Mercury

Mercury is the smallest planet in our solar system and the closest to the Sun. It is a rocky, airless world that experiences some of the most extreme temperatures anywhere in the solar system. Because of its nearness to the Sun, Mercury orbits faster than any other planet, completing one lap every 88 Earth days.

Despite its small size, Mercury is surprisingly dense. It has a large metallic core that makes up about 85% of the planet’s volume. Scientists believe this oversized core formed after a massive collision early in the solar system’s history stripped away much of Mercury’s original rocky mantle.

Extreme Temperatures and Surface

Mercury has almost no atmosphere to trap heat or protect its surface. During the day, temperatures can soar to 800°F (430°C) — hot enough to melt lead. At night, without an atmosphere to hold in warmth, the temperature plummets to as low as -290°F (-180°C). This creates the greatest temperature swing of any planet in the solar system.

The surface of Mercury is heavily cratered, resembling the Moon. Large impact basins such as the Caloris Basin stretch hundreds of miles across. Long, steep cliffs called scarps snake across the planet, formed as Mercury’s interior cooled and shrank over billions of years.

Key Facts About Mercury

Diameter: 3,032 miles (4,879 km) — slightly larger than Earth’s Moon
Mass: 0.055 times Earth’s mass
Distance from the Sun: 36 million miles (0.39 AU) on average
Day Length: 59 Earth days (one rotation)
Year Length: 88 Earth days
Surface Temperature: 800°F (day) to -290°F (night)

Exploration and Mysteries

Mercury is difficult to study from Earth because it stays so close to the Sun in the sky. Only two spacecraft have visited the planet in detail: NASA’s Mariner 10 in the 1970s and the MESSENGER probe, which orbited Mercury from 2011 to 2015.

MESSENGER revealed surprising findings, including water ice hidden in permanently shadowed craters near the poles and a surprisingly strong magnetic field for such a small planet. Scientists are still trying to understand how a planet so close to the Sun can retain any ice at all.

Today, Mercury remains one of the least explored planets. Its heavily cratered surface tells the story of a violent early solar system, while its unusual internal structure continues to challenge our understanding of how rocky planets form. As the closest world to our star, Mercury offers a unique window into the harsh conditions near the center of the solar system.