Mercury: The Swift Planet Closest to the Sun

Mercury is the smallest planet in our solar system and the closest to the Sun, racing around it in just 88 Earth days. Despite being closest to our star, it is not the hottest planet — that title belongs to Venus. Mercury's thin atmosphere and slow rotation create the most extreme temperature swings of any planet.

Why Is Mercury So Hot AND Cold?

Mercury has virtually no atmosphere to retain heat. During the day, the side facing the Sun reaches 430°C (800°F) — hot enough to melt lead. But without an atmospheric blanket, nighttime temperatures plunge to −180°C (−290°F). This 610-degree swing is the largest of any planet, far exceeding Mars (whose nights reach −125°C) or even the Moon.

Mercury's Giant Iron Core

Mercury's most surprising feature is its enormous metallic core, which makes up about 85% of the planet's radius — far larger, proportionally, than any other planet. Scientists believe Mercury may have been much larger in the distant past, and a massive collision stripped away most of its outer layers, leaving the dense iron core behind.

• Core radius: ~2,020 km (85% of Mercury's radius vs. 55% for Earth)
• Iron content: About 70% of Mercury's mass is iron — the highest iron fraction of any planet
• Magnetic field: Mercury has a weak magnetic field, just 1% of Earth's — surprising given its slow rotation
• MESSENGER discovery: NASA's MESSENGER probe confirmed the core is partially liquid, generating that faint magnetic field

Surface: A World of Craters and Cliffs

Without weather or plate tectonics, Mercury's surface preserves a 4-billion-year record of impacts. The Caloris Basin — 1,550 km across — formed when an asteroid roughly 100 km wide slammed into Mercury. The impact was so powerful that it created chaos terrain on the exact opposite side of the planet.

Mercury also features enormous lobate scarps — cliffs up to 3 km tall and 1,000 km long — formed as the planet's core cooled and contracted, wrinkling the crust like a dried apple.

Ice in Permanently Shadowed Craters

Despite Mercury's scorching days, water ice has been confirmed in permanently shadowed polar craters where sunlight never reaches. MESSENGER detected hydrogen signatures; radar observations from Earth showed bright reflections consistent with ice. The ice may have been delivered by comets and asteroids over billions of years.

Missions to Mercury

• Mariner 10 (1974–75): First spacecraft to visit Mercury; mapped 45% of the surface
• MESSENGER (2004–2015): First to orbit Mercury; mapped 100% of surface, discovered ice, confirmed liquid core
• BepiColombo (launched 2018): ESA/JAXA joint mission arriving at Mercury in 2026; will study the magnetic field, surface composition, and interior structure in unprecedented detail

Mercury vs. Moon: Spot the Difference

Many people confuse Mercury's appearance with Earth's Moon — both are grey, cratered worlds. But Mercury is larger (diameter 4,880 km vs. Moon's 3,474 km), denser, and has a much stronger magnetic field. The Moon lacks any magnetic field. Mercury's surface also has far fewer large flat "mare" plains than the Moon, which was flooded by ancient lava flows.

Mercury's Future: BepiColombo 2026

The BepiColombo mission — a collaboration between ESA and JAXA — is arriving at Mercury in 2026 after six years of complex gravity-assist maneuvers around Earth, Venus, and Mercury itself. Two orbiters will separate and study Mercury simultaneously: one mapping the surface and exosphere, the other probing the magnetic field and magnetosphere. BepiColombo is expected to revolutionize our understanding of this enigmatic, underexplored world.