Cooling fractures in impact melt deposits on the Moon and Mercury: Implications for cooling solely by thermal radiation

Abstract

We study the distribution, morphology, and geometrical properties of fractures in several young impact melt deposits on the Moon and Mercury, and the ways that these fractures may form from cooling by thermal radiation. In each impact melt complex, the topography of the underlying terrain determines the orientation of cooling fractures, such that interior fractures that formed in the relatively thick interior areas of the melt unit are wider and have a larger spacing than marginal fractures that formed in the relatively thin areas near the unit's margins. Solid debris entrained in molten deposits provides prefracture flaws that can seed cooling fractures, but too much solid debris prevents cooling fractures from growing to macroscopic sizes. The appearance of subparallel fractures is mainly caused by subsidence of the deposits during the process of cooling and solidification. Tensile stresses caused by thermal radiation are large enough to initiate cooling fractures on both the Moon and Mercury, which may represent the initial stage of columnar joints formation, but the cooling rate caused solely by thermal radiation is not large enough to form well-organized columnar joints that feature polygonal colonnades. We therefore propose that thermal conduction and convection are the major contributors in the formation of columnar joints on planetary bodies.

Examining the Potential Contribution of the Hokusai Impact to Water Ice on Mercury

“…Hokusai has a relatively smooth floor with cracking suggestive of cooling impact melt and has melt ponds in the terraces (Barnouin et al, ; Xiao, Zeng, et al, ). Hokusai crater has an unusually large amount of impact melt in its interior, as much as 200–400 m greater thickness than in other similar‐sized craters, on the basis of differences in peak‐ring heights (Barnouin et al, ; Susorney et al, ).…”

Geological Mapping of Lunar Crater Lalande: Topographic Configuration, Morphology and Cratering Process

“…Fractures can be found on almost all ponded surfaces. These fractures are probably due to thermal contraction of the surface during cooling, but may also be related to extension of the surface (Xiao et al, 2014). The largest terrace melt pond is shown in Fig.…”

“…7, yellow lines) on this unit. Fractures form in solidified rock during the cooling of impact melts coming from cratering process (Xiao et al, 2014). These fractures mostly gather in subparallel groups, along the border of crater floor and crater wall (Fig.…”

“…According to previous studies, the elevation differences on the crater floors, are a relatively general phenomenon, may be caused by the following reasons: (1) material from wall collapse piling up on the neighbouring floor (Margot et al, 1999), (2) differential subsidence due to cooling of the melt column (Xiao et al, 2014), (3) melt movements during the early stages of the crater formation and (4) impact conditions (Hawke and Head, 1977). Here, we evaluated the possible causes of the elevation differences on Lalande's floor based on some observations.…”

“…Therefore, elevation differences on the local scale (irregular topography) are caused by subsidence of melt cooling due to changing amounts of impact melt and rock mixtures at different locations on the crater floor. The patterns of cooling fracture groups formed on an impact melt deposit can represent their underlying terrain topographic differences (Xiao et al, 2014). Fractures in relatively deep areas have a mix of orientations and sometimes form polygonal patterns.…”