Venus, cloudy with a chance of life

Highlights

  • Venus may have been habitable to life before becoming inhospitable.
  • By studying Venus, scientists learn how Earth-like planets change over time.
  • There is a chance life exists in Venus' upper atmosphere.

Why We Study Venus

Venus is the hottest planet in the solar system, even though Mercury is twice as close to the Sun and receives four times more solar energy. The reason? Venus’ thick, carbon dioxide atmosphere causes a runaway greenhouse effect. At the surface, Venus has an atmosphere 50 times denser than Earth’s, and average surface temperatures of 470 degrees Celsius (878 degrees Fahrenheit) — hot enough to melt lead.

Venus is currently inhospitable, but it wasn’t always that way. Missions there have observed granite-like rocks that require abundant water to form. In the solar system's early days when the Sun was cooler, scientists think the planet may have had liquid water on the surface for 2 billion years — far longer than Mars, which had liquid water for a relatively shorter 300 million years. Water is the key to life as we know it, so did Venus once have life?

Scientists including Planetary Society co-founder Carl Sagan have predicted that life could currently exist in Venus' upper atmosphere, which has Earth-like temperatures and pressures roughly 50 kilometers (31 miles) above the planet's surface. There, mysterious dark patches absorb more than half the planet's solar energy. In 2020 scientists announced they had found phosphine, a chemical strongly associated with life, in Venus's clouds — though the existence of the signal is currently being reviewed.

We don't know how Venus transformed from a potentially habitable world to its current hellish state. By studying Venus, scientists learn how Earth-like planets evolve and what conditions exist on Earth-sized exoplanets. Venus also helps scientists model Earth’s climate, and serves as a cautionary tale on how dramatically a planet’s climate can change.

Venus surface panorama from Venera 14 rear camera
Venus' surface Only 4 spacecraft have ever returned images from Venus’ surface. The Soviet Union's Venera 14 probe captured this view in 1982.Image: Russian Academy of Sciences / Ted Stryk

Venus Facts

Surface temperature: 440°C (820°F) to 480°C (900°F)
Average distance from Sun: 108 million kilometers (67 million miles), or 38% closer to the Sun than Earth
Diameter: 12,104 kilometers (7,521 miles), Earth is just 5% wider
Volume: 928 billion km3 (223 billion mi3), Venus could fit inside Earth 1.1 times
Gravity: 8.9 m/s², or 90% that of Earth’s
Solar day: 243 Earth days
Solar year: 225 Earth days
Atmosphere: 96% carbon dioxide, 3% nitrogen, 1% other gases

What missions have gone to Venus?

Venus was the first planet to be visited by a spacecraft. In 1962, NASA’s Mariner 2 flew by the planet and discovered it was a hot world with no self-generated magnetic field. The Soviet Union became the world leader in early Venus exploration after that, sending multiple atmospheric probes and as many as ten landers to the planet. To this day, they remain the only nation to have landed spacecraft on the surface and transmitted both data and images back to Earth.

Due to thick clouds, it is impossible to see Venus’ surface without radar. NASA’s Magellan orbiter, launched in 1990, used radar to map Venus’ surface at the highest resolution to date. Magellan revealed that all of the planet’s impact craters are formed within the last 700 million years. This implies that Venus’ surface was completely reshaped by a worldwide volcanic event in its recent geologic past—but exactly what happened is still up for debate.

Magellan also found no sign of plate tectonics. On Earth, plate tectonics is a process in which sections of the planet’s outer crust glide over the mantle — the rocky inner layer above the core — allowing heat to escape through volcanism. Since we think Venus’s interior is similar to Earth's, the lack of plate tectonics means that volcanoes on Venus must work differently than on Earth.

The European Space Agency launched the Venus Express orbiter in 2006. By observing hotspots on the surface and changing sulfur dioxide levels in the atmosphere over 6 years, the spacecraft collected the best evidence yet of active volcanism on Venus. Venus Express also discovered granite-like rocks across the planet that require abundant liquid water to form, solidifying the idea of the planet having past oceans.

Japan’s Akatsuki spacecraft is the only probe currently orbiting Venus. It studies Venus’s atmosphere in frequencies of light that human eyes cannot see, which helps scientists paint a better picture of what’s happening above the planet’s surface. These images can be processed to create beautiful enhanced-color pictures of the planet.

Only three missions have visited Venus in the past 30 years. A spacecraft with a higher-resolution radar could help us solve the mystery of how Venus’ surface changed within the last billion years. Landers and atmospheric probes would analyze the extent that water may have existed on the surface, what the planet’s atmosphere was like, and how it changed into its present-day state.

India aims to launch a Venus orbiter called Shukrayaan in December 2024 equipped with a radar and infrared camera to map the surface. It will have a payload capacity of about 100 kilograms and study Venus for four years from a polar orbit of 200 x 600 kilometers. The spacecraft will carry both Indian and international science instruments. One, called VIRAL  (Venus Infrared Atmospheric Gases Linker), will be co-developed by the French and Russian space agencies.

Upcoming missions to Venus

In June 2021 NASA announced it had selected two Venus missions as part of its low-cost Discovery program. The spacecraft, which will launch between 2028 and 2030, are:

  • DAVINCI+ (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging Plus) consists of an orbiter and an atmospheric descent probe. The probe will make high precision measurements of trace gases in Venus’ atmosphere, helping firmly determine how much water Venus’ oceans had and how long they existed.
  • VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy) would have a radar instrument with up to 100 times higher resolution than Magellan. This will give scientists a better handle on Venus’ geology and evolution and also reveal why the planet lacks large-scale plate tectonics.

ESA also announced in June 2021 that it will send its own Venus orbiter, EnVision, to explore the planet in the early 2030s. EnVision seeks to provide a comprehensive view of Venus, from the core all the way to its upper atmosphere.

Magellan Drifts Away
Magellan Drifts Away NASA has not sent a spacecraft to Venus since the launch of Magellan, seen here in 1989 after release from Space Shuttle Atlantis.Image: NASA

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Acknowledgements: This page was initially written by Jatan Mehta in 2020.