The selection of Dragonfly enables regional Titan surface science, but global Titan geophysics, geology, hydrology, and meteorology require an orbiter. We consider the sources of signal and noise that would contribute to near-infrared surface imaging from such an orbiter both analytically and numerically. The fraction of light arriving at an orbiting camera directly from Titan’s surface, and therefore conveying full-resolution surface information, decreases at shorter wavelengths as additive atmospheric scatter and light blurred on the way out increase with higher haze optical depths. We apply the Monte Carlo radiative transfer model SRTC++ and show that up to 75% of observed flux in Titan’s 5 μ m window comes directly from the surface, up to 47% comes directly at 2 μ m, and up to just 7% comes directly in the 0.94 μ m window. We find that diffraction-limited surface imaging with 10 m pixels is possible with a signal-to-noise ratio for surface features of 100 in the near-infrared at 5 and 2 μ m using a 50 cm aperture. A Titan orbiter camera could image in color using 5 μ m, 2 μ m, and potentially other wavelengths using a pushbroom strategy with time-delay integration.

Titan was a mostly unknown world prior to the Cassini spacecraft’s arrival in July 2004. We review the major scientific advances made by Cassini’s Titan Radar Mapper (RADAR) during 13 years of Cassini’s exploration of Saturn and its moons. RADAR measurements revealed Titan’s surface geology, observed lakes and seas of mostly liquid methane in the polar regions, measured the depth of several lakes and seas, detected temporal changes on its surface, and provided key evidence that Titan contains an interior ocean. As a result of the Cassini mission, Titan has gone from an uncharted world to one that exhibits a variety of Earth-like geologic processes and surface-atmosphere interactions. Titan has also joined the ranks of “ocean worlds” along with Enceladus and Europa, which are prime targets for astrobiological research.

Saturn’s main ring system is associated with a set of small moons that either are embedded within it or interact with the rings to alter their shape and composition. Five close flybys of the moons Pan, Daphnis, Atlas, Pandora, and Epimetheus were performed between December 2016 and April 2017 during the ring-grazing orbits of the Cassini mission. Data on the moons’ morphology, structure, particle environment, and composition were returned, along with images in the ultraviolet and thermal infrared. We find that the optical properties of the moons’ surfaces are determined by two competing processes: contamination by a red material formed in Saturn’s main ring system and accretion of bright icy particles or water vapor from volcanic plumes originating on the moon Enceladus.

We report on the completion of a global control network of Enceladus containing Cassi-ni Imaging Science Subsystem (ISS) images, and asso-ciated photogrammetric and cartographic accomplish-ments. This effort was in support of a new global geo-logic map of Enceladus [1]. The resulting products include a preliminary global monochrome basemap and improved camera pointing for individual images. The basemap and ancillary data will be available through USGS Astrogeology Online Planetary Data and Services (PDS Annex/MAP2) [2-4]. The updated pointing is recorded in the Navigation and Ancillary Information Facility (NAIF)/SPICE ck kernel format [5] and released to the community via the Integrated Software for Imagers and Spectrometers (ISIS 3) [6] public release in February 2016.

Observations from Cassini VIMS and ISS show localized but extensive surface brightenings in the wake of the 2010 September cloudburst. Four separate areas, all at similar latitude, show similar changes: Yalaing Terra, Hetpet Regio, Concordia Regio, and Adiri. Our analysis shows a general pattern to the time-sequence of surface changes: after the cloudburst the areas darken for months, then brighten for a year before reverting to their original spectrum. From the rapid reversion timescale we infer that the process driving the brightening owes to a fine-grained solidified surface layer. The specific chemical composition of such solid layer remains unknown. Evaporative cooling of wetted terrain may play a role in the generation of the layer, or it may result from a physical grain-sorting process.

Large areas of Titan’s surface brightened at all wavelengths as seen from Cassini/VIMS for several months. The brightenings occurred after a large storm and rainfall event, and may relate to volatile refreezing due to evaporative cooling.

Titan's fluvial channels have been investigated based on data obtained by the Synthetic Aperture Radar (SAR) instrument and the Visible and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft. In this paper, a database of fluvial features is created based on radar-SAR data aiming to unveil the distribution and the morphologic and spectral characteristics of valleys on Titan on a global scale. It will also study the spatial relations between fluvial valleys and Titan's geologic units and spectral surface units which have become accessible thanks to Cassini-VIMS data. Several distinct morphologic types of fluvial valleys can be discerned by SAR-images. Dendritic valley networks appear to have much in common with terrestrial dendritic systems owing to a hierarchical and tree-shaped arrangement of the tributaries which is indicative of an origin from precipitation. Dry valleys constitute another class of valleys resembling terrestrial wadis, an indication of episodic and strong flow events. Other valley types, such as putative canyons, cannot be correlated with rainfall based on their morphology alone, since it cannot be ruled out that they may have originated from volcanic/tectonic action or groundwater sapping. Highly developed and complex fluvial networks with channel lengths of up to 1200 km and widths of up to 10 km are concentrated only at a few locations whereas single valleys are scattered over all latitudes. Fluvial valleys are frequently found in mountainous areas. Some terrains, such as equatorial dune fields and undifferentiated plains at mid-latitudes, are almost entirely free of valleys. Spectrally, fluvial terrains are often characterized by a high reflectance in each of Titan's atmospheric windows, as most of them are located on Titan's bright 'continents'. Nevertheless, valleys are spatially associated with a surface unit appearing blue due to its higher reflection at 1.3μm in a VIMS false color RGB composite with R: 1.59/1.27μm, G: 2.03/1.27μm, and B: 1.27/1.08μm; the channels either dissect pure bluish surface units or they are carved into terrain with a mixed spectral signature between bright and bluish surface materials. The global picture of fluvial flows clearly indicates a high diversity of parameters controlling fluvial erosion, such as climatic processes, as well as surface and bedrock types. Recent fluvial activity is very likely in the north polar region in contrast to more arid conditions at lower latitudes and at the south pole of Titan. This divergence is probably an indication of seasonal climatic asymmetries between the hemispheres. However, traces of previous fluvial activity are scattered over all latitudes of Titan, which is indicative of previous climatic conditions with at least episodic rainfall.

In the last few years Cassini-VIMS, the Visible and Infrared Mapping Spectrometer, returned to us a comprehensive view of the Saturn's icy satellites and rings. After having analyzed the satellites' spectral properties (Filacchione, G., Capaccioni, F., McCord, T.B., Coradini, A., Cerroni, P., Bellucci, G., Tosi, F., D'Aversa, E., Formisano, V., Brown, R.H., Baines, K.H., Bibring, J.P., Buratti, B.J., Clark, R.N., Combes, M., Cruikshank, D.P., Drossart, P., Jaumann, R., Langevin, Y., Matson, D.L., Mennella, V., Nelson, R.M., Nicholson, P.D., Sicardy, B., Sotin, C., Hansen, G., Hibbitts, K., Showalter, M., Newman, S. [2007]. Icarus 186, 259-290, paper I) and their distribution across the satellites' hemispheres (Filacchione, G., Capaccioni, F., Clark, R.N., Cuzzi, J.N., Cruikshank, D.P., Coradini, A., Cerroni, P., Nicholson, P.D., McCord, T.B., Brown, R.H., Buratti, B.J., Tosi, F., Nelson, R.M., Jaumann, R., Stephan, K. [2010]. Icarus 206, 507-523, paper II), we proceed in this paper to investigate the radial variability of icy satellites (principal and minor) and main rings average spectral properties. This analysis is done by using 2264 disk-integrated observations of the satellites and a 12 x 700 pixels-wide rings radial mosaic acquired with a spatial resolution of about 125 km/pixel. Using different VIS and IR spectral indicators, e.g. spectral slopes and band depths, we perform a comparative analysis of these data aimed to measure the distribution of water ice and red contaminant materials across Saturn's system. The average surface regolith grain sizes are estimated with different indicators through comparison with laboratory and synthetic spectra. These measurements highlight very striking differences in the population here analyzed, which vary from the almost uncontaminated and water ice-rich surfaces of Enceladus and Calypso to the metal/organic-rich and red surfaces of Iapetus' leading hemisphere and Phoebe. Rings spectra appear more red than the icy satellites in the visible range but show more intense 1.5-2.0 mu m band depths. Although their orbits are close to the F-ring, Prometheus and Pandora are different in surface composition: Prometheus in fact appears very water ice-rich but at the same time very red at VIS wavelengths. These properties make it very similar to A-B ring particles while Pandora is bluer. Moving outwards, we see the effects of E ring particles, generated by Enceladus plumes, which contaminate satellites surfaces from Mimas out to Rhea. We found some differences between Tethys lagrangian moons, Calypso being much more water ice-rich and bluer than Telesto. Among outer satellites (Hyperion, Iapetus and Phoebe) we observe a linear trend in both water ice decrease and in reddening. Hyperion being the reddest object of the population. The correlations among spectral slopes, band depths, visual albedo and phase permit us to cluster the saturnian population in different spectral classes which are detected not only among the principal satellites and rings but among co-orbital minor moons as well. These bodies are effectively the "connection" elements, both in term of composition and evolution, between the principal satellites and main rings. Finally, we have applied Hapke's theory to retrieve the best spectral fits to Saturn's inner regular satellites (from Mimas to Dione) using the same methodology applied previously for Rhea data discussed in Ciarniello et al. (Ciarniello, M., Capaccioni, F., Filacchione, G., Clark, R.N., Cruikshank, D.P., Cerroni, P., Coradini, A., Brown, R.H., Buratti, B.J., Tosi, F., Stephan, K. [2011]. Icarus 214, 541-555). (c) 2012 Elsevier Inc. All rights reserved.

During the 58th close flyby of Titan (T58), the Cassini Visual and Infrared Mapping Spectrometer (VIMS) observed a specular reflection of sunlight from Titan's Jingpo Lacus through the 5-mu m methane window (Stephan, K. et al. [2010]. Geophys. Res. Lett. 37, L07104). The maximum intensity of this reflection is controlled by three basic factors: (1) the shape of the reflecting surface (its overall geometry and roughness), (2) the reflectance of the surface, as controlled by the real refractive index of the material (and that of the atmosphere), and (3) attenuation due to absorption and scattering by atmospheric gases and aerosols along the pathlength. Herein we model the expected intensity of a specular reflection off of a convex mirror-like surface on Titan. We assume the specular reflection is from a body of liquid hydrocarbons on Titan's surface with optical properties consistent with CH4 and C2H6 with smaller amounts of nitrogen and heavier hydrocarbons (e.g., C3H8) admixed. We assume the 5-mu m opacity for the polar atmosphere is a factor of two higher than that of the tropical haze. For the geometry of the T58 observations, our model predicts a maximum I/F = 1-to-5; for a Lambertian surface at normal illumination I/F = 1. The maximum 5-mu m intensity observed during T58 was I/F similar to 2.6, from which we conclude that Jingpo Lacus is filled with a liquid that has a real index of refraction consistent with that of methane-ethane-nitrogen liquid and that the 5-mu m atmospheric opacity was tau = 0.5, consistent with the higher particle column expected in the winter polar atmosphere. Future VIMS observations will allow us to refine the refractive index of the liquid in the lakes and to place a quantitative constraint on the ratio of methane to ethane. (C) 2012 Elsevier Inc. All rights reserved.

Turtle et al. (2011) previously announced large-scale surface changes in Titan's tropics following a 2010 September cloudburst event. Those changes were areas that had darkened, and the darkening was attributed to surface wetting by rain. Here we will discuss the results of continued monitoring of the darkened areas by Cassini VIMS and ISS. These new observations show that instead of reverting to their previous state, the rain-darkened areas instead brightened beyond their original albedos starting a few months after the cloudburst event. The brightening was unexpected, and spectra show that it occurs in each of Titan's atmospheric wavelength windows. The brightened spectra show some similarity to he heretofore unique signature of Xanadu. The areas slowly revert to their original spectra over a period of a year. The two hypotheses that we have not eliminated involve (1) volatile frosts resulting from evaporative cooling of rain-derived surface methane that later sublime, (2) deposition of a thin surface layer of very-fine-grained particles (similar to terrestrial playa lakes) that degrade due to aeolean erosion. Future calculations and observations will serve to constrain the mechanism that drives the brightening.

Since Titan entered Northern spring in August 2009, the North Pole has been illuminated allowing observations at optical wavelengths. On June 5, 2010 the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini spacecraft observed the Northern Pole area with a pixel size from 3 to 7 km. Since, as we demonstrate, little of the solar flux at 5 pm is scattered by the atmosphere, these observations were obtained at relatively large incidence angles and allowed us to build a mosaic covering an area of more than 500,000 km(2) that overlaps and complements observations made by the Synthetic Aperture Radar (SAR) in 2007. We find that there is an excellent correlation between the shape of the radar dark area, known as Ligeia Mare and the VIMS 5-mu m dark unit. Matching most of the radar shoreline, the 2010 VIMS observations suggest that the 125,000-km(2) surface area of Ligeia Mare measured by RADAR in 2007 has not significantly changed. The VIMS observations complement the radar observations to the west of Ligeia Mare and suggest that Ligeia Mare is connected to Kraken Mare by either a diffuse network similar to a swamp area, or by well-defined, sub-pixel rivers. Considering the results of recent evaporation models of methane, our preferred interpretation of the relative constancy in surface area of Ligeia is that it is principally composed of ethane although we cannot rule out the possibility that methane evaporation is balanced with replenishment by either precipitation or underground seepage. There is also strong correlation between the location of the small radar lakes and the small VIMS 5-mu m dark patches. The geographic location of the small lakes are within a VIMS pixel of the SAR location, suggesting that the non-synchronous component of Titan's spin rate, if it exists, was less than 2.3 x 10(-4) deg/day between 2007 and 2010 in agreement with the recent T64 radar observations. These observations question the existence of non-synchronous rotation. Two radar-bright features appear dark at 5-mu m. The simplest interpretation is that these are very shallow lakes, less than one meter deep. Three new small lakes, named Freeman, Cardiel, and Towada by the IAU, are found outside of the area mapped with the SAR. A single-scattering model describing reflection of sunlight at 5-mu m suggests that the lake surface is mirror-like and that the albedo of the solid surfaces surrounding the lakes is about 8%. These observations together with information of the haze aerosols allow us to show that Titan's lakes, atmospheric ethane and aerosol haze are smaller carbon reservoirs than Titan's sand dunes and atmospheric methane. A simple model involving an outburst of methane a few hundreds of Myr ago followed by the dissociation of methane in the atmosphere leading to the formation of the haze particles that constitute the dune fields would be consistent with both the present observations and recent measurements of isotopic ratios in atmospheric methane (Mandt, K.E. et al. [2012]. Astrophys. J. 749(160), 14). (C) 2012 Elsevier Inc. All rights reserved.

Mapping the surface composition of Titan with the Cassini Visual and Infrared Mapping Spectrometer (VIMS) requires knowledge of the atmospheric absorption in the windows through which VIMS can observe the surface as well as the spectral properties of candidate materials. Recent atmospheric models are refining that knowledge. Titan's surface, in the VIMS spectral range, is seen in only a few spectral windows, near 0.94, 1.1, 1.3, 1.6, 2.0, 2.68-2.78, and 4.9-5.1 microns. Atmospheric models fail to fit the the observed spectra on the long wavelength side of the 2-micron window without invoking surface absorption at 2.1 microns. This new knowledge, along with the spectral shapes of the 2.68-2.78-micron, and 5-micron windows provide powerful constraints on Titan's surface composition. Water ice is incompatible with the observed 2.78/2.68 micron I/F ratio but likely exists below the surface. Many organic compounds have absorptions that are not seen in spectral windows of Titan, eliminating them as possible major components at the surface, including many polycyclic aromatic hydrocarbons (PAH). We find that some ring compounds and compounds with single N-H bonds have a close match to Titan's overall spectrum and can explain the relative intensities observed in the spectral windows, including the 2.68 and 2.78-micron double window, the low 3-5 micron reflectance, and increased absorption near 2.1-microns. Glycine is the only NH2 compound we have found that is also compatible. Combinations of coronene (C24H12), phenanthene (C14H12), pentacene (C22H14), indole (C8H7N), uracil (C4H4N2O2), and glycine (NH2CH2C00H) match the overall spectral structure of Titan spectra. We are searching for additional compounds that are also compatible. Indole, cytosine, and uracil, have 1.5-micron bands that are similar to the feature observed in Huygens DISR spectra of Titan's surface. These compounds, if present, can also help explain the pyrolysis results from the Huygens probe.

Opportunity has been traversing the Meridiani plains since 25 January 2004 (sol 1), acquiring numerous observations of the atmosphere, soils, and rocks. This paper provides an overview of key discoveries between sols 511 and 2300, complementing earlier papers covering results from the initial phases of the mission. Key new results include (1) atmospheric argon measurements that demonstrate the importance of atmospheric transport to and from the winter carbon dioxide polar ice caps; (2) observations showing that aeolian ripples covering the plains were generated by easterly winds during an epoch with enhanced Hadley cell circulation; (3) the discovery and characterization of cobbles and boulders that include iron and stony-iron meteorites and Martian impact ejecta; (4) measurements of wall rock strata within Erebus and Victoria craters that provide compelling evidence of formation by aeolian sand deposition, with local reworking within ephemeral lakes; (5) determination that the stratigraphy exposed in the walls of Victoria and Endurance craters show an enrichment of chlorine and depletion of magnesium and sulfur with increasing depth. This result implies that regional-scale aqueous alteration took place before formation of these craters. Most recently, Opportunity has been traversing toward the ancient Endeavour crater. Orbital data show that clay minerals are exposed on its rim. Hydrated sulfate minerals are exposed in plains rocks adjacent to the rim, unlike the surfaces of plains outcrops observed thus far by Opportunity. With continued mechanical health, Opportunity will reach terrains on and around Endeavour's rim that will be markedly different from anything examined to date.