T. Ohman

The Preliminary Analysis of Polygonal Impact Craters within Greater Hellas Region, Mars

The polygonal planimetric shape of impact craters has been known for a long time, but has not been discussed much in the past. Polygonal craters exist on all kinds of celestial bodies that have fractured rigid crusts. They are also found on the Earth (e.g., Meteor Crater, Soderfjarden). Polygonal craters are thought to have been formed by two possible mechanisms. Simple polygonal craters, such as the square-shaped Meteor Crater, result when the excavation flow opens the crater, tearing the target more easily along pre-existing fractures or other planes of weakness. Complex polygonal craters are supposed to have formed during the modification stage, when rocks of the crater rim slump along the fractures in the target. Both mechanisms lead to straight segments of the rim. However, in simple craters the straight rim is typically at some angle, usually about 45°, to the direction of the fractures, whereas in complex craters the rims are thought to be parallel to them. Thus, the regional fracture trends are easier to deduce from complex polygonal craters.

The basalts of Mare Frigoris

This paper discusses the methodology and results of a detailed investigation of Mare Frigoris using remote sensing data from Clementine, Lunar Prospector, and Lunar Reconnaissance Orbiter, with the objective of mapping and characterizing the compositions and eruptive history of its volcanic units. With the exception of two units in the west, Mare Frigoris and Lacus Mortis are filled with basalts having low-TiO2 to very low TiO2, low-FeO, and high-Al2O3 abundances. These compositions indicate that most of the basalts in Frigoris are high-Al basalts—a potentially undersampled, yet important group in the lunar sample collection for its clues about the heterogeneity of the lunar mantle. Thorium abundances of most of the mare basalts in Frigoris are also low, although much of the mare surface appears elevated due to contamination from impact gardening with the surrounding high-Th Imbrium ejecta. There are, however, a few regional thorium anomalies that are coincident with cryptomare units in the east, the two youngest mare basalt units, and some of the scattered pyroclastic deposits and volcanic constructs. In addition, Mare Frigoris lies directly over the northern extent of the major conduit for a magma plumbing system that fed many of the basalts that filled Oceanus Procellarum, as interpreted by Andrews-Hanna et al. (2014) using data from the Gravity Recovery and Interior Laboratory mission. The relationship between this deep-reaching magma conduit and the largest extent of high-Al basalts on the Moon makes Mare Frigoris an intriguing location for further investigation of the lunar mantle.

Characterization of melt and ejecta deposits of Kepler crater from remote sensing data

We used Moon Mineralogy Mapper (M3), Arecibo and Mini-RF radar, and Diviner radiometer data with Lunar Reconnaissance Orbiter (LRO) Camera and Kaguya Terrain Camera images to characterize the target, ejecta, and impact melt-rich lithologies in and around lunar central peak crater Kepler. M3 data indicate the impact melt rocks of crater floor to be high-Ca pyroxene dominated, distinct from the low-Ca pyroxene-dominated crater wall. The central uplift is high-Ca pyroxene dominated, and has higher albedo. These observations are consistent with thin mare basalts underlain by noritic Imbrium ejecta, underlain by gabbroic crustal material. M3 data reveal an enigmatic, splash-like feature of melt-rich material on the southeastern (uprange) crater wall and flank. M3 data also highlight halos around Kepler. In detail the halos are slightly variable, but in broad terms they define a consistent feature, offset to the inferred downrange direction, and interpreted to reflect the distribution of glass-bearing impact breccia. The radar data sets show most of the proximal ejecta to be radar-bright. However, Diviner rock abundance data do not indicate the presence of blocks on the surface nor can they be seen using LRO Narrow Angle Camera images. Thus, the blocks giving rise to the enhanced radar signal are buried. Beyond the radar-bright zone, a subtle radar-dark halo emerges, coincident with a region of very low rock abundance in Diviner data. This multidisciplinary approach provides a robust analysis of the main characteristics of a lunar complex crater and reveals previously unidentified features related to the distribution of impact melt.