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1.
The origin of the Martian moons, Phobos and Deimos, is still an open issue: either they are asteroids captured by Mars or they formed in situ from a circum-Mars debris disk. The capture scenario mainly relies on the remote-sensing observations of their surfaces, which suggest that the moon material is similar to outer-belt asteroid material. This scenario, however, requires high tidal dissipation rates inside the moons to account for their current orbits around Mars. Although the in situ formation scenarios have not been studied in great details, no observational constraints argue against them. Little attention has been paid to the internal structure of the moons, yet it is pertinent for explaining their origin. The low density of the moons indicates that their interior contains significant amounts of porous material and/or water ice. The porous content is estimated to be in the range of 30?C60% of the volume for both moons. This high porosity enhances the tidal dissipation rate but not sufficiently to meet the requirement of the capture scenario. On the other hand, a large porosity is a natural consequence of re-accretion of debris at Mars?? orbit, thus providing support to the in situ formation scenarios. The low density also allows for abundant water ice inside the moons, which might significantly increase the tidal dissipation rate in their interiors, possibly to a sufficient level for the capture scenario. Precise measurements of the rotation and gravity field of the moons are needed to tightly constrain their internal structure in order to help answering the question of the origin.  相似文献   

2.
The determination of the ephemeris of the Martian moons has benefited from observations of their plane-of-sky positions derived from images taken by cameras onboard spacecraft orbiting Mars. Images obtained by the Super Resolution Camera (SRC) onboard Mars Express (MEX) have been used to derive moon positions relative to Mars on the basis of a fit of a complete dynamical model of their motion around Mars. Since, these positions are computed from the relative position of the spacecraft when the images are taken, those positions need to be known as accurately as possible. An accurate MEX orbit is obtained by fitting two years of tracking data of the Mars Express Radio Science (MaRS) experiment onboard MEX. The average accuracy of the orbits has been estimated to be around 20–25 m. From these orbits, we have re-derived the positions of Phobos and Deimos at the epoch of the SRC observations and compared them with the positions derived by using the MEX orbits provided by the ESOC navigation team. After fit of the orbital model of Phobos and Deimos, the gain in precision in the Phobos position is roughly 30 m, corresponding to the estimated gain of accuracy of the MEX orbits. A new solution of the GM of the Martian moons has also been obtained from the accurate MEX orbits, which is consistent with previous solutions and, for Phobos, is more precise than the solution from the Mars Global Surveyor (MGS) and Mars Odyssey (ODY) tracking data. It will be further improved with data from MEX-Phobos closer encounters (at a distance less than 300 km). This study also demonstrates the advantage of combining observations of the moon positions from a spacecraft and from the Earth to assess the real accuracy of the spacecraft orbit. In turn, the natural satellite ephemerides can be improved and participate to a better knowledge of the origin and evolution of the Martian moons.  相似文献   

3.
CHOMIK is the name of a penetrator constructed for sampling and retrieval of Phobos surface material. It formed an integral part of the Phobos Sample Return Mission. In this paper we present its construction and intended mode of operation, since the concept is still viable for future missions either to Phobos or to other small bodies of similar dimensions. We take Phobos as an example to describe the science case for such an instrument and how it might be utilized to resolve important open issues regarding the origin of the Martian moons. Concerning the latter, we place emphasis on measurement techniques and analysis tools for mapping trace element concentrations in returned sample.  相似文献   

4.
The Phobos-Soil project, based on a new generation of spacecraft, is aimed at the study of Phobos, one of the Martian moons. The main goal of the project is to bring soil samples from the surface of Phobos to the Earth for a scrutinized analysis of the relic matter collected on one of the Solar System’s minor bodies. The project is considered to be a step in the long-scale extensive quest for data on planets, comets, asteroids, and solving a vast number of other theoretical and applied problems.  相似文献   

5.
The High-Resolution Imaging Science Experiment (HiRISE) onboard Mars Reconnaissance Orbiter (MRO) has been used to observe Phobos and Deimos at spatial scales of around 6 and 20 m/px, respectively. HiRISE (McEwen et al., JGR, 112, CiteID E05S02, DOI: 10.1029/2005JE002605, 2007) has provided, for the first time, high-resolution colour images of the surfaces of the Martian moons. When processed, by the production of colour ratio images for example, the data show considerable small-scale heterogeneity, which might be attributable to fresh impacts exposing different materials otherwise largely hidden by a homogenous regolith. The bluer material that is draped over the south-eastern rim of the largest crater on Phobos, Stickney, has been perforated by an impact to reveal redder material and must therefore be relatively thin. A fresh impact with dark crater rays has been identified. Previously identified mass-wasting features in Stickney and Limtoc craters stand out strongly in colour. The interior deposits in Stickney appear more inhomogeneous than previously suspected. Several other local colour variations are also evident.Deimos is more uniform in colour but does show some small-scale inhomogeneity. The bright “streamers” (Thomas et al., Icarus, 123, 536–556,1996) are relatively blue. One crater to the south-west of Voltaire and its surroundings appear quite strongly reddened with respect to the rest of the surface. The reddening of the surroundings may be the result of ejecta from this impact.The spectral gradients at optical wavelengths observed for both Phobos and Deimos are quantitatively in good agreement with those found by unresolved photometric observations made by the Imager for Mars Pathfinder (IMP; Thomas et al., JGR, 104, 9055–9068, 1999). The spectral gradients of the blue and red units on Phobos bracket the results from IMP.  相似文献   

6.
The origin of Saturn’s inner mid-sized moons (Mimas, Enceladus, Tethys, Dione and Rhea) and Saturn’s rings is debated. Charnoz et al. [Charnoz, S., Salmon J., Crida A., 2010. Nature 465, 752–754] introduced the idea that the smallest inner moons could form from the spreading of the rings’ edge while Salmon et al. [Salmon, J., Charnoz, S., Crida, A., Brahic, A., 2010. Icarus 209, 771–785] showed that the rings could have been initially massive, and so was the ring’s progenitor itself. One may wonder if the mid-sized moons may have formed also from the debris of a massive ring progenitor, as also suggested by Canup [Canup, R., 2010. Nature 468, 943–946]. However, the process driving mid-sized moon accretion from the icy debris disks has not been investigated in details. In particular, Canup’s (2010) model does not seem able to explain the varying silicate contents of the mid-sized moons (from 6% to 57% in mass). Here, we explore the formation of large objects from a massive ice-rich ring (a few times Rhea’s mass) and describe the fundamental properties and implications of this new process. Using a hybrid computer model, we show that accretion within massive icy rings can form all mid-sized moons from Mimas to Rhea. However in order to explain their current locations, intense dissipation within Saturn (with Qp < 2000) is required. Our results are consistent with a satellite origin tied to the rings formation at least 2.5 Gy ago, both compatible with either a formation concurrent to Saturn or during the Late Heavy Bombardment. Tidal heating related to high-eccentricity post-accretional episodes may induce early geological activity. If some massive irregular chunks of silicates were initially present within the rings, they would be present today inside the satellites’ cores which would have accreted icy shells while being tidally expelled from the rings (via a heterogeneous accretion process). These moons may be either mostly icy, or, if they contain a significant amount of rock, already differentiated from the ice without the need for radiogenic heating. The resulting inner mid-sized moons may be significantly younger than the Solar System and a ∼1 Gyr formation delay is possible between Mimas and Rhea. The rings resulting from this process would evolve to a state compatible with current mass estimates of Saturn’s rings, and nearly devoid of silicates, apart from isolated silicate chunks coated with ice, interpreted as today Saturn’s rings’ propellers and ring-moons (like Pan or Daphnis).  相似文献   

7.
One of the main goals of the Phobos-Grunt project is to analyze the surface composition of Phobos. Plasma methods of the studies make it possible to complete direct surface studies with measurements of the minor components of the solar wind, which are produced when a material sputtered from the surface of Phobos is ionized. The surface of Phobos is sputtered under the action of solar wind protons, energetic ions, hard solar radiation, and meteorites. In addition to the studies of Phobos, the experiment also includes the study of the interaction between the solar wind and Mars. An energy-mass spectrometer, which makes it possible to measure instantaneously complete unobscured distribution of the flux of different ions in the hemisphere, has been designed based on the new CAMERA analyzer of charged particles proposed previously (Vaisberg et al., 2001, 2005; Vaisberg, 2003). The instrument’s electro-optics model and the results of the numerical and laboratory tests are described in this paper. Such an instrument can be used in magnetospheric studies and to study different objects of the Solar System.  相似文献   

8.
The paper considers how a spacecraft can be put into orbit around a small asteroid to function as its artificial moon. We study the general behavior of perturbations that affect the current coordinates of an orbiting spacecraft and estimate the perturbations caused by the main perturbing factors, i.e., (1) the irregular shape of an asteroid and (2) celestial bodies of the Solar System. With specific orbital parameters, a long-term targeted operation of a spacecraft can be actualized in a mission to the asteroid Apophis where the spacecraft will carry a radio beacon transponder.  相似文献   

9.
The opportunities to study Phobos’ internal structure provided by radio methods are considered in this paper. The necessity of these studies is related to solution of the problem of the origin of the Martian moons. Radiosounding is one of the most efficient methods of analyzing the internal structure of small space objects and, in particular, that of Phobos. The new Boomerang project planned according to the Federal Space Program of Russia for 2016—2025 within the Expedition-M program aimed at the exploration of Phobos and delivery of soil samples from its surface to the Earth, as well as the specifics of a ballistic scenario of this expedition, provide a unique opportunity to carry out radioscopy of this space object to discover the internal structure Phobos and to solve the key problem of its origin. The model of Phobos’ internal structure, radiosounding ballistic conditions, analysis of optimum frequency range of sounding, and key parameters of the device required for the experiment are considered in this paper. The significance of proposed studies and opportunities for their implementation are discussed.  相似文献   

10.
Magnetic measurements of meteorites suggest that small bodies (e.g. asteroids) in the Solar System have small but distinct magnetic fields produced by the bulk remanent magnetisation (NRM) of the body. Here we report calculations of magnetic fields of small bodies, assuming that they can be approximated as homogeneously magnetised spheres with dipole moments derived from NRM data on known meteorites. The magnetic fields are compared with the field of the asteroid 951 Gaspra measured by spacecraft Galileo in 1991 (Kivelson et al., 1993). The result of this comparison suggests that the field of Gaspra could be caused by an L-, H- or E-chondritic or a pallasite body. The spectral reflectance data on Gaspra suggest, however, that it is a basaltic achondrite. The problem can be resolved if Gaspra is a differentiated body, its surface material being closer to that of basaltic achondrites, and the bulk closer to ordinary chondrites or pallasites. We also present magnetic anomaly profiles along the surface of Mars such as would be measured with a magnetometer installed on a Rover-type vehicle by assuming that the main sources of the surface anomalies are the NRMs of the boulders on the Martian surface. The NRM values are taken from the data measured on SNC meteorites. The results suggest large oscillations in magnetic field intensity at the Martian surface.  相似文献   

11.
Magnetic measurements of meteorites suggest that small bodies (e.g. asteroids) in the Solar System have small but distinct magnetic fields produced by the bulk remanent magnetisation (NRM) of the body. Here we report calculations of magnetic fields of small bodies, assuming that they can be approximated as homogeneously magnetised spheres with dipole moments derived from NRM data on known meteorites. The magnetic fields are compared with the field of the asteroid 951 Gaspra measured by spacecraft Galileo in 1991 (Kivelson et al., 1993). The result of this comparison suggests that the field of Gaspra could be caused by an L-, H- or E-chondritic or a pallasite body. The spectral reflectance data on Gaspra suggest, however, that it is a basaltic achondrite. The problem can be resolved if Gaspra is a differentiated body, its surface material being closer to that of basaltic achondrites, and the bulk closer to ordinary chondrites or pallasites. We also present magnetic anomaly profiles along the surface of Mars such as would be measured with a magnetometer installed on a Rover-type vehicle by assuming that the main sources of the surface anomalies are the NRMs of the boulders on the Martian surface. The NRM values are taken from the data measured on SNC meteorites. The results suggest large oscillations in magnetic field intensity at the Martian surface.  相似文献   

12.
《Planetary and Space Science》2006,54(9-10):844-854
It has long been suspected that Mars might be encircled by two faint rings, one originating from each of its moons Phobos and Deimos. Meteoroid impacts into these moons should release clouds of dust that quickly spread out to become rings; similar dust rings have been associated with several small inner moons of the gas giants. On May 28, 2001 Mars’ hypothetical ring plane appeared edge-on to Earth within weeks of its opposition, providing the best Earth-based opportunity to detect these rings in several decades. Using the Wide Field/Planetary Camera 2 (WFPC2) on the Hubble Space Telescope, we obtained a set of deep exposures off the east and west limbs of Mars to search for these hypothetical rings. No rings were detected. This result limits normal optical depths to ∼3×10−8 for the Phobos ring and ∼10−7 for the Deimos ring. These limits fall at the low end of prior dynamical predictions and a factor of 1000 below previous observational limits. However, our limit for the Deimos ring is more tentative because of large uncertainties about this ring's expected shape, size and orientation. Our data set is also sensitive to small, previously undetected inner moons. No moons were detected down to a radius limit of 75–125 m. Longitudinal coverage of the region near and between Phobos and Deimos is 40–80% complete. We conclude by describing a promising opportunity for further Martian ring viewing in December 2007.  相似文献   

13.
The Kaidun meteorite exhibits an incredible diversity of extraterrestrial material. The parent body of the meteorite is mainly composed of carbonaceous chondrite material of the second petrological type. The meteorite is specific in its composition: it contains numerous fragments and inclusions formed at an early stage of the Solar System evolution by nebular condensation, gaseous metasomatosis, agglomeration, and other processes, and two different fragments of alkaline-enriched differentiated material, which entered the parent body as a result of different events. The data on the lithologic composition of the Kaidun meteorite give strong arguments for considering the meteorite's parent body to be a carbonaceous chondrite satellite of a large differentiated planet. Phobos, the moon of Mars, is the most probable candidate. Many features of the Kaidun meteorite can be well explained within the framework of the popular hypothesis of Phobos' origin based on the nebular capture model.  相似文献   

14.
The origin of the Martian satellites presents a puzzle of long standing. Addressing the composition of Phobos will help constrain theories of its formation. Visible and near-infrared spectra of Phobos lack deep absorption features, making the compositional interpretation a tricky task. PFS/MEx and TES/MGS observations in the thermal infrared show several spectral features that can be used to investigate the composition of the surface. Our results show that the majority of the spectra are consistent with the presence of phyllosilicates, particularly in the area northeast of Stickney. This area corresponds to the “blue” region as defined by Murchie et al. (1999). Analysis of PFS and TES observations in the “red” region defined by Murchie et al. (1999) are consistent with tectosilicates, especially feldspars/feldspathoids. We discuss several physical and chemical mechanisms that can act to eliminate or reduce the strength of bands in the VIS/NIR spectra, with possibly little or no effect in the mid-IR. Comparison of the TES and PFS data to the meteorites shows that no class of chondritic meteorites provide significant agreement with the spectral features observed. The lack of consistency of the PFS and TES spectra to analogs of ultraprimitive materials (organic residues) suggests that an origin via capture of a transneptunian object is not supported by these observations, although it cannot be completely ruled out. Derived surface temperatures from PFS and TES data are in very good agreement with brightness temperatures derived from Viking orbiter measurements, Earth-based observations, and values predicted by numerical models. Our results show that the surface temperature of Phobos varies with solar incidence angle and heliocentric distance, reconciling the different results.We collect and summarize the compositional clues for the origin of Phobos discussed in this paper, including our results. Currently, the most likely scenario is the in-situ formation of Phobos, although a capture of achrondrite-like meteorites is not ruled out.  相似文献   

15.
Remote observations with the Chandra X-ray Observatory and the XMM-Newton Observatory have shown that the jovian system is a source of X-rays with a rich and complicated structure. The planet's polar auroral zones and its disk are both powerful sources of X-ray emission. Chandra observations revealed X-ray emission from the Io plasma torus and from the Galilean moons Io, Europa, and possibly Ganymede. The emission from the moons is due to bombardment of their surfaces by highly energetic magnetospheric protons, and oxygen and sulfur ions. These ions excite atoms in their surfaces leading to fluorescent X-ray emission lines. These lines are produced against an intense background continuum, including bremsstrahlung radiation from surface interactions of primary magnetospheric and secondary electrons. Although the X-ray emission from the Galilean moons is faint when observed from Earth orbit, an imaging X-ray spectrometer in orbit around one or more of these moons, operating from 200 eV to 8 keV with 150 eV energy resolution, would provide a detailed mapping of the elemental composition in their surfaces. Surface resolution of 40 m for small features could be achieved in a 100-km orbit around one moon while also remotely imaging surfaces of other moons and Jupiter's upper atmosphere at maximum regional resolutions of hundreds of kilometers. Due to its relatively more benign magnetospheric radiation environment, its intrinsic interest as the largest moon in the Solar System, and its mini-magnetosphere, Ganymede would be the ideal orbital location for long-term observational studies of the jovian system. Here we describe the physical processes leading to X-ray emission from the surfaces of Jupiter's moons and the properties required for the technique of imaging X-ray spectroscopy to map the elemental composition of their surfaces, as well as studies of the X-ray emission from the planet's aurora and disk and from the Io plasma torus.  相似文献   

16.
Matija ?uk 《Icarus》2004,167(2):369-381
To elucidate the capture of Jupiter's outer moons, we reverse-evolve satellites from their present orbits to their original heliocentric paths in the presence of Jupiter's primordial circumplanetary disk (Lubow et al., 1999, Astrophys. J. 526, 1001-1012; Canup and Ward, 2003, Astron. J. 124, 3404-3423). Our orbital histories use a symplectic integrator that allows dissipation. We assume that the present satellites Himalia, Elara, Lysithea, Leda, and S/2000 J11 are collisional fragments of a single parent. Our simulations show that this “prograde-cluster progenitor” (PCP) could be derived from objects with heliocentric orbits like those of the Hilda asteroid group. We show analytically that this capture is energetically possible. We also compare the spectroscopic characteristics of the prograde cluster members (Grav et al., 2003, Icarus, submitted for publication) with those of the Hildas, and conclude that the surface color of the prograde-cluster progenitor is consistent with an origin within the Hilda group. Accordingly, gas drag in the primordial jovian nebula is found to offer a plausible explanation for the origin of the prograde cluster. A similar capture mechanism is proposed for Saturn's Phoebe.  相似文献   

17.
We show how to calculate the impact orbits of dangerous asteroids using the freely available the OrbFit software, and compare our results with impact orbits calculated using Sitarski??s independent software (Sitarski, 1999; 2000; 2006). The new method is tested on asteroid 2009 FJ. Using the OrbFit package to integrate alternate orbits along the line of variation (Milani et al., 2002; 2005a; 2005b), we identify impact orbits and can plot paths of risk for the Earth or any other body in the Solar System. We present the orbital elements of asteroid 2009 FJ and its ephemerides, along with uncertainties, for the next 100 years. This paper continues a long-term research program on impact solutions for asteroids (Wlodarczyk, 2007; 2008; 2009).  相似文献   

18.
Recent Viking results indicate the Martian satellites are composed of carbonaceous chondritic material, suggesting that Phobos and Deimos were once asteroids captured by Mars. On the other hand, the low eccentricities and inclinations of their orbits on the equator of Mars argue against that hypothesis. This paper presents detailed calculations of the tidal evolution of Phobos and Deimos, considering dissipation in both Mars and its satellites simultaneously and using a new method applicable for any value of the eccentricity. In particular, including precession of the satellites' orbits indicates that they have always remained close to their Laplacian plane, so that the orbital planes of Phobos and Deimos switched from near the Martian orbital plane to the Martian equator once the perturbations due to the planetary oblateness dominated the solar perturbations, as they do presently. The results show that Deimos has been little affected by tides, but several billion (109) years ago, Phobos was in a highly eccentric orbit lying near the common plane of the solar system. This outcome is obtained for very reasonable values of dissipation inside Mars and inside Phobos. Implications for the origin of the Martian satellites are discussed.  相似文献   

19.
More than 490 elliptical aerobraking and science phasing orbits made by Mars Global Surveyor (MGS) in 1997 and 1998 provide unprecedented coverage of the solar wind in the vicinity of the orbits of the martian moons Phobos and Deimos. We have performed a comprehensive survey of magnetic field perturbations in the solar wind to search for possible signatures of solar wind interaction with dust or gas escaping from the moons. A total of 1246 solar wind disturbance events were identified and their distribution was examined relative to Phobos, the Phobos orbit, and the Deimos orbit. We find that the spatial distribution of solar wind perturbations does not increase near or downstream of Phobos, Phobos’ orbit, or Deimos’ orbit, which would have been expected if there is significant outgassing or dust escape from the martian moons. Of the 1246 magnetic field perturbation events found in the MGS data set, 11 events were found within 2000 km of the Phobos orbit, while three events were found within 2000 km of the Deimos orbit. These events were analyzed in detail and found to likely have other causes than outgassing/dust escape from the martian moons. Thus we conclude that the amount of gas/dust escaping the martian moons is not significant enough to induce detectable magnetic field perturbations in the solar wind. In essence we have not found any clear evidence in the MGS magnetic field data for outgassing or dust escape from the martian moons.  相似文献   

20.
The numerous problems related to the origin and evolution of Phobos and Deimos, as well as to specific features of their topography, are readily accounted for in the context of a concept presupposing the possibility of detonation (or burning) of electrolyzed ices.The explosion of an icy asteroid of mass m A \> 1023 g within the gravitational sphere of Mars resulted in the capture of secondary fragments into satellite orbits and the formation of a ring of icy and rocky particles. The motion of satellites in the ring reduced the eccentricities of their orbits and rendered their mutual collisions impossible. The thick regolith of Deimos is the material captured from the ring.The impact responsible for the Stickney crater on Phobos initiated a detonation in the material in the crater, and the detonation, in its turn, ignited the products of electrolysis in the bulk of the satellite. Phobos lost its regolith because of detonation-induced acceleration. As a result of the burning out of Phobos's ices, its density somewhat exeeds that of Deimos.Different grooves on Phobos have differing origins. The grooves of groups B, E, D (as specified by P. Thomas et al., J. Geophys. Res., 84, 8457–8477, 1979) are due to fracturing caused by the Sickney event; group C (which is normal to the minor axis of the Phobos ellipsoid) reflects the layered nature of the parent asteroid; while the closely arranged group A grooves, which are normal to Phobos's major axis, were formed during the contraction of the satellite as a result of the loss of burnt-out ices. A number of conclusions are drawn which can be tested by future missions to the satellites.  相似文献   

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