• High-latitude cold-based glacial deposits on Mars: Multiple superposed drop moraines in a crater interior at 70 degrees N latitude

      Garvin, James B.; Head, James W.; Marchant, David R.; Kreslavsky, Mikhail A. (The Meteoritical Society, 2006-01-01)
      An impact crater 26.8 km in diameter, located in the northern lowlands (70.32 degrees N, 266.45 degrees E) at the base of the flanking slopes of the shield volcano Alba Patera, is characterized by highly unusual deposits on its southeastern floor and interior walls and on its southeastern rim. These include multiple generations of distinctive arcuate ridges about 115-240 m in width and lobate deposits extending down the crater wall and across the crater floor, forming a broad, claw-like, ridged deposit around the central peak. Unusual deposits on the eastern and southeastern crater rim include frost, dunes, and a single distal arcuate ridge. Based on their morphology and geometric relationships, and terrestrial analogs from the Mars-like Antarctic Dry Valleys, the floor ridges are interpreted to represent drop moraines, remnants of the previous accumulation of snow and ice, and formation of cold-based glaciers on the crater rim. The configuration and superposition of the ridges indicate that the accumulated snow and ice formed glaciers that flowed down into the crater and across the crater floor, stabilized, covering an area of about 150 km^2, and produced multiple individual drop moraines due to fluctuation in the position of the stable glacier front. Superposition of a thin mantle and textures attributed to a recent ice-age period (~0.5-2 Myr ago) suggest that the glacial deposits date to at least 4-10 Myr before the present. At least five phases of advance and retreat are indicated by the stratigraphic relationships, and these may be related to obliquity excursions. These deposits are in contrast to other ice-related modification and degradation processes typical of craters in the northern lowlands, and may be related to the distinctive position of this crater in the past atmospheric circulation pattern, leading to sufficient preferential local accumulation of snow and ice to cause glacial flow.
    • Martian subsurface properties and crater formation processes inferred from fresh impact crater geometries

      Stewart, Sarah T.; Valiant, Gregory J. (The Meteoritical Society, 2006-01-01)
      The geometry of simple impact craters reflects the properties of the target materials, and the diverse range of fluidized morphologies observed in Martian ejecta blankets are controlled by the near-surface composition and the climate at the time of impact.Using the Mars Orbiter Laser Altimeter (MOLA) data set, quantitative information about the strength of the upper crust and the dynamics of Martian ejecta blankets may be derived from crater geometry measurements. Here, we present the results fromgeometrical measurements of fresh craters 3-50 km in rim diameter in selected highland (Lunae and Solis Plana) and lowland (Acidalia, Isidis, and Utopia Planitiae) terrains. We find large, resolved differences between the geometrical properties of the freshest highland and lowland craters. Simple lowland craters are 1.5-2.0 times deeper (greater than or equal to 5-sigma difference) with >50% larger cavities (greater than or equal to 2-sigma) compared to highland craters of the same diameter. Rim heights and the volume of material above the preimpact surface are slightly greater in the lowlands over most of the size range studied. The different shapes of simple highland and lowland craters indicate that the upper ~6.5 km of the lowland study regions are significantly stronger than the upper crust of the highland plateaus. Lowland craters collapse to final volumes of 45-70% of their transient cavity volumes, while highland craters preserve only 25-50%. The effective yield strength of the upper crust in the lowland regions falls in the range of competent rock, approximately 9-12 MPa, and the highland plateaus may be weaker by a factor of 2 or more, consistent with heavily fractured Noachian layered deposits. The measured volumes of continuous ejecta blankets and uplifted surface materials exceed the predictions from standard crater scaling relationships and Maxwell's Z model of crater excavation by a factor of 3. The excess volume of fluidized ejecta blankets on Mars cannot be explained by concentration of ejecta through nonballistic emplacement processes and/or bulking. The observations require a modification of the scaling laws and are well fit using a scaling factor of ~1.4 between the transient crater surface diameter to the final crater rim diameter and excavation flow originating from one projectile diameter depth with Z = 2.7. The refined excavation model provides the first observationally constrained set of initial parameters for study of the formation of fluidized ejecta blankets on Mars.
    • Modification of impact craters in the northern plains of Mars: Implications for Amazonian climate history

      Kreslavsky, M. A.; Head, J. W. (The Meteoritical Society, 2006-01-01)
      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 degrees 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 degrees) may have occurred episodically through the last 210-430 Myr. A sharp step in the frequency distribution of wall steepness at 20 degrees may indicate a geologically long period prior to that time where obliquity never exceeded 40-45 degrees.