The relationship between crustal tectonics and internal evolution in the moon and Mercury

Abstract

Faulting and volcanism on a planetary surface can be closely related to the thermal evolution of the planetary volume. Interior warming leads to global expansion, surface extensional tectonics and a crustal stress system that aids surface volcanism. Interior cooling leads to contraction, compressional tectonics and crustal stresses that act to shut off surface volcanism. Limits on the sign and magnitude of the change in volume of Mercury and the moon since the period of heavy bombardment on both bodies restrict possible early thermal histories. Mercury began hot throughout most of its volume, in order to have completed core-mantle differentiation in the first 0.6 b.y., and has cooled and contracted since. Solidification of an inner core in Mercury of up to 60% of the outer core radius is permitted by current limits on total contraction of the planet. The moon began with a 200–300 km thick outer shell at near melting conditions and a relatively cold deep interior, resulting in modest expansion for the first 2 b.y. and subsequent slight contraction. The duration of plains volcanism on a planet may be controlled by the length of the period of volumetric expansion and crustal extension in the planet's history. On this basis the youngest major units of plains volcanics on Mercury are predicted to be older than the youngest major mare units on the moon.