Comparison of Impact Crater Data in the Original and Revised Versions of the “catalog of Large Martian Impact Craters”

Introduction: The Catalog of Large Martian Impact Craters [1] was compiled from Viking 1:2M photomosaics around 1980 and contains information on 42,283 craters generally ≥5-km-diameter across the entire Martian surface. The Catalog is undergoing revision using MGS (MOLA) and Odyssey (THEMIS) data. We have completed the initial revision of northern hemisphere craters, updating latitude and longitude, revising interior and ejecta morphology classifications, and refining diameter measurements for both circular and elliptical craters. Method: The original Catalog data were converted into Geographic Information System (GIS) format by Trent Hare (USGS). Hare wrote an ArcView-based program for use in the Catalog revision. The base map of this program used Viking images corrected to the MDIM 2.1 (MOLA-based) control network [2]. The view can be switched between Viking images and MOLA shaded relief to confirm the exact location of the rim. The switch to MDIM 2.1 resulted in the Viking-based coordinates of each crater being offset from the crater center using the new control net. The ArcView program displays locations of the old coordinates and allows the user to move the crater center to the MDIM 2.1 location. Crater diameters are remeasured by determining a best-fit circle to the crater— diameters were typically measured at least twice and averaged. The program allows measurement of a minor diameter and azimuthal angle of orientation for elliptical craters. THEMIS data (daytime IR, and VIS when available) were used to classify interior and ejecta morphologies for each crater. Higher resolution imagery combined with improved atmospheric transparency has resulted in better classification of ejecta and interior features. Ejecta morphology was classified using recommended nomenclature from the Mars Crater Consoritum [3]. Information about crater preservation was revised using a 8-point numeric scale based on crater rim height, crater depth, ejecta blanket, interior morphology, and thermal inertia considerations [4]. On this scale, 0.0 represents a “ghost” crater whereas 7.0 is an extremely fresh crater. Diameters and Elliptical Craters: The revised Catalog (Catalog 2.0) contains 14,225 craters ≥5-kmdiameter, compared to 12,920 craters in this size range in the original Catalog (Catalog 1.0) [5]. This 10% increase in the number of craters results from (1) inclusion of craters missed in the original Catalog because of poor resolution or cloudy/dusty conditions, (2) improved detection of highly degraded or buried craters using MOLA, and (3) improved diameter measurements which resulted in additions of craters near the 5km-diameter cutoff. Most craters in Catalog 2.0 have diameters within 10% of their Catalog 1.0 values, which was the stated uncertainty level for Catalog 1.0. Elliptical craters were identified in Catalog 1.0 solely on their appearance—only craters which were obviously elliptical in planform were included. Elliptical craters were identified in the revision based on both obvious ellipticity in crater form and on the presence of non-symmetric ejecta deposits indicating a low angle of impact [6]. As a result, the number of elliptical craters in the northern hemisphere increased from 88 in Catalog 1.0 to 454 in Catalog 2.0. Most elliptical craters are only slightly elliptical: 75% of elliptical craters have a minor/major diameter ratio >0.70. Azimuthal orientation of the major axis is generally consistent over all angles, with a frequency peak in the 80°-100° range (Fig. 1).