Global Sub-Decameter-Scale Roughness of the Moon’s Surface

Surface roughness is an effective parameter for mapping geomorphological units and for quantifying the topographic evolution of the Moon’s surface, as it records the effects of impact cratering, regolith processes, and geological modification [1,2]. It highlights surface features that are often difficult to detect in optical images and conventional digital elevation models (DEMs). Additionally, roughness at small spatial scales is valuable for assessing landing site hazards and for interpreting radar remote sensing observations. However, existing global lunar roughness maps are largely limited to ~10 m and longer baselines, thereby hindering spatially detailed studies of surface geology.

We present novel estimates of global surface roughness for the Moon at ~5 m length scales, determined from Lunar Orbiter Laser Altimeter (LOLA) echo pulse width measurements. In addition to measuring surface elevations from time-of-flight ranging, LOLA recorded the width of reflected laser pulses, which is sensitive to vertical variations within the illuminated footprint of ~5 m diameter. LOLA pulses reflected from the Moon’s surface are broadened relative to the transmitted pulses due to surface slopes and small-scale roughness. We determine small-scale roughness from the amount of pulse broadening, after correcting for factors such as beam divergence and curvature, observation geometry, the temporal decline in transmitted power, and receiver misalignment during polar and nightside crossings [3,4].  

Roughness at sub-decameter scales (~5 m) reveals signatures of recent and ongoing surface processes on the Moon. The youngest impact craters, formed in the Copernican period, are distinctly rough, with interiors rougher than their ejecta blankets. The high-albedo swirl Reiner Gamma also appears unusually rough at these scales, despite lacking evident topographic expression, with on-swirl areas rougher than off-swirl. In the polar regions, permanently shadowed regions are smoother than nearby sunlit areas even on gentle slopes (<20°), suggesting potential for volatile preservation [5]. Among Artemis III candidate sites in the south pole, the Mons Mouton Plateau and Haworth are the smoothest and most favorable sites for rover navigation and extravehicular activities. Thus, our small-scale roughness map complements existing longer-baseline roughness products, captures topographic variability at spatial scales most relevant to upcoming surface missions, and provides new insight into recent modification of the lunar surface.

 

References: [1] Shepard M. K. et al. (2001) JGR, 106, 32777–32795. [2] Kreslavsky M. et al. (2013) Icarus, 226, 52-66. [3] Gardner C. S. (1992) IEEE Trans. Geosci. Remote Sens., 30, 1061–1072. [4] Neumann G. A. et al. (2003) GRL, 30(11). [5] Magaña L. O. et al. (2024) Planet. Sci. J., 5(2), 30.