Oral presentations and abstracts
Session assets
Icy bodies are the most numerous and diverse bodies in the Solar System, but only a few have been visited or will be visited by a space probe. The Galilean satellites are one of those, especially Ganymede which is the primary target of the future L-class mission JUICE (launch scheduled in 2022) in ESA’s Cosmic Vision programme. Ganymede's surface visually exhibits an important geological diversity, with young bright areas to older dark terrains. This diversity also expresses itself through the moon’s surface composition, which was studied extensively in situ by the NIMS instrument of the Galileo mission (NASA) in the late 90s; like the majority of giant planet satellites, Ganymede's surface is dominated by H2O-ice and some non-icy components, very likely hydrated salts based on the distorted shape of the spectral signatures (McCord et al., 2001). However, this binary composition has been obtained with a spectral sampling (~25 µm) not allowing to detect specific absorptions for the non-icy materials, thus not allowing their identification. Hence, many questions about Ganymede’s surface composition remain unanswered while important technical advances have been made since.
In preparation of the JUICE mission, and specifically of the near-infrared imaging spectrometer MAJIS of the JUICE mission, a ground-based campaign was performed using an instrument with a much finer, i.e. better, spectral sampling: SINFONI (SINgle Faint Object Near-IR Investigation). SINFONI is installed on the UT4 of the Very Large Telescope (VLT hereafter) at the European Southern Observatory (ESO hereafter) in Chile. It combines one adaptive optics module and an integral field spectrometer operating in the near-infrared covering from the beginning of the J-band (~1.1 µm) to the end of the K-band (~2.45 µm). Here we present the results derived from the analysis and the modeling of four observations acquired at different dates, from October 2012 to March 2015, all covering the 1.45 – 2.45 µm wavelength range with a spectral resolution about 0.5 µm and a spatial sampling of 12.5 x 12.5 mas2. These results were recently published in Icarus (Ligier et al., 2019).
The first result we obtained concerns the physical properties of Ganymede’s surface. Indeed, the data reduction process highlights that the Lambertian model is not sufficient to remove the photometric effects due to observations geometry. Instead, the Oren-Nayar model (Oren & Nayar, 1994), which generalizes the Lambertian law for rough surfaces, produces excellent results where no inclination residuals are observed up to inclination angles around 65°. The quality of the photometric correction is thus used as proxy to infer Ganymede’s surface roughness: from 16° ± 6° to 21° ± 6° depending on the observations.
Then, concerning the surface composition of the moon, our modeling confirms that it is dominated by H2O-ice, predominantly the crystalline form. The abundance maps of the ice show two main patterns: (1) a latitudinal gradient in terms of abundance, with large polar caps, and (2) a latitudinal gradient in terms of grain size, where the smaller grains (>50 µm) are located at the highest latitudes, showing a sharp transition around ±35°N coinciding with the transition between open and closed field lines of Ganymede’s own magnetic field (figure 1a). Ice sublimation explains this distribution, redistributing H2O-ice from the “hot” equator to the colder poles, very likely redeposited as finer grains.
Apart from the ice, another major compound is required to fit Ganymede’s spectra: a darkening agent. Similarly to previous studies, this darkening agent could not be identified, but we were able to provide new constraints on it. First of all, this unknown material cannot be organic matter since its reflectance level, about 0.25, is more or less five times higher than that of organic matter. Instead, the reflectance level suggests a silicate-type material, as already mentioned in a previous study about Callisto (Calvin & Clark, 1991). Its abundance map shows the highest abundances (up to 0.85) at equatorial latitudes in the trailing hemisphere (figure 1b). The well-known surface sputtering engendered by Jupiter’s magnetosphere is the simplest process to explain such distribution. Even with the magnetic field of the moon, it is possible for corotating singly-charged ions to become neutralized near the moon and continue to the surface as neutrals, sputtering mostly around the trailing apex.
Last but not least, our study highlights the necessity of secondary species, i.e. >10% overall, to better fit the measurements: sulfuric acid hydrate and salts, likely sulfates and chlorinated. While the sulfuric acid hydrate is, like H2O-ice, mostly located at high latitudes (figure 1c), the abundance map of the salts shows a heterogeneous distribution which seems neither related to the Jovian magnetospheric bombardment nor craters (figure 1d). These species are mostly detected on bright grooved terrains surrounding darker areas. Endogenous processes, such as freezing of upwelling fluids going through the moon’s ice shell, may explain this heterogeneous distribution.
How to cite: Ligier, N., Paranicas, C., Carter, J., Poulet, F., Calvin, W., Nordheim, T. A., and Snodgrass, C.: Pending the JUICE/ESA mission … surface composition and properties of Ganymede from near-infrared ground-based measurements, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-655, https://doi.org/10.5194/epsc2020-655, 2020.
Abstract
While the thickness of Europa’s ice shell is underconstrained by the current knowledge of the body, it is not unconstrained. That is, while there are many possible ice shell thicknesses, not all are equally plausible. In this study, I survey the current knowledge of Europa and quantify distributions in the parameters and processes that control ice shell thickness. I then create a Monte Carlo simulation of 107 plausible Europa’s with ice shells in thermodynamic equilibrium, and condition the result into a probability distribution for ice shell thickness. I predict a best estimate thickness of 22.0 km, but with great uncertainty. These results may inform future planning and inferences from NASA’s Europa Clipper mission and ESA’s JUICE mission later this decade [1].
Introduction
Within its predominantly water-ice shell, Jupiter’s moon Europa likely harbors a global saltwater ocean, placing it among an emerging class of celestial objects known as Ocean Worlds. The ice shell is thought to consist of a brittle upper lithosphere, where heat transfer occurs by thermal conduction, and a ductile interior asthenosphere that may be convecting in solid-state. The icy surface of this shell exhibits one of the youngest average ages in the solar system (~40 – 90 Myr) [2], requiring the recent or current geologic resurfacing.
The geologic processes within the ice shell responsible for the young surface may convey material between the surface and interior ocean, critically influencing chemical disequilibria within the watery interior and the habitability of Europa’s ocean [3], [4]. However, the thickness of the ice shell and therefore the potential for geologic material exchange is unknown, and estimates span three order of magnitude.
Following discoveries at Europa by the Galileo mission, a summary of ice shell thickness predictions was made by Billings and Kattenhorn [5], providing a name to the “Great Thickness Debate.” That study catalogued dozens of previous publications, collating estimates for icy thicknesses from a few hundred meters to many tens of kilometers.
In order to estimate a probability distribution for Europa’s ice shell thickness using the Monte Carlo method, several parameters must be estimated. Therefore, I first identify current best estimate (CBE) values and construct distributions that capture the uncertainty in those values for several key parameters. In each instance, I determine both the range of values for the parameter of interest and the form of the probability distribution for that parameter.
Method
Assuming an ice shell in thermodynamic equilibrium, the heat flux out of the icy lithosphere radiated to space is equal to the heat flux into the base of the lithosphere from the silicate interior plus the internal heat generated by tidal dissipation within a ductile convecting asthenosphere, integrated over the depth of the asthenosphere.
For each of these terms, I identify the underlaying uncertainties associated with their calculation. For Europa as a whole, I include uncertainties in total H2O layer and iron core thickness. In the silicate interior, I consider uncertainties in radiogenic heating associated with the material Europa accreted from, as well as the potential for silicate tidal dissipation based on mechanical constraints. For the ice shell, I consider uncertainties in convective and melting temperatures, grain size, empirical diffusion creep constants, non-ice composition, porosity, mechanical properties, and tidal response.
The thickness of the conductive and convective layer are then sampled according to the governing equations for heat transfer and tidal heat dissipation (Figure 1).
Results and Discussion
The CBE ice shell thickness for Europa is 22.0 km (Figure 2), though the spread in potential thicknesses is great. Only ~1% of possible configurations are thinner than 15 km, and the median thickness is ~40 km. Further, ice shells are primarily dominated by heat transfer through thermal conduction (Figure 3), with thermal convection being relegated most commonly to the bottom 1/3rd or less of the ice shell.
I will address several surprising results in the underlaying distributions, and their affect on the overall solution. Additionally, there are many key sensitivities built in to this model that may be retired through future laboratory analysis and spacecraft observation. In parallel, some unresolvable issues will persist until the ice shell is explored in situ.
Figures
Figure 1. Differential and cumulative probability distributions for Europa’s ice shell thickness. The steep rise is attributed to the inverse relationship between minimum ice shell thickness and total heat flux, and the tail to the right is controlled by silicate heat flux and H2O thickness.
Figure 2. Probability heat map showing conductive and convective thicknesses. Warmer colors denote higher probability. White dashed lines show lines of constant ice shell thickness. Note, for example, that the red line showing a constant thickness of 15 km falls outside the majority of solutions, while the line showing 20 km thickness passes through regions of high probability.
Acknowledgements
This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
References
[1] S. M. Howell and R. T. Pappalardo, “NASA’s Europa Clipper—a mission to a potentially habitable ocean world,” Nat. Commun., vol. 11, no. 1, Art. no. 1, Mar. 2020, doi: 10.1038/s41467-020-15160-9.
[2] E. B. Bierhaus, K. Zahnle, C. R. Chapman, R. T. Pappalardo, W. R. McKinnon, and K. K. Khurana, “Europa’s crater distributions and surface ages,” Europa, pp. 161–180, 2009.
[3] K. P. Hand, C. F. Chyba, J. C. Priscu, R. W. Carlson, and K. H. Nealson, “Astrobiology and the Potential for Life on Europa,” in Europa, R. T. Pappalardo, W. B. McKinnon, and K. Khurana, Eds. Tucson: University of Arizona Press, 2009, p. 589.
[4] S. M. Howell and R. T. Pappalardo, “Can Earth-like plate tectonics occur in ocean world ice shells?,” Icarus, 2019, doi: 10.1016/j.icarus.2019.01.011.
[5] S. E. Billings and S. A. Kattenhorn, “The great thickness debate: Ice shell thickness models for Europa and comparisons with estimates based on flexure at ridges,” Icarus, vol. 177, no. 2, pp. 397–412, Oct. 2005, doi: 10.1016/j.icarus.2005.03.013.
How to cite: Howell, S.: The Likely Thickness of Europa’s Icy Shell, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-173, https://doi.org/10.5194/epsc2020-173, 2020.
Abstract
It has long been postulated that Europa might have a sub-surface ocean covered by an icy crust. First clues for the existence of such a sub-surface ocean were obtained by the Galileo magnetometer in the late '90s [1]. If such an ocean indeed exists, it might sporadically erupt in plumes. Indeed, in 2014, [2] reported on increased Lyman-α and oxygen OI130.4 nm emissions, which the authors interpreted as transient water vapor measurements resembling water plumes. In this presentation we analyze different plume models to determine which model would result in an observation as the one presented by [2], and analyze what implications this might have for the upcoming measurements of the Neutral and Ion Mass spectrometer (NIM) onboard JUICE.
1. Plume observations
[2] characterized the source of the increased Lyman-α and oxygen OI130.4 nm emissions through modeling. According to their results, the emissions are best described by two individual water vapor plumes that are located at 90°W/55°S and 90°W/75°S, respectively, both of which exhibit a radial expansion of ~200 km and a latitudinal expansion of ~270 km, and the surface densities of which amount to 1.3x1015 m-3 and 2.2x1015 m-3, respectively.
2. Plume model
The source of the observed plume might either be a liquid or a solid (icy) reservoir that evaporates or sublimates as it comes into contact with space. Figure 1 shows different scenarios which could lead to the localized release of H2O particles resulting in a plume-like structure. In the first scenario, a surface, or near-surface, liquid reservoir is exposed to near-vacuum conditions upon which water directly evaporates into space. In the second scenario, a crack in the ice shell all the way to the bottom leads to the exposure of oceanic water to space, resulting in the formation of an oceanic plume. If the flow is chocked (by the conduit's geometry), the plume may become a supersonic jet. In the third scenario a rising diapir results in the warming of local surface ice (indicated by the shaded region in Figure 1), which sublimates into space. The water temperature was set to 280 K in scenarios one and two, whereas the ice temperature was set to 250 K in scenario three, and the reservoir areas were set to ~1'000 m2 and ~20'000 m2, respectively.
Figure 1: The three analyzed scenarios: Scenario one shows evaporation of a surface, or near-surface, liquid, scenario two shows evaporation of oceanic water (in form of a jet), and scenario three shows the sublimation of surface ice heated by diapirs.
3. Monte-Carlo Model
To simulate Europa's plumes, we use a 3D Monte Carlo model originally developed to model Mercury's exosphere [3]. In this model particles are created ab initio, travel on collision-less trajectories, and are removed as they are either ionized, fragmented, or lost either to space or by freezing out on the surface. The grids are ~25x25x25 km3 in size, thus almost a factor 10 higher in resolution than the [2] measurements. For comparison with the [2] observations we also merged our model results into 200x200x200 km3 bins.
4. Model Results
Figure 2 shows our model results for the three different scenarios presented above. For each scenario, we present the [2] measurement on the left, the reduced resolution result in the middle, and the high resolution result on the right. All measurements are normalized to one and span six orders of magnitude.
Figure 2: Model results for the three analyzed scenarios. The top row shows the surface liquid scenario, the middle row shows the oceanic water jet scenario, and the bottom row shows the diapir scenario. The [2] measurement is shown on the left, the reduced resolution model results are shown in the middle, and the high resolution model results are shown on the right.
5. Discussion
Whereas the observed ~200 km scale height is met by all three modeled scenarios, only scenario number two, the oceanic water jet scenario, results in a narrow enough plume structure. Both scenarios number one and three are too broad to be in good agreement with the [2] observations. It thus seems that for the high radial scale height but the low latitudinal expansion a narrowing factor needs to be present, as for example a conduit-like geometry provides. The presence of a nozzle does not necessarily require that the liquid stems from the ocean, though. If a throat exists close to Europa's surface, it is also possible that a water inclusion close to the surface results in a jet-like geometry.
6. NIM Plume Observations
NIM is a highly sensitive neutral gas and ion mass spectrometer designed to measure the exospheres of the Europa, Ganymede, and Callisto. The detection limit is at 10-16 mbar for a 5 second integration time, which translates to a particle density of ~1 cm-3. NIM's mass resolution is M/ΔM > 1100 in the mass range 1-1000 amu. In addition to the modeled 3D plume density profiles, we will also present modeled mass spectra for the individual scenarios, and discuss their implications for positive plume identification possibilities.
7. Conclusion
The origin of the observed Europa water vapor plumes is of high scientific interest because the plume's chemical composition directly represents the reservoir's chemical composition. If the plume thus indeed originates in the ocean expected to lie underneath Europa's surface ice layer, analysis of its chemical omposition with a mass spectrometer would offer us direct information about the chemical composition of the water ocean itself. Such information would in turn teach us more about Europa's habitability, and about the possibility of Europa harboring life.
[1] Khurana, K. K., et al.: Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto, Nature, Vol. 395, pp. 777-780, 1998.
[2] Roth, L., et al.: Transient water vapor at Europa's south pole, Science, Vol. 343, pp. 171-174, 2014.
[3] Wurz, P., and Lammer, H.: Monte-Carlo simulation of Mercury's exosphere, Icarus, Vol. 164, pp. 1-13, 2003.
How to cite: Vorburger, A. and Wurz, P.: Monte-Carlo Model of Europa's Water Vapor Plumes, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-78, https://doi.org/10.5194/epsc2020-78, 2020.
Abstract
We carried out a regional study of Ganymede and Callisto’s photometry by estimating the parameters of the Hapke photometric models on a handful of regions of interest. We found that the photometry was diverse across the surface and highlighted areas with a distinct behavior that could indicate localized activity on Ganymede
Introduction
Jupiter’s icy moons are at the center of future space exploration missions such as ESA’s JUpiter ICy moons Explorer [1] and NASA’s Europa Clipper [2]. Ganymede, in particular, will be the primary target of the JUICE mission. Knowledge of the surface of Ganymede and Callisto is paramount to best plan the mission, help with navigation [3] and understand the its geology. In this study, we focus on the photometry of the surface and how we can describe it using the Hapke photometric model [4].
1. Dataset
We use images from the Voyager probes dataset taken with the Imaging Science System (ISS) [5] and from the New Horizons spacecraft taken with the LOng Range Reconnaissance Orbiter (LORRI) [6] with a ground resolution between 10 km and 30 km at the subspacecraft point. All Voyager images are limited to the clear filter.
2. Method
We need two elements to carry out this study: the reflectance and the geometry of observation. The first can be obtained after radiometric calibration of the images. The second necessitates accurate projections of each pixel.
2.1 Correction of metadata
We simulated images with SurRender [7] and compared those simulations to the real images to correct for spacecraft pointing and moon attitude. Additional distortion and distance corrections were needed and implemented on the Voyager images [8].
2.2 Model and Bayesian inversion
For this study we are considering the Hapke model detailed in Hapke, 1993 [4]. Six parameters are estimated: b, c, ω, θ, h and B0.
We have developed an inversion tool using a Bayesian approach based on previous work done on Mars [9, 10]. No a priori knowledge of the parameters were inferred except for their physical domain of variation. We also include in the model uncertainties on the absolute level reflectance (radiometric uncertainties) to correct for potential bias. This work is detailed in our previous study of Europa [11].
3. Results
3.1. Ganymede's regional photometry
We realized a regional photometric study of 15 areas of Ganymede with a very limited dataset of 16 images matching our criteria (see section 1) for which we corrected the metadata (spacecraft position and orientation) and radiometric calibration discrepancies.
We found that most of our areas are consistent with a global backscattering behavior of the surface with two notable exceptions - ROIs #2 and #4 (see fig. 1). They are situated in the polar latitudes, respectively in the north and in the south hemispheres. They are both very bright and rough and exhibit a strong and narrow forward scattering. This could be due to them being particularly favorable areas for ice redistribution at the poles or signs of possibly localized activity.
3.2. Callisto's regional photometry
We have estimated the photometric behavior of 16 regions of interest on Callisto's surface using 47 images per our selection criteria (see section 1).
Our phase coverage of Callisto is sparse, especially on the leading side, which makes it difficult to have reliable results. However, we have obtained fairly well constrained results for some areas. We have noted that θ - although in average higher than on Europa and Ganymede - shows some variation. ROI #2 is the palimpsest at the center of crater Valhalla. It is the brightest and roughest area that we sampled across Callisto's surface.
Although Callisto is the darkest of the three icy Galileans, we have identified a couple of bright and rough areas that show a strong and narrow forward scattering. We think they might be fields of knobs with frost on the crest as it is common to find frost deposits on topographical highs on Callisto [12].
Conclusion
The preliminary results of this study are very encouraging and show areas of particular interest that could be targeted by future missions. Overall, the general trends of our results are consistent with past integrated photometric studies [13, 14].
We plan on extending this work with more photometric models to find one that best describe the local photometry of both Ganymede and Callisto.
Acknowledgements
This work is supported by Airbus Defence & Space, Toulouse (France), the "IDI 2016" project funded by the IDEX Paris-Saclay, ANR-11-IDEX-0003-02 and the INSU, CNRS and CNES through the Programme National de Planétologie (PNP). Participation to EPSC 2020 is supported by ESAC.
References
[1] O. Grasset et al. An ESA mission to orbit Ganymede and to Characterize the Jupiter system, Planetary and Space Science, 2012
[2] Phillips and Pappalardo, Europa Clipper mission concept: Exploring Jupiter’s ocean moon, Eos, Transactions American Geophysical Union, 2014.
[3] G. Jonniaux et al. Autonomous vision-based navigation for JUICE. IAC, 2014
[4] B. Hapke, Theory of reflectance and emittance spectroscopy. Topics in Remote Sensing, Cambridge University Press, 1993
[5] A.Cheng , NEW HORIZONS Calibrated LORRI JUPITER ENCOUNTER V2.0, NH-J-LORRI-3-JUPITER-V2.0, NASA PDS, 2014.
[6] M. Showalter, VG1/VG2 JUPITER ISS PROCESSED IMAGES V1.0 VGISS_5101-5214, NASA PDS, 2013.
[7] R. Brochard et al. Scientific Image Rendering for Space Scenes with SurRender software, IAC, 2018.
[8] I. Belgacem et al. Image processing for precise geometry determination, under revision for Planetary and Space Science.
[9] J. Fernando, F. Schmidt, and S. Douté, Martian Surface Microtexture from Orbital CRISM Multi-Angular Observations: A New Perspective for the Characterization of the Geological Processes, Planetary and Space Science, 2016.
[10] F. Schmidt and S. Bourguignon, Efficiency of BRDF sampling and bias on the average photometric behavior Icarus, 2019.
[11] I. Belgacem et al. Regional study of Europa’s photometry, Icarus, 2019.
[12] J. M. Moore et al. Callisto in : Jupiter : The planet, Satellites and Magnetosphere, Cambridge University Press, 2004.
[13] B. J. Buratti, Photometry and Surface Structure of the Icy Galilean Satellites, Journal of Geophysical Research, 1995.
[14] D. L. Domingue and A. Verbiscer, Re-Analysis of the Solar Phase Curves of the Icy Galilean Satellites, Icarus, 1997.
How to cite: Belgacem, I., Schmidt, F., and Jonniaux, G.: Regional photometric study of Ganymede and Callisto, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-83, https://doi.org/10.5194/epsc2020-83, 2020.
Abstract
The surface of Pluto is dominated by the Sputnik Planitia basin, possibly caused by an impact ~ 4 Gyr ago. To explain basin's unlikely position close to tidal axis with Charon, a reorientation driven by the post-impact uplift of the subsurface ocean was proposed recently. To maintain the reorientation until today, relaxation of the ice/ocean interface topography has to be sufficiently slow. Here we study the thermo-mechanical viscous relaxation of Pluto's ice shell of uneven thickness. We show that for a pure H2O ice shell the topography relaxes quickly (~ tens of Myr). On the other hand, if a layer of methane-clathrates is present at the base of the ice shell, the relaxation is much slower. With at least 5 km of clathrates, the ice/ocean interface topography can be maintained for 4 Gyr.
Introduction
Pluto, the largest dwarf planet in the Kuiper belt, is probably differentiated into a rocky core and a hydrosphere [9]. Its surface is dominated by the enigmatic Sputnik Planitia basin, which has likely been produced by a giant impact. The basin that covers about 7 % of Pluto's surface is located very close to the Charon tidal axis. To explain this unlikely position, [7] suggested that it may be the result of a whole body reorientation, which requires the crater to be a positive gravity anomaly. Isostatic uplift of the subsurface ocean was thus suggested to compensate the crater's negative gravity and an observed nitrogen layer [6] at the crater's surface to provide the positive signature necessary for the reorientation. However, it is not clear whether the ice/ocean interface topography is stable on long time scales. Here, we study the thermo-mechanical viscous relaxation of Pluto's ice shell of uneven thickness.
Numerical model
We solve thermal convection of ice in 2D cartesian geometry with time-evolving bottom boundary (interface with the ocean). The initial height of the ice/ocean interface is evaluated assuming Airy isostasy and a 6 km deep basin at surface. The governing equations are following:
Free surface is prescribed at the bottom boundary and free slip is prescribed elsewhere. Temperature is fixed at the top and bottom boundary (40 and 265 K) and the side boundaries are kept thermally insulating. The initial condition is steady-state conductive profile. In some simulations, a layer of methane-clathrate [4] of thickness hc is assumed. We use strongly nonlinear viscosity for ice dependent on temperature, grain size and stress using the composite law [2], and similarly for clathrates [1]. As a result, the viscosity of clathrate layer is at least one order of magnitude higher than the viscosity of ice immediately above. We prescribe temperature dependent thermal conductivity for ice [6] that gives values of 2.3 to 13 Wm-1K-1 at the ice/ocean interface and surface respectively. In the clathrate layer of thickness hc we prescribe constant thermal conductivity of 0.6 Wm-1K-1 [10]. We use the Arbitrary Lagrangian-Eulerian formulation to adress the moving boundary [8]. The problem is implemented in freely available Finite Element Method library FEniCS [5].
Results
We performed series of simulations with ice shell thickness h=150 km and clathrate layer thickness hc= 0, 5 and 10 km. The initial height of the ice/ocean interface topography is 87 km and grain size is d=1mm in all simulations.
Figure 1 shows the time evolution of temperature in the ice shell in case without clathrate hydrates layer (hc=0 km). We observe that the ice shell is relatively warm which results in low viscosities and thus relatively fast relaxation. In this setting, the ice/ocean interface topography disappears in about 5 Myr. Note that in this relatively warm, low-viscosity ice shell, thermal convection further increases the temperature (panels c and d).
Figure 2 shows the temperature (top) and viscosity (bottom) in the ice shell with hc=10 km after 100 Myr of relaxation. The insulating layer of clathrates at the ocean interface leads to significantly colder ice shell than in the case without clathrates. Therefore, the viscosity is also very high and no significant relaxation is observed.
Conclusions
We investigated the thermo-mechanical viscous relaxation of Pluto's ice shell using a numerical model with moving boundary. We showed that if Pluto's ice shell is made of pure H2O ice, the ice/ocean interface topography cannot be maintained for longer than a few tens of Myr. If an insulating layer of methane-clathrates is present at the ice shell/ocean interface, it may significantly delay the topography relaxation.
Future study will take into account a more realistic initial temperature condition resulting from the impact. In such case, one might expect that the effect of clathrates would be reduced and the relaxation would proceed faster. These results will be presented at the meeting.
Acknowledgements
This research was supported by the Czech Science Foundation through project No. 19-10809S. M.K. acknowledges the support from the Charles University projects SVV-2019-260447 and SVV-2020-260581. K.K. and O.S. were supported by the Charles University Research program No. UNCE/SCI/023.
References
[1] Durham et al. (2010). SSR. 153. 273-298.
[2] Goldsby Kohlstedt. (2001). JGR Solid Earth. 106. 11,017-11,030
[3] Hobbs (1974). Ice Physics. Clarendon Press, Oxford.
[4] Kamata et al. (2019). Nature Geoscience. 12. 407-410.
[5] Logg et al. (2012). The FEniCS Book, Springer-Verlag
[6] McKinnon et al. (2016), Nature, 534, 82-85.
[7] Nimmo et al. (2016). Nature. 540. 94-96.
[8] Scovazzi Hughes (2007), Sandia National Laboratories.
[9] Stern et al. (2015), Science, 350, 1815.
[10] Waite et al. (2006). GJI. 169. 767-774.
How to cite: Kihoulou, M., Kalousová, K., Souček, O., and Čadek, O.: Viscous relaxation of Pluto's ice shell below Sputnik Planitia, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-91, https://doi.org/10.5194/epsc2020-91, 2020.
Abstract
We present preliminary results of a numerical study of tidally induced deformation of Enceladus’s ice shell containing fissures (Tiger Stripes) in the south polar region (SPR). It is a follow-up study of Souček et al. (2016), which brings in a more realistic description of the faults' behavior. In particular, the fault zone is described by a model of effectively visco-elasto-plastic material with yield stress depending on the stress state. Such a model mimics the Coulomb-type friction contact at idealized fault surfaces. We show the basic features of the model and discuss the importance of fault description on the predicted tidal deformation and stress field within Enceladus's ice shell.
Introduction
In Souček et al. (2016), we have numerically demonstrated a profound effect of faults (Tiger Stripes) on the tidally induced deformation and stress fields in the south polar region of Enceladus. In Běhounková et al. (2017), we showed how this effect can be further enhanced in a synergetic way, when combined with a realistic thinning of the ice shell in the SPR for a shell thickness model based on inversion of gravity and libration data (Čadek et al., 2016). In both these studies, the faults were modelled as zones with significantly reduced elastic moduli. Effectively, such description corresponds to faults being described as narrow slots partially filled with water (compensating the overburden pressure within the layer). Such description of the faults is oversimplified since it does not take into account any friction at the faults. We attempt to circumvent this deficiency in the new study.
Model
We solve equations of deformation of a 3D Maxwell visco-elastic planetary shell containing faults in the SPR. The shell's deformation is driven by the incremental tidal potential and at the fault, Coulomb, type response is considered. The Coulomb-type friction is mimicked by a continuum model of a fault zone of finite thickness with stress limiting viscosity. The stress limiter is given by the yield stress, composed of (hydro)static contribution due to overburden pressure and a dynamic contribution due to normal stress variations during the tidal period.
Results
We present preliminary results with one active fault in the SPR, corresponding to the Baghdad Sulcus. In Figure 1, we plot the displacement in SPR for four instants during the tidal period, colors indicate the amplitude of normal displacement, vectors show the horizontal displacement. Compared to the case with frictionless faults, the model with Coulomb-type friction exhibits asymmetry - the lateral motion is facilitated during the opening phase and suppressed during the closing (compressional) phase of the period.
Acknowledgements
The research leading to these results received funding from the Czech Science Foundation through project No. 19-10809S. The computations were carried out in IT4Innovations National Supercomputing Center (project no. LM2015070). The study was supported by the Charles University, project GA UK No. 304217, SVV-2019-260447 and SVV 115-09/260581 (K.S.), and by Charles University Research program No. UNCE/SCI/023 (O.S.).
References
Běhounková, M., Souček, O., Hron, J.and O. Čadek (2017), Plume activity and tidal deformation of Enceladus influenced by faults and variable ice shell thickness, Astrobiology, 17(9), 941-954, https://doi.org/10.1089/ast.2016.1629.
Čadek, O., Tobie, G., van Hoolst, T., Massé, M., Choblet, G., Lefèvre, A., Mitri, G., Baland, R.-M., Běhounková, M., Bourgeois, O., and Trinh, A. (2016) Enceladus’s internal ocean and ice shell constrained from Cassini gravity, shape, and libration data. Geophys Res Lett 43:5653–5660.
Souček, O., Hron, J., Běhounková, M., and O. Čadek (2016), Effect of the tiger stripes on the deformation of Saturn's moon Enceladus, Geophysical Research Letters, 43(14), 7417–7423, https://doi.org/10.1002/2016GL069415.
How to cite: Souček, O., Sládková, K., Běhounková, M., and Čadek, O.: Tiger stripes revisited - effect of fault zone rheology, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-95, https://doi.org/10.5194/epsc2020-95, 2020.
Introduction: The potential habitability of the icy moons of Jupiter and Saturn, in particular Europa and Enceladus, has been hotly debated in recent years. There is evidence of subsurface liquid water oceans [1], and complex organic molecules (>200 amu) have been detected in the plumes from Enceladus [2]. Plumes are also thought to arise from the surface of Europa [3], which will be the target of the Europa Clipper spacecraft, due to be launched in a few years. In a similar manner to the Ion and Neutral Mass Spectrometer (INMS) on Cassini [4], the Europa Clipper will also carry a mass spectrometer, MASPEX [5], enabling molecular detection of material in the plumes.
Extremophile Archaea and Bacteria, besides being some of the simplest known forms of life, can withstand extreme conditions of temperature, salinity and acidity, and have been observed in hydrothermal vents on Earth. Similar organisms are the most likely candidates for life on icy moons.
The purpose of our current work is to study the mass spectral fingerprint of earthly simple Archaea and Bacteria to understand the spectra that would arise from similar organisms in the vicinity of icy moons and distinguish them from abiotic compounds. Nonbiological organic materials generally have complete structural diversity and are characterized by the presence of racemic mixtures and polycyclic aromatic hydrocarbons (PAHs). These are absent in biological materials which tend to have specific structures and show chiral preference.
Methods: Different strains of Archaea and Bacteria, including cyanobacteria, were obtained in freeze-dried form (DSMZ GmbH). These include extremophiles isolated from sea ice (Shewanella frigidimarina) and hot springs (Metallosphaera hakonensis). They were analyzed in pure form with flash pyrolysis-GC-MS. This enabled identification of the decomposition fragments that are representative of the different components of bacteria.
Flash pyrolysis offers a good analogue of the impact fragmentation that occurs as the spacecraft flies through a plume. In our experiments pyrolysis is coupled with gas chromatography-mass spectrometry (GC-MS). This separates the pyrolysis products, enabling identification of individual molecules. This GC stage is missing on the spacecraft instruments but simulated INMS or MASPEX spectra can be reconstructed from the chromatograms.
Additional experiments were carried out to understand the degradation of bacterial signals under conditions relevant to icy moons. Hydrous pyrolysis was used to simulate hydrothermal degradation, where bacteria are subjected to increased pressures and temperatures, in the presence of water [6].
Results: Our results show that pyrolysis-GC-MS analysis of intact archaea and bacteria produces distinctive fingerprints of biotic substances, shown in Figure 1. Decomposition products from protein, amino acid, carbohydrate and lipid species were detected from all the strains analyzed. The compound types detected are detailed in Table 1.
| Compound type | Possible origin |
| Alkylbenzenes, e.g. toluene, styrene, ethylbenzene | Protein |
| Nitrogen containing heterocycles, e.g. indole, benzyl nitrile, pyrrole | Proteins, amino acids |
| 2,5-Diketopiperazines | Peptides, proteins |
| Oxygen containing heterocycles (furans), e.g. methyl-furan, dimethyl-furan, furfural, maltol | Carbohydrates |
| Phenols, e.g. phenol, cresol | Protein, carbohydrates |
| Alkanes | Lipids |
| Fatty acid, e.g. hexadecenoic acid | Lipids |
Table 1: Possible origin of compound types detected from bacteria and archaea analyzed with pyrolysis-GC-MS.
Using GC-MS, we have been able to identify the molecules detected, shown in Figure 1A, and hence identify the ion series that represent specific compound types which are related to different components of the organisms. These ion series greatly help with the interpretation of complex mass spectra when no separation technique is present, i.e. in INMS and MASPEX. The chromatogram can be converted into a mass spectrum, Figure 1B, with lines joining the peaks in a specific series. This mass spectrum is more representative of the data acquired in space missions.
Although only a relatively small number of strains have been analyzed in this study, there is diversity among the results obtained with differing ratios of carbohydrate and protein products detected. However, all the strains also share common features, such as the presence of indole and phenols.
The mass spectra obtained from Archaea and Bacteria are clearly different from those of abiotic materials such as meteorites [7], which show features as discussed in the Introduction. This shows that mass spectrometry is a powerful technique for distinguishing between non-biological and biological materials.
As well as understanding the mass spectra of intact bacteria, it is important to understand how these spectra will change under different sample processing methods. Thermal processing is likely to be present on icy moons, both on and under the icy crust and as material is ejected into space. Hydrous pyrolysis can also simulate conditions near hydrothermal vents. Increased amounts of sample processing degrade the biotic fingerprint of the bacteria. Understanding the degradation pattern and being able to recognize where the spectral features originate from may play an important role in future spectral interpretation. It will also be important to note at what point the compounds detected become indistinguishable from abiotic materials.
Acknowledgments: We thank the MASPEX team for useful discussions.
References: [1] Pappalardo R. T. et al. (1999) J. Geophys. Res., 104 (E10), 24015-24055. [2] Postberg F. et al. (2018) Nature, 558, 564-568. [3] Paganini L. et al. (2019) Nat. Astron., doi:10.1038/s41550-019-0933-6. [4] Waite J. H. et al. (2004) Space Sci. Rev., 114, 113–231. [5] Brockwell T. G. et al. (2016) IEEE Aerospace Conference 2016. [6] Sephton M. A. et al. (1999) Planet. Space Sci., 47, 181-187. [7] Sephton M. A. et al. (2018) Astrobiology, 18, 7, 843-855.
How to cite: Salter, T., Waite, H., and Sephton, M.: Mass spectrometric fingerprints of Archaea and Bacteria for life detection on icy moons, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-143, https://doi.org/10.5194/epsc2020-143, 2020.
1. Introduction
The Saturnian moon Enceladus exhibits large cryovolcanic plumes sourced from a subsurface ocean which contain evidence of endogenic organic chemistry and ongoing hydrothermal activity [2-4]. Salt-rich ice particles observed in the plumes by the Cassini spacecraft are interpreted as rapidly frozen ocean spray [5] thus studying them provides a means of probing the chemistry, habitability and potential biology of an extraterrestrial ocean. A crucial step in understanding these particles is to determine the composition and sequence of solid phases that form upon freezing of Enceladus ocean fluids. We investigated the partitioning of ice and non-ice materials and the pH-dependence of cryogenic mineral formation using simulated Enceladus ocean fluids at contrasting cooling regimes: (i) flash freezing of ~4 µl droplets into liquid nitrogen (>10 K s-1), and (ii) slow freezing near equilibrium (~0.01 K s-1). Fluids were designed based on up-to-date constraints provided by Cassini data and modelling efforts [1,2,4,6]. Our findings provide new endmember constraints on the range of products produced by Enceladan cryovolcanism.
2. Results & Discussion
2.1. Kinetic inhibition of crystallisation during flash freezing
Imaging via cryo-scanning electron microscopy shows that flash-freezing produces a glass-like non-crystalline phase accumulated at ice grain boundaries (Fig. 1A). The formation of this ‘brine vein’ network demonstrates that ice had formed and solutes were excluded. Glasses represent a homogenous, unfractionated snapshot of fluid composition and would provide an ideal mechanism for preserving organics or microorganisms in their native state.
Fig. 1. A. Interior of flash-frozen droplet, showing ice (dark regions) and glass-like phase (lighter material). Imaged at 123 K, partially sublimated. B. Crystals formed through re-warming flash-frozen fluids. C. Crystals formed through slow freezing.
2.2. Production of crystalline materials within brine veins
We found that salt minerals can form within the confines of brine veins through two contrasting routes: direct crystallisation during cooling (during gradual freezing), or ‘cold crystallisation’ upon warming of flash-frozen samples. These contrasting mechanisms were reflected in the distribution of salt crystals; flash-frozen samples exhibited random crystal distributions, homogenous at the 10 µm scale, whereas crystals produced by slow freezing were arranged in clusters of locally distinct crystal types and highly heterogenous at the 10 µm scale (Fig. 1B,C). These crystallisation textures record the rapidity at which the system solidified, with crystal growth of different phases occurring either simultaneously (in flash-frozen samples) or in a thermodynamically defined sequence (in gradually-frozen samples).
2.3. Mineralogy of cryogenic salts
Measurements of bulk mineralogy using X-ray diffraction showed that at pH 11, carbonate assemblages were dominated by thermonatrite (Na2CO3.H2O), whereas at pH 9 trona (Na3H(CO3)2.2H2O) and nahcolite (NaHCO3) were the major phases (Fig. 2). This demonstrates that carbonate mineral assemblages can function as an independent pH probe for Enceladus’s ocean. Furthermore, at pH 9, nahcolite was kinetically inhibited by flash-freezing. Hence for lower pH fluids, the presence of nahcolite is an indicator of more gradual freezing whereas its absence may indicate kinetically-limited freezing.
Fig. 2. X-ray diffraction patterns collected from crystalline cryogenic salts. Identified peaks are: H, Halite (NaCl); S, Sylvite (KCl); Tr, Trona (NaH3(CO3)2.2H2O); Th, Thermonatrite (Na2CO3.H2O); N, Nahcolite (NaHCO3). Note largest halite peaks at 37.0 and 53.5° (2θ) are truncated for clarity.
3. Conclusions
Our experiments describe contrasting scenarios for solid phase production upon cooling of fluids with an Enceladus ocean composition, each of which leaves a compositional and textural signature of its formation route. Whether glass forms at Enceladus depends on the thermal history of ocean aerosols as they travel upwards from the liquid/vapour interface to space. Particles encountered by Cassini that consist of pure water ice or ice and organics imply that condensation must occur throughout some portion of the Enceladus crack depth and thus that re-warming is likely [1,3]. Due to the self-limiting nature of evaporative cooling and the likelihood of re-warming a conservative assumption is that crystalline salts are more likely than glass in the Enceladus plumes. If crystalline material exists, its mineralogical composition and micron-scale partitioning can record fluid pH and delineate between thermodynamically defined crystallization in sequence, and kinetically limited simultaneous crystallisation. Based on these findings, non-destructive measurements of plume particles could open powerful new avenues of investigation at Enceladus and other bodies that display evidence for cryovolcanic processes, such as Ceres or Jupiter’s moon Europa.
References:
[1] Postberg et al. Nature 459, 1098-1101 (2009)
[2] Waite, J. H. et al. Science 356, 155–159 (2017)
[3] Postberg, F. et al. Nature 558, 564–568 (2018)
[4] Hsu, H.-W. et al Nature 558, 564–568 (2018)
[5] Porco, C. C. et al. Science 311, 1393–401 (2006)
[6] Glein et al. Geochim. Cosmochim. Acta 162, 202–219 (2015)
How to cite: Fox-Powell, M. and Cousins, C.: Production of crystalline and amorphous phases during freezing of simulated Enceladus ocean fluids, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-199, https://doi.org/10.5194/epsc2020-199, 2020.
Introduction
Smooth bright terrains represent the stratigraphically youngest geological units on Ganymede. One plausible theory explaining their unusual smoothness is based on cryo-volcanic eruptions of low-viscosity water-ice lava, flooding the pre-existing rough terrain to an equipotential flat surface [1]. The cryo-volcanic resurfacing hypothesis is interesting since it implies the presence of local melting or liquid water in the shallow crust of Ganymede. However, due to lack of concrete evidence of source vents of cryo-magma, the theories of cryovolcanic origins of the bright terrain are debatable.
These ambiguities can be resolved by directly imaging the subsurface using low-frequency ice-penetrating radar sounders. This is the case of the Radar for Icy Moon Exploration (RIME) [2] on board the ESA’s Jupiter Icy Moons Explorer (JUICE). RIME is designed to achieve a penetration up to 9 km through the ice crust, by operating at a central frequency of 9 MHz with a programmable bandwidth (high-resolution 2.8 MHz, low-resolution 1 MHz). In order to understand the RIME capability in resolving the possible evidence of cryovolcanic resurfacing, it is necessary to model the radar response using radar sounder simulations.
The goal of this analysis is to model the geo-electrical hypotheses of cryovolcanic resurfacing on Ganymede and simulate the corresponding RIME radargrams. One of the main challenges to accomplish this task is the limited availability of high-resolution topographic data over the potential cryovolcanic targets required for generating the simulation inputs. This might be partially mitigated using manually sketched models, flexible enough for tuning the topographic parameters. However, the process is subjective, and manually generated models are a simplified approximation of reality. To address this limitation, in this paper we propose automatic modelling of the target, coupled with a simulation approach, which is described in the next section.
Modelling and Simulation Approach
Our model assumes that the smooth bright terrain represents the surface of a partially flooded pre-existing dark terrain. This underlying terrain is modelled as a superposition of sinusoids (large-scale undulations) and steps (horst-graben morphology). The overlying surface is a nearly-flat surface and positioned at the mean elevation of the pre-existing terrain.
Let Δx represent the required spacing between the topographic samples. Let x=0,Δx,2Δx,...X be the along-track position axis, where X is the required length of the profile. Let λS and AS be the short wavelengths and amplitudes, respectively (representing the spacing between the grabens), superimposed over long topographic wavelengths (λL) with amplitude AL (representing the ridges and troughs). The topographic profile in the along-track direction is given by:
z(x) = AS Rect( 2πx / λS ) + AL cos( 2πx / λL )
For the topography of the overlying surface, a Gaussian roughness of variable RMS height is added in order to avoid unrealistically smooth interfaces that can result in spurious specular reflections. For the complex dielectric permittivity, we consider 2.9 + 3x10-4i for the top layer and 3.5 + 3x10-3i for the buried layer.
By varying the topographic parameters, we generate target models having different depths of resurfacing (average thickness between the surface and the buried layer) and RMS roughness. Further variations are introduced in the orientation of the track and the selection of the RIME bandwidth. The models are summarized in Table 1 and their geometries are visualized in Figure 1 (a) and (b). For comparison, the hypothesis CR 1 is chosen as the baseline model, while the other cases are generated by varying one parameter (yellow cells) relative to the baseline. For simulating the radar response, a multi-layer coherent simulator is considered [3], using the generated target models and the parameters of RIME [2] as input. Figure 2 shows the simulated radargram for the baseline case model (CR 1). The radargrams are processed using unfocussed synthetic aperture radar technique (coherent summing of consecutive radar traces). In this example, the buried layer is clearly detectable as a relatively lower intensity reflector (indicated by white arrows).
Results and Discussions
In the simulation chain, the contribution of the Jovian radio noise has not been considered by assuming anti-Jovian acquisitions. To assess the detectability of the terrain buried under cryovolcanic resurfacing, the subsurface to surface power ratio (SSR) is used. Figure 3 shows the average power of the radar traces of the simulated radargrams. In all the cases, the buried terrain is clearly detectable above the sidelobes, with an SSR in the range between -28 dB and -38 dB. In particular, the deepest reflections in the baseline case models (CR1, CR4, CR5, CR7) appear as clearly distinguishable prominent peaks, irrespective of the differences in either the track orientation or the bandwidth. Furthermore, in case of the shallow (CR3) and the smooth (CR6) models, the buried terrain is weakly detectable, thus pointing the role of off-nadir clutter and subsurface scattering in controlling the performance.
Conclusions
We have presented an analysis of the RIME capability to detect the subsurface evidence of cryovolcanic resurfacing of the smooth bright terrains on Ganymede. The dielectric contrast between the cryo-lava and the older terrain is not expected to be significant (since both are primarily composed of ice having different fractions of impurity). However, the subsurface interface is found to be clearly detectable, owing to the scattering caused by the interface roughness of the underlying layer. This work will be further developed by: (1) studying the effects of small-scale roughness and dielectric contrast using more advanced simulators, and (2) using fully-focused SAR processing to improve the assessment of the detectability.
References
[1] Schenk, Paul M., et al. "Flooding of Ganymede's bright terrains by low-viscosity water-ice lavas." Nature 410.6824 (2001)
[2] Bruzzone, Lorenzo, et al. "Jupiter Icy Moon Explorer (JUICE): Advances in the design of the Radar for Icy Moons (RIME)." 2015 IEEE International Geoscience and Remote Sensing Symposium (IGARSS). IEEE (2015)
[3] Gerekos, Christopher, et al. "A coherent multilayer simulator of radargrams acquired by radar sounder instruments." IEEE Transactions on Geoscience and Remote Sensing 56.12 (2018)
Acknowledgements
This work is supported by the Italian Space Agency under Grant No ASI 2018-25-HH.0 in the framework of “Attività scientifiche per JUICE fase C/D”.
How to cite: Thakur, S. and Bruzzone, L.: Subsurface radar evidence of cryovolcanic resurfacing on the Jovian moon Ganymede: RIME detectability analysis, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-259, https://doi.org/10.5194/epsc2020-259, 2020.
Introduction: Europa’s surface and subsurface will be explored by robotic spacecraft in the late 2020s and early 2030s, but significant advances from astronomical facilities will be possible in the coming decade. Its surface is composed of water ice, with significant contamination from sulfuric acid hydrates and potentially salts [2,10]. Infrared spectra from the Galileo orbiter Near-Infrared Mapping Spectrometer (NIMS) provide the highest spatial resolution IR spectra of Europa but with limited coverage in many locations. In recent years, ground-based adaptive optics observations in the infrared with Keck/OSIRIS and VLT/SINFONI have provided new insights into the distributions of surface materials on Europa [2,4,10].
Datasets: Near-IR observations of Europa were taken during VLT/SPHERE [1] science verification in December 2014. Europa’s anti-jovian hemisphere was observed, with a sub-observer longitude of 192°W. Data were taken in IRDIFS_EXT mode, allowing simultaneous imaging with the Integral Field Spectrograph (IFS) and Infrared Differential Imaging Spectrometer (IRDIS) sub-systems of the SPHERE instrument. IFS [5,12] produces image cubes with spectra from 0.95 to 1.65 μm (R∼30). It has a high spatial resolution, with a pixel size of 7.46 mas/px, corresponding to ∼25 km/px at Europa. Accounting for diffraction, this allows features ∼150 km across to be resolved. For example, Europa’s largest lineae are resolved in Figure 1, the first time this has been possible from Earth. IRDIS produced simultaneous imaging through two filters, with transmissions centred on 2.11 and 2.25 μm. We have also analysed a series of Galileo/NIMS observations with similar spectral and spatial coverage to our SPHERE data. One of the detectors on the NIMS instrument failed early in the Galileo mission at Jupiter, meaning the NIMS coverage of the SPHERE spectral range is limited.
The datasets have been reduced and cleaned to produce mapped spectral cubes of Europa’s anti-Jovian hemisphere. Images are photometrically corrected to remove the variation in brightness towards the edge of the observed disc caused by varying viewing angles and illumination levels of the surface. Our photometric correction uses the Oren-Nayar model, which generalizes the Lambertian model to more accurately represent rough surfaces [13]. This enables regions at large emission angles to be mapped accurately, providing significant improvements over the Lambertian model which overcorrects the brightness towards the edge of the disc. The Oren-Nayar correction allows our mapping to reach emission angles ∼70°, higher than previous studies that extend to 50° to 60° [2,7,10].
Spectral modelling: We analyse the mapped cubes by fitting to laboratory spectra from reference cryogenic libraries. These reference spectra include water ice, sulfuric acid, and hydrated salts [3,8,11]. We have developed an implementation of the Hapke bidirectional reflectance model [9] which we use to model a range of ice grain sizes.
Our fitting routine is run for each observed location to produce compositional maps of Europa’s anti-jovian hemisphere. We treat each observed spectrum as a linear combination of discrete endmembers Ei(λ) with respective abundances ai, where the modelled spectrum is calculated as M(λ)=∑aiEi(λ). Our routine uses Markov Chain Monte Carlo techniques [6] to model an observed spectrum, producing a posterior distribution of fitted abundance values for each endmember, and combinations of different endmembers (Figure 2).
The median of each distribution is used as the best estimate abundance for each endmember, and the width of the distribution is used to estimate the uncertainty on that central value. The posterior distributions of combinations of endmembers can likewise be calculated, accounting for correlations in the uncertainties of the summed endmembers. Whilst the uncertainty on a specific endmember’s abundance may be large (e.g. a specific ice grain size), the uncertainty of the abundance of a combination of endmembers is often much smaller (e.g. the total abundance of all ice endmembers).
The use of Monte Carlo techniques allows better exploration of the endmember parameter space than a simple linear fit, and the uncertainty estimates allow more detailed understanding of potential detections.
Modelling results from the SPHERE and NIMS data show strong similarities and are consistent with previously observed compositional features. These include the leading-trailing hemisphere difference in water ice fraction and the structure of non-ice material (mainly sulfuric acids) from the anti-jovian point to Powys Regio. Salts have a lower abundance and appear spatially constrained to geological features including Dyfed and Powys Regio, and the region around the northern hemisphere lineae.
Acknowledgments: We would like to thank the Royal Society for supporting this work.
References: [1] Beuzit et al 2019, arXiv: 1902.04080. [2] Brown & Hand 2013, The Astronomical Journal, 145(4):110. [3] Carlson, Johnson, & Anderson, 1999, Science, 286(5437):97–99. [4] Carlson et al., 1992, The Galileo Mission, pages 457–502. Springer. [5] Claudi et al., 2008: Ground-based and Airborne Instrumentation for Astronomy II, volume 7014, page 70143E. International Society for Optics and Photonics, SPIE. [6] Foreman-Mackey et al., 2013, Publications of the Astronomical Society of the Pacific 125(925):306. [7] Grundy et al., 2007, Science, 318(5848):234–237. [8] Hanley et al., 2014, Journal of Geophysical Research: Planets, 119(11):2370–2377.[9] Hapke, 1993, Theory of reflectance and emittance spectroscopy,Cambridge University Press. [10] Ligier et al., 2016, The Astronomical Journal, 151(6):163. [11] McCord et al., 1999, Journal of Geophysical Research: Planets, 104(E5):11827–11851. [12] Mesa et al., 2015, Astronomy & Astrophysics, 576:A121. [13] Oren & Nayar, 1944, Proceedings of the 21st annual conference on Computer graphics and interactive techniques, pages 239–246. ACM.
How to cite: King, O. and Fletcher, L.: Compositional mapping of Europa with VLT/SPHERE and Galileo/NIMS using Markov Chain Monte Carlo Fitting , Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-298, https://doi.org/10.5194/epsc2020-298, 2020.
1. Introduction
The surface of Dione, Saturn’s fourth-largest moon, is affected by a variety of processes, both exogenic and endogenic, although the satellite does not seem to be active today [e.g., 1]. Like its neighboring mid-sized icy airless satellites Tethys and Rhea, it exhibits a leading/trailing dichotomy, observed at UV to IR wavelengths [e.g., 2].
This dichotomy has also been reported at 2.2 cm wavelengths, in the unique resolved observation acquired by the Cassini Radar on Dione [3, Fig. 1b]. The leading hemisphere is more radar-bright than the trailing hemisphere, implying greater water ice purity on the leading side. This asymmetry may be caused by the deposition of E-ring material on the leading side, and/or by contamination by a non-icy material on the trailing side.
Herein, we examine Cassini radiometry observations of Dione.
2. Dataset and Methods
The active radar observations of Dione conducted by the Cassini Radar have been presented and analyzed by [3–5]. Spatially resolved data were acquired during flyby DI163 in 2012, simultaneously in active (radar) and passive (radiometry) modes. The resulting images are shown in Fig. 1; the resolved radiometry data is deconvolved following the method described in [6,7]. A preliminary analysis of the resolved radiometry also points to a leading/trailing dichotomy. Indeed, the leading hemisphere, during late afternoon, should be radiometrically warmer than the trailing hemisphere, assuming uniform albedo, thermal inertia, and emissivity. The fact that the reverse is observed indicates variations in at least one of these properties.
Distant radiometry scans have been acquired during four flybys. These data and the disk-integrated antenna temperature they yield are summarized in Table 1. We note cooler disk-integrated temperatures at high latitudes. However, any further interpretations require the use of a thermal model.
We simulate the Cassini radiometry antenna temperatures using a combination of thermal, radiative, and emissivity models, similar to the method already applied to Iapetus [8], Enceladus [9], and Rhea [7]. By fitting the simulated antenna temperatures to the observations, we hope to better constrain the thermal, structural, and compositional properties of Dione’s leading and trailing sides. In particular, using the thermal model with the bolometric Bond albedo map derived by [10], it is possible to separate temperature variations caused by local time, albedo, and emissivity, and thus derive a partial emissivity map.
References
[1] C. J. A. Howett et al. (2018), GRL, 45, pp. 5876-5898
[2] P. Schenk et al. (2011), Icarus, 211, pp. 740-757
[3] A. Le Gall et al. (2019), GRL, 46, pp. 11747-11755
[4] S. J. Ostro et al. (2006), Icarus, 41, pp. 381-388
[5] S. J. Ostro et al. (2010), Icarus, 183, pp. 479-490
[6] Z. Zhang et al. (2017), Icarus, 281, pp. 297-321
[7] L. E. Bonnefoy et al. (2020), Icarus, accepted
[8] A. Le Gall et al. (2014), Icarus, 241, pp. 221-238
[9] A. Le Gall et al. (2017), Nature Astronomy, 1, 0063
How to cite: Bonnefoy, L. E., Le Gall, A., Azevedo, M., Lellouch, E., Leyrat, C., and Janssen, M. A.: Dione's leading/trailing dichotomy at 2.2 cm , Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-329, https://doi.org/10.5194/epsc2020-329, 2020.
Introduction
The subsurface environment of Enceladus is potentially habitable: there is a global subsurface ocean [1], energy from hydrothermal activity [2] and bioessential elements [3]. Carbon, as a fundamental bioessential element, is critical for life, so understanding how it is processed within the Enceladus environment is crucial in assessing this moon’s potential habitability. Evidence from the south polar plumes suggests that carbon is likely to be bound within the silicate interior [4] and liberated through water-rock (silicate-ocean) interactions.
Methods
We have undertaken thermochemical modelling (CHIM-XPT) [5] of these interactions and tested different hypotheses for the formation of Enceladus. Both models reacted the silicate interior (with a CI chondrite composition [6]) with a fluid representative of the subsurface ocean: model 1) a dilute sodium chloride solution, based upon the assumption that the subsurface ocean originated as almost pure water [7]; model 2) a solution with a cometary composition (using data collected from 67P [8]), based upon the assumption that the ocean originated from melted cometary ice [9]. We have explored the full temperature and pressure ranges anticipated at the rock-water interface [2]. We have modelled the rock-water interactions: at the rock-water interface in the areas of hydrothermal activity; within the silicate interior, and in cooler areas of the ocean floor.
In addition, we have run thermochemical models to cool the subsurface ocean fluid as it ascends from the ocean floor to the water-ice boundary; these results should be more comparable to the Cassini observations, as the plume material emanates from the ocean surface.
Figure 1 - A cross section of the subsurface of Enceladus' South Polar Region, highlighting the area of study. The stars represent the different regions of study for the water-rock interaction models; each location represents different temperatures and pressure regimes. The arrows represent cooling and change in the chemical composition of the fluid as it moves from the ocean floor to the ice/ocean interface. Image Credit: R. E. Hamp
Results/ Discussion
With regards to the plausible chemistry of the Enceladus ocean, the model output agrees with the observations from the Cassini data; the dominant chemical species are sodium chloride, potassium chloride and sodium carbonates. The model has also revealed a possible formation mechanism for the silica nanoparticles within the subsurface ocean that could account for the detection of such particles in the Enceladus plumes [2]. This, in turn, validates our model and the simulant chemistry inputted.
The cooling model results show that the pH of the ocean at the water/rock interface was significantly lower than that at the ocean/ice interface (8 compared with 10.5). Furthermore, the concentration of carbonates and sulfates decrease as they ascend though the water column, but the concentration of silica particles increases. The output from this work provides results very closely aligned to the concentrations of the chemical species detected by the Cassini Spacecraft.
This work has also determined that a carbon cycle may operate at the rock-water interface, which could include a carbonate pump system and the formation of hydrocarbons up to C4. The availability of these chemical species and the potential for further ongoing reactions supports the evidence that the subsurface environment of Enceladus may be habitable.
Summary
We will present the outcomes from this modelling, which includes a theoretical composition for a modern day subsurface ocean, potential carbon cycling pathways and the effect of carbon species on the pH of the subsurface ocean fluid. We will also present the initial findings from modelling of the cooling of ocean fluids as they are transported from the ocean floor to the ocean/ice interface surface.
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How to cite: Hamp, R., Olsson-Francis, K., Schwenzer, S., and Pearson, V.: Thermochemical modelling of the subsurface environment of Enceladus to derive potential carbon reaction pathways , Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-550, https://doi.org/10.5194/epsc2020-550, 2020.
Abstract
We used photoclinometry to estimate the topography on Europa, where stereoscopy cannot be used. We used sfs, an open source function from NASA Ames Stereo Pipeline [1]. We tested the robustness of photoclinometry alone to evaluate topography. Then we tested the impact of free parameters in sfs on the results. We checked tuning and modeling parameters and propagated their uncertainties on the results, with errors under 10% on the final volumes estimated.
Photoclinometry, or shape-from-shading is a method to estimate the shape of an object using a 2D image and modeling its photometric behavior to reconstruct a 3D shape. In planetology, it is usually combined with stereoscopy. Stereoscopy recreates the 3D of an object using the parallax between 2 images of the same object viewed from slightly different directions.
Stereoscopy is very robust because it can give absolute heights, and does not need assumptions on the reflective properties of the surface itself. Nevertheless, one needs two images of the same place at the same moment, in different viewing axes to use it. On Jupiter’s moon Europa, such data is poorly available. As topography estimation is a key to study active processes such as cryovolcanism [2], we chose to test the robustness of photoclinometry alone to estimate topography on Europa. Test areas were selected with objects of different scales, and easily identifiable shadow, allowing to validate the relative heights estimations.
Photoclinometry is based on the variations of light intensity from a pixel to its neighbors to estimate local slopes and reconstructing relative heights. It does not allow the estimation of absolute heights, but in our application, we need to estimate the volume of observed objects, so the estimation of relative heights variations is enough.
Figure 1: Gal
ileo SSI image 0526r that shows objects at different scales and clear shadows is adapted to test the robustness of the method
We used sfs, from NASA Ames Stereo Pipeline [1] in the photoclinometry mode only to estimate local topography. It starts from an initialization of the digital elevation model h0(x, y) and uses the light intensity variations I(h(x, y)) to estimate local slopes, considering a uniform surface photometric behavior (often Hapke model in planetology, hidden in the function in R in the cost function). Topography is then estimated iteratively minimizing the following cost function :
where h(x, y) describes the local topography, h0 (x, y) is the initialization for the topography, I (h(x, y)) is measured intensity interpolated at pixels obtained by projecting into the camera 3D points from the terrain h(x, y), T is the image exposure, A is the albedo, R (h (x, y)) is the reflectance, and μ and λ are two tuning coefficients controlling respectively the smoothness and the initialization weight [3]. The algorithm stops when a tolerance parameter is satisfied, meaning it has converged.
In our methodology, as we have no initial information, the algorithm is initialized as perfectly flat, of elevation 0, and the parameter λ that penalizes getting too far from the initialization is put to 0 (no penalization). The influence of the remaining tuning smoothness parameter µ is evaluated generating various DEM of the same area for a wide range of µ. Figure 2 shows an estimation of local elevation that is consistent with the cast shadows of various objects in image 0526r. The smoothness coefficient µ used in this case was 1.6, and the Hapke photometric model was used with the following parameters: ω=0.9, b=0.35, c=0.65, B0=0.5, h=0.6 [4].
Figure 2: Estimated DEM for image 0526r (from [3]), using only photoclinometry.
The topographic profiles AB, CD and DE in figure 2 were selected (i) to evaluate the consistency of the DEM evaluation with cast shadows at three different scales and (ii) to test the influence of µ at theses different levels. With µ=1.6, the DEM is consistent with cast shadows at every scale. Figure 3 shows the influence of µ on three topographic profiles. Low values seem to uniformize the height distribution: larger objects appear the same size as the smallest. Large values may exaggerate the local topography, or on the contrary smooth completely the area. We found that the coefficient µ must be tuned specifically for each image, as its influence depends on the dynamics of the data. The influence of model parameters has also been studied, and three different photometric models have been tested: Hapke, LunarLambert, and Lambertian. For each model, uncertainties on their parameters (such as albedo, backscattering fraction etc) have been propagated. All parameters considered, we showed an uncertainty under 10% for the estimation of volumes of the small objects we were interested in.
Figure 3: Estimated DEM on three topographic profiles from image 0526r (from [3]) using various smoothness parameters µ.
We tested the robustness of photoclinometry alone to evaluate volumes of planetary objects. We propagated uncertainties both from tuning and model parameters to the final results. We showed that when other techniques are not available volumes can be evaluated with a 10% error.
Acknowledgements
We acknowledge support from the “Institut National des Sciences de l’Univers” (INSU), the "Centre National de la Recherche Scientifique" (CNRS) and "Centre National d’Etudes Spatiales" (CNES) through the "Programme National de Planétologie »
References
[1] Beyer, R. A., Alexandrov, O., McMichael, S., 2018. The ames stereo pipeline: NASA's open source software for deriving and processing terrain data. Earth and Space Science 5
[2] Lesage, E., Massol, H., Schmidt, F., 2020. Cryomagma ascent on europa. Icarus 335, 113369.
[3] Lesage, E., Schmidt, F., Andrieu, F, Massol, H., 2020. Constraints on effusive cryovolcanic eruptions on Europa using topography from Galileo images, Icarus, in revisions
[4] Belgacem, I., Schmidt, F., Jonniaux, G., mar 2020. Regional study of europa’s photometry. Icarus 338, 113525.
How to cite: Andrieu, F., Lesage, E., Schmidt, F., and Massol, H.: DEM of Europa using photoclinometry only, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-1123, https://doi.org/10.5194/epsc2020-1123, 2020.
