The effect of impact angle on craters formed by hypervelocity particles

The Space Power Institute (SPI) at Auburn University has conducted experiments on the effects of impact angle on crater morphology and impactor residue retention for hypervelocity impacts. Copper target plates were set at angles of 30 deg, 45 deg, 60 deg, and 75 deg from the particle flight path. For the 30 deg and 45 deg impacts, in the velocity regime greater than 8 km s(exp -1) the resultant craters are almost identical to normal incidence impacts. The only difference found was in the apparent distribution of particle residue within the crater, and further research is needed to verify this. The 60 deg and 75 deg impacts showed marked differences in crater symmetry, crater lip shape, and particle residue distribution in the same velocity regime. Impactor residue shock fractionation effects have been quantified in first-order. It is concluded that a combination of analysis techniques can yield further information on impact velocity, direction, and angle of incidence

Characterization and petrologic interpretation of olivine-rich basalts at Gusev Crater, Mars

Rocks on the floor of Gusev crater are basalts of uniform composition and mineralogy. Olivine, the only mineral to have been identified or inferred from data by all instruments on the Spirit rover, is especially abundant in these rocks. These picritic basalts are similar in many respects to certain Martian meteorites (olivine-phyric shergottites). The olivine megacrysts in both have intermediate compositions, with modal abundances ranging up to 20–30%. Associated minerals in both include low-calcium and highcalcium pyroxenes, plagioclase of intermediate composition, iron-titanium-chromium oxides, and phosphate. These rocks also share minor element trends, reflected in their nickel-magnesium and chromium-magnesium ratios. Gusev basalts and shergottites appear to have formed from primitive magmas produced by melting an undepleted mantle at depth and erupted without significant fractionation. However, apparent differences between Gusev rocks and shergottites in their ages, plagioclase abundances, and volatile contents preclude direct correlation. Orbital determinations of global olivine distribution and compositions by thermal emission spectroscopy suggest that olivine-rich rocks may be widespread. Because weathering under acidic conditions preferentially attacks olivine and disguises such rocks beneath alteration rinds, picritic basalts formed from primitive magmas may even be a common component of the Martian crust formed during ancient and recent times.Additional co-authors: PR Christensen, BC Clark, JA Crisp, DJ DesMarais, T Economou, JD Farmer, W Farrand, A Ghosh, M Golombek, S Gorevan, R Greeley, VE Hamilton, JR Johnson, BL Joliff, G Klingelhöfer, AT Knudson, S McLennan, D Ming, JE Moersch, R Rieder, SW Ruff, PA de Souza Jr, SW Squyres, H Wnke, A Wang, A Yen, J Zipfe

Overview of the Opportunity Mars Exploration Rover mission to Meridiani Planum: Eagle crater to Purgatory ripple

The Mars Exploration Rover Opportunity touched down at Meridiani Planum in January 2004 and since then has been conducting observations with the Athena science payload. The rover has traversed more than 5 km, carrying out the first outcrop-scale investigation of sedimentary rocks on Mars. The rocks of Meridiani Planum are sandstones formed by eolian and aqueous reworking of sand grains that are composed of mixed fine-grained siliciclastics and sulfates. The siliciclastic fraction was produced by chemical alteration of a precursor basalt. The sulfates are dominantly Mg-sulfates and also include Ca-sulfates and jarosite. The stratigraphic section observed to date is dominated by eolian bedforms, with subaqueous current ripples exposed near the top of the section. After deposition, interaction with groundwater produced a range of diagenetic features, notably the hematite-rich concretions known as ‘‘blueberries.’’ The bedrock at Meridiani is highly friable and has undergone substantial erosion by wind-transported basaltic sand. This sand, along with concretions and concretion fragments eroded from the rock, makes up a soil cover that thinly and discontinuously buries the bedrock. The soil surface exhibits both ancient and active wind ripples that record past and present wind directions. Loose rocks on the soil surface are rare and include both impact ejecta and meteorites. While Opportunity’s results show that liquid water was once present at Meridiani Planum below and occasionally at the surface, the environmental conditions recorded were dominantly arid, acidic, and oxidizing and would have posed some significant challenges to the origin of life.Additional co-authors: J Farmer, WH Farrand, W Folkner, R Gellert, TD Glotch, M Golombek, S Gorevan, JA Grant, R Greeley, J Grotzinger, KE Herkenhoff, S Hviid, JR Johnson, G Klingelhöfer, AH Knoll, G Landis, M Lemmon, R Li, MB Madsen, MC Malin, SM McLennan, HY McSween, DW Ming, J Moersch, RV Morris, T Parker, JW Rice Jr, L Richter, R Rieder, M Sims, M Smith, P Smith, LA Soderblom, R Sullivan, NJ Tosca, H Wnke, T Wdowiak, M Wolff, A Ye

Lava-Rise Plateaus and Inflation Pits in the McCartys Lava Flow Field, New Mexico: An Analog for Pahoehoe-Like Lava Flows on Planetary Surfaces

Basaltic lava flows are common on the surface of the Earth and other terrestrial bodies. However, inflation-including a combination of initially rapid molten core thickening and gradual crustal growth-must be accounted for to enable accurate reconstructions of eruption parameters from observed lava flow morphologies. The shape of an inflated lava flow can change significantly over time. Therefore, incorrectly attributing the flow's final thickness to its dimensions in an initially fully molten state will yield excessively high flow rates, erroneous rheological properties, and unreasonably short eruption durations. To develop improved criteria for identifying inflated lava flows, we examined the McCartys lava flow field in New Mexico, USA. This locality provides an example of how pahoehoe-like lava lobes can coalesce and coinflate to form interconnected lava-rise plateaus with internal inflation pits. These structures were examined using a combination of field observations, low-altitude kite-based imaging, and quantitative geomorphology using high-resolution (1.47 cm/pixel) orthomosaics and stereo-derived digital terrain models. These observations were used to identify characteristics and diagnostics of inflation, thereby facilitating the interpretation of comparable landforms on other planetary surfaces. Lava-cooling models were also used to estimate the lava emplacement duration of the similar to 20-m-thick flows by demonstrating that the similar to 8-m-thick upper crust exposed within inflation clefts in the southern part of the McCartys lava flow field would have required 1.2-2.5 years of continuous lava supply to form. This places a minimum bound on the total eruption duration, and implies that comparably thick inflated flows on Mars required years to form. Plain Language Summary Lava is common throughout the solar system and can provide information about the geologic history of terrestrial planets and moons. However, reconstructing information about volcanic eruptions from lava flows requires an understanding of how their shapes change during an eruption. Much like an inflating balloon, or rising bread, lava can stretch and break apart as it grows. With lava, inflation is driven by the supply of molten lava into the flow's interior, which cools and adds new material to the crust, lifting the upper crust like a car jack. To accurately determine eruption conditions, it is necessary to rewind the inflation process and distinguish between the lava's initial shape and gradual changes that took place as the eruption progressed. To address this problem, we examined the McCartys lava flow field in New Mexico, USA, using hands-on observations and digital aerial photography-enabled by use of a camera mounted onto a kite. By mapping the three-dimensional shape of the lava and its structures, we determined that the 20-m-thick flow includes 8 m of crust and took about 2 years of continuous lava supply to form. This suggests that similar lava flows on other planets, like Mars, also involve eruptions that last for years.6 month embargo; first published online 27 April 2020This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Geochemistry of Martian soil and bedrock in mantled and less mantled terrains with gamma ray data from Mars Odyssey

Surficial materials, including soil and dust, are abundant in the upper tens of centimeters of the Martian surface sensed by the Mars Odyssey Gamma Ray Spectrometer (GRS). Seven large areas (14% of the Martian surface) that represent possible compositional end-members were selected, including three regions heavily mantled with surficial materials. The selection process included mapping the ratio of exposed rocky terrain to surficial materials using high-resolution imagery. GRS data for H, Cl, Fe, Si, K, and Th were obtained for each area. The areas are chemically homogeneous within each area, given the spatial resolution and analytical uncertainty of the GRS data. However, substantial chemical differences exist among the areas, including the different mantled terrains, contrary to earlier assumptions that surficial materials are globally homogeneous due to aeolian mixing. The observed chemical differences among the areas may be due to variations in the protolith compositions, extent of alteration of the protolith regions, or post soil formation processes. The abundances of Cl, K, and Th in rockier (but still soil-rich) areas such as Syrtis Major Planum can be explained by mixing between a soil with higher concentrations of Cl, K, and Th, similar to the abundances in the mantled terrains (and some of the landing sites), and crustal rocks containing lower abundances of these elements, similar to Martian meteorites. Copyright 2007 by the American Geophysical Union