Why is Venus hotter than Mercury, when Mercury is closer to the Sun?

Why is Venus hotter than Mercury, when Mercury is closer to the Sun?

Proximity to the Sun usually dictates a planet's temperature. Mercury is the innermost planet, orbiting between 46 and 69 million kilometres from its surface. However, the title of the hottest planet in the solar system belongs to Venus, which orbits at an average distance of 108 million kilometres.

The discrepancy lies in atmospheric composition. Mercury has no significant atmosphere, possessing only a thin exosphere of atoms and molecules created by solar wind interactions with its surface and meteoroid impacts. Because it cannot retain heat, Mercury experiences violent temperature swings. Its sunlit side can reach 800 degrees Fahrenheit (430 degrees Celsius) — hot enough to melt lead — but its night side can plummet to minus 290 degrees F (minus 180 degrees C).

Venus, by contrast, is shrouded in a dense atmosphere that acts as a massive thermal blanket. This atmosphere is primarily composed of carbon dioxide, with the gas making up about 96% of the air. Carbon dioxide is a greenhouse gas that absorbs infrared radiation emitted from the planet's surface, preventing heat from escaping into space. This creates a nearly constant surface temperature of 867 F (464 C) regardless of whether it is day or night.

The Runaway Greenhouse Effect

Scientists believe that 4.5 billion years ago, Venus may have been a tropical paradise with puffy clouds and oceans of water. Because Venus and Earth are similar in size and proximity to the Sun, they likely started with comparable levels of carbon dioxide and water. On Earth, carbon dioxide remains largely dissolved in oceans or locked in rocks like limestone.

Venus followed a different path. Scientific models indicate the Sun has grown about 40% brighter since the early history of the solar system, emitting only about 70% of its current energy four billion years ago. As the Sun grew brighter, water on Venus evaporated. Without surface water to absorb and store carbon dioxide, the gas accumulated in the atmosphere.

This triggered a feedback loop known as a runaway greenhouse effect: increased temperatures caused more evaporation, which trapped more heat, further raising the temperature. The result is a crushing atmosphere with a surface pressure more than 90 times greater than Earth's pressure at sea level. This pressure is comparable to diving 3,000 feet (940 meters) underwater.

A Hostile World

The environment on Venus is chemically active and extreme. Thick, yellowish clouds of sulfuric acid droplets reflect sunlight, giving the planet its brightness but trapping heat below. In the upper atmosphere, winds can reach speeds over 350 kilometres per hour, circling the globe faster than the planet rotates. Venus itself spins very slowly, taking 243 Earth days to complete one rotation, which is longer than its 225-Earth-day year.

The surface is a volcanic landscape featuring vast lava plains and "pancake" domes. Some scientists believe these volcanoes continue to belch gases into the atmosphere, replenishing the greenhouse gases that sustain the heat. Additionally, Venus lacks a significant magnetic field, leaving it vulnerable to solar particles that may have helped strip away its water over time.

Lessons for Earth

The state of Venus serves as a cosmic cautionary tale. Earth also relies on a moderate greenhouse effect to remain habitable; without greenhouse gases, Earth's average temperature would drop from about 59 F (15 C) to closer to 0 F (minus 18 C).

However, human activities, such as burning oil, coal, and natural gas, are releasing large amounts of carbon dioxide. A further risk comes from thawing permafrost in the Arctic, which exposes ancient organic matter. Bacteria decomposing this matter release methane, another potent greenhouse gas, creating a vicious cycle of warming.

While scientists state Earth's temperature will not reach that of Venus in our lifetime, the comparison highlights the fragility of planetary habitability. For researchers, Venus remains a natural laboratory for testing theories on atmospheric chemistry and planetary evolution.

The study of the planet is expected to intensify. NASA, the European Space Agency, and other organizations have announced new missions for the next decade to map the surface, sample the atmosphere, and search for current volcanic activity. There is also ongoing debate regarding the detection of phosphine gas in the upper clouds in 2020, which some suggest could be a sign of acid-loving microbes.