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1.
3D simulations of basin-scale lunar impacts are carried out to investigate: (a) the origins of strong crustal magnetic fields and unusual terrain observed to occur in regions antipodal to young large basins; and (b) the origin of enhanced magnetic and geochemical anomalies along the northwest periphery of the South Pole-Aitken (SPA) basin. The simulations demonstrate that a basin-forming impact produces a massive, hot, partially ionized cloud of vapor and melt that expands thermally around the Moon, converging near the basin antipode approximately 1 h after the impact for typical impact parameters. In agreement with previous work, analytic calculations of the interaction of this vapor-melt cloud with an initial ambient magnetic field predict a substantial temporary increase in field intensity in the antipodal region. The time of maximum field amplification coincides with a period when impacting ejecta also converge near the antipode. The latter produce antipodal shock stresses within the range of 5-25 GPa where stable shock remanent magnetization (SRM) of lunar soils has been found experimentally to occur. Calculated antipodal ejecta thicknesses are only marginally sufficient to explain the amplitudes of observed magnetic anomalies if mean magnetization intensities are comparable to those produced experimentally. This suggests that pre-existing ejecta materials, which would also contain abundant metallic iron remanence carriers, may be important anomaly sources, a possibility that is consistent with enhanced magnetic anomalies observed peripheral to SPA. The latter anomalies may be produced by amplified secondary ejecta impact shock waves in the thick SPA ejecta mantle occurring near the antipodes of the Imbrium and Serenitatis impacts. Together with converging seismic compressional waves, these antipodal impact shocks may have produced especially deep fracture zones along the northwest edge of SPA near the Imbrium antipode, allowing the ascent of magma with enhanced KREEP concentrations.  相似文献   

2.
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   

3.
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   

4.
Abstract— We survey the magnetic fields of lunar multi‐ring impact basins using data from the electron reflectometer instrument on the Lunar Prospector spacecraft. As for smaller lunar craters, the primary signature is a magnetic low that extends to ?1.5–2 basin radii, suggesting shock demagnetization of relatively soft crustal magnetization. A secondary signature, as for large terrestrial basins, is the presence of central magnetic anomalies, which may be due to thermal remanence in impact melt rocks and/or shock remanence in the central uplift. The radial extent of the anomalies may argue for the former possibility, but the latter or a combination of the two are also possible. Central anomaly fields are absent for the oldest pre‐Nectarian basins, increase to a peak in early Nectarian basins, and decrease to a low level for Imbrian basins. If basin‐associated anomalies provide a good indication of ambient magnetic fields when the basins formed, this suggests the existence of a “magnetic era” (possibly due to a lunar core dynamo) similar to that implied by paleointensity results from returned lunar samples. However, the central basin anomalies suggest that the fields peaked in early Nectarian times and were low in Imbrian times, while samples provide evidence for high fields in Nectarian and early Imbrian times.  相似文献   

5.
We measured the velocity distributions of impact ejecta with velocities higher than ∼100 m s−1 (high-velocity ejecta) for impacts at variable impact angle α into unconsolidated targets of small soda-lime glass spheres. Polycarbonate projectiles with mass of 0.49 g were accelerated to ∼250 m s−1 by a single-stage light-gas gun. The impact ejecta are detected by thin aluminum foils placed around the targets. We analyzed the holes on the aluminum foils to derive the total number and volume of ejecta that penetrated the aluminum foils. Using the minimum velocity of the ejecta for penetration, determined experimentally, the velocity distributions of the high-velocity ejecta were obtained at α=15°, 30°, 45°, 60°, and 90°. The velocity distribution of the high-velocity ejecta is shown to depend on impact angle. The quantity of the high-velocity ejecta for vertical impact (α=90°) is considerably lower than derived from a power-law relation for the velocity distribution on the low-velocity ejecta (less than 10 m s−1). On the other hand, in oblique impacts, the quantity of the high-velocity ejecta increases with decreasing impact angle, and becomes comparable to those derived from the power-law relation. We attempt to scale the high-velocity ejecta for oblique impacts to a new scaling law, in which the velocity distribution is scaled by the cube of projectile radius (scaled volume) and a horizontal component of impactor velocity (scaled ejection velocity), respectively. The high-velocity ejecta data shows a good correlation between the scaled volume and the scaled ejection velocity.  相似文献   

6.
Experiments to investigate the effect of impacts on side-walls of dust detectors such as the present NASA/ESA Galileo/Ulysses instrument are reported. Side walls constitute 27% of the internal area of these instruments, and increase field of view from 140° to 180°. Impact of cosmic dust particles onto Galileo/Ulysses Al side walls was simulated by firing Fe particles, 0.5-5 μm diameter, 2-50 km s−1, onto an Al plate, simulating the targets of Galileo and Ulysses dust instruments. Since side wall impacts affect the rise time of the target ionization signal, the degree to which particle fluxes are overestimated varies with velocity. Side-wall impacts at particle velocities of 2-20 km s−1 yield rise times 10-30% longer than for direct impacts, so that derived impact velocity is reduced by a factor of ∼2. Impacts on side wall at 20-50 km s−1 reduced rise times by a factor of ∼10 relative to direct impact data. This would result in serious overestimates of flux of particles intersecting the dust instrument at velocities of 20-50 km s−1. Taking into account differences in laboratory calibration geometry we obtain the following percentages for previous overestimates of incident particle number density values from the Galileo instrument [Grün et al., 1992. The Galileo dust detector. Space Sci. Rev. 60, 317-340]: 55% for 2 km s−1 impacts, 27% at 10 km s−1 and 400% at 70 km s−1. We predict that individual particle masses are overestimated by ∼10-90% when side-wall impacts occur at 2-20 km s−1, and underestimated by ∼10-102 at 20-50 km s−1. We predict that wall impacts at 20-50 km s−1 can be identified in Galileo instrument data on account of their unusually short target rise times. The side-wall calibration is used to obtain new revised values [Krüger et al., 2000. A dust cloud of Ganymede maintained by hypervelocity impacts of interplanetary micrometeoroids. Planet. Space Sci. 48, 1457-1471; 2003. Impact-generated dust clouds surrounding the Galilean moons. Icarus 164, 170-187] of the Galilean satellite dust number densities of 9.4×10−5, 9.9×10−5, 4.1×10−5, and 6.8×10−5 m−3 at 1 satellite radius from Io, Europa, Ganymede, and Callisto, respectively. Additionally, interplanetary particle number densities detected by the Galileo mission are found to be 1.6×10−4, 7.9×10−4, 3.2×10−5, 3.2×10−5, and 7.9×10−4 m−3 at heliocentric distances of 0.7, 1, 2, 3, and 5 AU, respectively. Work by Burchell et al. [1999b. Acceleration of conducting polymer-coated latex particles as projectiles in hypervelocity impact experiments. J. Phys. D: Appl. Phys. 32, 1719-1728] suggests that low-density “fluffy” particles encountered by Ulysses will not significantly affect our results—further calibration would be useful to confirm this.  相似文献   

7.
《Icarus》1987,71(1):19-29
From counts of postbasin craters larger than 30 km in diameter, lying within or near to seven giant front face lunar basins, relative ages for the basins may be obtained. These relative ages correlate well with absolute basin ages found from viscosity arguments in R. B. Baldwin (1987, Icarus 70, □□□-□□□). From crater counts the basins are in the following sequence of increasing relative age: Orientale, Imbrium, Crisium, Serenitatis, Nectaris, Humorum, and the unnamed basin lying between Werner and the Altai ring. The absolute ages from Baldwin (1987) range from 3.80 to 4.30 × 109 years while a correlation with the relative ages of this paper yields a range of 3.79 to 4.27 × 109 years. The discrepancy is largely due to Serenitatis where the debris from Imbrium has presumably buried some post-Serenitatis craters. From both sets of data there is no evidence that a “Terminal Lunar Cataclysm” ever occured.  相似文献   

8.
Did a large impact reorient the Moon?   总被引：3，自引：0，他引：3  
The Moon is currently locked in a spin–orbit resonance of synchronous rotation, of which one consequence is that more impacts should occur near the Moon's apex of motion (0° N, 90° W) than near its antapex of motion (0° N, 90° E). Several of the largest lunar impact basins could have temporarily unlocked the Moon from synchronous rotation, and after the re-establishment of this state the Moon would have been left in either its initial orientation, or one that was rotated 180° about its spin axis. We show that there is less than a 2% probability that the oldest lunar impact basins are randomly distributed across the lunar surface. Furthermore, these basins are preferentially located near the Moon's antapex of motion, and this configuration has less than a 0.3% probability of occurring by chance. We postulate that the current “near side” of the Moon was in fact its “far side” when the oldest basins formed. One basin with the required size and temporal characteristics to account for a 180° reorientation is the Smythii basin.  相似文献   

9.
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   

10.
Areas of lunar surface magnetic field are observed to ‘mirror’ low energy electrons present in the normal lunar space environment. The ambient electrons provide, in effect, a probe along the ambient magnetic field lines down to the lunar surface for remote sensing of the presence of surface fields. This probe, unlike direct measurement by the magnetometer, does not require low altitude or a very stable (magnetotail) ambient field to provide a mapping of regions of occurrence of such fields. Use of the on-board vector magnetometer measurements of the ambient magnetic field orientation allows accurate projection of such mapping onto the lunar surface. Preliminary maps of the lunar surface magnetic areas underlying the orbit of the ‘Particles and Fields Satellite deployed from Apollo-16’ have been generated, obtaining 40% coverage from partial data to demonstrate feasibility of the technique. As well as providing independent verification of areas such as Van de Graaff already discovered in the magnetometer data, these maps reveal many previously unreported areas of surface magnetism. The method is sensitive to fields of less than 0.1γ at the surface. Application to the full body of available PFS-1 & 2 electron data is expected to provide complete mapping of the lunar surface for areas of magnetization up to latitudes of 35–40 deg. The surface field regions observed are generally due to sources smaller than 10–50 km in size, although many individual regions are often so close together as to give much larger regions of effectively continuous mirroring. Absence of consistent mirroring by any global field places an upper limit on the size of any net lunar dipole moment of less than 1010 γ km3. Much additional information regarding the magnetic regions can be obtained by correlated analysis of both the electron return and vector magnetometer measurements at orbital altitude, the two techniques providing each other with directly complimentary measurements at the satellite and along the ambient field lines to the surface.  相似文献   

11.
Javier Ruiz 《Icarus》2005,177(2):438-446
The heat flow from Europa has profound implications for ice shell thickness and structure, as well as for the existence of an internal ocean, which is strongly suggested by magnetic data. The brittle-ductile transition depth and the effective elastic thickness of the lithosphere are here used to perform heat flow estimations for Europa. Results give preferred heat flow values (for a typical geological strain rate of 10−15 s−1) of 70-110 mW m−2 for a brittle-ductile transition 2 km deep (the usually accepted upper limit for the brittle-ductile transition depth in the ice shell of Europa), 24-35 mW m−2 for an effective elastic thickness of 2.9 km supporting a plateau near the Cilix impact crater, and >130 mW m−2 for effective elastic thicknesses of ?0.4 km proposed for the lithosphere loaded by ridges and domes. These values are clearly higher than those produced by radiogenic heating, thus implying an important role for tidal heating. The ?19-25 km thick ice shell proposed from the analysis of size and depth of impact structures suggests a heat flow of ?30-45 mW m−2 reaching the ice shell base, which in turn would imply an important contribution to the heat flow from tidal heating within the ice shell. Tidally heated convection in the ice shell could be capable to supply ∼100 mW m−2 for superplastic flow, and, at the Cilix crater region, ∼35-50 mW m−2 for dislocation creep, which suggests local variations in the dominant flow mechanism for convection. The very high heat flows maybe related to ridges and domes could be originated by preferential heating at special settings.  相似文献   

12.
Magma genesis in the Moon could have been significantly altered by large impacts if they melted solidified residual liquids and late cumulates from the ‘magma ocean’. Calculations of the heat required to melt these materials, under different assumed conditions, are compared to estimates of the total kinetic energy of the Imbrium impact. For a significant amount of these materials to have been melted, they must have been near their solidus temperatures, the impacts must have been very large, and the lunar lithosphere must have been locally heated at depths of 70 to 140 km. Unless the Imbrium impact released at least the maximum estimated kinetic energy, only larger impacts, e.g., the proposed ‘Gargantuan’ impact, could have augmented the intrinsic lunar heat budget enough to locally alter the abundance, timing of eruption, and chemical compositions of lunar magmas. The mechanical and thermal energy generated by such an impact could have been critical in creating (1) the higher concentrations of radioactive elements in the Imbrium/Procellarum area by migration of residual liquids driven by differential lithospheric thickness; and (2) hybrid mare basalts (representing varying proportions of late cumulates and/or residual liquids incorporated into primitive magmas rising from the partially molten lunar interior). Complete compositional spectra of lunar basalts are to be expected, from primitive mare basalts to pure KREEP and to Ti-rich varieties. Comparison of the Gargantuan/Imbrium area with ancient basins in the eastern nearside area suggests that the interplay between the Moon's internal heat engine and the timing of large impacts was a crucial factor in determining the time of tunar volcanism and the chemical composition of the lavas.  相似文献   

13.
The 1300-km-diameter Caloris impact basin is surrounded by well-defined ejecta units that can be recognized from more than 1000 km, radially outward from the basin edge. A formal rock stratigraphic nomenclature is proposed for the Caloris ejecta units, which are collectively called the Caloris Group. Each of the individual formations within the Group are described and compared to similar rock units associated with the lunar Imbrium and Orientale basins. A crater degradation chronology, linked the the Caloris event, is also proposed to assist in stratigraphic correlation on a Mercury-wide basis.  相似文献   

14.
I. Pat-El 《Icarus》2009,201(1):406-411
From recent close encounters with Comets Wild-2 and Tempel 1 we learned that their surfaces are very rugged and no simple uniform layers model can be applied to them. Rather, a glaciological approach should be applied for describing their surface features and behavior. Such intrinsically rugged surface is formed in our large scale experiments, where an agglomerate of ∼200 μm gas-laden amorphous ice particles is accumulated to form a 20 cm diameter and few cm high ice sample. The density, tensile strength and thermal inertia of our ice sample were found to be very close to those found by Deep Impact for Comet Tempel 1: density 250-300 kg m−3 vs DI 350-400 kg m−3; tensile strength 2-4 kPa vs DI 1-10 kPa; thermal inertia 80 W K−1 m−2 s1/2 vs <100 W K−1 m−2 s1/2 and <50 W K−1 m−2 s1/2. From the close agreement between the thermal inertias measured in our ice sample, which had no dust coverage and that of Comet Tempel 1, we deduce that the low thermal inertia is an intrinsic property of the fluffy structure of the ice as a result of its low density, with an addition by the broken terrain and not due to the formation of a dust layer. Upon warming up of the ice, water vapor migrates both outward into the coma and inward. Reaching cooler layers, the water vapor condenses, forming a denser ice crust, as we show experimentally. We also demonstrate the inward and outward flow of water vapor in the outer ice layers through the exchange between layers of D2O ice and H2O ice, to form HDO.  相似文献   

15.
The crater populations of 18 lunar light plains (Cayley plains) show a variation in relative ages by a factor of about 4 in crater frequency of regions in the surroundings of the Orientale resp. Imbrium basin, and by a factor of greater than 25 for more distant sites. Thus the idea of a Moonwide synchronism in the emplacement of the lunar light plains with the formation of the basins Imbrium or Orientale cannot be supported.Some light plains are younger than the youngest basin Orientale. Since these plains cannot have been emplaced by any other basin-forming event and local impact-derived origin can certainly be excluded, an endogenic (magmatic) origin is proposed for these plains.Age determination data (D L -values) by Soderblom and Lebofsky (1972) and Soderblom and Boyce (1972) are shown to be correlated with own cumulative crater frequency data (N) for surfaces younger than 3.8 × 109 years; we findD L × N 0.6, different from the originally proposed relationD L × N. For ages > 3.8 × 109 years, theD L data by those authors, especially their light plains data, are incompatible with our crater frequency data.  相似文献   

16.
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   

17.
Matija ?uk  Brett J. Gladman 《Icarus》2010,207(2):590-7225
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   

18.
In this paper we analyze near-infrared thermal emission spectra of the spatially resolved nucleus of Comet 9P/Tempel 1 obtained by the NASA spacecraft Deep Impact. Maps of spectral reddening, the product X′ between the beaming function and directional emissivity, as well as surface temperature are constructed. Thermophysical modeling is used to estimate the degree of small scale surface roughness and thermal inertia by detailed reproduction of the empirical temperature map. Mie and Hapke theories are used in combination with numerically calculated beaming functions to analyze the X′ map and place constraints on composition and grain size of the surface material. We show that it is absolutely mandatory to include small scale surface roughness in thermophysical modeling of this object, since the resulting self heating is vital for reproducing the measured temperatures. A small scale self heating parameter in the range 0.6?ξ?0.75 is common, but smoother areas where 0.2?ξ?0.3 are also found. Contrary to models neglecting small scale surface roughness, we find that the thermal inertia of Comet 9P/Tempel 1 generally is high (1000-3000 J m−2 K−1 s−1/2), although it may be substantially lower (40-380 J m−2 K−1 s−1/2) in specific areas. We obtain a disk-averaged reddening of 3.5% kÅ−1, with statistically significant local variations around that value on a ±1.0% kÅ−1 level. Vast regions appear covered by small (∼0.1 μm) highly absorbing grains such as carbon or iron-rich silicates. Other regions appear dominated by somewhat larger (∼0.5 μm) and/or less absorbing grains such as troilite or magnesium-rich silicates. Surface variations in reddening, roughness, thermal inertia, composition and/or grain size are moderately to strongly correlated to the locations of morphological units on the surface. The existence of morphological units with differing physical properties may be primordial, hence reflecting a diversity in the building block cometesimals, or resulting from evolutionary processes.  相似文献   

19.
The Deep Impact (DI) spacecraft encountered Comet 9P/Tempel 1 on July 4th, 2005 and observed it with several instruments. In particular, we obtained infrared spectra of the nucleus with the HRI-IR spectrometer in the wavelength range of 1.0-4.9 μm. The data were taken before impact, with a maximum resolution of ∼120 m per pixel at the time of observation. From these spectra, we derived the first directly observed temperature map of a comet nucleus. The surface temperature varied from 272±7 to 336±7 K on the sunlit hemisphere, matching the surface topography and incidence angle. The derived thermal inertia is low, most probably <50 W K−1 m−2 s1/2. Combined with other arguments, it is consistent with the idea that most of rapidly varying thermal physical processes, in particular the sublimation of volatiles around perihelion, should occur close to the surface. Thermal inertia is sufficient to explain the temperature map of the nucleus of Comet Tempel 1 to first order, but other physical processes like roughness and self-radiation are required to explain the details of the temperature map. Finally, we evaluated that the Standard Thermal Model is a good approximation to derive the effective radius of a cometary nucleus with an uncertainty lower than ∼10% if combined with a thermal infrared light curve.  相似文献   

20.
Photometry and thermal lightcurves of six large asteroids (1-Ceres, 2-Pallas, 3-Juno, 12-Victoria, 85-Io and 511-Davida) have been observed at 870 μm (345 GHz) using the MPIfR 19-Channel Bolometer of the Heinrich-Hertz Submillimeter Telescope. Only Ceres displayed a lightcurve with an amplitude (∼50%, peak to peak) that was significantly greater than the uncertainty in the observations. When thermal fluxes and brightness temperatures are corrected for heliocentric distance and albedo, there is a significant relation with the sub-solar latitude of the asteroid, or the local season of the asteroid. No such trend can be found between observations with solar phase angle. These results are evidence that most of the submillimeter thermal radiation is emitted from below the diurnal thermal wave. Comparing the observed trend with model output suggests that the submillimeter radiation from all the asteroids we observed is best modeled by surface material with low thermal inertia (<15 J m−2 s−0.5 K−1, consistent with mid-infrared observations of large main-belt asteroids) and a refractive index closer to unity relative to densities inferred from radar experiments, implying a veneer of material over the asteroid surface with a density less than 1000 kg m−3. More data with better signal-to-noise and aspect coverage could improve these models and constrain physical properties of asteroid surface materials. This would also allow asteroids to be used as calibration sources with accurately known and stable, broadband fluxes at long wavelengths.  相似文献   

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