Venus is classified as a terrestrial planet and it is sometimes called Earth's "sister planet" due to the similar size, gravity, and bulk composition. Venus is covered with an opaque layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light. Venus has the densest atmosphere of all the terrestrial planets in our solar system, consisting mostly of carbon dioxide. Venus has no carbon cycle to lock carbon back into rocks and surface features, nor does it seem to have any organic life to absorb it in biomass. A younger Venus is believed to have possessed Earth-like oceans,[11] but these evaporated as the temperature rose. Venus's surface is a dusty dry desertscape with many slab-like rocks, periodically refreshed by volcanism. The water has most likely dissociated, and, because of the lack of a planetary magnetic field, the hydrogen has been swept into interplanetary space by the solar wind.[12] The atmospheric pressure at the planet's surface is 92 times that of the Earth.
The Venusian surface was a subject of speculation until some of its secrets were revealed by planetary science in the twentieth century. It was finally mapped in detail by Project Magellan in 1990–91. The ground shows evidence of extensive volcanism, and the sulfur in the atmosphere may indicate that there have been some recent eruptions.[13][14] However, the absence of evidence of lava flow accompanying any of the visible caldera remains an enigma. The planet has few impact craters, demonstrating that the surface is relatively young, approximately 300–600 million years old.[15][16] There is no evidence for plate tectonics, possibly because its crust is too strong to subduct without water to make it less viscous. Instead, Venus may lose its internal heat in periodic massive resurfacing events.[15]
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