Mercury

About

Mercury is the smallest planet in our solar system and the closest to the Sun. Its orbit around the Sun takes only 87.97 Earth days, making it the fastest planet. Despite being nearest to the Sun, Mercury is not the hottest planet — that title belongs to Venus, whose thick atmosphere traps far more heat.

The planet has virtually no atmosphere to retain heat, which results in the most extreme temperature variation of any planet. The dayside can reach a scorching 430°C while the nightside plummets to -180°C. Mercury's surface is heavily cratered and resembles Earth's Moon, bearing the scars of billions of years of asteroid and comet impacts.

Mercury possesses a surprisingly large iron core that occupies about 85% of the planet's radius, making it the most iron-rich planet in the solar system. This oversized core generates a weak but detectable magnetic field, about 1% the strength of Earth's, which was one of the great surprises of early planetary exploration.

Key Facts

• Mercury has no moons and no rings, making it one of only two planets (with Venus) without natural satellites.
• A single solar day on Mercury (sunrise to sunrise) lasts 176 Earth days — longer than its year.
• Despite its proximity to the Sun, radar observations have confirmed water ice exists in permanently shadowed craters near its poles.
• Mercury's Caloris Basin is one of the largest impact craters in the solar system, spanning 1,550 km in diameter.
• The planet's core makes up approximately 85% of its radius, proportionally the largest core of any planet.
• Mercury has a 3:2 spin-orbit resonance, rotating exactly three times for every two orbits around the Sun.
• The planet is shrinking — as its iron core slowly cools and contracts, Mercury has lost about 7 km of its radius over billions of years.

Exploration History