Lunar sinuous rilles: Distribution, characteristics, and implications for their origin

Abstract

Lunar sinuous rilles (SRs) are enigmatic features interpreted to have formed as the result of lava erosion into the lunar surface. While specific SRs have been studied in detail over the past few decades, the most recent general survey of these features was conducted in 1969 using Lunar Orbiter IV and V photographs. The current global study updates the catalog of SRs, using higher resolution SELENE and LRO image and topography data to provide detailed observations and measurements of the rilles observed across the lunar surface. The new survey catalogs more than 200 SRs that vary in length from 2km to 566km (median length 33.2km), in width from 160m to 4.3km (median width 480m), in depth from 4.8m to 534m (median depth 49m), in slope from −1.4° to 0.5° (median slope −0.2°), and in sinuosity index from 1.02 to 2.1 (median sinuosity 1.19). Oceanus Procellarum contains 48% of the rilles mapped in this survey, and these rilles are typically associated with the known centers of volcanism within the Procellarum-KREEP Terrain, the Aristarchus Plateau and the Marius Hills. The current study also constrains the timing of the formation of lunar SRs, using the assumptions that the incised unit represents an upper age limit and the terminal or embaying unit represents a lower age limit. Results indicate that the distribution of ages of rille formation is highly correlated with the emplacement ages of mare units, where the majority of rilles are observed to have formed between 3.0Ga and 3.8Ga ago, though some of the features associated with the Aristarchus Plateau may have formed as recently as 1.0Ga to 1.5Ga ago. The documented observations can be used to better understand how SRs formed; for example, the range of slopes observed for all rilles (−1.4° to 0.5°) indicates that thermal erosion is likely dominant during the formation of the 78% of rilles that are observed to have formed in solidified mare basalt material, though mechanical erosion is likely to have been a more significant process during the formation of the 25% of observed rilles that originate in the highlands (20% of the mapped rilles crossed from the highlands into the mare), where a thicker regolith is expected to have been more easily eroded.