1. Introduction to the Sequenced Catalog of the Moons Largest Craters and Basins
Why sequence and not age ? Very few absolute ages are available for lunar features, but more features have been positioned in time sequence using such clues as stratigraphy or degradation. While additional sample collection and analysis would provide more evidence of absolute ages, that is for some time in the future. There is now a great deal of remote sensing data that can contribute to the assignment of the sequence of features, supplemented by the limited ground truth provided from the Apollo and Luna missions. For some purposes, described in the early chapters of this book, sequence can be used as a surrogate for time.
It is important to sequence nearly all of a class of features in order to emulate the continuity of time. Otherwise, the sequence will have gaps of uncertain duration. Therefore this catalog includes the set of impact features that are greater than 200 km in rim crest diameter to include large craters in addition to the basins (impact features greater than 300 km in diameter) that were included in earlier proposed sequences. A total of 72 impact features are in this catalog. Features which may well be caused by impact but are too degraded for their apparent diameter and apparent depth to be measured and to be placed in sequence are not included.
Data collected for each feature: This catalog includes a wide class of information in a compact form to allow comparisons and also to be a useful source of data for other purposes. References and notes are provided in a separate section after the chapters and indicated with a superscript in the text. This Catalog includes images, topographic maps, Bouguer gravity maps, and a set of parameters that include center location. The cataloged impact features include many of those in traditional lists1.1 as well as those identified by diverse researchers from later missions, especially the Lunar Reconnaissance Orbiter (LRO) and the Gravity Recovery and Interior Laboratory (GRAIL). Parameters are measured from Radial Elevation Profiles based on Kaguya topographic data by methods described in The Moons Near Side Megabasin and far side bulge1.2 and revised for this paper in the light of LRO data, recent literature, and discussions.
Criteria for Sequence Numbers : Assigning Sequence Numbers to the features requires a great deal of judgment. This first assignment for a reasonably comprehensive class of features is put forward as a starting point. The judgments have been based on a careful but not rigorous comparison of images and topographic maps derived from an excellent online interactive mapping tool supported by Arizona State University1.3. The images and topographic maps of the catalog are based on screen captures from this tool.
Two objective measures have been used in assigning Sequence Numbers: stratigraphic layers as defined by the USGS and densities of small primary craters (>20 km diameter) superposed on the features determined recently by C. I. Fassett and others1.4. Where objective measures were lacking, judgment of relative degrees of degradation was applied to interpolate Sequence Numbers between those of features with objective measures. The result is a sequence that conforms to the stratigraphy data and also provides a smooth relation between the available densities of small superposed primary craters and Sequence Numbers.
In summary, the criteria used in the assignment of Sequence Numbers are as follows:
Stratigraphic relationships.
Qualitative estimates of degradation of the features crater, peak ring, rim, and ejecta field.
Superposed Crater Densities when available.
New proposals for sequencing in the literature1.5.
In addition to the Sequence Numbers, parameters of the features in the catalog were derived from Radial Elevation Profiles, relative to a model1.6. These profiles are provided for each feature and are the basis for the Apparent Diameter, Rim Crest Diameter, and Apparent Depth assigned here to each feature. The values of these parameters are similar but not identical to the parameters in the literature and in the compilations of the International Astronomical Union.
Bouguer gravity anomalies detected by GRAIL and analyzed by Greg Neumann1.7 and others were especially helpful in the identification of main rings and peak rings. This was important because often the degradation of peak rings was used in assigning Sequence Numbers because the main rings were compromised, by interaction with neighboring features. Further analysis of GRAIL data by Jeff Andrews-Hanna is discussed in Appendix B, the Procellarum Region1.8.
In this catalog, all of the information for each impact feature is presented in two pages, including an image, parameters, topographic and gravity maps, and short discussions. For examples, see any of the figures in Chap ., which are organized by the major geographical periods of the Moon.
Applications of the Sequenced Feature Catalog : Chapters discuss applications of the Sequence Numbers by using the sequence as a surrogate for time and exploring the relationships of other properties of the features to the sequence.
In Chap. , superposed Crater Density (>20 km) is graphed against Sequence Number, showing a smooth decline in the density as the Sequence Number increases. The smoothness increases confidence in the assignment of Sequence Numbers, although Sequence Numbers could be exchanged within a small range without a major disruption.
In Chap. , the Apparent Diameter is graphed against Sequence Number. The graph shows an early bombardment of very large basins, subsiding to a period of features in the 200300 km range. This is followed by a dichotic set of features, with more in the 4001000 km range than in the 200300 range. This is one indication of the Late Heavy Bombardment. A second set of very large impactors follows.
In Chap. , the thermal history of the crust is examined by graphing Relative Compensation against Sequence Number. There is an early period when Relative Compensation was high, inferring high plasticity of the crust while the newly solid crust was cooling, and a later period of high plasticity when the crust was heated by radioactivity and the heat of the Late Heavy Bombardment of large impactors.
In Chap. , the Bouguer gravity pattern is discussed. There is a very high positive anomaly with peak rings, and a very low negative anomaly from peak ring to the edge of the apparent cavity. The difference, called the Bouguer Anomaly Contrast (BAC), increases for features with larger diameters than 200 km. The pattern is clearly related to certain features of the recent 3D simulations of large impacts .
The Sequenced Catalog chapters: The large craters and basins (Large Impact Features) are cataloged in Chaps. according to the periods that correspond to their stratigraphic systems:
In Chap. are the pre-Nectarian impact features (Sequence Numbers 144). These are the features that are believed to predate the Nectarian Basin because its ejecta field is still visible over a large part of the Moon, establishing a major distinction between those features that precede it and those that follow it. The ejecta fields of earlier basins that might have served this function are obscured, either by the Nectaris Basin or by later large basins, especially the Imbrium and Orientale Basins. These pre-Nectarian impact features tend to be degraded by neighboring features if not the Nectaris, Imbrium, and Orientale basins or mare flooding. Judgment of their sequence is often a challenge. Even the existence of these very old features has been in the past and still is a problem.
