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1.
Peter Thomas 《Icarus》1979,40(2):223-243
Viking Orbiter images have provided nearly complete coverage of the two satellites of Mars and have been used to construct maps of the surface features of Phobos and Deimos. The satellites have radically different appearances although nearly all features on both objects were formed directly or indirectly by impact cratering. Phobos has an extensive network of linear depressions (grooves) that probably were formed indirectly by the largest impact recorded on Phobos. Deimos lacks grooves as well as the large number of ridges that occur on Phobos. Craters on Deimos have substantial sediment fill; those on Phobos have none. Evidence of downslope movement of debris is prominent on Deimos but is rare on Phobos. Many of the differences between Phobos and Deimos may be caused by modest differences in mechanical properties. However, the lack of a very large crater on Deimos may be responsible for its lack of grooves. 相似文献
2.
Nathalia Alzate 《Icarus》2011,211(2):1274-1283
Central pit craters are common on Mars, Ganymede and Callisto, and thus are generally believed to require target volatiles in their formation. The purpose of this study is to identify the environmental conditions under which central pit craters form on Ganymede. We have conducted a study of 471 central pit craters with diameters between 5 and 150 km on Ganymede and compared the results to 1604 central pit craters on Mars (diameter range 5-160 km). Both floor and summit pits occur on Mars whereas floor pits dominate on Ganymede. Central peak craters are found in similar locations and diameter ranges as central pit craters on Mars and overlap in location and at diameters <60 km on Ganymede. Central pit craters show no regional variations on either Ganymede or Mars and are not concentrated on specific geologic units. Central pit craters show a range of preservation states, indicating that conditions favoring central pit formation have existed since crater-retaining surfaces have existed on Ganymede and Mars. Central pit craters on Ganymede are generally about three times larger than those on Mars, probably due to gravity scaling although target characteristics and resolution also may play a role. Central pits tend to be larger relative to their parent crater on Ganymede than on Mars, probably because of Ganymede’s purer ice crust. A transition to different characteristics occurs in Ganymede’s icy crust at depths of 4-7 km based on the larger pit-to-crater-diameter relationship for craters in the 70-130-km-diameter range and lack of central peaks in craters larger than 60-km-diameter. We use our results to constrain the proposed formation models for central pits on these two bodies. Our results are most consistent with the melt-drainage model for central pit formation. 相似文献
3.
D.W.G Sears P.H Benoit S.W.S McKeever D Banerjee T Kral W Stites L Roe P Jansma G Mattioli 《Planetary and Space Science》2002,50(9):821-828
The recently established Arkansas-Oklahoma Center for Space and Planetary Science has been given a large planetary simulation chamber by the Jet Propulsion Laboratory, Pasadena, California. When completely refurbished, the chamber will be dubbed Andromeda and it will enable conditions in space, on asteroids, on comet nuclei, and on Mars, to be reproduced on the meter-scale and surface and subsurface processes monitored using a range of analytical instruments. The following projects are currently planned for the facility. (1) Examination of the role of surface and subsurface processes on small bodies in the formation of meteorites. (2) Development of in situ sediment dating instrumentation for Mars. (3) Studies of the survivability of methanogenic microorganisms under conditions resembling the subsurface of Mars to test the feasibility of such species surviving on Mars and identify the characteristics of the species most likely to be present on Mars. (4) The nature of the biochemical “fingerprints” likely to have been left by live organisms on Mars from a study of degradation products of biologically related molecules. (5) Testing local resource utilization in spacecraft design. (6) Characterization of surface effects on reflectivity spectra for comparison with the data from spacecraft-borne instruments on Mars orbiters. 相似文献
4.
The Tractus Fossae region of Mars is a wide area dominated by grabens, normal faults and pit crater chains. In this work, based on previous studies on the area and the new interpretation of topographic data and morphological units based on images as a geologic framework we present a new insight on the origin of the graben structures as well as on the processes associated with the formation of the volcanic features. Here we propose a new model for this region, on the basis of new measurements of graben extension and geological interpretations, together with a reinterpretation of the stratigraphy and the geologic history of the area. Finally, it is postulated that this region underwent a tectonic regime analog to an asymmetric rift system on Earth. 相似文献
5.
6.
Mohammad J. Mashayekhi Arun K. Misra 《Celestial Mechanics and Dynamical Astronomy》2016,125(3):363-380
Potentially hazardous near-Earth objects which can impose a significant threat on life on the planet have generated a lot of interest in the study of various asteroid deflection strategies. There are numerous asteroid deflection techniques suggested and discussed in the literature. This paper is focused on one of the non-destructive asteroid deflection strategies by attaching a long tether–ballast system to the asteroid. In the existing literature on this technique, very simplified models of the asteroid-tether–ballast system including a point mass model of the asteroid have been used. In this paper, the dynamical effect of using a finite size asteroid model on the asteroid deflection achieved is analyzed in detail. It has been shown that considering the finite size of the asteroid, instead of the point mass approximation, can have significant influence on the deflection predicted. Furthermore the effect of the tether-deployment stage, which is an essential part of any realistic asteroid deflection mission, on the predicted deflection is studied in this paper. Finally the effect of cutting the tether on the deflection achieved is analyzed and it has been shown that depending on the orbital properties of the asteroid as well as its size and rotational rate, cutting the tether at an appropriate time can increase the deflection achieved. Several numerical examples have been used in this paper to elaborate on the proposed technique and to quantitatively analyze the effect of different parameters on the asteroid deflection. 相似文献
7.
Analytical estimates of melt volumes produced by a given projectile and contained in a given impact crater are derived as a function of impact velocity, impact angle, planetary gravity, target and projectile densities, and specific internal energy of melting. Applications to impact events and impact craters on the Earth, Moon, and Mars are demonstrated and discussed. The most probable oblique impact (45°) produces ～1.6 times less melt volume than a vertical impact, and ～1.6 and 3.7 times more melt volume than impacts with 30° and 15° trajectories, respectively. The melt volume for a particular crater diameter increases with planetary gravity, so a crater on Earth should have more melt than similar-size craters on Mars and the Moon. The melt volume for a particular projectile diameter does not depend on gravity, but has a strong dependence on impact velocity, so the melt generated by a given projectile on the Moon is significantly larger than on Mars. Higher surface temperatures and geothermal gradients increase melt production, as do lower energies of melting. Collectively, the results imply thinner central melt sheets and a smaller proportion of melt particles in impact breccias on the Moon and Mars than on Earth. These effects are illustrated in a comparison of the Chicxulub crater on Earth, linked to the Cretaceous–Tertiary mass extinction, Gusev crater on Mars, where the Mars Exploration Rover Spirit landed, and Tsiolkovsky crater on the Moon. The results are comparable to those obtained from field and spacecraft observations, other analytical expressions, and hydrocode simulations. 相似文献
8.
We review the present status of how astronomical observations allow to constrain the evolution of fundamental constants of
nature. The main observational constraints on the variation of the gravitational constant, on the fine structure constant
and on the proton-to-electron mass ratio are reviewed. We also elaborate on some theoretical schemes which naturally lead
to such variations. 相似文献
9.
NG. Ibohal 《Astrophysics and Space Science》1997,249(1):73-93
The existence of skew symmetric Killing-Yano (KY) tensors of order 2 has been investigated on curved spaces. The integrability conditions of KY tensor including Carter's algebraic relation of symmetric tensor with electromagnetic field have been transcribed in Newman-Penrose formalism. The KY bivectors are classified according to their nullity in electrovac space-times. It is shown that the non-null (or null) electromagnetic field implies to the existence of non-null (or null) KY tensor. Thus Collinson's theorems on the existence of KY tensors on vacuum space-times have been generalized on electrovac space-times. Chandrasekhar's theorem on vacuum type D space-times has also been generalized on the existence of non-null KY tensor on electrovac type D or non-vacuum type D space-time filled with dust. All theorems presented here have been strengthened by giving examples for known space-times. It is also shown that most of KY tensors discussed here are eigen-KY-bivectors of the respective curvature tensors. 相似文献
10.
Spectroscopic and infrared observations of Comet Sugano-Saigusa-Fujikawa (1983V) were obtained during its close approach to the Earth on 11-14 June 1983. The [O I] production rates of 1.8 +/- 0.9 x 10(26) atoms/s observed on 12.3 June and 7 +/- 3.5 x 10(26) atoms/s on 13.4 June lead to derived water-production rates of 3 x 10(27) mol/s on 12 June and 1.1 x 10(28) mol/s on 13 June. The abundances of the minor species NH2, CN, C2, and C3 are unusually low relative to [O I]. The upper limit to the average nuclear radius from our infrared and visual photometry on 12-13 June (assuming that the entire signal came from the nucleus) is approximately 370 m. The dust/gas mass ratio was <0.01 on June 12 and <0.005 on June 13. 相似文献
11.
D. R. Bates
R. J. Moffett
《Planetary and Space Science》1968,16(12):1531-1537Using Cummack's model (in which non-linear terms are ignored) detailed calculations are carried out on the diurnal variations of planetary thermospheres. The dependences on the speed of rotation of the atmosphere; on the specific heat, the thermal conductivity and the absorption coefficient of the air; and on the pressure at the lower boundary, are separated. The diurnal variation of the terrestrial thermosphere is discussed. It is not yet possible to reach a firm conclusion as to whether or not non-linear terms have a major effect on this variation. 相似文献
12.
《Icarus》1975,26(3):377-385
In March 1974 the Board of the Scientific Council on the Radio Astronomy Problem Area, Academy of Sciences of the USSR, considered and approved a Research Program on the Problem of Communication with Extraterrestrial Civilizations. The Program was developed by the Search for Cosmic Signals of Artificial Origin section of the Council on Radio Astronomy, on the basis of recommendations made at the Soviet National Conference on the Problem of Comunication with Extraterrestrial Civilizations held at the Byurakan Astrophysical Observatory in Armenia in May 1964, and the Soviet-American CETI conference held at Byurakan in September 1971. The projected program was reported to the 7th Soviet National Conference on Radio Astronomy, which convened at Gor'kii in 1972. 相似文献
13.
In this paper the tidal phenomena on the Earth are concisely specified, including solid tides, ocean tides and atmospheric tides due to the luni-solar tide-generating force, and the Earth pole tide due to the motion of the Earth's rotation axis (polar motion); as well as their effects on the Earth rotation. The outcomes of scientific researches of Chinese astronomers on these topics are described in some detail. These researches deal with the mechanisms responsible for tidal effects on the earth rotation, and on the measurements of the Earth rotation parameters. Finally, the effects discovered by Chinese researchers on the measurements of the period and change in period of pulsars are discussed. These effects are very small in magnitude but not negligible. 相似文献
14.
Sarah J. Morrison Peter C. Thomas Matthew S. Tiscareno Joseph A. Burns Joseph Veverka 《Icarus》2009,204(1):262-270
High-resolution images from the Cassini Imaging Science Subsystem (ISS) show parallel sets of grooves on Epimetheus and Pandora. Grooves have previously been observed on other satellites and asteroids, including Phobos, Gaspra, Ida, Eros, and minor occurrences on Phoebe. Sets of parallel grooves are so far observed only on satellites known or likely to be subject to significant tidal stresses, such as forced librations. Grooves on asteroids and on satellites not subject to significant forced librations occur in more globally disorganized patterns that may reflect impacts, varying internal structures, or even thermal stresses. The patterns and individual morphologies of grooves on the tidally-affected satellites suggest fracturing in weak materials due to tidal stresses and forced librations. 相似文献
15.
M. Fränz J.D. Winningham E. Roussos S. Barabash M. Holmström M. Yamauchi R.A. Frahm J.R. Scherrer Y. Soobiah D.O. Kataria T. Säles W. Schmidt J. Kozyra E. Roelof S. Livi K.C. Hsieh M. Grande J.-A. Sauvaud J.-J. Thocaven S. Orsini M. Maggi P. Bochsler K. Asamura 《Icarus》2006,182(2):406-412
Using data of the ASPERA-3 instrument on board the European Mars Express spacecraft we investigate the effect of the martian crustal fields on electrons intruding from the magnetosheath. For the crustal field strength we use published data obtained by the Mars Global Surveyor MAG/ER instrument for a fixed altitude of 400 km. We use statistics on 13 months of 80-100 eV electron observations to show that the electron intrusion altitude determined by a probability measure is approximately linearly dependent on the total field strength at 400 km altitude. We show that on the dayside the mean electron intrusion altitude describes the location of the Magnetic Pile-Up Boundary (MPB) such that we can quantify the effect of the crustal fields on the MPB. On the nightside we quantify the shielding of precipitating electrons by the crustal fields. 相似文献
16.
Sébastien Picaud Emmanuel Chereul Annie C. Robin Francisco Garzón 《Astrophysics and Space Science》2003,284(2):523-526
We present the results of two studies of the inner Galaxy : on the one hand, a maximum likelihood fitting method using the Besançon model of Galaxy to constrain the density laws of the outer bulge and the holed disc; on the other hand, the comparison of Besançon simulations with infrared data on fields near longitude 20–27 degrees to verify the existence of a long and thin in-plane bar. These two parallel studies seem to show the co-existence of a structure (the bar) in addition with a very oblate outer bulge. 相似文献
17.
18.
Tisserand's definition of the “sphere of action” of a planet is based on the equality of tidal vs. gravitational acceleration ratios of the sun and planet. Öpik and others based their relation on equating the differential solar and planetary forces on the particle.Neither expression was formulated to describe the zone of influence surrounding a planet when considering the small but significant (i.e. long-term) perturbative effects of the planets on a particle's orbital elements. For the purpose of determining these effects on interplanetary dust we dervive a zone of influence based on equating the gravitational forces of the sun and planet, and demonstrate its applicability by utilizing the particle's closest approach to the planet as a measure of the zone of influence. 相似文献
19.
Larry A. Lebofsky 《Icarus》1975,25(2):205-217
Calculations on the stability of various frosts (against evaporation) for solar system objects in circular and elliptical orbits are made. It is found that the stability of these frosts is dependent on the rate of rotation of the object, the latitude of the area on the object being considered, and the eccentricity of the orbit as well as its mean distance from the Sun. These factors greatly influence the amount of solar radiation incident and reradiated from a given area on the object. The likelihood of finding these frosts on the surfaces of objects and the lifetimes of objects composed of these frosts is discussed. 相似文献
20.
V. V. Prokof’eva-Mikhailovskaya V. V. Busarev N. N. Gor’kavyi A. N. Rublevskii 《Kinematics and Physics of Celestial Bodies》2011,27(6):291-298
A new spectral-frequency method (SFM) for the study of solid body surfaces is briefly described. This method allows estimation
of the sizes of various spots. Estimates for the sizes of spots on asteroid surfaces made by the SFM and other methods are
compared and discussed. The sizes of spots on the surface of asteroid 1620 Geographos determined by the SFM are well consistent
with those of the craters obtained from radar data. The sizes of hydrosilicate spots on the surface of asteroid 21 Lutetia
found by the SFM agree with those of the craters determined by the Rosetta spacecraft. The size of a blue spot on the surface
of asteroid 4 Vesta found by the SFM is consistent with the size of the well-known crater on the south pole of the asteroid.
It is inferred that the SFM is a promising method for the estimation of the sizes of spots on asteroid surfaces. 相似文献