Strange relatives - Earth and her sibling neighbours

Abstract

Comparative planetology has brought to light some general themes in the geological evolution of planets and planetary bodies. Particularly the study of the Moon and the Earth-like planets Venus and Mars has contributed much to the understanding of the early evolution of the Earth. All have sizes and distances to the Sun of the same order of magnitude, are primarily composed of cosmically rare silicates and metals, and underwent differentiation into core, mantle and crust. The crust is basaltic on Venus and Mars, while on the Moon the crust is anorthositic in the highlands and basaltic in the maria. Earth has, besides a basaltic crust in the ocean basins, as the only planet in the Solar System a continental crust of average dioritic composition. This continental crust owes its genesis to the interaction between ocean water, basaltic crust, and mantle dynamics. All planetary bodies suffered an intense bombardment of meteoroids early in their histories. Impacting was probably the dominant geological process in that time, but on Earth virtually no traces of the early impacts are preserved because of later geological activities. The differences in environmental conditions and geological evolution between Venus, Mars and Earth are primarily determined by differences in size (cooling rate and gravity) and distance to the Sun (solar energy input). Only Earth has a hospitable environment suitable for life - conditions that are maintained by the biosphere. On Venus infernal conditions prevail, with a surface temperature of 480°C, an atmosphere almost entirely composed of CO2, an atmospheric surface pressure of 88 bar, and no water. Very little is known about tectonic activities because of the dense sulfuric-acid cloud cover, but volcanism appears to be still active. Mars is a barren desert with extreme variations in temperature, and a very thin atmosphere with a surface pressure of 0.0064 bar almost entirely composed of CO2. Water is present as ice in polar caps and in permafrost. Both Venus and Mars possessed early in their histories copious amounts of water (and Mars possibly also life ?). Venus lost all water because of 'runaway greenhouse' conditions, while Mars probably lost most of his volatiles because of the early episode of large-scale impacting - which ravaged all planets, but was more effective on the martian atmosphere because of the planet's smaller gravity. There are no indications that plate tectonics was ever in operation on Mars, but until fairly recently (and maybe up to now) there was active volcanism, giving rise to the largest volcanoes in the Solar System.