Query: Venus is not hot because of a runaway greenhouse.

Relevant passages:
Venus <=SEP=> Venus is the second planet from the Sun , orbiting it every 224.7 Earth days . It has the longest rotation period ( 243 days ) of any planet in the Solar System and rotates in the opposite direction to most other planets . It has no natural satellites . It is named after the Roman goddess of love and beauty . It is the second-brightest natural object in the night sky after the Moon , reaching an apparent magnitude of − 4.6 , bright enough to cast shadows at night and , though rare , occasionally be visible in broad daylight . Because Venus orbits within Earth 's orbit it is an inferior planet and never appears to venture far from the Sun ; its maximum angular distance from the Sun ( elongation ) is 47.8 ° .   Venus is a terrestrial planet and is sometimes called Earth 's `` sister planet '' because of their similar size , mass , proximity to the Sun , and bulk composition . It is radically different from Earth in other respects . It has the densest atmosphere of the four terrestrial planets , consisting of more than 96 % carbon dioxide . The atmospheric pressure at the planet 's surface is 92 times that of Earth , or roughly the pressure found 900 m underwater on Earth . Venus is by far the hottest planet in the Solar System , with a mean surface temperature of 735 K , even though Mercury is closer to the Sun . Venus is shrouded by an opaque layer of highly reflective clouds of sulfuric acid , preventing its surface from being seen from space in visible light . It may have had water oceans in the past , but these would have vaporized as the temperature rose due to a runaway greenhouse effect . The water has probably photodissociated , and the free hydrogen has been swept into interplanetary space by the solar wind because of the lack of a planetary magnetic field . Venus 's surface is a dry desertscape interspersed with slab-like rocks and is periodically resurfaced by volcanism .   As one of the brightest objects in the sky , Venus has been a major fixture in human culture for as long as records have existed . It has been made sacred to gods of many cultures , and has been a prime inspiration for writers and poets as the `` morning star '' and `` evening star '' . Venus was the first planet to have its motions plotted across the sky , as early as the second millennium BC .   As the closest planet to Earth , Venus has been a prime target for early interplanetary exploration . It was the first planet beyond Earth visited by a spacecraft ( Mariner 2 in 1962 ) , and the first to be successfully landed on ( by Venera 7 in 1970 ) . Venus 's thick clouds render observation of its surface impossible in visible light , and the first detailed maps did not emerge until the arrival of the Magellan orbiter in 1991 . Plans have been proposed for rovers or more complex missions , but they are hindered by Venus 's hostile surface conditions .

Greenhouse effect <=SEP=> The greenhouse effect is the process by which radiation from a planet 's atmosphere warms the planet 's surface to a temperature above what it would be without its atmosphere .   If a planet 's atmosphere contains radiatively active gases ( i.e. , greenhouse gases ) the atmosphere will radiate energy in all directions . Part of this radiation is directed towards the surface , warming it . The downward component of this radiation -- that is , the strength of the greenhouse effect -- will depend on the atmosphere 's temperature and on the amount of greenhouse gases that the atmosphere contains .   On Earth , the atmosphere is warmed by absorption of infrared thermal radiation from the underlying surface ( heated by the sun ) , absorption of shorter wavelength radiant energy from the sun , and convective heat fluxes from the surface . Greenhouse gases in the atmosphere radiate energy , some of which is directed to the surface and lower atmosphere . The mechanism that produces this difference between the actual surface temperature and the effective temperature is due to the atmosphere and is known as the greenhouse effect .   Earth 's natural greenhouse effect is critical to supporting life . Human activities , primarily the burning of fossil fuels and clearing of forests , have intensified the natural greenhouse effect , causing global warming .   The mechanism is named after a faulty analogy with the effect of solar radiation passing through glass and warming a greenhouse . The way a greenhouse retains heat is fundamentally different , as a greenhouse works by reducing airflow and retaining warm air inside the structure .

Carl Sagan <=SEP=> Carl Edward Sagan ( -LSB- ˈseɪɡən -RSB- November 9 , 1934 -- December 20 , 1996 ) was an American astronomer , cosmologist , astrophysicist , astrobiologist , author , science popularizer , and science communicator in astronomy and other natural sciences . He is best known for his work as a science popularizer and communicator . His best known scientific contribution is research on extraterrestrial life , including experimental demonstration of the production of amino acids from basic chemicals by radiation . Sagan assembled the first physical messages sent into space : the Pioneer plaque and the Voyager Golden Record , universal messages that could potentially be understood by any extraterrestrial intelligence that might find them . Sagan argued the now accepted hypothesis that the high surface temperatures of Venus can be attributed to and calculated using the greenhouse effect .   Sagan published more than 600 scientific papers and articles and was author , co-author or editor of more than 20 books . He wrote many popular science books , such as The Dragons of Eden , Broca 's Brain and Pale Blue Dot , and narrated and co-wrote the award-winning 1980 television series Cosmos : A Personal Voyage . The most widely watched series in the history of American public television , Cosmos has been seen by at least 500 million people across 60 different countries . The book Cosmos was published to accompany the series . He also wrote the science fiction novel Contact , the basis for a 1997 film of the same name . His papers , containing 595,000 items , are archived at The Library of Congress .   Sagan advocated scientific skeptical inquiry and the scientific method , pioneered exobiology and promoted the Search for Extra-Terrestrial Intelligence ( SETI ) . He spent most of his career as a professor of astronomy at Cornell University , where he directed the Laboratory for Planetary Studies . Sagan and his works received numerous awards and honors , including the NASA Distinguished Public Service Medal , the National Academy of Sciences Public Welfare Medal , the Pulitzer Prize for General Non-Fiction for his book The Dragons of Eden , and , regarding Cosmos : A Personal Voyage , two Emmy Awards , the Peabody Award and the Hugo Award . He married three times and had five children . After suffering from myelodysplasia , Sagan died of pneumonia at the age of 62 , on December 20 , 1996 .

Runaway greenhouse effect <=SEP=> A runaway greenhouse effect is a process in which a net positive feedback between surface temperature and atmospheric opacity increases the strength of the greenhouse effect on a planet until its oceans boil away . An example of this is believed to have happened in the early history of Venus . On the Earth , the IPCC states that `` a ` runaway greenhouse effect ' -- analogous to -LSB- that of -RSB- Venus -- appears to have virtually no chance of being induced by anthropogenic activities . ''   Other large-scale climate changes are sometimes loosely called a `` runaway greenhouse effect '' although it is not an appropriate description . For example , it has been hypothesized that large releases of greenhouse gases may have occurred concurrently with the Permian -- Triassic extinction event or Paleocene -- Eocene Thermal Maximum . Other terms , such as `` abrupt climate change '' , or tipping points could be used when describing such scenarios .