Venus, often called Earth’s twin due to its similar size and mass, currently holds no liquid oceans on its surface. It is a scorching, dry world where liquid water cannot exist. Despite this harsh present, scientific evidence suggests that early Venus may have been a much different world, potentially hosting vast surface water reserves billions of years ago. Scientists are working to solve the mystery of past oceans by analyzing the planet’s extreme atmosphere and surface composition.
The Current State of Venus’s Surface
The conditions on Venus’s surface today are hostile, making the presence of liquid water impossible. The average surface temperature is approximately \(464^\circ \text{C}\) (\(867^\circ \text{F}\)), hot enough to melt lead. This intense heat results from an extreme greenhouse effect driven by a thick, dense atmosphere.
The atmospheric pressure is overwhelming, reaching about 93 times the pressure found at sea level on Earth. This pressure is equivalent to the force felt nearly one kilometer beneath Earth’s oceans. The atmosphere is composed primarily of carbon dioxide, accounting for about 96.5% of its total volume.
This carbon dioxide atmosphere traps heat with incredible efficiency, creating the hottest surface temperatures of any planet in the solar system. Thick cloud layers, which contain sulfuric acid droplets, permanently shroud the planet. These extreme conditions ensure that any water present exists only as trace amounts of vapor high in the atmosphere.
Evidence Suggesting Past Water
The most compelling data supporting the theory of past water on Venus comes from the high concentration of deuterium found in its atmosphere. Deuterium is a heavier isotope of hydrogen, containing one neutron in addition to the single proton found in normal hydrogen. Water molecules containing deuterium (HDO) are heavier than normal water (\(\text{H}_2\text{O}\)).
The ratio of deuterium to hydrogen (D/H) in Venus’s atmosphere is measured to be about 120 times greater than the ratio found in Earth’s oceans. This extreme enrichment is considered a strong chemical fingerprint of massive water loss over time. When water vapor reaches the upper atmosphere, solar radiation splits the molecules into hydrogen and oxygen atoms.
The lighter, normal hydrogen atoms can more easily escape the planet’s gravitational pull and drift into space. The heavier deuterium atoms are less able to escape, causing them to be preferentially retained in the remaining atmosphere. The current, highly enriched D/H ratio indicates that Venus must have started with a substantial amount of water, which has since been lost.
Scientists estimate that the initial amount of water was enough to cover the entire planet with an ocean several meters deep. This estimate is based on the measured level of deuterium enrichment.
The Runaway Greenhouse Effect
The mechanism that transformed Venus from a potentially wet world into a searing desert is known as the runaway greenhouse effect. Due to Venus’s closer proximity to the Sun, the planet received a greater amount of solar radiation than Earth. This increased solar energy caused initial surface water to begin evaporating, introducing immense amounts of water vapor into the atmosphere.
Water vapor is a highly effective greenhouse gas, and its accumulation began a devastating positive feedback loop. Rising temperatures caused more water to evaporate, which trapped more heat, causing further evaporation and spiraling out of control. This process effectively boiled the oceans completely, transferring all the planet’s water into the atmosphere as superheated vapor.
Once the water was in the upper atmosphere, intense ultraviolet radiation from the Sun broke the water molecules apart into hydrogen and oxygen. The light hydrogen atoms were subsequently stripped away by the solar wind, permanently removing the water from the planet. This catastrophic sequence also affected the planet’s carbon cycle.
On Earth, liquid water facilitates the sequestration of carbon dioxide from the atmosphere into rocks; without liquid water on Venus, this process stopped. Carbon dioxide trapped within Venus’s surface rocks was baked out and released into the atmosphere as the planet heated. This massive influx of \(\text{CO}_2\) completed the transformation, creating the extremely hot, high-pressure atmosphere that persists today.