Modification Of Impact Craters Research Articles

Modification of impact craters in the northern plains of Mars: Implications for Amazonian climate history

Abstract— We measured the depth, wall steepness, and ejecta roughness and surveyed the wall and floor morphology of all craters 10–25 km in diameter within the typical Vastitas Borealis Formation in the northern lowlands of Mars north of 52°N. Two of the 130 craters have unusually rough ejecta; they are deep, have steep walls, and are apparently the youngest in the population. Icy mantles filling the local subkilometer‐scale topographic lows is the main contribution to ejecta smoothing, which occurs at a time scale on the order of tens of Myr. Wall degradation and crater shallowing generally occur at longer time scales, comparable to the duration of the Amazonian period. Many craters are shallow due to filling of the crater with specific ice‐rich material of uncertain origin. We use our collected data to infer the nature of the past climate back through the Amazonian, a period prior to ˜10–20 Myr ago, when orbital parameter solutions are chaotic and one must rely on geological data to infer climate conditions. We conclude that moderately high obliquity and wide obliquity variations were probable during the last 40–160 Myr. We tentatively conclude that high obliquity peaks (>40–45°) may have occurred episodically through the last 210–430 Myr. A sharp step in the frequency distribution of wall steepness at 20° may indicate a geologically long period prior to that time where obliquity never exceeded 40–45°.

The role of fallback ejecta in the modification of impact craters

Fragments of target material ejected from an impact crater's transient cavity may be deposited within the cavity as fallback due to in-flight particle collisions and atmospheric deceleration. The characteristics of aerodynamically decelerated fallback ejecta are inferred by determining the size of the smallest fragment that is aerodynamically capable of traveling beyond the transient cavity rim at successive stages of cavity excavation. This technique has been applied to Meteror Crater, Arizona (MCA), a 1.2-km diameter crater that formed within a group of flat--lying, sedimentary rock units. Model results predict that: (i) ∼5% of the primary ejecta excavated from the crater was deposited as fallback (volume percent), (ii) average fallback fragment size is 1.4 cm, (iii) fallback ejecta is derived from a region encompassing all sedimentary formations intersected by the crater, and (iv) portions of the fallback source region were subjected to large shock stresses, on the order of several hundred kilobars. These results are in excellent agreement with empirical studies of the MCA fallback deposit. This agreement is interpreted to be an indication that atmospheric deceleration is the principal mechanism responsible for fallback deposition on the Earth. Model results for larger terrestrial craters indicate that as excavation cavity diameter ( D e) increases from 1 to 100 km, the proportion of primary ejecta deposited as fallback increases from 5 to 11% (average fragment size increases from 1.5 cm to 1.3 m over the same cavity diameter range). Fallback deposition is inferred to be a major cavity modification process during Venusian cratering events. Model estimates suggest that more than half of the primary ejecta excavated from large scale Venusian cavities ( D e > 10 km) should be deposited as fallback. In contrast, relatively small quantities of fallback are anticipated within lunar and Martian craters formed in coherent target media.

Modification of premare impact craters by volcanism and tectonism

Many lunar craters greater than 10 km in diam exhibit a variety of morphological characteristics which are not produced by meteorite impact or meteorite erosion. Most such craters are located in or near the margins of the maria. Although some could have resulted from processes such as cauldron resurgence, caldera formation, or ring dike emplacement, most have formed by modification of impact craters by endogenic processes including erosion by flowing lava, fissure volcanism, plutonism and uplift of crater floors along ring fractures of impact origin.