Thermal Histories of Earth, Moon, Mars and Vesta, and A Thermal Signal for the Onset of Terrestrial-like Plate Tectonics

Abstract:

Mantle potential temperatures (Tp) reflect mantle circulation and planetary cooling, and thus may provide clues as to why Earth is unique amongst the terrestrial planetary bodies. A re-evaluation of Tp estimates for Earth, Mars, Moon, and Vesta, reveals an anticipated decrease in maximum Tp as planet size decreases, at least if we apply a 4.3 Ga age to shergottites (e.g., Bouvier et al., 2009; Werner et al., 2014), and use the highest MgO komatiites from Earth’s Archean eon (27-30% MgO, e.g., Green et al., 1975). With these assumptions, Earth and Mars yield time-Tp paths indicative of secular cooling that appears to be largely monotonic following the end of accretion. These two cooling trends further provide intriguing support for Stevenson’s (2003) hypothesis that smaller planets cool at similar rates, but always maintain lower temperatures. One key difference between Earth and the other bodies, though, is that Earth exhibits a clear split in Phanerozoic terrestrial mantle temperatures, into plume- and ambient-Tp, although there is a hint of a parallel process on Mars at 1.3 Ga, if we accept that the high MgO Nakhla D composition is a liquid (Longhi and Pan, 1989; Treiman 1986). The onset of Tp bi-modality may be a harbinger of a transition from a stagnant lid to a plume/plate convective regime; in the former, only hot plumes from the core-mantle boundary contribute to volcanism, whereas in a plume/plate mode, small-scale convection of cooler ambient mantle, mostly at rifts and subduction zones, also contributes to surface volcanism. The contrasts between Earth and the other bodies imply that planetary radii and initial temperatures exert the primary control on whether or not modern terrestrial tectonics is able to develop. Exploration of Mars is still at too early a stage to infer much regarding its internal thermal state at 1.3 Ga, but clearly the study of young volcanic features there are of prime importance, perhaps much more so for understanding planetary and bioligical evolution than our current search for microbiotic life.