Rotational Properties of Cometary Nuclei

We review several techniques used to retrieve rotational parameters from observations. The spin period of a dozen of comets retrieved with these techniques are summarized. We describe how the spin period of comet Hale-Bopp (C/1995 O1) has been calculated with a high accuracy (11.30–11.34 h). Although several authors converged to a spin axis orientation at (α,δ) = (275 ± 15°, -55 ± 5°), detailed studies indicate that the dust jets morphology in 1996–1997 may be incompatible with this orientation. Comet 19P/Borrelly has been recently observed by the Deep Space 1 spacecraft. At the same time, its spin axis orientation and period have been determined by several authors to be respectively (α,δ) = (225 ± 15°, -10 ± 10°)and 26h. These two comets are likely to be in (or close to) a principal axis spin state. We discuss new modeling of the spin state of comet 46P/Wirtanen, the target of the Rosetta mission. The model involves a three-dimensional shape and thermal model, from which the torque of the non gravitational force is calculated at each time step. The moments of inertia are computed for each irregular shape. The results from numerical integrations show that this comet can remain in a principal axis spin state during more than 10 orbits if the spin period does not get above～6 h. If the spin period increases, its nucleus gets rapidly into excited spin states. It shows that even small and very active short-period comets are not necessarily in non principal axis spin states. In the last section, the consequences of recent observations and modeling of the rotational parameters of comet nuclei are discussed, and unsolved problems are presented.     

Radar observations of asteroid 25143 Itokawa (1998 SF36)

Abstract— We observed 25143 Itokawa, the target of Japan's Hayabusa (MUSES‐C) sample‐return mission, during its 2001 close approach at Arecibo on twelve dates during March 18‐April 9 and at Goldstone on nine dates during March 20‐April 2. We obtained delay‐Doppler images with range resolutions of 100 ns (15 m) at Arecibo and 125 ns (19 m) at Goldstone. Itokawa's average circular polarization ratio at 13 cm, 0.26 ± 0.04, is comparable to that of Eros, so its cm‐to‐m surface roughness probably is comparable to that on Eros. Itokawa's radar reflectivity and polarization properties indicate a near‐surface bulk density within 20% of 2.5 g cm^?3. We present a preliminary estimate of Itokawa's shape, reconstructed from images with rather limited rotation‐phase coverage, using the method of Hudson (1993) and assuming the lightcurve‐derived spin period (12.132 hr) and pole direction (ecliptic long., lat. = 355°, ?84°) of Kaasalainen et al. (2003). The model can be described as a slightly asymmetrical, slightly flattened ellipsoid with extents along its principal axes of 548 times 312 times 276 m ± 10%. Itokawa's topography is very subdued compared to that of other asteroids for which spacecraft images or radar reconstructions are available. Similarly, gravitational slopes on our Itokawa model average only 9° and everywhere are less than 27°. The radar‐refined orbit allows accurate identification of Itokawa's close planetary approaches through 2170. If radar ranging planned for Itokawa's 2004 apparition succeeds, then tracking of Hayabusa during its 2005 rendezvous should reveal Yarkovsky perturbation of the asteroid's orbit.     

On the oscillatory motions of the principal plane of the solar corona

We define the principal plane of the solar corona as the equatorial plane of the dipole component of the solar magnetic field at the source surface. The position of this plane defines the orientation of the heliospheric current sheet and the outer solar corona in 3D space, in particular, their inclination with respect to the solar equatorial plane. Their tilt varies from almost zero at solar minimum to almost 90° at solar maximum. But this change is not monotonic; more or less regular oscillations are superimposed onto the average curve. Data on the inclination of the coronal principal plane during 1994–2000 were analyzed. Oscillations with a quasiperiod of 1.3 years were clearly revealed. These oscillations were found to be correlated with variations of the solar rotation rate at the base of the solar convection zone. The latter variations have the same quasi-period of 1.3 years, but they are 1 month out of phase.     

A Model for the Common Origin of Jupiter Family and Halley Type Comets

A numerical simulation of the Oort cloud is used to explain the observed orbital distributions and numbers of Jupiter-family (JF) and Halley-type (HT) short-period (SP) comets. Comets are given initial orbits with perihelion distances between 5 and 36 au, and evolve under planetary, stellar and Galactic perturbations for 4.5 Gyr. This process leads to the formation of an Oort cloud (which we define as the region of semimajor axes a > 1,000 au), and to a flux of cometary bodies from the Oort cloud returning to the planetary region at the present epoch. The results are consistent with the dynamical characteristics of SP comets and other observed cometary populations: the near-parabolic flux, Centaurs, and high-eccentricity trans-Neptunian objects. To achieve this consistency with observations, the model requires that the number of comets versus initial perihelion distance is concentrated towards the outer planetary region. Moreover, the mean physical lifetime of observable comets in the inner planetary region (q < 2.5 au) at the present epoch should be an increasing function of the comets’ initial perihelion distances. Virtually all observed HT comets and nearly half of observed JF comets come from the Oort cloud, and initially (4.5 Gyr ago) from orbits concentrated near the outer planetary region. Comets that have been in the Oort cloud also return to the Centaur (5 < q < 28 au, a < 1,000 au) and near-Neptune high-eccentricity regions. Such objects with perihelia near Neptune are hard to discover, but Centaurs with characteristics predicted by the model (e.g. large semimajor axes, above 60 au, or high inclinations, above 40°) are increasingly being found by observers. The model provides a unified picture for the origin of JF and HT comets. It predicts that the mean physical lifetime of all comets in the region q < 1.5 au is less than ～200 revolutions.     

Jovian plasma sheet morphology: particle and field observations by the Galileo spacecraft

We present results from an investigation of the plasma sheet encounter signatures observed in the Jovian magnetosphere by the Energetic Particles Detector (EPD) and Magnetometer (MAG) onboard the Galileo spacecraft. Maxima in ion flux were used to identify over 500 spacecraft encounters with the plasma sheet between radial distances from Jupiter from 20 to 140R_J during the first 25 orbits (4 years of data). Typical signatures of plasma sheet encounters show a characteristic periodicity of either 5 or 10 hours that is attributed to an oscillation in the relative distance between the spacecraft and the plasma sheet that arises from the combination of planetary rotation and offset magnetic and rotational axes. However, the energetic particle and field data also display much variability, including instances of intense fluxes having little to no periodicity that persist for several Jovian rotation periods. Abrupt changes in the mean distance between the plasma sheet and the spacecraft are suggested to account for some of the transitions between typical flux periodicities associated with plasma sheet encounters. Additional changes in the plasma sheet thickness and/or amplitude of the plasma sheet displacement from the location of the spacecraft are required to explain the cases where the periodicity breaks down but fluxes remain high. These changes in plasma sheet characteristics do not display an obvious periodicity; however, the observations suggest that dawn/dusk asymmetries in both the structure of the plasma sheet and the frequency of anomalous plasma sheet encounters are present. Evidence of a thin, well-ordered plasma sheet is found out to 110R_J in the dawn and midnight local time sectors, while the dusk magnetosphere is characterized by a thicker, more disordered plasma sheet and has a potentially more pronounced response to an impulsive trigger. Temporal variations associated with changing solar wind conditions are suggested to account for the anomalous plasma sheet encounters there.     

Reorientation of planets with lithospheres: The effect of elastic energy

It is commonly assumed that internal energy dissipation will ultimately drive planets to principal axis rotation, i.e., where the rotation vector is aligned with the maximum principle axis, since this situation corresponds to the minimum rotational energy state. This assumption simplifies long-term true polar wander (TPW) studies since the rotation pole can then be found by diagonalizing the appropriate (non-equilibrium) inertia tensor. We show that for planets with elastic lithospheres the minimum energy state does not correspond to principal axis rotation. As the planet undergoes reorientation elastic energy is stored in the deforming lithosphere, and the state of minimum total energy is achieved before principal axis rotation. We find solutions for the TPW of planets that include this effect by calculating the elastic stresses associated with deformation, and then minimizing the total (rotational and elastic) energy. These expressions indicate that the stored elastic energy acts to reduce the effective size of the driving load (relative to predictions which do not include this energy term). Our derivation also yields expressions for the TPW-induced stress field that generalizes several earlier results. As an illustration of the new theory, we consider TPW driven by the development of the Tharsis volcanic province on Mars. Once the size of the Tharsis load and the Mars model is specified, the extended theory yields a more limited range on the possible TPW.     

On the origin of the solar system,I

The principal dynamical properties of the planetary and satellite systems listed in Section 2 require these bodies to have condensed in highly-flattened nebulae which provided the dissipation forces that produced the common directions of orbital motion, and the lowe andi values. Minimum masses of these nebulae can be estimated on the assumption that the initial solar abundances apply, starting from the empirical data on present planetary and satellite compositions and masses. The asteroids and comets are assumed to be direct condensations and accretion products in their respective zones (2–4 AU and 20–50 AU), without the benefit of gravitational instability in the solar nebula, owing to the comparatively low density there; with gravitational instability accelerating and ultimately dominating the accretion of the planets and major satellites, in zones approaching and exceeding the local Roche density. Only in the case of Jupiter, gravitational instability appears to have dominated from the outset; the other planets are regarded as hybrid structures, having started from limited accretions. In Section 3 the empirical information on protostars is reviewed. ‘Globules’ are described, found to have the typical range of stellar masses and with gaseous compositions now well known thanks largely to radio astronomy. They contain also particulate matter identified as silicates, ice, and probably graphite and other carbon compounds. The measured internal velocities would predict a spread of total angular momenta compatible with the known distribution of semi-major axes in double stars. The planetary system is regarded as an ‘unsuccessful’ binary star, in which the secondary mass formed a nebula instead of a single stellar companion, with 1–2% of the solar mass. This mass fraction gives a basis for an estimate of thefrequency of planetary systems. The later phases of the globules are not well known empirically for the smaller masses of solar type; while available theoretical predictions are mostly made for non-rotating pre-stellar masses. Section 5 reviews current knowledge of the degree of stability of the planetary orbits over the past 4.5×10⁹ yr, preparatory to estimates of their original locations and modes of origin. The results of the Brouwer and Van Woerkom theory and of recent numerical integrations by Cohenet al. indicate no drastic changes in Δa/a over the entire post-formation history of the planets. Unpublished numerical integrations by Dr P. E. Nacozy show the remarkable stability of the Jupiter-Saturn system as long as the planetary masses are well below 29 times their actual values. Numerical values of Δa/a are collected for all planets. The near resonances found for both pairs of planets and of satellites are briefly reviewed. Section 6 cites the statistics on the frequency and masses of asteroids and information on the Kirkwood gaps, both empirical and theoretical. An analogous discussion is made for the Rings of Saturn, including its extension observed in 1966 to the fourth Saturn satellite, Dione. The reality, or lack of it, of the divisions in the Rings are considered. The numbers of Trojan asteroids are reviewed, as is the curious, yet unexplained, bimodal distribution of their orbital inclinations. Important information comes from the periods of rotation of the asteroids and the orientation of their rotational axes. The major Hirayama families are considered as remnants of original asteroid clusterings whose membership has suffered decreases through planetary perturbations. Other families with fewer large members may be due to collisions. The three main classes of meteorites, irons, stones, and carbonaceous chondrites all appear to be of asteroidal origin and they yield the most direct evidence on the early thermal history of the solar system. While opinion on this subject is still divided, the author sees in the evidence definite confirmation of thecold origin of the planetary system, followed by ahot phase due to the evolving sun that caused the dissolution of the solar nebula. This massive outward ejection, that included the smaller planetesimals, appears to have caused the surface melting of the asteroids by intense impact, with the splashing responsible for the formation of the chondrules. The deep interiors of the asteroids are presumably similar to the C1 meteorites which have recently been found to be more numerous in space by two orders of magnitude than previously supposed.     

Tectonic patterns on reoriented and despun planetary bodies

Several processes may produce global tectonic patterns on the surface of a planetary body. The stresses associated with distortions of biaxial figures due to despinning or reorientation were first calculated by Vening Meinesz [Vening Meinesz, F.A., 1947. Trans. Am. Geophys. Union 28 (1), 1-23]. We adopt a mathematically equivalent, but physically more meaningful treatment for distortions associated with rotation. The new approach allows us to find analytic solutions for the general case of stresses associated with distortions of biaxial or triaxial planetary figures. Distortions of biaxial figures may be driven by variations in rotation rate, rotation axis orientation, or the combination of both. Distortions of triaxial figures may be driven by the same mechanisms and/or variations in tidal axis orientation for tidally deformed satellites. While the magnitude of the resulting stresses depends on the adopted elastic and physical parameters, the expected tectonic pattern is independent of these parameters for these mechanisms. Reorientation of the rotation/tidal axis alone is expected to produce normal/thrust faulting provinces enclosing the initial rotation/tidal poles, and thrust/normal faulting provinces enclosing the final rotation/tidal poles. Reorientation of both the rotation and tidal axis results in a wide variety of tectonic patterns for different reorientation geometries. On Europa, the tidal axis reorientation which generally accompanies rotation axis reorientations may provide an alternative explanation for tectonic features that have been interpreted as evidence for non-synchronous rotation. The observed tectonic pattern on Enceladus is more easily explained by a large reorientation (∼90°) of the rotation axis, than by rotation rate variations.     

Maxwell Montes and Thessera Fortuna: A study of Venera 15 and 16 radar images

The radar images of Maxwell Montes and Thessera Fortuna obtained by Venera 15 and 16 were analyzed. It was concluded that the structures are not aeolian but are tectonic deformations. Because of the lack of large-scale erosion, these deformations must have been formed near the surface and, therefore, one of the principal stress axes must have been vertical. The orientations of the stress ellipsoid in several localities are presented. Differences between localities in the types of deformation can be explained by changes in the orientation of the intermediate stress axis, while the major stress axis remains constant. The latter is generally horizontal and oriented EW. Other differences may be caused by a shift from a compressional to an extensional regime. Inhomogeneities in the stress field have caused shear zones. Block diagrams are developed to explain the postulated structures.The described features are not unique to the studied areas; thus the method used and the conclusions reached have planetary implications.     

Nightglow OH (8, 3) band intensities and rotational temperature at 23°S

The OH (8, 3) band airglow emission has been observed over 1 year at a latitude of 23°S. The average band intensity observed was 385 Rayleighs with a nocturnal range typically less than 100 R. The nocturnal variation in rotational temperature was usually less than 10°K, and the mean temperature was 179°K. The nocturnal variation of intensity is usually uncorrelated with that of the rotational temperature. Time average values of these parameters do, however, show some correlation. On some occasions large post-twilight and pre-dawn intensity enhancements are observed.     

Application of a textural geospeedometer to the late‐stage magmatic history of MIL 03346

Abstract— Dynamic crystallization experiments performed on Fe‐rich, Al‐poor basalt are employed as a textural calibration set to quantify the late‐stage igneous history of nakhlite Miller Range (MIL) 03346. The ratio of crystal‐melt surface area to volume typifying morphologically distinct populations of Ca‐pyroxene has been shown to vary as a strong function of cooling rate (Hammer 2006). Furthermore, a texture of phenocrysts surrounded by finer‐grained groundmass crystals arises by sequential nucleation events during constant‐rate cooling, but multiple populations nucleate only if the cooling rate is 72 °C h^?1. Textural analysis of meteorite MIL 03346 reveals at least two distinct populations. The Ca‐pyroxene phenocryst and microphenocryst three dimensional (3D) aspect ratios are 112 ± 8.3 and 1530 ± 160 mm^?1, respectively. By comparison with the calibration set, the range of cooling rates consistent with 3D aspect ratios of both populations in MIL 03346 is ?20 °C h^?1An additional experiment was performed approximating a conductive heat transfer profile in order to interpret and apply results of constant‐rate cooling experiments to the natural cooling of magma. Results suggest that the textures of constant‐rate experiments parallel the initial period of rapid cooling in natural magma. Initial cooling rates of ?20 °C h^?1in the lava hosting MIL 03346 occur in conductively solidifying lava at depths of?0.4 m, constraining the minimum total thickness to 0.8 m. Crystal accumulation beginning in a subsurface reservoir and continuing after lava emplacement as an inflated pahoehoe sheet satisfies all textural constraints on the late‐stage igneous history of MIL 03346.     

Rotational modeling of Hyperion

Saturn’s moon, Hyperion, is subject to strongly-varying solid body torques from its primary and lacks a stable spin state resonant with its orbital frequency. In fact, its rotation is chaotic, with a Lyapunov timescale on the order of 100 days. In 2005, Cassini made three close passes of Hyperion at intervals of 40 and 67 days, when the moon was imaged extensively and the spin state could be measured. Curiously, the spin axis was observed at the same location within the body, within errors, during all three fly-bys—~ 30° from the long axis of the moon and rotating between 4.2 and 4.5 times faster than the synchronous rate. Our dynamical modeling predicts that the rotation axis should be precessing within the body, with a period of ~ 16 days. If the spin axis retains its orientation during all three fly-bys, then this puts a strong constraint on the in-body precessional period, and thus the moments of inertia. However, the location of the principal axes in our model are derived from the shape model of Hyperion, assuming a uniform composition. This may not be a valid assumption, as Hyperion has significant void space, as shown by its density of 544± 50  kg m⁻³ (Thomas et al. in Nature 448:50, 2007). This paper will examine both a rotation model with principal axes fixed by the shape model, and one with offsets from the shape model. We favor the latter interpretation, which produces a best-fit with principal axes offset of ~ 30° from the shape model, placing the A axis at the spin axis in 2005, but returns a lower reduced χ ² than the best-fit fixed-axes model.     

Coma Morphology And Constraints On The Rotation Of Comet Hale–Bopp (C/1995 O1)

We present constraints on the spin state of comet Hale-Bopp based on coma morphology. Three cases of rotational states are compatible with near perihelion observations: (1) principal-axis rotation, (2) complex rotational state with a small precessional angle, or (3) complex rotational state with a large ratio between the component periods. For principal axis rotators, images from 1996 (pre-perihelion) are consistent with a rotational angular momentum vector, M, directed at ecliptic longitude and latitude (250°, -5°) while images from late 1997 (post-perihelion) indicate (310°, -40°). This may suggest a change in M. A complex rotational state with small precessional angle requires only a small or no change in M over the active orbital arc. In this case, M is directed near ecliptic longitude and latitude (270°, -20°). A rotationally excited nucleus with a large ratio between component periods requires the nucleus to be nearly spherical. The transformation of dust coma morphology from near-radial jets to bright arcs and then again to near-radial jets is interpreted as a heliocentric and geocentric distance dependent evolutionary sequence. The spiral structures seen in CN filters near perihelion (in contrast to sunward side arcs seen in continuum) can be explained if the precursor of CN molecules (likely sub-micron grains) are emitted from the nucleus at low levels (≈ 10% of the peak daytime emission) during the nighttime. This may be indicative of a nucleus with a CO-rich active area(s). This revised version was published online in July 2006 with corrections to the Cover Date.     

Statistical and Physical Study of One-Sided Planetary Nebulae

We have investigated the spatial orientation of one-sided Planetary Nebulae. Most of them if not all are interacting with the interstellar medium. Seventy percent of the nebulae in our sample have inclination angles larger than 45^° to the galactic plane and 30% of the inclination angles are less than 45^°. Most of the selected objects are old, evolved planetary nebulae with large dimensions, and not far away from the galactic plane. Seventy-five percent of the objects are within 160 pc from the galactic plane. The enhanced concavity arc can be explained physically as a result of the `planetary nebulae – interstellar matter' interaction. We have discussed the possible effect of the interstellar magnetic field in the concavity regions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Atmospheric tides and the rotation of Venus II. Spin evolution

Tides in the atmosphere of Venus may help to stabilize its slow retrograde rotation. The frequency dependence of the body tides also affects its rotational stability. However, the obliquity is probably maintained near 180° by friction between the core and mantle of Venus. In any case, it appears most likely that Venus originated with an obliquity greater than 90°.     

Lava flooding of ancient planetary crusts: Geometry,thickness, and volumes of flooded lunar impact basins

Estimates of lava volumes on planetary surfaces provide important data on the lava flooding history and thermal evolution of a planet. Lack of information concerning the configuration of the topography prior to volcanic flooding requires the use of a variety of techniques to estimate lava thicknesses and volumes. A technique is described and developed which provides volume estimates by artificially flooding unflooded lunar topography characteristic of certain geological environments, and tracking the area covered, lava thicknesses, and lava volumes. Comparisons of map patterns of incompletely buried topography in these artificially flooded areas are then made to lava-flooded topography on the Moon in order to estimate the actual lava volumes.This technique is applied to two areas related to lunar impact basins; the relatively unflooded Orientale basin, and the Archimedes-Apennine Bench region of the Imbrium basin. Artificially flooding the Orientale basin to the Cordillera Mountains (outer basin ring) produces a lava fill geometry with two components; (a) thebasin interior (within the inner Rook ring) where the area covered is small but lava thicknesses are large (6–8 km), and (b) thebasin, edges where larger areas are covered but thicknesses are less, averaging about 2 km. Detailed examination of the Archimedes-Apennine Bench area (Imbrium basin edge) also shows average thicknesses in this region of basins of approximately 2 km.On the basis of these analyses it is concluded that early flooding of the basin interior places a major load on the lithosphere in the same geographic region where mascon gravity anomalies are concentrated. Mare ridges and arches are concentrated at the outer edge of the region of thickset fill and appear to be related to tectonic activity accompanying basin loading and downwarping. Lava thicknesses in most areas of flooded, impact basins (>2 km) exceed the thickness of lava where vertical mixing of underlying non-mare material is possible. Thus, vertical mixing is not likely to have been an important process in mare deposits within flooded impact basins. Thickness estimates derived from this technique exceed those derived from the morphometry of buried or partially buried craters by at least a factor of two. Examination of the assumptions employed in the latter technique show several sources of variability (e.g., initial rim height variability in a fresh crater) which may result in significant underestimation of lava thicknesses.     

Noble gases in presolar diamonds I: Three distinct components and their implications for diamond origins

Abstract— High-purity separates of presolar diamond were prepared from 14 primitive chondrites from 7 compositional groups. Their noble gases were measured using stepped pyrolysis. Three distinct noble gas components are present in diamonds, HL, P3, and P6, each of which is found to consist of five noble gases. P3 , released between 200 °C and 900 °C, has a “planetary” elemental abundance pattern and roughly “normal” isotopic ratios. HL , consisting of isotopically anomalous Xe-HL and Kr-H, Ar with high ³⁸Ar/³⁶Ar, and most of the gas making up Ne-A2 and He-A, is released between 1100 °C and 1600 °C. HL has “planetary” elemental ratios, except that it has much more He and Ne than other known “planetary” components. HL gases are carried in the bulk diamonds, not in some trace phase. P6 has a slightly higher median release temperature than HL and is not cleanly separated from HL by stepped pyrolysis. Our data suggest that P6 has roughly “normal” isotopic compositions and “planetary” elemental ratios. Both P3 and P6 seem to be isotopically distinct from P1, the dominant “planetary” noble-gas component in primitive chondrites. Release characteristics suggest that HL and P6 are sited in different carriers within the diamond fractions, while P3 may be sited near the surfaces of the diamonds. We find no evidence of separability of Xe-H and Xe-L or other isotopic variations in the HL component. However, because ～10¹⁰ diamonds are required to measure a Xe composition, a lack of isotopic variability does not constrain diamonds to come from a single source. In fact, the high abundance of diamonds in primitive chondrites and the presence of at least three distinct noble-gas components strongly suggest that diamonds originated in many sources. Relative abundances of noble-gas components in diamonds correlate with degree of thermal processing (see companion paper), indicating that all meteorites sampled essentially the same mixture of diamonds. That mixture was probably inherited from the Sun's parent molecular cloud.     

Tectonic patterns on a reoriented planet: Mars

Both geologic and free-air-gravity data suggest that the positive mass anomaly associated with the Tharsis volcanoes may have reoriented Mars' lithosphere by as much as 25°. Since Mars is oblate (with flattening $\text{? ?0.005}$), rotation of the lithosphere over the equatorial bulge by 25° produces membrane stresses of several kilobars, large enough to initiate faulting. These stresses were first evaluated by F.A. Vening-Meinesz (1947, Trans. Amer. Geophys. Union28, 1–61) who treated the lithosphere as a thin elastic shell. The fracture patterns which result from these stresses are determined by the relation between stress and faulting proposed by E.M. Anderson (1951, The Dynamics of Faulting, Oliver & Boyd, Edinburgh). Plots of the magnitude and direction of stresses in a reoriented planet show that near Tharsis the dominant fault type should be north-south- trending normal faults. This normal fault province is centered about 30°N latitude and extends about 45° east and west in longitude. Similar faults should occur at the antipodes, north of Hellas Planitia. The polar regions should be occupied by roughly north-south-trending thrust faults which extend close to the equator south of Tharsis and north of Hellas. The regions between Tharsis and Hellas are subject to compression on a NE-trending axis and extension along a NW axis east of Tharsis (west of Tharsis the directions are NW compression and NE extension), thus predicting a zone of NNW and ENE strike slip faults east of Tharsis (NNE and WNW west of Tharsis). Although these patterns, except for the north-south normal faults north of Tharsis, have not yet been recognized, the discovery of such a tectonic system of the same age as Tharsis would provide strong support for the reorientation idea. Stresses due to reorientation appear to have little to do with Valles Marineris, since the stress normal to the axis of the Valles is predicted to be compressive, whereas geologic evidence suggests extension.     

关于北斗G2卫星的溯往追终

2022年1月, 失效的北斗G2卫星被实践21号卫星从地球静止轨道拖入了坟墓轨道. 为了这项捕获任务的安全实施, 需要预先确定北斗G2的旋转状态. 基于过去10 yr的测光观测数据展示了北斗G2卫星自转的演化过程. 根据北斗G2的自转速度和轨道的演化, 确认了在过去的10 yr里发生的6次异常事件. 据推测, 2012年的小碎片碰撞事件, 是随后几年燃油泄漏的导火索. 2017年之后剩余燃油完全释放, 再也没有出现转速异常. 将2014年太阳能帆板损坏和2016年的解体事件后建立的旋转动力学模型外推1 yr, 转轴的标准偏差小于3°, 转速标准偏差为0.11°· s^-1, 能够有效地满足捕获任务时刻旋转状态的精度要求.     

A tidal hypothesis about the origin of planetary rotation

We assume that the solar tidal action on accreting protoplanets forced them to corotation. If this is true, and assuming a subsequent conservation of spin angular momentum, we can easily get a rough explanation of: (1) the actual rotation periods of the planets, (2) the trend toward small inclination angles for the rotation axes, (3) the existence of a relation between rotational angular momenta and planetary masses similar to the empirical ones due to McDonald (1964). Hartmann and Larson (1967) and Fish (1967).