Fun Facts About Mercury, the Smallest Planet

What kind of planet is Mercury?

Mercury is the first planet from the Sun and the smallest of the eight. It's a rocky planet, like Earth, Venus and Mars, with a solid surface you could stand on. NASA says it's only slightly larger than Earth's Moon: if Earth were a nickel, Mercury would be about the size of a blueberry.

From Earth, Mercury is hard to spot because it never strays far from the Sun. You can sometimes see it low in the sky just after sunset or just before sunrise, a little bright dot that ancient sky watchers noticed moving quickly. The Romans named it for their fast messenger god, and the name fits.

Mercury's size, distance, day and year

Source: NASA Planetary Fact Sheet and NASA Science.

Mercury is the fastest planet, racing around the Sun at about 47 kilometers per second. Its orbit is the most stretched-out of all the planets: it swings in to about 46 million kilometers from the Sun and out to about 70 million.

Its day is the strangest part. Mercury spins once every 59 Earth days, but because it's also moving fast around the Sun, one full day from sunrise to sunrise lasts 176 Earth days. That's two whole Mercury years! NASA notes that from some spots on Mercury, the Sun would rise, set briefly, and rise again.

How hot and cold is Mercury?

Being closest to the Sun, Mercury gets sunlight up to seven times brighter than Earth's, and the Sun would look more than three times bigger in its sky. Daytime temperatures reach about 430 °C (800 °F), hot enough to melt lead.

But Mercury has almost no atmosphere to hold that heat. As soon as the Sun sets, the warmth escapes into space and temperatures fall to about −180 °C (−290 °F). That's a swing of more than 600 degrees, the biggest of any planet.

Surprisingly, Mercury is not the hottest planet. That title belongs to Venus, whose thick clouds trap heat so well that it stays about 465 °C day and night.

Instead of a real atmosphere, Mercury has a very thin exosphere made of atoms knocked off its surface by the solar wind and tiny meteoroids: oxygen, sodium, hydrogen, helium and potassium.

What's inside Mercury?

Tap the cut-away wedge in the diagram to explore Mercury's three layers.

1. Crust: a thin, rocky shell, gray-brown and covered in craters.
2. Mantle: a rocky layer that is unusually thin. NASA says the crust and mantle together are only about 400 kilometers (250 miles) thick.
3. Iron core: Mercury's core is huge. Its radius is about 2,074 kilometers, roughly 85 percent of the way from the center to the surface.

That giant metal core makes Mercury the second densest planet, after Earth. Part of the core may still be melted. Mercury also has a magnetic field, but it is only about 1 percent as strong as Earth's.

Why is the core so big? Scientists are still working on it. One idea is that a giant impact long ago blasted away much of Mercury's outer rock; another is that the hot young Sun boiled off lighter material. The BepiColombo mission is designed to help answer questions like this.

Craters, cliffs and ice

Mercury's surface looks a lot like our Moon's: billions of years of space rocks have left it covered in craters. Mercury's features are named after famous artists, musicians and writers who have died, so you can find craters named for Dr. Seuss and the dancer Alvin Ailey.

• Caloris Basin is one of the biggest impact craters in the solar system, about 1,550 kilometers (960 miles) across. That's wider than Texas.
• Cliffs (scarps) stretch for hundreds of kilometers and stand up to a mile high. They formed as Mercury's insides cooled and shrank, wrinkling the surface like the skin of a drying apple.
• Crater rays are bright streaks of crushed rock splashed out from young craters. They darken with age, so bright rays mean a crater is fairly new.
• Polar ice: craters near the poles are always in shadow. MESSENGER found strong evidence of water ice hiding inside them, on the planet closest to the Sun.

Missions to Mercury

Mercury is surprisingly hard to reach. A spacecraft falling toward the Sun speeds up, so it has to use lots of fuel or gravity flybys to slow down enough to stay in orbit.

• Mariner 10 (NASA) flew past three times in 1974 and 1975 and photographed part of the surface.
• MESSENGER (NASA) became the first spacecraft to orbit Mercury, from 2011 to 2015, and mapped the whole planet.
• BepiColombo, a joint mission of the European Space Agency and Japan's JAXA, launched in 2018. As of October 2026, ESA says it is due to enter orbit around Mercury on November 21, 2026, with science operations starting in 2027.

Fun facts about Mercury

• Mercury has no moons and no rings.
• A year on Mercury (88 days) is shorter than one of its days (176 days).
• Mercury's axis is tilted only about 2 degrees, so it has no seasons.
• If you weigh 80 pounds on Earth, you'd weigh about 30 pounds on Mercury. Try your own weight in the profile above.
• Mercury is shrinking: as its core cools, the whole planet has contracted, creating its giant cliffs.

Learning about Mercury at home

Start with the diagram and the "at a glance" profile, then take the facts quiz until you get a perfect score. The labeled Mercury diagram makes a good notebook page, and the planets in order sheet shows where Mercury fits.

To compare Mercury with its neighbors, visit Venus and Earth, or go back to the planets in order explorer to see all eight together in 2D and 3D.