GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 59-8
Presentation Time: 4:15 PM


CRISS, Robert E., Earth and Planetary Sciences, Washington University in St. Louis, 1 Brookings Drive, Saint Louis, MO 63130-4899 and HOFMEISTER, Anne, Department of Earth and Planetary Sciences, Washington University, Campus Box 1169, St. Louis, MO 63130

Profound differences in the surface morphologies, tectonics, geologic histories and atmospheres of the rocky bodies in the Solar System have perplexed scientists, yet can be mostly explained by gravitational processes. This key driver has been overshadowed by emphasis on Laplace’s disk model, hot homogeneous accretion, and mantle convection. Instead, accretion was likely cold and heterogeneous, and resultant planetary assembly directly converted gravitational potential energy into axial spin, a prominent feature of planets that is otherwise unexplained. Frictional dissipation of this spin contributed up to 1 MJ/kg of specific energy to Earth and Venus over geologic time, compared to ~ 2 MJ/kg that would have been contributed by the decay of K, U and Th. Spin dissipation can be nearly as important as radioactivity. The combined specific energies of the rocky bodies go as:

Earth ~ Venus >> Moon > Mars > Mercury > Vesta

Small objects generate little heat by spin loss, and lose their internal heat rapidly. Thus, this order is nearly congruent with object mass, as well as with the gamma ray intensities of these bodies, and their relative magmatic and tectonic activities. Power produced by radionuclides and spin loss both decline exponentially on Ga timescales and is preferentially delivered to upper zones where rocks are weakest. Specific energy of the Moon is inordinately large and perhaps overestimated, due to its surficial Th anomaly.

Activity is affected by distance (r) from the Sun. (1) Proximity rapidly despun Venus and Mercury, so their tectonic activity ceased long ago, in contrast to the Earth and even Mars. (2) Post-accretion impacts of km-sized bodies promoted early surface process, especially on the innermost bodies since impactor flux varies as ~1/r2, and the energy of each goes as ~1/r. (3) Blackbody temperatures go as 1/√r and determine the physical state of H2O and other volatiles, which affect planetary magmatism and differentiation. This temperature and object mass explain atmosphere retention and constitution across the Solar System, with volatile loss limiting igneous differentiation. (4) Earth is far enough from the Sun to have retained its water, spin, and Moon, whose presence drives surficial plate tectonics. All other rocky objects are long-since dead, except for Io which is also gravitationally pumped.