively. Titan's surface is bifurcated into two albedo regimes, particularly at 2.01 μm. Analysis of these two regimes to understand the physical character of the surface was accomplished with a macroscopic roughness model. We find that the two types of surface have substantially different roughness, with the low-albedo surface exhibiting mean slope angles of ∼18°, and the high-albedo terrain having a much more substantial roughness with a mean slope angle of ∼34°. A single-scattering phase function approximated by a one-term Henyey–Greenstein equation was also fit to each unit. Titan's surface is back-scattering ( g∼0.3–0.4), and does not exhibit substantially different backscattering behavior between the two terrains. Our results suggest that two distinct geophysical domains exist on Titan: a bright region cut by deep drainage channels and a relatively smooth surface. The two terrains are covered by a film or a coating of particles perhaps precipitated from the satellite's haze layer and transported by eolian processes. Our results are preliminary: more accurate values for the surface albedo and physical parameters will be derived as more data is gathered by the Cassini spacecraft and as a more complete radiative transfer model is developed from both Cassini orbiter and Huygens Lander measurements.