Descent Imager/Spectral Radiometer Research Articles

A New Digital Terrain Model of the Huygens Landing Site on Saturn's Largest Moon, Titan

AbstractRiver valleys have been observed on Titan at all latitudes by the Cassini‐Huygens mission. Just like water on Earth, liquid methane carves into the substrate to form a complex network of rivers, particularly stunning in the images acquired near the equator by the Huygens probe. To better understand the processes at work that form these landscapes, one needs an accurate digital terrain model (DTM) of this region. The first and to date the only existing DTM of the Huygens landing site was produced by the U.S. Geological Survey (USGS) from high‐resolution images acquired by the DISR (Descent Imager/Spectral Radiometer) cameras on board the Huygens probe and using the SOCET SET photogrammetric software. However, this DTM displays inconsistencies, primarily due to nonoptimal viewing geometries and to the poor quality of the original data, unsuitable for photogrammetric reconstruction. We investigate a new approach, benefiting from a recent reprocessing of the DISR images correcting both the radiometric and geometric distortions. For the DTM reconstruction, we use MicMac, a photogrammetry software based on automatic open‐source shape‐from‐motion algorithms. To overcome challenges such as data quality and image complexity (unusual geometric configuration), we developed a specific pipeline that we detailed and documented in this article. In particular, we take advantage of geomorphic considerations to assess ambiguity on the internal calibration and the global orientation of the stereo model. Besides the novelty in this approach, the resulting DTM obtained offers the best spatial sampling of Titan's surface available and a significant improvement over the previous results.

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

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

Eight-color maps of Titan’s surface from spectroscopy with Huygens’ DISR

During the descent of the Huygens probe in Titan’s atmosphere, the Descent Imager/Spectral Radiometer (DISR) acquired spectra of 3660 locations within 250km of the landing site. Each spectrum consisted of 200 resolution elements between 480 and 960nm wavelength. With the help of radiative transfer models, contributions from the atmosphere and surface were separated. In eight methane windows, the data were combined into a map of Titan’s surface reflectivity with 250km diameter near the landing site. Principal component analysis revealed three significant components, a brightness component that is consistent with a mosaic based on DISR imaging of much higher spatial resolution, a spectral slope component, and a spectral curvature component. The brightness component has stronger contrasts at longer wavelengths, or brighter areas have a larger spectral slope, consistent with previous results (Keller et al. [2008]. Planet. Space Sci. 56, 728–752). The second component corresponds to small differences in spectral slopes that are not correlated with features seen before except for an area with unusual high spectral slope found by the same authors and confirmed here. Our map of the second component gives another important parameter in characterizing and understanding Titan’s surface. The third principal component is somewhat noisy and describes variation in the spectral curvature that have never seen before at similar wavelengths. These variations require processes to differentiate surface spectra. To extend this work to longer wavelengths, 62 spectra from 850 to 1600nm wavelength were investigated too, although the much lower number of spatial resolution points revealed only two significant components in the principal component analysis. They correlate with the first two components found in the shorter wavelength data. We also compare our results with an observation by Cassini’s Visible Imager/Mapping Spectrometer (VIMS) that imaged part of our investigated area with 4096 spatial resolution elements. Both data sets are complementary. DISR data extend to about 1500nm wavelength while most surface features are seen in the VIMS data beyond 1500nm.

The methane mole fraction in Titan’s stratosphere from DISR measurements during the Huygens probe’s descent

We present a determination of the methane mole fraction in Titan’s stratosphere using spectral measurements by the Upward Looking Infrared Spectrometer (ULIS) of the Descent Imager/Spectral Radiometer (DISR) aboard the Huygens probe. We analyzed the 1.4-μm band of methane for which a complete linelist, down to very low intensities, was recently made available. The DISR/ULIS measurements were used to derive the methane transmittance along the path from the probe to the Sun during the descent from 135 to 29km. Fitting the transmittance ratios to remove residual instrumental effects, we derived a stratospheric methane mole fraction of 1.44 (+0.27/−0.11)%, taking into account random errors and uncertainties in the spectroscopic parameters. This value is fully consistent with the simultaneous measurement of the methane profile by the Huygens Gas Chromatograph/Mass Spectrometer (GCMS). It disagrees with the ∼1% mixing ratio recently inferred from Cassini/CIRS spectra at low latitudes near 85km. Possible reasons for the discrepancy and uncertainties in the methane spectroscopic parameters are discussed.

Morphology of fluvial networks on Titan: Evidence for structural control

Although Titan’s surface shows clear evidence of erosional modification, such as fluvial incision, evidence for tectonism has been less apparent. On Earth, fluvial networks with strongly preferred orientations are often associated with structural features, such as faults or joints, that influence flow or erodibility. We delineated and classified the morphologies of fluvial drainages on Titan and discovered evidence of structural control. Fluvial networks were delineated both on synthetic aperture radar (SAR) images covering ∼40% of Titan from the Cassini Titan Radar Mapper up through T71 and on visible light images of the Huygens landing site collected by the Descent Imager/Spectral Radiometer (DISR). The delineated networks were assigned to one of three morphologic classes—dendritic, parallel or rectangular—using a quantitative terrestrial drainage pattern classification algorithm modified for use with Titan data. We validated our modified algorithm by applying it to synthetic fluvial networks produced by a landscape evolution model with no structural control of drainage orientations, and confirmed that only a small fraction of the networks are falsely identified as structurally controlled. As a second validation, we confirmed that our modified algorithm correctly classifies terrestrial networks that are classified in multiple previous works as rectangular. Application of this modified algorithm to our Titan networks results in a classification of rectangular for one-half of the SAR and DISR networks. A review of the geological context of the four terrestrial rectangular networks indicates that tensional stresses formed the structures controlling those terrestrial drainages. Based on the similar brittle response of rock and cryogenic ice to stress, we infer that structures formed under tension are the most likely cause of the rectangular Titan networks delineated here. The distribution of these rectangular networks suggests that tensional stresses on Titan may have been widespread.

Titan’s surface and atmosphere from Cassini/VIMS data with updated methane opacity

We present an analysis of Titan data acquired by the Cassini Visual and Infrared Mapping Spectrometer (VIMS), making use of recent improvements in methane spectroscopic parameters in the region 1.3–5.2μm. We first analyzed VIMS spectra covering a 8×10-km2 area near the Huygens landing site in order to constrain the single scattering albedo (ω0) of the aerosols over all of the VIMS spectral range. Our aerosol model agrees with that derived from Huygens Probe Descent Imager/Spectral Radiometer (DISR) in situ measurements below 1.6μm. At longer wavelengths, ω0 steadily decreases from 0.92 at 1.6μm to about 0.70 at 2.5μm and abruptly drops to about 0.50 near 2.6μm, a spectral variation that differs from that of Khare et al.’s (Khare, B.N., Sagan, C., Arakawa, E.T., Suits, F., Callcott, T.A., Williams, M.W. [1984]. Icarus 60, 127–137) laboratory tholins. Our analysis shows that the far wings of the strong methane bands on both sides of the transparency windows provide a significant source of opacity in these windows, and that their unknown sub-Lorentzian behavior limits our ability to determine precisely the surface albedos. Below 1.6μm, the retrieved surface albedos agree with those derived from Huygens/DISR. The VIMS spectrum at 2.0μm indicates a surface albedo of 0.11±0.01, larger than derived in previous studies, and inconsistent with the signature of water ice. A series of VIMS data taken from 2004 to 2010 between 40°S and 40°N were then analyzed to monitor the latitudinal and temporal evolution of the atmospheric aerosol content. In the 2004–2008 period, the haze extinction is larger at Northern mid-latitudes by ∼20% with respect to the Huygens site, whereas Southern mid-latitudes are depleted by ∼15–20%. In 2009–2010, a progressive decline of the haze content in the Northern hemisphere is observed but no reversal of the North-to-South asymmetry is seen till mid-2010. Finally, data from five regions in Tui Regio and Fensal that show markedly different spectral behaviors and morphologies were analyzed to investigate the wavelength dependence of their surface albedo. The difference between bright and dark regions can be explained by different contents of small-sized tholins at the surface, brighter regions being more tholin-rich than dark regions, including the Huygens landing site. On the other hand, the albedo spectrum of the so-called blue regions, either dark or bright, can be explained by an excess of water ice particles, compared with the Huygens landing site. The spectrum of a 5-μm bright region in Tui Regio indicates a large excess of small-sized tholins relative to the Huygens site, but does not point to any particular surface composition.

Fluvial features on Titan: Insights from morphology and modeling

Fluvial features on Titan have been identified in synthetic aperture radar (SAR) data taken during spacecraft flybys by the Cassini Titan Radar Mapper (RADAR) and in Descent Imager/Spectral Radiometer (DISR) images taken during descent of the Huygens probe to the surface. Interpretations using terrestrial analogs and process mechanics extend our perspective on fluvial geomorphology to another world and offer insight into their formative processes. At the landscape scale, the varied morphologies of Titan’s fluvial networks imply a variety of mechanical controls, including structural influence, on channelized flows. At the reach scale, the various morphologies of individual fluvial features, implying a broad range of fluvial processes, suggest that (paleo-)flows did not occupy the entire observed width of the features. DISR images provide a spatially limited view of uplands dissected by valley networks, also likely formed by overland flows, which are not visible in lower-resolution SAR data. This high-resolution snapshot suggests that some fluvial features observed in SAR data may be river valleys rather than channels, and that uplands elsewhere on Titan may also have fine-scale fluvial dissection that is not resolved in SAR data. Radar-bright terrain with crenulated bright and dark bands is hypothesized here to be a signature of fine-scale fluvial dissection. Fluvial deposition is inferred to occur in braided channels, in (paleo)lake basins, and on SAR-dark plains, and DISR images at the surface indicate the presence of fluvial sediment. Flow sufficient to move sediment is inferred from observations and modeling of atmospheric processes, which support the inference from surface morphology of precipitation-fed fluvial processes. With material properties appropriate for Titan, terrestrial hydraulic equations are applicable to flow on Titan for fully turbulent flow and rough boundaries. For low-Reynolds-number flow over smooth boundaries, however, knowledge of fluid kinematic viscosity is necessary. Sediment movement and bed form development should occur at lower bed shear stress on Titan than on Earth. Scaling bedrock erosion, however, is hampered by uncertainties regarding Titan material properties. Overall, observations of Titan point to a world pervasively influenced by fluvial processes, for which appropriate terrestrial analogs and formulations may provide insight.

Bouncing on Titan: Motion of the Huygens probe in the seconds after landing

While landing on Titan, several instruments onboard Huygens acquired measurements that indicate the probe did not immediately come to rest. Detailed knowledge of the probe's motion can provide insight into the nature of Titan's surface. Combining accelerometer data from the Huygens Atmospheric Structure Instrument (HASI) and the Surface Science Package (SSP) with photometry data from the Descent Imager/Spectral Radiometer (DISR), we develop a quantitative model to describe motion of the probe, and its interaction with the surface. The most likely scenario is the following. Upon impact, Huygens created a 12cm deep hole in the surface of Titan. It bounced back, out of the hole onto the flat surface, after which it commenced a 30–40cm long slide in the southward direction. The slide ended with the probe out of balance, tilted in the direction of DISR by around 10°. The probe then wobbled back and forth five times in the north–south direction, during which it probably encountered a 1–2cm sized pebble. The SSP provides evidence for movement up to 10s after impact. This scenario puts the following constraints on the physical properties of the surface. For the slide over the surface we determine a friction coefficient of 0.4. While this value is not necessarily representative for the surface itself due to the presence of protruding structures on the bottom of the probe, the dynamics appear to be consistent with a surface consistency of damp sand. Additionally, we find that spectral changes observed in the first four seconds after landing are consistent with a transient dust cloud, created by the impact of the turbulent wake behind the probe on the surface. The optical properties of the dust particles are consistent with those of Titan aerosols from Tomasko et al. (2008). We suggest that the surface at the landing site was covered by a dust layer, possibly the 7mm layer of fluffy material identified by Atkinson et al. (2010). The presence of a dust layer contrasts with the dampness measured just centimeters below the surface, and suggests a recent spell of dry weather at the landing site.

Radiative transfer analyses of Titan’s tropical atmosphere

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 92km 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., Bézard, 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 5.0μm, 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 5km error. VIMS spectra of Titan at 2.6–3.2μm indicate not only spectral features due to CH4 and CH3D (Rannou, P., Cours, T., Le Mouélic, 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 4.8μm spectrum point to a haze optical depth of 0.2 at that wavelength. Cloud spectra at 2μm indicate that the far wings of the Voigt profile extend 460cm−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 8×12km2 area surrounding the Huygens landing site. Within the 0.4–1.6μm spectral region our surface albedos match DISR measurements, indicating that DISR and VIMS measurements are consistently calibrated. These values together with albedos at longer 1.9–5.0μm 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.

Condensation in Titan’s atmosphere at the Huygens landing site

We present a self-consistent description of Titan’s aerosols-clouds-gases system and compare our results with the optical properties retrieved from measurements made by the Descent Imager/Spectral Radiometer (DISR) experiment on the Huygens probe (Tomasko, M.G. et al. [2008]. Planet. Space Sci. 56, 669–707). Our calculations include the condensation of methane, ethane and hydrogen cyanide on photochemical aerosols produced in the thermosphere. Our results suggest that the two distinct extinction layers observed by DISR below 80 km are produced by HCN and methane condensation, while for the Huygens’ equatorial conditions simulated here, the contribution of ethane clouds to the total opacity is negligible. The HCN mass flux is comparable to the mass flux of aerosols, thus the majority of the HCN cloud particles have a similar size with the aerosol particles, they are HCN-coated aerosols. Ethane cloud particles have sizes between 2 and 10 μm depending on altitude, and methane clouds grow to an average size of ∼100 μm before starting to evaporate below 10 km. The reproduction by the simulation of the main features observed by DISR suggests that the atmospheric snapshot acquired by the Huygens instruments corresponds to a condition very close to the steady state simulated here. This points to the stability of the equatorial atmospheric conditions at the time of the descent. Moreover, we investigate the resulting abundance of ethane in the lower part of the atmosphere and its impact on the flux of condensates to the surface. Our results suggest that under the steady state conditions investigated, ethane condensates evaporate before reaching the surface and that the ethane gas abundance close to the surface is well below its saturation limit.

Titan’s aerosol and stratospheric ice opacities between 18 and 500 μm: Vertical and spectral characteristics from Cassini CIRS

Vertical distributions and spectral characteristics of Titan’s photochemical aerosol and stratospheric ices are determined between 20 and 560 cm −1 (500–18 μm) from the Cassini Composite Infrared Spectrometer (CIRS). Results are obtained for latitudes of 15°N, 15°S, and 58°S, where accurate temperature profiles can be independently determined. In addition, estimates of aerosol and ice abundances at 62°N relative to those at 15°S are derived. Aerosol abundances are comparable at the two latitudes, but stratospheric ices are ∼3 times more abundant at 62°N than at 15°S. Generally, nitrile ice clouds (probably HCN and HC 3N), as inferred from a composite emission feature at ∼160 cm −1, appear to be located over a narrow altitude range in the stratosphere centered at ∼90 km. Although most abundant at high northern latitudes, these nitrile ice clouds extend down through low latitudes and into mid southern latitudes, at least as far as 58°S. There is some evidence of a second ice cloud layer at ∼60 km altitude at 58°S associated with an emission feature at ∼80 cm −1. We speculate that the identify of this cloud may be due to C 2H 6 ice, which in the vapor phase is the most abundant hydrocarbon (next to CH 4) in the stratosphere of Titan. Unlike the highly restricted range of altitudes (50–100 km) associated with organic condensate clouds, Titan’s photochemical aerosol appears to be well-mixed from the surface to the top of the stratosphere near an altitude of 300 km, and the spectral shape does not appear to change between 15°N and 58°S latitude. The ratio of aerosol-to-gas scale heights range from 1.3–2.4 at about 160 km to 1.1–1.4 at 300 km, although there is considerable variability with latitude. The aerosol exhibits a very broad emission feature peaking at ∼140 cm −1. Due to its extreme breadth and low wavenumber, we speculate that this feature may be caused by low-energy vibrations of two-dimensional lattice structures of large molecules. Examples of such molecules include polycyclic aromatic hydrocarbons (PAHs) and nitrogenated aromatics. Finally, volume extinction coefficients N χ E derived from 15°S CIRS data at a wavelength of λ = 62.5 μm are compared with those derived from the 10°S Huygens Descent Imager/Spectral Radiometer (DISR) data at 1.583 μm. This comparison yields volume extinction coefficient ratios N χ E (1.583 μm)/N χ E (62.5 μm) of roughly 70 and 20, respectively, for Titan’s aerosol and stratospheric ices. The inferred particle cross-section ratios χ E (1.583 μm)/ χ E (62.5 μm) appear to be consistent with sub-micron size aerosol particles, and effective radii of only a few microns for stratospheric ice cloud particles.

Limits on the size of aerosols from measurements of linear polarization in Titan’s atmosphere

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.

New insights on Titan's plasma-driven Schumann resonance inferred from Huygens and Cassini data

After a preliminary analysis of the low-frequency data collected with the electric antenna of the Permittivity, Wave and Altimetry (PWA) experiment onboard the Huygens Probe that landed on Titan on 14 January, 2005, it was anticipated in a previous article [Béghin et al., 2007. A Schumann-like resonance on Titan driven by Saturn's magnetosphere possibly revealed by the Huygens Probe. Icarus, 191, 251–266] that the Extremely Low-Frequency (ELF) signal at around 36 Hz observed throughout the descent, might have been generated in the upper ionosphere of Titan, driven by a plasma instability mechanism associated with the co-rotating Kronian plasma flow. The involved process was proposed as the most likely source of a Schumann resonance in the moon's atmospheric cavity, the second eigenmode of which is actually found by models to occur at around 36 Hz. In this paper, we present a thorough analysis of this signal based upon the Huygens Probe attitude data deduced from the Descent Imager Spectral Radiometer (DISR), and relevant measurements obtained from the Radio Plasma Wave Science (RPWS) experiment and from the magnetometer (MAG) onboard Cassini orbiter during flybys of Titan. We have derived several coherent characteristics of the signal which confirm the validity of the mechanism initially proposed and provide new and significant insights about such a unique type of Schumann resonance in the solar system. Indeed, the 36 Hz signal contains all the characteristics of a polarized wave, with the measured electric field horizontal component modulated by the antenna rotation, and an altitude profile in agreement with a Longitudinal Section Electric (LSE) eigenmode of the atmospheric cavity. In contrast to Earth's conditions where the conventional Transverse Magnetic mode is considered, the LSE mode appears to be the only one complying with the observations and the unexpected peculiar conditions on Titan. These conditions are essentially the lack of any lightning activity that can be ascertained from Cassini observations, the presence of an ionized layer centered around 62 km altitude that was discovered by the PWA instrumentation, and the existence of a subsurface conducting boundary which is mandatory for trapping ELF waves. A simple theoretical model derived from our analysis places tentatively consequential constraints on the conductivity profile in the lower ionosphere. It is also consistent with the presence of a conductive water ocean below an icy crust some tens of kilometers thick.

Rain and dewdrops on titan based on in situ imaging

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.

Heat balance in Titan's atmosphere

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.

Measurements of methane absorption by the descent imager/spectral radiometer (DISR) during its descent through Titan's atmosphere

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.

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

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

The properties of Titan's surface at the Huygens landing site from DISR observations

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 aerosols: Comparison between our model and DISR findings

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 C 2H 2 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 C 2H 2, C 2H 4 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., Bézard, 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 6.9×10 6 m −3 at Z=100 km, as compared with DISR's finding of 5.0×10 6 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 CH 4 on the highly fractal structures could begin at the altitude where thin methane clouds were observed, filling somewhat the new open fractal structures.

DISR imaging and the geometry of the descent of the Huygens probe within Titan's atmosphere

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 2 ∘ north of east from 145 to 50 km altitude, turning to 5 ∘ 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 15 ∘ 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 5 ∘ / s at high and low altitudes, but 13 ∘ / s between 110 and 30 km altitude. Maximum swing rates were often above 40 ∘ / s , far above the design limit of 6 ∘ / 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 180 ∘ , 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 2 ∘ . 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., Bézard, B., Bushroe, M., Combes, M., Cook, D., Coustenis, A., de Bergh, C., Dafoe, L.E., Doose, L., Douté, S., Eibl, A., Engel, S., Gliem, F., Grieger, B., Holso, K., Howington-Kraus, E., Karkoschka, E., Keller, H.U., Kirk, R., Kramm, R., Küppers, M., Lanagan, P., Lellouch, E., Lemmon, M., Lunine, J., McFarlane, E., Moores, J., Prout, G.M., Rizk, B., Rosiek, M., Rueffer, P., Schröder, 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 3 ∘ , changing by a small fraction of a degree. The apparent horizon looking south to SSW from the landing site was 1– 2 ∘ above the theoretical horizon, sloping by 1 ∘ 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.