Inferred heat transfer mechanisms and tectonic development of planets and satellites

A schematic diagram showing the relative importance of conduction, convection and hotspots as heat transfer mechanisms on planets has been previously described by Solomon and Head (1982). In their construction they assumed that the majority of heat transfer on Earth involved mantle convection (and hence, plate recycling), with Io and Mercury dominated by hotspot and conduction, respectively. This diagram is here quantified and used to deduce the tectonic regime of Jovian and Saturnian satellites.     

Possible satellites of Mercury and Venus

On the basis of works of King and Innanen, the limiting direct and retrograde orbits around the planets Mercury and Venus have been calculated. Synthesizing this concept with the concept of synchronous orbits around the planets and tidal drags acting within them it is shown that Venus may not have retained any satellite direct or retrograde but Mercury may have retained a retrograde satellite at a distance between 225000 and 252700 km from its center. It is urged that this satellite may be investigated observationally.     

Stable satellites around extrasolar giant planets

In this work, we study the stability of hypothetical satellites of extrasolar planets. Through numerical simulations of the restricted elliptic three-body problem we found the borders of the stable regions around the secondary body. From the empirical results, we derived analytical expressions of the critical semimajor axis beyond which the satellites would not remain stable. The expressions are given as a function of the eccentricities of the planet, e _P, and of the satellite, e _sat. In the case of prograde satellites, the critical semimajor axis, in the units of Hill's radius, is given by a _E≈ 0.4895   (1.0000 − 1.0305 e _P− 0.2738 e _sat). In the case of retrograde satellites, it is given by a _E≈ 0.9309  (1.0000 − 1.0764 e _P− 0.9812 e _sat). We also computed the satellite stability region ( a _E) for a set of extrasolar planets. The results indicate that extrasolar planets in the habitable zone could harbour the Earth-like satellites.     

Far-infrared photometry of planets: Saturn and Venus

We present far-infrared observations of Saturn and Venus made within four spectral bands (31 to 38, 47 to 67, 71 to 94, and 114 to 196 μm) using a 32-cm airborne telescope during May 1977. The set of brightness temperatures obtained from Saturn is analyzed on the basis of thermal models of the atmosphere of this planet. The best agreement is obtained with an effective temperature of about 95°K for the planet itself and a ring contribution corresponding to brightness temperatures ranging from 55 to 70°K. These values of the temperature of the ring system are smaller than the ones measured at shorter wavelengths and could be indicative of a decreasing emissivity of the rings in the far infrared.     

A survey for satellites of Venus

We present a systematic survey for satellites of Venus using the Baade-Magellan 6.5 m telescope and IMACS wide-field CCD imager at Las Campanas observatory in Chile. In the outer portions of the Hill sphere the search was sensitive to a limiting red magnitude of about 20.4, which corresponds to satellites with radii of a few hundred meters when assuming an albedo of 0.1. In the very inner portions of the Hill sphere scattered light from Venus limited the detection to satellites of about a kilometer or larger. Although several main belt asteroids were found, no satellites (moons) of Venus were detected.     

Atmospheric dynamics on the outer planets and some of their satellites

A short review of the atmospheric dynamics for the outer planets and some of their satellites with atmospheres is presented. Their physical properties are discussed. A survey of observational data for atmospheric motions on the large planets is presented and similarity parameters are given for all objects. General problems of the vertical structure of atmospheres are then considered with some detailed discussion for rarefied atmospheres on Io and Ganymede. The low densities of these atmospheres make their dynamics similar to those of the thermospheres of the terrestrial planets but with a specific boundary layer. The atmospheric temperature regime must be strongly coupled to that of their surface, and so winds should be of the order of the velocity of sound. Similarities and differences are noted between the dynamics of Titan and possibly of Pluto and the circulation on Venus. For large and rapidly rotating planets, some analogies with the oceans are pointed out. The “soliton” hypothesis is discussed in some detail for circulation perturbations observed on Jupiter's disk. Finally, it is noted that the bimodal rotation period found for Neptune [D.P. Cruikshank, Astrophys. J. 220, 157–159 (1978)] may be interpreted as an indication of an equatorial jet on the planet with a relative velocity of about 140 m sec^?1.     

The escape of natural satellites from Mercury and Venus

Kumar's (1977) suggestion that the slow rotations of Mercury and Venus are in part due to natural satellites that subsequently escaped is discussed. A more useful criterion for the escape of such satellites than that previously proposed is derived, and it is shown that this distance is sufficiently small for Mercury and Venus to make the escape of satellites a likely possibility.     

The escape of natural satellites from Mercury and Venus

It is suggested that the slow rotations of Mercury and Venus may be connected with the absence of natural satellites around them. If Mercury or Venus possessed a satellite at the time of formation, the tidal evolution would have caused the satellite to recede. At a sufficiently large distance from the planet, the Sun's gravitational influence makes the satellite orbit unstable. The natural satellites of Mercury and Venus might have escaped as a consequence of this instability.     

Velocity distribution of meteoroids in the vicinity of planets and satellites

Collisions in the Solar System play an important role in its history. Impact processes depend essentially on the velocity distribution of meteoroids colliding with a chosen planet. According to Carleman's theorem it is sufficient to find the set of M _k = mathematical expectation of v ^k , v being the collisional velocity. We suppose that M _k for meteoroids of asteroidal nature differs slightly from that for asteroids themselves. So among all numbered minor planets we select those which may potentially collide with the chosen major planet. Then we calculate v at intersection points and count the average over all such points and all selected asteroids. The gravitation of a body-target may be taken into account or not. Numerical results are collected in four Tables.     

Velocity distribution of meteoroids in the vicinity of planets and satellites

Collisions in the Solar System play an important role in its history. Impact processes depend essentially on the velocity distribution of meteoroids colliding with a chosen planet. According to Carleman's theorem it is sufficient to find the set of M _k = mathematical expectation of v ^k , v being the collisional velocity. We suppose that M _k for meteoroids of asteroidal nature differs slightly from that for asteroids themselves. So among all numbered minor planets we select those which may potentially collide with the chosen major planet. Then we calculate v at intersection points and count the average over all such points and all selected asteroids. The gravitation of a body-target may be taken into account or not. Numerical results are collected in four Tables.St.Petersburg University     

Orbital evolution of the distant satellites of the giant planets

We study the evolution of several distant satellite orbits. These are the orbits (including the improved ones)of the recently discovered Neptunian satellites S/2002 N1, N2, N3, N4; S/2003 N1 and the orbits of Jovian, Saturnian, and Uranian satellites with librational variations in the argument of the pericenter: S/2001 J10 (Euporie), S/2003 J20; S/2000 S5 (Kiviuq), S/2000 S6 (Ijiraq), and S/2003 U3. The study is performed using mainly an approximate numerical-analytical method. We determine the extreme eccentricities and inclinations as well as the periods of the variations in the arguments of pericenters and longitudes of the ascending nodes on time intervals ～10⁵?10⁶ yr. We compare our results with those obtained by numerically integrating the rigorous equations of satellite motion on time intervals of the order of the circulation periods of the longitudes of the ascending nodes (10²?10³ yr).     

On the stability of the inner planets,satellites and close binaries

I consider the range of Hill stability in the restricted circular problem of three bodies when the larger one of the two principal bodies has a finite oblateness. I show that the range r satisfies the equation

$\text{r =}\text{1? μ}\text{C}{}_{\mathrm{cr}}\text{? 3μ + μr}{}^2\text{? v (1? μ)r}{}^{\mathrm{?3}}\text{±(2 + 3v)}\text{(1 ? μ}\text{1 +}\text{3v}\text{r}{}^2\text{r}\text{,}$

where μ is the mass parameter and v is an oblateness parameter. This result is applied to the solar system, the Earth-Moon system and binary star systems. It is then shown that, all the inner planets of the solar system, the great majority of asteroids and some short-period comets are Hill stable, that direct artificial satellites of the Earth are more stable than retrograde ones, and that contact binaries possess cores between which no mass exchange takes place.     

Computer simulation of regolith vertical structure and exposure age of small satellites of planets

The simulation has shown that the regolith layers several hundred meters thick can be formed on the small satellites of planets of Phobos and Deimos type providing that almost all the material lost at meteorite impacts return to the satellite. The existence of a global layer on Deimos that filled the craters for the depth of about 5 m can be explained by the crater presence of diameter of about 4 km and the age of less than 350 Myr on its hemisphere almost unstudied. Dust belts with dust concentration by 3 orders greater than round the Earth can exist in the region of satellite orbits.     

Microdunes and other aeolian bedforms on Venus: Wind tunnel simulations

Previous studies confirm that sand can be entrained at the wind velocities recorded on Venus. Present results describe bedforms produced in the Venus Wind Tunnel (VWT) simulating the average Venusian environment. Even at the low wind speeds measured on Venus, dunelike structures form in fine-grained quartz sands (particles 50–200 μm in diameter). The dunelike structures, referred to as microdunes, are considered to be true dunes analogous to those on Earth because they have (1) slip faces, (2) a lack of particle-size sorting, (3) a low ratio of saltation path length to dune length, and (4) internal cross-bedding. The microdunes typically produced in the VWT are 9 cm long and 0.75 cm high. It is proposed that there may be fields of microdunes on Venus that are capable of very fast rates of migration and that they may grow into features much larger than those observed in the VWT. However, neither dunes nor other types of features develop above a wind speed of ～ 1.5 m/sec; at this wind speed, the bed is flat and featureless. Thus, it is predicted that relatively short periods of higher winds may destroy microdunes and other small bedforms which could account for the sparsity of definitive aeolian features observed in Venera images. Some apparent cross-bedding observed in Venera images, however, could represent preserved aeolian structures.     

Formation of the regular satellites of giant planets in an extended gaseous nebula I: subnebula model and accretion of satellites

We model the subnebulae of Jupiter and Saturn wherein satellite accretion took place. We expect each giant planet subnebula to be composed of an optically thick (given gaseous opacity) inner region inside of the planet’s centrifugal radius (where the specific angular momentum of the collapsing giant planet gaseous envelope achieves centrifugal balance, located at rCJ ∼ 15R_J for Jupiter and rCS ∼ 22R_S for Saturn) and an optically thin, extended outer disk out to a fraction of the planet’s Roche-lobe (R_H), which we choose to be ∼R_H/5 (located at ∼150 R_J near the inner irregular satellites for Jupiter, and ∼200R_S near Phoebe for Saturn). This places Titan and Ganymede in the inner disk, Callisto and Iapetus in the outer disk, and Hyperion in the transition region. The inner disk is the leftover of the gas accreted by the protoplanet. The outer disk may result from the nebula gas flowing into the protoplanet during the time of giant planet gap-opening (or cessation of gas accretion). For the sake of specificity, we use a solar composition “minimum mass” model to constrain the gas densities of the inner and outer disks of Jupiter and Saturn (and also Uranus). Our model has Ganymede at a subnebula temperature of ∼250 K and Titan at ∼100 K. The outer disks of Jupiter and Saturn have constant temperatures of 130 and 90 K, respectively.Our model has Callisto forming in a time scale ∼10⁶ years, Iapetus in 10⁶-10⁷ years, Ganymede in 10³-10⁴ years, and Titan in 10⁴-10⁵ years. Callisto takes much longer to form than Ganymede because it draws materials from the extended, low density portion of the disk; its accretion time scale is set by the inward drift times of satellitesimals with sizes 300-500 km from distances ∼100R_J. This accretion history may be consistent with a partially differentiated Callisto with a ∼300-km clean ice outer shell overlying a mixed ice and rock-metal interior as suggested by Anderson et al. (2001), which may explain the Ganymede-Callisto dichotomy without resorting to fine-tuning poorly known model parameters. It is also possible that particulate matter coupled to the high specific angular momentum gas flowing through the gap after giant planet gap-opening, capture of heliocentric planetesimals by the extended gas disk, or ablation of planetesimals passing through the disk contributes to the solid content of the disk and lengthens the time scale for Callisto’s formation. Furthermore, this model has Hyperion forming just outside Saturn’s centrifugal radius, captured into resonance by proto-Titan in the presence of a strong gas density gradient as proposed by Lee and Peale (2000). While Titan may have taken significantly longer to form than Ganymede, it still formed fast enough that we would expect it to be fully differentiated. In this sense, it is more like Ganymede than like Callisto (Saturn’s analog of Callisto, we expect, is Iapetus). An alternative starved disk model whose satellite accretion time scale for all the regular satellites is set by the feeding of planetesimals or gas from the planet’s Roche-lobe after gap-opening is likely to imply a long accretion time scale for Titan with small quantities of NH₃ present, leading to a partially differentiated (Callisto-like) Titan. The Cassini mission may resolve this issue conclusively. We briefly discuss the retention of elements more volatile than H₂O as well as other issues that may help to test our model.     

Aqueous alteration affecting the irregular outer planets satellites: Evidence from spectral reflectance

The surface reflectance properties of the irregular outer planets satellites are probed for evidence for the presence of aqueous alteration products on their surfaces using the strong correlation between the 3.0-μm water of hydration absorption feature and the 0.7-μm Fe²⁺ → Fe³⁺ oxidized iron feature seen in low-albedo asteroid reflectances, in an effort to expand our understanding of the composition of the precursor bodies from which the dynamical satellite clusters are derived. Equations converting Johnson V and Kron-Cousins RI photometry to Eight Color Asteroid Survey v (0.550 μm), w (0.701 μm), and x (0.853 μm) photometry are derived from relationships defined by Howell (1995, Ph.D. thesis), and coupled with an algorithm previously defined to detect the presence of the 0.7-μm absorption feature in ECAS asteroid photometry [Vilas, F., 1994. Icarus 111, 456-467]. Broadband VRI photometry of Ch-class Asteroid 19 Fortuna acquired during 2004 confirms the efficacy of this method of identifying the presence of the 0.7-μm feature. Photometric observations of many recently discovered irregular outer jovian, saturnian, uranian, and neptunian satellites, coupled with limited asteroid spectroscopy, were examined for the presence of aqueous alteration. The dynamical clusters of outer irregular jovian satellites are mixed between objects that do and do not show this absorption feature. Multiple observations of some objects test both positively and negatively, similar to the surface variegation that has been observed among many C-class asteroids in the main asteroid belt. Evidence for aqueous alteration on these jovian satellites augers for an origin in or near the same location as the asteroids now occupying the aqueous alteration zone (2.6-3.5 AU), at heliocentric distances internal to Jupiter's orbit. Among the saturnian irregular satellites, only S IX Phoebe shows limited evidence of aqueous alteration from ground-based observations. The other satellites show no sign of this feature, and have general reflectance properties very similar to the D-class asteroids, supporting an origin for their precursor bodies in the outer Solar System, perhaps the Centaur region. Only two uranian satellites were tested: U XVII Caliban tests positively for the feature. The differences in surface reflectance properties support the idea that Caliban and U XVI Sycorax derive from separate parent bodies. One observation of neptunian satellite N II Nereid shows no sign of this absorption feature.