How Big Is Mercury?
QUICK ANSWER
Mercury has a diameter of about 3,032 miles (4,879 km), making it the smallest planet in our solar system. It's roughly 38 percent the size of Earth and only about 1.4 times larger than our Moon. Two moons in the solar system, Ganymede and Titan, are actually larger than Mercury.
Mercury isn't just the closest planet to the Sun, it's also the smallest. And not by a little. Mercury is small enough that two moons in our own solar system are bigger than it, which raises a fair question about where the line between planet and moon really sits.
What is Mercury's actual size?
Mercury's diameter is about 3,032 miles (4,879 kilometers), according to NASA. For comparison, Earth's diameter is roughly 7,918 miles, so Mercury is about 38 percent of Earth's size. Mercury's surface area is around 28.9 million square miles, which sounds like a lot until you realize that's less than the land area of the Americas combined. By volume, you could fit about 18 Mercurys inside Earth.
Is Mercury smaller than the Moon?
Mercury is bigger than Earth's Moon, but not by much. The Moon's diameter is about 2,159 miles, so Mercury is roughly 1.4 times the Moon's size. From a distance, the two would look like cousins more than parent and child. Mercury is also surprisingly dense for its size, with a much heavier interior than the Moon, which means it packs more mass into that small frame than its appearance suggests.
Are any moons larger than Mercury?
Yes, two. Ganymede, the largest of Jupiter's moons, has a diameter of about 3,273 miles, making it bigger than Mercury. Saturn's largest moon, Titan, comes in at about 3,200 miles across, also larger than Mercury. Both moons would technically qualify as planets if they orbited the Sun directly instead of orbiting their parent planets. Size alone isn't what makes something a planet; orbit and how it cleared its surroundings matter too.
Why is Mercury so small?
Mercury's small size comes down to where and how it formed. Closer to the young Sun, the intense heat prevented lighter materials like ice and gases from sticking around, so only heavier rock and metal could accumulate. Mercury also lost a significant portion of its outer mantle, possibly from a massive impact early in its history. What's left is a small, dense, mostly metallic world with an iron core that takes up most of its interior.
Mercury holds the record for smallest planet in our solar system, at about 3,032 miles across. Its size is the result of being too close to the Sun for lighter materials to gather, and possibly losing part of itself to an ancient impact. The result is a compact, heavy little world that's smaller than two of the moons orbiting its own neighbors.
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