The terrestrial planets
Mercury, Venus, Earth, and Mars share a rocky makeup, but their histories diverged dramatically, leaving one world teeming with life.
Mercury
Mercury, the smallest planet and closest to the Sun, has almost no atmosphere and a heavily cratered surface. Temperatures swing from about 430 degrees C in daytime to about -180 degrees C at night. It has a surprisingly large iron core and a weak magnetic field. A day-night cycle there lasts about 176 Earth days.
Venus
Venus is nearly Earth's size but has a crushing carbon dioxide atmosphere about 90 times as dense as Earth's, with clouds of sulfuric acid. A runaway greenhouse effect makes its surface about 465 degrees C, hotter than Mercury. Venus rotates very slowly and backward (retrograde), so the Sun rises in the west.
Earth
Earth is the only known world with liquid water on its surface and life. Its plate tectonics recycles crust and regulates carbon dioxide over long timescales; its magnetic field shields the surface from charged particles; and its large Moon stabilises its axial tilt. Its atmosphere is about 78% nitrogen and 21% oxygen, the oxygen produced by life.
Mars
Mars is about half Earth's diameter, with a thin carbon dioxide atmosphere about 1% as dense as Earth's. It has the solar system's largest volcano, Olympus Mons, and a huge canyon system, Valles Marineris. Its red colour comes from iron oxide (rust) in its dust. Polar caps contain water ice and frozen carbon dioxide.
Water on Mars
Dry river valleys, ancient lake beds, and minerals that form in water show Mars once had liquid water on its surface, probably over 3 billion years ago. Rovers such as Curiosity (landed 2012) and Perseverance (landed 2021) found evidence of ancient habitable environments, and Perseverance is collecting samples for possible return to Earth.
Why the planets differ
Size matters: small planets like Mercury and Mars cooled quickly and lost much of their geological activity and atmosphere. Distance from the Sun matters: Venus became too hot to keep oceans. Magnetic fields matter: Mars lost its global field early, letting the solar wind strip much of its atmosphere.
The greenhouse effect
Greenhouse gases such as carbon dioxide and water vapour let sunlight in but trap outgoing infrared radiation, warming a planet. Earth's natural greenhouse effect keeps it about 33 degrees C warmer than it would otherwise be. Venus shows the extreme case, and human emissions are strengthening Earth's greenhouse effect.
Worked example
Venus and Earth formed at similar sizes from similar materials. But Venus, closer to the Sun, grew warm enough that water evaporated, adding greenhouse gas; it got hotter, evaporating more water in a runaway loop. Without liquid water, carbon dioxide could not be locked into rocks as on Earth, so it built up in the atmosphere, creating today's inferno.
Key takeaways
Mercury is small, airless, cratered, and has extreme temperatures. Venus has a dense CO2 atmosphere and a runaway greenhouse effect, at about 465 degrees C. Earth has liquid water, life, plate tectonics, and a protective magnetic field. Mars is cold, with a thin atmosphere, giant volcanoes, and evidence of ancient water. Size, distance, and magnetic fields explain their differences. The greenhouse effect warms planets; Venus shows its extreme.
Flashcard
Smallest planet, closest to the Sun
Check your understanding
Why is Venus hotter than Mercury, even though Mercury is closer to the Sun?