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.

Soil Thermophysical Properties Near the InSight Lander Derived From 50 Sols of Radiometer Measurements

Measurements from the InSight lander radiometer acquired after landing are used to characterize the thermophysical properties of the Martian soil in Homestead hollow. This data set is unique as it stems from a high measurement cadence fixed platform studying a simple well-characterized surface, and it benefits from the environmental characterization provided by other instruments. We focus on observations acquired before the arrival of a regional dust storm (near Sol 50), on the furthest observed patch of soil (i.e., ∼3.5 m away from the edge of the lander deck) where temperatures are least impacted by the presence of the lander and where the soil has been least disrupted during landing. Diurnal temperature cycles are fit using a homogenous soil configuration with a thermal inertia of 183 ± 25 J m^-2 K^-1 s^-1/2 and an albedo of 0.16, corresponding to very fine to fine sand with the vast majority of particles smaller than 140 μm. A pre-landing assessment leveraging orbital thermal infrared data is consistent with these results, but our analysis of the full diurnal temperature cycle acquired from the ground further indicates that near surface layers with different thermophysical properties must be thin (i.e., typically within the top few mm) and deep layering with different thermophysical properties must be at least below ∼4 cm. The low thermal inertia value indicates limited soil cementation within the upper one or two skin depths (i.e., ∼4-8 cm and more), with cement volumes <<1%, which is challenging to reconcile with visible images of overhangs in pits.

Simultaneous Measurements of Heat Capacity, Electrical Resistivity, and Hemispherical Total Emittance by a Pulse Heating Technique: Vanadium, 1500 to 2100 K

Simultaneous measurements of heat capacity, electrical resistivity, and hemispherical total emittance of vanadium in the temperature range 1500 to 2100 K by a subsecond duration, pulse heating technique are described. The results are expressed by the relations: c p = 56.34 - 3.839 × 10-2 T + 1.563 × 10-5 T 2 ρ = 8.794 + 6.282 × 10-2 T - 6.804 × 10-6 T 2 where c p is in J · mol-1 · K-1, ρ is in 10-8 Ω · m, and T is in K. The values for the hemispherical total emittance are: 0.313 at 1900 K and 0.332 at 2000 K. Estimated inaccuracies of the measured properties are: 3 percent for heat capacity, 0.5 percent for electrical resistivity and 5 percent for hemispherical total emittance.

Measurement of Melting Point, Normal Spectral Emittance (at Melting Point) and Electrical Resistivity (Near Melting Point) of Some Refractory Alloys

A subsecond duration pulse heating method is used to measure the melting point, normal spectral emittance (at the melting point, corresponding to 650 nm). and electrical resistivity (near the melting point) of the following refractory alloys: 90 Ta-10 W, 99 Nb-1 Zr, and 80 Nb-10 Ta-10 W (numbers indicate nominal composition in percentage by weight). The melting point and the normal spectral emittance are: 3286 ± 15 K and 0.396 for 90 Ta-10 W, 2737 ± 10 K and 0.351 for 99 Nb-1 Zr, and 2814 ± 10 K and 0.333 for 80 Nb-10 Ta-10 W. The inaccuracy of the normal spectral emittance and electrical resistivity results is estimated to be 3 percent and 0.5 percent, respectively.