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1.
Paleocratering of the Moon: Review of post-Apollo data 总被引：1，自引：0，他引：1
William K. Hartmann 《Astrophysics and Space Science》1972,17(1):48-64
As a result of the dating of lunar samples, we are in a position to utilize the lunar surface as a recorder of environmental conditions in the Earth-Moon neighborhood in the past. Plots of crater density vs rock age at different lunar landing sites can be used to date unexplored lunar provinces. These plots also demonstrate evolution in the population of planetesimals that struck the Moon. Prior to 4.1 aeons ago, the cratering rate on the Moon was at least 103 times the present rate, and the rate declined with a half-life less than 8×107 yr. During the interval from 4.1 to 3.2 aeons ago, the number of planetesimals showed an exponential decay with a half-life about 3×108 yr, corresponding to sweep-up of particles from solar orbits somewhat similar to those of Apollo asteroids. A more nearly constant cratering rate applied in the last three aeons. These data indicate that the Moon displays at least the final stages of an ancient accretion process; they also set certain conditions on possible capture processes relating to the Moon's origin. Pre-Apollo expectations that the Moon would provide a Rosetta Stone for interpreting solar system history and planet formation thus appear justified.Paper given at Philadelphia meeting of American Association for Advancement of Science, December, 1971. 相似文献
2.
William K. Hartmann 《Icarus》2005,174(2):294-320
This paper reviews and refines the technique of dating martian surfaces by using impact-crater isochrons (defined as size distributions of impact craters on undisturbed martian surfaces of specified ages). In the 1970s, this system identified not only abundant ancient martian volcanic surfaces, but also extensive lava plains with ages of a few 108 y-old; this dating was initially controversial but confirmed in the 1980s and 90s by martian meteorites. The present update utilizes updated estimates of the Mars/Moon cratering ratio (the most important calibration factor), improves treatment of gravity and impact velocity scaling effects, combines aspects of the crater size distribution data from earlier work by both Neukum and Hartmann, and for the first time applies a correction for loss of small meteoroids in the martian atmosphere from Popova et al. (2003, Meteorit. Planet. Sci. 38, 905-925). The updated isochrons are not radically different from the previous “2002 iteration” but fit observed data better and give somewhat older model ages for features dated from small craters (diameter D<100 m). Crater counts from young lava flows in various areas give good fits to the new isochrons over as much as 3 orders of magnitude in D, confirming the general isochron shape and giving crater retention ages in the range of some 106 to some 108 y, interpreted as lava flow ages. More complex, older units are also discussed. Uncertainties are greatest if only small craters (D?100 m) are used. Suggestions by other workers of gross uncertainties, due to local secondary craters and deposition/exhumation, are discussed; they do not refute our conclusions of significant volcanic, fluvial, and other geologic activity in the last few percent of martian geologic time or the importance of cratering as a tool for studying processes such as exhumation. Indeed, crater count data suggest certain very recent episodes of deposition, exhumation, and ice flow, possibly associated with obliquity cycles of ∼107 y timescale. Evidence from ancient surfaces suggests higher rates of volcanism, fluvial activity, glaciation, and other processes in Noachian/Hesperian time than in Amazonian time. 相似文献
3.
Ralph B. Baldwin 《Icarus》1974,23(2):157-166
Arguments based on changes in viscosity of the lunar outer layers, changes in crater shapes due to a cold flow process and changes in the flux of crater forming planetesimals indicate that the period of premare time during which the vast majority of craters, large and small, were formed was several hundred million years long. The craters of all sizes were formed in exponentially declining numbers during this period with a half life of about 88×106 yr.Recent work done by numerous students of RbSr, UPb and KAr ages of lunar rocks are beginning to recognize numerous events as having occured prior to the formation of Mare Imbrium and after the crust of the moon solidified.There was no major series of events which produced the “terminal lunar cataclysm” approximately 3.95×109 yr ago. The magnitude and timing of the Imbrium collision was the single overwhelming event at that time. 相似文献
4.
The lunar maria reflect two to five times less Earth-based radar power than the highlands, the spectrally blue maria surfaces returning the lowest power levels. This effect of weakening signal return has been attributed to increased signal absorption related to the electrical and magnetic characteristics of the mineral ilmenite (FeTiO3). The surface of Mare Imbrium contains some of the most distinct red-blue colorimetric boundaries and depolarized 70 cm wavelength reflectivity variations on the near side of the Moon. The weakest levels of both 3.8 cm and 70 cm reflectivity within Imbrium are confined to regional mare surfaces of the blue spectral type that can be recognized as stratigraphically unique flow surfaces. Frequency distributions of the 70 cm polarized and depolarized radar return power for five mare surfaces within the basin indicate that signal absorption, and probably the ilmenite content, increases generally from the beginning of the Imbrian Period to the end of the Eratosthenian Period with slight reversal between the end of the Imbrian and beginning of the Eratosthenian. TiO2 calibrated radar reflectivity curves can be utilized for lunar maria geochemical mapping in the same manner as the TiO2 calibrated spectral reflectivity curves of Charetteet al. (1974). The long wavelength radar data may be a sensitive indicator of mare chemical variations as it is unaffected by the normal surface rock clutter that includes ray materials from large impact craters. 相似文献
5.
The Lunar Prospector Electron Reflectometer has obtained the first global map of lunar crustal magnetic fields, revealing that the effects of basin-forming impacts dominate the large-scale distribution of remanent magnetic fields on the Moon. The weakest surface magnetic fields (<0.2 nT) are found within two of the largest and most recent impact basins, Orientale and Imbrium. Conversely, the largest concentrations of strong surface fields (>40 nT) are diametrically opposite to these same basins. This pattern is present though less pronounced for several other post-Nectarian impact basins larger than 500 km in diameter. The reduced strength and clarity of the pattern for older basins may be attributed to: (1) demagnetization from many smaller impacts, which erases antipodal magnetic signatures over time, (2) superposition effects from other large impacts, and (3) variation in the strength of the ambient magnetizing field. The absence of fringing fields stronger than 1 nT around the perimeter of the Imbrium basin or associated with craters within the basin implies that any uniform magnetization of the impact melt must be weaker than ∼10−6 G cm3 g−1. This limits the strength of any steady ambient magnetic field to no more than ∼0.1 Oe at the lunar surface while the basin cooled for tens of millions of years following the Imbrium impact 3.8 billion years ago. 相似文献
6.
A. Tyrie 《Earth, Moon, and Planets》1988,42(3):265-275
Although researchers in the last decade have been primarily concerned with the exotic findings of the more distant planets and moons in our solar system, as given by the Voyager series, there is still much work to be done on our nearer neighbours, including the Moon. This paper summarizes some basic age dating of a portion of the lunar surface, namely the mare in the crater Tsiolkovsky on the lunar far side.Using the Apollo 15 panoramic camera photographs, the cumulative crater frequency (N km-2) relative to crater diameter (D) distribution has been obtained for the mare in the crater Tsiolkovsky. The diameter size range sampled was 0.07 km < D < 1 km. A total of 12 604 craters were counted and their average apparent diameters measured. There were 85 sample areas on the mare surface which were chosen at random, after exclusion of blanketed, volcanic or secondary cratered areas. It was found that a large proportion of the crater floor contains endogenic features, especially volcanic vents at approximately D = 0.3 km. An additional 7 areas of interest were also examined in detail for comparison with areas of purely primary impact craters. Evidence for up to 8 lava floodings can be detected from the size-frequency distributions although no visual data, e.g., flow lobes, can be seen on the mare surface.The total size-frequency distribution for all the areas is coincident with Neukum et al. (1975a and b) Calibration Distribution in the size range 0.25 km < D < 1 km which is at the smallest crater diameters that they obtained. Neukum et al. (1975a and b) give their distribution as a polynomial of 7th degree. However, in this present study a variation is indicated in the steepening of the curve for D < 0.1 km.The results also approximate (but only for D < 0.6 km) the distribution obtained by Shoemaker et al. (1970) in the range 100 m < D < 3 km where N ~ D
-2.9. The best fit line reached for the data given here is N ~ D
-2.682.Comparison of the distribution with plots for the maria at Apollo 11, 12, and 15 landing sites show that Tsiolkovsky mare is 3.51 ± 0.1 × 109 yr old. This agrees with other workers (see Gornitz, 1973) who place it between Mare Tranquillitatis (Apollo 11 radiometric dating: 3.5 to 3.9 aeons) and Oceanus Procellarum (Apollo 12: 3.5 to 3.4 aeons). There are no rock samples from Tsiolkovsky to given an absolute age.This places Tsiolkovsky mare within the weighted mean of the age range (1.0 to 4.3 × 109 yr old) of the maria on the Moon. From this it can be concluded that the processes producing the vast basalt outpourings seen on the Moon's face apply for the far side also and that there is a linking factor for the whole Moon. 相似文献
7.
Karl R. Blasius 《Icarus》1976,29(3):343-361
Mariner 9 images of the four great volcanic shields of the Tharsis region of Mars show many circular craters ranging in diameter from 100mm to 20 km. Previous attempts to date the volcanoes from their apparent impact crater densities yielded a range of results. The principal difficulty is sorting volcanic from impact craters for diameters . Many of the observed craters are aligned in prominent linear and concentric patterns suggestive of volcanic origin. In this paper an attempt is made to date areas of shield surface, covered with high resolution images using only scattered small (?1 km) craters of probable impact origin. Craters of apparent volcanic origin are systematically excluded from the dating counts.The common measure of age, deduced for all surfaces studied, is a calculated “crater age” F′ defined as the number of craters equal to or larger than 1 km in diameter per 106km2. The conclusions reached from comparing surface ages and their geological settings are: (1) Lava flow terrain surfaces with ages, F′, from 180 to 490 are seen on the four great volcanoes. Summit surfaces of similar ages, F′ = 360 to 420, occur on the rims of calderas of Arsia Mons, Pavonis Mons, and Olympus Mons. The summit of Ascraeus Mons is possibly younger; F′ is calculated to be 180 for the single area which could be dated. (2) One considerably younger surface, F′ < 110, is seen on the floor of Arsia Mon's summit caldera. (3) Nearly crater free lava flow terrain surfaces seen on Olympus Mons are estimated to be less than half the age of a summit surface. The summit caldera floor is similarly young. (4) The pattern of surface ages on the volcanoes suggests that their eruption patterns are similar to those of Hawaiian basaltic shields. The youngest surfaces seem concentrated on the mid-to-lower flanks and within the summit calderas. (5) The presently imaged sample of shield surface, though incomplete, clearly shows a broad range of ages on three volcanoes—Olympus, Arsia, and Pavonis Mons.Estimated absolute ages of impact dated surfaces are obtained from two previously published estimates of the history of flux of impacting bodies on Mars. The estimated ranges of age for the observed crater populations are 0.5 to 1.2b.y. and 0.07 to 0.2b.y. Areas which are almost certainly younger, less than 0.5 or 0.07b.y., are also seen. The spans of surface age derived for the great shields are minimum estimates of their active lifetimes, apparently very long compared to those of terrestrial volcanoes. 相似文献
8.
The lunar Late Heavy Bombardment (LHB) defines a time between ∼3.8 to possibly 4.1 Gy ago when the Nectarian and early-Imbrium basins on the Moon with reasonably well-constrained ages were formed. Some have argued that these basins were produced by a terminal cataclysm that caused a spike in the inner Solar System impactor flux during this interval. Others have suggested the basins were formed by the tail end of a monotonically decreasing impactor population originally produced by planet formation processes in the inner Solar System. Here we investigate whether this so-called declining bombardment scenario of the LHB is consistent with constraints provided by planet formation models as well as the inferred ages of Nectaris, Serenitatis, Imbrium, and Orientale. We did this by modeling the collisional and dynamical evolution of the post-planet formation population (PPP) for a range of starting PPP masses. Using a Monte Carlo code, we computed the probability that the aforementioned basins were created at various times after the Moon-forming event approximately 4.54 Ga. Our results indicate that the likelihood that the declining bombardment scenario produced Nectaris, Serenitatis, Imbrium, and Orientale (or even just Imbrium and Orientale) at any of their predicted ages is extremely low and can be ruled out at the 3σ confidence level, regardless of the PPP's starting mass. The reason is that collisional and dynamical evolution quickly depletes the PPP, leaving behind a paucity of large projectiles capable of producing the Moon's youngest basins between 3.8-4.1 Gy ago. If collisions are excluded from our model, we find that the PPP produces numerous South Pole-Aitken-like basins during the pre-Nectarian period. This is inconsistent with our understanding of lunar topography. Accordingly, our results lead us to conclude that the terminal cataclysm scenario is the only existing LHB paradigm at present that is both viable from a dynamical modeling perspective and consistent with existing constraints. 相似文献
9.
Multiple impact basins formed on the Moon about 3.8 Gyr ago in what is known as the lunar cataclysm or Late Heavy Bombardment. Many workers currently interpret the lunar cataclysm as an impact spike primarily caused by main-belt asteroids destabilized by delayed planetary migration. We show that morphologically fresh (class 1) craters on the lunar highlands were mostly formed during the brief tail of the cataclysm, as they have absolute crater number density similar to that of the Orientale basin and ejecta blanket. The connection between class 1 craters and the cataclysm is supported by the similarity of their size-frequency distribution to that of stratigraphically-identified Imbrian craters. Majority of lunar craters younger than the Imbrium basin (including class 1 craters) thus record the size-frequency distribution of the lunar cataclysm impactors. This distribution is much steeper than that of main-belt asteroids. We argue that the projectiles bombarding the Moon at the time of the cataclysm could not have been main-belt asteroids ejected by purely gravitational means. 相似文献
10.
LARRY A. HASKIN RANDY L. KOROTEV KAYLYNN M. ROCKOW BRADLEY L. JOLLIFF 《Meteoritics & planetary science》1998,33(5):959-975
Abstract— Mafic, Th-rich impact-melt breccias, most of which are identified with the composition known as low-K Fra Mauro (LKFM), are the most common rock type in the nonmare regoliths of the Apollo lunar landing sites. The origin of mafic impact-melt breccias bears on many lunar problems: the nature of the late meteoroid bombardment (cataclysm); the spatial distribution of KREEP, both near the surface and at depth; the ages of the major basins; and the composition of the early crust of the nearside lunar highlands. Thus, it is crucial that the origin of mafic impact-melt breccias be accurately understood. Because of both intra- and intersite differences in compositions of mafic impact-melt breccia samples, apparent differences in crystallization age, and differences in siderophile-element ratios, previous studies have argued that either (1) most mafic impact-melt breccias are the products of several large craters local to the site at which they were found but that some are of basin origin or that (2) they are all from the Imbrium (Apollos 14 and 15), Nectaris (Apollo 16), and Serenitatis (Apollo 17) basins. Here, we reconsider the hypothesis that virtually all of the Th-rich, mafic impact-melt breccias from the Apollo missions are products of the Imbrium impact. Ejecta deposit modeling based on modern crater scaling indicates that the Imbrium event produced ejecta deposits that average hundreds of meters thick or more at all Apollo highland sites, which is thicker than some previous estimates. Substantial amounts of Imbrium ejecta should have been sampled at every Apollo highland site. We suggest that the mafic impact-melt breccias may be the principal form of those ejecta. The Imbrium projectile impacted into Th-rich material that we regard as part of a unique, mafic, lunar geochemical province we call the High-Th Oval Region. Based on the surface distribution of Th, only basins within the High-Th Oval Region excavated Th-rich material; the Th concentrations of the highlands as observed by the Apollo orbiting γ-ray experiments are consistent with the estimates from ejecta modeling. Of the younger basin-forming impacts, only Imbrium was large enough to produce the copious amount of melt required by the ubiquitous presence of mafic impact-melt breccias in the Apollo-sampled regolith. The High-Th Oval Region still may have been molten or hot at shallow depths ～4 Ga ago when the Imbrium projectile struck. We reason that compositional heterogeneity of ejected melt breccia is to be expected under these circumstances. We argue that siderophile-element “fingerprints” of mafic impact-melt breccias are not inconsistent with production of all common types by a single projectile. We suggest that the narrow range of ages of 3.7–4.0 Ga for all successfully dated mafic impact-melt breccias may reflect a single event whose age is difficult to measure precisely, rather than a number of discrete impact events closely spaced in time, such that reported age variations among mafic impact-melt breccias reflect the ability to measure 40Ar/39Ar ages with greater precision than the accuracy with which measured portions of mafic impact-melt breccias have recorded the time of their formation. 相似文献
11.
Alfred S. McEwen Brandon S. Preblich Natalia A. Artemieva Michelle Hurst Devon M. Burr 《Icarus》2005,176(2):351-381
A 10-km diameter crater named Zunil in the Cerberus Plains of Mars created ∼107 secondary craters 10 to 200 m in diameter. Many of these secondary craters are concentrated in radial streaks that extend up to 1600 km from the primary crater, identical to lunar rays. Most of the larger Zunil secondaries are distinctive in both visible and thermal infrared imaging. MOC images of the secondary craters show sharp rims and bright ejecta and rays, but the craters are shallow and often noncircular, as expected for relatively low-velocity impacts. About 80% of the impact craters superimposed over the youngest surfaces in the Cerberus Plains, such as Athabasca Valles, have the distinctive characteristics of Zunil secondaries. We have not identified any other large (?10 km diameter) impact crater on Mars with such distinctive rays of young secondary craters, so the age of the crater may be less than a few Ma. Zunil formed in the apparently youngest (least cratered) large-scale lava plains on Mars, and may be an excellent example of how spallation of a competent surface layer can produce high-velocity ejecta (Melosh, 1984, Impact ejection, spallation, and the origin of meteorites, Icarus 59, 234-260). It could be the source crater for some of the basaltic shergottites, consistent with their crystallization and ejection ages, composition, and the fact that Zunil produced abundant high-velocity ejecta fragments. A 3D hydrodynamic simulation of the impact event produced 1010 rock fragments ?10 cm diameter, leading to up to 109 secondary craters ?10 m diameter. Nearly all of the simulated secondary craters larger than 50 m are within 800 km of the impact site but the more abundant smaller (10-50 m) craters extend out to 3500 km. If Zunil is representative of large impact events on Mars, then secondaries should be more abundant than primaries at diameters a factor of ∼1000 smaller than that of the largest primary crater that contributed secondaries. As a result, most small craters on Mars could be secondaries. Depth/diameter ratios of 1300 small craters (10-500 m diameter) in Isidis Planitia and Gusev crater have a mean value of 0.08; the freshest of these craters give a ratio of 0.11, identical to that of fresh secondary craters on the Moon (Pike and Wilhelms, 1978, Secondary-impact craters on the Moon: topographic form and geologic process, Lunar Planet. Sci. IX, 907-909) and significantly less than the value of ∼0.2 or more expected for fresh primary craters of this size range. Several observations suggest that the production functions of Hartmann and Neukum (2001, Cratering chronology and the evolution of Mars, Space Sci. Rev. 96, 165-194) predict too many primary craters smaller than a few hundred meters in diameter. Fewer small, high-velocity impacts may explain why there appears to be little impact regolith over Amazonian terrains. Martian terrains dated by small craters could be older than reported in recent publications. 相似文献
12.
Barbara A. Cohen 《Meteoritics & planetary science》2013,48(5):771-785
Crystalline impact‐melt samples were created in high‐temperature environments by relatively large craters and, as such, give additional constraints on the nature of the impacts that created them. This article provides new 40Ar‐39Ar ages of impact‐melt clasts in howardites and shows that these clasts formed on the HED parent body, 4 Vesta, within the time period 3.3–3.8 Ga. Rather than resulting from an increased number of impacts, however, impact‐melted material in howardites may result from unusually high‐velocity impacts occurring in the asteroid belt during this period. This scenario is similar to the late heavy bombardment of the Moon, pointing to an unusual dynamical event at this time across the inner solar system. Therefore, impact‐melt rocks in howardites uniquely record a Vestan cataclysm. 相似文献
13.
Compositional and temporal investigation of exposed lunar basalts in the Mare Imbrium region 总被引：2，自引：0，他引：2
This paper presents an updated stratigraphical and compositional study of the exposed maria within the Imbrium basin on the Moon. Clementine multispectral data were employed to derive TiO2 and FeO wt% abundance estimates of potentially distinct basaltic flows. Additionally, NASA Lunar Orbiter images were used to estimate flow ages using crater count statistics. Mare Imbrium shows evidence of a complex suite of low to high-Ti basaltic lava units infilling the basin over an 800 million year timescale. More than a third (37%) of identified mare basalts were found to contain 1-3 wt% TiO2. Two other major mare lithological units (representing about 25% of the surface each) show TiO2 values between 3-5 and 7-9 wt%. The dominant fraction (55%) of the sampled maria contain FeO between 16 and 18 wt%, followed by 27% of maria having 18-20 wt% and the remaining 18%, 14-16 wt% FeO. A crater frequency count (for diameters >500 m) shows that in three quarters of the sampled mare crater counts range between 3.5 and 5.5×10−2 per km2, which translates, according to a lunar cratering model chronology, into estimated emplacement ages between ∼3.3 and 2.5 Ga. A compositional convergence trend between the variations of iron and titanium oxides was identified, in particular for materials with TiO2 and FeO content broadly above 5 and 17 wt%, respectively, suggesting a related petrogenesis and evolution. According to these findings, three major periods of mare infill are exposed in the Imbrium basin; despite each period showing a range of basaltic compositions (classified according to their TiO2 content), it is apparent that, at least within these local geological settings, the igneous petrogenesis generally evolved through time towards more TiO2- and FeO-rich melts. 相似文献
14.
RALPH B. BALDWIN 《Meteoritics & planetary science》1987,22(2):111-119
Careful examination of seven giant front face basins on the moon will show that the basins most densely covered by younger craters are the oldest. With increasing age they exhibit lower external rims, not scarp heights. The rims are progressively more subdued with age. This paper proposes that absolute ages for these basins can be obtained by calculating an effective viscosity of the moon's outer layers from 3.85 × 109 y, the date of Imbrium, to the present. Similarly viscosity measures can be determined for the oldest basin. To do this we need the present and the original rim heights. The present values are observed. The original heights are calculated by extrapolating the relationship between diameter and rim height for normal Class I craters. It turns out that as long as the larger basins have proportionately higher original heights than the smaller, the absolute values are of little importance and the ages are definitive. There are many similar families of viscosity changes with age and they yield similar absolute ages. In each case equations relating viscosity changes with age were derived and for each basin there is only one age that will yield the final rim height. Ages, × 109 y, of the basins are: Orientale 3.82, Imbrium 3.85, Crisium 4.00, Nectaris 4.07, Serenitatis 4.14, Humorum 4.23 and an Unnamed basin between Werner and the Altai ring 4.30. 相似文献
15.
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
16.
L. B. Ronca A. T. Basilevsky V. P. Krychov B. A. Ivanov 《Earth, Moon, and Planets》1981,24(2):209-229
In order to study the geomorphic evolution and lifetimes of lunar craters, data were collected from (i) 32mare andterra provinces of the nearside of the Moon using the L.P.L. catalog; (ii) amare area in Sinus Medii, using direct observations of Lunar Orbiter photos, and (iii) aterra area on the farside using direct observations of Zond-8 photos. The theory presented in a previous publication is expanded and applied to the data.The following conclusions are obtained. (1) Steady-state conditions occur on the studiedmare surfaces for craters of diameter up to approximately 220 m, and on the studiedterra surfaces for craters of diameter up to at least 50 km. (2) The average lifetime of a crater, in addition of being a function of the meteoroidal flux, is a steep function of the diameter of the crater. (3) The correlation is good between a geomorphic classification of craters based on visual comparison with standard craters and a classification of craters based on their depth-diameter ratio, resulting in a coefficient of rank correlation of 0.64. (4) When craters are classified as young, mature, and old, the length of time spent as young is less than a few percent of the total lifetime of the crater; the time spent as mature is 10 to 30%; and as much as 80% is spent as an old crater. Within the error of the calculations, these values are independent of crater diameter and apply to both pre-mare and post-mare craters, indicating that they are also independent of the intensity of the meteoroidal flux. (5) The average lifetime of a 50 km crater in pre-mare times is estimated to be less than 0.3×109 years. (6) The average lifetime of a 50 km crater in post-mare times is estimated to be between 3×1011 and 1014 years. (7) The average meteoroidal flux in pre-mare times is estimated to be three to six orders of magnitude more intense than in post-mare times. 相似文献
17.
E. L. Ruskol 《Earth, Moon, and Planets》1973,6(1-2):190-201
It is suggested that the overall early melting of the lunar surface is not necessary for the explanation of facts and that
the structure of highlands is more complicated than a solidified anorthositic ‘plot’. The early heating of the interior of
the Moon up to 1000K is really needed for the subsequent thermal history with the maximum melting 3.5 × 109 yr ago, to give the observed ages for mare basalts. This may be considered as an indication that the Moon during the accumulation
retained a portion of its gravitational energy converted into heat, which may occur only at rapid processes. A rapid (t < 103 yr) accretion of the Moon from the circumterrestrial swarm of small particles would give necessary temperature, but it is
not compatible with the characteristic time 108 yr of the replenishment of this swarm which is the same as the time-scale of the accumulation of the Earth. It is shown that
there were conditions in the circumterrestial swarm for the formation at a first stage of a few large protomoons. Their number
and position is evaluated from the simple formal laws of the growth of satellites in the vicinity of a planet. Such ‘systems’
of protomoons are compared with the observed multiple systems, and the conclusion is reached that there could have been not
more than 2–3 large protomoons with the Earth. The tidal evolution of protomoon orbits was short not only for the present
value of the tidal phase-lag but also for a considerably smaller value.
The coalescence of protomoons into a single Moon had to occur before the formation of the observed relief on the Moon. If
we accept the age 3.9 × 109 yr for the excavation of the Imbrium basin and ascribe the latter to the impact of an Earth satellite, this collision had
to be roughly at 30R, whereR is the radius of the Earth, because the Moon at that time had to be somewhere at this distance. Therefore, the protomoons
had to be orbiting inside 20–25R, and their coalescence had to occur more than 4.0x109 yr ago. The energy release at coalescence is equivalent to several hundred degrees and even 1000 K. The process is very rapid
(of the order of one hour). Therefore, the model is valid for the initial conditions of the Moon. 相似文献
18.
Fiona Thiessen Alexander A. Nemchin Joshua F. Snape Martin J. Whitehouse Jeremy J. Bellucci 《Meteoritics & planetary science》2017,52(4):584-611
Secondary ion mass spectrometry (SIMS) U‐Pb ages of Ca‐phosphates from four texturally distinct breccia samples (72255, 76055, 76015, 76215) collected at the Apollo 17 landing site were obtained in an attempt to identify whether they represent a single or several impact event(s). The determined ages, combined with inferences from petrologic relationships, may indicate two or possibly three different impact events at 3920 ± 3 Ma, 3922 ± 5 Ma, and 3930 ± 5 Ma (all errors 2σ). Searching for possible sources of the breccias by calculating the continuous ejecta radii of impact basins and large craters as well as their expected ejecta thicknesses, we conclude that Nectaris, Crisium, Serenitatis, and Imbrium are likely candidates. If the previous interpretation that the micropoikilitic breccias collected at the North Massif represent Serenitatis ejecta is correct, then the average 207Pb/206Pb age of 3930 ± 5 Ma (2σ) dates the formation of the Serenitatis basin. The occurrence of zircon in the breccias sampled at the South Massif, which contain Ca‐phosphates yielding an age of 3922 ± 5 Ma (2σ), may indicate that the breccia originated from within the Procellarum KREEP terrane (PKT) and the Imbrium basin appears to be the only basin that could have sourced them. However, this interpretation implies that all basins suggested to fall stratigraphically between Serenitatis and Imbrium formed within a short (<11 Ma) time interval, highlighting serious contradictions between global stratigraphic constraints, sample interpretation, and chronological data. Alternatively, the slightly older age of the two micropoikilitic breccias may be a result of incomplete resetting of the U‐Pb system preserved in some phosphate grains. Based on the currently available data set this possibility cannot be excluded. 相似文献
19.
The mineralogy of mare basalts reflects the chemical composition of the magma source, as well as the physical and chemical environment of the rocks' formation. This is significant for understanding the thermal evolution of the Moon. In this study, the spatial distribution of mineralogy on the lunar northern nearside basalts was mapped using the Moon Mineralogy Mapper(M~3) data. The study area, which is an elongated mare, Mare Frigoris and northern Mare Imbrium, was mapped and characterized into 27 units based on multi-source data, including spectrum, terrain and element abundance. We extracted 177 M~3 spectra from fresh craters. Spectral parameters such as absorption center and band area ratio(BAR)were obtained through data processing. The variation of mafic minerals in this region was acquired by analyzing these parameters. The basaltic units in eastern Mare Frigoris, which are older, have been found to be dominated by clinopyroxene with lower CaO compared to the returned lunar samples; this is similar to older basaltic units in Mare Imbrium. The basaltic units of western Mare Frigoris and Sinus Roris which are younger have been found to be rich in olivine. The late-stage basalts in Oceanus Procellarum and Mare Imbrium show the same feature. These widespread olivine-rich basalts suggest uniqueness in the evolution of the Moon. Geographically speaking, Mare Frigoris is an individual mare, but the magma source region has connections with surrounding maria in consideration of mineral differences between western and eastern Frigoris, as well as mineral similarities with maria at the same location. 相似文献
20.
Grooved and hilly terrains occur at the antipode of major basins on the Moon (Imbrium, Orientale) and Mercury (Caloris). Such terrains may represent extensive landslides and surface disruption produced by impact-generatedP-waves and antipodal convergence of surface waves. Order-of-magnitude calculations for an Imbrium-size impact (1034 erg) on the Moon indicateP-wave-induced surface displacements of 10 m at the basin antipode that would arrive prior to secondary ejecta. Comparable surface waves would arrive subsequent to secondary ejecta impacts beyond 103 km and would increase in magnitude as they converge at the antipode. Other seismically induced surface features include: subdued, furrowed crater walls produced by landslides and concomitant secondary impacts; emplacement and leveling of light plains units owing to seismically induced ‘fluidization’ of slide material; knobby, pitted terrain around old basins from enhancement of seismic waves in ancient ejecta blankets; and perhaps the production and enhancement of deep-seated fractures that led to the concentration of farside lunar maria in the Apollo-Ingenii region. 相似文献