PIIOTOGEOLOGY OF ARATUS CAY MARE SERENITATIS. R. Greeley, Univ. Sarltn Clara - NASA Arnes Research Center, M/S 245-5, Moffett Field, CA 94035
Aratus CA is a possible volcanic vent in western Mare Serenitatis. A preliminary geological report of this feature and a comparison withapossible terrestrial analog was made shortly after the Apollo 17 mission (1). Since then, 1:250,000 scale and 1:10,000 scale topographic maps have been made for the area(%)and field studies for the possible analog have been completed (3). Aratus CA is about 9.5 km long, 3 km wide, and has a maximum depth of about 400 m. It is composed of several coalescing depressions named Christel, Krishna, Lorca, and Sung-Mei (2), some of which display outcrops of mare basalts in the walls. Topographic profiles across the depressions show slopes that range from 170 to 37". The shallow slopes are typical of lunar talus slopes; however, the steepest slopes (measured on Sung-Mei) may indi- cate prominent outcrop resulting from collapse. Different stages of degrada- tion and cross-cutting relations suggest that individual elements of Aratus CA are of different ages. A shallow slightly sinuous rille, about 250 m wide and interpreted as a lava channel, trends south from the southern rim of Aratus CA. Topographic profiles across the channel show no appreciable changes in slope away from the axis of the channel. Aratus CA is within the younger (4), "intermediatett color mare lavas (5, 6). It is on a gentle western-sloping surface and, within the limits of the 100 m contour interval, Aratus CA topographically does not appear to be a prominent constructional feature. Dorsum Owen, a north-south mare ridge, terminates at Aratus CA. The ridge has a typical two-part profile: a broad element several km wide and a superposed narrow element several hundred m wide. In some places the narrow element transects normal impact craters that are on the broad element, indicating that the narrow and broad elements formed independently, as noted in other areas (7). Four possible origins are suggested for Aratus CA: 1) tectonic collapse, 2) volcanic vent, 3) psuedo volcanic vent, and 4) volcanic vent-tectonic collapse. Photogeological studies and comparisons with terrestrial analogs support the fourth hypothesis and suggest the following geological history: Earliest history involves the emplacement of most of the mare lavas in the area and the development of the broad elements of Dorsum Owen and other mare ridges. From its size, relation to the ridges, and regional topography, it seems unlikely that Aratus CA was the source for mast of these lavas. The oldest elements of Aratus CA (Krishna and Christel) truncate Dorsum Owen and must have formed after the mare lavas had developed a sufficient crust to preserve the structures. At this stage, Aratus CA erupted relatively small volumes of lava, indicated by the minor sinuous rille. Lorca appears to be a collapse pit crater younger than Christel and Krishna, and older than Sung-Mei; by terrestrial analogy, subsidence at a central vent is often accompanied by magmatic activity elsewhere. In this case, the formation of Lorca might be coincident with the extrusion of lava that developed the narrow ridge element on Dorsum Owen.
0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System PHOTOGEOLOGY OF ARATUS CA R. Greeley
The final endogenic event at Aratus CA appears to be the formation of Sung-Mei. The steep (37') slopes on its walls indicate an origin by collapse, possibly by tectonic processes. Alternatively, Sung-Mei may represent an early-formed lava tube that did not collapse until after Lorca formed. There is no other evidence in the area, however, to suggest that Sung-Mei is a lava tube.
DORSUM OWEN
- - - DORSUM OWEN - - -
0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System PHOTOGEOLOGY OF ARATUS CA R. Greeley
- DORSUM OWEN - C' Raised rim ------
4700 CHRiSTEC Raised rim
D 4500
KRiSHNA Raised rim
mVertical scale in meters; 0 500 1000 1500 Vertical exaggeration: X3 meters
(1) Greeley, R., 1973. Apollo 17 Prelim. Sci. Rpt. NASA SP 330, 30-1 to 30-6. (2) Defense Mapping Agency, 1: 250,000 (Linne , Hornsby) , 1: 10,000 (Lorca East, Lorca West) . (3) Kelley, L. M., 1975. Unpub. Masters Thesis, Univ. New York at Buffalo. (4) Boyce, J. M., Dial, A. L. and Soderblom, L. A., 1974. Geochim. Cosmo- chim. Acta, Suppl. 5, -1, 11-23. (5) Strom, R., 1972. The Moon - Symp. 47, Inter. Ast. Union, 187-215. (6) Whitaker, E., 1972. Apollo 15 Prelim. Sci. Rpt. NASA SP 289, 25-83 to 25-84. (7) Schultz, P., 1972. Ph. D. Thesis, Univ. Texas and 1976 Moon Md~phology, Univ. Texas Press.
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