Evidence of young volcanic vents in the lunar maria

Images from the Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) contain >70 examples of small (10s of m to km), young mare features [1-4] with morphologies similar to the relatively well-studied Ina-D (18.65°N, 5.30°E) [1,5-10]. These features, here called Irregular Mare Patches (IMPs), exhibit sharp, meter-scale morphologies and are distinct from adjacent mare units. Studies of IMPs indicate two common units termed uneven units and smooth units. The uneven unit is present in each IMP and varies in reflectance, surface morphology, and boulder density. The smooth units are mare-like but are often found isolated from the mare, surrounded by the uneven unit. Sharp contacts are found at the boundaries between the uneven and smooth units, or between the uneven unit and the surrounding mare (common in smaller IMPs where there is no distinct occurrence of the smooth unit). Both uneven and smooth units within IMPs have relatively few superposed impact craters with diameters (D) >10 m, consistent with a young age. A combination of crater counts and topographic analyses of four IMPs indicate ages of <100 Ma. We used topography derived from LROC digital terrain models to compare 500x500 m areas from IMPs, impact melt deposits (4 Ma - 100 Ma), and older surfaces (1.6 - 3.6 Ga). Properties measured include the average and maximum slopes, standard deviation of slope (roughness), and relief (maximum - minimum elevation). Most of the IMPs have topographic properties similar to young impact melt deposits such as those from Giordano Bruno, Larmor Q, and Moore F. Constraints on the ages of IMPs are important for bounding the age of the youngest mare eruptions and thus improving our understanding of the lunar heat inventory over time. The morphologies and distinct flow fronts of IMPs may represent the remnants of late-stage volcanic vents. The rate of extrusion and temperature required to produce the sharp flow boundaries found in the IMPs are likely different from broad areas of mare basalt flows. Changes in extrusion rate and temperature commonly occur at vents, which could explain the IMPs' unique morphology. Furthermore, IMPs are often spatially associated with volcanic features such as mare domes (e.g., Cauchy 5), collapse calderas (e.g., Hyginus crater), and large pyroclastic deposits (e.g., Mare Vaporum). Multispectral analysis on a subset of the largest IMPs using combined Clementine UV-VIS data (950/750 nm ratio) and LROC Wide Angle Camera images (320/415 nm ratio) show that the uneven unit is consistent with a mare basalt composition. The multispectral data also suggest the presence of pyroclastic material in the area directly surrounding some of the IMPs, which is consistent with a vent hypothesis. Thus, the morphology, composition, and distribution of IMPs imply that lunar volcanic activity may be much younger than generally recognized. References: [1] Whitaker E. (1972) NASA SP-289. [2] Schultz P. H. (1976) Moon Morphology, 626 pp., Univ. of Texas. [3] Stooke P. J. (2012) LPSC 43, abst 1011. [4] Braden S. E. et al. (2013) LPSC 44, abst. 2843. [5] El-Baz, F. (1973) NASA SP-330. [6] Strain P. L. and El-Baz F. (1980) PLPSC 11, 2437-2446. [7] Schultz P. H. (2006) Nature, 444, 184-186. [8] Staid M. et al. (2011) LPSC 42, abst. 2499. [9] Garry W. B. et al. (2012) JGR, 117, E00H31. [10] Robinson M. S. et al. (2010) LPSC 41, abst. 2592.