Rotationally-resolved spectra of Ceres in the 3-μm region     

Andrew S. Rivkin  Eric L. Volquardsen 《Icarus》2010,206(1):327-333

We present observations of Ceres over the 2.2-4.0 μm region taken using the SpeX instrument on the NASA IRTF in 2005. The observations cover Ceres’ entire longitude range and show evidence for a relatively uniform surface in terms of Ceres’ composition, however there is a subtle but consistently shallower band depth over longitudes associated with bright regions in HST maps, suggesting those areas are slightly less carbonate- and brucite-rich. We also find Ceres’ beaming parameter, a measure of its thermal properties, to have changed with its viewing aspect.  相似文献   

Ceres - Neither a porous nor salty ball     

Julie C. Castillo-Rogez 《Icarus》2011,215(2):599-602

This study explores the geophysical implications of two compositional models recently proposed for Ceres, which assume that the dwarf planet is a homogeneous mixture of chondritic material devoid with free water. In order to reproduce Ceres’ density, the rock density has to be offset by the presence of porosity and/or an abundance of hydrated salts resulting from the extensive hydration and oxidation of the chondritic material. Thermal modeling shows that a mixture of hydrated minerals is bound to compact and partly dehydrate as a consequence of long-lived radioisotope decay heat. The resulting interior structure is differentiated in a silicate-rich core and water-rich shell, with little porosity. Hence, this study confirms previous suggestion that Ceres contains a large fraction of free water.  相似文献   

On the composition and differentiation of Ceres     

Mikhail Yu. Zolotov 《Icarus》2009,204(1):183-193

The dwarf planet Ceres has a density of 2040-2250 kg m⁻³, and a dark non-icy surface with signs of hydrated minerals. As opposed to a differentiated internal structure with a nonporous rocky core and a water mantle, there are arguments for undifferentiated porous interior structure. Ceres’ mass and dimensions are uncertain and do not exclude undifferentiated interior even if hydrostatic equilibrium is attained. The rocky surface may be inconsistent with a large-scale water-rock differentiation. A differentiated structure with a thick water mantle below a rocky crust is gravitationally unstable and an overturn would have led to abundant surface salt deposits, which are not observed. A formation of hydrated surface minerals caused by internal heating implies a major density increase through devolatilization of the interior. A later accumulation of hydrated materials is inconsistent with anhydrous surfaces of many asteroids and with a low rate of the cosmic dust deposition in the inner Solar System. Ceres’ internal pressures (<140-200 MPa) are insufficient to significantly reduce porosity of chondritic materials and there is no need for abundant water phases to be present to account for the bulk density. Having the porosity of ordinary chondrites (∼10%), Ceres can consist of rocks with the grain density of pervasively hydrated CI carbonaceous chondrites. However, additional low-density phases (e.g., water ice) require to be present in the body with the grain density of CM chondrites. The likely low-density mineralogy of the interior implies Ceres’ accretion from pervasively aqueously altered carbonaceous planetesimals depleted in short-lived radionuclide ²⁶Al. Abundant water ice may not have accreted. Limited heat sources after accretion may not have caused major mineral dehydration leading to formation of water mantle. These inferences can be tested with the Dawn spacecraft in 2015.  相似文献   

Water,heat, bombardment: The evolution and current state of (2) Pallas     

Britney Elyce Schmidt  Julie C. Castillo-Rogez 《Icarus》2012,218(1):478-488

Using recent constraints on the shape and density of (2) Pallas, we model the thermal evolution of the body as a function of possible formation scenarios that differ in the time of formation and composition assumed for the protoplanet. We develop possible evolution scenarios for Pallas and compare these to available observations. Our models imply two distinct types of end states: those with a hydrosphere and silicate core, and those where the body is dominated by hydrated silicates. We show that for an initial ice-rock mixture with density 2400 kg/m³, Pallas is likely to differentiate and form a rocky core and icy shell. If Pallas accreted from material with lower initial ice content, our models indicate that Pallas’s interior is dominated by hydrated silicates, possibly with a core of anhydrous silicates.We also investigate the possibility that Pallas’s initial density was similar to Ceres’, i.e., that it formed from an ice–rock mixture of density 2100 kg/m³. This implies that the object lost a significant fraction of its hydrosphere as a consequence of thermal oscillations and impacts, a distinct possibility given its density, evidence for impact excavation and current orbital parameters. Its blue spectral slope and observed surface variation may also be evidence for such a process (e.g. Jewitt, D.C. [2002]. Astron. J. 123, 1039–1049; Schmidt, B.E. et al. [2009]. Science 326, 275–279; Yang, B., Jewitt, D. [2010]. Astron. J. 140, 692–698). If Pallas still contains a thin layer of water ice, then that layer corresponds to the bottom of a former icy shell, and as such, could be enriched in non-ice materials such as organics. We evaluate the likeliness of each scenario and show the general magnitude of water loss processes for Pallas. Given a balance of observational and theoretical constraints, we favor a water-rich accretion for Pallas that implies that Pallas has lost a significant fraction of its initial water content through exogenic processes since its internal evolution ceased. We also discuss implications of this work to other hydrated asteroids.  相似文献   

Asteroids 1 ceres and 4 vesta: Objects of the Dawn space mission     

D. F. Lupishko  R. A. Mohamed 《Solar System Research》2009,43(6):475-482

The Dawn spacecraft of the NASA space mission to asteroids 1 Ceres and 4 Vesta was launched in September 2007. The choice of these two asteroids is deeply grounded: they are the largest and most massive objects of the main belt that are completely different in material composition, evolution history, and internal structure. Recently, the results of observations and numerical modeling have shown their amazing uniqueness: they both have experienced the complex process of thermal evolution and differentiation of their internal mineral resources, but have a completely different internal structure. Being the largest bodies, have they managed to resist the process of collisional evolution in the asteroid belt and have survived in their “primitive form.” Because of this, their study is very important from the point of view of cosmogonic problems regarding the asteroid belt and the Solar System as a whole. The present paper shortly reviews the recent progress in the study of Ceres and Vesta achieved due to observations performed on the Earth (including the polarimetric observations made by the authors) and from the Hubble Space Telescope (HST) before the long-term orbital investigations performed by the Dawn spacecraft.  相似文献   

Ceres internal structure from geophysical constraints          下载免费PDF全文

Scott D. King  Julie C. Castillo‐Rogez  M. J. Toplis  Michael T. Bland  Carol A. Raymond  Christopher T. Russell 《Meteoritics & planetary science》2018,53(9):1999-2007

Thermal evolution modeling has yielded a variety of interior structures for Ceres, ranging from a modestly differentiated interior to more advanced evolution with a dry silicate core, a hydrated silicate mantle, and a volatile‐rich crust. Here we compute the mass and hydrostatic flattening from more than one hundred billion three‐layer density models for Ceres and describe the characteristics of the population of density structures that are consistent with the Dawn observations. We show that the mass and hydrostatic flattening constraints from Ceres indicate the presence of a high‐density core with greater than a 1σ probability, but provide little constraint on the density, allowing for core compositions that range from hydrous and/or anhydrous silicates to a mixture of metal and silicates. The crustal densities are consistent with surface observations of salts, water ice, carbonates, and ammoniated clays, which indicate hydrothermal alteration, partial fractionation, and the possible settling of heavy sulfide and metallic particles, which provide a potential process for increasing mass with depth.  相似文献   

Glaciopanspermia: Seeding the terrestrial planets with life?     

Joop M. Houtkooper 《Planetary and Space Science》2011,59(10):1107-1111

The question whether life originated on Earth or elsewhere in the solar system has no obvious answer, since Earth was sterilized by the Moon-forming impact and possibly also during the LHB, about 700 Ma after the formation of the solar system. Seeding by lithopanspermia has to be considered. Possible sources of life include Earth itself, Mars, Venus (if it had a more benign climate than today) and icy bodies of the solar system. The first step of lithopanspermia is the ejection of fragments of the surface into space, which requires achieving at least escape velocity. As the velocity distribution of impact ejecta falls off steeply, attention is drawn to bodies with lower escape velocities. Ceres has had, or still has, an ocean more than 100 km deep, with hydrothermal activity at its rocky core. The possible presence of life, its relative closeness to the terrestrial planets and Ceres' low escape velocity of 510 m/s suggest that Ceres could well be a parent body for life in the solar system.Icy impact ejecta - hence glaciopanspermia - from Ceres will be subject to evaporation of volatiles. Spores may be loosened by evaporation and enter the atmospheres of the terrestrial planets as micrometeorites.The seeding of the terrestrial planets from Ceres would result in (1) detection of life in the crustal layers of Ceres; (2) a commonality of Cerean life with Terran and possible Martian and Venusian life and (3) biomarkers of Cerean life, which might be found in the ice at the Moon's poles and on the surface of other main belt asteroids.  相似文献   

Ceres’s internal evolution: The view after Dawn          下载免费PDF全文

T. B. McCord  J. C. Castillo‐Rogez 《Meteoritics & planetary science》2018,53(9):1778-1792

As the Dawn mission approaches a successful conclusion at Ceres, it seems time to assess how its findings have sharpened the picture of Ceres’s evolution. Before Dawn, we inferred from Ceres's bulk density of about 2100 kg m^?3 that Ceres contained about 25% water by mass. Thermodynamic modeling of the interior evolution suggested that the original accreted ice had to melt even if only long‐lived radionuclides were present, leading to the aqueous alteration of the original chondritic silicates and differentiation of the altered silicates from any remaining water, consistent with telescopic detection of aqueously altered silicates (serpentine and clay minerals) on Ceres’s surface. Earth‐based observations of Ceres’s shape were not accurate enough to constrain the extent of differentiation of its interior. Dawn's results confirm these early findings and extend them dramatically to reveal an evolved and active small planet, probably even today, due to water/ice‐driven processes. A nearly uniform global distribution of surface mineralogy, which includes Mg‐serpentines, ammoniated clays, and salts including carbonates, suggests extensive, endogenous, planet‐wide aqueous alteration. Local exceptions show salt‐rich deposits of varied composition, which suggests subsurface heterogeneities. Concentration of Fe below carbonaceous chondrite levels suggests chemical fractionation, leading to Ceres being chemically differentiated. The high spatial uniformity of element abundance measurements of equatorial regolith also indicates that some ice‐rock fractionation occurred on a global scale. Even some local exposures of ice are seen, especially in higher latitudes and in low‐illumination regions that must be very young, as surface water ice is unstable on time scales of 1–1000 years under Ceres’s surface temperatures. Subsurface ice is also likely in abundance at higher latitudes in at least the upper few meters of the surface, as suggested by near‐surface H‐rich polar deposits. Observations of bright ice deposits in permanently shadowed regions suggest cold‐trapping of migrating H₂O across the surface. Gravity field measurements indicate a concentration of mass toward the center and near isostatic equilibrium, consistent with at least some mass differentiation driven by water‐related processes. Abundant small and midsize craters but relaxed or missing large craters suggest a stiff upper crust with water abundance lower than 30 vol%. A sharp decrease in viscosity at ~40 km depth suggests the occurrence of a small fraction of liquid, consistent with earlier thermophysical models. Surface cryogenic features, such as flows, extrusions, and domes, some geologically very recent, are evidence of active water/ice‐driven subsurface processes. Ceres experienced extensive water‐related processes and at least some mass and chemical fractionation and is probably active today, consistent with previous moderate heating thermodynamic models. Clearly, Ceres is a “wet,” evolved planet at the edge of the inner solar system, as described in this special issue. We conclude with a list of questions suggested by the Dawn findings; they especially regard the state and fate of water and its role in driving past and possibly current chemical and physical activity in this dwarf planet.  相似文献   

Long-term and large-scale viscous evolution of dense planetary rings     

J. Salmon  S. Charnoz 《Icarus》2010,209(2):771-785

Planetary rings are common in the outer Solar System but their origin and long-term evolution is still a matter of debate. It is well known that viscous spreading is a major evolutionary process for rings, as it globally redistributes the disk’s mass and angular momentum, and can lead to the disk’s loosing mass by infall onto the planet or through the Roche limit. However, describing this process is highly dependent on the model used for the viscosity. In this paper we investigate the global and long-term viscous evolution of a circumplanetary disk. We have developed a simple 1D numerical code, but we use a physically realistic viscosity model derived from N-body simulations (Daisaka et al., 2001), and dependent on the disk’s local properties (surface mass density, particle size, distance to the planet). Particularly, we include the effects of gravitational instabilities (wakes) that importantly enhance the disk’s viscosity. This method allows to study the global evolution of the disk over the age of the Solar System.Common estimates of the disk’s spreading time-scales with constant viscosity significantly underestimate the rings’ lifetime. We show that, with a realistic viscosity model, an initially narrow ring undergoes two successive evolutionary stages: (1) a transient rapid spreading when the disk is self-gravitating, with the formation of a density peak inward and an outer region marginally gravitationally stable, and with an emptying time-scale proportional to (where M₀ is the disk’s initial mass), (2) an asymptotic regime where the spreading rate continuously slows down as larger parts of the disk become non-self-gravitating due to the decrease of the surface density, until the disk becomes completely non-self-gravitating. At this point its evolution dramatically slows down, with an emptying time-scale proportional to 1/M₀, which significantly increases the disk’s lifetime compared to the case with constant viscosity. We show also that the disk’s width scales like t^1/4 with the realistic viscosity model, while it scales like t^1/2 in the case of constant viscosity, resulting in much larger evolutionary time-scales in our model. We find however that the present shape of Saturn’s rings looks like a 100 million-years old disk in our simulations. Concerning Jupiter’s, Uranus’ and Neptune’s rings that are faint today, it is not likely that they were much more massive in the past and lost most of their mass due to viscous spreading alone.  相似文献   

On the composition of ices incorporated in Ceres     

Olivier Mousis  Yann Alibert 《Monthly notices of the Royal Astronomical Society》2005,358(1):188-192

We use the clathrate hydrate trapping theory and gas drag formalism to calculate the composition of ices incorporated in the interior of Ceres. Utilizing a time-dependent solar nebula model, we show that icy solids can drift from beyond 5 au to the present location of the asteroid and be preserved from vaporization. We argue that volatiles were trapped in the outer solar nebula in the form of clathrate hydrates, hydrates and pure condensates prior to having been incorporated in icy solids and subsequently in Ceres. Under the assumption that most of volatiles were not vaporized during the accretion phase and the thermal evolution of Ceres, we determine the per mass abundances with respect to H₂O of CO₂, CO, CH₄, N₂, NH₃, Ar, Xe and Kr in the interior of the asteroid. The Dawn space mission, scheduled to explore Ceres in August 2014, may have the capacity to test some predictions. We also show that an in situ measurement of the D/H ratio in H₂O in Ceres could constrain the distance range in the solar nebula where its icy planetesimals were produced.  相似文献   

Geophysical evolution of Saturn’s satellite Phoebe,a large planetesimal in the outer Solar System     

Julie C. Castillo-Rogez  T.V. Johnson  P.C. Thomas  M. Choukroun  D.L. Matson  J.I. Lunine 《Icarus》2012,219(1):86-109

Saturn’s satellite Phoebe is the best-characterized representative of large outer Solar System planetesimals, thanks to the close flyby by the Cassini spacecraft in June 2004. We explore the information contained in Phoebe’s physical properties, density and shape, which are significantly different from those of other icy objects in its size range. Phoebe’s higher density has been interpreted as evidence that it was captured, probably from the proto-Kuiper-Belt. First, we demonstrate that Phoebe’s shape is globally relaxed and consistent with a spheroid in hydrostatic equilibrium with its rotation period. This distinguishes the satellite from ‘rubble-piles’ that are thought to result from the disruption of larger proto-satellites. We numerically model the geophysical evolution of Phoebe, accounting for the feedback between porosity and thermal state. We compare thermal evolution models for different assumptions on the formation of Phoebe, in particular the state of its water, amorphous or crystalline. We track the evolution of porosity and thermal conductivity as well as the destabilization of amorphous ice or clathrate hydrates. While rubble-piles may never reach temperatures suitable for porous ice to creep and relax, we argue that Phoebe’s shape could have relaxed due to heat from the decay of ²⁶Al, provided that this object formed less than 3 Myr after the production of the calcium–aluminum inclusions. This is consistent with the idea that Phoebe could be an exemplar of planetesimals that formed in the transneptunian region and later accreted onto outer planet satellites, either during the satellite’s formation stage, or still later, during the late heavy bombardment.  相似文献   

Electromagnetic heating of minor planets in the early solar system     

Floyd Herbert  Charles P. Sonett 《Icarus》1979,40(3):484-496

Electromagnetic processes occurring in the primordial solar system are likely to have significantly affected planetary evolution. In particular, electrical coupling of the kinetic energy of a dense T-Tauri-like solar wind into the interior of the smaller planets could have been a major driver of thermal metamorphism. Accordingly a grid of asteroid models of various sizes and solar distances was constructed using dc transverse magnetic induction theory. Plausible parameterizations with no requirement for a high environmental temperature led to complete melting for Vesta (and others with sizes down to 50 km diameter and distance out to 2.8 AU thus approximately reproducing the observed distributions of S objects) with no melting for Pallas and Ceres. Fairly high temperatures were reached in the Pallas model, perhaps implying nonmelting thermal metamorphosis as a cause of its anomalous spectrum (somewhat similar to but distinct from C type). A reversal of this temperature sequence seems implausible, suggesting that the Ceres-Pallas-Vesta dichotomy is a natural outcome of the induction mechanism. Highly localized heating is expected to arise due to an instability in the temperature-controlled current distribution. Localized metamorphosis resulting from this effect may be relevant to the production and evolution of pallasites, the large presumed metal component of S object spectra, and the formation of the lunar magma ocean.  相似文献   

Mid-infrared lightcurve of Vesta     

M.A. Chamberlain  M.V. Sykes  E.F. Tedesco 《Icarus》2011,215(1):57-61

We present a thermal mid-infrared lightcurve of Asteroid 4 Vesta and use this to infer variations in thermophysical properties over the surface. Vesta was observed over three nights during the May 2007 opposition with the Infrared Telescope Facility on Mauna Kea. Mid-infrared observations are compared to a model based on the Standard Thermal Model which is draped over a Vesta shape model derived from Hubble Space Telescope observations.A visible lightcurve with similar aspect was used to estimate the albedo as Vesta rotates. Shape and albedo can explain some of the features observed in the mid-infrared lightcurve. However, variations in the thermophysical properties, such as the “beaming parameter,” over Vesta’s surface are required to completely explain the observations.In order to match the mid-infrared magnitudes observed of Vesta, a beaming parameter of ∼0.862 is required which is higher than other Main Belt Asteroids such as Ceres and Pallas (0.756), indicating a smoother and/or rockier surface on Vesta. Variations in the beaming parameter with longitude are invoked to reproduce the observed thermal variations. Surface materials with relatively high beaming values, indicating a smoother and/or rockier surface, in the eastern hemisphere of Vesta coincide with locations where impact excavations may have produced surfaces that are younger and brighter relative to the western hemisphere.  相似文献   

10 μm polarimetry of ceres     

Paul E. Johnson  James C. Kemp  Marcia J. Lebofsky  George H. Rieke 《Icarus》1983,56(3):381-392

Linear polarimetry of Ceres at 10 μm is presented. These data represent the first published polarization measurements of an asteroid in the thermal infrared. It is found that Ceres is polarized at the 0.2-0.6% level. This data set is compared with theoretical models of the linear polarization of emitted radiation from a spherical plane. These models are used to derive the pole position and thermal inertia of Ceres. Ceres is best fit with a thermal inertia of $\text{0.0010±0.0003}\text{cal}\text{cm}{}^{\mathrm{?2}}\text{°}\text{K}{}^{\mathrm{?1}}\text{sec}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and a pole orientation of β_p = 36° ± 5°, λ_p = 270° ± 3°. It is concluded that 10μm polarimetry is a potentially powerful technique for remotely sensing the pole orientation and thermal inertia of asteroids.  相似文献   

Dynamical portrait of the Lixiaohua asteroid family     

B. Novaković  K. Tsiganis  Z. Knežević 《Celestial Mechanics and Dynamical Astronomy》2010,107(1-2):35-49

In this paper we present a comprehensive analysis of the dynamics in the region of the (3556) Lixiaohua asteroid family. The family lies in a particularly interesting region of the phase space, crossed by several two-body and three-body mean motion resonances. Also, members of this family can have close encounters with large asteroids, such as Ceres. We have identified the mean motion resonances which contribute to the long-term dynamical evolution of the family and our results confirm that the members of this family can be classified into a number of groups, exhibiting different dynamical behavior. We show for the first time that in the Lixiaohua region, apart from the chaotic diffusion in proper eccentricity and inclination (e _p and I _p ), there is at least one extended chaotic zone where several resonances overlap, thus giving rise to chaotic diffusion in proper semi-major axis (a _p ) as well. Using a code of Monte Carlo type, we simulate the evolution of the family, according to the model which combines the chaotic diffusion (in a _p , e _p and I _p ), Yarkovsky/YORP thermal effect and random walk in a _p due to the close encounters with massive asteroids. These simulations show that all these effects should be taken into account in order to accurately explain the observed distribution of family members in the space of proper elements, although a “minimal” model that accounts for chaotic diffusion in (e _p , I _p ), Yarkovsky-induced drift in a _p and random walk in a _p due to the close encounters with the most massive asteroids is enough to grossly characterize the shape of the family.  相似文献   

Radar observations of asteroid 1 Ceres     

Steven J. Ostro  Gordon H. Pettengill  Irwin I. Shapiro  Donald B. Campbell  Richard R. Green 《Icarus》1979,40(3):355-358

Radar observations of asteroid 1 Ceres were made at a 12.6-cm wavelength from the Arecibo Observatory in March/April 1977. The measurements, made with a received circular polarization orthogonal to that transmitted, yield a radar cross section of (0.04 ± 0.01)πR², for R = 510 km. The corresponding radar reflectivity is less than that measured for any other celestial body. Within the accuracy of measurement, no significant variation of cross section with rotational phase is apparent. The shape of the power spectrum suggests that Ceres is rougher at the scale of the observing wavelength than the Moon and inner planets, but smoother than the outer three Galilean satellites.  相似文献   

Long-term evolution of the spin of Venus: I. theory     

Alexandre C.M. Correia  Jacques Laskar 《Icarus》2003,163(1):1-23

  相似文献   

Insights into Ceres's evolution from surface composition          下载免费PDF全文

Julie Castillo‐Rogez  Marc Neveu  Harry Y. McSween  Roger R. Fu  Michael J. Toplis  Thomas Prettyman 《Meteoritics & planetary science》2018,53(9):1820-1843

Inspired by the recent results of the Dawn mission, thermodynamic models of rock alteration and brine evaporation have been used to help understand the conditions under which water–rock interaction took place within the dwarf planet Ceres. This analysis constrains Ceres's early history and offers a framework within which future observations may be interpreted. A broad range of alteration conditions have been simulated using the Geochemist's Workbench and PHREEQC software, associated with the FREZCHEM model that constrains the consequences of freezing the liquid phase in equilibrium with the observed mineralogical assemblage. Comparison of the modeling results with observed surface mineralogy at Ceres indicates advanced alteration under a relatively high fugacity of hydrogen, a conclusion that is consistent with predictions for, and observations of, large ice‐rich bodies. The simulations suggest production of methane that could help regulate the redox environment and possibly form clathrate hydrates upon freezing of the early ocean. The detection of localized occurrences of natrite (sodium carbonate) at the surface of Ceres provides key constraints on the composition of fluids that are necessarily alkaline. In addition, the combined hydrothermal and freezing simulations suggest that hydrohalite may be abundant in Ceres's subsurface, similar to Earth's polar regions. The global homogeneity of Ceres's surface, made of material formed at depth, suggests a large‐scale formation mechanism, while local heterogeneities associated with impact craters and landslides suggest that some form of sodium carbonate and other salts are accessible in the shallow subsurface.  相似文献   

A refined “standard” thermal model for asteroids based on observations of 1 Ceres and 2 Pallas     

《Icarus》1986,68(2):239-251

We present ground-based thermal infrared observations of asteroids 1 Ceres and 2 Pallas made over a period of 2 years. By analysing these data in light of the recently determined occultation diameter of Ceres (933–945 km) and Pallas (538 km) and their known small-amplitude lightcurves, we have determined a new value for the infrared beaming parameter used in the “standard” thermal emission model for asteroids. The new value is significantly lower than that previously used, and when applied in the reduction of thermal infrared observations of other asteroids, should yield model diameters that are closer to actual diameters. In our formulation, we also incorporate the recently adopted IAU magnitude convention for asteroids, which uses the zero-phase magnitudes (including the opposition effect) the same as is used for satellites.  相似文献   

The rotation of Janus and Epimetheus     

Matthew S. Tiscareno  Peter C. Thomas 《Icarus》2009,204(1):254-261

Epimetheus, a small moon of Saturn, has a rotational libration (an oscillation about synchronous rotation) of 5.9°±1.2°, placing Epimetheus in the company of Earth’s Moon and Mars’ Phobos as the only natural satellites for which forced rotational libration has been detected. The forced libration is caused by the satellite’s slightly eccentric orbit and non-spherical shape.Detection of a moon’s forced libration allows us to probe its interior by comparing the measured amplitude to that predicted by a shape model assuming constant density. A discrepancy between the two would indicate internal density asymmetries. For Epimetheus, the uncertainties in the shape model are large enough to account for the measured libration amplitude. For Janus, on the other hand, although we cannot rule out synchronous rotation, a permanent offset of several degrees between Janus’ minimum moment of inertia (long axis) and the equilibrium sub-Saturn point may indicate that Janus does have modest internal density asymmetries.The rotation states of Janus and Epimetheus experience a perturbation every 4 years, as the two moons “swap” orbits. The sudden change in the orbital periods produces a free libration about synchronous rotation that is subsequently damped by internal friction. We calculate that this free libration is small in amplitude (<0.1°) and decays quickly (a few weeks, at most), and is thus below the current limits for detection using Cassini images.  相似文献