MERCURY
Mercury is the closest planet to the Sun, the smallest planet in the solar system, and the fastest. It’s a tiny, gray, cratered world only a little bigger than Earth’s Moon. Mercury is mostly made of iron, has wilder day-and-night temperature swings than any other planet, has zero moons and zero rings, and is the only place where one day actually lasts longer than one year. Here’s everything that makes Mercury one of the strangest worlds in our solar system.
Quick Facts About Mercury
| Fact | Value |
|---|---|
| Position | First planet from the Sun (innermost) |
| Type | Terrestrial (rocky) planet |
| Diameter | 4,879 km (3,032 miles) — about 38% of Earth’s |
| Distance from Sun | 58 million km (36 million miles) on average |
| Length of day (sunrise to sunrise) | 176 Earth days |
| Length of year | 88 Earth days |
| Surface temperature | Day: +430 °C (+800 °F); Night: -180 °C (-290 °F) |
| Atmosphere | Almost none — an extremely thin exosphere |
| Gravity | 38% of Earth’s |
| Mass | 5.5% of Earth’s |
| Moons | 0 |
| Rings | 0 |
| Named after | Mercury, the Roman messenger god |
| First recorded | Around 3000 BCE by the Sumerians |
Where Is Mercury in the Solar System?
Mercury sits in first place — closer to the Sun than any other planet. As the innermost planet, it sits right next to the heat. Mercury is one of the four rocky worlds called the terrestrial planets: Mercury, Venus, Earth, and Mars. They all have hard surfaces you could walk on, unlike Jupiter or Saturn, which are basically giant balls of gas.
From an average distance of 58 million km (36 million miles), Mercury races around the Sun once every 88 Earth days. That’s the shortest year of any planet. If you lived on Mercury, you’d celebrate a new birthday every three Earth months. A 10-year-old kid on Earth would already be over 41 in Mercury years.
Sunlight, traveling at the speed of light, takes only 3.2 minutes to reach Mercury. The same sunlight takes 8 minutes to reach Earth and more than 5 hours to reach Pluto. From Mercury’s surface, the Sun would look about three times bigger than it does from Earth, and seven times brighter.
How Mercury Compares to Earth
Mercury is much smaller than Earth. If Earth were the size of a basketball, Mercury would be about the size of a tennis ball. Earth’s diameter is around 7,918 miles (12,742 km). Mercury’s diameter is just 3,032 miles (4,879 km) — only 38% of Earth’s. In fact, Mercury is so small that it’s only a little bigger than Earth’s Moon. Some moons in the solar system, like Jupiter’s Ganymede and Saturn’s Titan, are actually larger than the planet Mercury. But Mercury is heavy for its size. Mercury’s density is 5.43 g/cm³, just slightly less than Earth’s 5.51 g/cm³, which makes Mercury the second densest planet in the solar system — because so much of it is solid iron.
Gravity on Mercury is only 38% as strong as Earth’s. A kid who weighs 100 pounds on Earth would weigh just 38 pounds on Mercury. Your jumps would also go much higher.
A Day on Mercury Lasts Longer Than a Year
This is one of the strangest things about Mercury. A single day on Mercury — from one sunrise to the next sunrise — lasts 176 Earth days. But a Mercury year, the time it takes the planet to go around the Sun, is only 88 Earth days. So a day on Mercury equals two Mercury years.
How can that be? Mercury spins slowly on its axis, taking 59 Earth days to make one full rotation. But it also zips around the Sun fast, once every 88 days. Because the planet moves so quickly around the Sun while spinning so slowly, the Sun appears to crawl across the Mercury sky and takes 176 Earth days to come back to a new sunrise.
There’s an even weirder twist. When Mercury is at perihelion (the closest point in its orbit to the Sun), the Sun actually appears to rise, set, and rise again from some places on the surface. It’s the only planet where the Sun does this strange dance. Mercury also has almost no axial tilt — only about 2 degrees — so it doesn’t have real seasons the way Earth does.
The wow stat: Mercury day = 176 Earth days. Mercury year = 88 Earth days. A day on Mercury is twice as long as a year on Mercury.
How Hot — and How Cold — Is Mercury?
Mercury has the wildest temperature range of any planet in the solar system. During the day, the surface bakes at +800 °F (+430 °C) — hot enough to melt lead. At night, the same surface plunges to -290 °F (-180 °C), colder than the coldest spot ever recorded on Earth. The temperature swings nearly 1,100 °F between day and night.
Why such extreme temperatures? Because Mercury has almost no atmosphere. Earth’s atmosphere works like a blanket, holding heat in at night and keeping things mostly comfortable. Mercury has no blanket. Sunlight pours straight onto the surface during the day with nothing to slow it down, and the moment the Sun sets, all that heat escapes back into space.
Even more surprising, Mercury isn’t the hottest planet in the solar system — Venus is. Even though Mercury sits closer to the Sun, Venus has a thick, heavy atmosphere full of carbon dioxide that traps heat like a greenhouse, keeping its surface around +870 °F (+465 °C) day and night. Mercury’s heat just leaks straight back out into space.
What Does Mercury Look Like? Surface, Craters & the Caloris Basin
Mercury looks a lot like Earth’s Moon. The surface is gray, rocky, and covered with craters from billions of years of space rocks crashing into it. With no atmosphere to slow incoming meteoroids and no rain or wind to wear craters down, every impact stays scarred into the surface essentially forever. Mercury is a terrestrial planet like Earth, with the same basic layers: a metal core in the middle, a rocky mantle around it, and a thin crust on the outside. What makes Mercury different is the size of that core. Mercury’s iron core takes up about 85% of the planet’s radius. Earth’s core is only about 17%. Mercury is essentially a giant iron ball with a thin layer of rock around it.
The Caloris Basin — The Biggest Crater on Mercury
The biggest crater on Mercury is called the Caloris Basin. It’s about 960 miles (1,550 km) across — big enough to fit the entire state of Texas inside. Caloris was made about 3.9 billion years ago, when an asteroid roughly 60 miles (100 km) wide slammed into the planet during a period scientists call the Late Heavy Bombardment.
The impact was so powerful that seismic waves traveled all the way through the planet. On the exact opposite side of Mercury, the rocks were jumbled into a strange landscape that scientists actually call the “weird terrain” (its formal name is chaotic terrain). The shock from the Caloris hit moved through the inside of the planet and scrambled the surface on the far side.
At the center of Caloris is a feature called Pantheon Fossae — a network of long radial cracks that fan out from a central crater like spokes on a wheel. From space, it almost looks like a giant spider lying on the planet’s face. No other crater in the solar system looks quite like it.
Craters Named After Authors and Musicians
Mercury has thousands of craters — far more than Earth, because Mercury has no atmosphere or weather to wear them down. There’s an official rule for naming them: each crater is named after an artist, writer, or musician from any culture, who has been dead for at least three years and famous for at least 50. So far, named craters include Tolkien (J. R. R. Tolkien, author of The Lord of the Rings), Disney (Walt Disney), Mark Twain, Lennon (John Lennon of the Beatles), and Dr. Seuss (Theodor Seuss Geisel). Somewhere out there, on a tiny gray world close to the Sun, there is officially a crater named after the guy who wrote Green Eggs and Ham.
Mercury Is Shrinking
Mercury also has long, curving cliffs called lobate scarps. These formed because Mercury is shrinking. Yes — the actual planet is getting smaller. As its iron core slowly cools down, it contracts, and the rocky outer shell crumples to fit. Mercury’s radius has shrunk by about 4 miles (7 km) since it formed, and the planet is still shrinking today, kind of like an apple drying out and getting wrinkly.
Mercury Has Almost No Air
Mercury barely has an atmosphere. What it does have is called an exosphere — a layer of gas so thin that it’s almost a vacuum. It’s made of small amounts of oxygen, sodium, hydrogen, helium, potassium, and calcium. These atoms are constantly being knocked off the surface by sunlight and tiny meteoroids, then drifting back down or escaping into space.
Because Mercury’s exosphere is so thin, it can’t scatter sunlight the way Earth’s atmosphere does. So if you stood on Mercury at noon, the sky would still be black, with the Sun blazing in the middle of it and stars visible all around. You also couldn’t breathe there — there’s no real air to fill your lungs.
Mercury does have a magnetic field, even though it’s tiny — only about 1% as strong as Earth’s. The bubble of space carved out around the planet by that magnetic field is called a magnetosphere, and Mercury’s magnetosphere is the smallest of any planet that has one. That little magnetic field still does something amazing: when the solar wind from the Sun crashes into it, it creates magnetic tornadoes — twisting funnels of charged particles that spiral down to the surface. NASA’s MESSENGER spacecraft was the first to spot them.
Does Mercury Have Moons or Rings?
No and no. Mercury has zero moons and zero rings. Out of the eight planets in the solar system, only Mercury and Venus have no moons at all. Earth has 1 moon, Mars has 2, Saturn and Jupiter have dozens, and Mercury has none.
Why not? Mercury is so close to the Sun that the Sun’s gravity is far stronger than Mercury’s. Any moon that ended up in orbit around Mercury would be pulled away by the Sun. Mercury’s small size and weak gravity also make it hard to capture a passing object. So if there ever were moons around Mercury, they’re long gone.
Is There Water on Mercury?
Yes — and that’s one of the most surprising facts in modern space science. Mercury has water, but only as ice, and only in deep craters near its north and south poles.
Here’s the trick. Mercury barely tilts on its axis, so the floors of the deepest craters at its poles are in permanent shadow. Sunlight has never reached the bottom of those craters. So even though the rest of Mercury bakes at 800 °F during the day, those crater floors stay below -280 °F (-170 °C) — cold enough for ice to last for billions of years. Scientists call them cold traps.
NASA’s MESSENGER spacecraft confirmed in 2012 that there are at least 100 billion tons of frozen water tucked away in those shadowed craters. Some of those craters have great names: Prokofiev, Kandinsky, Tolkien, and Gordimer. The water probably came from comets crashing into Mercury over billions of years and leaving water behind in the only places it could survive.
That doesn’t mean Mercury could support life. The conditions are far too harsh — wild temperature swings, almost no air, and intense radiation from the Sun. No life has ever been found on Mercury, and humans couldn’t survive there without a heavily shielded base.
Three Spacecraft Have Visited Mercury
Reaching Mercury is harder than reaching almost any other planet. The Sun’s gravity keeps pulling spacecraft toward it, and slowing down enough to enter orbit around Mercury takes years of careful flying. Only three missions have ever made the trip.
Mariner 10 (1974–1975)
NASA’s Mariner 10 was the first. It flew past Mercury three times in 1974 and 1975, mapping about 45% of the surface. Mariner 10 also discovered that Mercury has a magnetic field, which was a big surprise at the time.
MESSENGER (2011–2015)
MESSENGER (short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging) was the first spacecraft ever to enter orbit around Mercury. The MESSENGER probe launched in 2004 and took 6.5 years to arrive. Once in orbit, it mapped 100% of the surface, photographed the Caloris Basin in fine detail, and confirmed the polar water ice. After running out of fuel in 2015, MESSENGER crashed into the planet, adding one final crater of its own.
BepiColombo (2018–present)
BepiColombo is the newest mission, run by the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). It launched in October 2018 and made its sixth and final flyby of Mercury on January 8, 2025, photographing the planet’s north pole and the dark, possibly icy craters there. BepiColombo will enter orbit around Mercury in November 2026, and its main science mission begins in early 2027. The mission is named after Giuseppe “Bepi” Colombo, the Italian mathematician who first figured out Mercury’s strange 3:2 spin pattern.
How Did Mercury Get Its Name?
People have been watching Mercury for at least 5,000 years. The Sumerians and Babylonians tracked it across the sky as far back as 3000 BCE. Ancient observers noticed something strange about it: Mercury moved faster than any other star or planet. They actually thought it was two different objects — a “morning star” that appeared just before sunrise and an “evening star” that appeared just after sunset. Around 350 BCE, Greek astronomers figured out it was the same object.
The Romans named the planet after Mercury, their swift-footed messenger god. (The Greeks had called him Hermes.) Mercury wore winged sandals that let him move faster than anyone else. Naming the speediest planet after the speediest god made perfect sense. The same name shows up in lots of other places. The chemical element mercury, the silver liquid metal that used to be inside thermometers, is named after the same god. NASA’s first crewed spaceflight program, Project Mercury (1958–1963), borrowed the name too.
Mind-Blowing Wow Facts About Mercury
- Mercury is mostly metal. Its iron core takes up about 85% of the planet’s radius. Earth’s core is only about 17%.
- Mercury has a tail like a comet. Sodium atoms knocked off the surface by sunlight stream behind the planet, forming a long, glowing tail that can stretch for millions of miles.
- Mercury spins three times for every two trips around the Sun — a pattern called 3:2 spin-orbit resonance. No other planet does this.
- Some craters are named after children’s book authors, including Tolkien, Dr. Seuss, John Lennon, Walt Disney, and Mark Twain.
- The next transit of Mercury — when Mercury crosses in front of the Sun — happens on November 13, 2032.
- Mercury’s day-to-night temperature change is bigger than on any other planet: about 1,100 °F (600 °C) of swing.
- From Mercury, the Sun looks about three times bigger than it does from Earth, and seven times brighter.
- The Greek philosopher Heraclitus, around 500 BCE, was probably the first person to suggest that Mercury and Venus orbit the Sun rather than the Earth.
- Galileo Galilei first looked at Mercury through a telescope in 1610 — though it was so small and so close to the Sun that he couldn’t see surface details.
- Mercury has frost made of water ice tucked into permanently shadowed craters at its poles, even though the rest of the planet bakes at 800 °F during the day.
Mercury Glossary — Big Words Made Simple
- Exosphere: a layer of gas around a planet so thin that it’s almost a vacuum. Mercury’s atmosphere is an exosphere.
- Perihelion: the point in a planet’s orbit when it’s closest to the Sun.
- Aphelion: the point in a planet’s orbit when it’s farthest from the Sun.
- Terrestrial planet: a rocky planet (Mercury, Venus, Earth, or Mars).
- Innermost planet: another name for Mercury, the planet closest to the Sun.
- Lobate scarp: a long, curving cliff caused by a planet’s surface crumpling. Mercury has many of these because it’s still shrinking.
- Cold trap: a deep, permanently shadowed crater where ice can survive even on a hot planet, because sunlight never reaches the floor.
- Magnetosphere: the bubble of space carved out around a planet by its magnetic field. Mercury’s is the smallest of any planet that has one.
- Spin-orbit resonance: when a planet’s spin is exactly synced to its trips around the Sun. Mercury has a 3:2 resonance — three spins for every two orbits.
Frequently Asked Questions About Mercury
Why is Mercury so hot during the day and so cold at night?
Because Mercury has almost no atmosphere to hold in heat. Daytime sunlight bakes the surface up to 800 °F (430 °C), but the moment the Sun sets, that heat escapes straight into space and the surface drops to -290 °F (-180 °C). Earth’s atmosphere acts like a blanket; Mercury has no blanket.
Is Mercury the hottest planet in the solar system?
No. Even though Mercury is the closest planet to the Sun, Venus is hotter. Venus has a thick atmosphere full of carbon dioxide that traps heat like a greenhouse. Mercury has no atmosphere to trap heat, so it loses warmth fast at night.
How big is Mercury compared to Earth?
Mercury is about 38% the size of Earth, with a diameter of 3,032 miles (4,879 km) compared to Earth’s 7,918 miles (12,742 km). Mercury is barely bigger than Earth’s Moon — in fact, Jupiter’s moon Ganymede and Saturn’s moon Titan are both larger than Mercury.
How long is a year on Mercury?
A year on Mercury is just 88 Earth days — the shortest year of any planet. That’s because Mercury is the closest planet to the Sun and orbits the fastest, at about 29 miles (47 km) per second.
How long is a day on Mercury?
Mercury spins once on its axis every 59 Earth days. But the time from one sunrise to the next is 176 Earth days — twice as long as a Mercury year.
Does Mercury have any moons?
No. Mercury has zero moons and zero rings. The Sun’s powerful gravity makes it impossible for Mercury to hold onto a moon.
Is there water on Mercury?
Yes, but only as ice, and only inside permanently shadowed craters near Mercury’s north and south poles. NASA’s MESSENGER spacecraft confirmed it in 2012.
Can humans live on Mercury?
No. Daytime hits 800 °F, nighttime drops to -290 °F, there’s no air to breathe, and the Sun’s radiation is intense. Astronauts would need a fully sealed, heavily shielded habitat to survive even a short visit.
What is the biggest crater on Mercury?
The Caloris Basin — about 960 miles (1,550 km) across, big enough to hold the entire state of Texas. It was made by an asteroid impact about 3.9 billion years ago.
How did Mercury get its name?
Mercury is named after the Roman messenger god, who was the swiftest of all the gods. Ancient stargazers noticed this little planet moved across the sky faster than any other, so they named it after the fastest god they knew. Mercury orbits the Sun in just 88 Earth days — the shortest year of any planet.
Why does Mercury look like the Moon?
Both worlds have almost no atmosphere, no weather, and no moving tectonic plates, so craters from billions of years ago are still there today. Mercury and the Moon are also similar in size. Together, those things make their surfaces look almost identical — gray, rocky, and covered in craters.
What missions have visited Mercury?
Three missions have visited Mercury: NASA’s Mariner 10 (1974–75, the first flybys), NASA’s MESSENGER (2011–15, the first orbiter), and the joint ESA/JAXA mission BepiColombo, which completed its sixth and final flyby in January 2025 and will enter orbit around Mercury in November 2026.
Learn More About Mercury and Other Planets
- Why Is Mercury So Hot AND So Cold?
- Mercury vs Earth — How Different Are They?
- The Caloris Basin — Mercury’s Giant Crater
- A Day on Mercury Lasts Longer Than a Year
- Mercury’s Atmosphere — Almost No Air at All
- Does Mercury Have Moons?
- Missions to Mercury — Mariner 10, MESSENGER & BepiColombo
- Water Ice on Mercury
- Strange and Surprising Mercury Facts
- Mercury Quiz for Kids
