GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L05201, doi:10.1029/2010GL046450, 2011 Distant secondary craters from Lyot crater, Mars, and implications for surface ages of planetary bodies Stuart J. Robbins1 and Brian M. Hynek1,2 Received 9 December 2010; revised 20 January 2011; accepted 28 January 2011; published 2 March 2011. [1] The population of secondary craters ‐ craters formed by recent work has shown that secondary fields can be several the ejecta from an initial impact event ‐ is important to thousand kilometers from the primary [Preblich et al., 2007]. understand when deriving the age of a solid body’s With current and forthcoming high‐resolution imagery of surface. Only one crater on Mars, Zunil, has been studied Mars, the Moon, and Mercury, case studies of fields of in‐depth to examine the distribution, sizes, and number of secondaries from large primary craters are necessary to these features. Here, we present results from a much larger assess their effects on the overall crater population. and older Martian crater, Lyot, and we find secondary [4] Here, we discuss the identification of thousands of crater clusters at least 5200 km from the primary impact. secondary craters originating from one of the largest fresh Individual craters with diameters >800 m number on the craters on Mars: Lyot crater is a 222‐km‐diameter peak‐ring order of 104. Unlike the previous results from Zunil, these crater centered at 50.5°N, 29.3°E (Figure 1), with a Middle craters are not contained in obvious rays, but they are Amazonian age of ∼1.6–3.3 Ga [Tanaka et al., 2005; linked back to Lyot due to the clusters’ alignment along Dickson et al., 2009]. The secondaries are up to 25% of the great circles that converge to a common origin. These way across the planet, and there could be countless more widespread and abundant craters from a single impact not clearly linked to Lyot due to degradation, resurfacing, limit the accuracy of crater age‐dating on the Martian resolution limitations, or lack of easily identifiable tight surface and beyond. Citation: Robbins, S. J., and B. M. Hynek clusters. Besides the basic identification of Lyot secondaries (2011), Distant secondary craters from Lyot crater, Mars, and in the near‐ and far‐field (Section 2), we detail their size‐ implications for surface ages of planetary bodies, Geophys. Res. frequency distribution relative to primary craters (Section 3), Lett., 38, L05201, doi:10.1029/2010GL046450. and discuss implications (Section 4). 1. Introduction 2. Identification of Craters and Secondary Clusters [2] Crater counting is the only way to estimate absolute ages on solid surfaces without returned samples, akin to the [5] Over 500,000 craters ≳1‐km‐diameter have been Apollo missions. This practice has been refined and utilized identified on Mars in a nearly completed global Mars crater for decades [e.g., Arvidson et al., 1979; Hartmann, 2005]. A database [Robbins and Hynek, 2010]. Craters were visually fundamental assumption of crater age‐dating is that crater identified in ArcGIS software using 100 m/pix global formation is a stochastic process. Nearly half a century ago, THEMIS Daytime IR mosaics [Christensen et al., 2004], Shoemaker [1965] identified the issue of secondary craters and rims were mapped using ArcGIS’s edit tools. Polygons (craters that form from the ejecta of a larger primary impact representing crater rims were imported into Igor Pro where a event and are necessarily smaller). The subject of secondary non‐linear least‐squares (NLLS) circle‐fit algorithm was craters was controversial but generally ignored in the liter- used to calculate each crater’s diameter and center latitude ature until Bierhaus et al. [2005] identified >104 secondary and longitude. The NLLS algorithm corrects for map pro- craters that contaminate crater statistics on Jupiter’s moon jection by converting the polygon’s geographic coordinates Europa, and McEwen et al. [2005] identified >106 around into meters from the polygon’s centroid, accounting for the the fresh crater Zunil on Mars. first‐order spherical surface of Mars. [3] Understanding the role secondary craters have on local [6] The global database’s small crater distribution con- and global crater statistics is important, especially because tains numerous clusters that appear to trend radially from the crater population with diameters D < 1 km on Mars may Lyot crater at distances up to ∼5200 km. Over 90% are in be significantly contaminated by secondary craters [McEwen the quadrant southeast of Lyot (Figure 2). Determining if and Bierhaus, 2006]. Secondary fields are typically manifest these clusters likely originated from Lyot was not aided by as a visible and tight annulus around the primary, though observable rays nor a nearly crater‐free landscape as is the case with Zunil [McEwen et al., 2005]. Great circles were traced between the potential clusters and Lyot to determine if the cluster was aligned with it (i.e., a circle described by 1Laboratory for Atmospheric and Space Physics, University of the intersection of the surface of a sphere with a plane Colorado at Boulder, Boulder, Colorado, USA. passing through the center). 2 Department of Geological Sciences, University of Colorado at [7] The formula prescribed by Vincenty [1975] was used Boulder, Boulder, Colorado, USA. to construct great circle arcs via an iterative approach: An initial bearing along the great circle connecting the starting Copyright 2011 by the American Geophysical Union. 0094‐8276/11/2010GL046450 position (the cluster) and ending position (Lyot) was cal- L05201 1of5 L05201 ROBBINS AND HYNEK: SECONDARY CRATERS FROM LYOT, MARS L05201 Figure 1. THEMIS Daytime IR mosaic [Christensen et al., 2004] showing the region around Lyot crater in a local sinusoidal projection. The continuous ejecta blanket of Lyot is outlined in black and the identified near‐field secondaries are shown as white circles. Black regions correspond to gaps in the THEMIS data. culated. Then, the point 1 km from the starting point along clusters are not as clearly linked to Lyot. These two popula- the bearing was calculated (∼1/60° at the equator; resolution tions are examined separately in Section 3. tests at 10 km and 100 m showed differences <0.0005°, or <30 m). At this point, a new bearing was calculated and the process was repeated until Lyot was reached. 3. Size‐Frequency Distributions of Secondary [8] Potential Lyot secondary clusters were initially identi- Clusters fied by visual inspection from the database, but a more robust [10] Crater size‐frequency distributions (SFDs) were cal- clustering algorithm was run to isolate potential groups: culated following Arvidson et al. [1979] with some mod- Every crater D ≤ 5 km was analyzed for its distance to all 1/8 ‐ ifications: Craters were binned in multiplicative 2 D other craters within 2°. If the fifth nearest crater was within intervals for purposes of slope‐fitting. Finer binning than the 10 crater diameters, it was considered to be in a potential 1/2 ‐ more standard 2 D was used to bring out detail in the cluster; otherwise it was removed. The fifth nearest was SFDs that would otherwise be obscured. Craters were chosen to eliminate random over‐densities; in the clusters binned such that Dbin−1 < Dcrater ≤ Dbin, putting all craters in observed initially, several dozen craters are often closely a diameter bin that is the largest crater size in that bin. Since packed with rims touching. Candidate clusters were visually inspected in THEMIS mosaics to validate the cluster and determine, based upon morphology, if the cluster was com- posed of secondaries. Great circle arcs were drawn between the secondary clusters and Lyot to test orientation (e.g., Figure 2). This only identifies clusters of craters and fre- quently fields of smaller craters were observed that were also likely from Lyot but not included in this study. In this case, “fields” define regions of craters that show an over‐ density from the background that are probably secondaries, and “clusters” are craters that are packed closely with rims often touching. [9] The search identified 143 distinct clusters of 10– 300 craters each, where craters have diameters D > ∼800 m. A total of 5341 craters from the global database were extracted as members of these distant secondaries, similar in magnitude to the number identified on Europa [Bierhaus et al., 2005]. The closest cluster is ∼700 km away (∼6 crater radii) and the farthest is ∼5200 km (∼46 crater radii). Close Figure 2. Distribution of identified secondary crater clus- secondaries occur in an annulus around Lyot with generally ters from Lyot. Arcs are representative examples of great distinct morphologies [Shoemaker, 1962, 1965; Oberbeck circles between Lyot and distal secondary craters. Dark out- and Morrison, 1974] (Figure 1), while “distant” or “far” lines show each cluster. 2of5 L05201 ROBBINS AND HYNEK: SECONDARY CRATERS FROM LYOT, MARS L05201 the primary’s diameter. The largest alternative is an 11‐km crater to the northwest which fits the canonical 5% of pri- mary size relation [Melosh, 1989]. 1719 of these nearby secondary craters were extracted and their SFD is displayed in Figure 3. The diameter range 3.2 < D < 7.0 km has a power‐law slope of −5.6 ± 0.3. This is significantly steeper than a normal crater population with slopes between −2 and −3, but it is typical for secondary crater populations [McEwen and Bierhaus, 2006]. 3.2. Distant Secondary Craters [13] All 5341 craters in Lyot’s far‐field clusters were combined into one SFD (Figure 3); based on SFD slopes, D > 2.5 km craters are likely mostly primaries, but the overall contamination by primaries is minimal since D > 2.5 km craters comprise 0.3% of the total. The power‐law − Figure 3.