A comparison of rayed craters on the Moon and Mercury

Abstract

[1] Observations of rayed craters at optical and radar wavelengths provide insight into the processes that lead to ray formation and degradation on terrestrial planets. We have compared optical and S-Band radar data for several large (> 20 km diameter), young craters on the Moon and Mercury and find evidence that secondary cratering plays a significant role in the formation of crater rays. Regions where rays appear bright to optical and radar sensors correspond to dense concentrations of secondary craters, and the observed radar enhancement appears to be a result of the deposition of blocky, immature ejecta from the secondary craters and/or the rocky, immature interior walls of the secondary craters. We define a new optical maturity index for Mercury and find that rays in radar and optical images correspond closely, indicating that the rays are rich in centimeter-to decimeter-sized clasts. Rays on the Moon are less prominent at radar wavelengths, suggesting that they are currently composed of smaller clasts, centimeter sized or less. This difference suggests that secondary craters are larger on Mercury and capable of excavating more decimeter-sized clasts. Furthermore, observations of rayed craters provide an opportunity to assign relative ages to the youngest craters on the Moon and Mercury. Although rayed craters on Mercury appear most similar to the youngest craters on the Moon, the apparent ages are more likely influenced by inherent differences in impact velocity, surface gravitational acceleration, and target properties that result in the formation of larger secondary craters on Mercury.

Distal ejecta from lunar impacts: Extensive regions of rocky deposits

“…In some ways, this is unsurprising and we only need to observe the rays of Tycho and Giordano Bruno craters to understand the widespread effects of impacts on the Moon (e.g., Wells et al, 2010;Neish et al, 2013). However, the surface features and deposits described here appear to be a separate manifestation of the impact process.…”

Global distribution of lunar impact melt flows

“…13). Glushko has a fresh impact melt flow (Campbell et al, 2010;Neish et al, 2013) that appears to be covered by very little to no regolith (unlike other melt flows, such as that at Gerasimovich D, shown in Fig. 1), so any scattering we see is from the impact melt flow surface, not a buried interface (which would tend to increase the DLP).…”

“…This suggests that the melts are 'rougher' at the centimeter to decimeter scale than most types of terrestrial lavas, which have average CPRs of 0.2 (pahoehoe), 0.5 (a 0 a), and 0.8-1.0 (blocky lavas) at similar incidence angles to Mini-RF (Campbell, 2002). It is notable that lavas on Mars (Harmon et al, 2012) and some impact melts on Mercury (Neish et al, 2013) also have high CPR. The cause of this increased CPR is unknown, but could be the result of either surface or subsurface scattering.…”

Bistatic radar observations of the Moon using Mini-RF on LRO and the Arecibo Observatory

“…4e) is commonly used in analyses of lunar radar data [e.g., Ghent et al, 2005Ghent et al, , 2008Ghent et al, , 2010Campbell et al, , 2010Campbell et al, , 2012Spudis et al, 2010, Carter et al, 2012;Raney et al, 2012;Thomson et al, 2012;Neish et al, 2011Neish et al, , 2013Neish et al, , 2014Cahill et al, 2014], and is a representation of surface roughness at the wavelength scale of the radar. In other words, surfaces that are smoother at the wavelength scale will have lower CPR values and surfaces that are rougher will have higher CPR values.…”