Venus is often called Earth’s “twin,” but the resemblance is superficial. Beyond its similar size and mass, Venus is a world of extremes, a reputation that has long fueled scientific curiosity about the possibility of life. The question of whether our nearest planetary neighbor could harbor life has prompted a scientific investigation into how a planet so like our own evolved into such a hostile place.
The Inhospitable Surface
The surface of Venus is one of the most extreme environments in the solar system. The atmospheric pressure at the surface is over 90 times that of Earth at sea level, a force equivalent to being nearly 900 meters (3,000 feet) underwater. This pressure is due to a dense carbon dioxide atmosphere that created a runaway greenhouse effect, trapping heat and leading to surface temperatures averaging 464°C (867°F), hot enough to melt lead and zinc.
The planet is perpetually shrouded in thick clouds of sulfuric acid, which prevent direct observation of the surface in visible light. These clouds reflect the majority of sunlight, yet the heat they trap ensures little temperature difference between day and night, despite a Venusian day lasting longer than its year. The surface itself is a dry, rocky landscape shaped by extensive volcanism. Analysis from Soviet Venera landers in the 1970s and 1980s confirmed a basaltic composition for the volcanic plains that dominate the surface.
A Potentially Habitable Past
Despite its current state, scientific models suggest Venus may have once been a more temperate world. Evidence gathered by NASA’s Pioneer Venus mission in 1978 first hinted that the planet might have possessed shallow oceans. For billions of years, Venus may have hosted liquid water on its surface, creating conditions suitable for life.
Computer simulations support this theory, indicating that early in its history, Venus’s slow rotation could have generated persistent cloud cover. This cloud layer would have reflected enough solar radiation to keep the surface cool, with some models suggesting average temperatures around 15°C (59°F). These more Earth-like conditions could have persisted before a massive resurfacing event, possibly linked to widespread volcanism, triggered the runaway greenhouse effect that transformed the planet about 700 million years ago.
The Search in the Clouds
Given the harsh conditions on the ground, the search for life has shifted to the Venusian atmosphere. Scientists have identified a temperate layer between 48 and 60 kilometers (30 to 37 miles) above the surface where conditions are less extreme. In this region, the atmospheric pressure is similar to Earth’s at sea level, and temperatures range from 30°C to 80°C (86°F to 176°F).
This has led to the concept of a potential “aerial biosphere,” where microscopic life could exist within the droplets of sulfuric acid that make up the clouds. On Earth, microbes are known to thrive in highly acidic environments, and some even produce ammonia to neutralize their surroundings. A proposed life cycle for Venusian microbes involves them residing within these droplets, drying out and falling to a lower haze layer as spores, and then being lifted back up to the habitable cloud layer to rehydrate.
The Phosphine Controversy
The search for life in the clouds intensified in 2020 with the reported detection of phosphine gas. On Earth, phosphine (PH3) is a rare molecule primarily produced by anaerobic organisms, meaning life that does not require oxygen. The discovery was unexpected, as no known non-biological chemical processes on Venus could account for its presence at concentrations of about 20 parts per billion.
This initial announcement sparked a significant scientific debate, and subsequent analyses by other research groups have contested the finding. Some scientists proposed that the signal attributed to phosphine could have been a misidentification of sulfur dioxide. Others pointed to potential errors in the data processing, specifically the method used to subtract the bright background signal from Venus.
The original team has since reanalyzed their data, addressing some criticisms and maintaining that a phosphine signal, albeit at a lower concentration, is still present. However, the scientific community remains divided, with some studies finding no evidence of the gas. The controversy underscores the difficulty of remote detection when searching for signs of extraterrestrial life.
Future Venus Missions
To resolve these questions and gather more definitive data, a new wave of missions to Venus is planned for the coming decade. NASA is preparing two missions for launch no earlier than 2031: DAVINCI (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging) and VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy).
The DAVINCI mission includes an atmospheric probe that will descend through the clouds, directly measuring the chemical composition and searching for gases like phosphine. It will also be the first to image one of Venus’s “tesserae,” rugged highland regions that may be remnants of ancient continents. The VERITAS orbiter will map the planet’s surface in high resolution, helping to determine if Venus ever had water and whether volcanic activity is still occurring. Joining these efforts is the European Space Agency’s EnVision mission, which will also orbit the planet to study its surface and interior.