Development of low cost ablative nozzles for large solid propellant rocket motors

Evaluation of selected low-cost ablative materials for large solid rocket engine nozzle desig

Reconstruction of eolian bed forms and paleocurrents from cross-bedded strata at Victoria Crater, Meridiani Planum, Mars

Outcrop exposures imaged by the Opportunity rover at Victoria Crater, a 750 m diameter crater in Meridiani Planum, are used to delineate sedimentary structures and further develop a dune-interdune depositional model for the region. The stratigraphy at Victoria Crater, observed during Opportunity's partial traverse of its rim, includes the best examples of meter-scale eolian cross bedding observed on Mars to date. The Cape St. Mary promontory, located at the southern end of the rim traverse, is characterized by meter-scale sets of trough cross bedding, suggesting northward migrating sinuous-crested bed forms. Cape St. Vincent, which is located at the opposite end of the traverse, shows tabular-planar stratification indicative of climbing bed forms with meter- to decameter-scale dune heights migrating southward. Promontories located between Cape St. Mary and Cape St. Vincent contain superposed stratigraphic units with northward and southward dipping beds separated by outcrop-scale bounding surfaces. These bounding surfaces are interpreted to be either reactivation and/or superposition surfaces in a complex erg sea. Any depositional model used to explain the bedding must conform to reversing northward and southward paleomigration directions and include multiple scales of bed forms. In addition to stratified outcrop, a bright diagenetic band is observed to overprint bedding and to lie on an equipotential parallel to the preimpact surface. Meter-scale cross bedding at Victoria Crater is similar to terrestrial eolian deposits and is interpreted as a dry dune field, comparable to Jurassic age eolian deposits in the western United States

Reply

The authors respond to Hoffman et al. (2001), who acknowledged that methane may have played an important role in unusual events associated with Neoproterozoic glaciation, but questioned the authors' permafrost gas hydrate hypothesis for 13C-depleted cap carbonate formation. The critique focused on three issues: (1) an interpretation for tube structures in cap carbonates unrelated to gas migration; (2) the absence of a suitable source for methane gas; and (3) the degree of 13C depletion in sheet-crack cements

Reply

The authors address additional comments on their hypothesis for the origin of Neoproterozoic postglacial cap carbonates and their isotopic excursions

Considering a Neoproterozoic Snowball Earth

P. F. Hoffman et al. and N. Christie-Blick et al. discuss Hoffman et al.'s paper that "developed a modified 'snowball Earth' hypothesis (2) to explain the association of Neoproterozoic low-latitude glaciation with the deposition of 'cap carbonate' rocks bearing highly depleted carbon isotopic values (δ13C ≤ −5‰). According to Hoffman et al., the ocean became completely frozen over as a result of a runaway albedo feedback, and primary biological productivity collapsed for an interval of geological time exceeding the carbon residence time (greater than 105 years). During this interval, continental ice cover is inferred to have been thin and patchy owing to the virtual elimination of the hydrological cycle.

Are Proterozoic Cap Carbonates and Isotopic Excursions a Record of Gas Hydrate Destabilization Following Earth’s Coldest Intervals

Regionally persistent, thin intervals of carbonate rock directly and ubiquitously overlie Proterozoic glacial deposits on almost every continent, and are commonly referred to as cap carbonates. Their unusual facies, stratigraphically abrupt basal and upper contacts, and strongly negative carbon isotopic signature (δ13C values between ∼0‰ and −5‰) suggest a chemical oceanographic origin, the details of which remain unresolved. Here we propose that these enigmatic deposits are related to the destabilization of gas hydrate in terrestrial permafrost following rapid postglacial warming and flooding of widely exposed continental shelves and interior basins. Supporting evidence for this hypothesis includes (1) the common occurrence within the cap carbonates of unusual fabrics, similar to those produced by cold methane seeps; (2) a distinctive time evolution for the carbon isotopic excursions indicative of a pulse addition of isotopically depleted carbon to the ocean- atmosphere system; and (3) agreement between mass-balance estimates of carbon released by hydrate destabilization and carbon buried in the cap carbonate. We infer that during times of low-latitude glaciation, characteristic of the Neoproterozoic, gas hydrates may have been in greater abundance than at any other time in Earth history