Mercury is the closest planet to the Sun, orbiting at only 57.9 million km. It’s classified as one of the “rocky” or terrestrial planets of the Solar System. But what is Mercury made of? How does Mercury’s composition compare to the other planets in the Solar System.
Scientists believe that Mercury has an interior composition similar to Earth. Mercury has a large core of liquid metal surrounded by a mantle of silica and a solid outer crust. In the case of Mercury, though, the core accounts for 42% of the planet, while Earth’s core is only 17%.
It’s difficult to know the actual interior composition of Mercury. Scientists know that it has a density of 5.427 grams per cubic centimeter. This is only a little less than our own planet. That means that Mercury has an interior metal core, and the rest is rock. In fact, Mercury’s core is thought to have a higher iron content than any other planet in the Solar System. It’s believed that Mercury used to be similar in composition to a chondrite meteorite, but then it was struck by a planetesimal early in its history that stripped away its outer crust and mantle. The remaining planet was much denser.
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Astronomers were unsure if Mercury’s core was solid or liquid until recently. They bounced radio signals off the surface of the planet and measured how long they took to return. They determined that Mercury was wobbling at a rate you would expect if its core was liquid and not solid.
Mercury’s surface is similar in appearance to the Moon, with large basins and impact craters. The largest crater is Caloris Basin, measuring 1,550 km across. The impact that created Caloris Basin sent shock waves around the planet, causing volcanic activity on the opposite point of the impact.
We have written many stories about what is Mercury made out of here on Universe Today. Here’s an article about a the discovery that Mercury’s core is liquid. And how Mercury is actually less like the Moon than previously believed.
We have also recorded a whole episode of Astronomy Cast that’s just about planet Mercury. Listen to it here, Episode 49: Mercury.