Cassini Imaging Science Subsystem Research Articles

A Novel Approach to Impact Crater Mapping and Analysis on Enceladus, Using Machine Learning

AbstractImpact cratering is one of the most important processes shaping planetary surfaces, offering valuable clues about the target body's geologic history and composition. However, crater mapping has historically been done manually, a process that has proven to be both arduous and time consuming. This paper outlines a machine learning crater mapping approach for bodies with limited elevation data available (Digital Elevation Models). We applied a Convolutional Neural Network for the detection and morphometry of impact craters on Saturn's moon Enceladus using light‐shadow labels trained on data from the Cassini Imaging Science Subsystem. Our algorithm identified a total of 5,240 features which were used to quantify crater distribution; this included the highest number of small craters (<1–2 km in diameter) recorded on Enceladus by any previous published study. The pool of features was later down‐selected to craters between 0 and 30°N (latitude) imaged at high incidence (>60°) and phase angles (>26°). The down selection was necessary to accurately perform diameter measurements and derive depths from shadow estimation techniques to calculate depth–diameter ratios (d/D); a well‐studied relationship used to constrain planetary surface properties. Results show that the d/D ratio of craters in the equatorial region of Enceladus range from ∼0.06 to 0.37, with a median of 0.19. Our results will inform efforts to constrain the surface properties of this region of Enceladus, potentially also supporting future mission concept design for the Saturnian moon. Future work will explore the simple‐to‐complex crater transition and differences between this area's d/D and Enceladus' northern and southern latitudes.

Titan’s Atmospheric Albedo Asymmetry and Seasonal Variability Observed through the Cassini Imaging Science Subsystem

Using images from Cassini, we analyzed the north–south albedo asymmetry that has been observed in the atmosphere of Saturn’s moon, Titan. Suitable images from the Cassini Imaging Science Subsystem taken at 889 nm spanned from 2004 to 2017—around half of a Titan year—and revealed seasonal changes in the characteristics and orientation of the north–south asymmetry boundary. Such circumglobal features provide insight into the dynamics and circulation of the atmosphere more broadly. The albedo asymmetry has been observed to reverse for part of the Titan year, inverting the brighter and darker hemispheres; we also observed this inversion, along with the formation of additional banding briefly during the transition (around 2014–2016). A tilt in the rotation axis of Titan’s atmosphere with respect to the solid body rotation has previously been noted. Using robust edge-detection techniques, we likewise identified a tilt offset of a few degrees in the albedo transition boundaries. The azimuth of this tilt axis remained roughly fixed in inertial space, with some smaller possible seasonal fluctuations around the fixed direction noted.

A study of Jupiter's UV Great Dark Spot and tropopause to stratosphere winds in the high northern latitudes as seen by Cassini imaging

Images of Jupiter taken during the year 2000 Cassini flyby revealed the early stages and evolution of a high northern-latitude UV Great Dark Spot over a three-month period. They also provided a window on the circulation of Jupiter's polar stratosphere. Jupiter's rarely-detected UV Great Dark Spot (UVGDS) is unusual in several respects. It is an ephemeral feature, twice appearing as an oval with size and shape similar to Jupiter's Great Red Spot (GRS). Like the GRS, it is an anticyclonic feature. It is unlike the GRS in that: (1) A gas, and/or very small particles (radius < < 1 micro meter, of unknown composition but likely hydrocarbon) is responsible for its contrast with its surroundings; (2) It appears to be a shallow feature, confined to the stratosphere; (3) It evolves (moves in System III longitude, elongates and tilts) over a period of months; and (4) It may be produced by an auroral event as inferred by its proximity to the southern extent of Jupiter's main auroral oval. However, we did not identify a specific auroral trigger event. We measure the rotation rate for the UVGDS to have been ∼20° per Earth day. To investigate the dynamical regime of this feature, we applied an automated cloud feature tracking algorithm to Cassini Imaging Science Subsystem images of Jupiter's northern hemisphere (30°N – 80°N) in the near-IR continuum, methane band, and ultraviolet filters. From these images we were able to derive zonal wind profiles from the middle troposphere cloud tops to the lower stratosphere and track the evolution of the UV Great Dark Spot. Differences in mean zonal wind speed between the continuum and methane band filters indicate a weakening of jets with altitude as far north as 75°N latitude at the altitudes sampled by these filters. We also observe notable differences between the ultraviolet and the other two filters: (1) Winds are significantly weaker and mostly westward in the ultraviolet relative to lower altitudes at all latitudes equatorward of 60°N latitude; (2) Poleward of 60°N latitude, winds observed in the ultraviolet may be faster than those observed in the continuum and methane bands, though we are less confident in this result as Jupiter's haze morphology and the lack of small, discrete, unambiguous trackable features present unique challenges to obtaining reliable mean zonal winds in the ultraviolet at high latitudes.

Titan’s North–South Haze Asymmetry Ratio and Boundary at Visible Wavelengths over the Cassini Mission

We document the evolution of the north–south asymmetry (NSA) of Titan’s haze albedo during the Cassini mission between 2004 and 2017. We analyze coadded cube images taken at 96 distinct wavelengths between 0.35 and 1.05 μm by the Cassini Visual and Infrared Mapping Spectrometer (VIMS-V) instrument from 14 Titan flybys. Over half of a Titan year, we observe a near-complete transition in the NSA boundary latitude across the geographic equator from the southern to the northern hemisphere, including a 3 yr fading of the boundary for several years after the equinox. The fading transition of the NSA matches previous observations of a reversal of the NSA in Hubble Space Telescope images of Titan before the winter solstice between 1997 and 2000. A comparison of NSA images taken at similar times but different phase angles shows the NSA boundary is detectable, albeit with less contrast, at moderately high phase angles (∼90°). Analysis of the NSA boundary in T61 and T67 VIMS images further supports a small tilt between the superrotating atmosphere and the solid body of Titan, as suggested in a previous analysis of 0.890 μm images from the Cassini Imaging Science Subsystem.

Astrometric Reduction of Saturnian Satellites with Cassini-ISS Images Degraded by Trailed Stars

The Imaging Science Subsystem (ISS) mounted on the Cassini spacecraft has taken a lot of images, which provides an important source of high-precision astrometry of some planets and satellites. However, some of these images are degraded by trailed stars. Previously, these degraded images cannot be used for astrometry. In this paper, a new method is proposed to detect and compute the centers of these trailed stars automatically. The method is then performed on the astrometry of ISS images with trailed stars. Finally, we provided 658 astrometric positions between 2004 and 2017 of several satellites that include Enceladus, Dione, Tethys, Mimas and Rhea. Compared with the JPL ephemeris SAT427, the mean residuals of these measurements are 0.11 km and 0.26 km in R.A. and decl., respectively. Their standard deviations are 1.08 km and 1.37 km, respectively. The results show that the proposed method performs astrometric measurements of Cassini ISS images with trailed stars effectively.

Characterising the interior of five inner Saturnian moons using Cassini ISS data

Context. The Cassini-Huygens space mission provided a huge amount of data for the Saturnian system. While the mission ended in September 2017, there is still much information to be retrieved from the data. Aims. Here, from their orbital motion, we infer the interior of the five inner Saturnian moons, Atlas, Prometheus, Pandora, Janus, and Epimetheus. Our results benefit from a recent study of the shape of these moons, which provides a much better estimation of their 3D shape and volume. Methods. Using Cassini Imaging Science Subsystem (ISS) astrometric data, we again reduced the Cassini ISS images with an improved shape modelling. We then integrated the orbital motion of the inner Saturnian moons and solved for a large set of physical parameters for this system, including the masses and physical libration amplitudes of the moons. Results. We obtain a density value (in g cm−3 with a 1σ error bar based on the digital terrain models and spherical harmonics available today) of 0.3935 ± 0.0033, 0.4873 ± 0.0026, 0.5045 ± 0.0017, 0.6233 ± 0.0015, and 0.6413 ± 0.0002 for Atlas, Prometheus, Pandora, Epimetheus, and Janus, respectively. We were able to determine the physical libration amplitudes of Prometheus, Pandora, Epimetheus, and Janus to be 0.98 ± 0.08, −5.2 ± 0.7, −6.0 ± 0.6, and −0.39 ± 0.16 (degrees and 1σ error bar), respectively. Our solutions for Epimetheus and Janus are in full agreement with a former estimation performed directly from the measurement of the rotation of these two moons. Conclusions. We confirm the large porosity of these five moons and their increase in density as a function of their size. Our estimations of the physical librations of Prometheus, Pandora, Janus, and Epimetheus are consistent with a homogeneous interior, but for Pandora a heterogeneous interior is also plausible.

Complementary astrometry of Cassini Imaging Science Subsystem images of Phoebe

Phoebe is the only major satellite of Saturn with a retrograde orbit. The Cassini Imaging Science Subsystem (ISS) took a lot of Phoebe images between 2004 and 2017, but only a selection of them has been reduced. In this paper, we reduced the remaining ISS images of Phoebe. In the reduction, the Gaia EDR3 catalogue was used to provide the reference stars' positions, and the modified moment was used to measure the centre of image stars and Phoebe. Finally, a total of 834 ISS images of Phoebe have been reduced successfully. Compared with the JPL ephemeris SAT375, Phoebe's positions are consistent. The average residuals in the right ascension and declination are 0.08" and $-0.05$", and the standard deviations of the residuals are about 0.2". In terms of residuals in linear units, the means in the right ascension and declination are about 5 km and $-2$ km, respectively; The standard deviations are about 11 km. Compared with the JPL ephemeris SAT427 and IMCCE ephemeris PH20, our measurements show a strong bias and a large dispersion.

Multiple convective storms within a single cyclone on Saturn

Here we describe a cyclone at 50∘N in the atmosphere of Saturn that was the source of eight separate convective storms. The storm clouds that appeared bright in scattered sunlight were identified using images from the Cassini Imaging Science Subsystem (ISS) and ground-based telescopes. The Cassini Radio and Plasma Wave Science (RPWS) instrument detected Saturn Electric Discharge (SED) signals from the lightning within the storms, which were correlated to the sudden appearance of bright storm clouds in and surrounding the cyclone. Using the number and flash rate of SEDs we infer the duration and intensity of the storms. Although previous storms have been observed to erupt from within cyclones, the cyclone studied here is unusual in that it survived six storms before being disrupted by the seventh and strongest storm. An eighth storm occurred in the remnant of the disrupted cyclone. We conjecture that the anticyclonic circulation generated by the seventh storm was enough to overwhelm the cyclone’s gradient wind balance.

Modeling global-scale mass flows on the Lagrangian satellites of Dione and Tethys

We perform geomorphological analysis and modeling of the Lagrangian satellites of Dione and Tethys, based on images obtained by Cassini's Imaging Science Subsystem camera and a digital elevation model covering Helene's Saturn-facing and leading sides. Helene's cratered surface has been heavily modified by mass flows as indicated by pronounced crater rim degradation and downslope-oriented chutes, ridges and mesas, which reach >5 km long and ~ 100–300 m wide, and merge onto broad featureless basins at lower elevations with concave lateral boundaries. Mesas are elevated 20–25 m above adjacent parallel oriented chutes that merge with smooth basin materials, suggesting that they define an earlier mass flow surface that has undergone dissection by renewed mass flows. Similar landforms are observed on Calypso. Telesto also exhibits concave seams in the center of its smooth plains, which are interpreted as sites where opposing mass flows converge. We employ landform evolution modeling in order to investigate how these features might form via mass wasting. We test two hypothetical scenarios of the mass flow mechanics. The first scenario postulates shallow diffusive flows, and involves cessation of the mass flow followed by a quiescent period, during which a thin, indurated surface layer forms that armors the loose regolith underneath. Upon resumption of flow, the downslope motion of avalanching materials erodes the hardened surface to create chutes and high standing mesas. A disadvantage of this scenario is the slow formation of an encrusted surface layer. The second scenario assumes deep flows with a Bingham rheology, and we find that development of chutes and mesas appears to be an intrinsic consequence of progressive, differential cessation of Bingham flows as surface relief declines. We conclude that Bingham flow modeling more successfully replicates the observed morphology of the Lagrangian satellites. The preferential location of long mass flows on the leading side of Helene suggests the accumulation of exogenous granular ice as a major source of the mobile material. Estimated rates of deposition derived from the current E-ring system are very low, suggesting alternative sources such as cryovolcanic or impact-generated debris from Dione or Tethys, or a catastrophic recent event in the inner satellite system of Saturn.

A comparison of centring algorithms in the astrometry of Cassini imaging science subsystem images and Anthe’s astrometric reduction

ABSTRACT In the caviar software package, a standard tool for astrometry of images from the Cassini imaging science subsystem (ISS), Gaussian fitting is used to measure the centre of point-like objects, achieving a typical precision of about 0.2 pixels. In this work, we consider how alternative methods may improve on this. We compare three traditional centroiding methods: two-dimensional Gaussian fitting, median, and modified moment. Results using 56 selected images show that the centroiding precision of the modified moment method is significantly better than the other two methods, with standard deviations for all residuals in sample and line of 0.065 and 0.063 pixels, respectively, representing a factor of over 2 improvement compared to Gaussian fitting. Secondly, a comparison of observations using Cassini ISS images of Anthe is performed. Anthe results show a similar improvement. The modified moment method is then used to reduce all ISS images of Anthe during the period 2008–2017. The observed-minus-calculated residuals relative to the JPL SAT393 ephemeris are calculated. In terms of α × cos(δ) and δ in the Cassini-centred international celestial reference frame, mean values of all residuals are close to 0 km, and their standard deviations are less than 1 km for narrow angle camera images, and about 4 km for wide angle camera images.

Tropopause and lower stratosphere winds and eddy fluxes on Saturn as seen by Cassini imaging

An automated cloud feature tracking algorithm is applied to 2004 Cassini Imaging Science Subsystem images of Saturn's southern hemisphere and equatorial region (5°N-70°S) in continuum, methane band, and ultraviolet filters to derive zonal wind profiles and eddy momentum fluxes from the middle troposphere cloud tops to the lower stratosphere. Zonal winds in the eastward and westward jets decrease in strength from the cloud tops (≳ 1 bar) to the upper troposphere (~60–600 mb), but do not measurably change in strength from the upper troposphere to the tropopause (~60 mb) to the lower stratosphere (≲ 60 mb). A narrow (~3°N-3°S) equatorial jet is detected at the cloud tops, but a previously reported strengthening of this jet from the cloud top level to the upper troposphere/tropopause region cannot be verified due to an absence of easily trackable features at the equator and conflicting results for the few features available. Eddy momentum fluxes on either side of the jet cores outside the tropics (poleward of ~20°S) converge into the eastward jets and diverge from the westward jets. Whether the flux convergence into the eastward jets weakens from the visible cloud level to the upper troposphere cannot be determined for this limited time period, but the eddy convergence appears to increase from the troposphere to the lower stratosphere. The divergence of the eddy flux from the westward jets seems to more clearly increase from the middle to the upper troposphere to the stratosphere, but this does not explain the slight weakening of the westward jets with height.

Ray-tracing thermal modelling of Saturn's main rings

The observations of the Cassini spacecraft gave new perspectives to our understanding of the rings of Saturn and their thermal studies due to their large variety of observation geometries. However, we still know little about the ring particles' physical and, particularly, thermal properties, which is the reason why a number of observations are not easy to explain. Nevertheless, the observed temperature profiles of the rings can be reproduced by considering the rings' general structural properties like local optical depth, τ, and, for the optically-thick rings, wake properties like the ratios H/W or wake thickness to wake width, and S/W or wake separation to wake width; as well as their main energy input sources (e. g., direct and reflected solar energy and Saturn's infrared energy).In this work, we assume that the main rings (C, B, Cassini Division and A rings) are monolayers and study the temperature variations with solar elevation angle, B′, of ten of their regions with a simple thermal balance equation that depends on the rings' shadowing behaviour due to the illumination geometry (B′) and the arrangement of particles across the rings (τ). The shadowing is a key feature, which we reproduce with a numerical function obtained via ray tracing from arrays of spherical particles (C(B′,τ)). Using the former model, we fit Cassini Composite Infrared Spectrometer (CIRS) temperature data with the Bond albedo, AV, and the so-called rotation parameter, f (which represents the rotation status of the ring particles) as only parameters. These parameters are two of the most important physical properties that define the rings' temperatures.With our method, we are able to reproduce the CIRS data temperature trends of the selected regions reasonably well. For evaluation purposes we compare the derived Bond albedo values with those obtained with the Cassini Imaging Science Subsystem (ISS) at wavelengths in the range 451 ≲ λ ≲ 650nm. The comparison suggests a dominance of slow rotators. Additionally, we evaluate the differences obtained when using either a numerical or an analytical shadowing function (in particular, that by (Altobelli et al., 2008)) in the thermal model. The differences between these two approaches reflect some details of the structure of the ring regions we study.

Automatic removal of false image stars in disk-resolved images of the Cassini Imaging Science Subsystem

Taking a large number of images, the Cassini Imaging Science Subsystem(ISS) has been routinely used in astrometry. In ISS images, disk-resolved objects often lead to false detection of stars that disturb the camera pointing correction. The aimof this study was to develop an automated processingmethod to remove the false image stars in disk-resolved objects in ISS images. The method included the following steps: extracting edges, segmenting boundary arcs, fitting circles and excluding false image stars. The proposed method was tested using 200 ISS images. Preliminary experimental results show that it can remove the false image stars in more than 95% of ISS images with disk-resolved objects in a fully automatic manner, i.e., outperforming the traditional circle detection based on Circular Hough Transform (CHT) by 17%. In addition, its speed is more than twice as fast as that of the CHT method. It is also more robust (no manual parameter tuning is needed) when compared with CHT. The proposed method was also applied to a set of ISS images of Rhea to eliminate the mismatch in pointing correction in automatic procedure. Experiment results showed that the precision of final astrometry results can be improve by roughly 2 times that of automatic procedure without the method. It proved that the proposed method is helpful in the astrometry of ISS images in a fully automatic manner.

Contour detection in Cassini ISS images based on Hierarchical Extreme Learning Machine and Dense Conditional Random Field

In Cassini ISS (Imaging Science Subsystem) images, contour detection is often performed on disk-resolved objects to accurately locate their center. Thus, contour detection is a key problem. Traditional edge detection methods, such as Canny and Roberts, often extract the contour with too much interior details and noise. Although the deep convolutional neural network has been applied successfully in many image tasks, such as classification and object detection, it needs more time and computer resources. In this paper, a contour detection algorithm based on H-ELM (Hierarchical Extreme Learning Machine) and DenseCRF (Dense Conditional Random Field) is proposed for Cassini ISS images. The experimental results show that this algorithm’s performance is better than both traditional machine learning methods, such as Support Vector Machine, Extreme Learning Machine and even deep Convolutional Neural Network. The extracted contour is closer to the actual contour. Moreover, it can be trained and tested quickly on the general configuration of PC, and thus can be applied to contour detection for Cassini ISS images.

Interior properties of the inner saturnian moons from space astrometry data

During the thirteen years in orbit around Saturn before its final plunge, the Cassini spacecraft provided more than ten thousand astrometric measurements. Such large amounts of accurate data enable the search for extremely faint signals in the orbitalmotion of the saturnian moons. Among these, the detection of the dynamical feedback of the rotation of the inner moons of Saturn on their respective orbits becomes possible. Using all the currently available astrometric data associated with Atlas, Prometheus, Pandora, Janus and Epimetheus, we first provide a detailed analysis of the Cassini Imaging Science Subsystem (ISS) data, with special emphasis on their statistical behavior and sources of bias. Then, we give updated estimates of the moons' averaged densities and try to infer more details about their interior properties by estimating the physical librations for Prometheus, Pandora, Epimetheus and Janus from anomalies in their apsidal precession. Our results are compatible with a homogeneous interior for Janus and Epimetheus, within the uncertainty of the measurements. On the other hand, we found some inconsistency for Pandora and Prometheus, which might result from a dynamical mismodeling of Saturn's gravity field. Last, we show how the synergistic introduction of libration measurements directly derived from imaging should allow the moons' moments of inertia to be better constrained.

Cassini CIRS and ISS opposition effects of Saturn’s rings – I. C ring narrow or broad surge?

Abstract We focus on the thermal and optical opposition effects of Saturn’s C ring seen by Cassini CIRS (Composite InfraRed Spectrometer) at 15.7 ${\mu}$m and ISS (Imaging Science Subsystem) at 0.6 ${\mu}$m. The opposition surge is a brightness peak observed at low phase angle (α → 0°). Saturn rings’ opposition surge was recently observed in reflected light and thermal infrared emission by Cassini. There is debate on whether the C ring’s thermal opposition surge width is narrow (≲1°) or broad (≳30°). This surge is important because its width was used to define the scale of ring properties driving the thermal peak. We parametrize the CIRS and ISS phase curves with several morphological models to fit the surge shape. For five of the largest C ring’s plateaus, we find that their thermal surge is 10 times wider than the optical surge and that the thermal surge width (∼4°) is neither narrow, nor broad. We compare radial differences between CIRS and ISS surge morphologies with the optical depth τ (from UVIS, UltraViolet Imaging Spectrograph) and water ice band depth (from VIMS, Visual and Infrared Mapping Spectrometer) profiles. We find that: water ice band depths (microscopic ring signatures) and τ (macroscopic ring signatures) show respectively little and large contrasts between the background and the plateaus. The thermal surge amplitude and τ are correlated, and we found no band depth dependence, contrary to the optical surge amplitude, which shows no correlation with τ. These correlations suggest a macroscopic scale dominance in controlling the C ring’s thermal opposition effect.

Potential Vorticity of Saturn's Polar Regions: Seasonality and Instabilities

AbstractWe analyze the potential vorticity of Saturn's polar regions, as it is a fundamental dynamical tracer that enables us to improve our understanding of the dynamics of these regions and their seasonal variability. In particular, we present zonally averaged quasi‐geostrophic potential vorticity maps between 68° planetographic latitude and the poles at altitudes between 500 and 1 mbar for three different epochs: (i) June 2013 (early northern summer) for the north polar region, (ii) December 2008 (late northern winter) for both polar regions, and (iii) October 2006 (southern summer) for the south, computed using temperature profiles retrieved from Cassini Composite Infrared Spectrometer data and wind profiles obtained from Cassini's Imaging Science Subsystem. The results show that quasi‐geostrophic potential vorticity maps are very similar at all the studied epochs, showing positive vorticities at the north and negative at the south, indicative of the dominance of the Coriolis parameter 2Ωsinϕ at all latitudes, except near the pole. The meridional gradients of the quasi‐geostrophic potential vorticity show that dynamical instabilities, mainly due to the barotropic term, could develop at the flanks of the Hexagon at 78°N, the jet at 73.9°S, and on the equatorward flank of both polar jets. There are no differences in potential vorticity gradients between the two hemispheres that could explain why a hexagon forms in the north and not in the south. No seasonal variability of the potential vorticity and its meridional gradient has been found, despite significant changes in the atmospheric temperatures over time.

Astrometric reduction of Cassini ISS images of Helene

Helene is a small irregular Saturnian satellite. It is difficult to observe Helene by ground-based telescopes because of its weak lightness (18.5 mag) and proximity to Saturns rings. The number of historical observations of the satellite is small, but a large number of images of Helene were obtained between 2004 and 2017 using the Imaging Science Subsystem (ISS) in Cassini Spacecraft. In this paper, astrometry reduction was performed based on these observations. Specifically, we first corrected the initial pointing of the Narrow Angle Camera in ISS using the referencing Gaia DR2, and then measured the Helenes center position in image coordinates using the two-dimensional Gaussian fitting method. Finally the position was transformed to the Right Ascension ($\alpha$) and Declination ($\delta$) in International Celestial Reference Frame (ICRF) centered at Cassini. The result was compared with JPL SAT393 ephemerides, and the position residual was calculated. Totally 890 images were effectively measured. The results showed that compared with SAT393, the mean values of all residuals in $\alpha$$*$cos$(\delta)$ and $\delta$ directions were $0.23$ and $-0.63$ km, respectively, and the standard deviations were $3.48$ and $2.23$ km, respectively. This precision was much higher than that of ground observations, indicating the advantage of space-based observations.

A New Enceladus Global Control Network, Image Mosaic, and Updated Pointing Kernels From Cassini's 13‐Year Mission

AbstractNASA's Cassini spacecraft spent 13 years exploring the Saturn system, including 23 targeted flybys of the small, geologically active moon Enceladus. These flybys provided a wealth of image data from Cassini's Imaging Science Subsystem. To improve the usability of the Enceladus data set, we created a new, global photogrammetric control network for Enceladus that enabled compilation of a versatile cartographic package to support geologic mapping and other investigations. The network used 586 images in four image filters with a pixel scale generally between 50 and 500 m per pixel and a phase angle less than 120° and consisted of 10,362 tie points and 173,704 individual image measures, averaging nearly 17 measures per tie point. Least squares bundle adjustment resulted in a root‐mean‐square residual of 0.45 pixel, corresponding to root‐mean‐square ground point uncertainties of 66, 51, and 46 m in latitude, longitude, and radius, respectively. Using our geodetic control network, we created new global image mosaics, coregistered flyby mosaics to support geologic mapping, and updated pointing kernels for every image used in the solution. These products, including the updated pointing kernels, are available to the community through NASA's Planetary Data System Imaging Annex. The bundle adjustment solution also yielded independently determined shape information, resulting in radii within the stated uncertainty of International Astronomical Union values. The challenges of the data set, and the technical methodology described here are applicable to bodies imaged during multiple flybys with variable viewing and illumination geometry, including other midsized satellites of Saturn, and the Europa Clipper mission.

The Cassini VIMS archive of Titan: From browse products to global infrared color maps

We have analyzed the complete Visual and Infrared Mapping Spectrometer (VIMS) data archive of Titan. Our objective is to build global surface cartographic products, by combining all the data gathered during the 127 targeted flybys of Titan into synthetic global maps interpolated on a grid at 32 pixels per degree (∼1.4 km/pixel at the equator), in seven infrared spectral atmospheric windows. Multispectral summary images have been computed for each single VIMS cube in order to rapidly identify their scientific content and assess their quality. These summary images are made available to the community on a public website (vims.univ-nantes.fr). The global mapping work faced several challenges due to the strong absorbing and scattering effects of the atmosphere coupled to the changing observing conditions linked to the orbital tour of the Cassini mission. We determined a surface photometric function which accounts for variations in incidence, emergence and phase angles, and which is able to mitigate brightness variations linked to the viewing geometry of the flybys. The atmospheric contribution has been reduced using the subtraction of the methane absorption band wings, considered as proxies for atmospheric haze scattering. We present a new global three color composite map of band ratios (red: 1.59/1.27 µm; green: 2.03/1.27 µm; blue: 1.27/1.08 µm), which has also been empirically corrected from an airmass (the solar photon path length through the atmosphere) dependence. This map provides a detailed global color view of Titan's surface partially corrected from the atmosphere and gives a global insight of the spectral variability, with the equatorial dunes fields appearing in brownish tones, and several occurrences of bluish tones localized in areas such as Sinlap, Menvra and Selk craters. This kind of spectral map can serve as a basis for further regional studies and comparisons with radiative transfer outputs, such as surface albedos, and other additional data sets acquired by the Cassini Radar (RADAR) and Imaging Science Subsystem (ISS) instruments.