LUNAR CONCENTRIC CRATERS. C. A. Wood, Dept. of Geological Sciences, Brown University, Providence, RI 029 12. It is generally accepted that most lunar craters formed by impact proc- esses; however, a relatively small number of craters with peculiar morpholo- gies and spatial distributions may have other origins. Physical character- istics of one of these anomalous crater types - concentric craters - are here documented and it is concluded that volcanism produced some of their unique features. Concentric craters are small (average diameter 'L8 km) craters con- taining an inner ring about & the diameter of the main crater. Morphologies of the inner rings vary from donut-like, rounded ridges to steep crater rims to flattened mounds. Table 1 lists the principal facts for the 51 lunar con- centric craters now known. Concentric structures in multi-ring basins1, large craters such as Vitello, Sabine and ~osidonius~'~,and subkilometer size craters4 are believed to have different origins than those in the cra- ters described here. Morphologies. The most common concentric crater morphology is exempli- fied by Hesiodus A (Fig. la), a 14.9 km wide fresh crater on the periphery of Mare Nubium. The inner slopes of the main crater are smooth and terrace-free (Fig. 2a), similar to normal impact craters of this diameter. The outer slopes of the inner ring appear to be slightly convex, whereas normal impact craters have concave outer slopes. According to shadow measurements the floor of the inner ring is $250 m deeper than the lowest point of the moat between the rim and ring. Hesiodus A is Q1.7 km deep, only 55% as deep as a typical fresh impact crater of the same diameter. A degraded ejecta blanket surrounds the crater5. 70% of the known concentric craters are morphologi- cally similar to Hesiodus A, but there are many variations: Lagrange T (Figs. lb, 2b) has a rounded, donut-like inner ring. The rings within Marth, Nos. 11 and 32, resemble cratered domes. Both the inner and outer rims of Nos. 47 and 51 are elliptical, whereas Crozier H is a circular crater with an elliptical inner ring. Gruithuisen K (Fig. lc) , Nos. 8, 37, and 4 1, are especially in- triguing because each has two inner rims concentric to the main crater: i.e., they are triple craters. Three concentric craters on the Imbrium ejecta blanket near Montes Jura have rounded concentric rings that are fractured and cracked. These breadcrust-ring craters are more degraded than normal Hesi- odus A type craters. Two other distinctly different types of concentric craters occur. One, represented by Gambart J, is characterized by low, inconspicuous rings, sepa- rated from the main wall by a portion of the crater floor. Whereas the height of the inner ring of Hesiodus A is Q45% of the main crater's depth, the flat ring of Gambert J is only ~10%of the crater's depth. Only 3 Gambert J type craters have been detected; others probably exist. A third type of concentric structure (probably unrelated to the above types), is exemplified by No. 16, a crater with a broad concentric collar high on its rim. Statistics. The diameter distribution of concentric craters is peaked at 7 km (Fig. 3) and the average is 8.3 km. Ratios of the diameters of inner rings to crater rims are commonly U.5 (Fig. 41, with values >0.7 occurring in'bollared" craters and triple craters. 50% of the concentric craters are LPL class 2; neither very sharp (young), nor very degraded (old). For com- parison, only 24% of normal lunar craters with diameters between 2 and 15 km6 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System LUNAR CONCENTRIC CRATERS Wood, C. A. are class 2 or 3. There is no correlation between ring to rim ratios and diameter or class, however concentric craters formed on mare surfaces tend to be slightly smaller and younger, and.have smaller inner rings than similar craters on the lunar highlands. Distribution. 70% of concentric craters are located near the margins of maria - both on the maria and on the adjacent highlands. Concentric craters are not found in the central regions of maria. A further 20% of the known concentric craters occur on smooth floors of larger craters (e.g., No. 13 within Hurnboldt) considered to be lava flooded, or near other areas where lava reached the surface within the highlands (e.g., No. 42 near Cruger). Only 10% are on pure highlands. Discussion. The possibility that concentric craters formed by impacts of tidally split meteoroids7 seems ruled out by their concentration around mare margins. It is likewise improbable that a single impact into layered terrain (a scaled-up version of experiments4) formed these enigmatic craters because they occur on both the mare and highland sides of mare margins. The concentration of concentric craters near mare margins suggests an association with volcanism and possible association with basin related fractures. Exam- ples of the intimate associations with volcanism include: (a) Marth, a con- centric crater located on a slight, low albedo mound that has flooded 4 near- by rilles; (b) a concentric crater and rilles on the 300 m high domed floor of Mersenius; (c) rille and dome associations in Sinus Roris. This evidence for volcanism is supported by the syn-mare age (class 2 = %3.8 to 3.5 b.y.8), restricted diameter distribution, and unique morphology of concentric craters. Evidence such as this prompted suggestions that concentric craters formed by successive eruptions from the same ~ent~'~,~.However, the outer rims of concentric craters are indistinguishable from those of impact craters, and one of the largest and freshest examples, Hesiodus A, appears to have a rem- nant ejecta blanket. It is thus proposed that many concentric craters are polygenetic structures - impact craters colonized by lava that used crater breccia and fracture zones'as conduits to the surface. For the lava to form rings rather than ponds (e.g., Cruger) requires a higher viscosity or a lower 10 extrusion rate than normal mare lavas. The magma may have been mare basalts erupted under unusual conditions, or mare basalt that differentiated within pockets, or in some cases, a non-mare magma type. The variations of concen- tric crater morphologies suggest a variety of styles of volcanic modification. The descriptions of non-mare style volcanic landforms and speculations of non- mare magmas complement similar conclusions for the material that forms some steep lunar domes1'. The occurrence of concentric craters (10%) in highland terrains further suggests that unfamiliar forms of volcanism may have played a more important role in forming or modifying the lunar highlands than cur- rently suspected. References: (1) Hartmaan, W. K. and Wood, C. A. (1971) The lbon 3, 3. (2) Thornton, F. H. (1950) Brit. Astron. Ass. &. lJ, 9. (3) Warner, B. (1961) J. Brit. Aatron.~s.,-115. (4) Quide, W. L. and Oberbeck, V. R. (1968) JGR 11, 5247. (5) S~hultz,P. H. (l~-rp~o~U. Texas Press, p. 10-15. (6) Wood, C. A. and Andersson, L. (1978) LPL Catalo of Lunar Craters (in prep.). (7) Sekiguchi, N. (1970) The Moon 1, 429. (B) wood, C. A., Head, J.7and Cl:tala, M. Jn)Proc. Lmar Sci. Conf. Bth, 3503. (FS=,-E.I. (1973) The Ebon 6, 3. (10) Walker, G. P. L. (1973) Phil TZ.7~oc.-i&-~274107. (11) Head, J. U. and McCord, T. B. (1978) x. (in press). (12) L. Andersson and J. V. Head are thanked for help. 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System LUNAR CONCENTRIC CRATERS Wood, C. A. TABLE I: LIDUR CONCEXTRIC CRATERS & Dcaipation Long. Lac. Diaikn Ratio Class Photo 1 Archyras C 0.4E 55.81 6.8 .54 2M 116-655 . -. ..-~---.-- ~.~~ .. 6 rrr 8eamnt P 29.6's 19.1s 11.1 .45 3W 77-518 7 Crozier E 49.48 14.1s 11.3 .37 2C 60-277 8 nr Endymion 50.68 51.7N 6.9 .36..64 2M 79-735 Fig. 3 9 Apollonivs N 64.08 4.7N 9.4 .46 2C 1l126-431 10 or Legendre 68.28 27.5s 4.6 .33 X 38-346 11 nr Dubiego 69.OE 3.7N 4.6 .36 34 178-796 12 nr Sshubert N 74.OE 2.ON 9.9 .45 3W 178-795 13.~ in- Hdldt- - 83.2E 26.55 6.7 .47 ll4 27-981 14 nr tlamilton 84.71 44.25 9.6 .55 21 ml-047 15 nr Jeans 94.48 53.15 23.8 .70 32 6154 169 nr Chamberlin 102.5E 58,85 8.9 .75 2UC 9-632 17 in Pasreur 104.9E 11.8s 5.4 .57 X 2Hl96-320 18 or Jules Veroe 144.18 37.59 6.4 .55 2C W75-454 19 nr Geiger 159.08 16.25 6.5 .39 X 2875-275 20 nr Aitken 172.68 20.68 10.0 .50 X A17-150-22961 MAJOR CRATER DIAMETER (km) 21 Archimedes F 7.8U 24.1N 7.4 .54 2HC 1%-323 22 Aeoiodur A 17.OY 30.15 14.9 .41 U! 115-8866 23 Gambarr J 18.2Y 0.7s 7.1 .44 24 38120-772 24* or Laplace E 21.2Y 50.ON 7.6 .56 5K 134-961 25 Fonrenelle D 23.3U 62.5s 17.2 .46 W 128-213 26' in Blancanus C 29.1V 66.2s 13.0 .42 X 130-444 27 Harrh 29.3Y 31.19 6.1 .44 2U 136-213 28 nr La Cond. F 31.3U 57.29 5.1 .37 2U 145-384 29 Hsiiozel B 33.1U 36.95 11.2x8.9 .43 3C 136224 30 nr Bouguer B 33.8U 53.5N 5.8x6.7 .43 X 145-395 31 nr Bougver A 34.1Y 53.21 7.1 .47 U 145-396 Fig.