Mercury is the smallest and nearest planet to the sun in the solar system. Despite its size, it moves through space at incredible speed, completing a journey around the sun faster than any other planet, which is why ancient civilizations associated it with swiftness.
This rocky world, devoid of natural satellites, has a dense metallic core that accounts for about 75% of its diameter. The core is surrounded by a thin outer shell, and its composition has long puzzled researchers. The planet’s surface lacks significant atmospheric protection, leaving it heavily marked by craters.
Ancient Observations
Historical records show Mercury was observed thousands of years ago. Early astronomers noted its dual appearance as a morning and evening star. Around 500 B.C., scholars correctly theorized that both Mercury and Venus orbit the sun.
Extreme Temperatures
Being so close to the sun, Mercury experiences dramatic temperature swings. During the day, temperatures can soar to 840°F (450°C), while nighttime conditions drop to a chilling -275°F (-170°C). This fluctuation of over 1,100°F (600°C) is the most extreme among the planets.
Mercury: The Scorched Planet | The Planets | Earth Science
Video by BBC Earth Science
Size and Impact History
Mercury is only slightly larger than Earth's moon and has a history of massive impacts. Around 4 billion years ago, a collision with an asteroid created the Caloris Basin, a crater vast enough to fit entire large regions on Earth.
Frozen Surprises
Despite its proximity to the sun, Mercury has regions near its poles that remain permanently shaded. In these areas, spacecraft have detected water ice. This discovery suggests that ice may have been delivered by space objects or emerged from Mercury’s interior.
Geologic Activity
Though small, Mercury continues to evolve. Its iron core is cooling and shrinking, causing the surface to form towering cliffs and valleys. These features suggest that tectonic processes are still reshaping the planet.
A Magnetic Puzzle
Mercury’s magnetic field was an unexpected discovery. Despite the planet’s slow rotation and small size, it has an active field, albeit weaker than Earth's. Theories suggest the magnetic activity may be influenced by unique processes in its molten core, creating magnetic interactions with the solar wind.
A Thin Exosphere
Instead of a robust atmosphere, Mercury has a tenuous exosphere composed of particles blasted from its surface. These elements quickly escape into space, forming a faint trail around the planet.
Orbit and Retrograde Motion
Mercury has an elliptical orbit, bringing it as close as 29 million miles (47 million km) and as far as 43 million miles (70 million km) from the sun. Its rotation and orbital dynamics create unusual solar views, with the sun appearing to rise, set briefly, and rise again before crossing the sky.
Approximately three or four times a year, Mercury appears to move backward across Earth’s sky due to its orbit. This phenomenon, called retrograde motion, has been a source of fascination for observers.
