The prospect of diking on the Moon and Mercury (Invited)

Both Mercury and the Moon host extensional graben that have previously been suggested to be the surface expressions of dikes at depth. Whereas on the Moon graben occur globally, with a denser concentration of structures near or within mare units, graben on Mercury are exclusively confined to smooth plains units within impact craters and basins. Observed surface displacements associated with these graben, obtained from detailed along-track topographic profiles recorded by the Mercury Laser Altimeter (MLA) and Lunar Orbiter Laser Altimeter (LOLA), can be fit to modeled displacements to test the inference that dikes are present beneath these graben. Topographic profiles across graben usually show characteristic concave-upward graben flanks with topographic maxima directly at the graben rim on one or both sides of the structure. In contrast, profiles across dike-induced graben show generally concave-downward profiles at their flanks that lack pronounced topographic highs at graben rims. On the Moon, the formation of some graben has been attributed to extensional flexural stresses within and around mare-filled basins, such as those graben concentric to the mare-filled Serenitatis and Humorum basins. Other graben are superposed by volcanic pits and so an origin due to diking was invoked. Surface displacement models fit to LOLA topography measurements across 11 selected large lunar graben show that their topographic signatures are consistent with dike intrusions at depth. Importantly, graben in the Schrödinger crater, as well as Rima Hyginus, and Rimae Daniell specifically require surface displacements from dikes at depth to account for their observed topographic signatures. Mercury experienced a substantial amount of global contraction due to interior cooling, the onset of which caused unfavorable conditions for graben formation and dike intrusions in its lithosphere. Large impacts can reset stresses imposed by global contraction, which likely allowed for the formation of graben within the smooth plains infill of the Caloris basin, the largest recognized impact basin on Mercury. Debate exists as to the formation mechanism of the most prominent graben set in Caloris, the radial Pantheon Fossae, with one hypothesis suggesting them to be indicative of a radial dike complex. However, MLA profiles across several Pantheon Fossae graben have typical normal fault signatures and show no evidence of dike intrusion. Diking may have occurred on Mercury during planetary contraction, but ascending magma was likely constrained to preexisting weaknesses in the lithosphere such as large, deeply penetrating thrust faults, which explains the occurrence of many pyroclastic vents and pits along or near thrust fault-related landforms.