Derivation of thermal, physical, and compositional subsurface properties from Cassini radiometry

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Inner icy satellite formation and age

There are three leading models, each with different variants and caveats, of the formation of Saturn’s inner icy moons. First, they may have formed in place about 4.5 Ga years ago, within Sat-urn’s subnebula; this model cannot, however, explain the variations in density between satellites (e.g. Mosqueira and Estrada, 2003a,b; Canup and Ward, 2006). Second, they may have formed from a larger ring system (Charnoz et al., 2011), itself originating from the disruption of a large satellite entering the Roche zone of Saturn (e.g. Canup, 2010). Third, they may have formed from the reac-cretion of debris from a collision between two satellites (Sekine and Genda, 2012; Asphaug and Reufer, 2013; Cuk et al., 2016; Hyodo and Charnoz, 2017). The accretion from rings and from debris models can both be compatible with very young icy satellites, whereas the subnebula model implies old satellites. However, the age of the icy moons of Saturn is remarkably difficult to constrain.
Various clues indicate that the inner mid-sized icy satellites of Saturn could be relatively young, of the order of 100 million years old. More specifically, these clues include Saturn’s low tidal Q implying a fast orbital expansion of the satellites (e.g., Lainey et al., 2012), high past heat fluxes on Tethys, Dione, and Rhea indicating past resonances between different satellites (e.g., Cuk et al., 2016), the scarcity of large impact basins on the inner moons compared to Iapetus (e.g., Charnoz et al., 2011; Schenk et al., 2018), the anomalously high heat flow from Enceladus (e.g., Spencer and Nimmo, 2013), and the low mass and low non-icy contaminant fraction of Saturn’s main rings as possible evidence of their youth (Iess et al., 2019). Together, these observations suggest that the satellites formed either from a large ring system or from reaccretion of collisional debris.
Using crater densities to date the surfaces of Saturn’s icy satellites is particularly difficult, as crater statistics in the outer Solar System are not well known (e.g., Zahnle et al., 2003). This is especially true within the Saturn system, where the dominant source of impactors may be planetocen-tric debris, i.e., objects orbiting Saturn rather than the Sun. Furthermore, secondary and sesquinary (ejecta that escapes into Saturn orbit before re-impacting the surface) impacts may contribute a signif-icant amount of craters on the mid-sized icy satellites (Alvarellos et al., 2005; Bierhaus et al., 2012; Alvarellos et al., 2017; Bierhaus et al., 2018). Although crater statistics do not allow absolute aging of the inner icy moons, the presence of several large impact basins such as Mimas’s Herschel and Tethys’s Odysseus, which likely formed from comets, seems to indicate an ancient (∼ 4 billion years ago) formation (e.g., Kirchoff et al., 2018). Relative crystalline and amorphous ice fractions near the apparently recent (relative to the rest of the surface) Obatala crater on Rhea lead to an age of ∼450 Ma for this crater, implying that the rest of the surface is significantly older (Dalle Ore et al., 2015). Thus the cratering record, though not robust enough to clearly discern between the old and young satellite models, does point towards old surfaces (Kirchoff et al., 2018; Castillo-Rogez et al., 2018).

Icy satellite surface processes

The processes that altering the (sub)surfaces of Saturn’s icy satellites are summarized hereafter; the depths affected are illustrated in Fig. 1.12.
• Endogenic activity, namely tectonic, cryovolcanic, and relaxation processes, has modified all of Saturn’s mid-sized icy satellites to varying degrees, as described above.
• Impacts clearly have affected these surfaces throughout their histories, although cratering his-tory in the Saturn system is not robust enough to allow confident surface dating from crater statistics (see discussion above). Heliocentric impactors dominantly affect the leading hemi-sphere; however, little to no apex-antapex asymmetry impact crater density has been found, implying either crater saturation or a dominantly nonheliocentric impactor population (Kir-choff and Schenk, 2010; Leliwa-Kopystynski et al., 2012; Hirata, 2016a; Kirchoff et al., 2018). Planetocentric impactors would be more uniformly distributed (Kirchoff et al., 2018).
• Micrometeoroid impacts likely contribute to the formation of a loose regolith (impact « gar-dening »), and would dominantly affect the leading hemisphere (e.g., Buratti et al., 1990; Ries and Janssen, 2015). The Cassini Cosmic Dust Analyzer (CDA; Srama et al., 2004) measured an important dust population in the ring plane, as well as interplanetary particles (Srama et al., 2006; Altobelli et al., 2016). While these particles very likely affect the satellites’ surfaces, this effect is difficult to disentangle from others (in particular E ring particles and high-energy electrons) and has not been clearly observed (Szalay et al., 2018). Micrometeoroid impacts also introduce non-icy materials to the surface.
• Deposition of E ring particles, which are mainly composed of high-purity water ice from Enceladus’s subsurface ocean, bombard the moons of the inner Saturn system (Kempf et al., 2010; Schenk et al., 2011). The mean visual geometric albedos of the inner mid-sized satellites are well correlated with the expected E ring flux at their orbits (Verbiscer et al., 2007); infrared water ice band depths (Filacchione et al., 2012) and radar brightness (Ostro et al., 2006; Le Gall et al., 2019) follow the same trend. E ring brightening affects preferentially the trailing hemisphere of Mimas and the leading hemispheres of Tethys, Dione, and Rhea, and is therefore the likely cause of hemispheric dichotomies (e.g., Hamilton and Burns, 1994; Schenk et al., 2011; Royer and Hendrix, 2014; Howett et al., 2018), although recent work indicates that the distribution of infalling E ring particles may be more complex (Juhasz and Horanyi, 2015; Kempf et al., 2018). E ring material dominantly affects the satellites closest to Enceladus (Mimas, Enceladus itself, and Tethys), whereas as at most millimeter depths are expected on Dione, and less on Rhea (Juhasz and Horanyi, 2015; Hendrix et al., 2018).
• Deposition of Phoebe ring particles are the most likely origin for the dark material on Iapetus and Hyperion (see Section 1.4). It is also a possible origin for the dark material on the trailing hemispheres of Rhea, Dione, and Tethys, although Phoebe dust is expected to be considerably more rare in the inner Saturn system (Clark et al., 2012).
• Deposition of a dark red material seems to be affecting the trailing hemispheres of Tethys, Dione, and Rhea, and to a lesser degree their leading hemispheres as well as Mimas and Ence-ladus (Clark et al., 2008; Stephan et al., 2010; Schenk et al., 2011; Stephan et al., 2012; Hendrix et al., 2018). The nature and origin of this material are not clear; candidates include complex organic dust (tholins) (e.g., Hendrix et al., 2018), nano-sized iron particles (Clark et al., 2008, 2012), and processed salts from the E ring (Hendrix et al., 2018).
• High-energy (MeV) electrons circulating in Saturn’s magnetosphere are the best explanation for the thermal inertia anomaly and UV-bright lens-shaped regions on the leading sides of Tethys (Howett et al., 2012; Howett et al., 2019), Mimas (Fig. 1.11b; Howett et al., 2011a, 2020), and possibly Dione (Howett et al., 2014; Howett et al., 2018). These MeV electrons penetrate up to centimeter depths into the subsurface, which may lead to physical (sintering, amorphization) and/or chemical (new molecules, coloring) changes detectable both in the UV and in the thermal infrared (Schenk et al., 2011; Paranicas et al., 2014; Schaible et al., 2017; Howett et al., 2018).
• Cold plasma (ions and keV electrons) bombardment would affect preferentially the icy satel-lites’ trailing sides (Nordheim et al., 2017; Howett et al., 2018; Verbiscer et al., 2018). Like MeV electrons, keV electrons may affect the structure of the surface (through annealing or sput-tering), but as they would only affect the top millimeters of the surface, their effect should often be obscured by other processes such as infalling E ring grains (Nordheim et al., 2017; Howett et al., 2018). Radiolysis of silicates and organics by keV electrons may however be reddening and darkening the trailing hemispheres, though this effect is also competing with infalling clean E ring grains, especially for Mimas and Enceladus (Hendrix et al., 2012; Hendrix et al., 2018). Further, cold plasma may play a role in the presence of CO2 and O3 on all five inner mid-sized moons (Johnson et al., 2008; Hendrix et al., 2018).
• Temperature variations lead to winter adsorption and condensation and summer desorption and sublimation of CO2 at Rhea’s and Dione’s poles (Teolis and Waite, 2016). On Iapetus, the long day length and stark temperature contrasts between bright and dark regions lead to thermal migration of water ice (e.g., Spencer and Denk, 2010). Thermal stresses likely also modify the structure of the surface, possibly creating stresses and cracks (Baragiola, 2003).
In the last four hundred years, we have progressed from discovering Saturn’s icy satellites to investigating their surfaces in detail. The Cassini-Huygens mission, during its 13.5 years in the Sat-urn system, revealed diverse and complex worlds, interacting with each other and with the rings and magnetosphere of their planet. Yet, the formation and evolution of the icy satellites remains largely mysterious, as contradictory information points to young or old systems. A variety of processes affect the composition and structure of the satellites’ surfaces: these processes come into competition both in space (depths and regions) and over time, and their relative contributions are still unconstrained. The thesis work described herein aims to contribute to our understanding of Saturn’s icy satellite sub-surfaces, informing on their thermal, physical, and compositional properties down to metric depths.

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Microwaves, which can peer under the icy surfaces of Saturn’s satellites, offer unique informa-tion on their properties, history, and evolution.

This chapter begins by reviewing the principles of microwave remote sensing and the way mi-crowaves propagate through matter. We then describe both the active (radar) and passive (radiometry) modes of microwave observations. Finally, this chapter details the microwave radar and radiometry observations of Jupiter’s and Saturn’s icy satellites.

Principles of microwave remote sensing

Advantages of microwave remote sensing

By convention, the microwave region of the electromagnetic spectrum includes wavelengths from 1 mm to 1 m (i.e., frequencies from 300 GHz to 3 GHz; Fig. 2.1). Just beyond the far infrared (also called sub-millimeter), it encompasses the high-frequency end of the radio spectrum: the (con-fusingly named) Ultra High Frequency (UHF, 0.3–3 GHz), Super High Frequency (SHF, 3–30 GHz), and Extremely High Frequency (EHF, 30–300 GHz) bands. Common uses of microwaves include communication systems, microwave ovens, cellular telephone, TV broadcasting, radar, radio astron-omy, and surface remote sensing. The latter three applications are particularly of interest to planetary science.
Microwave remote sensing observations, which can be both active (radar) and passive (radiom-etry), offer information complementary to observations at other wavelengths (including UV, optical, and IR). Microwave radiometry measures the passive continuum blackbody emission of an object (e.g., a surface) in the microwave domain, which gives an indication of its thermal, physical, and to some extent compositional properties. Especially at shorter (millimetric) wavelengths, microwave radiometry can also observe gas emission and absorption bands from atmospheres. Radars observe the signal scattered by a medium from a transmitter to a receiver; it informs on the target’s geometry and its absorbing and scattering properties. Both active and passive observations can be obtained regardless of target illumination from the Sun. This offers a considerable advantage with respect to UV and optical wavelengths: the possibility to « see » the surface during the night and polar winter.
In the case of planetary bodies with an atmosphere opaque to most wavelengths, microwaves permit to peer through the clouds and haze to examine the surface. This is the case of Venus and Titan, whose surfaces were revealed in unprecedented detail by, respectively, the Magellan Venus Radar Mapper and the Cassini radar/radiometer. Observing the surface is also possible within several infrared windows, although these observations generally include a greater atmospheric contribution: the Cassini VIMS and ISS instruments, for example, mapped Titan’s surface in the near- and mid-infrared.
Microwave radiometry is also particularly well adapted to the study of very cold objects (∼ 20 − 70 K), whose thermal radiation is difficult or impossible to detect in the infrared. Furthermore, in most cases, UV to IR wavelengths only give information on the top few nanometers to millimeters of the surface, although in a few cases meter depths can be probed as well (i.e., with very long ob-servation timescales in polar regions where daily temperature variations are negligible; Howett et al., 2016). Meanwhile, microwaves can generally penetrate down to several meters under the surface. They can thus access key pages in a surface’s history, providing insight into thermal, compositional, and structural variations with depth.
Microwave instruments are, however, hampered by their low spatial resolution, which can only be improved by building large antennas. There are consequently few microwave instruments able to resolve Saturn’s icy satellites from Earth. Spaceborne radars and radiometers, which can achieve better resolutions from their proximity to the target, are large and heavy. Their use on space missions has therefore been limited, and the Cassini radar/radiometer is one of the most complex, multi-faceted, and high-performance microwave instruments on any interplanetary mission.

Radar/radiometer antennas

Both radars and radiometers include antennas and receivers but radars differ from radiometers in that they include a transmitter as well. Radar/radiometer antennas can take many different shapes, best suited to their wavelength, polarization, modes of operation, and target. The data used in this thesis were obtained either from the Cassini radar/radiometer, the Karl G. Jansky Very Large Array (VLA), or the Institut de Radioastronomie Millimétrique (IRAM) 30m telescope, which all consist in parabolic dish antennas. The main characteristics of such antennas are described herein.

Table of contents :

1 Saturn’s icy satellites: discovery and exploration 
1.1 First discoveries
1.2 Spacecraft exploration
1.2.1 Before Cassini
1.2.2 The Cassini-Huygens mission
1.2.3 After Cassini
1.3 A variety of worlds
1.4 Outer system satellites: Phoebe, Iapetus, and Hyperion
1.4.1 Phoebe
1.4.2 Iapetus
1.4.3 Hyperion
1.5 Inner mid-sized icy satellites
1.5.1 Rhea
1.5.2 Dione
1.5.3 Tethys
1.5.4 Enceladus
1.5.5 Mimas
1.6 Inner icy satellite formation and age
1.7 Icy satellite surface processes
1.8 Conclusion
2 Microwave remote sensing: from theory to icy satellite observations 
2.1 Principles of microwave remote sensing
2.1.1 Advantages of microwave remote sensing
2.1.2 Radar/radiometer antennas
2.2 Interactions of microwaves with matter
2.2.1 Medium electromagnetic properties
2.2.2 Electromagnetic waves in a homogeneous medium
2.2.3 Wave reflection and transmission
2.2.4 Wave scattering
2.2.5 Subsurface scattering
2.3 Radars for planetary exploration
2.3.1 Types of radars used in planetary exploration
2.3.2 Fundamental radar equation
2.3.3 Radar albedo
2.4 Microwave radiometry
2.4.1 Planetary exploration with microwave radiometry
2.4.2 Thermal emission from a surface and near subsurface
2.4.3 Microwave radiometers
2.5 Icy satellite microwave observations
2.5.1 Icy Galilean satellites: radiometry observations
2.5.2 Galilean satellites: radar observations
2.5.3 Galilean satellites: future microwave exploration
2.5.4 Titan radar/radiometry observations
2.5.5 Saturn’s icy satellite radar/radiometry observations
3 Saturn’s icy satellites seen by the Cassini radar 
3.1 The radar/radiometer on the Cassini spacecraft
3.1.1 Characteristics
3.1.2 Observation strategy
3.1.3 Observation geometry recalculation
3.2 Distant radar observations: Disk-integrated radar albedos
3.2.1 Observations and derivation of the disk-integrated radar albedo
3.2.2 Results
3.3 Resolved radar observations: scatterometry
3.3.1 Observations
3.3.2 Data reduction
3.3.3 Results
3.4 Interpretations
3.4.1 Radar albedo and water ice purity
3.4.2 Interactions with Saturn’s E ring
3.4.3 Structure of the regoliths
4 Saturn’s icy satellites seen by the Cassini radiometer 
4.1 Observations
4.2 Calibration
4.2.1 Radiometry calibration
4.2.2 Removal of the far sidelobe contribution
4.2.3 Determination of the baseline offset
4.2.4 Pointing and time offset correction
4.2.5 Measurement uncertainties
4.3 Preliminary analysis: disk-integrated temperatures
4.3.1 Extraction of disk-integrated temperatures
4.3.2 Results
4.4 Towards a resolved analysis: deconvolution
4.4.1 Deconvolution method
4.4.2 Deconvolution results
4.5 Conclusion
5 Simulation of microwave radiometry observations 
5.1 Thermal model
5.1.1 Incident flux
5.1.2 Derivation of the temperature profile below the surface
5.1.3 Model parameters
5.1.4 Model outputs
5.2 Radiative transfer model
5.2.1 Model hypotheses
5.2.2 Calculating the effective temperature Te f f
5.2.3 Model parameters
5.2.4 Numerical application
5.2.5 Model outputs
5.3 Emissivity model
5.3.1 Combined emissivity-backscatter model
5.3.2 Model parameters
5.3.3 Application to Rhea, Dione, and Iapetus
5.4 Simulating the antenna temperature
5.4.1 Obtaining the brightness temperature
5.4.2 Convolution with the beam pattern
5.4.3 Data fitting method
5.4.4 Deriving subsurface thermal, physical, and compositional properties
5.4.5 Application to Rhea, Dione, and Iapetus
5.4.6 Model limitations
5.5 Conclusion
6 Derivation of thermal, physical, and compositional subsurface properties from Cassini radiometry
6.1 Disk-integrated emissivities
6.1.1 Method
6.1.2 Application to Iapetus
6.1.3 Application to Enceladus
6.1.4 Application to Dione
6.1.5 Application to Rhea
6.2 Thermal, structural, and compositional properties of Rhea’s subsurface
6.2.1 Method
6.2.2 Results
6.2.3 Interpretations and discussion
6.2.4 Summary and conclusion
6.3 Preliminary results for Dione
6.3.1 Method
6.3.2 Results
6.3.3 Preliminary interpretations
6.4 Preliminary results for Iapetus
6.4.1 Method
6.4.2 Results
6.4.3 Preliminary interpretations
6.5 Conclusion
7 Radiotelescope observations of Iapetus and Phoebe
7.1 The hemispherical dichotomy of Iapetus in the microwaves
7.1.1 Radar observations
7.1.2 Ground-based microwave radiometry
7.1.3 Iapetus ground-based radiometry: pre-existing observations and interpretations
7.1.4 Outstanding questions and motivations for the present study
7.2 Disk-integrated observations of Iapetus from the IRAM 30 meter telescope
7.2.1 The NIKA2 camera on the IRAM 30-meter telescope
7.2.2 Observation strategy
7.2.3 Calibration and flux derivation
7.2.4 Results
7.3 Observations of Iapetus and Phoebe from the VLA
7.3.1 The Karl G. Jansky Very Large Array (VLA) interferometer
7.3.2 Observations and calibration
7.3.3 Results
7.4 Discussion and interpretations
7.4.1 The LH and TH microwave spectra of Iapetus
7.4.2 Comparison with a thermal model
7.4.3 Emissivity variations with wavelength
7.5 Conclusion and perspectives
7.5.1 Future observations
7.5.2 Towards a resolved analysis of VLA observations
7.5.3 Need for a multi-layer thermal model
7.5.4 Emissivity modeling

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