Time-dependent flexure of the lithospheres on the icy satellites of Jupiter and Saturn

Previous analyses into flexural deformation on the icy satellites of Jupiter and Saturn have assumed static, elastic lithospheres. Viscous creep within the lithosphere, however, can cause evolution over time. Here, we apply a finite-element model that employs a time-dependent elastic–viscous-plastic rheology in order to investigate flexure on icy satellites. Factors that affect this time-dependent response are those that control creep rates; surface temperature, heat flow, and grain size. Our results show that surface temperature is by far the dominant factor. At higher surface temperatures (100–130 K), the evolution of the deformation is such that the thickness of a modeled elastic lithosphere could vary by up to an order of magnitude, depending on the time scale over which the deformation occurred. Because the flexure observed on icy satellites generally indicates transient high heat flow events, our results indicate that the duration of the heat pulse is an important factor. For the icy worlds of Jupiter and Saturn, static models of lithospheric flexure should be used with caution.     

A mechanism for the production of hydrogen peroxide and the hydroperoxyl radical on icy satellites by low-energy electrons

Hydrogen peroxide (H₂O₂) is one of the minor constituents of the water ice covered surfaces of the jovian satellites Europa, Ganymede, and Callisto. Here we demonstrate that H₂O₂ production may be initiated by the dissociative electron attachment (DEA) of low-energy electrons (LEEs) to water molecules. Electronic excitation or ionization by electrons also contributes to H₂O₂ formation at higher electron energies. Finally, we show that hydroperoxyl (HO₂) radicals could be formed on the surfaces of icy satellites by LEE impact.     

Meteoritical and dynamical constraints on the growth mechanisms and formation times of asteroids and Jupiter

Thermal models and radiometric ages for meteorites show that the peak temperatures inside their parent bodies were closely linked to their accretion times. Most iron meteorites come from bodies that accreted <0.5 Myr after CAIs formed and were melted by ²⁶Al and ⁶⁰Fe, probably inside 2 AU. Rare carbon-rich differentiated meteorites like ureilites probably also come from bodies that formed <1 Myr after CAIs, but in the outer part of the asteroid belt. Chondrite groups accreted intermittently from diverse batches of chondrules and other materials over a 4 Myr period starting 1 Myr after CAI formation when planetary embryos may already have formed at ∼1 AU. Meteorite evidence precludes accretion of late-forming chondrites on the surface of early-formed bodies; instead chondritic and non-chondritic meteorites probably formed in separate planetesimals. Maximum metamorphic temperatures in chondrite groups are correlated with mean chondrule age, as expected if ²⁶Al and ⁶⁰Fe were the predominant heat sources. Because late-forming bodies could not accrete close to large, early-formed bodies, planetesimal formation may have spread across the nebula from regions where the differentiated bodies formed. Dynamical models suggest that the asteroids could not have accreted in the main belt if Jupiter formed before the asteroids. Therefore Jupiter probably reached its current mass >3-5 Myr after CAIs formed. This precludes formation of Jupiter via a gravitational instability <1 Myr after the solar nebula formed, and strongly favors core accretion. Jupiter probably formed too late to make chondrules by generating shocks directly, or indirectly by scattering Ceres-sized bodies across the belt. Nevertheless, shocks formed by gravitational instabilities or Ceres-sized bodies scattered by planetary embryos may have produced some chondrules. The minimum lifetime for the solar nebula of 3-5 Myr inferred from the total spread of CAI and chondrule ages may exceed the median lifetime of 3 Myr for protoplanetary disks, but is well within the 1-10 Myr observed range. Shorter formation times for extrasolar planets may help to explain their unusual orbits compared to those of solar giant planets.     

Some results from the National Observatory of Turkey, Kazan State University, and Nikolaev Astronomical Observatory on small bodies of the solar system

A scientific collaboration between TÜB?TAK National Observatory (Turkey), Kazan State University (Russia) and Nikolaev Astronomical Observatory (Ukraine) involves observations of minor planets and near-Earth asteroids (NEAs) with the 1.5 m Russian-Turkish telescope (RTT150). Regular observations of selected asteroids in the range of 11-18 magnitudes began in 2004 with the view of determining masses of selected asteroids, improving the orbits of the NEAs, and studying physical characteristics of selected asteroids from photometric observations. More than 3000 positions of 53 selected asteroids and 11 NEAs have been obtained with an internal error in the range of 30-300 mas for a single determination. Photometric reductions of more than 4000 CCD frames are in progress. Masses of 21 asteroids were estimated through dynamical method using the ground-based optical observations, mainly from the RTT150 and Minor Planet Center. A comparison of the observational results from the RTT150 in 2004-2005 with observations of the same objects at other observatories allows us to conclude that RTT150 can be used for ground-based support in astrometry for the space mission GAIA.     

Possible patterns in the distribution of planetary formation regions

Eris, an object larger than Pluto, is known to reside in the transneptunian region further away than Pluto. One can wonder whether its semimajor orbital axis fits in a generalized Titius–Bode law, in the same way as Pluto does. We performed a new least-squares fit to a generalized Titius–Bode law including Eris and found that not only does Eris fit in the trend, but also the correlation coefficient improves. In addition, there is a remarkable symmetry of the location of the planetary formation regions with respect to Jupiter when the natural logarithm of the heliocentric distance is used as the metric. The issue of whether the observed patterns have some physical meaning or are due to mere chance is addressed using a Monte Carlo approach identical to that of Lynch. Although the probability of chance occurrence is highly dependent on the way in which the random configurations of synthetic planetary systems are selected, we find that in all reasonable scenarios of random planetary systems the probability of chance occurrence of the observed patterns is small (below 1 per cent in most cases). If the trend were used as a prediction tool, one might expect another planet or dwarf planet or a swarm of bodies with semimajor orbital axis of 120 ± 20 au. Simple calculations show that the protoplanetary nebula most likely had enough mass to allow the accretion of at least a dwarf planet at that distance. We also found that if the surface density of the nebula decayed with heliocentric distance ( r ) as a power of −2, the regular spacing in ln  r in the Solar system could be a natural consequence of the existence of a threshold mass for planetary formation.     

Constraints on the Formation Regions of Comets from their D:H Ratios

Studies of the D:H ratio in H₂O within the Solar nebula provide a relationship between the degree of enrichment of deuterium and the distance from the young Sun. In the context of cometary formation, such models suggest that comets which formed in different regions of the Solar nebula should have measurably different D:H ratios. We aim to illustrate how the observed comets can give information about the formation regions of the reservoirs in which they originated. After a discussion of the current understanding of the regions in which comets formed, simple models of plausible formation regions for two different cometary reservoirs (the Edgeworth–Kuiper belt and the Oort Cloud) are convolved with a deuterium-enrichment profile for the pre-solar nebula. This allows us to illustrate how different formation regions for these objects can lead to great variations in the deuterium enrichment distributions that we would observe in comets today. We also provide an illustrative example of how variations in the population within a source region can modify the resulting observational profile. The convolution of a deuterium-enrichment profile with examples of proto-cometary populations gives a feel for how observations could be used to draw conclusions on the formation region of comets which are currently fed into the inner Solar system from at least two reservoirs. Such observations have, to date, been carried out on only three comets, but future work with instruments such as ALMA and Herschel should vastly improve the dataset, leading to a clearer consensus on the formation of the Oort cloud and Edgeworth–Kuiper belt.     

Simulation of Io’s auroral emission: Constraints on the atmosphere in eclipse

We study the morphology of Io’s aurora by comparing simulation results of a three-dimensional (3D) two-fluid plasma model to observations by the high-resolution Long-Range Reconnaissance Imager (LORRI) on-board the New Horizons spacecraft and by the Hubble Space Telescope Advanced Camera for Surveys (HST/ACS). In 2007, Io’s auroral emission in eclipse has been observed simultaneously by LORRI and ACS and the observations revealed detailed features of the aurora, such as a huge glowing plume at the Tvashtar paterae close to the North pole. The auroral radiation is generated in Io’s atmosphere by collisions between impinging magnetospheric electrons and various neutral gas components. We calculate the interaction of the magnetospheric plasma with Io’s atmosphere-ionosphere and simulate the auroral emission. Our aurora model takes into account not only the direct influence of the atmospheric distribution on the morphology and intensity of the emission, but also the indirect influence of the atmosphere on the plasma environment and thus on the exciting electrons. We find that the observed morphology in eclipse can be explained by a smooth (non-patchy) equatorial atmosphere with a vertical column density that corresponds to ∼10% of the column density of the sunlit atmosphere. The atmosphere is asymmetric with two times higher density and extension on the downstream hemisphere. The auroral emission from the Tvashtar volcano enables us to constrain the plume gas content for the first time. According to our model, the observed intensity of the Tvashtar plume implies a mean column density of ∼5 × 10¹⁵ cm⁻² for the plume region.     

Constraints on the location, magnitude, and dimensions of Ganymede's mass anomalies

Previously, radio Doppler data, generated with NASA's Galileo spacecraft during its second encounter with Jupiter's moon Ganymede, were used to infer the locations and magnitudes of mass anomalies on Ganymede using point-mass models. However, the point-mass solutions do not provide the vertical and horizontal extent of the anomalous mass concentrations. Here, we provide the results of a new study using spherical cap disks to model Ganymede's mass anomalies. The spherical cap disk models not only provide the locations and magnitudes of the mass anomalies, but also their vertical and horizontal dimensions. The new models show that three disks, a positive mass located at (53.0° N, 127.0° W) and two negative masses located at (22.0° N, 87.0° W) and (49.0° N, 219.0° W), can explain the data. The magnitudes of the mass anomalies are on the order of 10¹⁸ kg. The diameters of the anomalies are a few thousand kilometers. The positive anomaly is about 100 meters thick and both negative anomalies have a thickness of less than a kilometer. We use the additional information provided by the disk models to investigate the viability of mass anomalies at Ganymede's surface by comparing the diameters of the anomalies to the sizes of regiones and sulci and the anomalies' thicknesses to accumulated layers of rock and clean ice on the surface. We find that the dimensions of the mass anomalies could be explained by concentrations of rock in the regio and rock-free ice in the sulci. These results confirm that mass anomalies may reside on or near Ganymede's surface and that positive mass anomalies are contained within areas of dark terrain and negative mass anomalies within bright terrain.     

Constraints on gas giant satellite formation from the interior states of partially differentiated satellites

An icy satellite whose interior is composed of a homogeneous ice/rock mixture must avoid melting during its entire history, including during its formation when it was heated by deposition of accretional energy and short-lived radioisotopes. Estimates of the temperature rise associated with radiogenic and accretional heating, coupled with limits on satellite melting can be used to constrain the timing of formation of a partially differentiated satellite relative to the origin of the calcium-aluminum-rich inclusions (CAI's) as a function of its accretion timescale and the protosatellite disk temperature (Td). Geological characterization and spacecraft radio tracking data suggest that Callisto, the outermost regular satellite of Jupiter, and Saturn's mid-sized satellite Rhea, are partially differentiated if their interiors are in hydrostatic equilibrium. Because the specific conditions during the satellites' formation are uncertain, we determine accretional temperature profiles for a range of values for Td and accretion time scales with the limiting assumption that impactors deposit all their energy close to the surface, leading to maximally effective radiative cooling. We find that Callisto can remain unmelted during formation if it accreted on a time scale longer than 0.6 Myr. Considering both radiogenic and accretional heating, Callisto must have finished accreting no earlier than ∼4 Myr after formation of CAI's for Td=100 K. Warmer disks or larger impactors that deposit their energy at depth in the satellite would require longer and/or later formation times. If Rhea accreted slowly (in 10⁵ to 10⁶ years), its growth must have finished no earlier than ∼2 Myr after CAI's for 70 K?Td?250 K to avoid early melting. If Rhea formed quickly (?10³ yr), its formation must have been delayed until at least 2 to 7 Myr after CAI's and in a disk with Td<190 K in the small impactor limit. If the satellites form in slow-inflow-supplied disks as proposed by Canup and Ward [Canup, R.M., Ward, W.R., 2002. Astron. J. 124, 3404-3423], the implied satellite ages suggest that gas inflow to the giant planets ceased no earlier than ∼4 Myr after CAI's, comparable to average nebular lifetimes inferred from observations of circumstellar disks.     

Dust in the solar system and in extra-solar planetary systems

Among the observed circumstellar dust envelopes a certain population, planetary debris disks, is ascribed to systems with optically thin dust disks and low gas content. These systems contain planetesimals and possibly planets and are believed to be systems that are most similar to our solar system in an early evolutionary stage. Planetary debris disks have been identified in large numbers by a brightness excess in the near-infrared, mid-infrared and/or submillimetre range of their stellar spectral energy distributions. In some cases, spatially resolved observations are possible and reveal complex spatial structures. Acting forces and physical processes are similar to those in the solar system dust cloud, but the observational approach is obviously quite different: overall spatial distributions for systems of different ages for the planetary debris disks, as opposed to detailed local information in the case of the solar system. Comparison with the processes of dust formation and evolution observed in the solar system therefore helps understand the planetary debris disks. In this paper, we review our present knowledge of observations, acting forces, and major physical interactions of the dust in the solar system and in similar extra-solar planetary systems.     

The radiolysis of SO2 and H2S in water ice: Implications for the icy jovian satellites

Spectra of Europa, Ganymede, and Callisto reveal surfaces dominated by frozen water, hydrated materials, and minor amounts of SO₂, CO₂, and H₂O₂. These icy moons undergo significant bombardment by jovian magnetospheric radiation (protons, electrons, and sulfur and oxygen ions) which alters their surface compositions. In order to understand radiation-induced changes on icy moons, we have measured the mid-infrared spectra of 0.8 MeV proton-irradiated SO₂, H₂S, and H₂O-ice mixtures containing either SO₂ or H₂S. Samples with H₂O/SO₂ or H₂O/H₂S ratios in the 3-30 range have been irradiated at 86, 110, and 132 K, and the radiation half-lives of SO₂ and H₂S have been determined. New radiation products include the H₂S₂ molecule and HSO⁻₃, HSO⁻₄, and SO²⁻₄ ions, all with spectral features that make them candidates for future laboratory work and, perhaps, astronomical observations. Spectra of both unirradiated and irradiated ices have been recorded as a function of temperature, to examine thermal stability and phase changes. The formation of hydrated sulfuric acid in irradiated ice mixtures has been observed, along with the thermal evolution of hydrates to form pure sulfuric acid. These laboratory studies provide fundamental information on likely processes affecting the outer icy shells of Europa, Ganymede, and Callisto.     

Planetesimals to protoplanets – II. Effect of debris on terrestrial planet formation

In this paper, we extend our numerical method for simulating terrestrial planet formation to include dynamical friction from the unresolved debris component. In the previous work, we implemented a rubble pile planetesimal collision model into direct N -body simulations of terrestrial planet formation. The new collision model treated both accretion and erosion of planetesimals but did not include dynamical friction from debris particles smaller than the resolution limit for the simulation. By extending our numerical model to include dynamical friction from the unresolved debris, we can simulate the dynamical effect of debris produced during collisions and can also investigate the effect of initial debris mass on terrestrial planet formation. We find that significant initial debris mass, 10 per cent or more of the total disc mass, changes the mode of planetesimal growth. Specifically, planetesimals in this situation do not go through a runaway growth phase. Instead, they grow concurrently, similar to oligarchic growth. The dynamical friction from the unresolved debris damps the eccentricities of the planetesimals, reducing the mean impact speeds and causing all collisions to result in merging with no mass loss. As a result, there is no debris production. The mass in debris slowly decreases with time. In addition to including the dynamical friction from the unresolved debris, we have implemented particle tracking as a proxy for monitoring compositional mixing. Although there is much less mixing due to collisions and gravitational scattering when dynamical friction of the background debris is included, there is significant inward migration of the largest protoplanets in the most extreme initial conditions (for which the initial mass in unresolved debris is at least equal to the mass in resolved planetesimals).     

Current state and development of the minor planets research in Nikolaev Astronomical Observatory

Regular positional observations of minor planets in Nikolaev Astronomical Observatory have been begun with installation of photographic Zone Astrograph in 1961. The observations of 19 selected minor planets up to 12 magnitude were obtained for 36 years. Accuracy of the photographic positions of minor planets is rather high, 0.15^′′-0.19^′′. These positions were used for improvement of the system of fundamental catalogue and determination of its orientation to the dynamical reference frame. CCD observations of asteroids have been begun at the Zone Astrograph in 2000. There was obtained about the same accuracy, as in photographic observations. During 2004-2006 NAO participated in international collaboration with TUBITAK National Observatory (Turkey) and Kazan State University (Russia) in positional and photometric observations of small Solar system bodies. About four thousands of CCD images for 58 asteroids of 11-18 mag were obtained with internal and external errors of 30-80 mas of a single determination. Some of these observations, as well as the observations of the Minor Planet Center, are being used for the current asteroid mass determinations in Nikolaev observatory. Available results allow us to consider the Russian-Turkish telescope RTT150 as a good candidate for ground-based astrometry support of the future space mission GAIA, moreover in the period before GAIA.