Venus

Venus is the second planet from the sun (about 0.7 astronomical units away), which it orbits every 224.7 days. It is the closest planet to Earth, as well as the one that is the most similar in size and gravity. Venus’s mass is about 0.815 Earths, while it is 0.866 Earths in volume, making it slightly less dense than the Earth. Its rotation is both slow and retrograde; it takes 243 days to complete a rotation.

The surface of Venus is desert-like and covered in smooth volcanic plains (which account for about 80 percent of its surface). Its two main highland areas (called continents despite the lack of oceans) are named for love gods, as is the planet itself: Ishtar Terra in the north is about the size of Australia, while Aphrodite Terra in the south is the size of South America. Most surface features were formed by volcanic activity and exploration by probes has suggested that volcanic activity continues. The planet’s internal structure is believed to be similar to Earth’s—a partially liquid dense core surrounded by a mantle and crust. Previously, there was no evidence of plate tectonics on Venus, but a 2023 study published in the journal Nature Astronomy suggested that at one point in time Venus may have experienced a period of tectonic activity.

Venus’s slow rotation preserves its spherical shape. It is one of only two planets without natural satellites. Three asteroids are in temporary quasi-orbital relationships, none of which appear stable enough to become permanent.

Atmosphere and Climate

Venus’s atmosphere is dense, due to a high concentration (more than 96 percent) of carbon dioxide, and despite resembling Earth in other respects, its surface is subject to atmospheric pressures ninety-two times as strong. Its temperature is about 462 degrees Celsius, almost hot enough to ignite magnesium. It is the hottest planet in the solar system by a considerable margin (even Mercury, which is closest to the sun, has a maximum surface temperature of 420 degrees Celsius). Nitrogen is the second most common element in the Venusian atmosphere, followed by traces of sulfur and other elements.

Venus may once have had oceans, but the greenhouse effect of its atmosphere would long since have evaporated them. Sulfuric acid clouds shroud the atmosphere, making the surface impossible to see from space except through special instruments. Rain falls from these clouds as well, but evaporates in the high heat long before it reaches the surface of the planet. The clouds reflect about 90 percent of sunlight, which keeps the daytime surface dim. Below these sulfuric clouds is the dense carbon dioxide layer. These factors along with surface conditions contribute to the reasons Venus retains a constant temperature from day to night. Probes have reported lightning, which would seem to result not from rainstorms but from airborne ash following volcanic eruptions.

In 2013, the European Space Agency (ESA) discovered that Venus’s ionosphere streams outward, like the tail of a comet.

History of Observation and Exploration

Venus is the second brightest celestial object in the night sky, after the moon, and is bright enough to cast shadows. Its position in the sky is never far from the sun and because it is brightest just before sunrise or just after sunset (depending on its position in its orbit around the sun), many early cultures called it the evening or morning star. In some cases it was mistaken for two separate objects; until Pythagoras, the ancient Greeks believed the morning and evening stars were two different objects. In the modern era, Venus has frequently been mistaken for an unidentified flying object (UFO) because of its brightness.

Like the moon, Venus displays phases when seen from the Earth. This was first noted by Galileo, using a telescope, in 1610. The phases result from Venus’s orbit being inside Earth’s orbit. When Venus is in its crescent phase, an “ashen light” of unknown origin can sometimes be seen. It may be the result of electrical activity on the planet or may be an optical illusion caused by factors on Earth.

Telescopes allowed for the discovery of Venus’s atmosphere in the eighteenth century, but otherwise little was understood about the planet until the twentieth century, when technological advances allowed it to be studied through spectroscopy and ultraviolet images. The retrograde rotation of the planet was discovered in the 1950s.

The Soviet Venera program was the first probe mission to another planet, with the first probe launched in 1961. It was followed by the launch of the Mariner by the United States the following year. Early probe explorations discovered Venus’s extremely high temperatures and lack of a magnetic field. Venera probes later in the 1960s took extensive measurements of the atmosphere, and in 1970, Venera 7 was the first successful landing on Venus. In the 1970s, both the US National Aeronautics and Space Administration (NASA) and the Soviets deployed lander/orbiter pairs to Venus—the Pioneer mission and more Venera missions, respectively.

One of the most ambitious Venus missions was the Magellan probe launched by NASA in 1990, which used detailed radar mapping to map 98 percent of the surface at a fairly high resolution. The Magellan images remained some of the most detailed of Venus. Magellan orbited the planet for four years, transmitting data back to Earth, before its orbit decayed as planned and it crashed to the surface. More than a decade passed before the next major Venus mission: the Venus Express, an ESA probe that entered Venus’s orbit in 2006 and remained, collecting data on Venusian atmosphere and climate with the goal of better understanding terrestrial climate, until its fuel was exhausted in 2014.

Venus has also been used for a gravitational slingshot by several probes conducting flybys en route to the outer planets, including the Galileo mission to Jupiter in 1989 and the Cassini mission to Saturn in 1997. Venus rover missions have been planned by NASA and other agencies but the conditions of the planet make the challenge greater than for the Mars rover missions.

In September 2020, Venus made headlines once more as it was reported that astronomers had published papers detailing a discovery in the planet's atmosphere that they believed was a possible indication of life. The astronomers, using a high-powered telescope and observatory on Earth, claimed that they had picked up on a signature of phosphine in the clouds of Venus, a gas that has been scientifically argued as being produced by living organisms on such rocky planets; upon further analysis of the detection via computer modeling, the team of scientists and researchers involved hypothesized that the existence of a life-form was the likeliest explanation for it. They also asserted that this possibility had support in the fact that the discovery was made within an area of Venus's clouds that had been thought to be more habitable for life because of lower temperature ranges. However, other experts and scientists expressed skepticism of the life theory regarding the detection of phosphine, largely because many aspects of Venus's geology and atmosphere were still not well understood. Regardless, most agreed that this was a substantial finding that warranted more attention and observation.

In 2021, NASA announced two missions, DAVINCI (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging) and VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy), that aimed to further scientific understanding of the planet.

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