The Schrödinger impact basin near the southern pole on the lunar farside (134°E, 75°S) is a young multiring impact basin, and it is well preserved and exposed for scientific study. A crewed sortie-reconnaissance mission to Schrödinger Basin would allow (1) collection of samples in order to obtain an absolute age date for the Schrödinger impact event and to constrain the ages of volcanic events, (2) detailed analysis of pyroclastic materials that mantle the basin's impact melt sheet, (3) study of lunar explosive volcanism mechanics, and (4) installation of a passive seismic array for study of interior activity. The region's diversity of geologic materials and features make it a prime target for human and robotic exploration. A landing site located within the pyroclastic deposit (139.6°E, 75.7°S) allows access to the volcanic vent and inner ring of the basin. Sampling the inner ring, which may be composed of South Pole–Aitken Basin uplift material, would allow absolute dating of the South Pole–Aitken Basin event. Engineering objectives necessary for extending surface stay time for sortie missions or a lunar outpost can be met at this locale. Pyroclastic material is optimal for in situ oxygen production. Demonstrating oxygen production and storage at the landing site would prove technologies for an outpost and leave a cache of consumables for use by future longer-term expeditions. Mission planning is based on Lunar Reconnaissance Orbiter , Lunar Orbiter , Clementine , and SELENE mission data. Extravehicular activities necessary for completing the science objectives require long traverses (24 km and 7.5 h per traverse) for a four-member crew over a 4 d mission.

The motivation for this study was to create lunar surface exploration scenarios that would support current science needs, as captured in the Lunar Exploration Analysis Group (LEAG) Roadmap for Lunar Exploration. A science-driven capability to meet those needs required enhanced capability, relative to the Apollo J missions, to provide a broader field context for (1) improved interpretation of samples and measurements; (2) greater flexibility in the selection and nature of activities at field stations; as well as (3) greater potential for breakthrough science. Here, we offer advanced regional-scale (hundreds of kilometers) surface exploration scenarios, essentially design reference missions, for three high-priority targets representing the broadest differences in the nature and distribution of geological features. South Pole–Aitken Basin is the largest and oldest confirmed lunar impact basin. Covering most of the farside southern hemisphere and >2000 km in diameter, it contains extraordinarily diverse features and geochemical anomalies that are widely scattered and thus would require several regional-scale missions. Tsiolkovsky is an anomaly among farside craters: It is mare-filled in the thickest portion of farside crust, young, and has well-preserved impact structures, yet it is surrounded by the ancient Tsiolkovsky-Stark Basin. Aristarchus Plateau is a tectonically uplifted plateau associated with the formation of Imbrium Basin, and it is found on a concentric ring of basin. Features encompassing the entire range of mare volcanism activity in style and age are found either on the relatively compact plateau or within hundreds of kilometers in surrounding western Oceanus Procellarum. Our regional-scale architecture would allow science objectives for study of Aristarchus Plateau or Tsiolkovsky to be addressed from one landing site conveniently located on the target, while South Pole–Aitken Basin would require several missions to achieve such objectives. We describe the geological context and resulting investigations, as well as the required tools, instruments, and activities. We assumed, as initially instructed, science need–driven capabilities at least a generation beyond the Apollo J missions, i.e., the availability of a minimum of two pressurized rovers capable of hundreds of kilometers driving range at average speeds of 10–15 kph (without recharge), four crew, and 700 kg of science payload. The implications of such science-conducive architecture in the context of other architectures under consideration are discussed.

The lunar south polar region (60°S–90°S) is being mapped at 1:2,500,000 scale using spacecraft data ( Lunar Reconnaissance Orbiter , Clementine , Lunar Prospector , and Lunar Orbiter ) to characterize geologic units, recognize contacts and structures, and identify impact craters (diameter [ D ] >2 km) for age dating. Most of the map area is located within the South Pole–Aitken basin, the largest (~2600 km) and oldest basin known on the Moon. At 18 km deep, South Pole–Aitken basin is believed to have exposed materials from the Moon's lower crust or upper mantle. Several large impact basins, such as Schrödinger basin ( D = 334 km), are superposed on the floor of South Pole–Aitken and may have excavated through the floor of the basin. Thus, the materials that form the primary basin structures (rim and peak-ring) of Schrödinger, as well as the materials that cover its floor, may be used as proxies for the ancient lunar crustal and/or upper-mantle materials. Characterization of the materials that constitute Schrödinger and geologic mapping of the basin have identified nine units within the Schrödinger assemblage organized into three groups: basin materials, the plains formation, and the volcanic formation. The volcanic and plains materials found on the floor of Schrödinger exhibit flat expanses with smooth to rough surfaces and are dissected by floor fractures. These materials are interpreted to consist of impact melt and/or were emplaced by effusive eruptions of mafic materials, and they are some of the youngest materials in the basin, ranging from early Imbrian to early Eratosthenian in age.

To test the lunar cataclysm hypothesis and anchor the beginning of the basin-forming epoch on the Moon, which are high science priorities for lunar exploration, we evaluated potential landing sites within Schrödinger basin. This impact site is the second youngest basin-forming event and lies within the South Pole–Aitken basin, which is the oldest and largest impact basin on the Moon. Thus, landing sites within Schrödinger should provide access to impact lithologies with ages of each event, providing a bracket of the entire basin-forming epoch and resolving both of the leading science priorities. Additionally, the floor of Schrödinger basin has been partially covered by younger mare and pyroclastic units. The volcanic materials, as well as impact-excavated and uplifted units, will provide chemical and lithologic samples of the lunar crust and potentially the upper mantle. Collectively, the impact and volcanic lithologies will provide calibration points to the entire lunar stratigraphic column.

The interior of the enigmatic South Pole-Aitken basin has long been recognized as being compositionally distinct from its exterior. However, the source of the compositional anomaly has been subject to some debate. Is the source of the iron-enhancement due to lower-crustal/upper-mantle material being exposed at the surface, or was there some volume of ancient volcanism that covered portions of the basin interior? While several obvious mare basalt units are found within the basin and regions that appear to represent the original basin interior, there are several regions that appear to have an uncertain origin. Using a combination of Clementine and Lunar Orbiter images, several morphologic units are defined based on albedo, crater density, and surface roughness. An extensive unit of ancient mare basalt (cryptomare) is defined and, based on the number of superimposed craters, potentially represents the oldest volcanic materials within the basin. Thus, the overall iron-rich interior of the basin is not solely due to deeply derived crustal material, but is, in part due to the presence of ancient volcanic units.

Mare basalts cover much of the Earth-facing side of the Moon. The underlying cause for this distribution has been attributed to an ancient nearside megabasin, asymmetric accretion, and differential tidal effects. While each hypothesis is plausible, the hypothesis for a megabasin also accounts for a subconcentric and radial system of graben and ridges that centers on a region southwest of Imbrium basin. Moreover, such a nearside megabasin could account for the distribution of nearside geochemical anomalies related to localized igneous intrusions. The farside South Pole–Aitken basin, however, is a well-established impact megabasin exceeding 2200 km in diameter. Here, we propose an oblique collision scenario for this basin on the farside that would have created the initial conditions for localized deep-seated and long-lasting weaknesses on the nearside. Laboratory and computational experiments demonstrate that a large oblique collision generates asymmetric shock waves that converge in a region offset from the basin-center antipode. The resulting damage would have provided pathways for deep magma to reach shallow reservoirs.

This paper is a current status report on a project focused on understanding the formation of large impact basins on terrestrial planetary bodies. A set of preliminary two-dimensional axisymmetric numerical models of collisions of asteroids with diameters from 150 to 800 km with the Moon, Mars, and Mercury illustrates the main mechanical effects of planetary-scale impacts. The target body is modeled on a regular grid with a spatial resolution of 5–10 km. Self-gravity is included in the hydrocode. The main consequence of such an impact is a deep melt pool at the center of the basin. Model results are tentatively compared with known impact basins such as South Pole–Aitken on the Moon and Hellas on Mars.

The four largest well-preserved impact basins in the solar system, Borealis, Hellas, and Utopia on Mars, and South Pole–Aitken on the Moon, are all significantly elongated, with aspect ratios >1.2. This population stands in contrast to experimental studies of impact cratering that predict <1% of craters should be elliptical, and the observation that ~5% of the small crater population on the terrestrial planets is elliptical. Here, we develop a simple geometric model to represent elliptical crater formation and apply it to understanding the observed population of elliptical craters and basins. A projectile impacting the surface at an oblique angle leaves an elongated impact footprint. We assume that the crater expands equally in all directions from the scaled footprint until it reaches the mean diameter predicted by scaling relationships, allowing an estimate of the aspect ratio of the final crater. For projectiles that are large relative to the size of the target planet, the curvature of the planetary surface increases the elongation of the projectile footprint for even moderate impact angles, thus increasing the likelihood of elliptical basin formation. The results suggest that Hellas, Utopia, and South Pole–Aitken were formed by impacts inclined at angles less than ~45° from horizontal, with a probability of occurrence of ~0.5. For the Borealis Basin on Mars, the projectile would likely have been decapitated, with the topmost portion of the projectile on a trajectory that did not intersect with the surface of the planet.