60 Fun Facts About Mercury

Where does Mercury rank in distance from the Sun?

It is also the smallest of the eight planets, smaller even than the moons Ganymede and Titan.

How long is a year on Mercury, in Earth days?

Its orbit is the fastest of any planet, which is why the Romans named it after their swift messenger god.

How long does one solar day (sunrise to sunrise) last on Mercury?

That is twice its year: one side bakes for a full 88-day orbit and then freezes for the next.

Mercury rotates three times for every two orbits of the Sun. What is this relationship called?

Astronomers had assumed it was tidally locked, like the Moon, until 1965 radar observations proved otherwise.

Despite being closest to the Sun, Mercury is only the second-hottest planet. Which is hotter?

Mercury has almost no atmosphere to trap heat, whereas Venus is wrapped in a thick greenhouse blanket.

Roughly what temperature range does Mercury's equator experience between night and day?

The swing of nearly 600 degrees is the largest of any planet.

What is the name of Mercury's largest crater, about a third the diameter of the planet?

Its rim of mountains stands about 2 km high and the impact sent shock waves right around the planet.

What is the diameter of Mercury's largest impact basin?

The planet itself is only 4,880 km across.

What is the nickname of the hilly region on the exact opposite side of Mercury from the Caloris Basin?

One theory is that shock waves from the impact travelled round the planet and met there.

Which crater on Mercury is nicknamed 'the Spider' for its radiating troughs?

It sits inside the Caloris Basin and its origin is still debated.

Craters on Mercury are named after what kind of people?

Hence craters called Beethoven, Tolstoj and Rembrandt; ridges get scientists and valleys get abandoned ancient cities.

Mercury's mountains (montes) are named after what?

The escarpments, or rupes, are the ones named for ships of scientific expeditions.

Roughly what fraction of Mercury's volume is taken up by its iron core?

Earth's core is only about 17% of its volume; Mercury's may be the leftover of a planet stripped by a giant impact.

Mercury has the second-highest density of any planet. Which is first?

Factor out gravitational squeezing and Mercury's raw material is actually denser than Earth's.

What has been confirmed to exist in permanently shadowed craters at Mercury's poles?

Radar from Earth first spotted the bright patches in the early 1990s; orbiter images later confirmed the ice.

Mercury's axial tilt is the smallest of any planet, at about how much?

An observer at the poles would never see the centre of the Sun rise above the horizon.

Mercury's orbit is the most eccentric of the eight planets. What is its eccentricity?

Its distance from the Sun swings between about 46 and 70 million km.

Mercury's magnetic field, first measured in the 1970s, is roughly what fraction of Earth's strength?

Its existence surprised scientists, who thought the planet spun far too slowly to run a dynamo.

Which was the first spacecraft to fly past Mercury, in 1974?

It was also the first probe to use a gravitational slingshot, swinging past Venus on the way.

Which planet's gravity did the first Mercury flyby probe use as a slingshot to get there?

The manoeuvre was proposed by Italian scientist Giuseppe Colombo, after whom BepiColombo is named.

Which spacecraft became the first to orbit Mercury, in March 2011?

Its name is a backronym for Mercury Surface, Space Environment, Geochemistry, and Ranging.

How did the first Mercury orbiter's mission end in April 2015?

With its propellant gone, controllers used the helium pressurant to squeeze out a few extra weeks first.

BepiColombo, launched in 2018 to orbit Mercury, is a joint mission between ESA and which other agency?

It carries two orbiters, one to map the planet and one to study its magnetosphere.

The ESA-JAXA Mercury orbiter is named after Giuseppe Colombo, who first proposed what technique?

Mariner 10 used his idea in 1974, and nearly every planetary probe has used it since.

How many Mercury flybys did BepiColombo complete by its last one in January 2025?

The first was in October 2021 and the sixth in January 2025; its two orbiters were then due to enter orbit around Mercury in late 2026.

What was the hidden planet Le Verrier proposed in 1859 inside Mercury's orbit called?

It was never found; Einstein's general relativity explained the extra precession in the 20th century.

Which theory finally explained the unexplained 43 arcseconds per century in Mercury's perihelion advance?

It became one of the three classic tests of the theory, alongside light bending and gravitational redshift.

Transits of Mercury across the Sun can only happen in which two months?

They occur about every seven years on average, when Mercury crosses the ecliptic while between Earth and the Sun.

Who made the first telescopic observation of a planetary transit, watching Mercury cross the Sun in 1631?

Kepler had predicted the transit; Gassendi was the one who saw it.

Who, in 1639, used a telescope to show Mercury has phases like the Moon, proving it orbits the Sun?

Marius had already suspected phases in 1612 from the planet's changing brightness.

Which observatory's radar showed in 1965 that Mercury rotates in about 59 days, not synchronously?

Gordon Pettengill and Rolf Dyce made the measurement; Giuseppe Colombo then worked out the 3:2 resonance.

Which nation's scientists were the first to bounce a radar signal off Mercury, in 1962?

Vladimir Kotelnikov's team opened the era of radar planetary astronomy.

Mercury is named after the Roman god of what?

The Romans equated him with Hermes, and chose him because the planet crosses the sky faster than any other.

What did the ancient Greeks call Mercury, meaning 'twinkling'?

By about 350 BC they had realised the morning and evening 'stars' were the same object.

The Babylonians named Mercury after which god, the herald of their pantheon?

The even older MUL.APIN tablets called it 'the jumping planet'.

The MUL.APIN tablets, the earliest records of Mercury, call it what?

An Assyrian astronomer probably made the observations around the 14th century BC.

Modern Chinese, Japanese, Korean and Vietnamese names for Mercury literally mean what?

In the Five Phases system the planet was linked with water and the direction north.

Which day of the week was linked to Mercury by both Hindu mythology (Budha) and Germanic paganism (Odin)?

French mercredi and Spanish miércoles still carry the planet's name.

The Maya may have represented Mercury as which animal, an envoy to the underworld?

Possibly four owls, two each for its morning and evening appearances.

Mercury's astronomical symbol is a stylised version of what object?

A Christian cross was added to the bottom in the 16th century.

Mercury's tenuous exosphere is thought to be replenished partly by micrometeorites shed by which body?

Sodium, potassium and calcium in the exosphere come largely from vaporised surface rock.

The small crater Hun Kal serves what purpose on Mercury?

The intended zero meridian was in darkness when Mariner 10 arrived, so a nearby crater was chosen as the reference.

From which part of Earth is Mercury more often and easily seen?

Its best elongations fall in southern autumn and late winter, when the planet's orbit meets the horizon steeply.

Mercury has the largest what among the eight planets, at 7°?

That tilt is why transits are rare: the planet usually passes above or below the Sun's disc.

At certain spots on Mercury an observer could watch the Sun do what within a single day?

Near perihelion the planet's orbital motion briefly outpaces its spin, so the Sun appears to reverse.

The Roman festival of the god Mercury, the Mercuralia, was held on which date?

Merchants sprinkled water from his sacred well near the Porta Capena on their heads and their goods.

In Roman myth, the god Mercury was the son of Jupiter and which daughter of the Titan Atlas?

Maia gave her name to the month of May, when his festival was held.

What is the chemical symbol of the element mercury?

It abbreviates hydrargyrum, the Latinised Greek for 'water-silver'; the metal is also called quicksilver.

What is the atomic number of the element mercury?

It is the only metallic element known to be liquid at standard temperature and pressure.

In which red mineral does the element mercury mostly occur in nature?

Ground up, the same mercuric sulfide gives the pigment vermilion.

Which Spanish mine, opened some 2,500 years ago, long dominated the world's mercury production?

From 1558 the patio process made mercury essential for extracting silver in New Spain and Peru.

Dumping of mercury waste into which Japanese bay from 1932 to 1968 caused a notorious poisoning disaster?

More than 3,000 people suffered deformities, severe poisoning or death from what became known as Minamata disease.

The felt-hat process using mercuric nitrate, blamed for 'mad hatters', was known as what?

The name came from the orange colour of the solution used to separate fur from pelt from the mid-18th century.

What was the first US human spaceflight programme originally going to be called before Eisenhower objected?

Eisenhower felt the original name gave too much attention to the pilot, so a name from classical mythology was chosen.

Which company was chosen in January 1959 as prime contractor to build the Mercury spacecraft?

Twelve firms bid for the $20 million contract; the capsules were built in clean rooms in St. Louis.

Project Mercury flights were launched on rockets modified from which two missiles?

The capsule carried about a day's supply of water, food and oxygen in a pressurised cabin.

During which years did Project Mercury, the first US human spaceflight programme, run?

Its goal was to put a man into Earth orbit and return him safely, ideally before the Soviet Union.

The Roman god Mercury appears related to which Etruscan deity in his earliest forms?

Both gods share characteristics with the Greek Hermes, and Mercury is usually shown holding the caduceus.

The name of the god Mercury is possibly linked to which Latin word for merchandise?

He subsumed the earlier Dei Lucrii, gods of profit, as Roman religion absorbed Greek influences from around the 4th century BC.

What is the equatorial radius of the planet Mercury?

That makes it smaller, though more massive, than the largest moons Ganymede and Titan.