Venus

• 1. Introduction

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  Venus is the second planet from the Sun, and it is the brightest object in Earth's sky after the Sun and the Moon. On any clear night, it outshines every star and every other planet, glowing so intensely that it has been spotted in broad daylight by people who had no idea what they were looking at. French emperor Napoleon Bonaparte was one such observer, encountering it unexpectedly at a reception in Luxembourg. At the inauguration of Abraham Lincoln on the 4th of March 1865 in Washington, D.C., Venus hung visible in the daytime sky above the crowd.

  For centuries, that blazing light felt familiar, even intimate. Poets from Homer to William Wordsworth wrote verses about it. Cultures from ancient Babylon to the Maya tracked its cycles with painstaking care. And yet, hidden beneath its brilliant clouds, Venus is a place of almost unimaginable hostility. Its surface temperature averages 737 Kelvin. Its atmospheric pressure is 92 times greater than Earth's at sea level. Its clouds are made of sulfuric acid.

  How did a planet so similar to Earth in size and mass become the most extreme environment in the Solar System? Why does it spin backward, and so slowly that a single day lasts longer than its entire year? Could anything be alive in its clouds right now? Those are the questions this documentary sets out to answer.

• 2. A World Like No Other

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  Venus has a diameter of 12,103.6 km, only 638.4 km less than Earth's, and its mass is 81.5% of Earth's. Scientists sometimes call it Earth's "sister" or "twin." But the resemblance stops at the surface.

  The atmosphere of Venus is 96.5% carbon dioxide, with most of the remaining 3.5% being nitrogen. Together they generate the strongest greenhouse effect in the Solar System, pushing surface temperatures to at least 462 degrees Celsius. That is hotter than Mercury, even though Venus is nearly twice as far from the Sun. Mercury receives about four times as much solar energy per square meter as Venus does, yet Venus wins the heat contest.

  At ground level, the atmosphere reaches a pressure of 9.3 MPa, equivalent to being nearly 1 kilometer beneath Earth's ocean surface. The air itself is so dense, at 65 kilograms per cubic meter, that it would feel more like wading through liquid than breathing a gas. Surface winds are only a few kilometers per hour, but because of that density, they carry enough force to push dust and small stones across the landscape.

  Above the surface, thick cloud layers sit between 45 and 70 km altitude, composed mainly of sulfuric acid formed when UV radiation reacts with sulfur dioxide and water. These clouds cover the entire planet. They are so reflective that Venus bounces back roughly 70% of incoming sunlight, giving it one of the highest albedos in the Solar System. Yet only about 10% of sunlight ever makes it through to the ground, where daytime illumination averages around 14,000 lux, comparable to an overcast day on Earth.

  The sky seen from the surface is orange-yellow. Higher in the atmosphere it turns white. No observer has ever seen the Sun from Venus directly, because the clouds never part.

• 3. Backward and Slow

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  Venus rotates clockwise on its axis, opposite to the direction of nearly every other planet, including Earth. This retrograde rotation means that on Venus, the Sun rises in the west and sets in the east, though the permanent cloud deck makes it impossible to watch from the surface.

  The rotation is also extraordinarily slow. A Venusian sidereal day lasts 243 Earth days, which is longer than a Venusian year of 224.7 Earth days. That peculiar situation means a day on Venus is longer than its year. A Venusian solar day, the time from one noon to the next, works out to 116.75 Earth days, about half a Venusian year.

  Scientists believe Venus may have formed with a different rotation period and orientation. Over billions of years, competing forces reshaped it: the Sun's gravitational tidal pull tended to slow and lock the rotation, while the thick atmosphere, unevenly heated by the Sun, pushed back in the opposite direction. The result is a kind of dynamic equilibrium between these two forces. The day length is not even perfectly stable; it fluctuates by up to 20 minutes due to the same atmospheric effects.

  Measurements by the Magellan spacecraft across a 500-day observing period showed a rotation period slightly different from what a 16-year comparison between Magellan and Venus Express suggested. The difference was about 6.5 minutes, a reminder that the planet's spin is still not fully settled.

  Also missing from Venus is any natural satellite. A 2006 study by Alex Alemi and David Stevenson at the California Institute of Technology proposed that Venus once had at least one moon, formed by a massive ancient impact. A later impact, they argued, reversed the planet's spin direction, and the resulting tidal forces caused that moon to spiral inward until it collided with the planet.

• 4. Volcanoes Everywhere

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  Venus has more than 85,000 identified and mapped volcanoes. That number dwarfs Earth's volcanic inventory, not because Venus is more volcanically active, but because its crust does not recycle through plate tectonics the way Earth's does. Earth's oceanic crust turns over on a timescale of roughly 100 million years; the Venusian surface is estimated to be 300 to 600 million years old.

  Of those tens of thousands of volcanoes, 167 are large ones exceeding 100 km across. The only volcanic structure of comparable scale on Earth is the Big Island of Hawaii. About 80% of the Venusian surface consists of smooth volcanic plains.

  In 2008 and 2009, the Venus Express spacecraft detected four transient infrared hot spots within the rift zone Ganis Chasma, near the shield volcano Maat Mons. Three of those spots appeared in more than one successive orbit, strongly suggesting freshly released lava. The actual temperatures could not be measured precisely, because the size of the hot spots was unknown, but scientists estimated them to be in the range of 800 to 1,100 Kelvin, well above the normal surface temperature of about 740 Kelvin.

  In 2023, scientists reexamined topographical images of the Maat Mons region taken by Magellan and used computer simulations to establish that the terrain had changed during an 8-month interval. Active volcanism was their conclusion. Then in 2024, evidence of recent lava flows appeared on Sif Mons, a shield volcano, and on Niobe Planitia, a flat plain.

  Sulfur dioxide concentrations in the upper atmosphere also tell a story. Between 1978 and 1986, those concentrations dropped by a factor of 10. They jumped again in 2006, then declined 10-fold once more. Each spike may correspond to a major volcanic eruption flushing fresh sulfur dioxide into the upper air.

  Venus also has surface features found nowhere else in the Solar System. Among them are flat-topped volcanic structures called farra, ranging from 20 to 50 km across and 100 to 1,000 meters high, resembling giant pancakes. There are also radial fracture systems called novae, spider-web patterns of concentric and radial fractures called arachnoids, and circular rings of fractures known as coronae.

• 5. A Planet That Forgets Its Craters

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  There are almost a thousand impact craters on Venus, spread fairly evenly across its surface. On the Moon, craters accumulate and overlap, preserving a record of bombardment stretching back billions of years. On Venus, about 85% of craters are in pristine condition, as if they just arrived.

  No crater on Venus is smaller than 3 km in diameter. That lower size limit exists because of the atmosphere. Any incoming projectile less than 50 meters across will fragment and burn before reaching the ground. Objects with insufficient kinetic energy are slowed so much by the dense air that they never punch through to create a crater at all.

  The near-perfect preservation of so many craters points to a global resurfacing event that wiped the surface clean roughly 300 to 600 million years ago. After that reset, volcanism declined, and the craters that arrived since have mostly survived intact.

  Without plate tectonics, Venus cannot bleed off internal heat the way Earth does through the slow creep of tectonic plates. Instead, the planet appears to store heat in its mantle until pressure builds to a critical threshold. Then, over a period of about 100 million years, subduction occurs on an enormous scale, completely overturning the crust in a single catastrophic episode. The next such event has not yet arrived; when it does, the surface record will reset again.

  The planet's internal structure resembles Earth's in broad outline: a core, a mantle, and a crust. The crust averages about 40 km thick and reaches at most 65 km. The core radius is estimated at 3,500 km, based on measurements of the rate of axial precession taken between 2006 and 2020.

• 6. The Question of Life

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  Carl Sagan and Harold J. Morowitz raised the possibility of life on Venus in a 1967 article in Nature, suggesting that tiny objects detected in Venus's clouds might be organisms similar to Earth's bacteria, which are of approximately the same size.

  The idea was not idle speculation. At altitudes between 48 and 59 km above the Venusian surface, temperatures fall between 30 and 80 degrees Celsius, pressure is roughly equivalent to Earth's surface, and radiation levels are comparable as well. Those are among the most Earth-like conditions found anywhere in the Solar System outside Earth itself. The drawback is that the clouds are made of sulfuric acid, and any organism would need to tolerate that acidity.

  In August 2019, a team led by Yeon Joo Lee reported that unknown absorbers in the Venusian atmosphere were altering the planet's light absorption and albedo in patterns that closely resembled the behavior of microorganisms found in Earth's clouds.

  Then in September 2020, a detection of what appeared to be an absorption line of phosphine in Venus's atmosphere attracted widespread attention. Phosphine is not produced by any known chemical process on or in Venus. Later research attributed the spectroscopic signal to sulfur dioxide, or found that the absorption line may not have been present at all. The question remains unresolved.

  Studies reported in 2023 suggested for the first time that Venus may have had plate tectonics in ancient times, which would imply a more habitable past environment, possibly one capable of supporting life. Earlier in its history, the planet may have held liquid surface water for a period of 600 million to several billion years. A runaway greenhouse effect, triggered when greenhouse gases in the atmosphere reached a critical concentration, evaporated that water and drove Venus to its current state.

  The Venus Life Finder mission, a private project developed through MIT and the rocket company Rocket Lab, is scheduled to launch no earlier than the summer of 2026, and aims to probe the atmosphere directly for organics.

• 7. Centuries of Watching

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  Ancient Babylonian records, including the Venus tablet of Ammisaduqa from the First Babylonian dynasty, show that the Sumerians already knew the morning and evening star were the same object. They called it Ninsi'anna, meaning "divine lady, illumination of heaven." Earlier spellings used the cuneiform sign meaning "to be red," suggesting the original name referred to the redness of the morning and evening sky.

  The ancient Greeks initially believed they were looking at two separate stars. Phosphorus was the morning apparition, Hesperus the evening one. Pliny the Elder credited Pythagoras in the sixth century BC with recognizing them as one body; Diogenes Laertius instead credited Parmenides in the early fifth century. The Romans kept the two names, translating them as Lucifer (light-bringer) and Vesper.

  The Maya considered Venus the most important celestial body after the Sun and Moon, calling it Chac ek or Noh Ek', meaning "the Great Star." Venus cycles appear in their calendrical system and were recorded in books such as the Maya Codex of Mexico and the Dresden Codex. In Chinese astronomical tradition, Venus was the metal star, connected to the element metal in the five-element system, a classification still embedded in the Chinese, Japanese, Korean, and Vietnamese words for the planet today.

  In December 1610, Galileo Galilei observed Venus through a telescope and saw that it displayed phases like the Moon. He recorded this in his 1613 Letters on Sunspots, providing clear evidence against the geocentric model. The first transit of Venus to be successfully predicted was that of 1631, calculated by Johannes Kepler and published in 1629. The following transit in 1639 was observed by Jeremiah Horrocks and his friend William Crabtree, each watching from their own homes on the 4th of December 1639. Captain Cook sailed to Tahiti in 1768 to record the third observed transit of Venus, and that voyage led to the exploration of the east coast of Australia.

  Only seven Venus transits have ever been observed. The most recent pair occurred on the 8th of June 2004 and the 5th-the 6th of June 2012. The next will not come until December 2117.

• 8. The Age of Spacecraft

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  Venera 1, launched by the Soviet Union in 1961, was the first spacecraft ever sent toward another planet. It lost contact en route. The first mission to succeed was the American Mariner 2, which passed Venus on the 14th of December 1962 at a distance of 34,833 km and returned the first scientific data from another planet.

  Venera 4, launched in 1967, measured the surface temperature at almost 500 degrees Celsius, hotter than Mariner 2 had estimated, and determined that the atmosphere was 95% carbon dioxide. Its data was combined with readings from the American Mariner 5 mission and analyzed by a joint Soviet-American science team, an unusual act of cooperation during the Space Race.

  On the 15th of December 1970, Venera 7 became the first spacecraft to soft-land on another planet and to transmit data from the surface back to Earth. In 1974, Mariner 10 used Venus as a gravitational waypoint toward Mercury, the first interplanetary gravity assist ever performed, a technique that has since become standard practice.

  Venera 9 and 10 transmitted the first images from the Venusian surface in 1975, in black and white. In 1985, the Soviet Vega 1 and Vega 2 missions deployed the first aerobots ever used beyond Earth, inflatable balloons that achieved atmospheric flight in the Venusian clouds.

  Between 1990 and 1994, NASA's Magellan orbiter mapped the Venusian surface in detail. In April 2006, the European Space Agency's Venus Express entered orbit, studying the atmosphere until the mission ended with a controlled deorbit in January 2015. Japan's Akatsuki probe operated in Venusian orbit from 2015 through 2024.

  NASA has approved two future missions to Venus, VERITAS and DAVINCI, both planned for launch no earlier than 2031. The European Space Agency's EnVision mission is targeted for the same year. Parker Solar Probe is scheduled to make repeated Venus flybys through 2030, and the Indian Space Research Organisation's Venus Orbiter Mission aims to launch in 2028.