Mercury is small, gray, and easy to overlook in pictures of the solar system. But once you start looking closely, it turns out to be one of the strangest worlds out there. Here are 15 of the weirdest things about it — many of which most kids' Mercury articles never mention.
15 Weird and Wonderful Facts About Mercury
1. Mercury is mostly metal
About 85% of Mercury's radius is taken up by a giant iron core. The rocky outer shell on top is only about 420 km (260 miles) thick. For comparison, Earth's iron core is only 17% of its radius. Mercury is essentially a metal ball with a thin layer of rock around it. The outer part of that core is even liquid, which is what gives Mercury its surprisingly active magnetic field.
Why is Mercury so iron-rich? Scientists aren't completely sure, and they call the puzzle the iron mystery. There are three main theories:
- The giant impact theory. Early in Mercury's history, an enormous object slammed into it and blasted away most of its rocky outer layer, leaving the heavy iron core behind. Similar to how Earth's Moon is thought to have formed.
- The vaporization theory. The young Sun was so hot that it baked Mercury's lighter material until it turned to vapor, and the solar wind then blew that "rock vapor" away.
- The solar nebula theory. When the planets were forming, lighter particles got pushed outward by the swirling cloud of gas and dust around the young Sun, leaving Mercury with a higher-than-normal share of heavy iron.
BepiColombo is partly designed to help work out which of these is right. The MESSENGER probe found more potassium and sulfur on Mercury than expected, which makes the giant impact and vaporization theories a bit harder to defend — but the case isn't closed yet.
2. The planet is shrinking
Mercury is getting smaller. As its iron core slowly cools down, it contracts, and the rocky surface on top has to crumple to fit. That crumpling shows up as long, curved cliffs called lobate scarps, some of them hundreds of miles long. Mercury's radius has shrunk by about 4 miles (7 km) since the planet formed, and it's still shrinking today. Read more about Mercury's shrinking surface.
3. Mercury has a tail like a comet
The solar wind blowing past Mercury constantly knocks sodium atoms off the surface. Those atoms get carried away in a long, thin stream behind the planet, pointing away from the Sun — just like a comet's tail. Mercury's sodium tail can stretch millions of miles behind it. With the right kind of telescope, scientists on Earth can actually photograph it.
4. A day on Mercury is twice as long as a year
Mercury orbits the Sun in 88 Earth days, but it takes 176 Earth days from one sunrise to the next. So one day on Mercury is exactly two Mercury years long. Find out how that's even possible.
5. Mercury is not the hottest planet
Even though it's the closest planet to the Sun, Mercury isn't the hottest. Venus is. Venus has a thick carbon-dioxide atmosphere that traps heat like a greenhouse, keeping its surface around 870°F day and night. Mercury's heat just leaks back into space at night.
6. Mercury's craters are named after artists, writers, and musicians
The International Astronomical Union has a rule for Mercury: every named crater has to be named for a dead artist, writer, or musician (and they had to be famous for at least 50 years). The list includes:
- Tolkien — J. R. R. Tolkien, author of The Lord of the Rings
- Disney — Walt Disney, founder of Disney
- Mark Twain — American author
- Lennon — John Lennon of the Beatles
- Dr. Seuss — the creator of The Cat in the Hat and Green Eggs and Ham
- Kandinsky, Prokofiev, Gordimer — all near Mercury's north pole, all probably containing water ice
7. Mercury has magnetic tornadoes
Mercury has a magnetic field, even though it's small (only 1% as strong as Earth's). When the solar wind crashes into that magnetic field, it sometimes creates huge twisting funnels of charged particles that spiral down to the surface. NASA's MESSENGER spacecraft was the first to spot these magnetic tornadoes in 2008. More on Mercury's atmosphere and magnetic tornadoes.
8. The Sun rises twice in the same day on Mercury (sometimes)
When Mercury is at the closest point in its orbit to the Sun, the planet's orbital speed actually beats its rotation speed. From certain places on the surface, this means the Sun can rise, briefly set, and then rise again over the course of a single Mercury morning. It's the only planet where the Sun does this.
9. Mercury was once thought to always show the same face to the Sun
Until 1965, scientists believed Mercury was tidally locked to the Sun, with one side always in daylight and one side always in darkness. That turned out to be wrong. Radar measurements made in 1965 revealed that Mercury actually spins three times for every two orbits around the Sun — the unique 3:2 spin-orbit resonance that no other planet does.
10. Mercury is the second densest planet
Even though Mercury is small, it's heavy for its size. Earth is the densest planet at 5.51 g/cm³. Mercury is right behind at 5.43 g/cm³ — almost the same density as Earth, but in a much smaller package. That's because so much of Mercury is made of dense iron metal.
11. Ancient astronomers thought Mercury was two planets
Because Mercury is always close to the Sun, it can only be seen briefly just after sunset or just before sunrise. Ancient observers (going back to the Sumerians around 3000 BC) noticed Mercury appearing in the morning sky and the evening sky and thought they were watching two different objects: the "morning star" and the "evening star." Around 350 BC, Greek astronomers figured out it was the same planet. Even more remarkably, the Greek philosopher Heraclitus (around 500 BC) was probably the first person to suggest that Mercury and Venus actually orbit the Sun, not the Earth — an idea that didn't catch on properly for another 2,000 years.
The next big leap came in 1610, when Galileo Galilei turned his new telescope toward Mercury. He couldn't see surface details (Mercury's too small and too close to the Sun for a 17th-century telescope), but he could watch it move and confirm it was a planet, not a star. Later astronomers would track Mercury's strange orbital wobble for centuries, until Einstein's theory of general relativity finally explained it in 1915.
12. Mercury was named after the fastest god
The Romans named Mercury after their swift-footed messenger god (the Greek version is Hermes). Why him? Because Mercury moves faster across the sky than any other planet — it laps the Sun in just 88 Earth days. The fastest planet got the fastest god's name. Mercury's planetary symbol — ☿ — is actually a tiny picture of his winged staff, the caduceus.
13. The element mercury is named for the same god
The chemical element mercury (chemical symbol Hg) — the silvery liquid metal that used to fill old thermometers — is named after the same Roman god the planet is named after. The element was named that way because it's the only metal that's liquid at room temperature, and it slips and runs around like a swift, slippery messenger. The connection between the metal and the planet is one of the oldest crossovers between astronomy and chemistry.
14. Project Mercury was NASA's first crewed space program
From 1958 to 1963, NASA ran a program called Project Mercury. It was the program that put the first American astronauts into space, including John Glenn, who became the first American to orbit Earth in 1962. The program was named after the same Roman god, who was also the god of travelers. The name was meant to evoke speed and exploration.
15. The next transit of Mercury is November 13, 2032
A "transit of Mercury" happens when Mercury passes directly between Earth and the Sun, so we see Mercury as a tiny black dot crossing the face of the Sun. With special solar filters (or a properly equipped telescope), people can actually watch it. Transits of Mercury only happen about 13 times each century. The most recent one was on November 11, 2019. The next one will be on November 13, 2032.
Why Mercury Was Named After a Roman God
People have been watching Mercury for at least 5,000 years. The Sumerians and Babylonians watched it move across the sky as far back as 3000 BC. Different cultures gave it different names: Stilbon in ancient Greece (meaning "the gleamer"), Hermes when Greek astronomers worked out it was the same as the morning and evening star, and Mercury when the Romans took the same idea and renamed everything in Latin.
The link between the speedy planet and the speedy messenger god has stuck for over two thousand years. Most other planets in the solar system also got their names from Roman gods (Venus, Mars, Jupiter, Saturn, Uranus, Neptune), and the modern English names are essentially the Roman ones translated into English.
The Coolest Mercury Fact, Save the Last for Best
Here's the one most kids' books leave out. Mercury is the only planet besides Earth where you can stand on the surface and see your home planet shining in the sky as a tiny blue-white dot. From Mercury, Earth would look much smaller than the Moon does from Earth — just a pale dot. But it would be unmistakably blue.
If a future astronaut ever stands at the rim of Mercury's Caloris Basin and looks up, they'll see Earth from farther away than any human has ever seen it before — tens of millions of miles closer to the Sun than home, looking back at the only planet we know of that has people on it.
Frequently Asked Questions
How did Mercury get its name?
Mercury is named after the Roman messenger god, who was the fastest of all the gods. Ancient stargazers noticed that this little planet moved across the sky faster than any other planet — it orbits the Sun in just 88 days — so naming it after the swiftest god was a perfect fit.
What is Mercury made of?
Mercury is mostly metal. Its iron core takes up about 85% of the planet's radius. Earth's core is only about 17%. Mercury is basically a giant iron ball with a thin rocky shell about 420 km thick on top.
Why is Mercury shrinking?
Mercury's giant iron core is slowly cooling down. As it cools, it contracts, and the rocky outer shell crumples to fit. That crumpling makes the long curved cliffs called lobate scarps. Mercury's radius has shrunk by about 4 miles (7 km) since it formed.
Are Mercury's craters really named after Dr. Seuss?
Yes! Mercury's craters are officially named after artists, writers, and musicians who have been dead for at least three years and famous for at least 50. There are craters named for Tolkien, Walt Disney, Mark Twain, John Lennon, and Dr. Seuss, among many others.
What is a transit of Mercury?
It's when Mercury passes directly between Earth and the Sun, and we see Mercury as a tiny black dot crossing the face of the Sun. They only happen about 13 times each century. The next one is on November 13, 2032.
Why does Mercury have a tail like a comet?
The solar wind constantly knocks sodium atoms off Mercury's surface. Those atoms get pushed away from the Sun, forming a long, thin glowing tail behind the planet that can stretch for millions of miles. With the right telescope, you can actually photograph it from Earth.