The Effects of Terrain Properties Upon the Small Crater Population Distribution at Giordano Bruno: Implications for Lunar Chronology.

The distribution of impact craters on the ejecta of Giordano Bruno, a recent (<10 Ma) 22-km diameter crater within the lunar highlands, exhibits substantial variations. We surveyed craters D ≥ 10 m across a 1,323 km2 area of Giordano Bruno's ejecta and compared the distribution of craters with variations in thermophysical properties derived from the Lunar Reconnaissance Orbiter Diviner instrument. We used Diviner-derived rock abundance and nighttime regolith temperatures along with thermal model-predicted surface temperatures for a diversity of terrains to identify and isolate areas of the ejecta based on thermophysical properties such as bulk density and thermal conductivity. We found that thermophysical properties of the ejecta vary considerably both laterally and vertically, and consistently differ from typical regolith, indicating the presence of higher thermal inertia materials. Crater-size frequencies are significantly lower in areas with terrain properties exhibiting higher: rock abundance, nighttime temperatures, and/or modeled thermal inertia. This discrepancy in crater distribution increases for craters smaller than ∼25 m. These thermophysical variations indicate changes in the mechanical properties of the target materials. We suggest that these variations-specifically, terrain-dependent crater scaling variations and impactor-scale heterogeneities in material properties such as the presence or absence of large boulders-may influence crater diameters or inhibit crater production altogether in Giordano Bruno's ejecta; furthermore, these factors are size-dependent.

Silicate Volcanism on Europa's Seafloor and Implications for Habitability.

Habitable ocean environments on Europa require an influx of reactants to maintain chemical disequilibrium. One possible source of reactants is seafloor volcanism. Modeling has shown that dissipation of tidal energy in Europa's asthenosphere can generate melt, but melt formation cannot be equated with volcanism. Melt must also be transported through Europa's cold lithosphere to erupt at the seafloor. Here, we use two models of dike propagation to show that dikes can only traverse the lithosphere if either the fracture toughness of the lithosphere or the flux into the dike is large (>500 MPa m1/2 or ∼1 m2 s-1, respectively). We conclude that cyclic volcanic episodes might provide reactants to Europa's ocean if magma accumulates at the base of the lithosphere for several thousand years. However, if dikes form too frequently, or are too numerous, the magma flux into each will be insufficient, and volcanism cannot support a habitable ocean environment.

Asteroid (101955) Bennu's weak boulders and thermally anomalous equator.

Thermal inertia and surface roughness are proxies for the physical characteristics of planetary surfaces. Global maps of these two properties distinguish the boulder population on near-Earth asteroid (NEA) (101955) Bennu into two types that differ in strength, and both have lower thermal inertia than expected for boulders and meteorites. Neither has strongly temperature-dependent thermal properties. The weaker boulder type probably would not survive atmospheric entry and thus may not be represented in the meteorite collection. The maps also show a high-thermal inertia band at Bennu's equator, which might be explained by processes such as compaction or strength sorting during mass movement, but these explanations are not wholly consistent with other data. Our findings imply that other C-complex NEAs likely have boulders similar to those on Bennu rather than finer-particulate regoliths. A tentative correlation between albedo and thermal inertia of C-complex NEAs may be due to relative abundances of boulder types.

Adenylate cyclase activity of bovine spermatozoa during maturation in the epididymis and the activation of sperm particulate adenylate cyclase by GTP and polyamines.

The adenylate cyclase activity of bovine caput and cauda epididymal spermatozoa was measured in intact- and in broken-cell preparations. Cyclase activity was 4-fold greater in caput than in cauda cells and total cyclase activity in broken-cell preparations was 3-5 times greater than that in intact cells. A particulate fraction derived from sonically disrupted caput spermatozoa was used to study the effects of compounds that might be physiologically important modulators of adenylate cyclase. Activity was stimulated by GTP, 5-guanylyl imidophosphate and by the polyamines, spermine and spermidine.