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, where it appears 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, meaning that a day on Mercury (from sunrise to sunrise) lasts about two of its years. This causes one side of the planet to bake in the Sun’s heat for 88 Earth days, then stay dark and cold for another 88 days. Temperatures swing wildly, from about -170 °C at night to 420 °C during the day.

Scientists believe Mercury may hold surprises, including 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, which gave us much more information about its mysterious surface and history.

Nomenclature

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

Mercury appears to have a solid silicate crust and mantle over a solid, metallic outer core layer, a deeper liquid core layer, and a solid inner core. The planet's density is the second highest in the Solar System, which helps scientists understand its inner structure. Mercury's core is large and rich in iron, making up about 57% of its volume.

The surface of Mercury looks similar to the Moon, with many craters and extensive plains. It also has unique features like narrow ridges called rupes, formed as Mercury's core and mantle cooled and contracted. These features help scientists learn about the planet's history and geological activity.

Orbit, rotation, and longitude

Mercury has the most stretched-out path around the Sun of any planet, stretching 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 experiences strong pulls from the Sun's gravity, creating big changes in temperature.

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 and often gets hidden by the Sun's bright light. You can usually spot Mercury only during the short periods of dawn or dusk. It shines brightest when it is a "full" phase, even though it is farthest from Earth at that time.

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 known records of Mercury come 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 and meanings for Mercury. In China, it was known as the "Hour Star" and linked to the direction north. In Hindu mythology, Mercury was Budha, and in Germanic traditions, the god Odin was associated with 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, similar to the Moon. In 1631, Pierre Gassendi watched Mercury pass in front of the Sun, an event called a transit.

Studying Mercury was difficult 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 revealed that Mercury’s day was about 59 days, not exactly 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, surprising scientists.

A later mission, MESSENGER, orbited Mercury from 2011 to 2015. It mapped almost the whole planet, studied its surface and magnetic field, and confirmed 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 on its way to Mercury. It will study the planet’s surface and magnetic field when it arrives in 2026.