AS15-9299(P)

Figure 224

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On January 16, 1973, the Soviet unmanned roving vehicle Lunokhod 2 was landed by Luna 21 in or near this area in the southeastern part of the crater Le Monnier. This crater is a large (61 -km) pre-Imbrian crater cut into terra at the eastern edge of Mare Serenitatis before Serenitatis was flooded by mare laves. Part of Le Monnier's southern wall fills the lower part of the picture. A conspicuous chain of elongate depressions has formed in the lava-filled floor of the crater. The chain trends 22 km northward and its pattern is quite surely controlled by an underlying fracture system. Regionally, the inferred fracture system is concentric to the grossly circular Serenitatis basin, and in this area trends northward. No comparably young structural features having the same trend cut the terrae surrounding Le Monnier. However, older structures having this trend occur in the southern and northern walls and rims of Le Monnier. The alined depressions on the mare are mostly 300 to 400 m wide and 30 to 60 m deep. The three deepest stretches are 1 to 2 km long and about 50 to 65 m deep. These depressions probably were the locus of fissure eruptions of mare basalt. Withdrawal of the last lava back into the fissure may have created subsurface voids into which collapse took place, causing the depressions and accounting for the absence of raised rims on the depressions.   -R.E.E.