Search

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 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.

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.

We analyzed 134 images of Saturn taken by the Hubble Space Telescope between 1991 and 2004. The images cover wavelengths between 231 and 2370 nm in 30 filters. We combined some 10 million calibrated reflectivity measurements into 18,000 center-to-limb curves. We used the method of principal component analysis to find the main latitudinal and temporal variations in Saturn's atmosphere and their spectral characteristics. The first principal variation is a strong latitudinal variation of the aerosol optical depth in the upper troposphere. This structure shifts with Saturn's seasons, but the structure on small scales of latitude stays constant. The second principal variation is a variable optical depth of stratospheric aerosols. The optical depth is large at the poles and small at mid- and low latitudes with a steep gradient in-between. This structure remains essentially constant in time. The third principal variation is a variation in the tropospheric aerosol size, which has only shallow gradients with latitude, but large seasonal variations. Thus, aerosol sizes and their phase functions inferred at a particular season are not representative of Saturn's atmosphere at other seasons. Aerosols are largest in the summer and smallest in the winter. The fourth principal variation is a feature of the tropospheric aerosols with irregular latitudinal structure and fast variability, on the time scale of months. Spherical aerosols do not display the spectral characteristic of that feature. We suspect that variations in the shape of aerosols may play a role. We found a spectral feature of the imaginary index of aerosols, which darkens them near 400 nm wavelength. While we can describe Saturn's variations quite accurately, our presented model of Saturn's average atmosphere is still uncertain due to possible systematic offsets in methane absorption data and limitations of the knowledge about the shape of aerosols. In order to compare our results with those from comparable investigations, which used less than 30 filters, we fit models to spectral subsets of our data. We found very different best-fitting models, depending on the subset of filters, indicating a high sensitivity of results on the spectral sampling.

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) was the first instrument to measure the atmospheric water on Mars from its surface. It took the images of the Sun through the Martian atmosphere in five narrowband filters, two in the 0.94 μm H2O band and three in the continuum around it. The observations were carried out in the mornings and in the evenings when the Sun was between 3° and 8° above the horizon. The absorption due to the atmospheric water vapor did not exceed 2%. An average column density of 6±4 precipitated microns (pr μm) was derived from the IMP data. The dependence of the observed H2O transmittance on Sun elevation tentatively implies that the water vapor is not uniformly mixed in the atmosphere but is rather confined to a layer 1–3 km thick near the surface. IMP observations also indicate a horizontal inhomogeneity of the layer but show no significant morning-to-evening variations of the water vapor amount.

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.

The Galileo probe net flux radiometer measured radiation within Jupiter's atmosphere over the 125-kilometer altitude range between pressures of 0.44 bar and 14 bars. Evidence for the expected ammonia cloud was seen in solar and thermal channels down to 0.5 to 0.6 bar. Between 0.6 and 10 bars large thermal fluxes imply very low gaseous opacities and provide no evidence for a deep water cloud. Near 8 bars the water vapor abundance appears to be about 10 percent of what would be expected for a solar abundance of oxygen. Below 8 bars, measurements suggest an increasing water abundance with depth or a deep cloud layer. Ammonia appears to follow a significantly subsaturated profile above 3 bars. Unexpectedly high absorption of sunlight was found at wavelengths greater than 600 nanometers.

Recent observations demonstrate that near-infrared spectroscopy can probe Titan's surface through its haze. In a 5-week period during September and October 1993 we observed Titan's methane windows at 1.1, 1.3, 1.6, and 2 μm. At 1.1 and 1.3 μm observations were consistent with observations in 1992 at the same phases reported by Lemmon et al. (1993, Icarus 103, 329-332). Our new observations indicate that Titan was brighter near eastern elongation than near western elongation by 23 ± 2% at 1.6 μn and 32 ± 3% at 2 μn. With almost daily observations at 2 μn during one orbit, we observed Titan to be dark near western elongation, to brighten as it approached eastern elongation, and to darken as it returned to western elongation. We determine that the observed light-curve is due to surface albedo variations and Titan's rotational period is 15.950 ± 0.025 days. By considering the work of other observers we constrain the rotational period to be 15.949 ± 0.006 days; this constraint is consistent with synchronous rotation. Models of Titan's surface reflectivity are inconsistent with the presence of a strong 2-μm water ice absorption feature and do not require the presence of any surface absorption features; however, we cannot rule out models of the surface as dirty water ice or silicates that have been suggested by others.

This work describes observations of Saturn's atmosphere in the visible and near-infrared (460–940 nm) including 4 hydrogen quadrupole lines, 17 methane absorption bands ranging over 3 orders of magnitude in absorption strength, an ammonia absorption band, and the absolute calibrated continuum spectrum. All observations have complete coverage of Saturn's disk, in latitude as well as in center-to-limb position. A new method describing center-to-limb information is presented. The accuracy of the data is comparable to or better than that of previous data. This data set gives a quite complete description of Saturn's atmosphere in the visible and near infrared at the spatial resolution of ground-based observations. While the main data were acquired in 1988, small changes between 1986 and 1989 were determined also. Weak absorption features of hydrogen, methane, and ammonia show a significant enhancement in the North Polar Region compared to the rest of the planet. An atmospheric model is given which fits all observations within estimated errors. It has clear gas at the top of the atmosphere, an extended haze layer, and a reflective cloud at the bottom. Pressure levels and the haze optical depth were determined as a function of latitude. The single-scattering albedo spectrum of the particles (most likely ammonia ice crystals) is also given for each latitude. The methane mixing ratio is (3.0 ± 0.6) × 10−3, the ammonia mixing ratio is (1.2 + 0.8/−0.6) × 10−3 below the ammonia condensation level. A cold temperature methane absorption spectrum is determined under the assumption that methane band strengths are temperature invariant. It indicates that the absorption coefficients in band centers are typically 20–30% stronger than at room temperature. This spectrum should be useful in the interpretation of methane observations of all the giant planets and Titan.

Aerosols in Titan's atmosphere play important roles in the transfer of solar and thermal radiation, in Titan's heat balance, in forcing atmospheric dynamics, and as a sink for photochemical reactions. In this chapter we briefly summarize the history of our knowledge of their distribution and optical properties before concentrating in greater detail on current knowledge of their properties and roles in the physics and chemistry in Titan's atmosphere. We discuss the size, shape, optical properties, and the vertical distribution of Titan's aerosols. We discuss variations of the optical properties of the aerosols with wavelength, variations of the distribution of the aerosols over the disk of Titan, and the seasonal and long-term variations in structure. We discuss the visible and thermal opacity of the aerosols, and their roles in Titan's heat balance. We summarize the history and current state of laboratory simulations of these particles, and present our understanding of their formation and life cycles in one and two-dimensional microphysical models. We also discuss the presence, location, and variations in condensation clouds over Titan's disk. Finally, we indicate the prospects for further progress in understanding the origin, distribution, and properties of Titan's aerosols in the future.

International audience; The recent measurements of the vertical distribution and optical properties of haze aerosols as well as of the absorption coefficients for methane at long paths and cold temperatures by the Huygens entry probe of Titan permit the computation of the solar heating rate on Titan with greater certainty than heretofore. We use the haze model derived from the Descent Imager/Spectral Radiometer (DISR) instrument on the Huygens probe [Tomasko, M.G., Doose, L., Engel, S., Dafoe, L.E., West, R., Lemmon, M., Karkoschka, E., See, C., 2008a. A model of Titan's aerosols based on measurements made inside the atmosphere. Planet. Space Sci., this issue, doi:10.1016/j.pss.2007.11.019] to evaluate the variation in solar heating rate with altitude and solar zenith angle in Titan's atmosphere. We find the disk-averaged solar energy deposition profile to be in remarkably good agreement with earlier estimates using very different aerosol distributions and optical properties. We also evaluated the radiative cooling rate using measurements of the thermal emission spectrum by the Cassini Composite Infrared Spectrometer (CIRS) around the latitude of the Huygens site. The thermal flux was calculated as a function of altitude using temperature, gas, and haze profiles derived from Huygens and Cassini/CIRS data. We find that the cooling rate profile is in good agreement with the solar heating profile averaged over the planet if the haze structure is assumed the same at all latitudes. We also computed the solar energy deposition profile at the 10°S latitude of the probe-landing site averaged over one Titan day. We find that some 80% of the sunlight that strikes the top of the atmosphere at this latitude is absorbed in all, with 60% of the incident solar energy absorbed below 150 km, 40% below 80 km, and 11% at the surface at the time of the Huygens landing near the beginning of summer in the southern hemisphere. We compare the radiative cooling rate with the solar heating rate near the Huygens landing site averaging over all longitudes. At this location, we find that the solar heating rate exceeds the radiative cooling rate by a maximum of 0.5 K/Titan day near 120 km altitude and decreases strongly above and below this altitude. Since there is no evidence that the temperature structure at this latitude is changing, the general circulation must redistribute this heat to higher latitudes.

International audience; The irreversible conversion of methane into higher hydrocarbons in Titan's stratosphere implies a surface or subsurface methane reservoir. Recent measurements from the cameras aboard the Cassini orbiter fail to see a global reservoir, but the methane and smog in Titan's atmosphere impedes the search for hydrocarbons on the surface. Here we report spectra and high-resolution images obtained by the Huygens Probe Descent Imager/Spectral Radiometer instrument in Titan's atmosphere. Although these images do not show liquid hydrocarbon pools on the surface, they do reveal the traces of once flowing liquid. Surprisingly like Earth, the brighter highland regions show complex systems draining into flat, dark lowlands. Images taken after landing are of a dry riverbed. The infrared reflectance spectrum measured for the surface is unlike any other in the Solar System; there is a red slope in the optical range that is consistent with an organic material such as tholins, and absorption from water ice is seen. However, a blue slope in the near-infrared suggests another, unknown constituent. The number density of haze particles increases by a factor of just a few from an altitude of 150 km to the surface, with no clear space below the tropopause. The methane relative humidity near the surface is 50 per cent.

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.

Attention is given to the solar and thermal radiation fields of Venus. Direct measurements and the results of numerical models based on direct measurements are presented. Radiation outside the atmosphere is considered with emphasis placed on global energy budget parameters, spectral and angular dependences, spatial distribution, and temporal variations of solar and thermal radiation. Radiation fluxes inside the atmosphere below 90 km are also considered with attention given to the solar flux at the surface, solar and thermal radiation fluxes from 100 km to the surface, and radiative heating and cooling below 100 km.

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.

An assessment is made of the development status of concepts for cloud and aerosol compositions, vertical and horizontal distributions, and microphysical properties, in the Jovian upper troposphere and stratosphere. Attention is given to several key photochemical species' relationships to aerosol formation as well as their transport process implications, treating photochemistry in the context of comparative planetology and noting differences and similarities among the outer planet atmospheres; since this approach emphasizes observational data, a variegated assortment of ground-based and spacecraft observations is assembled. Current views on the tropospheric distribution of clouds are challenged, and a rationale is presented for alternative accounts.

The paper presents cloud structure models for Jupiter's Great Red Spot, Equatorial and North Tropical Zones, North and South Temperate Zones, and North and South Polar Regions. The models are based on images of Jupiter in three methane bands and nearby continuum; radiative transfer calculations include multiple scattering and absorption from three aerosol layers. The model results include the transition in the upper-cloud altitude to 3 km lower altitude from the tropical zones to temperate zones and polar regions, a N/S asymmetry in cloud thickness in the tropical and temperature zones, and the presence of aerosols up to about 0.3 bar in the Great Red Spot and Equatorial Zone. It is concluded that polarization data are sensitive to aerosols in and above the upper cloud layer but insensitive to deeper cloud structure.

Three series of spectra of Jupiter covering the spectral range from 0.22 to 0.33 μm were obtained with the International Ultraviolet Explorer (IUE) satellite in November 1979. The absolute reflectivity of Jupiter was obtained in 50-A-wide regions centered at 0.221, 0.233, 0.252, and 0.330 μm from these observations. One of the three spectral series includes 7 spectra at various latitudes along Jupiter's central meridian. These data show a strong decrease in reflectivity for latitudes greater than about 30°, in agreement with measurements made by Voyager ( C. W. Hord, R. A. West, K. E. Simmons, D. L. Coffeen, M. Sato, A. L. Lane, and J. T. Bergstrahl, 1979 , Science ( Washington, D.C. ) 206, 956–959). A total of 24 spectra were also obtained in a west-east series along the equator and another near 40°N latitude. Both west-east series of spectra were obtained by using the motion of a Galilean satellite to pull the 3-arcsec-diameter IUE aperture across the disk of Jupiter. Spectra which straddled the edge of the disk were used to determine the locations of all the spectra in both west-east series to high accuracy. The west-east series show limb darkening at high latitudes and brightening toward the illuminated limb at low latitudes. Comparisons of model calculations with the data obtained near 40°N indicate a significant absorption optical depth (increasing from ∼0.3 at 0.25 μm to nearly 0.6 at 0.22 μm) centered near pressure levels of 20 to 30 mbar. Models in which the haze particles have effective radii within a factor of about 2 of 0.2 μm are favored. Smaller particles have difficulty fitting the variation with wavelength in our data (even with rapidly varying amounts of absorption with wavelength) and larger particles rapidly fall out of the high atmospheric layers. The aerosol mass loading of the atmosphere at high latitudes is estimated at 20 μm/cm 2 above the 50-mbar level. The required variation of the imaginary index of refraction of the aerosol material with wavelength is derived for several possible aerosol distributions. The variation measured by M. Podolak, N. Noy, and A. Bar-Nun (1979 , Icarus 40 , 193–198) for polyacetylene photochemical products is in reasonable agreement with the IUE observations for one of the vertical haze distributions presented, although mixtures of materials produced by irradiating various combinations of methane, hydrogen, and some nitrogen-bearing compounds with energetic particles may also be able to reproduce the observations. Near the equator, the haze aerosols produce much less absorption than near 40°N, and the derived aerosol distributions and optical properties are more dependent on the assumed location and reflectivity of the top of the tropospheric cloud. The equatorial haze aerosols can be as optically thick as the high-latitude aerosols only if they are concentrated much deeper in the atmosphere (near 150 mbar). However, if the haze aerosols extend up to pressures as low as 50 mbar or less at low latitudes as suggested by the eclipse studies of D.W. Smith (1980 , Icarus 44 , 116–133), then they have 5 to 10 times less absorption optical depth near the equator than at 40°N. Comparisons with the satellite eclipse studies and analyses of polarimetry near the limb at large phase ( P.H. Smith and M.G. Tomasko, 1984 , Icarus 58 , 35–73) indicate that the haze aerosols at low latitudes can have sizes in the same range as found near 40°N. A radius estimate of 0.2 μm yields a mass loading of some 3 μm/cm 2 for the haze aerosols near the equator above the 150-mbar pressure level. Assuming that the haze aerosols have the same composition at high and low latitudes implies that the single-scattering albedo of the tropospheric cloud particles at low latitudes decreases strongly from 0.33 to 0.22 μm.

The paper presents a summary of the data currently available (June 1984) describing the planet-enshrouding particulate matter in the Venus atmosphere. A description and discussion of the state of knowledge of the Venus clouds and hazes precedes the tables and plots. The tabular material includes a precis of upper haze and cloud-top properties, parameters for model-size distributions for particles and particulate layers, and columnar masses and mass loadings.

The Solar Flux Radiometer (LSFR) experiment on the large probe of the Pioneer Venus (PV) mission made detailed measurements of the vertical profile of the upward and downward broadband flux of sunlight at a solar zenith angle of 65.7°. These data have been combined with cloud particle size distribution measurements on the PV mission to produce a forward-scattering model of the Venus clouds. The distribution of clouds at high altitudes is constrained by measurements from the PV orbiter. Below the clouds the visible spectrum and flux levels are consistent with Venera measurements at other solar zenith angles. The variations in the optical parameters with height and with wavelength are summarized in several figures. The model is used to evaluate the solar heating rate at cloud levels as a function of altitude, solar longitude, and latitude for use in dynamical studies.

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.

The solar flux radiometer (LSFR) aboard the Pioneer Venus (PV) sounder probe measured the intensity of sunlight in five directions to the vertical using narrow angular fields of view. The measurements in a narrow spectral channel (0.59–0.67 µm) and two broad channels (0.4–1.0 and 0.4–1.8 µm) were reduced to yield the profiles of upward and downward solar flux with a vertical resolution of 100–500 m. All the flux profiles show three distinct cloud layers with bottoms at altitudes of 57.5, 49.7, and 47.9 km. The azimuthal structure of the intensity samples at the highest altitudes implies a cloud optical depth between 3.5 and 4.0 above 64.3 km. The narrow-band data were interpreted to give the optical depths and single scattering albedos of the clouds at a wavelength of 0.63 µm. The results for the upper cloud are generally consistent with those reported by the cloud particle size spectrometer (LCPS) experiment on PV. In the middle and lower cloud the LSFR data imply that the largest particle size mode has about one third the optical depth deduced from the LCPS data under the assumption that these particles are spherical. The total optical depth of the clouds was found to be about 25. Models of the broad-band fluxes confirm these conclusions and indicate that about half the sunlight absorbed by Venus is absorbed above our first measurements. In addition, absorption of solar radiation occurs in the upper cloud and below 35 km; the middle and lower cloud layers absorb remarkably little sunlight. Models consistent with our measured flux profiles and the spherical albedo of Venus were used to scale our measured net fluxes to a bolometric globally averaged solar energy deposition profile, assuming that the sounder site is typical of planetwide conditions. Averaged over the planet, about 17 W/m² are absorbed at the ground (some 2.5% of the total solar energy incident on the planet). The solar net flux profile in the lower atmosphere is not very sensitive to changes in the thickness of the nearly conservative lower cloud.

Instruments flown on the Pioneer Venus orbiter and probes measured many of the properties of the atmosphere of Venus which control its thermal balance and support its high surface temperature. Estimates based on orbiter measurements place the effective radiating temperature of Venus at 228±5 K, corresponding to an emission of 153±13 W/m², and the bolometric Bond albedo at 0.80±0.02, corresponding to a solar energy absorption of 132±13 W/m². Uncertainties in these preliminary values are too large to interpret the flux difference as a true energy imbalance. A mode of submicron particles is suggested as an important source of thermal opacity near the cloud tops to explain the orbiter and probe thermal flux measurements. Comparison of the measured solar flux profile with thermal fluxes computed from the measured temperature structure and composition shows that the greenhouse mechanism explains essentially all of the 500 K difference between the surface and radiating temperatures of Venus. Precise comparison of the observed and computed value of this difference is hindered by uncertainties in the local variability of H_(2)O and in the thermal opacity of CO_2 and H_(2)O at high temperature and pressure. The directly measured thermal flux profiles at the small probe sites are surprisingly large and variable in the lower atmosphere. Observed zonal and meridional circulation are qualitatively as required to produce the observed uniformity of temperature structure. However, the present lack of quantitative estimates of the horizontal and vertical dynamical heat transports implied by these measurements is a significant gap in the understanding of the thermal balance of the atmosphere of Venus.

The model predictions were compared with the Pioneer Venus probes and orbiter to determine the composition of the UV absorbing materials. The simulations were carried out with radiative transfer codes which included spacecraft constraints on the aerosol and gas characteristics in the Venus atmosphere; gaseous SO2 (a source of opacity at the wavelengths below 0.32 microns), and a second absorber (which dominates above 0.32 microns) were required. The UV contrast variations are due to the optical depth changes in the upper haze layer producing brightness variations between equatorial and polar areas, and to differences in the depth over which the second UV absorber is depleted in the highest portion of the main clouds.