Mercury (planet)

Mercury is the closest planet to the Sun and the smallest in our Solar System. It is a rocky planet with a thin atmosphere and a surface covered in craters, much like Earth's Moon. Because it orbits so close to the Sun, Mercury can only be seen from Earth just before sunrise or after sunset. It looks like a bright star in the sky.

Mercury has some very unusual features. Its day and year lengths are in a special 3:2 ratio. This means a day on Mercury lasts about two of its years. One side of the planet gets very hot in the Sun’s heat for 88 Earth days, then stays dark and cold for another 88 days. Temperatures change a lot, from about -170 °C at night to 420 °C during the day.

Scientists think Mercury may hold surprises, such as possible pockets of water ice in permanently shadowed craters. The planet was first explored by the Mariner 10 spacecraft in 1974, and later by the MESSENGER and BepiColombo orbiters. These gave us more information about its surface and history.

Nomenclature

People called Mercury different names depending on when they saw it in the sky. Around 350 BC, ancient Greeks realized these were the same object. They called it Stilbōn, meaning "twinkling," and also Hermēs because it moves quickly. The Romans named it after their messenger god, Mercury, for the same reason, linking it to the Greek Hermes. The planet's symbol is based on Hermes' caduceus, with a Christian cross added later.

Physical characteristics

Mercury is one of the four rocky planets in our Solar System, like Earth. It is the smallest planet, with a radius of 2,439.7 kilometers. Mercury is made of about 70% metal and 30% silicate material.

Mercury has a solid outer layer made of rock over a metallic core. This core has a liquid layer and a solid center. The planet is very dense, which helps scientists study its inside. Mercury's core is large and full of iron, making up about 57% of the planet.

The surface of Mercury looks like the Moon, with many craters and wide flat areas. It also has special features called narrow ridges, known as rupes. These ridges formed as Mercury cooled and shrank. They help scientists understand the planet's history.

Orbit, rotation, and longitude

Mercury has a very stretched-out path around the Sun, ranging from 46 million to 70 million kilometers away. It completes one full trip around the Sun in just under 88 Earth days. Because Mercury moves faster when it is closest to the Sun, its surface gets very hot and cold.

Mercury's path around the Sun is tilted by about 7 degrees compared to Earth's path. This means we only see Mercury pass directly between Earth and the Sun about every seven years. Mercury spins very straight up and down, with almost no tilt, so from its poles, the Sun never appears very high in the sky. Because of how Mercury spins and moves, there are special spots where the Sun seems to move backward in the sky for a short time, making these areas the hottest on the planet.

Observation

Mercury is hard to see because it is close to the Sun. The Sun's bright light often hides Mercury. You can usually see Mercury only during dawn or dusk. It looks brightest when it is a "full" phase, even though it is farthest from Earth then.

Mercury shows phases like the Moon and Venus. It is easier to see from the Southern Hemisphere than from the Northern. The best times to look for it are during its greatest elongation from the Sun. You can also try looking at Mercury during the daytime with a telescope, but be very careful not to look at the Sun. Mercury can also be seen during a total solar eclipse.

Observation history

Ancient astronomers

The earliest records of Mercury are from ancient Assyrian astronomers around the 14th century BC. In Babylonian culture, Mercury was called Nabu, the messenger to the gods.

Different cultures had their own names for Mercury. In China, it was called the "Hour Star" and linked to the direction north. In Hindu mythology, Mercury was Budha, and in Germanic traditions, the god Odin was linked to the planet.

Ground-based telescopic research

The first telescopic views of Mercury were made in 1610 by Thomas Harriot and Galileo. Later observations showed that Mercury’s brightness changed as it moved, like the Moon. In 1631, Pierre Gassendi watched Mercury pass in front of the Sun, an event called a transit.

Studying Mercury was hard because it is close to the Sun. Early maps made by astronomers like Giovanni Schiaparelli and Eugenios Antoniadi showed its bumpy surface. Radar observations in the 1960s found that Mercury’s day was about 59 days, not the same as its year, as once thought.

Research with space probes

Reaching Mercury from Earth is hard because it is very close to the Sun. Only three space probes have visited Mercury.

The first was NASA’s Mariner 10, which flew by Mercury in 1974–1975. It showed Mercury’s cratered surface and discovered the planet had a magnetic field, which surprised scientists.

A later mission, MESSENGER, orbited Mercury from 2011 to 2015. It mapped almost the whole planet, studied its surface and magnetic field, and found that Mercury had water ice in some shadowed craters.

Currently, a joint mission by the European Space Agency and the Japanese Space Agency called BepiColombo is traveling to Mercury. It will study the planet’s surface and magnetic field when it arrives in 2026.