This survey explores the evolution of thinking about the climates of Venus and Mars as life-support systems, in comparison to Earth. While hopes for Venus as an abode of life ended, the search for evidence of past life on Mars, possibly microbial, remains a central theme in space exploration. The high temperatures and pressures found on Venus supported a "runaway greenhouse theory," and Mars harbored an apparently lifeless landscape similar to the surface of the Moon. Missions to Venus and Mars revealed strikingly different worlds. A core element of this belief rested with the climatology of these two planets, as observed by astronomers, but these ideas were significantly altered, if not dashed during the space age. Seasonal changes on Mars were interpreted as due to the possible spread and retreat of ice caps and lichen-like vegetation. Mars also harbored interest as a place where life had or might still exist. These attributes brought myriad speculations about the nature of Venus, its climate, and the possibility of life existing there in some form. It was also the closest planet to Earth, with nearly the same size and surface gravity. Venus had long enchanted humans-all the more so after astronomers realized it was shrouded in a mysterious cloak of clouds permanently hiding the surface from view.
Venus-Earth-Mars: comparative climatology and the search for life in the solar system.īoth Venus and Mars have captured the human imagination during the twentieth century as possible abodes of life. Finally, the middle periods in the history of the terrestrial planets are compared, and future prospects for the exploration of the inner planets as well as other rocky bodies in the solar system are discussed. The origin and evolution of the moon are discussed on the basis of the Apollo results, and current knowledge of Mercury and Mars is examined in detail. Mechanisms for the modification of planetary surfaces by external factors and from within the planet are examined, including surface cycles, meteoritic impact, gravity, wind, plate tectonics, volcanism and crustal deformation. Past and current views of the origin of the earth, moon, Mercury, Venus and Mars are discussed, and the surface features characteristic of the moon, Mercury, Mars and Venus are outlined. The surfaces of the earth and the other terrestrial planets of the inner solar system are reviewed in light of the results of recent planetary explorations. Charts and graphs are presented on planetary surface properties, rotational parameters, atmospheric compositions, and astronomical characteristics.Įarthlike planets: Surfaces of Mercury, Venus, earth, moon, Mars Spacecraft- and Earth-based studies on the physical nature of the planets Mercury, Venus, and Mars are reported. Thus we are not constrained to a Mars-like model of Venus tectonics by craters and possible mantle dryness an Earth-like model is equally probableĮarth-type planets (Mercury, Venus, and Mars) Because erosional liberation of 40 Ar on Venus will be relatively inefficient, this range for 40 Ar abundance at least permits an active tectonic history, and if the 40 Ar abundance is towards the high end of the range, it may well require an active tectonic history.
The abundance of 40 Ar in the atmosphere of Venus lies somewhere between the Earth value and one-tenth of the Earth value. Geochemical models of solar system origin and petrological considerations suggest that K is about as abundant in Venus as in Earth. Due to the absence of liquid water, erosion and deposition will be much slower on Venus than on Earth, favoring retention of primordial cratered surfaces on portions of the crust that have not been destroyed or buried by tectonic and volcanic activity. If a trace of volatiles is present in the mantle, the lithosphere of Venus could be thinner. However, crust and upper mantle temperatures on Venus are very likely higher than on Earth so that a dry Venus could have a lithosphere with a thickness similar to that of Earth.
The presence of presumably primordial large craters has led to the suggestion that Venus may have a thick lithosphere like that of Mars despite its similarities to Earth in size and density. International Nuclear Information System (INIS) Venus tectonics: another Earth or another Mars
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