Venus is the second-closest planet to the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love. It is the brightest natural object in the night sky, except for the Moon, reaching an apparent magnitude of −4.6. Because Venus is an inferior planet from Earth, it never appears to venture far from the Sun: its elongation reaches a maximum of 47.8°. Venus reaches its maximum brightness shortly before sunrise or shortly after sunset, for which reason it is often called the Morning Star or the Evening Star.
Classified as a terrestrial planet, it is sometimes called Earth's "sister planet," because the two are similar in 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; this was a subject of great speculation until some of its secrets were revealed by planetary science in the twentieth century. Venus has the densest atmosphere of all the terrestrial planets, consisting mostly of carbon dioxide, as it has no carbon cycle to lock carbon back into rocks and surface features, nor organic life to absorb it in biomass. It has become so hot that the earth-like oceans that the young Venus is believed to have possessed have totally evaporated, leaving a dusty dry desertscape with many slab-like rocks. The best hypothesis is that the evaporated water has dissociated, and with the lack of a planetary magnetic field, the hydrogen has been swept into interplanetary space by the solar wind. The atmospheric pressure at the planet's surface is 92 times that of the Earth.
Venus's surface has been mapped in detail only in the last 22 years, by Project Magellan. It shows evidence of extensive volcanism, and the sulfur in the atmosphere is taken by some experts to show that there has been some recent volcanism, but it is an enigma as to why no evidence of lava flow accompanies any of the visible caldera. There is a surprisingly low number of impact craters, demonstrating that the surface is relatively young, approximately half a billion years old. There is no evidence for plate tectonics, possibly because its crust is too strong to subduct without water to make it less viscous, and some suggest that instead Venus loses its internal heat in periodic massive resurfacing events.
The adjective Venusian is commonly used for items related to Venus, though the Latin adjective is the rarely used Venerean; the archaic Cytherean is still occasionally encountered. Venus is the only planet in the Solar System named after a female figure, although two dwarf planets — Ceres and Eris — also have feminine names.
Venus is one of the four solar terrestrial planets, meaning that, like the Earth, it is a rocky body. In size and mass, it is very similar to the Earth, and is often described as its 'sister'. The diameter of Venus is only 650 km less than the Earth's, and its mass is 81.5% of the Earth's. However, conditions on the Venusian surface differ radically from those on Earth, due to its dense carbon dioxide atmosphere. The mass of the atmosphere of Venus is 96.5% carbon dioxide, with most of the remaining 3.5% composed of nitrogen.
Internal structure
Without seismic data or knowledge of its moment of inertia, there is little direct information about the internal structure and geochemistry of Venus. However, the similarity in size and density between Venus and Earth suggests that they share a similar internal structure: a core, mantle, and crust. Like that of Earth, the Venusian core is thought to be at least partially liquid. The slightly smaller size of Venus suggests that pressures are significantly lower in its deep interior than Earth. The principal difference between the two planets is the lack of plate tectonics on Venus, likely due to the dry surface and mantle. This results in reduced heat loss from the planet, preventing it from cooling and providing a likely explanation for its lack of an internally generated magnetic field.
