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Variability of Polarization on the Surface of Jupiter’s and Saturn’s Moons

Written By

Nikolai Kiselev and Marina Shcherbina

Submitted: 15 September 2025 Reviewed: 09 October 2025 Published: 04 March 2026

DOI: 10.5772/intechopen.1013560

Planetary Science - Explorations, Missions, and Mapping IntechOpen
Planetary Science - Explorations, Missions, and Mapping Explorations, Missions, and Mapping Edited by Yann-Henri Chemin

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Planetary Science - Explorations, Missions, and Mapping [Working Title]

Dr. Yann-Henri Chemin

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Abstract

We present the disk-integrated longitude-polarization functions of Jupiter’s and Saturn’s satellites. The observed degree of linear polarization, P(obs), of atmosphereless/airless solar system bodies (ASSBs) – asteroids and planetary satellites – depends on many quantities and observation circumstances: P(obs) = P(phase angle, planetocentric longitude, latitude, time, and wavelength). The dependence of polarization, P(α), on the phase angle is most sensitive to the properties of the regolith surfaces. However, in some cases, it is possible to detect polarization changes over the body surface, the so-called longitudinal dependence of polarization (LDP). Accurate measurements of P(obs) at different phase angles and longitudes have made it possible to establish the LDP of Jupiter’s satellites (Europa, Io, Ganymede, and Callisto) and Saturn’s satellites (Iapetus and Rhea). This may be evidence of optical/structural inhomogeneities on the surface of the bodies. A long-term change in the longitude dependence of Io’s polarization may indicate a change in global and/or local volcanic activity on the satellite’s surface.

Keywords

  • polarization
  • phase angle dependence of polarization
  • longitudinal dependence of polarization
  • planetary satellites
  • properties of surface

1. Introduction

Scattered light produced when unpolarized radiation interacts with various particles and media exhibits a number of properties, one of the most important being linear polarization. The observed degree of linear polarization, Pobs, for airless/atmosphereless solar system bodies (ASSBs) – such as asteroids, planetary satellites, and comets – generally depends on multiple quantities and observing conditions. This can be written as

P(obs)=P(α,L,t,λ)E1

where α is the phase angle; L is the planetocentric longitude (for planetary satellites and asteroids); t is the time of observation (for non-stationary objects such as comets or active asteroids); and λ is the wavelength.

The phase-angle dependence of polarization, P(α) (the polarization phase curve [PPC]), is especially sensitive to the properties of regolith-covered surfaces of ASSBs. In addition, in a number of cases, it is possible to detect variations in polarization across a body’s surface – the so-called longitudinal dependence of polarization (LDP) – which can indicate optical/structural inhomogeneities on the surface. In this review, we present examples of LDP detections for several satellites of Jupiter and Saturn, for which the most accurate PPCs to date have been obtained.

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2. Phase dependence of the polarization of scattered radiation

Before discussing spatial variations in the polarization degree across planetary satellites, we briefly recall the key parameters of the polarization phase dependence that must be accounted for when searching for an LDP.

Electromagnetic waves (EM waves) are transverse to the direction of propagation. For thermal radiation sources, in the absence of magnetic fields, the oscillations of the electric vectors in the image plane have no preferred direction. However, the interaction of transverse waves with a surface or medium introduces an asymmetry in the distribution of electric-vector orientations in the image plane. The degree of linear polarization, P, quantifies this asymmetry:

P=Imax−IminImax+Imin=IpIE2

where Imax and Imin are the maximum and minimum intensities (squared EM amplitudes), and Ip and I are the polarized and total intensities, respectively. By definition, 0 ≤ P ≤ 1. The angle at which the intensity is maximal in the image plane, θ, is the position angle of the polarization plane. The quantities P and θ are usually specified in a chosen reference frame. In Figure 1, P and θ are shown with respect to the equatorial system, in which the position angle is measured from celestial north through east (counterclockwise). The polarization parameters P and θ are commonly expressed via the Stokes parameters Q and U [1]:

Figure 1.

Schematic illustrating the principal characteristics of polarized scattered radiation. P and θ are the degree and position angle of linear polarization in the equatorial frame. A is the phase angle; φ is the position angle of the scattering plane. I|| and I⊥ are the intensities parallel and perpendicular to the scattering plane.

Q=Pcos2θ,U=Psin2θE3

In planetary astrophysics, the scattering plane is adopted as the reference frame. If φ is the position angle of the scattering plane, the transformation to this frame is

θr=θ−(φ±90°),Pr≡Qr=Pcos2θr,U=Psin2θrE4

Because the perpendicular to the scattering plane can be chosen in two ways, the sign in (φ ±90°) is selected so that its value does not exceed 180°.

In the scattering-plane frame, the (signed) polarization can also be written as.

Pr=(I⊥-III)/(I⊥+III)=(I⊥-III)/IE5

where I⊥ and I|| are the intensities of the scattered light with electric vectors oscillating perpendicular and parallel to the scattering plane, respectively (see Figure 1). As noted above, P is non-negative by definition; however, according to Eq. (5), if I⊥ < I||, the quantity Pr is formally negative. Physically, this means that the electric vectors oscillate predominantly in the scattering plane. This behavior is characteristic of most airless bodies (planetary satellites, asteroids) and of comets observed near opposition, at phase angles α ≤ αinv ≈9° – 30°.

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3. Instrumentation and methodology for determining the LDP in planetary satellites

In 2018, two aperture two-channel polarimeters, “POLSHAKH,” were built at the Crimean Astrophysical Observatory (CrAO) [2]. They are used for polarimetric observations on the 2.6-m CrAO telescope and the 2-m telescope of the Peak Terskol Observatory. The instruments are designed to measure the parameters of linear and circular polarization both in broadband UBVRI filters and in dedicated narrowband filters for various celestial objects. The determination of polarization parameters is based on measuring the intensity of light transmitted through a continuously rotating (≈30 Hz) half-wave/quarter-wave retarder, followed by splitting the beam with a Wollaston prism into two orthogonally polarized components. A synchronous-detection scheme is employed, that is, photon counts are accumulated in computer memory cells in step with the retarder’s rotation angle. As a result, the observational outcomes are practically insensitive to short-term atmospheric conditions. The instrumental polarization of both polarimeters does not exceed 0.04% and varies only weakly from season to season, which – after removal – allows us to investigate subtle polarimetric effects of celestial bodies.

As noted above, after correcting for instrumental effects and transforming to the scattering-plane reference system, the observed degree of linear polarization, Pr, depends on the observing geometry, namely on the phase angle, α, and other factors. Observations over different phase angles yield the PPC, Pr(α).

Because most satellites are tidally locked, they always present the same face to their parent planet. Consequently, as viewed from Earth, different hemispheres are seen at eastern versus western elongations. The planetocentric longitude L of a satellite is defined as the angle measured from the point of superior geocentric conjunction, increasing counterclockwise as seen from the north celestial pole. Over 0°≤ L ≤ 180°, the leading hemisphere (facing the direction of orbital motion) is observed; whereas, for 180°< L < 360°, the trailing hemisphere is seen. If the physical/structural properties of the surface vary with location, the observed linear polarization Pr may also depend on longitude L.

Because of the satellites’ orbital inclinations relative to the line of sight to Earth, the observed surface regions differ not only in longitude but also in latitude. Therefore, both longitudinal and latitudinal polarization effects are possible. We adopt as L and β the apparent planetographic longitude and latitude of the disk center, taken from JPL ephemerides [3].

Observing geometry allows Jupiter’s satellites to be studied over phase angles 0° −12°, and Saturn’s satellites over 0° − 6°. To reliably disentangle the phase and longitudinal dependences of polarization, measurements must therefore be obtained over a wide range of longitudes and latitudes and at multiple phase angles, which requires a substantial observational data set. In our material, the maximum latitude reached for Rhea was β = 22°; for the other satellites, β was limited to only a few degrees. Hence, latitudinal polarization effects are neglected in this work.

Because the dependence on phase angle α is the strongest, the relation Pr versus α is customarily termed the phase dependence of the polarization degree, although for some objects it may implicitly depend on longitude, that is, Pr(α, L). In the range 0 ≤ α ≤ αinv, the phase curve exhibits a negative polarization branch (NPB), whose schematic form is shown in Figure 2.

Figure 2.

Characteristic shape of the observed polarization dependence for airless solar system bodies and comets, and its parameters over a broad range of phase angles. Parameters of the negative branch include Pmin and αmin (the minimum polarization and the corresponding phase angle), the inversion angle αinv, and the polarimetric slope at the inversion angle h = ΔP/Δα (in percentage per degree).

3.1 General procedure for extracting the LDP from observations

To first order, the observed degree of linear polarization Pr(α, L) can be represented as a superposition of two dependencies:

Pr(α,L)=Pr(α)+A(α)Pr(L)E6

where Pr(α) is the PPC (PDP), free of longitudinal effects; Pr(L) is the dependence of polarization on orbital longitude at a given phase angle; and A(α) is the amplitude of the LDP, which may vary with phase. Over a narrow range of phase angles, the LDP can be treated as constant. Over the broader range 0≤ α ≤ αinv, Pr(α, L) is not linear in α, especially near αmin. Therefore, setting Pr(L) = 0

In Eq. (6), we selected a phase-angle interval Δα within which the values of Pr(α, L) can be approximated by a straight line. Then, to first order and separately for each hemisphere of a satellite, one can write.

Pr(α)=aPr(α,L)+bE7

where a and b are the coefficients of the linear fit, obtained by least squares. Consequently – accounting for the fitted slope a – the longitudinal dependences for the leading and trailing hemispheres were evaluated at the midpoint of the chosen. Δα as

Pr(L)=Pr(α,L)−aPr(α,L)(α−Δα/2)E8

Below, we present the polarization measurements of planetary satellites obtained with these polarimeters over 2018–2025.

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4. Results on the LDP for selected planetary satellites

4.1 Jupiter’s satellites

4.1.1 Europa

Europa’s surface is covered by an ice shell with a complex network of fractures discovered by the two Voyager spacecraft in 1979, along with a small number of impact craters. Beneath the thick ice crust lies an ocean of saline water. Cryovolcanic activity is observed at the surface, likely sourced from the subsurface saline ocean [4–6]. These active phenomena may modify the surface properties. The icy surface makes Europa one of the most reflective objects in the solar system: its disk-averaged geometric albedo in the V band is 0.68. The light scattered by Europa exhibits a steep, nonlinear increase in brightness toward opposition at phase angles α ≤2° for both the leading and trailing hemispheres. In the visible, the leading hemisphere is brighter than the trailing one [7], generally attributed to the preferential bombardment of the trailing side by charged ions, which darkens the surface.

Extensive polarimetric observations of Europa were conducted by Dollfus [8] and by Chigladze [9], but the most accurate PPCs in the UBVRI bands were obtained only recently by Kiselev et al. [10]. That study noted: “There is a slight difference (on average ~0.02%) in the polarization degree between Europa’s two hemispheres only in the R band: the polarization is systematically lower for the leading hemisphere. Most likely, if a longitudinal effect exists, it is small, but this requires verification with additional observations.” Motivated by this, we carried out additional observations of Europa, allowing us to refine the shape of the LDP. The final phase dependences of Europa’s polarization degree in the VRI bands are shown in Figure 3 (left panels). The right panels present the longitudinal dependences of polarization normalized to a phase angle of 9° after accounting for the PPC of each hemisphere over α = 5° − 12°. From the right panels (positive polarization branch), it is seen that in the V and R bands, the polarization of the leading hemisphere is systematically lower than that of the trailing hemisphere by ≈0.03% and ≈0.02%, respectively. In the left panels (NPB), the polarization of the leading hemisphere is systematically smaller in absolute value than that of the trailing hemisphere. Thus, the character of the LDP changes between the negative and positive branches of the phase curve, consistent with a phase-angle–dependent spectral behavior of the polarization. No longitudinal dependence is detected in the I band.

Figure 3.

Left panels: Phase dependences of Europa’s polarization degree in the VRI bands. Filled circles – data for the leading hemispheres, 0° ≤ L ≤180°; open circles – data for the trailing hemispheres, 180° < L < 360°. We combine results from Kiselev et al. [10] with additional observations obtained in the period 2022–2023. Right panels: Longitudinal dependences of Europa’s polarization after accounting for linear approximations to the observed PPC of each hemisphere over α = 5°−12°.

4.1.2 Io

Io is the most volcanically active object in the solar system; its disk-integrated geometric albedo in the V band is ≈0.63. Its compositionally heterogeneous surface is formed by lava flows from several hundred active volcanoes. Bright flows dominate over dark ones and are interpreted as sulfurous, whereas the dark areas are primarily silicate crust overlain by pyroclastics and mixed-composition flows. This structure produces a pronounced heterogeneity in optical properties [11]. Observations by Galileo [12, 13] showed that, due to the dynamic nature of volcanism on Io, its surface can change over time on a scale of years and even months. Allotropic sulfur compounds give different colors to Io’s surface [14]. A similar effect can occur for polarization due to the frost deposits on the surface of Io.

Our polarimetric observations in the period 2019–2024 [15, 16] showed (see Figure 4, left panel) that Io’s negative polarization decreases sharply in the phase angle range from 0° to about 2°, and then it increases almost linearly to ∼12°, reaching a value of about −0.15%. The extrapolated inversion angle in the V and R bands is about 26° ± 6°. For comparison, the right panel of Figure 4 shows Io’s PPC previously obtained by Veverka and Burns [17] over the same phase-angle range.

Figure 4.

Left panels – phase-angle dependences of polarization for Io in the V band. The data are taken from literature [15, 16]. Filled circles – data for the leading hemisphere (0° ≤ L ≤ 180°); open circles – data for the trailing hemisphere (180° < L < 360°). The right panel presents data taken from the study by Veverka and Burns [17] (Figure 10.6 of the study), which we adapted to the format of our observations. Filled circles are Io’s measurements made by Zellner and Gradie [18] at 0.52 μm. Open circles are data by Dollfus [8] at about 0.5 μm. The observations of Veverka [19] in the clear filter are marked by triangles, and the dashed curve is the fitting polarization curve by Dollfus.

According to Veverka and Burns [17], the large scatter of the degree of polarization, up to about 0.4%–0.5% at phase angles greater than 10°, obtained by Zellner and Gradie, may be caused by the spotty surface of Io. Figure 4 compares our measurements of Io’s polarization with those of Zellner and Gradie [18], obtained in April 1972 and November 1973, and with extensive polarization measurements of Io by Dollfus [8] in April 1972 and October 1973. The amplitude of the variations in P is 0.1%, 0.2%, and 0.45%, respectively. The accuracy of the measurements by Dollfus and Zellner and Gradie was better than 0.03% and was consistent with the accuracy of our measurements. Dollfus believed that the change in Io’s polarization was caused by the proximity of Jupiter, which introduced a background of scattered light.

Unfortunately, the results of the measurements by Zellner and Gradie are presented only as a figure in the paper by Veverka and Burns [17]. The lack of the measurement time and Io’s longitude precludes verifying Jupiter’s contribution to the diffuse light background. However, our observations of Io at different angular distances from Jupiter on the same night of measurements, that is, for close longitudes, showed that variations in the degree of polarization do not exceed the measurement errors. The influence of other potential causes (the positions of the orbiting moon and Io’s perihelion), causing polarization variations over a narrow range of phase angles, is difficult to assess. We therefore support the conclusion of Zellner and Gradie that the large scatter of their polarization measurements may be due to local surface properties, which, in turn, are due to Io’s volcanic activity.

In literature [12, 13], it has been shown that Io’s volcanic activity varies substantially over time. For example, Mekler and Eviatar [20] noted that late 1978 and the first half of 1979 were periods of anomalously high activity; Voyager-era observations likewise indicate the long-lived persistence and variability of volcanic plumes [21]. Taken together with the observed transience of hot spots and plume deposits, these facts indirectly suggest the possibility of seasonal changes in the longitudinal dependence of Io’s polarization, driven by long-term variations in both local and global volcanic activity.

Figure 5 shows all observed polarization of Io in the VR bands, in a narrow phase angle range of 10°–12° as a function of longitude L, since we can assume that the phase polarization curves in this range of angles α are constant. The average values of the degree of polarization of both hemispheres are almost the same. It is seen that the data scattering is within about 0.1%, which is close to values of 3*σp. Therefore, it can mainly be caused by observation errors, and the dependence of polarization on longitude, if any, is insignificant.

Figure 5.

Longitude dependence of polarization for Io in the V (left slide) and R (right slide) bands in the range of phase angles of 10°–12°.

Figure 6 shows a map of Io’s surface compiled from Galileo and Voyager data [22]. It clearly displays color variations and the nonuniform longitudinal distribution of volcanoes, including the largest centers, Pele, Gullan, and Loki. It is quite plausible that the transient activity of these centers can affect the properties of light reflected from Io’s surface.

Figure 6.

Cylindrical map of Io (north up), centered on the antijovian longitude (L = 180°). Major volcanic centers are labeled. Base map: USGS Astrogeology Science Center [22], derived from Galileo/Voyager mosaics.

Because of the intrinsically low spatial resolution of the aperture (disk-integrated) polarimetric method – since the measured polarization degree refers to the entire disk (±90° in longitude) at the instant of measurement – the distribution of polarization across the surface cannot be resolved in detail. Thus, even considering the scattering of data, the amplitude of the longitudinal dependence of Io’s polarization does not exceed 0.1%. However, the amplitude of Io’s LDP differs markedly from the value reported by Zellner and Gradie. Their result was likewise obtained from disk-integrated polarimetric measurements. This contradicts earlier estimates and indicates the variability of the longitudinal effect over large time intervals.

4.1.3 Ganymede

Ganymede is the largest satellite of Jupiter. Unlike Europa’s comparatively uniform surface, Ganymede’s surface is distinctly heterogeneous, partially ice-covered, and consists of a mixture of two terrain types. The dark, heavily cratered regions occupy roughly one-third of the surface. The brighter areas are younger and are covered with grooves and ridges [8, 19]. The leading hemisphere is brighter than the trailing one. At small phase angles, a brightness opposition effect (BOE) is observed.

New disk-integrated UBVRI polarimetric observations [15] show that Ganymede’s NPB is not bimodal: the curve is strongly asymmetric and has a single minimum, similar to Europa. In the V band, we obtained Pmin = −0.34% ± 0.01% at αmin = 0.52° ± 0.06°; the inversion occurs at αinv = 8.5° ± 0.2°. Figure 7 presents the phase dependences of Ganymede’s polarization degree in the VRI bands (left panels) and the longitudinal dependences after accounting for linear fits to the observed PPC of each hemisphere over α = 5° − 12° (right panels).

Figure 7.

Left panels: Phase dependences of Ganymede’s polarization degree in the VRI bands. Filled circles – data for the leading hemispheres, 0° ≤ L ≤ 180°; open circles – data for the trailing hemispheres, 180° < L < 360°. We combine results from literature [15] with additional observations obtained in the period 2022–2023. Right panels: Longitudinal dependences of Ganymede’s polarization after accounting for linear approximations to the observed PPC of each hemisphere over α = 5°−12°.

A clear longitudinal variation is revealed: the (absolute) polarization of the leading side exceeds that of the trailing side. The LDP amplitude increases with wavelength and, in the I band, reaches ∼0.15%, with the maximum near L∼270°. These features are interpreted as manifestations of genuine differences in regolith properties across the surface.

In sum, the combined photometry/spectroscopy and high-precision polarimetry yield a consistent picture: Ganymede is an icy satellite with a global longitudinal heterogeneity of its regolith. Its NPB has a single sharp minimum at small phase angles, and it exhibits stable longitudinal variations at the level of a few tenths of a percent, explainable by spatial variations in the fractions of ice and dark contaminants, grain size, and surface roughness.

4.1.4 Callisto

Polarimetric and photometric observations of Callisto have been carried out by many authors (see the review by Rosenbush et al. [23] and references therein). As a result, its photometric behavior, as well as the phase and longitudinal dependences of polarization, is well studied.

At phase angles α ≈2°−12°, the leading side is darker than the trailing one [7, 24, 25]. However, near opposition (α <2°), the brightnesses of the two hemispheres become comparable; consequently, the amplitude of the BOE for the leading hemisphere exceeds that for the trailing hemisphere. Following [23], we summarize the principal characteristics of Callisto’s polarized radiation needed for comparison with the other Galilean satellites.

The leading and trailing hemispheres differ markedly in the parameters of the NPB over different phase-angle intervals (see Figure 8, lower left panel). For the leading, darker hemisphere, |Pmin|, αmin, and αinv are larger than for the trailing hemisphere. Starting from α ≈3°, an inversion occurs – the degree of polarization of the leading side becomes systematically smaller in absolute value than that of the brighter trailing side, consistent with the change in the hemispheric brightness ratio.

Figure 8.

Top row: The observed degree of linear polarization for Callisto (V band) as a function of phase angle (the left-hand panel) and longitude of the satellite’s central meridian (the right-hand panel). Closed circles – data for the leading side (0° ≤ L ≤ 180°); open circles – data for the trailing side (180° < L < 360°). Bottom row: The left-hand panel – NPB for the leading and trailing hemispheres of Callisto after the correction for the orbital longitudinal variations. Solid curves represent the best fit to the data by a trigonometric expression of Lumme and Muinonen [26]. The right-hand panel – longitude dependence of polarization for Callisto in the V filter at the phase angle α = 6° after accounting for the phase-angle dependence of polarization.

After correcting the observed polarization (the procedure is described in Section 3 and in the study by Rosenbush et al. [23]), the LDP at a phase angle of 6° appears as a sinusoid-like function with an amplitude of about 0.2% and a polarization minimum of ≈−0.8% at a longitude of ≈90∘, that is, on the leading hemisphere.

Differences in the morphology of the NPBs of the two hemispheres and in their brightness amplitudes may be caused by differences in the microstructure and composition of their surface layers. The rather deep negative branch indicates that the surface is not only microscopically rough but also covered with fine, dark dust. High-resolution images of Callisto obtained by Galileo support this conclusion: the surface is heavily cratered, and much of the icy terrain is mantled by dark material. According to Klemaszewski et al. [27], these surface characteristics result from erosion of the upper layer due to the sublimation of volatiles.

4.2 Saturn’s satellites

4.2.1 Rhea

Rhea, the second-largest satellite of Saturn, is one of the high-albedo airless bodies of the solar system (ρv = 0.95) that exhibits a BOE. Its surface is covered with water ice [28, 29]. Images of the satellite obtained by the Voyager spacecraft showed that the surface of Rhea’s trailing hemisphere is geologically younger than that of the leading hemisphere. The leading hemisphere is brighter than the trailing one [30]. Therefore, one might expect the polarization degrees of the two hemispheres to differ. Polarimetric observations of Rhea are extremely scarce, owing to the challenging observing conditions caused by the satellite’s proximity to Saturn’s disk. Zaitsev [31] reported preliminary data on Rhea’s longitudinal polarization dependence (LDP) in the V band.

According to our data (see Figure 9), the amplitude of Rhea’s LDP in the V band does not exceed 0.02%, but it increases with wavelength and becomes significant at ≈0.1% and ≈0.15% in the R and I bands, respectively.

Figure 9.

Left panels: Polarization phase curves in the V (top), R (middle), and I (bottom) bands. Filled circles – data for the leading sides, 0° ≤ L ≤ 180°; open circles – data for the trailing sides, 180° < L < 360°. Right panels: Longitudinal dependences of Rhea’s polarization (LDP) after applying a linear approximation to the phase curve of each hemisphere over α = 2°−6°.

4.2.2 Dione

Dione (ρv = 0.998) is compositionally similar to Rhea, with heavily and weakly cratered regions. The leading side of the satellite is about 25% brighter than the trailing side, possibly due to intensive micrometeoritic bombardment on this hemisphere [32]. Kulyk [33] measured the polarization of Dione and Rhea and found that the PPCs of their leading and trailing hemispheres display deep minima of negative polarization (between −1.5% and −2.0%) at phase angles below 2°. However, with uncertainties of 0.1%–0.3% and limited coverage in phase angle and longitude, the available data were insufficient to define the phase dependence with confidence. Figure 10 shows the results of our measurements of the phase (left panel) and longitudinal (right panel) dependences of Dione’s polarization in the R band. Both figures indicate the absence of a noticeable longitudinal dependence of Dione’s polarization.

Figure 10.

Dione. Left panel – phase dependence of polarization in the R band. Filled circles – data for the leading side, 0° ≤ L ≤ 180°; open circles – data for the trailing side, 180° < L < 360°. Right panel – longitudinal dependence of Dione’s polarization after accounting for a linear approximation to the phase curve of each hemisphere over α = 3°−6°.

Despite the fact that Dione is similar to Rhea, there is no significant longitudinal dependence on the polarization.

4.2.3 Iapetus

Iapetus exhibits a pronounced albedo dichotomy: a dark leading hemisphere (geometric albedo ≈0.04) and a bright trailing hemisphere (≈0.39). On the bright side, a narrow polarization opposition effect (POE) is observed near opposition: at α = 0.77∘ the polarization is P ≈−0.9%, decreasing to ≈−0.3% by α ≈5.2° (in all BVRI bands), with a very weak spectral dependence [34]. On the dark side, by contrast, as the phase angle increases from ≈0.5° to ≈6.0°, the absolute value of the negative polarization grows from ≈0.5% to ≈1.3%, with the branch minimum near α ≈5.9°. Over the same phase interval, 3°−6°, the dark side is polarized (in absolute value) roughly three times more strongly than the bright side; for α <1°, the bright side is more deeply polarized, consistent with a POE for a high-albedo surface [35].

Scattering-model inversions using the radiative-transfer coherent-backscattering approach (RT–CB) and MSTM confirm differing surface properties on the two hemispheres. RT–CB yields geometric albedos of ∼0.40 for the bright (trailing) side and ∼0.08 − 0.10 for the dark (leading) side, with inversion angles αinv∼7° (bright) and ∼11°−13.5° (dark). MSTM fits indicate characteristic particle radii of ∼0.10−0.20 μm; for the dark material, an effective refractive index consistent with a mixture of Fe2O3 + Fe + H2O, whereas weakly absorbing ice suffices for the bright side. In the models, stronger absorption and/or reduced multiple scattering make the negative branch shallower, explaining the contrasting polarization behavior of the two hemispheres [36].

A strong LDP on Iapetus has long been noted, but on the leading side, it is difficult to separate from the phase trend because of geometric constraints (ideally, one needs a series at fixed phase and varying longitudes). This motivates continued polarimetric monitoring.

Figure 11 shows the phase and longitudinal dependences of Iapetus’s polarization in the V and R bands, obtained at CRAO and Peak Terskol Observatory and compiled from the literature [31, 35−37]. A linear approximation to the phase curve was fitted over α = 3°−6°; after removing this trend, the LDP is evaluated at a fixed phase angle of 4°.

Figure 11.

Phase (left panels) and longitude (right panels) dependencies of Iapetus’s polarization in the V and R bands.

After linear detrending of the phase curves over α = 3°−6°, the LDP is referenced to α = 4°. As seen in the figure, the degree of polarization of the leading, dark side at longitudes L ≈90° is about −1.3%, while for the trailing, bright side at L ≈270° it is about −0.3%. The large LDP amplitude reflects the strong albedo contrast between the two hemispheres, as illustrated in Figure 12.

Figure 12.

Map of Iapetus showing the distribution of dark (leading side) and bright terrains (trailing side) across the surface [37].

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5. Discussion

The principal photometric and polarimetric characteristics of the satellites’ hemispheres are summarized in Table 1.

Object Region of α Albedo’s dichotomy of surface Morphology of LDP
Jupiter family
Europa Positive branch: 5 ≤ α ≤ 12° <a= = 9° The leading side is brighter than the trailing side Polarization degree of the leading side is less than that of the trailing side
Io Negative branch: 10 ≤ α ≤ 12° Unknown The LDP is insignificant
Ganymede Negative branch: 5 ≤ α ≤ 12° < α > = 8° The leading side is brighter than the trailing side The |Pr| of the leading side is larger than that of the trailing side
Callisto Negative branch: < α > = 6° The leading side is darker than the trailing side The |Pr| of the leading side is larger than that of the trailing side
Negative branch: α ≤ 3° The leading side is brighter than the trailing side The longitudinal dependence is not determined due to the lack of a sufficient number of observations
Saturn family
Rhea Negative branch: 2 ≤ α ≤ 6° < α > = 4° The leading side is brighter than the trailing side The |Pr| of the leading side is smaller than that for the trailing side
Dione Negative branch: 2 ≤ α ≤ 6° < α > = 4° The leading side is brighter than the trailing side There is no longitudinal dependence
Iapetus Negative branch: 2 ≤ α ≤ 6° < α > = 4° The leading side is darker than the trailing side The |Pr| of the leading side is larger than that of the trailing side

Table 1.

Albedo’s dichotomy and morphology of LDP, observed for the leading and trailing sides of the selected Jupiter’s and Saturn’s satellites in the opposition region.

Composition (ratio of volatile to non-volatile components), surface structure (degree of cratering), magnetospheric processing, and the leading–trailing asymmetry of meteoritic erosion are the main factors shaping differences in regolith particle properties – and hence the observed photometric and polarimetric behavior. Because these factors may act in combination, disentangling them solely from observations is often difficult. Nonetheless, several patterns can be identified using the data in Table 1.

For Europa, the photometric and longitudinal polarization changes on the positive branch of the phase curve are consistent with Umov’s law [38, 39]. According to Buratti [25], for Europa, which was previously shown to have a less compacted trailing side, magnetospheric alterations predominate.

Io, by contrast, shows a small LDP: over α ≈4°−12° its amplitude did not exceed 0.1% during our observing interval, yet it was notably larger about 50 years ago. For a continually renewed volcanic surface with a sublimation–volcanic atmosphere, local and temporal changes in surface-layer composition can lead to both short-term and long-term variations in LDP.

Ganymede exhibits the most prominent LDP among the icy Galileans. The amplitude increases with wavelength and reaches ∼0.15% in the I band, while a net longitudinal modulation of ∼0.2% is robustly retrieved after removing P(α). The negative branch P(α) has a single minimum at small phase angles; together with independent constraints on the ice/dark-contaminant distribution, this indicates genuine longitudinal heterogeneity of the regolith.

Callisto’s phase curve does not display a POE at exact backscattering, but it shows substantial differences in the negative-branch parameters between the leading and trailing sides. Buratti [25] attributes the difference in Callisto to enhanced meteoritic erosion on the leading side. The amplitude of the orbital modulation grows with α. These traits are consistent with differing microtexture/albedo of the two hemispheres’ regolith and point to marked longitudinal optical heterogeneity.

For Saturn’s satellite Rhea, despite the challenging proximity to Saturn’s disk, earlier work noted substantial absolute minima of P(α) at small phase angles and preliminary indications of longitudinal variations – underscoring the need for systematic longitude coverage at fixed α.

Unlike Rhea, Dione has not been found to exhibit an LDP, at least in the R band. At the same time, both satellites have leading hemispheres that are lighter than their trailing ones.

For Iapetus, a narrow POE is found on the bright (high-albedo) side at small α, whereas on the dark side, the negative branch is deep and strengthens with phase; this demonstrates that the LDP here directly reflects the albedo dichotomy and the associated differences in regolith microphysics.

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6. Conclusion

The analysis of high-precision polarimetric observations from 2018 to 2025, obtained with the two-channel POLSHAKH polarimeters, demonstrates convincingly that a number of airless satellites of Jupiter and Saturn exhibit longitudinal variability of surface polarization (LDP) superposed on the phase dependence P(α). The adopted decomposition of P(α, L) into a phase term and an orbital sinusoidal modulation, followed by linearization over a narrow α-range, enables the extraction of LDP amplitudes at the level of a few hundredths of a percent and allows a like-for-like comparison of the leading and trailing hemispheres at similar phase angles.

Overall, the results support the following:

  1. The LDP is an informative diagnostic of optical and structural inhomogeneities in the regolith of airless bodies.

  2. The amplitude and spectral behavior of the LDP differ substantially among satellites and correlate with geological and geophysical factors (icy versus dark contaminants, volcanism, albedo dichotomies, magnetospheric processing, and meteoritic erosion).

  3. Robust reconstruction of the polarization phase dependence (PDP) and the LDP from the observed P(α, L) requires multiband time series spanning wide ranges of phase angles and longitudes – especially critical for systems with temporally variable surfaces (e.g., Io) and for objects with a limited observable phase domain (Saturnian satellites).

These conclusions set the stage for further joint analyses of photometry, spectroscopy, and polarimetry, combined with physically consistent scattering models (RT–CB), to retrieve regolith microstructural parameters and to map disk-resolved optical inhomogeneities.

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Written By

Nikolai Kiselev and Marina Shcherbina

Submitted: 15 September 2025 Reviewed: 09 October 2025 Published: 04 March 2026

© 2026 The Author(s). Licensee IntechOpen. This content is distributed under the terms of the Creative Commons Attribution 4.0 International License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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