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Introduction &amp... more Introduction Europa's surface is the youngest of the icy satellites and shows signs of recent activity [1]. The main driver of this activity is Jupiter&#8217;s tidal forces which are responsible for the existence of a global water ocean underneath Europa&#8217;s icy crust [2]. This ocean is thought to be in a direct contact with the rocky mantle [3] creating a potential environment for the emergence of life. This places Europa at the center of future space exploration with NASA&#8217;s Europa Clipper mission [4] as well as ESA&#8217;s JUpiter ICy moons Explorer (JUICE) that will more generally study the Jovian icy moons [5]. To prepare for those missions, it is crucial to get the most out of the data we currently have. Photometry plays a key role in deriving remote sensing science products. As such, photometric correction is often the first step of any remote sensing analysis such as mapping or spectroscopy. In itself, photometry is closely linked to the surface microtexture and can help us better understand its physical state [e.g. 6, 7].&#160; In this work, we are deriving photometric parameters for selected areas of Europa - we analyze their spectral dependency and how that can be translated into physical properties. &#160; Dataset We are revisiting data from Galileo&#8217;s Near Infrared Mapping Spectrometer (NIMS) [8] that operated between 0.7 &#8211; 5.2 m. Calibrated and georeferenced data cubes (g-cubes) archived in the PDS Imaging Node have been stored in a database [9] with relevant information such as longitude, latitude, wavelengths, reflectance value, photometric angles etc. This tool allows for a comprehensive view of the dataset and quick exploration to assess the surface and angle coverage available for our study. Due to changes in the wavelengths calibration and detector failures during operations, the phase angle coverage is very variable across the spectrum (from one cube to the other) with ranges from a few degrees to 80 degrees. We favor spectral intervals with a dense phase coverage. Moreover, to accommodate the shift in the wavelengths reference over time as well as to maximize our geometric diversity, we will not be looking at specific wavelengths but at 40 nm wide ranges of wavelengths across the spectrum. We retrieved the reflectance and geometry information for a selection of regions of interest across the surface (Figure 1) by finding a compromise between interesting regions and relevant phase angle and spectral coverage. <img src="" alt="" width="490" height="292" /> Figure 1: Map of Europa (credits: Bj&#246;rn J&#243;nsson) with studied areas highlighted &#160; Estimation of photometric parameters For each area we are considering, we estimated a set of photometric parameters for every 40 nm wavelength interval we have defined. For this work, we are considering Hapke&#8217;s direct model detailed in [10] and [11]. Six parameters are to be estimated: phase function (b, c), single-scattering albedo (&#969;), rugosity (&#952; &#773;), opposition effect (h and B_0). We used an estimation method based on Bayesian statistics and that we developed for our regional studies of Jupiter&#8217;s icy moons in the visible [12, 13, 14]. This allows for of the parameters was inferred except for their physical domain of variation. The posterior Probability Density Functions (PDFs) are sampled with a Monte Carlo Markov Chain algorithm. Results We analyzed the different parameters over our three areas of study. Results were very heterogeneous in terms of accuracy across the spectrum but it is clear that the photometric parameters do vary somewhat with the wavelength, as also observed in laboratory experiment [15]. Figure 2 shows the example of the single-scattering albedo and macroscopic roughness estimations between 700 nm and 5 &#956;m for ROI#1 in the leading hemisphere and a sample of wavelengths for which we have well constrained results. Even though with na&#239;ve theoretical considerations we would expect a constant roughness - independent of wavelength - we found here that &#952; &#773; seems to become more important with higher wavelengths and lower reflectance. This could be related to specific absorption patterns or the presence of certain inclusions that would influence the photometric…

<strong&amp... more Introduction: Spectroscopic data is rich and powerful to study surfaces. Besides, planetary surfaces are often made of intimately mixed components [1, 2], so modelling a reflectance spectrum will mean fitting a relatively large number of inter-related parameters: at least, one proportion and one grain size for each component in the case of a granular mixture, plus structure parameters (roughness, porosity). In these large parameters spaces, it is highly possible that multiple solutions give an equally satisfactory fit [3]. It is crucial to evaluate the ability of a method to retrieve multiple solutions when setting up an inversion strategy. &#160; Data: This work was done in the context of revisiting the Europa Galileo NIMS dataset. We compared the solutions of different methods on a radiative transfer model, based on Hapke modeling [1] and an observation of a bright region of Europa NIMS cube 14e006ci [4] with 4 components : crystalline ice, hexahydrite, magnetite and sulfuric acid octahydrate. The uncertainty on the data is assumed to be gaussian with a standard deviation at 10% with a minimum at 0.01 in reflectance. Method: The parameters space is on dimension 9, with 8 independent parameters: 4 abundances, 4 grain sizes and the surface roughness. We noted early in our work that while reproducing the data equally well, two minimisations algorithm would give different results for the parameters, meaning the solution is not unique. To explore the set of possible solutions, four methods were compared: (i) home-made Markov Chain Monte-Carlo (MCMC) method with metropolis hasting sampler [5] (ii) home-made MCMC method with improved metropolis-hasting sampler [this work] (iii) open-source multi-chain MCMC algorithm with "snooker" sampler [6] (iv) multiple minimizations, using &#8221;L-BFGS-B&#8221; bound-constrained algorithm with random initialisations [7]. To be able to compare the efficiency of the different methods, we limited the number of direct model evaluations to 1.5.106. This number was chosen for 2 reasons: (i) each algorithm tested seemed to have reached "convergence": increasing the number of iterations did not change significantly the result, and (ii) identical and affordable computational cost between the methods. This means that for MCMC methods, we set the number of iterations to 1.5.106. For the multiple minimizations method, each minimization resulted in approximately 1500 model evaluations to reach the result. So we performed 1000 minimisation with 1000 different random initialisations. For each of the model evaluation, a likelihood is computed, making the comparison between this method and the bayesian ones possible. <img src="" alt="" /> Figure 1: Corner plot and best fit for home-made MCMC method with Metropolis-Hasting sampler with a converged chain of 1.5 106 iterations. The sampling is done with 3 cases: agnostic uniform distribution over the full prior space, in far neighbourhood, in close neighbourhood. The corner plot represents the pairwise posterior distributions, and the marginal posterior distributions for each parameter. Parameters are from left to right roughness, 4 abundances, 4 grain sizes and from top to bottom 4 abundances and 4 grain sizes. Acceptance rate is 0.217. &#160; <img src="" alt="" /> Figure 2: Same as fig 1 but in this case the sampling is done with 4 cases: agnostic uniform distribution over the full prior space, in far neighbourhood, in close neighbourhood, 1 single parameter modification.&#160; Acceptance rate is 0.288. <img src="" alt="" /> Figure 3: Same as Fig1 but&#160; for 1000…

&lt;p&gt;&lt;strong&gt;Introduction&amp;#160;&lt;... more &lt;p&gt;&lt;strong&gt;Introduction&amp;#160;&lt;/strong&gt;&lt;/p&gt; &lt;p&gt;The explosive volcanism of a planet is a key process for understanding its thermal and geochemical evolution and for assessing its volatile budget over time. The MESSENGER (MErcury&amp;#8217;s Surface, Space ENvironment GEochemistry and Ranging) spacecraft highlighted products of Mercury's explosive activity. Pit landforms are interpreted to be endogenic pyroclastic vents on the basis of their irregular and often elongated morphology and their lack of raised rim &lt;strong&gt;[1]&lt;/strong&gt;. These endogenic pits are often surrounded by high albedo deposits with diffuse borders, named faculae. They are interpreted as pyroclastic deposits formed by the fragmentation and ejection of magma particles from the central volcanic vent. Faculae exhibit redder spectral slope and stronger downturn of reflectance in the ultraviolet (UV) than their background terrains &lt;strong&gt;[2]&lt;/strong&gt;. Here, we present a detailed spectral analysis of 26 faculae using all the available data of the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) onboard MESSENGER. In order to constrain the eruptive style and the pyroclast composition, we investigated the spectral variability between the faculae.&amp;#160;&lt;/p&gt; &lt;p&gt;&lt;strong&gt;Material and method&lt;/strong&gt;&lt;/p&gt; &lt;p&gt;We used the MASCS Derived Data Record (DDR) data products available on the Planetary Data System (PDS). These data have been radiometrically and photometrically calibrated by the MESSENGER team. Additional processing, using the method developed by &lt;strong&gt;[3]&lt;/strong&gt;, is applied to the data to obtain a continuous spectrum from 300 to 1450 nm. The MASCS observations are filtered by instrument temperature and incidence angle. The data carried out in the highest temperature regime of the instrument (temperature exceeding 40&amp;#176;C) are discarded. Observations made at incidence angles greater than 75&amp;#176; have also been removed in order to limit variations due to photometry at high phase angles. With the aim to highlight spectral properties of faculae, we computed spectral parameters: the UV-downturn &lt;strong&gt;[2]&lt;/strong&gt;, the reflectance at 750 nm (R750) and the slope in the visible (VIS-slope) &lt;strong&gt;[4]&lt;/strong&gt;. The parameters UV-downturn and VIS-slope are normalized by the Mercury&amp;#8217;s reference spectrum &lt;strong&gt;[5]&lt;/strong&gt;, thus the VIS-slope and UV-downturn are respectively equal to 1 and 3.0 (reevaluated at 3.1 by Besse et al., 2020) for the average surface of Mercury.&amp;#160;&lt;/p&gt; &lt;p&gt;The faculae with the highest number of observations, spatial coverage and spatial distribution of observations have been selected for this study &lt;strong&gt;[4]&lt;/strong&gt;. The size of 26 of them has been determined with MASCS data allowing optimal spectral analysis&lt;strong&gt; [4]&lt;/strong&gt;. As in &lt;strong&gt;[6]&lt;/strong&gt;, the spectral properties of the faculae are taken at the midpoint between the limit of the vent and edges of the facula. This allows to minimize the issue due to the intrinsic variability across the faculae (spectral parameters decrease from the limit of the vent to the edge of the facula)&lt;strong&gt; [3,6].&amp;#160;&lt;/strong&gt;&lt;/p&gt; &lt;p&gt;&lt;strong&gt;Results and discussion&lt;/strong&gt;&lt;/p&gt; &lt;p&gt;The 26 faculae are located over a latitudinal range between 60&amp;#176;S and 50&amp;#176;N and a longitudinal range between -165&amp;#176;E and 160&amp;#176;E (&lt;strong&gt;Fig. 1&lt;/strong&gt;). About three quarters of them are located within impact craters.&lt;/p&gt; &lt;p&gt;&lt;img src="data:image/png;base64,…

5th Planetary Data Workshop & Planetary Science Informatics & Analytics, Jun 1, 2021

ABSTRACT Titan, the largest satellite of Saturn, is the only satellite in the solar system with a... more ABSTRACT Titan, the largest satellite of Saturn, is the only satellite in the solar system with a dense atmosphere. The close and continuous observations of Titan by the Cassini spacecraft, in orbit around Saturn since July 2004, bring us evidences that Titan troposphere and low stratosphere experience an exotic, butcomplete meteorological cycle similar to the Earth hydrological cycle, with hydrocarbons evaporation, condensation in clouds, and rainfall. Cassini monitoring campaigns also demonstrate that Titan's cloud coverage and climate vary with latitude. Titan's tropics, with globally weak meteorological activity and widespread dune fields, seem to be slightly more arid than the poles, where extensive and numerous liquid reservoirs and sustained cloud activity were discovered. Only a few tropospheric clouds have been observed at Titan's tropics during the southern summer [2-4]. As equinox was approaching (in August 2009), they occurred more frequently and appeared to grow in strength and size [5-7].

Geophysical Research Letters, 2012

Advances in Space Research, 2013

Introduction &amp... more Introduction Titan is the only moon in the solar system with a thick atmosphere, dominated by nitrogen and organic compounds and methane- and ethane-based climatic cycles similar to the hydrological cycle on Earth. Hence, Titan is a prime target for planetary and astrobiological researches. Heaviest organic materials resulting from atmospheric chemistry (including high atomic number aerosols) precipitate onto the surface and are subject to geological processes (e.g., eolian and fluvial erosion) that lead to the formation of a variety of landscapes, including dune fields, river networks, mountains, labyrinth terrains, canyons, lakes and seas analogous to their terrestrial counterparts but in an exotic context. Its optically thick atmosphere, however, prevents the surface from being probed in the entirety of the near-infrared (NIR) range, and its composition is still largely unknown, or largely debated at the least, preventing to fully understand and quantify the geological processes at play. Incident and reflected solar radiations are indeed strongly affected by gaseous absorption and aerosol scattering in the NIR. Only where the methane absorption is the weakest, a few transmission windows allow the detection of radiation coming from the low atmosphere and the surface, making possible to retrieve the surface albedo. In the 0.88-5.11 &#956;m range (VIMS-IR channel), the Visual and Infrared Mapping Spectrometer (VIMS) instrument on board the Cassini spacecraft has shown that the surface can be observed in eight narrow transmission windows centered at 0.93, 1.08, 1.27, 1.59, 2.03, 2.69, and 2.78 &#956;m, and in the 5.0-5.11 &#956;m interval. Even in these transmission windows, residual gaseous absorption and increasing scattering from aerosols with decreasing wavelength make the analysis of the surface signal and the retrieving of surface albedo complex and delicate.&#160; In order to retrieve the surface albedo in the atmospheric windows in the most possible rigorous way, we have developed a radiative transfer (RT) model with up-to-date gaseous abundances profiles and absorption coefficients and improved photochemical aerosol optical properties. We validated our model using in situ observations of Huygens-DISR (Descent Imager / Spectral Radiometer) acquired during descent and once landed. We then applied our RT model to the Selk crater area (the Dragonfly mission landing area) in order to map the surface albedo and discuss the surface properties of the different geomorphological units of the region. Radiative transfer Our RT model is based on the SHDOM solver to solve the RT equations using the plan-parallel approximation. Vertical abundance profiles and absorption lines of CH4 and isotopes, CO, C2H2 and HCN are implemented using the most recent studies. Correlated-k coefficients are used to calculate gases absorption coefficients at VIMS-IR spectral sampling and resolution. Aerosols extinction profile and single scattering albedo are described using a fractal code developed by [1], allowing the aerosol fractal dimension to be varied. Aerosols phase function is modified using a multi-angular VIMS sequence (S&#233;bastien Rodriguez, personal communication). Our model is validated using the in situ observations of Huygens-DISR acquired during the complete descent sequence and once landed. Application We applied our RT model to the Selk crater region by inverting aerosol opacity and surface albedo over 4 VIMS cubes (1578266417_1, 1575509158_1, 1578263500_1, 1578263152_1) acquired over the area. We built local maps of aerosol opacities and surface albedos of the Selk region by combining the 4 VIMS cubes on a geographically projected mosaic (see the mosaic of the 4 raw VIMS observations in Fig. 1). A few longitudinal profiles of the retrieved atmospheric properties are shown in Fig. 2. Slopes and seams between cubes of the aerosol opacities, originally due to varying observation geometries between flybys, have been entirely corrected, confirming the robustness of our RT model and making the retrieved surface albedo more reliable. Retrieved surface albedo have been then corrected for the photometry using in-situ observations ([3]). The resulting albedo maps of the regions are highly contrasted and homogeneous, most of the seams between cubes (due to residual surface photometry) being corrected (Fig. 3).&#160; &#160; <img src="" alt="Fig1 : I/F mosaics of 4 overlapping cubes in an atmospheric band (left) and an atmospheric window (right)." width="700" height="350" /> <img src="" alt="Observation angles (top), I/F (middle) and aerosol opacity factors…

We have analyzed the complete archive of the Visual and Infrared Mapping Spectrometer (VIMS) data... more We have analyzed the complete archive of the Visual and Infrared Mapping Spectrometer (VIMS) data in order to monitor and analyze the evolution of the clouds and haze coverage at both poles of Titan during the entire Cassini mission. Our objective is to give a cartographic synopsis from a VIMS perspective, to provide a global view of the seasonal evolution of Titan's atmosphere over the poles. We leave the detailed comparison with the Imaging Science Subsystem (ISS) and the Composite Infrared Spectrometer (CIRS) data sets to further studies. We have computed global hyperspectral mosaics for each of the 127 targeted flybys of Titan to produce synthetic color maps emphasizing the main atmospheric features. The north pole appears fully covered by a huge cloud as soon as the first observations in 2004 and up to the equinox in 2009 (Le Mouélic et al. 2012). The northern skies then became progressively clearer, after the circulation turnover in 2009, revealing the underlying lakes and...

Titan's polar surface is dotted with hundreds of lacustrine depressions. Based on the hypothe... more Titan's polar surface is dotted with hundreds of lacustrine depressions. Based on the hypothesis that they are karstic in origin, we aim at determining the efficiency of surface dissolution as a landshaping process on Titan, in a comparative planetology perspective with the Earth as reference. Our approach is based on the calculation of solutional denudation rates and allow inference of formation timescales for topographic depressions developed by chemical erosion on both planetary bodies. The model depends on the solubility of solids in liquids, the density of solids and liquids, and the average annual net rainfall rates. We compute and compare the denudation rates of pure solid organics in liquid hydrocarbons and of minerals in liquid water over Titan and Earth timescales. We then investigate the denudation rates of a superficial organic layer in liquid methane over one Titan year. At this timescale, such a layer on Titan would behave like salts or carbonates on Earth dependin...

5th Planetary Data Workshop & Planetary Science Informatics & Analytics, Jun 1, 2021

Geophysical Research Letters, 2012

Advances in Space Research, 2013

Global mosaics of Titan have been produced using VIMS hyperspectral images. The properties at 5 µ... more Global mosaics of Titan have been produced using VIMS hyperspectral images. The properties at 5 µm are studied using a systematic comparison with the corresponding mosaic of viewing angles.

The Visual and Infrared Mapping Spectrometer (VIMS) onboard Cassini can see the surface of Titan ... more The Visual and Infrared Mapping Spectrometer (VIMS) onboard Cassini can see the surface of Titan in seven narrow atmospheric windows in the infrared at 0.93, 1.08, 1.27, 1.59, 2.01, 2.68-2.78, and 4.9-5.1 microns. In addition to the strong absorption by atmospheric gases (mainly methane and nitrogen), the presence of aerosols in the atmosphere blurs the images at short wavelengths due to a very strong scattering effect, which acts as an additive component to the signal coming from the surface. We have produced a global hyperspectral mosaic of the complete VIMS archive between T0 (July 2004) and T66 flyby (January 2010), by merging all the data cubes sorted by increasing spatial resolution in order to put the high resolution images on top and to use the low resolution images as background. We filtered out the observing geometry in order to remove the pixels acquired in too extreme illuminating and viewing conditions, which systematically amplify the atmospheric artifacts. We used thresholds of 80° both on the incidence and emission angles, and 100° on the phase angle. We first focused our study on the 5 microns window, where the additive component from the haze scattering is negligible, in order to investigate the multiplicative factor which should be used to normalize the viewing geometry between all flybys, and thus to reconcile observations acquired in very different viewing conditions. Indeed, if we exclude transient phenomena (mainly clouds and possible surface changes), the seams between all individual images should disappear after correction. We then investigated other windows at shorter wavelengths, where the additive scattering component cannot be neglected. We found that the wings of the atmospheric windows can be used as a proxy for the amount of additive scattering present in the center of these windows, where the surface is best seen by VIMS. In the band wings, the solar flux never reaches the ground. The corresponding signal therefore gives the contribution of the upper layers of the atmosphere, which is also present in the center of the atmospheric window (which probes the full atmospheric path up to the surface). The ultimate goal would be to produce homogeneous mosaics in each surface window corrected from the additive scattering component, in order to use the band ratio technique. Band ratios emphasize very subtle compositional heterogeneities provided that no additive component is present in the numerator and denominator. Examples will be given at the meeting.

Radar images from the Cassini spacecraft reveal closed, smooth and flat depressions above norther... more Radar images from the Cassini spacecraft reveal closed, smooth and flat depressions above northern and southern latitudes of 60° on Titan, Saturn’s largest moon. These depressions have been interpreted as lakes of liquid hydro-carbons and dissolved nitrogen, resting on the icy crust that covers this moon. The depressions include large (over 100,000 square kilometers) seas with dendritic or poorly defined contours, small (1-10 km wide) circular steep-sided depressions, and medium-sized (20-50 km wide) depressions, the contours of which are composed of adjacent circular segments. Some depressions are completely filled with radar-dark material, while others are partially filled and some are empty. Most of these depressions lie in flat plains. By comparison with a terrestrial analogue located in the Etosha Basin (Namibia), we introduce here a dissolution-precipitation model for the formation of these lakes at the expense of a superficial soluble layer. The Etosha Basin is a flat sedimentary basin located at the western border of the Kalahari desert. The climate is semi-arid, with an average annual precipitation rate of 400 mm/yr and an average annual potential evaporation rate of 2200 mm/yr. Sediments in the basin include clays and silts; they are covered by a layer of soluble calcrete a few meters in thickness. The calcrete has formed by precipitation, in the subsurface, of calcium carbonate dissolved in groundwater. Precipitation of calcium carbonate from groundwater is due to the average annual dominance of groundwater evaporation over precipitation. The calcrete layer is dotted with dozens of so-called pans: these are closed, steep-sided, flat and smooth depressions, 1 to 200 km wide and a few meters deep. Relict boulders of calcrete rest on the silty, clayey and evaporitic floors of the pans and provide evidence that the pans grow by radial regressive dissolution of the calcrete layer. By comparison with the development of pans at the expense of the calcrete layer of Etosha, we infer that the small and medium-sized lakes of Titan grow by regressive radial dissolution, during flooding episodes associated with rainstorms, of a superficial soluble layer. The formation of this layer can be explained, as for the superficial calcrete layer of Namibia, by precipitation at or near the topographic surface of non-volatile materials, during evapo-ration after rainstorms of liquids accumulated in the ground.

In June 2004 and July 2005, the ISS multispectral camera onboard the Cassini spacecraft imaged a ... more In June 2004 and July 2005, the ISS multispectral camera onboard the Cassini spacecraft imaged a 235 km-long and 75 km-wide dark feature near the south pole of Titan (McEwen et al., 2005). By comparison with other landforms observed near Titan’s north pole with the Radar instrument (Stofan et al., 2007), this feature has been interpreted as an hydrocarbon lake and named Ontario Lacus. Other observations of the lake, by the VIMS hyperspectral camera in December 2007 and the Radar altimeter in December 2008 are consistent with a liquid filled lake (Brown et al., 2008, Barnes et al., 2009), which lies in an extremely flat depression (Lorenz et al., 2009). In March 2009, VIMS acquired new hyperspectral cubes with a spatial resolution similar to the first ones. Finally, the new Radar observations in SAR mode in June and July 2009, 3 months after the VIMS observation, provided the first spatially resolved images of the lake. By merging all these data sets, we performed an integrated geomorphological and compositional study of Ontario Lacus and its surroundings. Comparisons with optical and radar satellite images of analogous landforms in the Etosha Basin, a semi-arid region of Namibia, allowed us to produce an interpretative geological map of Ontario Lacus in 2009. We also checked for potential surface changes of the lake between 2005 and 2009, i.e. during the austral summer and autumn. To achieve this work, we developed a new empirical processing method to remove atmospheric effects in VIMS images and to improve the surface mapping. This correction pipeline is also applied to ISS images. Our interpretative geological map shows that the lake is surrounded mostly by flat plains, except in the North where mountains are present (rough areas with dendritic valleys and triangular facets in the SAR images). The typical radar-dark signature of liquids is present over half the surface area of the lake only. Channels draining the plains SW of Ontario Lacus can be followed on the lake floor on the Radar images. This suggests that the lake floor, most probably composed of (perhaps soggy) sediment, is not covered by significant amounts of liquids over its whole surface. A set of lines curving along the eastern shoreline of the lake can be interpreted, by analogy with similar landforms observed in Namibia and other semi-arid areas on Earth, as “lunette-dunes”, which form by accumulation at downwind lake shorelines, of fine sediments provided by wind deflation of exposed and desiccated lake floors. This unit can be reconciled with 5µm-bright areas in the VIMS images. Alternatively, this set of lines may be interpreted as a series of ancient shorelines, which would indicate past episodes of lake high-stands. If this interpretation is correct, it means that Ontario Lacus has been subject to drying episodes in the past. Finally, at the spatial resolution of ISS and VIMS, we observe no significant change of the lake contour between 2005 and 2009 in the common part of the lake.

The study of a terrestrial analog, the Etosha Pan (Namibia), for Ontario Lacus on Titan seems to ... more The study of a terrestrial analog, the Etosha Pan (Namibia), for Ontario Lacus on Titan seems to indicate that Ontario might be a partially liquid-filled basin.

We are investigating an empirical method to correct the photometric and atmospheric effects in VI... more We are investigating an empirical method to correct the photometric and atmospheric effects in VIMS images of Titan's surface. This method is applied to T38 and T51 Ontario Lacus observations to determine whether surface changes occured between these two flybys.