The Stress State of a Cooling

Introduction: Understanding the Moon's thermal and magmatic evolution entails understanding the origin of the earliest crust, in particular whether it formed from a magma ocean [1]. The energy of late stage planet-building impacts would be at least sufficient to form magma oceans and possibly enough to melt whole planets [2]. The solidification history of the magma ocean will place constraints on the tectonic evolution of the Moon. Lunar igneous activity starts from the formation of the original crust that developed in response to the radiative cooling effects of a body devoid of an atmosphere. There is still some uncertainty as to the precise age of the Moon and consequently the time span during which the crust formed. However, there is common agreement that the Moon formed between 4.55 and 4.5 Ga ago [3, 4, 5], approximately 50 Ma after the formation of the Solar System [6], [7], and that the magma ocean existed for between 40 and 200 Ma [3, 8, 9,10, 11, 12]. The Moon's crust initially contracted in response to the cooling of the magma ocean. Little attention has been given to the stresses built up within the developing crust as the magma ocean cooled during the Moon's early history. These stresses are likely to have been very large and should have manifested themselves as compressive surface features similar to those evident on Mercury [13, 14]. Unfortunately, tectonic features may have been obliterated by the tail end of heavy impact bombardment [15]. The surface of a magma ocean will quickly cool and a crust would form [9]. It has been suggested [8] that a crust approximately 10 km thick would act as an effective thermal insulator. This 10 km thick crust represents one of the initial conditions for the following model; it ignores the effect bombardment of the surface will have on the thermal properties of the crust. It is likely that the bombardment and the subsequent brecciation will enhance the insulating effects and only serve to slow the cooling process, although in the early stages of the development of this crust the bombardment may in fact increase the rate of cooling [9, 16].