Smooth mare surfaces on the Moon have long been known to be volcanic plains formed billions of years ago by massive effusive volcanic eruptions. Analysis of the eruption parameters and condition is a clue for understanding the geologic processes on the Moon in the past. A few large relatively young lobate lava flows in southern part of Mare Imbrium have been documented by G. Schaber (Proc. 4th LPSC, 73-92, 1973) using Apollo orbital images. Morphology of mare-forming lava flows is difficult to study: the flows surfaces accumulated a significant number of small craters and a few meters thick regolith, which obscure and mute the original flow morphology. The mentioned study of Imbrium flows was only possible because the images used were obtained under low-sun illumination, which is favorable for seeing subtle gently-sloping topography amid generally flat mare surfaces. However, low-sun imaging has several limitations: (1) except for the polar regions, low-sun illumination is only available from the east or from the west, which produces an azimuthal bias; (2) long shadows from tall topographic features obscure significant parts of the surface; (3) imaging under proper illumination conditions requires careful planning of observations and cumbersome mosaicking. The high-precision orbital laser altimeter data are free of these shortcomings and potentially can be used to study morphologies of subtle gently-sloping features.
We made use of the exceptionally high vertical precision of ranging data from LOLA instrument onboard LRO orbital mission to the Moon. To show low-amplitude gently-sloping topographic features in a way suitable for traditional geomorphological analysis, we applied a detrending procedure to the LOLA-derived topographic maps. The detrended topography at each pixel was calculated as a difference between the actual elevation at this pixel and the median elevation of all pixels within a 10-km circular window centered at this pixel. The detrending algorithm filters out topographic features much larger than the window, preserves topographic features much smaller than the window, and distorts features comparable to the window in size. The use of the median (rather than the mean) makes the result of the detrending procedure much better for visual perception.
The detrended topography maps reveal impressive previously unrecognized textures of mare-forming flows in several regions. In addition to the known large lobate flows in the southern and central Mare Imbrium, the map shows a distinctive flow pattern in the west-central part of Mare Imbrium. It appears to be formed by a hierarchic dendritic system of subtle ridges with characteristic topographic amplitude of 1 - 3 m, and a characteristic spatial scale of several kilometers and larger. We interpret this pattern as a compound lava flow field formed when the flow propagation is cooling-limited, in contrast to the younger large lobate flows formed under higher effusion rate conditions when the flow propagation is volume-limited. The western part of Mare Serenitatis shows another compound flow pattern in contact with a large smooth surface. Further detailed studies of mare-forming flows of different morphologies will reveal new information about evolution of volcanic styles on the Moon.