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1.
A new method is presented for estimating crustal thickness from gravity and topography data on the Moon. By calculating analytically the exterior gravitational field for a set of arbitrarily shaped polyhedra, relief along the crust-mantle interface can be inverted for that satisfies the observational constraints. As this method does not rely upon filtering the Bouguer anomaly, which was required with previous inversions performed in the spherical-harmonic domain, and as the dramatic variations in spatial quality of the lunar gravity field are taken into account, our crustal thickness model more faithfully represents the available data. Using our model results, we investigate various aspects of the prominent nearside impact basins. The crustal thickness in the central portion of the Orientale and Crisium basins is found to be close to zero, suggesting that these basins could have conceivably excavated into the lunar mantle. Furthermore, given our uncertain knowledge of the density of the crust and mantle, it is possible that the Humorum, Humboldtianum, Nectaris, and Smythii basins could have excavated all the way through the crust as well. The crustal structure for most of the young impact basins implies a depth/diameter ratio of about 0.08 for their excavation cavities. As noted in previous studies, however, the crustal structure of Imbrium and Serenitatis is anomalous, which is conceivably a result of enhanced rates of post-impact viscous relaxation caused by the proximity of these basins to the Procellarum KREEP Terrane. Impact basins older than Smythii show little or no evidence for crustal thinning, suggesting that these ancient basins were also affected by high rates of viscous relaxation resulting from higher crustal temperatures early in the Moon's evolution. The lithosphere beneath many young basins is found to be supporting a downward directed force, even after the load associated with the mare basalts is removed, and this is plausibly attributed to superisostatic uplift of the crust-mantle interface. Those basins that are close to achieving a pre-mare isostatic state are generally found to reside within, or close to, the Procellarum KREEP Terrane.  相似文献   

2.
Carl Bowin 《Icarus》1983,56(2):345-371
The gravity anomalies of Venus, although small by comparison with those on Mars and the Moon, are still much larger than those on Earth for large features. On Venus, even the low-degree spherical harmonic terms for Venus' gravity field indicate a close association of broad positive gravity anomalies with major topographic highs. This is striking contrast to the situation on Earth, where the broad regional gravity anomalies show little correlation with continental masses or plate tectonic features, but instead appear to be caused by deep mass anomalies.A method for estimating radial gravity anomalies from line-of-sight acceleration data, their interpolation, and use of iteration for improved radial anomaly estimates is outlined. A preliminary gravity anomaly map of Venus at spacecraft altitude prepared using first estimate values is presented. A profile across the western part of Aphrodite along longitude 85 E was analyzed using time-series techniques. An elastic plate model would require a plate thickness of about 180 to 200 km to match the general amplitude of the observed gravity anomaly (about 33 mgal): a thickness much greater than that found for earth structures and, because of high surface temperatures, unlikely for Venus. An Airy isostatic model convolved with the topography across Aphrodite, however, provides a better match between the predicted and observed gravity anomalies if the nominal crustal thickness is about 70 to 80 km. This thickness is over twice that for continental crust on the earth, and considerably greater than that of the earth's basaltic ocean crust (only 5 km). A different differentiation history for Venus than that of the earth thus is anticipated. High gravity anomalies (+110 mgal) occur over Beta Regio and over the topographic high in eastern Aphrodite; both highs are associated with regions where detected lightning is clustered, and thus the topographic features may be active volcanic constructs. The large gravity anomalies at these two sites of volcanic activity require an explanation different than that indicated for western Aphrodite.  相似文献   

3.
The interpretation of planetary anomalies in the gravity fields of Mars and the Moon in relationship to their inhomogeneous internal structure is considered. The Martian and lunar gravity field models up to order and degree 20, three-layer (crust, mantle, core) model parameters, and planetary parameters have been used as input data. Models of the three-dimensional density distribution have been constructed for Mars and the Moon. The maps of horizontal density inhomogeneities at depths of 50, 100, and 1700 km for Mars and 60, 100, and 1400 km for the Moon are interpreted.  相似文献   

4.
Images returned by the MESSENGER spacecraft from the Mercury flybys have been examined to search for anomalous high-albedo markings similar to lunar swirls. Several features suggested to be swirls on the basis of Mariner 10 imaging (in the craters Handel and Lermontov) are seen in higher-resolution MESSENGER images to lack the characteristic morphology of lunar swirls. Although antipodes of large impact basins on the Moon are correlated with swirls, the antipodes of the large impact basins on Mercury appear to lack unusual albedo markings. The antipodes of Mercury’s Rembrandt, Beethoven, and Tolstoj basins do not have surface textures similar to the “hilly and lineated” terrain found at the Caloris antipode, possibly because these three impacts were too small to produce obvious surface disturbances at their antipodes. Mercury does have a class of unusual high-reflectance features, the bright crater-floor deposits (BCFDs). However, the BCFDs are spectral outliers, not simply optically immature material, which implies the presence of material with an unusual composition or physical state. The BCFDs are thus not analogs to the lunar swirls. We suggest that the lack of lunar-type swirls on Mercury supports models for the formation of lunar swirls that invoke interaction between the solar wind and crustal magnetic anomalies (i.e., the solar-wind standoff model and the electrostatic dust-transport model) rather than those models of swirl formation that relate to cometary impact phenomena. If the solar-wind standoff hypothesis for lunar swirls is correct, it implies that the primary agent responsible for the optical effects of space weathering on the Moon is solar-wind ion bombardment rather than micrometeoroid impact.  相似文献   

5.
Eugene I. Smith 《Icarus》1976,28(4):543-550
New central peak-crater size data for Mars shows that a higher percentage of relatively unmodified Martian craters have central peaks than do fresh lunar craters below a diameter of 30 km. For example, in the diameter range 10 to 20 km, 60% of studied Martian craters have central peaks compared to 26% for the Moon. Gault et al. (1975, J. Geophys. Res.80, 2444–2460) have demonstrated that central peaks occur in smaller craters on Mercury than on the Moon, and that this effect is due to the different gravity fields in which the craters formed. Similar differences when comparing Mars and the Moon show that gravity has affected the diameter at which central peaks form on Mars. Erosion on Mars, therefore, does not completely mask differences in crater interior structure that are caused by differences in gravity. Effects of Mars' higher surface gravity when compared to the Moon are not detected when comparing terrace and crater shape data. The morphology-crater size statistics also show that a full range of crater shapes occur on Mars, and craters tend to become more morphologically complex with increasing diameter. Comparisons of Martian and Mercurian crater data show differences which may be related to the greater efficacy of erosion on Mars.  相似文献   

6.
A comparison of the lunar frontside gravity field with topography indicates that low-density ( 2.9 g cm–3) types of rock form a surface layer or crust of variable thickness: 40-60 km beneath terrae; 20-40 km beneath non-mascon maria; 0-20 km beneath mascon maria. The observed offset between lunar centers of mass and figure is consistent with farside crustal thicknesses of 40-50 km, similar to frontside terra thicknesses.The Moon is asymmetric in crustal thickness, and also in the distribution of maria and gamma radioactivity. Early bombardment of the Moon by planetesimals, in both heliocentric and geocentric orbits, is examined as a possible cause of the asymmetries. The presence of a massive companion (Earth) causes a spin-orbit coupled Moon to be bombarded non-uniformly. The most pronounced local concentration of impacts would have occurred on the west limb of the Moon, when it orbited close to the Earth, if low-eccentricity heliocentric planetesimals were still abundant in the solar system at that time.A very intense bombardment of this type could have redistributed crustal material on the Moon, thinning the west limb crust appreciably. This would have caused a change in position of the principal axes of inertia, and a reorientation of the spin-orbit coupled Moon such that the thinnest portion of its crust turned toward one of the poles. Erupting lavas would have preferentially flooded such a thin-crusted, low-lying area. This would have caused another readjustment of principal moments, and a reorientation of the Moon such that the mare areas tipped toward the equator. The north-south and nearside-farside asymmetries of mare distribution on the present Moon can be understood in terms of such a history.Paper dedicated to Prof. Harold C. Urey on the occasion of his 80th birthday on 29 April 1973.  相似文献   

7.
An analysis of the planetwide tectonic system of Mars provided by Harp (1976) reveals that the Hellas and Isidis impact basins have general tectonic systems similar to that of the Argyre impact basin. This implies that Mars does indeed have a lithospheric thickness which would have to be considered thinner than that of the Moon or Mercury but thicker than that of the Galilean satellite Callisto.  相似文献   

8.
S.C. Werner 《Icarus》2008,195(1):45-60
Impact basin formation ages give insight into the early evolution of a planet. The martian basins Hellas, Isidis and Argyre provide an important time-marker for the cessation of the magnetic dynamo and the crustal thickness distribution, both established before 4 Ga ago. No martian surfaces are older than 4.15 Ga based on crater count statistics, and all are younger than the oldest lunar ones. I show that the heavy bombardment period on the Moon and Mars evolved similarly, but endogenic processes have removed the oldest martian basin record. The basin-forming projectile population appears to be different from the impactor population observed today in the inner Solar System. It is yet uncertain whether the heavy bombardment period is cataclysmic or characterized by the decaying flux of planetary formation.  相似文献   

9.
Impact craters on planetary bodies transition with increasing size from simple, to complex, to peak-ring basins and finally to multi-ring basins. Important to understanding the relationship between complex craters with central peaks and multi-ring basins is the analysis of protobasins (exhibiting a rim crest and interior ring plus a central peak) and peak-ring basins (exhibiting a rim crest and an interior ring). New data have permitted improved portrayal and classification of these transitional features on the Moon. We used new 128 pixel/degree gridded topographic data from the Lunar Orbiter Laser Altimeter (LOLA) instrument onboard the Lunar Reconnaissance Orbiter, combined with image mosaics, to conduct a survey of craters >50 km in diameter on the Moon and to update the existing catalogs of lunar peak-ring basins and protobasins. Our updated catalog includes 17 peak-ring basins (rim-crest diameters range from 207 km to 582 km, geometric mean = 343 km) and 3 protobasins (137-170 km, geometric mean = 157 km). Several basins inferred to be multi-ring basins in prior studies (Apollo, Moscoviense, Grimaldi, Freundlich-Sharonov, Coulomb-Sarton, and Korolev) are now classified as peak-ring basins due to their similarities with lunar peak-ring basin morphologies and absence of definitive topographic ring structures greater than two in number. We also include in our catalog 23 craters exhibiting small ring-like clusters of peaks (50-205 km, geometric mean = 81 km); one (Humboldt) exhibits a rim-crest diameter and an interior morphology that may be uniquely transitional to the process of forming peak rings. A power-law fit to ring diameters (Dring) and rim-crest diameters (Dr) of peak-ring basins on the Moon [Dring = 0.14 ± 0.10(Dr)1.21±0.13] reveals a trend that is very similar to a power-law fit to peak-ring basin diameters on Mercury [Dring = 0.25 ± 0.14(Drim)1.13±0.10] [Baker, D.M.H. et al. [2011]. Planet. Space Sci., in press]. Plots of ring/rim-crest ratios versus rim-crest diameters for peak-ring basins and protobasins on the Moon also reveal a continuous, nonlinear trend that is similar to trends observed for Mercury and Venus and suggest that protobasins and peak-ring basins are parts of a continuum of basin morphologies. The surface density of peak-ring basins on the Moon (4.5 × 10−7 per km2) is a factor of two less than Mercury (9.9 × 10−7 per km2), which may be a function of their widely different mean impact velocities (19.4 km/s and 42.5 km/s, respectively) and differences in peak-ring basin onset diameters. New calculations of the onset diameter for peak-ring basins on the Moon and the terrestrial planets re-affirm previous analyses that the Moon has the largest onset diameter for peak-ring basins in the inner Solar System. Comparisons of the predictions of models for the formation of peak-ring basins with the characteristics of the new basin catalog for the Moon suggest that formation and modification of an interior melt cavity and nonlinear scaling of impact melt volume with crater diameter provide important controls on the development of peak rings. In particular, a power-law model of growth of an interior melt cavity with increasing crater diameter is consistent with power-law fits to the peak-ring basin data for the Moon and Mercury. We suggest that the relationship between the depth of melting and depth of the transient cavity offers a plausible control on the onset diameter and subsequent development of peak-ring basins and also multi-ring basins, which is consistent with both planetary gravitational acceleration and mean impact velocity being important in determining the onset of basin morphological forms on the terrestrial planets.  相似文献   

10.
We examine gravity, topography, and magnetic field data along the well-preserved Martian dichotomy boundary between 105° and 180°E to better understand the origin and modification of the dichotomy boundary. Admittance modeling indicates bottom-loading for the Amenthes region (105–135°E) with crustal and elastic thickness estimates of 15–40 km, and 15–35 km and top-loading for the Aeolis region (145–180°E) with crustal and elastic thickness estimates of 10–20 km and 10–15 km, respectively. There is a general trend from bottom-loading in the west, to top-loading in the east. The bottom-loading signature near Amenthes may reflect its proximity to the Isidis basin or a broad valley southeast of Isidis. Surface volcanic deposits may produce the top-loading seen at Aeolis. Additional processes such as erosion and faulting have clearly affected the dichotomy and may contribute to the loading signature. Low elastic thickness estimates are consistent with loading in the Noachian, when heat flow was high. Significant Bouguer and isostatic gravity anomalies in these areas indicate substantial variations in the crustal density structure. Crater age dating indicates that major surface modification occurred early in the Noachian, and the small elastic thickness estimates also suggest that subsurface modification occurred in the Noachian. Magnetic and gravity anomalies show comparable spatial scales (several hundred kilometers). The similarity in scale and the constant ratio of the amplitudes of the isostatic and Bouguer gravity to the magnetic anomalies along the dichotomy suggest a common origin for the anomalies. Igneous intrusion and/or local thinning or thickening of the crust, possibly with a contribution from hydrothermal alteration, are the most likely mechanisms to create the observed anomalies.  相似文献   

11.
We report on the first results of a large‐scale comparison study of central pit craters throughout the solar system, focused on Mars, Mercury, Ganymede, Rhea, Dione, and Tethys. We have identified 10 more central pit craters on Rhea, Dione, and Tethys than have previously been reported. We see a general trend that the median ratio of the pit to crater diameter (Dp/Dc) decreases with increasing gravity and decreasing volatile content of the crust. Floor pits are more common on volatile‐rich bodies while summit pits become more common as crustal volatile content decreases. Uplifted bedrock from below the crater floor occurs in the central peak upon which summit pits are found and in rims around floor pits, which may or may not break the surface. Peaks on which summit pits are found on Mars and Mercury share similar characteristics to those of nonpitted central peaks, indicating that some normal central peaks undergo an additional process to create summit pits. Martian floor pits do not appear to be the result of a central peak collapse as the median ratio of the peak to crater diameter (Dpk/Dc) is about twice as high for central peaks/summit pits than Dp/Dc values for floor pits. Median Dpk/Dc is twice as high for Mars as for Mercury, reflecting differing crustal strength between the two bodies. Results indicate that a complicated interplay of crustal volatiles, target strength, surface gravity, and impactor energy along with both uplift and collapse are involved in central pit formation. Multiple formation models may be required to explain the range of central pits seen throughout the solar system.  相似文献   

12.
The lack of distinct magnetic signatures observed by Mars Global Surveyor (MGS) over the impact craters and impact-related Quasi-Circular-Depressions (QCDs) with diameters greater than 200 km located on South Province, south of 30S and from almost the west of Hellas to Argyre basins, implies a weakly magnetized crust. Using MOLA topography and the recent JPL gravity model of Mars we determine the structure of the crust beneath the craters and impact-related QCDs, and show that the impacts that have created these features were capable of strongly disturbing the crust directly beneath. On the basis of theoretical magnetic anomaly modeling and shock demagnetization models, we demonstrate that the impacts are capable of demagnetizing the entire crust beneath and creating distinct magnetic anomalies at the satellite altitude of 400 km in case the crust was appreciably magnetized prior to the impacts. We derive the magnetic anomalies of these features using the radial component of the high-altitude nighttime MGS data. An upper limit of <2 × 104 A for the bulk magnetization of the crust beneath South Province is estimated, which is about 30 times less than that underlying Terra Cimmeria and Terra Sirenum. Similar weak bulk magnetization is obtained for part of the crust surrounding Hellas, Isidis, and Argyre basins.  相似文献   

13.
The study of peak-ring basins and other impact crater morphologies transitional between complex craters and multi-ring basins is important to our understanding of the mechanisms for basin formation on the terrestrial planets. Mercury has the largest population, and the largest population per area, of peak-ring basins and protobasins in the inner solar system and thus provides important data for examining questions surrounding peak-ring basin formation. New flyby images from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft have more than doubled the area of Mercury viewed at close range, providing nearly complete global coverage of the planet's surface when combined with flyby data from Mariner 10. We use this new near-global dataset to compile a catalog of peak-ring basins and protobasins on Mercury, including measurements of the diameters of the basin rim crest, interior ring, and central peak (if present). Our catalog increases the population of peak-ring basins by ∼150% and protobasins by ∼100% over previous catalogs, including 44 newly identified peak-ring basins (total=74) and 17 newly identified protobasins (total=32). A newly defined transitional basin type, the ringed peak-cluster basin (total=9), is also described. The new basin catalog confirms that Mercury has the largest population of peak-ring basins of the terrestrial planets and also places the onset rim-crest diameter for peak-ring basins at , which is intermediate between the onset diameter for peak-ring basins on the Moon and those for the other terrestrial planets. The ratios of ring diameter to rim-crest diameter further emphasize that protobasins and peak-ring basins are parts of a continuum of basin morphologies relating to their processes of formation, in contrast to previous views that these forms are distinct. Comparisons of the predictions of peak-ring basin-formation models with the characteristics of the basin catalog for Mercury suggest that formation and modification of an interior melt cavity and nonlinear scaling of impact melt volume with crater diameter provide important controls on the development of peak rings. The relationship between impact-melt production and peak-ring formation is strengthened further by agreement between power laws fit to ratios of ring diameter to rim-crest diameter for peak-ring basins and protobasins and the power-law relation between the dimension of a melt cavity and the crater diameter. More detailed examination of Mercury's peak-ring basins awaits the planned insertion of the MESSENGER spacecraft into orbit about Mercury in 2011.  相似文献   

14.
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   

15.
Thirteen-centimeter-wavelength radar observations of Mars made in 1982 at Arecibo Observatory yield accurate measurements of the full backscatter spectrum in two orthogonal polarizations. The data, which were obtained for several widely separated subradar longitudes at 24°N latitude, provide the first global view of the distribution of small-scale surface roughness on Mars. The diffuse component of the echo exhibits strong spatial variations. Areas of maximum depolarization correlate well with volcanic regions (Tharsis and Elysium), while the heavily cratered upland terrain yields relatively low depolarization. Parts of Tharsis give near-complete depolarization (polaziation ratio μc ? 1 when viewed at oblique angles of incidence). Northern Martian plains regions (Tharsis, Elysium, and Amazonis) may comprise the most extensive area of severe decimeter-scale surface roughness in the inner Solar System. On the average, the northern Martian tropics yield higher diffuse radar cross sections (σD = 0.05–0.12) and a higher of degree disk-integrated depolarization (μc = 0.1–0.4) than is found for the Moon, Mercury, and Venus. Comparisons between the Moon and Mars using radar data, ground truth, and simple scattering models suggest that Mars possesses a relatively high average coverage by decimeter-scale rocks. Also discussed are several of the more interesting quasispecular scattering results, the most unsual of which were obtained over the Olympus Mons aureole region.  相似文献   

16.
Origin of the late heavy bombardment   总被引：1，自引：0，他引：1  
J. Mottmann 《Icarus》1977,31(3):412-413
Cratering data from the Moon, Mars, and Mercury have been interpreted by some as evidence that the inner solar system underwent a period of intense bombardment that ended about 4 × 109 yr ago. Planetary perturbations of small objects within the solar system seem unable to account for the effect. This paper suggests that stellar perturbations from members of the original open cluster within which the Sun formed triggered the brief flux of objects responsible for the bombardment.  相似文献   

17.
Abstract— We propose that argon‐40 measured in the lunar atmosphere and that in Mercury's atmosphere is due to current diffusion into connected pore space within the crust. Higher temperatures at Mercury, along with more rapid loss from the atmosphere, will lead to a similar or smaller column abundance of argon at Mercury than at the Moon, given the same crustal abundance of potassium. Because the noble gas abundance in the mercurian atmosphere represents current effusion, it is a direct measure of the crustal potassium abundance. We assume a fractal distribution of distance to a connected pore space, with the shortest distance increasing with depth. Given this “rock size” distribution, we show that the diffusive flux is not a unique function of temperature. Even though the diffusion coefficient is an exponential function of temperature, the flux to the surface is fairly insensitive to the temperature.  相似文献   

18.
The differences between the surface structure of the near side and the far side of the Moon have been topics of interest ever since photographs of the far side have been available. One recurrent hypothesis is that a large impact on the near side has deposited ejecta on the far side, resulting in thicker crust there. Specific proposals were made by P.H. Cadogan for the Gargantuan Basin and by E.A. Whitaker for the Procellarum Basin. Despite considerable effort, no consensus has been reached on the existence of these basins. The problem of searching for such a basin is one of finding its signature in a somewhat chaotic field of basin and crater impacts. The search requires a model of the topographic shape of an impact basin and its ejecta field. Such a model is described, based on elevation data of lunar basins collected by the Lidar instrument of the Clementine mission and crustal thickness data derived from tracking Clementine and other spacecraft. The parameters of the model are scaled according to the principles of dimensional analysis and isostatic compensation in the early Moon. The orbital dynamics of the ejecta and the curvature of the Moon are also taken into account. Using such a scaled model, a search for the best fit for a large basin led to identification of a basin whose cavity covers more than half the Moon, including the area of all of the impact basins visible on the near side. The center of this basin is at 22 degrees east longitude and 8.5 degrees north latitude and its average radius is approximately 3,160 km. It is a megabasin, a basin that contains other basins (the far side South Pole-Aitken Basin also qualifies for that designation). It has been called the Near Side Megabasin. Much of the material ejected from the basin escaped the Moon, but the remainder formed an ejecta blanket that covered all of the far side beyond the basin rim to a depth of from 6 to 30 km. Isostatic compensation reduced the depth relative to the mean surface to a range of 1–5 km, but the crustal thickness data reveals the full extent of the original ejecta. The elevation profile of the ejecta deposited on the far side, together with modification for subsequent impacts by known basins (especially the far side South Pole-Aitken Basin) matches the available topographic data to a high degree. The standard deviation of the residual elevations (after subtracting the model from the measured elevations) is about one-half of the standard deviation of the measured elevations. A section on implications discusses the relations of this giant basin to known variations in the composition, mineralogy, and elevations of different lunar terranes.  相似文献   

19.
New crater size-shape data were compiled for 221 fresh lunar craters and 152 youthful mercurian craters. Terraces and central peaks develop initially in fresh craters on the Moon in the 0–10 km diameter interval. Above a diameter of 65 km all craters are terraced and have central peaks. Swirl floor texture is most common in craters in the size range 20–30 km, but it occurs less frequently as terraces become a dominant feature of crater interiors. For the Moon there is a correlation between crater shape and geomorphic terrain type. For example, craters on the maria are more complex in terms of central peak and terrace detail at any given crater diameter than are craters in the highlands. These crater data suggest that there are significant differences in substrate and/or target properties between maria and highlands. Size-shape profiles for Mercury show that central peak and terrace onset is in the 10–20 km diameter interval; all craters are terraced at 65 km, and all have central peaks at 45 km. The crater data for Mercury show no clear cut terrain correlation. Comparison of lunar and mercurian data indicates that both central peaks and terraces are more abundant in craters in the diameter range 5–75 km on Mercury. Differences in crater shape between Mercury and the Moon may be due to differences in planetary gravitational acceleration (gMercury=2.3gMoon). Also differences between Mercury and the Moon in target and substrate and in modal impact velocity may contribute to affect crater shape.  相似文献   

20.
Craters on the Earth, Mars, and the Moon show a spectrum of morphologies with diameter increasing from simple, bowl-shaped craters through craters with increasingly complex central peaks, to craters with “peak rings” and basins with multiple concentric scarps. In each category there is a range of diameters, centered around a characteristic diameter, Dc. It is found that Dc decreases as the size of the planet increases. Several possible explanations are considered. It is suggested that the effect results from a gravity scaling law derived here and having approximately from the Dc∂ 1/g1.25, where g is the surface gravity. All geological structures in which gravity is the dominant parameter affecting the morphology should follow such a law.  相似文献   

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