Hannah C.M. Susorney

The surface roughness of Mercury from the Mercury Laser Altimeter: Investigating the effects of volcanism, tectonism, and impact cratering

Surface roughness is a statistical measure of change in surface height over a given spatial horizontal scale after the effect of broad scale slope has been removed, and can be used to understand how geologic processes produce and modify a planets topographic character at different scales. The statistical measure of surface roughness employed in this study of Mercury was the root-mean-square (RMS) deviation, and was calculated from 45–90°N at horizontal baselines of 0.5-250 km with detrended topographic data from individual Mercury Laser Altimeter tracks. As seen in previous studies, the surface roughness of Mercury has a bimodal spatial distribution, with the cratered terrain (dominated by the intercrater plains) possessing higher surface roughness than the smooth plains. The measured surface roughness for both geologic units is controlled by a trade off between impact craters generating higher surface roughness values and flood-mode volcanism decreasing surface roughness. The topography of the two terrain types has self-affine-like behavior at baselines from 0.5–1.5 km; the smooth plains collectively have a Hurst exponent of 0.88 +/- 0.01, whereas the cratered terrains have a Hurst exponent of 0.95 +/- 0.01. Subtle variations in the surface roughness of the smooth plains can be attributed to differences in regional differences in the spatial density of tectonic landforms. The northern rise, a 1,000-km-wide region of elevated topography centered at 65° N, 40° E, is not distinguishable in surface roughness measurements over baselines of 0.5–250 km.

Analysis of MESSENGER high-resolution images of Mercury's hollows and implications for hollow formation

High resolution images from MESSENGER provide morphological information on the nature and origin of Mercurys hollows, small depressions that likely formed when a volatile constituent was lost from the surface. Because graphite may be a component of the low-reflectance material that hosts hollows, we suggest that loss of carbon by ion sputtering or conversion to methane by proton irradiation could contribute to hollows formation. Measurements of widespread hollows in 565 images with pixel scales <20 m indicate that the average depth of hollows is 24 ± 16 m. We propose that hollows cease to increase in depth when a volatile-depleted lag deposit becomes sufficiently thick to protect the underlying surface. The difficulty of developing a lag on steep topography may account for the common occurrence of hollows on crater central peaks and walls. Disruption of the lag, e.g., by secondary cratering, could restart growth of hollows in a location that had been dormant. Extremely high-resolution images (~3 m/pixel) show that the edges of hollows are straight, as expected if the margins formed by scarp retreat. These highest-resolution images reveal no superposed impact craters, implying that hollows are very young. The width of hollows within rayed crater Balanchine suggests that the maximum time for lateral growth by 1 cm is ~10,000 yr. A process other than entrainment of dust by gases evolved in a steady-state sublimation-like process is likely required to explain the high-reflectance haloes that surround many hollows.

The Surface Roughness of Large Craters on Mercury: SURFACE ROUGHNESS OF CRATERS

This study investigates how individual large craters on Mercury (diameters of 25–200 km) can produce surface roughness over a range of baselines (the spatial horizontal scale) from 0.5 to 250 km. Surface roughness is a statistical measure of change in surface height over a baseline usually after topography has been detrended. We use root mean square deviation as our measure of surface roughness. Observations of large craters on Mercury at baselines of 0.5–10 km found higher surface roughness values at the central uplifts, rims, and exteriors of craters, while the crater floors exhibit the lowest roughness values. At baselines <10 km, the regions exterior to large craters with diameters >80 km have the highest surface roughness values. These regions, which include the ejecta and secondary fields, are the main contributors to the increased surface roughness observed in high-crater density regions. For baselines larger than 10 km, the crater cavity itself is the main contributor to surface roughness. We used a suite of numerical models, utilizing the measured surface roughness obtained in the study, to model the cumulative effect of adding large craters to a surface. The results indicate that not all of the surface roughness on Mercury is due to fresh large craters but that impact craters likely contribute to the Hurst exponent from baselines of 0.5–1.5 km and the shape of the deviogram. The simulations show that the surface roughness varied around an asymptote at the baselines studied before the surface was covered in impact craters. Plain Language Summary Impact cratering is the main process by which many planetary bodies are roughened, where an increase in the number of craters is related to higher surface roughness. In this study, we use observations and artificial data to explore how individual complex craters on Mercury can change the surface topography and produce surface roughness. Observations of the surface roughness of complex craters on Mercury found surface roughness related to several different geologic features of the craters. We found that impact craters are the main source of surface roughness on Mercury. We modeled how impact crater density affects surface roughness and found that it is difficult to relate surface age to surface roughness.