摘要 :
Whether or not self-secondaries dominate small crater populations on continuous ejecta deposits and floors of fresh impact craters has long been a controversy. This issue potentially affects the age determination technique using c...
展开
Whether or not self-secondaries dominate small crater populations on continuous ejecta deposits and floors of fresh impact craters has long been a controversy. This issue potentially affects the age determination technique using crater statistics. Here the self-secondary crater population on the continuous ejecta deposits of the Hokusai crater on Mercury is unambiguously recognized. Superposition relationships show that this population was emplaced after both the ballistic sedimentation of excavation flows and the subsequent veneering of impact melt, but it predated the settlement and solidification of melt pools on the crater floor. Fragments that formed self-secondaries were launched via impact spallation with large angles. Complex craters on the Moon, Mercury, and Mars probably all have formed self-secondaries populations. Dating young craters using crater statistics on their continuous ejecta deposits can be misleading. Impact melt pools are less affected by self-secondaries. Overprint by subsequent crater populations with time reduces the predominance of self-secondaries.
收起
摘要 :
On average, secondary impact craters are expected to deepen and become more symmetric as impact velocity (vi) increases with downrange distance (L). We have used high-resolution topography (1-2 m/pixel) to characterize the morphom...
展开
On average, secondary impact craters are expected to deepen and become more symmetric as impact velocity (vi) increases with downrange distance (L). We have used high-resolution topography (1-2 m/pixel) to characterize the morphometry of secondary craters as a function of L for several well-preserved primary craters on Mars. The secondaries in this study (N = 2644) span a range of diameters (25 m ≤ D ≤ 400 m) and estimated impact velocities (0.4 km/s ≤v_i ≤2 km/s). The range of diameter-normalized rim-to-floor depth (d∕D) broadens and reaches a ceiling of d∕D ≈ 0.22 at L ≈ 280 km (v_i = 1-1.2 km/s), whereas average rim height shows little dependence on v_i for the largest craters (h∕D ≈ 0.02, D>60 m). Populations of secondaries that express the following morphometric asymmetries are confined to regions of differing radial extent: planform elongations (L < 110-160 km), taller downrange rims (L < 280 km), and cavities that are deeper uprange (L < 450-500 km). Populations of secondaries with lopsided ejecta were found to extend to at least L ~ 700 km. Impact hydrocode simulations with iSALE-2D for strong, intact projectile and target materials predict a ceiling for d∕D versus L whose trend is consistent with our measurements. This study illuminates the morphometric transition from subsonic to hypervelocity cratering and describes the initial state of secondary crater populations. This has applications to understanding the chronology of planetary surfaces and the long-term evolution of small crater populations.
收起
摘要 :Highlights?Crater population density increases away from rim on continuous ejecta blankets.?Late-arriving self-secondary fragments may be cause of crater density discrepancy.?Ghost craters observed in impact melt po...
展开Highlights?Crater population density increases away from rim on continuous ejecta blankets.?Late-arriving self-secondary fragments may be cause of crater density discrepancy.?Ghost craters observed in impact melt ponds provide evidence of self-secondaries.?Lunar cratering chronology calibration may overestimate small crater population.?Impact melt likely the best record of the inner Solar System impact flux of past Ga.收起
摘要 :
On average, secondary impact craters are expected to deepen and become more symmetric as impact velocity (v(i)) increases with downrange distance (L). We have used high-resolution topography (1-2m/pixel) to characterize the morpho...
展开
On average, secondary impact craters are expected to deepen and become more symmetric as impact velocity (v(i)) increases with downrange distance (L). We have used high-resolution topography (1-2m/pixel) to characterize the morphometry of secondary craters as a function of L for several well-preserved primary craters on Mars. The secondaries in this study (N = 2644) span a range of diameters (25m D400m) and estimated impact velocities (0.4km/s v(i)2km/s). The range of diameter-normalized rim-to-floor depth (d/D) broadens and reaches a ceiling of d/D approximate to 0.22 at L approximate to 280km (v(i)=1-1.2km/s), whereas average rim height shows little dependence on v(i) for the largest craters (h/D approximate to 0.02, D > 60m). Populations of secondaries that express the following morphometric asymmetries are confined to regions of differing radial extent: planform elongations (L<110-160km), taller downrange rims (L < 280km), and cavities that are deeper uprange (L<450-500km). Populations of secondaries with lopsided ejecta were found to extend to at least L approximate to 700km. Impact hydrocode simulations with iSALE-2D for strong, intact projectile and target materials predict a ceiling for d/D versus L whose trend is consistent with our measurements. This study illuminates the morphometric transition from subsonic to hypervelocity cratering and describes the initial state of secondary crater populations. This has applications to understanding the chronology of planetary surfaces and the long-term evolution of small crater populations.
收起
摘要 :
Impact craters are the predominant geological features on the lunar surface. Data about the craters are crucial for inferring information about surface age, the generation processes of the geological units, and the sequences of ge...
展开
Impact craters are the predominant geological features on the lunar surface. Data about the craters are crucial for inferring information about surface age, the generation processes of the geological units, and the sequences of geological events. The higher-resolution remote sensing datasets collected by recent lunar missions enable the investigation of impact craters of smaller size and provide more accurate information. This paper presents an investigation of the distribution and population characteristics of impact craters in and around the Orientale Basin based on high-resolution datasets. First, an update to the crater catalogue for the Orientale Basin and its surrounding area is provided, including craters as small as 1 km in diameter. Based on the updated crater catalogue, the crater densities and depth-to-diameter ratios in and around the Orientate Basin are investigated. The inclusion of small craters enables a crater density map with higher resolution, revealing a significantly higher crater density of the study area. Also, the surface age of Orientale Basin is estimated from the size-frequency distribution (SFD) using the new crater catalogue, showing an age of 3.75 Ga using the production function of Neukum et al. (2001), which is in good agreement with previous studies. Finally, distribution patterns of secondary craters around the Orientate Basin are investigated, indicating the Orientale Basin may be caused by an oblique impact with a downrange direction of about 235 degrees-260 degrees and an offset strength towards the direction of about 305 degrees-350 degrees.
收起
摘要 :
Crater statistics are used across a wide variety of applications on planetary surfaces, one of the most notable being estimating relative and absolute ages of those surfaces. This requires an assumed cratering rate over time and t...
展开
Crater statistics are used across a wide variety of applications on planetary surfaces, one of the most notable being estimating relative and absolute ages of those surfaces. This requires an assumed cratering rate over time and that craters be randomly distributed. Secondary craters ‐ craters that form from the ejecta of an impact event ‐ belie this assumption by creating greater crater density in a local area at a single time, significantly affecting crater statistics. There has been substantial debate over the relative importance of secondary craters, and our findings in this Mars study indicate that these events can be very significant and cannot be ignored when age‐dating surfaces. We have analyzed secondary crater fields found close to 24 primary craters on Mars. Among other findings such as terrain control over secondary crater field characteristics, we conclude that a single large impact event (>100 km) can significantly affect crater statistics at the ～1–5‐kmdiameter level over a non‐trivial fraction of a planetary surface (minimum secondary crater diameters examined were ～0.9 km; the minimum primary crater diameter was ～20 km). We also suggest a potential way to avoid significant contamination by the majority of secondary craters that occur close to the primary impact event without the need to manually classify every crater as primary or secondary. Our findings are specific to Mars, but further work may show the patterns are applicable to other solid bodies.
收起
摘要 :Highlights?New CSFDs on impact melts cover crater diameters in both strength- and gravity-scaling regimes.?Craters from each regime yield discrepant absolute model ages.?Calculations show target properties affect fi...
展开Highlights?New CSFDs on impact melts cover crater diameters in both strength- and gravity-scaling regimes.?Craters from each regime yield discrepant absolute model ages.?Calculations show target properties affect final crater diameters and the slope of the CSFD.?Coeval materials with differing target properties may exhibit discrepant absolute model ages.AbstractRecent work on dating Copernican-aged craters, using Lunar Reconnaissance O
收起
摘要 :
It is well known that asteroidal impacts on the Moon and Mars have ejected a large number of fragments that, after traveling in the inner planetary system for thousands or millions of years, occasionally fall on Earth and are reco...
展开
It is well known that asteroidal impacts on the Moon and Mars have ejected a large number of fragments that, after traveling in the inner planetary system for thousands or millions of years, occasionally fall on Earth and are recovered as meteorites.
It is of interest, therefore, to ask the question: what fraction of the mass excavated from the Chicxulub crater was ejected with escape velocities as the result of the 100 million megaton explosion? These fragments, similarly to what happened with lunar and martian ejecta, can fall onto the Moon, as well as back on the Earth as meteorites: Chicxulubites.
A 10 km-diameter asteroid, like the one at Chicxulub, could have produced a number of high velocity fragments with a total mass of about one thousandth of the mass of the projectile. From the work of Vickery (1987) on secondary craters on Mercury, the Moon and Mars, we estimated the mass and the diameter of the largest fragments that would have a velocity larger than the Earth's escape velocity. Assuming Dohnanyi's mass frequency distribution, we estimated that the number of fragments with sizes larger than 10 cm and 2 cm is about 4×10~(10) and 2×10~(12), respectively. We also estimated the expected fraction of these Chicxulubites to the total number of earth-crossing asteroids (ECA's) of similar diameter.
We conclude that a number of fragments from the Chicxulub crater have fallen onto the Moon and the Earth after becoming ECAs, and are waiting to be identified as Chicxulubites.
收起
摘要 :
Boulders larger than 2.2 m were studied on the rims and interior of 15 secondary craters of Copernicus Crater. The age of this Copernicus is 800 Ma, so that the age of its secondaries should be the same. This study continues the e...
展开
Boulders larger than 2.2 m were studied on the rims and interior of 15 secondary craters of Copernicus Crater. The age of this Copernicus is 800 Ma, so that the age of its secondaries should be the same. This study continues the earlier work of Basilevsky et al. (2013, 2015b, 2018b) and Li et al. (2018), providing a possibility to verify our conclusions on the lifetime of boulders at this scale on the lunar surface. The investigation of the abundance of boulders inside the considered secondaries provided information on the evolution of the boulder populations on the crater inner slopes where their abundance is often higher than on the rims. Our study found that the lifetime of these boulders on the lunar surface is indeed only several hundred million years. The rock abundance inside some of the studied craters was found to be higher than on their rims. An analysis of these cases led to the conclusion that the observed higher abundance of the interior boulders is due to downslope material movement. This leads to the appearance of new boulders, which were not outcropped earlier, replacing those on the surface which have been destroyed. The study of boulders in the polar areas of the Moon, where the factor of the boulder thermal fatigue is different compared to other areas may be the next stage of our analysis on the evolution of the populations of boulders on the lunar surface.
收起
摘要 :
Accumulation of impact craters is the major reason causing equilibrium of crater populations on airless planetary surfaces. Besides primary craters, the effect of widespread secondaries on the equilibrium of local crater populatio...
展开
Accumulation of impact craters is the major reason causing equilibrium of crater populations on airless planetary surfaces. Besides primary craters, the effect of widespread secondaries on the equilibrium of local crater populations is little studied. Here the different secondary crater populations formed by the Hokusai crater on Mercury are systematically studied, and they are compared with those on the Moon to investigate their contribution to the evolution of local crater populations. Self-secondaries cause equilibrium on continuous ejecta deposits in a short time, and the equilibrium crater population has a differential size-frequency distribution (SFD) slope of about -3. Background secondaries are abundant on Mercury, and equilibrium caused by a combination of primaries and potential background secondaries follows the same pattern on the Moon and Mercury. The spatial dispersion of fragments that form both near-field and distant secondaries is the major factor affecting the degree of mutual destruction and thus the final crater SFD. Some clustered distant secondaries on Mercury are likely formed by individual fragments considering their large spatial dispersion and identical morphology with same-sized primaries, and the SFD rollovers of these secondaries possibly reflect the inherent SFD rollovers of the impact fragments. Near-field secondaries and many other distant secondaries have morphology and spatial distribution that are consistent with being formed by clustered fragments, and mutual destruction of secondaries may be the major reason causing the observed SFD rollovers. Heterogeneous secondary impacts are a potential explanation for both different crater densities within the equilibrium diameter range and different regolith thicknesses on coeval surfaces.
收起
原文获取