THE LANDING SITE OF ORIENTALE BASIN
THE LARGEST AND BEST PRESERVED LUNAR IMPACT STRUCTURE
Located on the western limb of the nearside, Orientale contains at least four ring structures encompassing a diameter of 930 km making it one of the largest lunar impact structures. With an estimated age of 3.82 Ga, it is the youngest basin with an estimated age of 3.82 Ga.
The ring structures are visible at radial distances from, example, Montes Cordillera at 465 km, the Outer Rook Mountains at 310 km and, the basin center of Inner Rook Mountains at 240 km. It is also possible to see these structures with another less prominent ring, the Inner Shelf Ring, situated at 160 km.
As showing in the right diagram, outside the Montes Cordillera lays the Hevelius Formation. between the Rook Mountains is the Montes Rook Formation and within the Inner Rook Mountains lays the Maunder Formation. Gravity modeling suggests the center of the basin sits ~3 km below the lunar geoid, with the basin possessing a vertical height range of 5–6 km .
The Orientale's inner most zone, i.e. the Maunder Formation, is seen as its impact melt sheet, while the composition of the outermost zone, the Hevelius formation, is seen as a impact ejecta which is thought to be highly feldspathic. Between these two, the Montes Rook Formation has a weak mafic signature, suggesting a deeper crustal composition.
The most topographically prominent ring is the Montes Cordillera, which would suggest the final crater rim. However, pre-impact structures have been recognized relatively intact between the Cordillera and Outer Rook, implying the Cordillera Mountains are placed outside of the crater rim and are therefore an outer ring structure. The crust beneath the center of Orientale appears to be thinned relative to the crust outside of the basin. Gravity estimates suggest a crustal thickness between ~0 km and 15 km at the basin center, that is some scientists suggest the absent of the entire upper crust below the basin center.
A cross-section through the Orientale Basin. The Inner and Outer Rook rings, as well as the Cordillera Ring are clearly visible. The crust has been thinned under the basin center resulting in uplift of the Moho (the crust/mantle boundary).
In this model the rings are composed of either megaregolith or upper crustal material. The upper crust has been completely removed around the basin center (after Head et al. 1993; modified from LPI classroom illustrations). Credit: JPL/NASA
Elevation profile of the Orientale basin (19.4˚S, 92.8˚W) showing the sampling areas for different kind of impact materials. Vertical exaggeration is 25. The different colors in the profile shows where different kinds of impact material can be found on the surface, immediately after the Orientale impact event (i.e. ejecta material, melt material, uplifted material). Subsequent impacts might have mixed the surface material, laterally and vertically.
Sampling areas at complex craters follow a similar pattern, except that the uplifted material can be found in the central peak and that the melt material can be found over the crater floor and on the rims of the crater. Elevation data were generated by the LOLA instrument on the Lunar Reconnaissance Orbiter. Resolution is 64 pixels/deg, or approximately 500m/px at the equator.
When an impact occurs, material from beneath the zone of melting is uplifted above the crater floor. Investigation of this material, for example in central peaks, can help determine the composition of originally deeper lithologies beneath the lunar surface. Some of this deep, excavated and uplifted material may represent melt sheet material from older, larger impacts that the smaller crater impacted into; testing of the vertical heterogeneity of the uplifted melt could take place.
Figure in the right below shows the minimum depth of the origin for central peaks with respect to final crater diameter. These estimates are developed from the notion that the central peak must come from a depth below the zone of melting of the cratering event. Any material above this depth would have been incorporated into the melt. The subsequent craters used for differentiation analyses must lie within the limit of the transient crater because the melt sheet is proposed to be confined within this area.
Difference in the formation mechanism of a simple (a) and a complex (b) craters.
Using impact craters as natural drills to sample material from deeper layers
Large impact craters and impact basins have the capacity to excavate or uplift material from the lower crust and upper mantle. Figures at the right and below illustrate different stages of an impact event, for simple and complex craters, as well as for basins. Small craters are simple, bowl-shaped depressions. Complex craters (with diameters ~16–20 km on the Moon) display broad flat shallow floors, terraced walls, and central peaks. Larger craters or basins (generally >200km) can have multiple central rings, referred as peak rings, instead of a central peak. The transition between a central peak to peak rings and their precise origin is still unclear.
The structure of a central-peak complex crater
Another way of using subsequent craters to access the underlying melt sheet is to examine their ejecta. Distal ejecta material would have originated from shallower depths, while material proximal to the crater is thought to have originated from deeper in the crust. it has been Croft found that the depth of excavation is ~1/10 of the transient crater diameter. With a reasonable estimate of the transient crater diameter, calculations could be made to ensure that the impact melt of interest was ejected.
Above we see location, age, and size of craters within Orientale Basin. The inner white circle represents the transient crater ; the outer white circle represents the basin rim (930 km diameter). The ages of craters shown by closed white circles are undetermined.
The largest crater inside the estimated transient diameter of Orientale basin is Maunder. It is a complex crater with a diameter of 55 km suggesting that its central peak originated from a depth of ~7.3 km, therefore within the melt sheet of Orientale. The ejecta blanket of Maunder shows considerable variability compared to that of the surrounding areas and may be estimated for variations in compositions with distance.
Just on top, a Digital Terrain Model ("DTM") of the large Orientale Basin (1100 km diameter), located on the western hemisphere of the Moon, produced from stereo images obtained by LROC's Wide-Angle Camera. The image shows the hill-shaded, color-coded DTM with heights varying from approx. -4,700 meters to 9,400 meters. The small white boxes are areas without WAC coverage [NASA/GSFC/Arizona State University/ DLR].
To determine the structure of the Orientale multi-ring basin, two possible site sampling is proposed. These sites are on or near the Outer Rook Ring, that lies the Inner Rook and Montes Cordillera rings. The formation between the two Rook rings is known as the Montes Rook Formation.
Studies suggest that, the Outer Rook ring is an appropriate site to sample because it may represent the main rim or a peak ring of the Orientale Basin. The Montes Rook Formation is also a very interesting site because its spectral data suggests it possesses a weak mafic signature. This latter may showing a lower crustal material, inferring the removal of the upper crust during the basin-forming impact.
These sites were also chosen due to a break in the Outer Rook Ring at this location; if crew landed here they could visit the massif at either the northern or southern end of the valley, or both.
The locations of two possible sample sites, Sites A and B, within the Orientale Basin are showing at the right. The Site A is located in a break in the ring; from here either the northern or southern section of the ring could be sampled. The Site B is located in the Montes Rook formation near some isolated massif structures.
An overview of Orientale is presented in the top right for reference and highlights the area within which the sites reside. IR: Inner Rook Ring, OR: Outer Rook Ring, and MC: Montes Cordillera (LROC WAC mosaic of Orientale Basin. Arizona State University).