Conditions on the surface of Venus differ radically from those on Earth because its dense atmosphere is 96.5% carbon dioxide, causing an intense greenhouse effect, with most of the remaining 3.5% being nitrogen.

[40] The southern continent is called Aphrodite Terra, after the Greek mythological goddess of love, and is the larger of the two highland regions at roughly the size of South America.

[50][51] The stratigraphically oldest tessera terrains have consistently lower thermal emissivity than the surrounding basaltic plains measured by Venus Express and Magellan, indicating a different, possibly a more felsic, mineral assemblage.

[61][62] This massive volcanic activity is fuelled by a hot interior, which models say could be explained by energetic collisions when the planet was young, as well as radioactive decay as in the case of the earth.

Impacts would have had significantly higher velocity than on Earth, both because Venus moves faster due to its closer proximity to the Sun and because high-eccentricity objects colliding with the planet would have high speeds.

[63] In 2008 and 2009, the first direct evidence for ongoing volcanism was observed by Venus Express, in the form of four transient localized infrared hot spots within the rift zone Ganis Chasma,[64][note 1] near the shield volcano Maat Mons.

The number of craters, together with their well-preserved condition, indicates the planet underwent a global resurfacing event 300–600 million years ago,[44][45] followed by a decay in volcanism.

[12][76] The principal difference between the two planets is the lack of evidence for plate tectonics on Venus, possibly because its crust is too strong to subduct without water to make it less viscous.

This erosion process results in a steady loss of low-mass hydrogen, helium, and oxygen ions, whereas higher-mass molecules, such as carbon dioxide, are more likely to be retained.

[4] Because of its runaway greenhouse effect, Venus has been identified by scientists such as Carl Sagan as a warning and research object linked to climate change on Earth.

[98][99] Studies have suggested that billions of years ago, the atmosphere of Venus may have been much more like the one surrounding the early Earth, and there may have been substantial quantities of liquid water on the surface.

[100][101][102] After a period of 600 million to several billion years,[103] the rising luminosity of the Sun and possibly large volcanic resurfacing caused the evaporation of the original water.

[133] Although Venus has no seasons, in 2019 astronomers identified a cyclical variation in sunlight absorption by the atmosphere, possibly caused by opaque, absorbing particles suspended in the upper clouds.

The variation causes observed changes in the speed of Venus's zonal winds and appears to rise and fall in time with the Sun's 11-year sunspot cycle.

Alex Alemi's and David Stevenson's 2006 study of models of the early Solar System at the California Institute of Technology shows Venus likely had at least one moon created by a huge impact event billions of years ago.

[165] About 10 million years later, according to the study, another impact reversed the planet's spin direction and the resulting tidal deceleration caused the Venusian moon gradually to spiral inward until it collided with Venus.

[185] When Venus is sufficiently bright with enough angular distance from the sun, it is easily observed in a clear daytime sky with the naked eye, though most people do not know to look for it.

[196] Nonetheless, a cylinder seal from the Jemdet Nasr period and the Venus tablet of Ammisaduqa from the First Babylonian dynasty indicate that the ancient Sumerians already knew that the morning and evening stars were the same celestial object.

[207][note 4] When the Italian physicist Galileo Galilei first observed the planet with a telescope in the early 17th century, he found it showed phases like the Moon, varying from crescent to gibbous to full and vice versa.

This could be possible only if Venus orbited the Sun, and this was among the first observations to clearly contradict the Ptolemaic geocentric model that the Solar System was concentric and centred on Earth.

[210][211] The 1639 transit of Venus was accurately predicted by Jeremiah Horrocks and observed by him and his friend, William Crabtree, at each of their respective homes, on 4 December 1639 (24 November under the Julian calendar in use at that time).

Later, American astronomer Chester Smith Lyman observed a complete ring around the dark side of the planet when it was at inferior conjunction, providing further evidence for an atmosphere.

[223] Venera 3, launched in 1966, became humanity's first probe and lander to reach and impact another celestial body other than the Moon, but could not return data as it crashed into the surface of Venus.

[226] In 1974, Mariner 10 swung by Venus to bend its path towards Mercury and took ultraviolet photographs of the clouds, revealing the extraordinarily high wind speeds in the Venusian atmosphere.

[236][237][238] Such speculations go back to 1967, when Carl Sagan and Harold J. Morowitz suggested in a Nature article that tiny objects detected in Venus's clouds might be organisms similar to Earth's bacteria (which are of approximately the same size): In August 2019, astronomers led by Yeon Joo Lee reported that long-term pattern of absorbance and albedo changes in the atmosphere of the planet Venus caused by "unknown absorbers", which may be chemicals or even large colonies of microorganisms high up in the atmosphere of the planet, affect the climate.

[240] In September 2020, a team of astronomers led by Jane Greaves from Cardiff University announced the likely detection of phosphine, a gas not known to be produced by any known chemical processes on the Venusian surface or atmosphere, in the upper levels of the planet's clouds.

The discovery prompted NASA administrator Jim Bridenstine to publicly call for a new focus on the study of Venus, describing the phosphine find as "the most significant development yet in building the case for life off Earth".

Though by the Roman era they were recognized as one celestial object, known as "the star of Venus", the traditional two Greek names continued to be used, though usually translated to Latin as Lūcifer and Vesper.

Modern Chinese, Japanese, Korean and Vietnamese cultures refer to the planet literally as the "metal star" (金星), based on the Five elements.

The impenetrable Venusian cloud cover gave science fiction writers free rein to speculate on conditions at its surface; all the more so when early observations showed that not only was it similar in size to Earth, it possessed a substantial atmosphere.