Mercury is the smallest and innermost planet in the Solar System, orbiting closest to the Sun. It has no moons.

1. 1. Size and Composition

• • • Diameter: About 4,880 km (3,032 miles), making it slightly larger than Earth’s Moon.
    • Composition: Mercury is a terrestrial planet, with a rocky surface and a large metallic core. The core makes up about 75% of the planet’s radius, and scientists believe it’s primarily composed of iron.

1. 2. Orbit and Rotation

• • • Orbit around the Sun: Mercury has the shortest orbit of all the planets, taking only 88 Earth days to complete a full revolution around the Sun.
    • Day length: A single day (rotation) on Mercury lasts 59 Earth days. Interestingly, due to its slow rotation and fast orbit, a full day-night cycle (solar day) on Mercury is equivalent to 176 Earth days.

1. 3. Surface and Temperature

• • • Mercury’s surface: Heavily cratered, similar to the Moon, due to its lack of atmosphere to burn up meteors.
    • Temperature extremes: Mercury experiences some of the most extreme temperature variations in the Solar System. On the side facing the Sun, temperatures can reach up to 430°C (800°F), while on the night side, they can drop to -180°C (-290°F) due to its lack of atmosphere to retain heat.

1. 4. Atmosphere

Mercury has an extremely thin atmosphere, called an exosphere, composed mainly of oxygen, sodium, hydrogen, helium, and potassium. This is because the planet’s low gravity and proximity to the Sun prevent it from holding a thick atmosphere.

1. 5. Magnetic Field

Surprisingly, Mercury has a weak magnetic field, about 1% the strength of Earth’s. This was unexpected given its slow rotation, and scientists believe it may be generated by its partially molten core.