Mercury

Mercury is the closest planet to the Sun and the smallest planet in the solar system. It is a rocky world covered in craters, with almost no atmosphere and some of the most extreme temperature changes of any planet.

Because Mercury orbits so close to the Sun, it moves faster than every other planet, racing through space at incredible speed.

The Fastest Planet

Mercury completes one orbit around the Sun in just 88 Earth days.

This short year makes Mercury the fastest planet in the solar system. It travels at an average speed of about 107,000 miles per hour (172,000 km/h).

Despite its fast orbit, Mercury rotates slowly. One full rotation takes 59 Earth days.

This unusual relationship creates very long days and nights on the planet.

A World of Extreme Temperatures

Mercury has almost no atmosphere to trap heat or distribute warmth around the planet.

As a result, temperatures vary dramatically:

• Daytime: up to 800°F (430°C)
• Nighttime: down to -290°F (-180°C)

This is the largest temperature swing of any planet in the solar system.

Surface Features

Mercury’s surface looks similar to Earth’s Moon.

It is heavily cratered from billions of years of asteroid impacts and contains enormous impact basins formed during the early solar system.

Major features include:

• Caloris Basin — one of the largest impact basins in the solar system
• Scarps — giant cliffs formed as the planet cooled and shrank
• Ancient lava plains
• Bright crater rays from recent impacts

The scarps stretch for hundreds of miles across the surface, showing that Mercury has contracted over time as its interior slowly cooled.

Key Facts About Mercury

Diameter: 3,032 miles (4,879 km)
Mass: 0.055 Earth masses
Average Distance from the Sun: 36 million miles (0.39 AU)
Year Length: 88 Earth days
Day Length: 59 Earth days
Average Orbital Speed: 107,000 mph (172,000 km/h)

A Giant Metal Core

Mercury has an unusually large iron core.

The core makes up about 85% of the planet’s interior volume, far larger proportionally than Earth’s core.

Scientists think this may have happened because:

• A massive collision stripped away much of Mercury’s outer rock
• Or intense heat near the young Sun removed lighter material early in formation

This oversized core also helps generate a surprisingly strong magnetic field.

Ice Near the Poles

Even though Mercury is extremely hot, scientists have discovered water ice near its poles.

Some craters near the poles are permanently shadowed because sunlight never reaches their floors.

These regions remain incredibly cold, allowing ice to survive for billions of years.

Exploring Mercury

Mercury is difficult to observe because it always appears close to the Sun in the sky.

Only a few spacecraft have explored it closely:

• Mariner 10 — first flybys in the 1970s
• MESSENGER — orbited Mercury from 2011–2015
• BepiColombo — ongoing joint ESA/JAXA mission

These missions revealed detailed maps of the surface, evidence of volcanic activity, and the surprising existence of polar ice deposits.

Why Mercury Matters

Mercury provides scientists with important clues about:

• The formation of rocky planets
• Conditions near the early Sun
• Planetary magnetic fields
• The violent impacts of the early solar system

Its unusual structure and harsh environment make it one of the most scientifically important planets despite its small size.

The Solar System’s Extreme Inner World

Mercury is a planet of extremes.

It races around the Sun faster than any other planet, experiences scorching heat and freezing cold, and preserves ancient scars from the earliest days of the solar system.

Though small and difficult to observe, Mercury offers a fascinating glimpse into the powerful forces that shaped the inner planets billions of years ago.