Limits on the brittle strength of planetary lithospheres undergoing global contraction

The degree and depth of fracturing of the lithospheres of Mars, Mercury, and the Moon remain poorly known. It is these two properties, however, that govern the mechanical behavior of a planetary lithosphere. Considering the lithosphere as a cohesive rock mass that consists of small and large blocky, interlocked rock fragments, as opposed to an intact body or a body entirely lacking cohesion, provides insight into the effect of lithospheric fracturing on tectonic processes on these bodies. Characterization of the near‐surface lithospheric brittle strength that incorporates the degree of fracturing is necessary for a realistic assessment of the lithospheric response to global contraction resulting from interior secular cooling. Such an assessment shows that all of these bodies could have accommodated substantial amounts of global contraction prior to the formation of thrust fault‐related landforms. In fact, their lithospheres were sufficiently strong so as to experience changes in radius of as much as 2.2 ± 0.4 km (Mars), 2.1 ± 0.4 km (Mercury), and 1.4 ± 0.3 km (the Moon) prior to the onset of widespread thrust faulting. These values imply that the process of global contraction begins before any evidence of it is established in the geologic record, requiring an earlier onset, and for Mars and Mercury a faster initial strain rate, of global contraction than previously thought. Such results add a heretofore unrecognized component of planetary radial decrease with implications for timing and strain rate to studies of global contraction on Mars, Mercury, and the Moon, in particular, and to planetary bodies, in general.