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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 (

Titan’s optical and near-IR spectra result primarily from the scattering of sunlight by haze and its absorption by methane. With a column abundance of 92 km amagat (11 times that of Earth), Titan’s atmosphere is optically thick and only ∼10% of the incident solar radiation reaches the surface, compared to 57% on Earth. Such a formidable atmosphere obstructs investigations of the moon’s lower troposphere and surface, which are highly sensitive to the radiative transfer treatment of methane absorption and haze scattering. The absorption and scattering characteristics of Titan’s atmosphere have been constrained by the Huygens Probe Descent Imager/Spectral Radiometer (DISR) experiment for conditions at the probe landing site (Tomasko, M.G., Bezard, B., Doose, L., Engel, S., Karkoschka, E. [2008a]. Planet. Space Sci. 56, 624–247; Tomasko, M.G. et al. [2008b]. Planet. Space Sci. 56, 669–707). Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) data indicate that the rest of the atmosphere (except for the polar regions) can be understood with small perturbations in the high haze structure determined at the landing site (Penteado, P.F., Griffith, C.A., Tomasko, M.G., Engel, S., See, C., Doose, L., Baines, K.H., Brown, R.H., Buratti, B.J., Clark, R., Nicholson, P., Sotin, C. [2010]. Icarus 206, 352–365). However the in situ measurements were analyzed with a doubling and adding radiative transfer calculation that differs considerably from the discrete ordinates codes used to interpret remote data from Cassini and ground-based measurements. In addition, the calibration of the VIMS data with respect to the DISR data has not yet been tested. Here, VIMS data of the probe landing site are analyzed with the DISR radiative transfer method and the faster discrete ordinates radiative transfer calculation; both models are consistent (to within 0.3%) and reproduce the scattering and absorption characteristics derived from in situ measurements. Constraints on the atmospheric opacity at wavelengths outside those measured by DISR, that is from 1.6 to 5.0 μm, are derived using clouds as diffuse reflectors in order to derive Titan’s surface albedo to within a few percent error and cloud altitudes to within 5 km error. VIMS spectra of Titan at 2.6–3.2 μm indicate not only spectral features due to CH4 and CH3D (Rannou, P., Cours, T., Le Mouelic, S., Rodriguez, S., Sotin, C., Drossart, P., Brown, R. [2010]. Icarus 208, 850–867), but also a fairly uniform absorption of unknown source, equivalent to the effects of a darkening of the haze to a single scattering albedo of 0.63 ± 0.05. Titan’s 4.8 μm spectrum point to a haze optical depth of 0.2 at that wavelength. Cloud spectra at 2 μm indicate that the far wings of the Voigt profile extend 460 cm−1 from methane line centers. This paper releases the doubling and adding radiative transfer code developed by the DISR team, so that future studies of Titan’s atmosphere and surface are consistent with the findings by the Huygens Probe. We derive the surface albedo at eight spectral regions of the 8 × 12 km2 area surrounding the Huygens landing site. Within the 0.4–1.6 μm spectral region our surface albedos match DISR measurements, indicating that DISR and VIMS measurements are consistently calibrated. These values together with albedos at longer 1.9–5.0 μm wavelengths, not sampled by DISR, resemble a dark version of the spectrum of Ganymede’s icy leading hemisphere. The eight surface albedos of the landing site are consistent with, but not deterministic of, exposed water ice with dark impurities.

We determined Titan's reflectivity spectrum near the Huygens' landing site from observations taken with the Descent Imager/Spectral Radiometer below 500 m altitude, in particular the downward-looking photometer and spectrometers. We distinguish signal coming from illumination by sunlight and the lamp onboard Huygens based on their different spectral signatures. For the sunlight data before landing, we find that spatial variations of Titan's reflectivity were only ∼0.8%, aside from the phase angle dependence, indicating that the probed area within ∼100 m of the landing site was very homogeneous. Only the very last spectrum taken before landing gave a 3% brighter reflectivity, which probably was caused by one bright cobble inside its footprint. The contrast of the cobble was higher at 900 nm wavelength than at 600 nm. For the data from lamp illumination, we confirm that the phase function of Titan's surface displays a strong opposition effect as found by Schroder and Keller (2009. Planetary and Space Science 57, 1963–1974). We extend the phase function to even smaller phase angles (0.02°), which are among the smallest phase angles observed in the solar system. We also confirm the reflectivity spectrum of the dark terrain near the Huygens' landing site between 900 and 1600 nm wavelength by Schroder and Keller (2008. Planetary and Space Science 56, 753–769), but extend the spectrum down to 435 nm wavelength. The reflectivity at zero phase angle peaks at 0.45±0.06 around 750 nm wavelength and drops down to roughly 0.2 at both spectral ends. Our reflectivity of 0.45 is much higher than all previously reported values because our observations probe lower phase angles than others. The spectrum is very smooth except for a known absorption feature longward of 1350 nm. We did not detect any significant variation of the spectral shape along the slit for exposures after landing, probing a 25×4 cm2 area. However, the recorded spectral shape was slightly different for exposures before and after landing. This difference is similar to the spectral differences seen on scales of kilometers (Keller et al., 2008. Planetary and Space Science 56, 728–752), indicating that most observations may probe spatially variable contributions from two basic materials, such as a dark soil partially covered by bright cobbles. We used the methane absorption features to constrain the methane mixing ratio near the surface to 5.0±0.3%, in agreement with the 4.92±0.24% value measured in situ by Niemann et al. (2005. Nature 438, 779–784), but smaller than their revised value of 5.65±0.18% (Niemann et al., 2010. Journal of Geophysical Research 115, E12006). Our results were made possible by an in depth review of the calibration of the spectroscopic and photometric data.

We analyzed a unique, three-dimensional data set of Uranus acquired with the STIS Hubble spectrograph on August 19, 2002. The data covered a full afternoon hemisphere at 0.1 arc-sec spatial resolution between 300 and 1000 nm wavelength at 1 nm resolution. Navigation was accurate to 0.002 arc-sec and 0.02 nm. We tested our calibration with WFPC2 images of Uranus and found good agreement. We constrained the vertical aerosol structure with radiative transfer calculations. The standard types of models for Uranus with condensation cloud layers did not fit our data as well as models with an extended haze layer. The dark albedo of Uranus at near-infrared methane windows could be explained by methane absorption alone using conservatively scattering aerosols. Ultraviolet absorption from small aerosols in the stratosphere was strongest at high southern latitudes. The uppermost troposphere was almost clear, but showed a remarkable narrow spike of opacity centered on the equator to 0.2° accuracy. This feature may have been related to influx from ring material. At lower altitudes, the feature was centered at 1–2° latitude, suggesting an equatorial circulation toward the north. Below the 1.2 bar level, the aerosol opacity increased some 100 fold. A comparison of methane and hydrogen absorptions contradicted the standard interpretation of methane band images, which assumes that the methane mixing ratio is independent of latitude and attributes reflectivity variations to variations in the aerosol opacity. The opposite was true for the main contrast between brighter high latitudes and darker low latitudes, probing the 1–3 bar region. The methane mixing ratio varied between 0.014 and 0.032 from high to low southern latitudes, while the aerosol opacity varied only moderately with latitude, except for an enhancement at −45° latitude and a decrease north of the equator. The latitudinal variation of methane had a similar shape as that of ammonia probed by microwave observations at deeper levels. The variability of methane challenges our understanding of Uranus and requires reconsideration of previous investigations based on a faulty assumption. Below the 2 bar level, the haze was thinning somewhat. Our global radiative transfer models with 1° latitude sampling fit the observed reflectivities to 2% rms. The observed spectra of two discrete clouds could be modeled by using the background model of the appropriate latitude and adding small amounts of additional opacity at levels near 1.2 bar (southern cloud) and levels as high as 0.1 bar (northern cloud). These clouds may have been methane condensation clouds of low optical depth (∼0.2).

We analyze observations taken with Cassini’s Visual and Infrared Mapping Spectrometer (VIMS), to determine the current methane and haze latitudinal distribution between 60°S and 40°N. The methane variation was measured primarily from its absorption band at 0.61 μm, which is optically thin enough to be sensitive to the methane abundance at 20–50 km altitude. Haze characteristics were determined from Titan’s 0.4–1.6 μm spectra, which sample Titan’s atmosphere from the surface to 200 km altitude. Radiative transfer models based on the haze properties and methane absorption profiles at the Huygens site reproduced the observed VIMS spectra and allowed us to retrieve latitude variations in the methane abundance and haze. We find the haze variations can be reproduced by varying only the density and single scattering albedo above 80 km altitude. There is an ambiguity between methane abundance and haze optical depth, because higher haze optical depth causes shallower methane bands; thus a family of solutions is allowed by the data. We find that haze variations alone, with a constant methane abundance, can reproduce the spatial variation in the methane bands if the haze density increases by 60% between 20°S and 10°S (roughly the sub-solar latitude) and single scattering absorption increases by 20% between 60°S and 40°N. On the other hand, a higher abundance of methane between 20 and 50 km in the summer hemisphere, as much as two times that of the winter hemisphere, is also possible, if the haze variations are minimized. The range of possible methane variations between 27°S and 19°N is consistent with condensation as a result of temperature variations of 0–1.5 K at 20–30 km. Our analysis indicates that the latitudinal variations in Titan’s visible to near-IR albedo, the north/south asymmetry (NSA), result primarily from variations in the thickness of the darker haze layer, detected by Huygens DISR, above 80 km altitude. If we assume little to no latitudinal methane variations we can reproduce the NSA wavelength signatures with the derived haze characteristics. We calculate the solar heating rate as a function of latitude and derive variations of ∼10–15% near the sub-solar latitude resulting from the NSA. Most of the latitudinal variations in the heating rate stem from changes in solar zenith angle rather than compositional variations.

We use 11 data sets of methane transmission measurements within 0.4–5.5 μm wavelength to model the methane transmission for temperature and pressure conditions in the jovian planets. Eight data sets are based on published laboratory measurements. Another two data sets come from two spectrometers onboard the Huygens probe that measured methane absorption inside Titan’s atmosphere ( Tomasko et al., 2008b, PSS 56, 624 ), and we provide a refined analysis. The last data set is a set of new Jupiter images by the Hubble Space Telescope to measure atmospheric transmission with Ganymede as the light source. Below 1000 nm wavelength, our resulting methane absorption coefficients are generally close to those by Karkoschka (1998, Icarus 133, 134) , but we add descriptions of temperature and pressure dependence. One remaining inconsistency occurs between 882 and 902 nm wavelength where laboratory data predict larger absorptions in the jovian atmospheres than observed. We present possible explanations. Above 1000 nm, our analysis of the Huygens data confirms methane absorption coefficients by Irwin et al. (2006, Icarus 181, 309) at their laboratory temperatures. Huygens data also confirm Irwin’s model of extrapolation to Titan’s lower pressures. However, their model of extrapolation to Titan’s lower temperatures predicts absorption coefficients up to 100 times lower than measured by Huygens. For each of ∼3700 wavelengths, we present a temperature dependence that is consistent with all laboratory data and the Huygens data. Since the Huygens data probe similar temperatures as many observations of Saturn, Uranus, Neptune, and Titan, our methane model will allow more reliable radiative transfer models for their atmospheres.

The Descent Imager/Spectral Radiometer (DISR) instrument on the Huygens probe into the atmosphere of Titan yielded information on the size, shape, optical properties, and vertical distribution of haze aerosols in the atmosphere of Titan [Tomasko, M.G., Doose, L., Engel, S., Dafoe, L.E., West, R., Lemmon, M., Karkoschka, E., 2008. Planet. Space Sci. 56, 669–707] from photometric and spectroscopic measurements of sunlight in Titan’s atmosphere. This instrument also made measurements of the degree of linear polarization of sunlight in two spectral bands centered at 491 and 934 nm. Here we present the calibration and reduction of the polarization measurements and compare the polarization observations to models using fractal aggregate particles which have different sizes for the small dimension (monomer size) of which the aggregates are composed. We find that the Titan aerosols produce very large polarizations perpendicular to the scattering plane for scattering near 90° scattering angle. The size of the monomers is tightly constrained by the measurements to a radius of 0.04 ± 0.01 μm at altitudes from 150 km to the surface. The decrease in polarization with decreasing altitude observed in red and blue light is as expected by increasing dilution due to multiple scattering at decreasing altitudes. There is no indication of particles that produce small amounts of linear polarization at low altitudes.

著者人数：156名, Accepted: 2008-05-27, 資料番号: SA1000998000

The Descent Imager/Spectral Radiometer (DISR) of the Huygens probe was in an excellent position to view aspects of rain as it descended through Titan's atmosphere. Rain may play an important part of the methane cycle on Titan, similar to the water cycle on Earth, but rain has only been indirectly inferred in previous studies. DISR detected two dark atmospheric layers at 11 and 21 km altitude, which can be explained by a local increase in aerosol size by about 5–10%. These size variations are far smaller than those in rain clouds, where droplets grow some 1000-fold. No image revealed a rainbow, which implies that the optical depth of raindrops was less than ∼ 0.0002 / km . This upper limit excludes rain and constrains drizzle to extremely small rates of less than 0.0001 mm/h. However, a constant drizzle of that rate over several years would clear the troposphere of aerosols faster than it can be replenished by stratospheric aerosols. Hence, either the average yearly drizzle rate near the equator was even less ( 0.1 mm / yr ), or the observed aerosols came from somewhere else. The implied dry environment is consistent with ground-based imaging showing a lack of low-latitude clouds during the years before the Huygens descent. Features imaged on Titan's surface after landing, which might be interpreted as raindrop splashes, were not real, except for one case. This feature was a dewdrop falling from the outermost baffle of the DISR instrument. It can be explained by warm, methane-moist air rising along the bottom of the probe and condensing onto the cold baffle.

Our model [Dimitrov, V., Bar-Nun, A., 1999. A model of energy dependent agglomeration of hydrocarbon aerosol particles and implication to Titan's aerosol. J. Aerosol. Sci. 30(1), 35–49] describes the experimentally found polymerization of C2H2 and HCN to form aerosol embryos, their growth and adherence to form various aerosols objects [Bar-Nun, A., Kleinfeld, I., Ganor, E., 1988. Shape and optical properties of aerosols formed by photolysis of C2H2, C2H4 and HCN. J. Geophys. Res. 93, 8383–8387]. These loose fractal objects describe well the findings of DISR on the Huygens probe [Tomasko, M.G., Bezard, B., Doose, L., Engel, S., Karkoschka, E., 2008. Measurements of methane absorption by the descent imager/spectral radiometer (DISR) during its descent through Titan's atmosphere. Planet. Space Sci., this issue, doi:10.1016/j.pss.2007]. These include (1) various regular objects of R=(0.035–0.064)×10−6 m, as compared with DISR's 0.05×10−6 m; (2) diverse low and high fractal structures composed of random combinations of various regular and irregular objects; (3) the number density of fractal particles is 6.9×106 m−3 at Z=100 km, as compared with DISR's finding of 5.0×106 m−3 at Z=80 km; (4) the number of structural units per higher fractals in the atmosphere at Z∼100 km is (2400–2700), as compared with DISR's 3000, and their size being of R=(5.4–6.4)×10−6 m will satisfy this value and (5) condensation of CH4 on the highly fractal structures could begin at the altitude where thin methane clouds were observed, filling somewhat the new open fractal structures.

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/cm3 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.

International audience; New low-temperature methane absorption coefficients pertinent to the Titan environment are presented as derived from the Huygens DISR spectral measurements combined with the in-situ measurements of the methane gas abundance profile measured by the Huygens Gas Chromatograph/Mass Spectrometer (GCMS). The visible and near-infrared spectrometers of the descent imager/spectral radiometer (DISR) instrument on the Huygens probe looked upward and downward covering wavelengths from 480 to 1620 nm at altitudes from 150 km to the surface during the descent to Titan's surface. The measurements at continuum wavelengths were used to determine the vertical distribution, single-scattering albedos, and phase functions of the aerosols. The gas chromatograph/mass spectrometer (GCMS) instrument on the probe measured the methane mixing ratio throughout the descent. The DISR measurements are the first direct measurements of the absorbing properties of methane gas made in the atmosphere of Titan at the pathlengths, pressures, and temperatures that occur there. Here we use the DISR spectral measurements to determine the relative methane absorptions at different wavelengths along the path from the probe to the sun throughout the descent. These transmissions as functions of methane path length are fit by exponential sums and used in a haze radiative transfer model to compare the results to the spectra measured by DISR. We also compare the recent laboratory measurements of methane absorption at low temperatures [Irwin et al., 2006. Improved near-infrared methane band models and k-distribution parameters from 2000 to 9500 cm -1 and implications for interpretation of outer planet spectra. Icarus 181, 309-319] with the DISR measurements. We find that the strong bands formed at low pressures on Titan act as if they have roughly half the absorption predicted by the laboratory measurements, while the weak absorption regions absorb considerably more than suggested by some extrapolations of warm measurements to the cold Titan temperatures. We give factors as a function of wavelength that can be used with the published methane coefficients between 830 and 1620 nm to give agreement with the DISR measurements. We also give exponential sum coefficients for methane absorptions that fit the DISR observations. We find the DISR observations of the weaker methane bands shortward of 830 nm agree with the methane coefficients given by Karkoschka [1994. Spectrophotometry of the jovian planets and Titan at 300- to 1000-nm wavelength: the methane spectrum. Icarus 111, 174-192]. Finally, we discuss the implications of our results for computations of methane absorption in the atmospheres of the outer planets.

The descent imager/spectral radiometer (DISR) onboard the Huygens probe investigated the radiation balance inside Titan's atmosphere and took hundreds of images and spectra of the ground during the descent. The scattering of the aerosols in the atmosphere and the absorption by methane strongly influence the irradiation reaching the surface and the signals received by the various instruments. The physical properties of the surface can only be assessed after the influence of the atmosphere has been taken into account and properly removed. In the broadband visible images (660 to 1000 nm) the contrast of surface features is strongly reduced by the aerosol scattering. Calculations show that for an image taken from an altitude of 14.5 km, the corrected contrast is about three times higher than in the raw image. Spectral information of the surface by the imaging spectrometers in the visible and near infrared range can only be retrieved in the methane absorption windows. Intensity ratios from the methane windows can be used to make false color maps. The elevated bright ‘land’ terrain is redder than the flat dark ‘lake bed’ terrain. The reflectance spectra of the land and lake bed area in the IR are derived, as well as the reflectance phase function in the limited range from 20 ∘ to 50 ∘ phase angle. An absorption feature at 1.55 μ m which may be attributed tentatively to water ice is found in the lake bed, but not in the land area. Otherwise the surface exhibits a featureless blue slope in the near-IR region (0.9– 1.6 μ m ). Brightness profiles perpendicular to the coast line show that the bottoms of the channels of the large scale flow pattern become darker the further they are away from the land area. This could be interpreted as sedimentation of the bright land material transported by the rivers into the lake bed area. The river beds in the deeply incised valleys need not to be covered by dark material. Their roughly 10% brightness decrease could be caused by the illumination as illustrated by a model calculation. The size distribution of cobbles seen in the images after landing is in agreement with a single major flooding of the area with a flow speed of about 1 m s - 1 .

Titan's vast equatorial fields of RADAR-dark longitudinal dunes seen in Cassini RADAR synthetic aperture images correlate with one of two dark surface units discriminated as “brown” and “blue” in Visible and Infrared Mapping Spectrometer (VIMS) color composites of short-wavelength infrared spectral cubes (RGB as 2.0, 1.6, 1.3 μm). In such composites bluer materials exhibit higher reflectance at 1.3 μm and lower at 1.6 and 2.0 μm. The dark brown unit is highly correlated with the RADAR-dark dunes. The dark brown unit shows less evidence of water ice suggesting that the saltating grains of the dunes are largely composed of hydrocarbons and/or nitriles. In general, the bright units also show less evidence of absorption due to water ice and are inferred to consist of deposits of bright fine precipitating tholin aerosol dust. Some set of chemical/mechanical processes may be converting the bright fine-grained aerosol deposits into the dark saltating hydrocarbon and/or nitrile grains. Alternatively the dark dune materials may be derived from a different type of air aerosol photochemical product than are the bright materials. In our model, both the bright aerosol and dark hydrocarbon dune deposits mantle the VIMS dark blue water ice-rich substrate. We postulate that the bright mantles are effectively invisible (transparent) in RADAR synthetic aperture radar (SAR) images leading to lack of correlation in the RADAR images with optically bright mantling units. RADAR images mostly show only dark dunes and the water ice substrate that varies in roughness, fracturing, and porosity. If the rate of deposition of bright aerosol is 0.001–0.01 μm/yr, the surface would be coated (to optical instruments) in hundreds-to-thousands of years unless cleansing processes are active. The dark dunes must be mobile on this very short timescale to prevent the accumulation of bright coatings. Huygens landed in a region of the VIMS bright and dark blue materials and about 30 km south of the nearest occurrence of dunes visible in the RADAR SAR images. Fluvial/pluvial processes, every few centuries or millennia, must be cleansing the dark floors of the incised channels and scouring the dark plains at the Huygens landing site both imaged by Descent Imager/Spectral Radiometer (DISR).

The Descent Imager/Spectral Radiometer (DISR) provided 376 images during the descent to Titan and 224 images after landing. Images of the surface had scales between 150 m/pixel and 0.4 mm/pixel, all of which we assembled into a mosaic. The analysis of the surface and haze features in these images and of other data gave tight constraints on the geometry of the descent, particularly the trajectory, the tip and tilt, and the rotation of the Huygens probe. Huygens moved on average in the direction of 2ring operator north of east from 145 to 50 km altitude, turning to 5ring operator south of east between 30 and 20 km altitude, before turning back to east. At 6.5 km altitude, it reversed to WNW, before reversing back to SE at 0.7 km altitude. At first, Huygens was tilting slowly by up to 15ring operator as expected for a descent through layers of changing wind speeds. As the winds calmed, tilts decreased. Tilts were approximately retrieved throughout the main-parachute phase, but only for 160 specific times afterwards. Average swing rates were 5ring operator/s at high and low altitudes, but 13ring operator/s between 110 and 30 km altitude. Maximum swing rates were often above 40ring operator/s, far above the design limit of 6ring operator/s, but they caused problems only for a single component of DISR, the Sun Sensor. The excitation of such high swing rates on the stabilizer parachute is not fully understood. Before the parachute exchange, the rotational rate of Huygens smoothly approached the expected equilibrium value of 3 rotations per vertical kilometer, although clockwise instead of counterclockwise. Starting at 40 s after the parachute exchange until landing, Huygens rotated erratically. Long-term averages of the rotational rate varied between 2.0 and 4.5 rotations/km. On time scales shorter than a minute, some 100 strong rotational accelerations or decelerations created azimuthal irregularities of up to 180ring operator, which caused DISR to take most exposures at random azimuths instead of pre-selected azimuths. Nevertheless, we reconstructed the azimuths throughout the 360 rotations during the descent and for each of some 3500 DISR exposures with a typical accuracy near 2ring operator. Within seconds after landing, the parachute moved into the field of view of one of the spectrometers. The observed light curve indicated a motion of the parachute of 0.3 m/s toward the SSE. DISR images indicated that the probe did not penetrate into the surface, assuming a level ground. This impact of Huygens must have occurred on major rocks or some elevated area. The unexpected raised height increases ice-rock sizes by 40% with respect to estimations made in 2005 [Tomasko, M.G., Archinal, B., Becker, T., Bezard, B., Bushroe, M., Combes, M., Cook, D., Coustenis, A., de Bergh, C., Dafoe, L.E., Doose, L., Doute, S., Eibl, A., Engel, S., Gliem, F., Grieger, B., Holso, K., Howington-Kraus, E., Karkoschka, E., Keller, H.U., Kirk, R., Kramm, R., Kuppers, M., Lanagan, P., Lellouch, E., Lemmon, M., Lunine, J., McFarlane, E., Moores, J., Prout, G.M., Rizk, B., Rosiek, M., Rueffer, P., Schroder, S.E., Schmitt, B., See, C., Smith, P., Soderblom, L., Thomas, N., West, R., 2005. Rain, winds and haze during the Huygens probe's descent to Titan's surface. Nature 438, 765–778]. During the 70-min surface phase, the tilt of Huygens was 3ring operator, changing by a small fraction of a degree. The apparent horizon looking south to SSW from the landing site was 1–2ring operator above the theoretical horizon, sloping by 1ring operator up to the left (east). Our best guess puts the horizon as a 1–2 m high hill in 30–50 m distance. We detected the refraction from warm, rising air bubbles above our illuminated spot. Bright, elongated, cm-sized objects appear occasionally on the surface. If real, they could be rain drop splashes or fluffy particles blown across Titan's surface.

Low-latitude near-infrared spectral images of Titan reveal what are probably dark liquid lakes of methane. Saturn's moon Titan has a 'methane cycle' that is similar, in principle, to Earth's water cycle, although surface liquid seems relatively scarce on Titan, being detected mainly at high latitudes. The fact that Titan can supply its atmosphere with methane — together with signs of surface water erosion around the Huygens probe landing site in what seems to be an otherwise arid region of the tropics — suggests that there may be more surface liquid to be discovered. This paper reports near-infrared spectral images of an area in the tropics that reveal a dark region, which could indicate the presence of liquid methane on the moon's surface, supplied by subterranean sources. Titan has clouds, rain and lakes—like Earth—but composed of methane rather than water. Unlike Earth, most of the condensable methane (the equivalent of 5 m depth globally averaged1) lies in the atmosphere. Liquid detected on the surface (about 2 m deep) has been found by radar images only poleward of 50° latitude2,3, while dune fields pervade the tropics4. General circulation models explain this dichotomy, predicting that methane efficiently migrates to the poles from these lower latitudes5,6,7. Here we report an analysis of near-infrared spectral images8 of the region between 20° N and 20° S latitude. The data reveal that the lowest fluxes in seven wavelength bands that probe Titan's surface occur in an oval region of about 60 × 40 km2, which has been observed repeatedly since 2004. Radiative transfer analyses demonstrate that the resulting spectrum is consistent with a black surface, indicative of liquid methane on the surface. Enduring low-latitude lakes are best explained as supplied by subterranean sources (within the last 10,000 years), which may be responsible for Titan’s methane, the continual photochemical depletion of which furnishes Titan's organic chemistry9.

The Imager for Mars Pathfinder (IMP) returned sequences of images of the Martian sky characterizing the size distribution, optical constants, and nature of the aerosols suspended in the atmosphere of Mars. These sequences were executed when the solar elevation angle was approximately 15° and consisted of images near the elevation of the Sun, spanning a range in azimuth from about 4° to 180° from the Sun. Images were obtained at four wavelengths from 444 to 965 nm. From one sequence of observations, results are shown from a comparison of absolute photometry of the Martian sky with multiple scattering models. Results include the following. (1) The geometric cross-section-weighted mean particle radius is 1.6 ± 0.15 μm almost independent of the assumed width (variance) of the size distribution. (2) The imaginary refractive index shows a steep increase with wavelength from 670 nm to shorter wavelengths, and a shallow increase toward longer wavelengths, consistent with the reflection spectrum observed by IMP for Martian soil. (3) For each assumed variance, two parameters governing the slope and curvature of the portion of the phase function due to internally transmitted light are found uniquely as functions of wavelength. (4) The variance of the gamma size distribution is difficult to constrain from these observations alone. The shape of the single scattering phase functions derived from the IMP observations is compared to laboratory measurements of powder samples. One sample of irregular particles has a single scattering phase function quite similar to that derived for Mars. Overall, the results for the mean cross-section-weighted size and imaginary refractive index as a function of wavelength are in remarkably good agreement with the revised analysis by Pollack et al. [1995] of the observations made by the Viking lander 20 years earlier.

The Galileo probe net flux radiometer (NFR) measured radiation fluxes in Jupiter's atmosphere from about 0.44 to 14 bars, using five spectral channels to separate solar and thermal components. Onboard calibration results confirm that the NFR responded to radiation approximately as expected. NFR channels also responded to a superimposed thermal perturbation, which can be approximately removed using blind channel measurements and physical constraints. Evidence for the expected NH3 cloud was seen in the spectral character of spin-induced modulations of the direct solar beam signals. These results are consistent with an overlying cloud of small NH3 ice particles (0.5-0.75 microns in radius) of optical depth 1.5-2 at 0.5 microns. Such a cloud would have so little effect on thermal fluxes that NFR thermal channels provide no additional constraints on its properties. However, evidence for heating near 0.45 bar in the NFR thermal channels would seem to require either an additional opacity source beyond this small-particle cloud, implying a heterogeneous cloud structure to avoid conflicts with solar modulation results, or a change in temperature lapse rate just above the probe measurements. The large thermal flux levels imply water vapor mixing ratios that are only 6% of solar at 10 bars, but possibly increasing with depth, and significantly subsaturated ammonia at pressures less than 3 bars. If deep NH3 mixing ratios at the probe entry site are 3-4 times ground-based inferences, as suggested by probe radio signal attenuation, then only half as much water is needed to match NFR observations. No evidence of a water cloud was seen near the 5-bar level. The 5-microns thermal channel detected the presumed NH4SH cloud base near 1.35 bars. Effects of this cloud were also seen in the solar channel upflux measurements but not in the solar net fluxes, implying that the cloud is a conservative scatterer of sunlight. The minor thermal signature of this cloud is compatible with particle radii near 3 gm, but it cannot rule out smaller particles. Deeper than about 3 bars, solar channels indicate unexpectedly large absorption of sunlight at wavelengths longer than 0.6 microns, which might be due to unaccounted-for absorption by NH3 between 0.65 and 1.5 microns.

Images of the martian surface returned by the Imager for Mars Pathfinder (IMP) show a complex surface of ridges and troughs covered by rocks that have been transported and modified by fluvial, aeolian, and impact processes. Analysis of the spectral signatures in the scene (at 440- to 1000-nanometer wavelength) reveal three types of rock and four classes of soil. Upward-looking IMP images of the predawn sky show thin, bluish clouds that probably represent water ice forming on local atmospheric haze (opacity ∼0.5). Haze particles are about 1 micrometer in radius and the water vapor column abundance is about 10 precipitable micrometers.

With the goal of improving our understanding of how small mineral dust particles scatter light at visible and near-infrared wafelengths we measured the scattering phase function and linear polarization of small mineral dust particles over the scattering angle range 15 to 170 at three wavelengths (0.47, 0.652,and 0.937 m).

The imager for Mars Pathfinder (IMP), a stereoscopic, multispectral camera, is described in terms of its capabilities for studying the Martian environment. The camera's two eyes, separated by 15.0 cm, provide the camera with range-finding ability. Each eye illuminates half of a single CCD detector with a field of view of 14.4×14.0° and has 12 selectable filters. The ƒ/18 optics have a large depth of field, and no focussing mechanism is required; a mechanical shutter is avoided by using the frame transfer capability of the 512×512 CCD. The resolving power of the camera, 0.98 mrad/pixel, is approximately the same as the Viking Lander cameras; however, the signal-to-noise ratio for IMP greatly exceeds Viking, approaching 350. This feature along with the stable calibration of the filters between 440 and 1000 nm distinguishes IMP from Viking. Specially designed targets are positioned on the Lander; they provide information on the magnetic properties of wind-blown dust, measure the wind vectors, and provide radiometric standard reflectors for calibration. Also, eight low-transmission filters are included for imaging the Sun directly at multiple wavelengths, giving IMP the ability to measure dust opacity and potentially the water vapor content. Several experiments beyond the requisite color panorama are described in detail: contour mapping of the local terrain, multispectral imaging of the surrounding rock and soil to study local mineralogy, viewing of three wind socks, measuring atmospheric opacity and water vapor content, and estimating the magnetic properties of wind-blown dust. This paper is intended to serve as a guide to understanding the scientific integrity of the IMP data that will be returned from Mars starting on July 4, 1997.

An imaging photopolarimeter aboard Pioneer 11, including a 2.5-centimeter telescope, was used for 2 weeks continuously in August and September 1979 for imaging, photometry, and polarimetry observations of Saturn, its rings, and Titan. A new ring of optical depth2 x 10(-3) was discovered at 2.33 Saturn radii and is provisionally named the F ring; it is separated from the A ring by the provisionally named Pioneer division. A division between the B and C rings, a gap near the center of the Cassini division, and detail in the A, B, and C rings have been seen; the nomenclature of divisions and gaps is redefined. The width of the Encke gap is 876 +/- 35 kilometers. The intensity profile and colors are given for the light transmitted by the rings. A mean particle size less, similar 15 meters is indicated; this estimate is model-dependent. The D ring was not seen in any viewing geometry and its existence is doubtful. A satellite, 1979 S 1, was found at 2.53 +/- 0.01 Saturn radii; the same object was observed approximately 16 hours later by other experiments on Pioneer 11. The equatorial radius of Saturn is 60,000 +/- 500 kilometers, and the ratio of the polar to the equatorial radius is 0.912 +/- 0.006. A sample of polarimetric data is compared with models of the vertical structure of Saturn's atmosphere. The variation of the polarization from the center of the disk to the limb in blue light at 88 degrees phase indicates that the density of cloud particles decreases as a function of altitude with a scale height about one-fourth that of the gas. The pressure level at which an optical depth of 1 is reached in the clouds depends on the single-scattering polarizing properties of the clouds; a value similar to that found for the Jovian clouds yields an optical depth of 1 at about 750 millibars.

A line at 1280 A, due to Raman scattering of solar Lyman alpha (Ly-alpha) in the atmosphere of Uranus, has been detected by the Voyager Ultraviolet Spectrometer. The measured intensity of 40 + or - 20 R implies that 200 R to 500 R of the measured 1500 R Ly-alpha intensity at the subsolar point is due to Rayleigh scattering of the solar line. The presence of Rayleigh and Raman scattering at 1216 A suggests that the Uranian atmosphere is largely devoid of absorbing hydrocarbons above the 0.5 mbar level. The most natural explanation of this depletion is very weak vertical mixing equivalent to an eddy coefficient on the order of 200 sq cm/sec between 0.5 mbar and 100 mbar.

We have used a number of models to analyze Voyager images of Uranus obtained at several high phase angles to derive physical and chemical properties of particulate matter present in the planet's lower stratosphere. These models include a multiple‐scattering algorithm for plane parallel atmospheres, a spherical atmosphere code for performing limb inversions, a microphysical model of aerosol formation, growth, and sedimentation, and a photochemical model of methane photolysis. We obtain definitive evidence for the presence of aerosols at pressure levels ranging from a few millibars to about 100 mbar. There are two possible sets of particle properties that can fit radiances observed close to but somewhat interior to the limb at several phase angles in four visible wavelength bands. The low‐density solution is characterized by particles having a modal radius and number density equal to 0.13 ± 0.02 µm and 2 ± 1 particles/cm³, respectively, at a pressure level of 44 mbar. The alternative, high‐density solution is characterized by particles having a modal radius that is 0.6–0.7 times that of the low‐density solution at the reference level and a density that is 2 orders of magnitude larger. Since the high‐density solution implies a mass production rate for the stratospheric aerosols that is much larger than those that can plausibly be supplied by photochemically produced gases that condense, whereas the low‐density solution does not, we favor the low‐density solution. Inversion of narrow‐angle, high‐resolution images of the limb provides a definition of a variable that provides a measure of the amount of aerosol scattering at high phase angles. The vertical profile of this variable shows a decrease of several orders of magnitude from pressure levels of tens to a few millibars. This decrease is due chiefly to the particle size of the aerosols becoming small compared to a wavelength. Above the base of the stratosphere the aerosol optical depth is approximately 0.01 in the mid‐visible. A major source for the stratospheric aerosols is the condensation of ethane, acetylene, and diacetylene gas species at pressure levels of approximately 14, 2.5, and 0.1 mbar, respectively. These gases are produced at much higher altitudes by solar UV photolysis of methane and diffuse to the lower stratosphere, where they condense. In addition, diacetylene is also produced photochemically within its condensation region. Condensation of locally produced diacetylene may represent a significant fraction of the total hydrocarbon condensation. Such a local source of condensation may be required by the inversions to the limb profiles, which indicate that at least half of the ice condensation occurs at altitudes above the 5‐mbar level. The hypothesis that the stratospheric particles are made of hydrocarbon ices is supported by the approximate agreement between the total ice condensation rate predicted by the methane photochemical model and the aerosol mass production rate derived for the low‐density solution of the Voyager data. The aerosol mass production rate derived from the Voyager data is equal to 2–15 × 10−17 g/cm²/s. Additional but weaker support for this hypothesis is provided by the Voyager radio occultation temperature profiles. It is suggested that solar UV radiation promotes solid state chemistry within the lower order hydrocarbon ices, resulting in the production of polymers capable of absorbing at visible wavelengths. Thus this altered material could play a key role in the planet's heat budget. Ethane, acetylene, and diacetylene ices evaporate at approximately the 600‐, 900‐, and 3000‐mbar levels of the upper troposphere. The polymeric material is expected to evaporate at pressures in excess of the evaporation level for diacetylene.

The imaging photopolarimeter aboard Pioneer 10 produced hundreds of red and blue images of Jupiter covering a wide range of phase angles and having good linearity and signal-to-noise characteristics. In this preliminary analysis the limb darkening across two of the red images (at phase angles of about 34 and 109 deg) in both a prominent dark belt and a bright zone are compared with multiple-scattering models. Of the simple models tried, the smallest deviations from the observations result for ones consisting of a thin absorbing layer above a semiinfinite atmosphere of particles scattering according to the Henyey-Greenstein phase function. The asymmetry parameter for the best fits to both the belt and the zone data is in the range g = 0 to 0.25, corresponding to particles small in comparison with the wavelength of red light. The phase integral derived from the models lies in the range of about 1.5 to 1.6, implying a substantial internal heat source for Jupiter.

For 2 weeks continuous imaging, photometry, and polarimetry observations were made of Jupiter and the Galilean satellites in red and blue light from Pioneer 11. Measurements of Jupiter's north and south polar regions were possible because the spacecraft trajectory was highly inclined to the planet's equatorial plane. One of the highest resolution images obtained is presented here along with a comparison of a sample of our photometric and polarimetric data with a simple model. The data seem consistent with increased molecular scattering at high latitudes.

Several photometric measurements of Venus made from the Pioneer Venus orbiter and probes indicate that solar near-ultraviolet radiation is being absorbed throughout much of the main cloud region, but little above the clouds or within the first one or two optical depths. Radiative transfer calculations were carried out to simulate both Pioneer Venus and ground-based data for a number of proposed cloud compositions. This comparison rules out models invoking nitrogen dioxide, meteoritic material, and volatile metals as the source of the ultraviolet absorption. Models involving either small ( approximately 1 micrometer) or large ( approximately 10 micrometers) sulfur particles have some serious difficulties, while ones invoking sulfur dioxide gas appear to be promising.

The current state of knowledge of the Venusian clouds is reviewed. The visible clouds of Venus are shown to be quite similar to low level terrestrial hazes of strong anthropogenic influence. Possible nucleation and particle growth mechanisms are presented. The Pioneer Venus experiments that emphasize cloud measurements are described and their expected findings are discussed in detail. The results of these experiments should define the cloud particle composition, microphysics, thermal and radiative heat budget, rough dynamical features and horizontal and vertical variations in these and other parameters. This information should be sufficient to initialize cloud models which can be used to explain the cloud formation, decay, and particle life cycle.

The suggestion by Elliot et al., (1989) that a haze layer near the surface of Pluto may be photochemical in origin and similar to the aerosol hazes in the atmospheres of other outer solar system bodies is evaluated. The nature of hazes which may be produced in the Hubbard et al., (1989) atmosphere is explored as well. It is concluded that the very low pressure in Pluto's atmosphere requires an aerosol production rate equal to the total maximum methane photolysis rate expected at Pluto.

The solar flux radiometer aboard the Pioneer Venus large probe operated successfully during its descent through the atmosphere of Venus. Upward, downward, and net fluxes from 0.4 to 1.0 micrometers were obtained at more than 390 levels between 185 millibars (at an altitude of approximately 61 kilometers) and the surface. Fluxes from 0.4 to 1.8 micrometers were also obtained between 185 millibars and about the level at which the pressure was 2 atmospheres. Data from 80 to 185 millibars should be available after additional decoding by the Deep Space Network. Upward and downward intensities in a narrower band from 0.59 to 0.66 micrometers were also obtained throughout the descent in order to constrain cloud properties. The measurements indicate three cloud regions above the 1.3-atmosphere level (at an altitude of approximately 49 kilometers) and a clear atmosphere beneath that level. At the 67 degrees solar zenith of the probe entry site, some 15 watts per square meter are absorbed at the surface by a dark ground, which implies that about 2 percent of the solar energy incident on the planet is absorbed at the ground.