Impact craters on geologic units of northern Venus: Implications for the duration of the transition from Tessera to regional plains
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Using Magellan SAR images and the Schaber et al. [1998] crater data base we examined impact craters in the area north of 35°N and determined the geologic units on which they are superposed. The crater density of the regional plains with wrinkle ridges (Pwr) was found to be very close to the global average and thus the mean surface age of the plains is close to the mean surface age of the planet (T). About 80 to 97% of the craters superposed on a composite unit that includes materials of Tessera terrain (Tt), Densely fractured plains (Pdf), Fractured and ridged plains (Pfr), and Fracture Belts (FB), also postdate the regional plains. Thus, the time interval between the formation of these older units and emplacement of the regional plains (ΔT) should be geologically short, from a few percent to about 20% of T, or approximately 40 to 150 m.y. This means that in the area under study, volcanic and tectonic activity in the beginning of the morphologically recognizable part of the geologic history of Venus (about the last 750 m.y.) was much more active than in the subsequent time.A high‐resolution study of 18 lunar craters, including both primary and distant secondary craters, shows that the secondary craters produce larger ejecta fragments at a given crater size than do the primary craters. The maximum boulder diameter ( B ) increases with crater size ( D ) according to the power law B = KD 2/3 ; for primary craters, when B and D are in meters, K is 0.29, whereas for secondary craters, we find that K is 0.46 (60% larger). Next we show that impact fracture theory predicts that secondary craters, because of their lower impact velocity, will produce larger ejecta fragments than primary craters. This result provides an opportunity for distinguishing between primary and secondary craters in high resolution planetary images. The ability to identify distant secondary craters will help constrain primary production rates of small craters and improve surface age determination of small areas based on small crater counts.
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Lunar craters
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Abstract We investigate the elevated crater rims of lunar craters. The two main contributors to this elevation are a structural uplift of the preimpact bedrock and the emplacement of ejecta on top of the crater rim. Here, we focus on five lunar complex mare craters with diameters ranging between 16 and 45 km: Bessel, Euler, Kepler, Harpalus, and Bürg. We performed 5281 measurements to calculate precise values for the structural rim uplift and the ejecta thickness at the elevated crater rim. The average structural rim uplift for these five craters amounts to S RU = 70.6 ± 1.8%, whereas the ejecta thickness amounts to E T = 29.4 ± 1.8% of the total crater rim elevation. Erosion is capable of modifying the ratio of ejecta thickness to structural rim uplift. However, to minimize the impact of erosion, the five investigated craters are young, pristine craters with mostly preserved ejecta blankets. To quantify how strongly craters were enlarged by crater modification processes, we reconstructed the dimensions of the transient crater. The difference between the transient crater diameter and the final crater diameter can extend up to 11 km. We propose reverse faulting and thrusting at the final crater rim to be one of the main contributing factors of forming the elevated crater rim.
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Lunar craters
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The density of impact craters on large volcanoes on Venus is half the average crater density for the planet. The crater density on some classes of coronae is not significantly different from the global average density, but coronae with extensive associated volcanic deposits have lower crater densities. These results are inconsistent with both single-age and steady-state models for global resurfacing and suggest that volcanoes and coronae with associated volcanism have been active on Venus over the last 500 million years.
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The amount of obliteration suffered by Martian impact craters is quantified by comparing measurable attributes of the current crater shape to those values expected for a fresh crater of identical size. Crater diameters are measured from profiles obtained using photoclinometry across the structure. The relationship between the diameter of a fresh crater and a crater depth, floor width, rim height, central peak height, etc. was determined by empirical studies performed on fresh Martian impact craters. We utilized the changes in crater depth and rim height to judge the degree of obliteration suffered by Martian impact craters.
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The long‐lived 222 Rn decay products 210 Pb, 210 Bi and 210 Po have been monitored in the plumes of several vents at Mount Etna (Sicily) from May to October 1986. The results show that the four main craters of this volcano emit gases whose compositions are different from each other. The 210 Bi/ 210 Pb ratios for the plumes have similar mean values, (close to 25), which correspond to a degassing time of 1.5 to 2.7 days, according to the model of Lambert et al. (1985/86). In contrast, 210 Po/ 210 Pb ratios have very different mean values in each plume: 35 at the Voragine crater, 20 at the Bocca Nuova crater, and 14 at the South East crater. These figures enable us to calculate proportions of deep magma of 50%, 29% and 19% in the degassing cells of these craters respectively. Moreover, the SE crater appears to be a secondary degassing vent, not directly related to the main magma reservoir. The evolution of these ratios has been related to variations in volcanic activity.
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Crater lake
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Abstract Sakurajima Muography Observatory captured the formation of a volcanic plug beneath the Showa crater with a spatial resolution of ∼60 m in accordance with the end of the eruption episode of Showa crater and the reactivation of Minamidake crater. The increase of average density was observed with above 3 σ standard deviation for both above the floor of Minamidake crater and beneath the floor of Showa crater, respectively, being interpreted as volcanic ejecta deposition and the formation of a volcanic plug laterally extended within a few hundreds of meters.
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[1] 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-km-diameter 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.
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