Moon Iapetus Research Articles

Microwave spectra of the leading and trailing hemispheres of Iapetus

The leading hemisphere of Saturn’s synchronous moon Iapetus is covered by a low-albedo material, contrasting with its bright trailing hemisphere. This dichotomy is also apparent in radar and microwave radiometry observations, which are sensitive to the properties of the near subsurface. To better understand the regional properties of Iapetus and their variations with depth, we assemble the microwave spectra of its leading and trailing hemispheres. Pre-existing data are combined with new millimetric and centimetric observations acquired with the IRAM 30-meter dish, IRAM NOEMA interferometer, and VLA interferometer. These data, interpreted with the help of a model with vertically uniform thermal properties, reveal complex variations in structure and/or composition with depth on the leading side. Meanwhile, the trailing side emissivity is found to be especially low at all observed frequencies, indicating efficient scattering processes on subsurface structures, as observed on Saturn’s other icy moons. We also report the first observations of Saturn’s retrograde moon Phoebe at these frequencies, which has an emissivity higher than that of the trailing hemisphere of Iapetus and similar to its dark leading side, consistent with the theory that Phoebe is the source of the dark material on Iapetus.

The morphology and age of the Iapetus equatorial ridge supports an exogenic origin

Both endogenic and exogenic formation mechanisms have been proposed for the equatorial ridge on Saturn's moon Iapetus. With photogeological mapping and crater statistics, we find that the morphology of the ridge is best explained by an exogenic origin, principally by the accretion onto the moon's surface of an orbiting ring of material.

Probing the subsurface of the two faces of Iapetus

Saturn’s moon Iapetus, which is in synchronous rotation, is covered by an optically dark material mainly on its leading side, while its trailing side is significantly brighter. Because longer wavelengths probe deeper into the subsurface, observing both sides at a variety of wavelengths brings to light possible changes in thermal, compositional, and physical properties with depth. We have observed Iapetus’s leading and trailing hemispheres at 1.2 and 2.0 mm, using the NIKA2 camera mounted on the IRAM 30-m telescope, and compared our observations to others performed at mm to cm wavelengths. We calibrate our observations on Titan, which is simultaneously observed within the field of view. Due to the proximity of Saturn, it is sometimes difficult to separate Iapetus’s and Titan’s flux from that of Saturn, detected in the telescope’s side lobes. Preliminary results show that the trailing hemisphere brightness temperatures at the two wavelengths are equal within error bars, unlike the prediction made by Ries (2012)[1]. On the leading side, we report a steep spectral slope of increasing brightness temperature (by 10 K) from 1.2 to 2.0 mm, which may indicate rapidly varying emissivities within the top few centimeters of the surface. Comparison to a diffuse scattering model and a thermal model will be necessary to further constrain the thermophysical properties of the subsurface of Iapetus’s two faces.

Secular Resonance Between Iapetus and the Giant Planets

Using numerical integrations, we find that the orbital eccentricity of Saturn's moon Iapetus undergoes prominent multi-Myr oscillations. We identify the responsible resonant argument to be $\varpi-\varpi_{g5}+\Omega-\Omega_{eq}$, with the terms being the longitudes of pericenter of Iapetus and planetary secular mode $g_5$, Iapetus's longitude of the node and Saturn's equinox. We find that this argument currently (on a $10^7$~yr timescale) appears to librate with a very large amplitude. On longer timescales, the behavior of this resonant angle is strongly dependent on the resonant interaction between Saturn's spin axis and the planetary mode $f_8$, with long-term secular resonance being possible if Saturn's equinox is librating relative to the node of the $f_8$ eigenmode. We present analytical estimates of the dependence of the resonant argument on the orbital elements of Iapetus. We find that this Iapetus-$g_5$ secular resonance could have been established only after the passage of Iapetus through the 5:1 mean-motion resonance with Titan, possibly in the last Gyr. Using numerical simulations, we show that the capture into the secular resonance appears to be a low-probability event. While the Iapetus-$g_5$ secular resonance can potentially help us put new constraints on the past dynamics of the Saturnian system, uncertainties in both the spin axis dynamics of Saturn and the tidal evolution rate of Titan make it impossible to make any firm conclusions about the resonance's longevity and origin.

A Nonequilibrium Figure of Saturn’s Satellite Iapetus and the Origin of the Equatorial Ridge on Its Surface

The structure, dynamical equilibrium, and evolution of Saturn’s moon Iapetus are studied. It has been shown that, in the current epoch, the oblateness of the satellite e2 ≈ 0.046 does not correspond to its angular velocity of rotation, which causes the secular spherization behavior of the ice shell of Iapetus. To study this evolution, we apply a spheroidal model, containing a rock core and an ice shell with an external surface e2, to Iapetus. The model is based on the equilibrium finite-difference equation of the Clairaut theory, while the model parameters are taken from observations. The mean radius of the rock core and the oblateness of its level surface, e1 ≈ 0.028, were determined. It was found that Iapetus is covered with a thick ice shell, which is 56.6% of the mean radius of the figure. We analyze a role of the core in the evolution of the shape of a gravitating figure. It was determined that the rock core plays a key part in the settling of the ice masses of the equatorial bulge, which finally results in the formation of a large circular equatorial ridge on the surface of the satellite. From the known mean altitude of this ice ridge, it was found that, in the epoch of its formation, the rotation period of Iapetus was 166 times shorter than that at present, as little as T ≈ 11h27m. This is consistent with the fact that a driving force of the evolution of the satellite in our model was its substantial despinning. The model also predicts that the ice ridge should be formed more intensively in the leading (dark and, consequently, warmer) hemisphere of the satellite, where the ice is softer. This inference agrees with the observations: in the leading hemisphere of Iapetus, the ridge is actually high and continuous everywhere, while it degenerates into individual ice peaks in the opposite colder hemisphere.

Global albedos of Pluto and Charon from LORRI New Horizons observations

The exploration of the Pluto-Charon system by the New Horizons spacecraft represents the first opportunity to understand the distribution of albedo and other photometric properties of the surfaces of objects in the Solar System's “Third Zone” of distant ice-rich bodies. Images of the entire illuminated surface of Pluto and Charon obtained by the Long Range Reconnaissance Imager (LORRI) camera provide a global map of Pluto that reveals surface albedo variegations larger than any other Solar System world except for Saturn's moon Iapetus. Normal reflectances on Pluto range from 0.08–1.0, and the low-albedo areas of Pluto are darker than any region of Charon. Charon exhibits a much blander surface with normal reflectances ranging from 0.20–0.73. Pluto's albedo features are well-correlated with geologic features, although some exogenous low-albedo dust may be responsible for features seen to the west of the area informally named Tombaugh Regio. The albedo patterns of both Pluto and Charon are latitudinally organized, with the exception of Tombaugh Regio, with darker regions concentrated at the Pluto's equator and Charon's northern pole. The phase curve of Pluto is similar to that of Triton, the large moon of Neptune believed to be a captured Kuiper Belt Object (KBO), while Charon's is similar to that of the Moon. Preliminary Bond albedos are 0.25 ± 0.03 for Charon and 0.72 ± 0.07 for Pluto. Maps of an approximation to the Bond albedo for both Pluto and Charon are presented for the first time. Our work shows a connection between very high albedo (near unity) and planetary activity, a result that suggests the KBO Eris may be currently active.

Long-runout landslides and the long-lasting effects of early water activity on Mars: COMMENT

Long-runout landslides and the long-lasting effects of early water activity on Mars: COMMENT

Radial profiles of the Phoebe ring: A vast debris disk around Saturn

We present observations at optical wavelengths with the Cassini Spacecraft’s Imaging Science System of the Phoebe ring, a vast debris disk around Saturn that seems to be collisionally generated by its irregular satellites. The analysis reveals a radial profile from 80–260 Saturn radii (RS) that changes behavior interior to ≈110RS. We attribute this to either the moon Iapetus...sweeping up small particles, or to orbital instabilities that cause the ring to flare up vertically. Our study yields an integrated I/F at 0.635 µm along Saturn’s shadow in the Phoebe ring’s midplane from 80–250 RS of 2.7−0.3+0.9×10−9. We develop an analytical model for the size-dependent secular dynamics of retrograde Phoebe ring grains, and compare this model to the observations. This analysis implies that 1) the “Phoebe” ring is partially sourced by debris from irregular satellites beyond Phoebe’s orbit and 2) the scattered light signal is dominated by small grains (≲20 µm in size). If we assume that the Phoebe ring is generated through steady-state micrometeoroid bombardment, this implies a power-law size distribution with index >4, which is unusually steep among Solar System rings. This suggests either a steep size distribution of ejecta when material is initially released, or a subsequent process that preferentially breaks up large grains.

Despinning and shape evolution of Saturn’s moon Iapetus triggered by a giant impact

Iapetus possesses two spectacular characteristics: (i) a high equatorial ridge which is unique in the Solar System and (ii) a large flattening (a-c=34km) inconsistent with its current spin rate. These two main characteristics have probably been acquired in Iapetus’ early past as a consequence of coupled interior-rotation evolution. Previous models have suggested that rapid despinning may result either from enhanced internal dissipation due to short-lived radioactive elements or from interactions with a sub-satellite resulting from a giant impact. For the ridge formation, different exogenic and endogenic hypotheses have also been proposed, but most of the proposed scenarios have not been tested numerically. In order to model simultaneously internal heat transfer, tidal despinning and shape evolution, we have developed a two-dimensional axisymmetric thermal convection code with a deformable surface boundary, coupled with a viscoelastic code for tidal dissipation. The model includes centrifugal and buoyancy forces, a composite non-linear viscous rheology as well as an Andrade rheology for the dissipative part. By considering realistic rheological properties and by exploring various grain size values, we show that, in the absence of additional external interactions, despinning of a fast rotating Iapetus is impossible even for warm initial conditions (T>250K). Alternatively, the impact of a single body with a radius of 250–350km at a velocity of 2km/s may be sufficient to slow down the rotation from a period of 6–10h to more than 30h. By combining despinning due to internal dissipation and an abrupt change of rotation due to a giant impact, we determined the parameters leading to a complete despinning and we computed the corresponding shape evolution. We show that stresses arising from shape change affect the viscosity structure by enhancing dislocation creep and can lead to the formation of a large-scale ridge at the equator as a result of rapid rotation change for initial rotation periods of 6h.

EXCITATION OF THE ORBITAL INCLINATION OF IAPETUS DURING PLANETARY ENCOUNTERS

Saturn's moon Iapetus has an orbit in a transition region where the Laplace surface is bending from the equator to the orbital plane of Saturn. The orbital inclination of Iapetus to the local Laplace plane is ~8 deg, which is unexpected, because the inclination should be ~0 if Iapetus formed from a circumplanetary disk on the Laplace surface. It thus appears that some process has pumped up Iapetus's inclination while leaving its eccentricity near zero (e=0.03 at present). Here we examined the possibility that Iapetus's inclination was excited during the early solar system instability when encounters between Saturn and ice giants occurred. We found that the dynamical effects of planetary encounters on Iapetus's orbit sensitively depend on the distance of the few closest encounters. In four out of ten instability cases studied here, the orbital perturbations were too large to be plausible. In one case, Iapetus's orbit was practically unneffected. In the remaining five cases, the perturbations of Iapetus's inclination were adequate to explain its present value. In three of these cases, however, Iapetus's eccentricity was excited to 0.1-0.25, and it is not clear whether it could have been damped to its present value (0.03) by some subsequent process (e.g., tides and dynamical friction from captured irregular satellites do not seem to be strong enough). Our results therefore imply that only 2 out of 10 instability cases (~20%) can excite Iapetus's inclination to its present value (~30% of trials lead to >5 deg) while leaving its orbital eccentricity low.

Topographic constraints on the origin of the equatorial ridge on Iapetus

Saturn’s moon Iapetus has an equatorial ridge system, which may be as high as 20km, that may have formed by endogenic forces, such as tectonic and convective forces, or exogenic processes such as debris infall. We use high-resolution topographic data to conduct a topographic analysis of the ridge, which suggests a predominantly triangular morphology, with some ridge face slopes reaching 40°, allowing for an exogenic formation mechanism.

Polarization of Saturn’s moon Iapetus

Context. Like many other atmosphereless solar system bodies, Iapetus exhibits a phenomenon of negative polarization at small phase angles, which can be modeled using theoretical approaches that consider interaction of light with a complex medium.Aims. To retrieve information on the nature of Iapetus’ surface material, we carried out theoretical modeling analyses for the observed polarization of its two sides.Methods. We applied two light-scattering models. The first modeling approach is based on the utilization of the phenomenological single-particle scattering matrix parametrization using the double Henyey-Greenstein (2HG) scattering phase function to characterize the resulting multiple scattering by a medium composed of such discrete scatterers. With this approach we carried out radiative-transfer coherent-backscattering (RT-CB) computations for a random medium composed of phenomenological fundamental scatterers. The second model, called the multiple sphere T -matrix method, is based on the exact solutions of the Maxwell equations. Employing this method, we carried out simulations of the scattering and absorption properties of light by a medium represented by a spherical volume of randomly positioned monodisperse particles. The modeling entails physical characteristics of the particulate surface, such as the porosity of the medium; the number of constituent particles; the size, and optical properties of the scatterers. Results. While our RT-CB model suggests geometric albedo values in the neighborhood of 0.40 for Iapetus’ trailing side and ~0.10 for the leading one, our T -matrix model retrieves particles of radius ~0.10 ≤ r ≤ 0.20 μ m for both Iapetus’ leading and trailing surface materials.

Polarization of Saturn’s moon Iapetus

Context. The surface properties of atmosphereless solar system objects can be constrained by investigating the nature of the polarized light scattered from their surfaces. Aims. We provide new and precise measurements of the phase angle and the wavelength dependence of linear polarization on the leading side of Iapetus over the maximum phase angle range accessible from Earth (∼0.5−6.0 ◦ ) and over a broad spectral range (400–800 nm), thereby identifying the polarimetric characteristics of its dark material. Moreover, we provide circular spectral polarization measurement of the same side of Iapetus at one phase angle, which was performed aiming at detection of chiral signatures on its surface. We also compare our new polarization measurements with those of the trailing hemisphere of Iapetus which were obtained in our previous work. Methods. Using the FORS2 instrument of the ESO VLT, we performed spectro-polarimetric observations of Iapetus’ leading side. Results. While the linear polarization measurements of Iapetus’ leading side show an opposite trend in phase angle dependence to that of its trailing hemisphere, the polarization values measured for the two hemispheres around similar phase angles (in the range ∼3.0−6.0 ◦ )d iffer by a factor of three. Besides this, the degree of negative linear polarization of Iapetus’ leading hemisphere shows a slight dependence on the wavelengths of observations. Furthermore, like that of its trailing hemisphere, the circular polarization measurement of Iapetus’ leading side indicates no evidence of a significant amount of optically active (chiral) molecules on its surface.

Saturn's moon Iapetus may hold secrets behind unusual landslides

Saturn's moon Iapetus may hold secrets behind unusual landslides

Massive ice avalanches on Iapetus mobilized by friction reduction during flash heating

The great distance travelled by long-runout landslides, observed previously on the Earth and Mars, requires a mechanism of friction reduction. Identification and analysis of long-runout landslides on Saturn’s moon Iapetus suggests that the Iapetian landslides are enabled by flash heating of the icy sliding surface.

How did the equatorial ridge on Saturn's moon Iapetus form?

Saturn's moon Iapetus is one of the most unusual moons in our solar system. Perhaps the most bizarre feature of Iapetus is its equatorial ridge, a 20‐kilometer‐high, 200‐kilometer‐wide mountain range that runs exactly along the equator, circling more than 75% of the moon. No other body in the solar system exhibits such a feature; asDombard et al. show, previous models have been unable to adequately explain how the ridge formed. The authors propose that the ridge formed from an ancient giant impact that produced a subsatellite around Iapetus. Tidal interactions with Iapetus ultimately led to orbital decay, eventually bringing the subsatellite close enough that the same forces tore it apart, forming a debris ring around Iapetus. Material from this debris ring then rained down on Iapetus, the researchers say, creating the mountain ring along the equator. (Journal of Geophysical Research‐Planets, doi:10.1029/2011JE004010, 2012)

Delayed formation of the equatorial ridge on Iapetus from a subsatellite created in a giant impact

The great equatorial ridge on Saturn's moon Iapetus is arguably the most perplexing landform in the solar system. The ridge is a mountain range up to 20 km tall and sitting on the equator of Iapetus, and explaining its creation is an unresolved challenge. Models of its formation must satisfy three critical observations: why the ridge (1) sits exactly on the equator, (2) is found only on the equator, and (3) is thus far found only on Iapetus. We argue that all previously proposed models fail to satisfy these observations, and we expand upon our previous proposal that the ridge ultimately formed from an ancient giant impact that produced a subsatellite around Iapetus. The orbit of this subsatellite would then decay, once Iapetus itself had despun due to tides raised by Saturn, until tidal forces from Iapetus tore the subsatellite apart. The resultant debris formed a transient ring around Iapetus, the material of which rained down on the surface to build the ridge. By sequestering the material in a subsatellite with a tidally evolving orbit, formation of the ridge is delayed, which increases the likelihood of preservation against the high‐impact flux early in the solar system's history and allows the ridge to form on thick, stiff lithosphere (heat flow likely <1 mW m−2) required to support this massive load without apparent flexure. This mechanism thus explains the three critical observations.

Spectro-polarimetry of the bright side of Saturn’s moon Iapetus

Measurements of the polarized reflected sunlight from atmosphereless solar system bodies, over a range of phase angles, provide information about the surface structure and composition. With this work, we provide analysis of the polarimetric observations of the bright side of Iapetus at five different phase angles, and over the full useful wavelength range (400-800nm), so as to assess the light scattering behaviour of a typical surface water ice. Using FORS2 of the ESO VLT, we have performed linear spectro-polarimetric observations of Iapetus' bright side from 2009 to 2011 at five different phase angles, in the range from 0.80-5.20^{0}, along with circular spectro-polarimetric observations at one phase angle. By measuring, with high accuracy (~0.1% per spectral bin for each Stokes parameter), the spectral polarization of the bright trailing hemisphere of Saturn's moon Iapetus, we have identified the polarimetric characteristics of water ice, and found that its linear degree of negative polarization decreases with increasing phase angle of observation (varying from -0.9% to -0.3%), with a clear dependence on wavelengths of observation.

Tectonics on Iapetus: Despinning, respinning, or something completely different?

Saturn’s moon Iapetus is unique in that it has apparently despun while retaining a substantial equatorial bulge. Stresses arising from such a non-hydrostatic shape should in principle cause surface deformation (tectonics). As part of a search for such a tectonic signature, lineaments (linear surface features) on Iapetus were mapped on both its bright and dark hemispheres. Lineament orientations were then compared to model stress patterns predicted for spin-down from a rotation period of 16.5 h (or less) to its present synchronous period, and for a range of lithospheric thicknesses. Many lineaments are straight segments of crater rimwalls, which may be faults or joints reactivated during complex crater collapse. Most striking are several large troughs on the bright, trailing hemisphere. These troughs appear to be extensional and are distinctive on that hemisphere, because the interior floors and walls of the troughs contain dark material. Globally, no specific evidence of strike slip or thrust offsets are seen, but this could be due to the age and degraded nature of any such features. We find that observed lineament orientations do not correlate with predicted patterns due to despinning on either hemisphere (the equatorial ridge was specifically excluded from this analysis, and is considered separately). Modest evidence for preferred orientations ±40° from north could be construed as consistent with respinning, which is not necessarily far-fetched. Assuming the rigidity of unfractured ice, predicted maximum lithospheric differential stresses from despinning range from ∼1 MPa to ∼160 MPa for the elastic spheroid and thin lithosphere limits, respectively (although it is only for thicker elastic lithospheres that we expect a nonhydrostatic state to be maintained over geologic time against lithospheric failure). The tectonic signature of despinning may have been obscured over time because the surface of Iapetus is very ancient, Iapetus’ thick lithosphere may have inhibited the full tectonic expression of despinning, or both. Several prominent lineaments strike E–W, and are thus parallel to the equatorial ridge (though not physically close to it), but a tectonic or volcanic origin for the ridge is highly problematic.

Finding the trigger to Iapetus’ odd global albedo pattern: Dynamics of dust from Saturn’s irregular satellites

The leading face of Saturn’s moon Iapetus, Cassini Regio, has an albedo only one tenth that on its trailing side. The origin of this enigmatic dichotomy has been debated for over 40 years, but with new data, a clearer picture is emerging. Motivated by Cassini radar and imaging observations, we investigate Soter’s model of dark exogenous dust striking an originally brighter Iapetus by modeling the dynamics of the dark dust from the ring of the exterior retrograde satellite Phoebe under the relevant perturbations. In particular, we study the particles’ probabilities of striking Iapetus, as well as their expected spatial distribution on the Iapetian surface. We find that, of the long-lived particles (≳5 μm), most particle sizes (≳10 μm) are virtually certain to strike Iapetus, and their calculated distribution on the surface matches up well with Cassini Regio’s extent in its longitudinal span. The satellite’s polar regions are observed to be bright, presumably because ice is deposited there. Thus, in the latitudinal direction we estimate polar dust deposition rates to help constrain models of thermal migration invoked to explain the bright poles (Spencer, J.R., Denk, T. [2010]. Science 327, 432–435). We also analyze dust originating from other irregular outer moons, determining that a significant fraction of that material will eventually coat Iapetus—perhaps explaining why the spectrum of Iapetus’ dark material differs somewhat from that of Phoebe. Finally we track the dust particles that do not strike Iapetus, and find that most land on Titan, with a smaller fraction hitting Hyperion. As has been previously conjectured, such exogenous dust, coupled with Hyperion’s chaotic rotation, could produce Hyperion’s roughly isotropic, moderate-albedo surface.