ABSTRACT Most differences in the gross surface morphologies, tectonic styles, overall geologic histories, and atmospheres of the rocky bodies in the solar system can be explained by contributions and dissipation of gravitational and radiogenic energy over geologic time. These two energy sources are large and measurable and can be extrapolated back in time. Accretion was likely cold, and directly converted gravitational potential energy into axial spin, a prominent feature of planets that is otherwise unexplained. Impact heating was mostly limited to planetary surfaces in the final stages of accretion. Frictional dissipation of spin contributed sufficient energy to ignite the primordial Sun and heated Earth and Venus by nearly as much as has the radioactive decay of K, U, and Th over geologic time. Energy inputs have been continuously offset by loss of heat to the surroundings. The magnitudes of most important energy contributions depend on the planet radius R and also on the distance r to the Sun. Quantitative, albeit approximate, relationships show that the net specific energy (kJ/kg) contributed to the rocky bodies over geologic time goes as: Earth ~ Venus >> Mars ~ Mercury ~ Moon >> asteroids. Net energy inputs increased the average internal temperatures of Earth and Venus by ~3000 K but heated asteroids by only a few hundred kelvins.

ABSTRACT Layered deposits on Mars imaged by the three rovers are generally inferred to have been deposited by liquid water (or wind or volcanism), consistent with interpretations based on orbital imaging. This interpretation implies early Mars was warm and wet, despite long-standing problems with modeling this case. As an alternative hypothesis, rapid sediment deposition during Late Noachian impact bombardment followed by local hydration and alteration of sediment by surficial acid condensates and (at least in Gale Crater) by chemically neutral groundwater can explain all the observed sediment features, such as ubiquitous low-angle cross-bedding, primitive basaltic compositions, persistent acidic salts, abundant amorphous materials, immature clays, high friability with low bulk densities, planar scoured unconformities, and rounded cobbles from rock tumbling. In other words, the ground-observed mineralogy, geochemistry, and sedimentology do not require and even are inconsistent with deposition from liquid water. Unlike the Moon, early Mars is believed to have had an atmosphere and water, perhaps mostly frozen. If so, impacts should have formed turbulent ground-hugging impactoclastic density currents capable of traveling hundreds of kilometers, and even globally. As terrestrial analogs, smaller-scale density currents are widespread around explosive volcanoes and nuclear test sites, whereas terrestrial impact analogs are lacking. Steam condensation on particles causes accretionary lapilli to form, grow to a maximum size, and fall out on layered deposits, and similar spherules have been observed by two of three rovers. Explaining these spherules as normal sedimentary concretions at Meridiani Planum required ignoring some of the observations. Ancient sediments on Mars that superficially resemble terrestrial aqueous deposits could therefore actually have resulted from impact cratering, the dominant geologic process in the early solar system.