Venus is the second planet from the Sun and one of the four terrestrial planets of the Solar System. It is similar to Earth in approximate size, mass and bulk composition, which is why it is sometimes called Earths planetary sibling, but its surface environment is profoundly different. Venus has a dense atmosphere dominated by carbon dioxide, thick clouds containing sulphuric acid droplets, extremely high surface pressure and a surface temperature around 465 degrees Celsius on average.
Its dense atmosphere produces an exceptionally powerful greenhouse effect, making Venus the hottest planet in the Solar System despite Mercury being closer to the Sun. Venus rotates very slowly and in the retrograde direction relative to most planets, meaning that its rotation is opposite to the direction of its orbital motion. The planet has no known natural satellites and no intrinsic global magnetic field comparable to Earths. Its present environment reflects a complex history involving volcanism, atmospheric evolution, water loss, solar radiation and geological processes.
Venus has a predominantly rocky interior consisting of a metallic core, a silicate mantle and a rocky crust. Its interior structure is broadly comparable to that of Earth, but its tectonic and thermal evolution differs substantially. Venus lacks an Earth like global system of moving tectonic plates, although its surface contains extensive deformation, fractures, mountain belts, volcanic structures and other evidence of geological activity.
Heat from the planets interior must therefore escape through processes that differ from Earths plate tectonic recycling system. Large scale resurfacing events may have occurred in the geological past, although the timing and nature of such episodes remain subjects of scientific investigation. Venus has thousands of volcanic features, including shield volcanoes, volcanic plains, lava flows, coronae and other structures. The abundance of volcanic terrain demonstrates that volcanism has been a major force in shaping the planet and raises important questions about whether volcanism continues today.
The atmosphere of Venus is composed primarily of carbon dioxide, with nitrogen as the second most abundant major component and smaller quantities of other gases. Atmospheric pressure at the surface is roughly 92 times Earths sea level pressure. Thick clouds extend through the atmosphere and strongly affect how sunlight and infrared radiation move through the system. The carbon dioxide rich atmosphere absorbs outgoing infrared radiation very efficiently, while the massive atmospheric column also produces intense pressure at the surface.
Venus therefore experiences a runaway or near runaway greenhouse climate in which high temperatures and atmospheric water loss became linked through feedback processes. Water vapour is itself a greenhouse gas, so warmer conditions can increase atmospheric water vapour, which can amplify warming. The loss of water and subsequent atmospheric chemistry allowed Venus to evolve into an extremely dry world. Understanding this history helps scientists investigate possible climate trajectories of rocky planets and the conditions under which Earth like environments can diverge dramatically.
Venus has an unusual rotation. It takes approximately 243 Earth days to rotate once relative to distant stars, longer than its approximately 225 Earth day orbital period. Its retrograde rotation means that the Sun would appear to rise in the west and set in the east if observed from the surface under suitable conditions. Because of its atmospheric and rotational dynamics, the Venusian day has a complicated definition depending on whether one means sidereal rotation or solar day.
The planet has only a small axial tilt in one convention, but because the rotation is retrograde, its seasonal geometry differs from Earths. Venus orbit is comparatively close to circular. Its dense atmosphere, slow rotation and complex circulation produce atmospheric behaviour unlike Earths, including rapid upper atmospheric winds that move much faster than the solid planet rotates, a phenomenon called superrotation.
The surface of Venus is hidden from ordinary optical telescopes by its opaque cloud cover, so much of our detailed knowledge comes from radar mapping and spacecraft measurements. Radar can penetrate the clouds and reveal surface topography. Venus contains vast volcanic plains, mountain regions, highlands, impact craters, tectonic structures and numerous volcanic edifices.
Because the planet has relatively few obvious small impact craters compared with heavily cratered bodies such as Mercury, scientists infer that much of its surface has been geologically resurfaced at relatively recent times in planetary terms. The absence of extensive erosion by liquid water means that many landforms reflect volcanic and tectonic processes rather than fluvial erosion. Some radar observations have also provided evidence consistent with recent or possibly ongoing volcanic activity. Understanding the chronology of Venusian surface features remains difficult because direct radiometric dating of rocks across the planet is extremely limited.
Venus has no intrinsic global magnetic field generated by a present Earth like dynamo, but it does possess an induced magnetosphere created through interaction between its electrically conducting ionosphere and the solar wind. The absence of a strong intrinsic magnetic shield may have contributed to atmospheric escape processes, although atmospheric loss is controlled by many factors including solar ultraviolet radiation, particle interactions and the properties of the upper atmosphere.
Venus receives stronger solar radiation than Earth because it is closer to the Sun. The upper atmosphere contains complex photochemistry driven by ultraviolet sunlight. Sulphur compounds contribute to the formation of the planets thick clouds, and the atmospheric chemistry is closely linked to volcanic gases and photochemical reactions. Studying Venus therefore connects planetary geology, atmospheric chemistry, plasma physics and climate science.
Venus has been visited by numerous spacecraft, including Soviet Venera missions that achieved the first successful landings on another planets surface and returned measurements under the extreme conditions. Other missions performed atmospheric studies, imaging and radar mapping, including Magellan, which produced a detailed global radar map. More recent and planned missions are intended to investigate Venusian geology, atmospheric composition and climate history in greater detail.
Venus is particularly valuable for comparative planetology because Earth and Venus are similar in size and likely formed from broadly related materials, yet they evolved into dramatically different environments. Understanding why Venus lost most of its water, developed a dense carbon dioxide atmosphere and lacks Earth like plate tectonics can help scientists interpret the evolution of rocky exoplanets around other stars.
Venus is often confused with a star because it is exceptionally bright in Earths sky, but it is a planet and shines mainly by reflecting sunlight. It is commonly called the morning star or evening star depending on whether it appears before sunrise or after sunset, although it is neither a star nor physically producing its own visible light.
Venus is also sometimes described as Earths twin because of its size and mass, but this comparison should not imply similar habitability or surface conditions. Its surface is hot enough to melt some metals, its atmospheric pressure is enormous, and its clouds are composed of corrosive sulphuric acid droplets. Nevertheless, Venus is scientifically important precisely because it demonstrates how similar starting conditions can lead to radically different planetary climates. Its study provides insight into greenhouse physics, atmospheric escape, volcanic evolution and the limits of Earth like planetary environments.
