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Planetary atmospheres originated from gases present during the solar system’s birth. Light gases, such as hydrogen and helium, largely escaped from smaller planets, while heavier gases contributed to the modern atmospheres of Mercury, Venus, Earth, and Mars through a process known as outgassing.
Approximately 5 billion years ago, the Sun and planets condensed from a cloud of gas and dust—the solar nebula—rich in hydrogen and helium. Gas giants captured these light gases, but the inner planets were too small to retain them. According to Vanderbilt University, their primordial atmospheres were thin compared to the current ones.
Penn State University explains that early planets began as accreting blobs. The energy from billions of collisions kept them molten; it took several million years for their surfaces to cool and form a solid crust. Afterward, volcanic outgassing released CO₂, Ar, and N₂, building the secondary atmospheres that we observe today. The stronger gravity of the larger terrestrial planets retained most of these heavier gases.
The early Earth’s atmosphere was dominated by CO₂, a pattern shared with Venus. Life on Earth, however, transformed most of that CO₂ into O₂ through photosynthesis, whereas Venus—lacking life—remains CO₂‑rich, driving a runaway greenhouse effect that can melt lead. Today, Earth’s volcanoes emit over 130 million tons of CO₂ annually, a relatively small fraction of the planet’s atmospheric CO₂.
Mars has a surface pressure of roughly 0.6 % of Earth’s, a consequence of its weak gravity. Its atmosphere is 95 % CO₂ and 2.7 % N₂—very similar in composition to Venus’s 96 % CO₂ and 3.5 % N₂, though its density is far lower.
Mercury’s outgassing likely occurred early in its history, but the planet now holds almost no atmosphere. Its surface pressure is a hard vacuum, reflecting its inability to retain atmospheric gases due to its small size and weak gravity.
