Venus: A planetary window into early Earth

Abstract

Venus is the most similar planet to the Earth in the solar system in terms of planet size, mass and orbital velocity. Since the early 1960s, Venus has been an important target for human deep space exploration. In this contribution, the exploration history of Venus, the known and unknown of Venus, the main scientific problems of Venus, and the future international exploration plan of Venus are briefly reviewed. The early exploration of Venus is represented by the "Венера plan" of the Soviet Union and the "Mariner series" of the United States, whereas more recent missions are represented by the "Venus Express" and "Akatsuki" launched for other countries. These missions provide valuable data which significantly enhanced our understanding of Venus atmosphere composition, surface topography and internal structure. The atmosphere of Venus is dominated by CO2 and N2, which is different from the atmospheric composition of the present Earth, similar to that of the early Earth. Some modeling results suggest that Venus was once covered by ocean although no liquid water exists any longer on Venus. The analyses of the surface compositions of Venus by a series of explorers show that Venus surface is mainly composed of basalt. In terms of topography, due to the special surface environment of Venus, the surface weathering of Venus is very weak and does not affect surface topography. The topographies of Venus are mainly controlled by volcanic resurfacing and differ with the Earth significantly. The dominant topographies include regional plains, shield plains, corona and tessera, which are produced by mantle plume-lithosphere interaction or mantle convection. No topography related to plate tectonics has been found on Venus. At present, Venus lacks many large and active volcanic hotspots. Although it is difficult to estimate the accurate volcanism rate, the volcanism rate is much smaller than that of the current earth. In terms of internal structure, Venus has a core-mantle-crust structure similar to that of Earth, but there is no internal magnetic field on Venus. In addition, Venus and Earth have similar densities and have similar compositions, for example, the mantle of Venus is likely to be composed of peridotite which is similar to that of Earth. No internal magnetic field and lack of plate tectonics are two important characteristics of Venus. The most popular explanation for the absence of magnetic field on Venus is that Venus lacks convection in its core and cannot develop its own dynamo. Three explanations were proposed to account for the absence of plate tectonics on Venus, the extremely hot surface, the lack of asthenosphere, and the lack of water, among which the last one is the most widely accepted. Considering many aspects including atmosphere composition, surface rock assemblages and tectonics, Venus is very similar to the early Earth. Venus provides the best natural laboratory for us to understand the evolution of the Earth and other terrestrial planets. Understanding why Venus and Earth evolve into different directions is one of the most important means to study the genesis and evolution of planet habitability and has significant bearings for studying the potential habitability of exoplanets. Venus has been one of the highest priority of deep space exploration targets. In recent years, the United States, Russia, Europe and other countries have put forward their own Venus exploration plans for above scientific problems, such as whether there was a sea on the early Venus, the unhabitability of Venus, and the structure and gravity field. China should and must do something in the new international competition.

Keywords

• venus
• early earth
• venus atmosphere
• venus morphology
• plate tectonics