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1.
We have determined the global fracture patterns resulting from combinations of stresses due to tidal despinning and contraction or expansion. We find that Mercury's lineament pattern is consistent with a history of despinning and contraction. According to our model, the observed tectonic pattern implies that the despinning process reached completion before the planet ceased contracting. Our model predicts a stress due to contraction which is up to 1.8 times the maximum despinning stress on Mercury. The maximum contractional stress could be as large as 4 times the maximum despinning stress if the oldest fractures on the planet are N-S thrust faults in the equatorial region.  相似文献   

2.
H.J. Melosh  D. Dzurisin 《Icarus》1978,35(2):227-236
The surface of Mercury exhibits a global tectonic system consisting of an ancient set of NE and NW trending lineaments and a younger set of planimetrically arcuate escarpments interpreted as thrust or high-angle reverse faults. The trends, distribution, and age relations of these tectonic features can be explained by a combination of tidal despinning and global contraction of the planet. In our model, early tidal despinning resulted in conjugate shear fractures trending roughly N60°E and N60°W which were subsequently modified by a variety of surface processes to produce the presently visible set of lineaments. Continued despinning plus global contraction produced thrust faults with roughly north-south trends. Final contraction may have postdated despinning and produced randomly oriented thrust faults. All of these events predated the formation of Caloris basin, because basin-associated deposits blanket both lineaments and arcuate thrust faults.  相似文献   

3.
Faults provide a record of a planet’s crustal stress state and interior dynamics, including volumetric changes related to long-term cooling. Previous work has suggested that Mars experienced a pulse of large-scale global contraction during Hesperian time. Here we evaluate the evidence for martian global contraction using a recent compilation of thrust faults. Fault-related strains were calculated for wrinkle ridges and lobate scarps to provide lower and upper bounds, respectively, on the magnitude of global contraction from contractional structures observed on the surface of Mars. During the hypothesized pulse of global contraction, contractional strain of −0.007% to −0.13% is indicated by the structures, corresponding to decreases in planetary radius of 112 m to 2.24 km, respectively. By contrast, consideration of all recognized thrust faults regardless of age produces a globally averaged contractional strain of −0.011% to −0.22%, corresponding to a radius decrease of 188 m to 3.77 km since the Early Noachian. The amount of global contraction predicted by thermal models is larger than what is recorded by the faults at the surface, paralleling similar studies for Mercury and the Moon, which suggests that observations of fault populations at the surface may provide tighter bounds on planetary thermal evolution than models alone.  相似文献   

4.
Analysis of images obtained by the MESSENGER spacecraft during its three flybys of Mercury yields a new estimate for the planet's mean radius of 2439.25±0.69 km, in agreement with results from Mariner 10 and Earth-based observations, as well as with MESSENGER altimeter and occultation data. The mean equatorial radius and polar radius are identical to within error, suggesting that rotational oblateness is negligible when compared with other sources of topography. This result is consistent with the small gravitational oblateness of the planet. Minor differences in radius obtained at different locations reflect regional variations in topography. Residual topography along three limb profiles has a dynamic range of 7.4 km and a root-mean-square roughness of 0.8 km over hemispherical scales. Following MESSENGER's entry into orbit about Mercury in March 2011, we expect considerable additional improvements to our knowledge of Mercury's size and shape.  相似文献   

5.
On 14 January and 6 October 2008 the MESSENGER spacecraft passed within 200 km of the surface of Mercury. These flybys by MESSENGER provided the first observations of Mercury from a spacecraft since the Mariner 10 flybys in 1974 and 1975. Data from the Mercury Laser Altimeter (MLA) provided new information on the equatorial shape of Mercury, and Doppler tracking of the spacecraft through the flybys provided new data on the planet’s gravity field. The MLA passes were on opposite hemispheres of the planet and span collectively ∼40% of the equatorial circumference. The mean elevation of topography observed during flyby 1, in the longitude range 0-90°E, is greater than that seen during flyby 2 in the longitude range 180-270°E, indicating an offset between centers of mass and figure having a magnitude and phase in general agreement with topography determined by Earth-based radar. Both MLA profiles are characterized by slopes of ∼0.015° downward to the east, which is consistent with a long-wavelength equatorial shape defined by a best-fitting ellipse. The Doppler tracking data show sensitivity to the gravitational structure of Mercury. The equatorial ellipticity of the gravitational field, C2,2, is well determined and correlates with the equatorial shape. The S2,2 coefficient is ∼0, as would be expected if Mercury’s coordinate system, defined by its rotational state, is aligned along its principal axes of inertia. The recovered value of the polar flattening of the gravitational potential, J2, is considerably lower in magnitude than the value obtained from Mariner 10 tracking, a result that is problematic for internal structure models. This parameter is not as well constrained as the equatorial ellipticity because the flyby trajectories were nearly in the planet’s equatorial plane. The residuals from the Doppler tracking data suggest the possibility of mascons on Mercury, but flyby observations are of insufficient resolution for confident recovery. For a range of assumptions on degree of compensation and crustal and mantle densities, the allowable crustal thickness is consistent with the upper limit of about 100 km estimated from the inferred depth of faulting beneath a prominent lobate scarp, an assumed ductile flow law for crustal material, and the condition that temperature at the base of the crust does not exceed the solidus temperature. The MESSENGER value of C2,2 has allowed an improved estimate of the ratio of the polar moment of inertia of the mantle and crust to the full polar moment (Cm/C), a refinement that strengthens the conclusion that Mercury has at present a fluid outer core.  相似文献   

6.
Among the terrestrial planets, Mercury is the smallest and has the highest bulk density. Mercury exhibits a lunar-like surface, shaped by impact basins and craters. Rapid cooling and contraction as well as tidal despinning have resulted in a large inventory of tectonic scarps and faults visible on the surface. With plans for new orbiter missions to this intriguing planet taking shape, this paper presents a summary of our current knowledge on Mercury's geology and cratering history. On the basis of improved data on asteroid populations and crater scaling, we updated the time stratigraphic sequence for the planet and made new estimates for the time of formation of impact basins such as Tolstoj and Caloris, which generally are now thought to be younger than in previous estimates. In order to advance our understanding of the geology of the planet, imaging experiments on future missions must fill the gap in the global coverage left by the Mariner spacecraft, and increase the global multispectral spatial resolution to at least 100 m/pixel. Locally, the image resolution must reach approx. 10 m/pixel. Also, stereo topographic models with global and local resolutions of 200 and 20 m, respectively, are required.  相似文献   

7.
Mounting attention has focused on interplanetary transfer of microorganisms (panspermia), particularly in reference to exchange between Mars and Earth. In most cases, however, such exchange requires millions of years, over which time the transported microorganisms must remain viable. During a large impact on Earth, however, previous work (J.C. Armstrong et al., 2002, Icarus 160, 183-196) has shown that substantial amounts of material return to the planet of origin over a much shorter period of time (< 5000 years), considerably mitigating the challenges to the survival of a living organism. Conservatively evaluating experiments performed [by others] on Bacillus subtilis and Deinococcus radiodurans to constrain biological survival under impact conditions, we estimate that if the Earth were hit by a sterilizing impactor ∼ 300 km in diameter, with a relative velocity of 30 km s−1 (such as may have occurred during the Late Heavy Bombardment), an initial cell population in the ejecta of order 103-105 cells kg−1 would in most cases be sufficient for a single modern organism to survive and return to an again-clement planet 3000-5000 years later. Although little can be said about the characteristics or distribution of ancient life, our calculations suggest that impact reseeding is a possible means by which life, if present, could have survived the Late Heavy Bombardment.  相似文献   

8.
The Mercury Laser Altimeter on the NASA MESSENGER mission has ranged to several ridges and lobate scarps during two equatorial flybys of the planet Mercury. The tectonic features sampled, like others documented by spacecraft imaging and Earth-based radar, are spatially isolated and have vertical relief in excess of 1 km. The profiles also indicate that the faulting associated with their formation penetrated to tens of kilometers depth into the lithosphere and accommodated substantial shortening. To gain insight into the mechanism(s) of strain accommodation across these structures, we perform analytical and numerical modeling of representative dynamic localization mechanisms. We find that ductile localization due to shear heating is not favored, given our current understanding of thermal gradients and shallow thermal structure of Mercury at the time of ridge and scarp formation, and is likely to be of secondary importance at best. Brittle localization, associated with loss of resistance during fault development or with velocity weakening during sliding on mature faults, is weakly localizing but permits slip to accumulate over geological time scales. The range of shallow thermal gradients that produce isolated faults rather than distributed fault sets under the assumption of modest fault weakening is consistent with previous models for Mercury’s early global thermal history. To be consistent with strain rates predicted from thermal history models and the amount of shortening required to account for the underlying large-offset faults, ridges and scarps on Mercury likely developed over geologically substantial time spans.  相似文献   

9.
A basin with an interior about 1000 km across and an outer rim about 2000 km in diameter has been found on the side of Mercury that remained unknown after theMariner 10imaging in 1973–1975. In its size, which is as much as 1/2 of the Mercury's diameter, this formation is one of the largest basins on the terrestrial planets. The presence of other large features of global dimension on this side of the planet suggests their possible asymmetric allocation on Mercury, similar to that on the Moon. New methods of ground-based astronomical observations including data processing with a special software package have been used.  相似文献   

10.
《Planetary and Space Science》2007,55(11):1494-1501
In this work, we calculate the neutral Na production rates on the Moon and Mercury, as due to the impacts of meteoroids having an impact probability on the surface that can influence the daily observations of the exosphere: the meteoroids radius range considered for the Moon and Mercury are 10−8–0.15 and 10−8–0.10 m, respectively. We also estimate the mass of meteoroids that has impacted the surfaces of the Moon and Mercury in the last 3.8 Gy (after the end of the Late Heavy Bombardment).The results of our model are that (i) the Na production rates are ∼(3–4.9)×104 and ∼(1.8–2.3)×106 atoms cm−2 s−1, for Moon and Mercury, respectively, and (ii) in the last 3.8 Gy, the mass of meteoroids that has impacted the whole surface of the Moon and Mercury has been 8.86×1018 and 2.66×1019 g, respectively.  相似文献   

11.
Topographic data measured from the Mercury Laser Altimeter (MLA) and the Mercury Dual Imaging System (MDIS) aboard the MESSENGER spacecraft were used for investigations of the relationship between depth and diameter for impact craters on Mercury. Results using data from the MESSENGER flybys of the innermost planet indicate that most of the craters measured with MLA are shallower than those previously measured by using Mariner 10 images. MDIS images of these same MLA-measured craters show that they have been modified. The use of shadow measurement techniques, which were found to be accurate relative to the MLA results, indicate that both small bowl-shaped and large complex craters that are fresh possess depth-to-diameter ratios that are in good agreement with those measured from Mariner 10 images. The preliminary data also show that the depths of modified craters are shallower relative to fresh ones, and might provide quantitative estimates of crater in-filling by subsequent volcanic or impact processes. The diameter that defines the transition from simple to complex craters on Mercury based on MESSENGER data is consistent with that reported from Mariner 10 data.  相似文献   

12.
Large escarpments on the Uranian satellite Miranda are similar in structure to thrust faults on Mercury result of crustal contraction. The tectonic lineations may be considered evidence of a crustal contraction on Miranda result of differentiation of the satellite.  相似文献   

13.
In 2008 the MESSENGER spacecraft made the first direct observation of Mercury's magnetosphere in the more than 30 years since the Mariner 10 encounters. During MESSENGER's first flyby on 14 January 2008, the interplanetary magnetic field (IMF) was northward immediately prior to and following MESSENGER's equatorial passage through this small magnetosphere. The Energetic Particle Spectrometer (EPS), one of two sensors on the Energetic Particle and Plasma Spectrometer instrument that responds to electrons from ∼35 keV to 1 MeV and ions from ∼35 keV to 2.75 MeV, saw no increases in particle intensity above instrumental background (∼5 particles/cm2/sr/s/keV at 45 keV) at any time during the probe's magnetospheric passage. During MESSENGER's second flyby on 6 October 2008, there was a steady southward IMF, and intense reconnection was observed between the planet's magnetic field and the IMF. However, once again EPS did not observe bursts of energetic particles similar to those reported by Mariner 10 from its March 1974 encounter. On 29 September 2009, MESSENGER flew by Mercury for the third and final time before orbit insertion in March 2011. Although a spacecraft safe-hold event stopped science measurements prior to the outbound portion of the flyby, all instruments recorded full observations until a few minutes before the closest approach. In particular, the MESSENGER Magnetometer documented several substorm-like signatures of extreme loading of Mercury's magnetotail, but again EPS measured no energetic ions or electrons above instrument background during the inbound portion of the flyby. MESSENGER's X-Ray Spectrometer (XRS) nonetheless observed photons resulting from low-energy (∼10 keV) electrons impinging on its detectors during each of the three flybys. We infer that suprathermal plasma electrons below the EPS energy threshold caused the bremsstrahlung seen by XRS. In this paper, we summarize the energetic particle observations made by EPS and XRS during MESSENGER's three Mercury flybys, and we revisit the observations reported by Mariner 10 in the context of these new results.  相似文献   

14.
H. Jay Melosh 《Icarus》1977,31(2):221-243
Mercury, the Moon, and many large satellites of the major planets have been tidally despun from an initially faster rotation. These bodies probably possessed equatorial bulges which relaxed as they lost their spin. An analysis of the stresses induced in an elastic shell by the relaxation of an equatorial bulge indicates that differential stresses may reach a few kilobars and that the tectonic pattern developed depends mainly upon the shell thickness. In every model studied the azimuthal stress σ?? is larger (more compressive) than the meridional stress σθθ. For a thin elastic shell (thickness less than one-twentieth of the planet's radius) the zone from the equator to 48° latitude is characterized by strike-slip faulting. Poleward of this, normal faults and graben trending east-west are expected. Thicker elastic shells acquire an equatorial belt of thrust faults with east-west throw and rough north-south trends. These tectonic styles may be modified by a small (0.05-0.1%) radial expansion or contraction. Expansion shifts the polar normal faulting province toward the equator, while contraction shifts the equatorial provinces poleward. These patterns are not substantially altered by plastic yielding of the shell, although the equatorial thrust fault province is suppressed by strike-slip faulting until strike-slip faults occur poleward of 64.8° latitude. We conclude that there are many tectonic patterns consistent with despinning and radial contraction or expansion, but they must all be consistent with σ?? > σθθ. These results also indicate that the polar regions of a despun planet are of particular interest in deciding whether a given lineament system is due to stresses induced by the relaxation of the planet's equatorial bulge.  相似文献   

15.
Mercurian lobate scarps are interpreted to be the surface expressions of thrust faults formed by planetary cooling and contraction, which deformed the crust down to the brittle–ductile transition (BDT) depth at the time of faulting. In this work we have used a forward modeling procedure in order to analyze the relation between scarp topography and fault geometries and depths associated with a group of prominent lobate scarps (Santa Maria Rupes and two unnamed scarps) located in the Kuiper region of Mercury for which Earth-based radar altimetry is available. Also a backthrust associated with one of the lobate scarps has been included in this study. We have obtained best fits for depths of faulting between 30 and 39 km; the results are consistent with the previous results for other lobate scarps on Mercury.The so-derived fault depths have been used to calculate surface heat flows for the time of faulting, taking into account crustal heat sources and a heterogeneous surface temperature due to the variable insolation pattern. Deduced surface heat flows are between 19 and 39 mW m?2 for the Kuiper region, and between 22 and 43 mW m?2 for Discovery Rupes. Both BDT depths and heat flows are consistent with the predictions of thermal history models for the range of time relevant for scarp formation.  相似文献   

16.
Plans to send orbiter missions to Mercury (e.g., NASA's Messenger and ESA's BepiColombo) have prompted renewed efforts to investigate the surface of Mercury using ground-based remote sensing. While the highest resolution instrumentation optical telescopes (e.g. HST) cannot be used at small angular distances (<45°) from the Sun (Mercury's elongation never exceeds 28° seen from Earth), advanced ground-based astronomical techniques and modern processing software can be used to construct resolved images of the poorly known part of Mercury. Our observations of the planet presented here were carried out mainly in April and May, 2002, at evening elongation of the planet, at the Skinakas astrophysical observatory of Heraklion University (Crete, Greece). A synthesis of the acquired images of the hemisphere of Mercury, which was not observed by the Mariner 10 mission (1974-1975), is presented. A double rim basin with an internal diameter of about 1000 km and an external rim about 2000 km is suggested by the data. We present the observational method, the data analysis approach, and the resulting images.  相似文献   

17.
The primary crater population on Mercury has been modified by volcanism and secondary craters. Two phases of volcanism are recognized. One volcanic episode that produced widespread intercrater plains occurred during the period of the Late Heavy Bombardment and markedly altered the surface in many areas. The second episode is typified by the smooth plains interior and exterior to the Caloris basin, both of which have a different crater size-frequency distribution than the intercrater plains, consistent with a cratering record dominated by a younger population of impactors. These two phases may have overlapped as parts of a continuous period of volcanism during which the volcanic flux tended to decrease with time. The youngest age of smooth plains volcanism cannot yet be determined, but at least small expanses of plains are substantially younger than the plains associated with the Caloris basin. The spatial and temporal variations of volcanic resurfacing events can be used to reconstruct Mercury's geologic history from images and compositional and topographic data to be acquired during the orbital phase of the MESSENGER mission.  相似文献   

18.
Measurements of the disk-integrated reflectance spectrum of Mercury and the Moon have been obtained by the MESSENGER spacecraft. A comparison of spectra from the two bodies, spanning the wavelength range 220-1450 nm, shows that the absolute reflectance of Mercury is lower than that of the nearside waxing Moon at the same phase angle with a spectral slope that is less steep at visible and near-infrared wavelengths. We interpret these results and the lack of an absorption feature at a wavelength near 1000 nm as evidence for a Mercury surface composition that is low in ferrous iron within silicates but is higher in the globally averaged abundance of spectrally neutral opaque minerals than the Moon. Similar conclusions have been reached by recent investigations based on observations from both MESSENGER and Mariner 10. There is weak evidence for a phase-reddening effect in Mercury that is slightly larger in magnitude than for the lunar nearside. An apparent absorption in the middle-ultraviolet wavelength range of the Mercury spectrum detected from the first MESSENGER flyby of Mercury is found to persist in subsequent observations from the second flyby. The current model of space weathering on the Moon, which also presumably applies to Mercury, does not provide an explanation for the presence of this ultraviolet absorption.  相似文献   

19.
Mikael Beuthe 《Icarus》2010,209(2):795-817
Contraction, expansion and despinning have been common in the past evolution of Solar System bodies. These processes deform the lithosphere until it breaks along faults. Their characteristic tectonic patterns have thus been sought for on all planets and large satellites with an ancient surface. While the search for despinning tectonics has not been conclusive, there is good observational evidence on several bodies for the global faulting pattern associated with contraction or expansion, though the pattern is seldom isotropic as predicted. The cause of the non-random orientation of the faults has been attributed either to regional stresses or to the combined action of contraction/expansion with another deformation (despinning, tidal deformation, reorientation). Another cause of the mismatch may be the neglect of the lithospheric thinning at the equator or at the poles due either to latitudinal variation in solar insolation or to localized tidal dissipation. Using thin elastic shells with variable thickness, I show that the equatorial thinning of the lithosphere transforms the homogeneous and isotropic fault pattern caused by contraction/expansion into a pattern of faults striking east-west, preferably formed in the equatorial region. By contrast, lithospheric thickness variations only weakly affect the despinning faulting pattern consisting of equatorial strike-slip faults and polar normal faults. If contraction is added to despinning, the despinning pattern first shifts to thrust faults striking north-south and then to thrust faults striking east-west. If the lithosphere is thinner at the poles, the tectonic pattern caused by contraction/expansion consists of faults striking north/south. I start by predicting the main characteristics of the stress pattern with symmetry arguments. I further prove that the solutions for contraction and despinning are dual if the inverse elastic thickness is limited to harmonic degree two, making it easy to determine fault orientation for combined contraction and despinning. I give two methods for solving the equations of elasticity, one numerical and the other semi-analytical. The latter method yields explicit formulas for stresses as expansions in Legendre polynomials about the solution for constant shell thickness. Though I only discuss the cases of a lithosphere thinner at the equator or at the poles, the method is applicable for any latitudinal variation of the lithospheric thickness. On Iapetus, contraction or expansion on a lithosphere thinner at the equator explains the location and orientation of the equatorial ridge. On Mercury, the combination of contraction and despinning makes possible the existence of zonal provinces of thrust faults differing in orientation (north-south or east-west), which may be relevant to the orientation of lobate scarps.  相似文献   

20.
Because of its proximity to the Sun and its small size, Mercury has not been able to retain its atmosphere and only a thin exosphere surrounds the planet. The exospheric pressure at the planetary surface is approximately 10−10 mbar, set by the Mariner 10 occultation experiment. The existence of gaseous species H, He, and O has been established by Mariner 10. In addition Na, K, and Ca have been observed by ground based instrumentation. Other elements are expected to be found in Mercury's exosphere since the total pressure of the known species is almost two orders of magnitude less than the exospheric pressure.It is intended to measure these exospheric particle densities in situ with an instrument on board of ESA's BepiColombo Mercury Planetary Orbiter (MPO) spacecraft. Since the expected exospheric densities are very small we developed a Monte-Carlo computer model to investigate if such a measurement is feasible along the MPO spacecraft orbit. We model energy and ejection angle distributions of the particles at the surface, with the emission process determining the actual distribution functions. Our model follows the trajectory of each particle by numerical integration until the particle hits Mercury's surface again or escapes from the calculation domain. Using a large set of these trajectories bulk parameters of the exospheric gas are derived, e.g., particle densities for various atomic and molecular species. Our study suggests that a mass spectrometric measurement is feasible and, at least at MPO's periherm, all species that are released from the surface will be observed.  相似文献   

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