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Mass and Density

In 1841, the German astronomer Johann Franz Encke determined Mercury’s mass by measuring its gravitational effect on a comet that now bears his name. This measurement was within 20% of the best modern measurement of 3.3 x 1023 kilograms, or about 5.5% of Earth’s mass.

Knowing the mass of Mercury doesn't tell us its exact composition, but since we know the planet's volume as well, we can estimate what the interior is made of. We start by dividing the planet's mass by its volume to find its average density. Each element or compound has its own characteristic density, and a planet is made up of many materials of different densities. From the average density, scientists can guess what materials, and how much of each, the planet contains.

For example, Earth has an average density of 5.5 g/cm3, which is between the density of iron (8 g/cm3) and that of the silicate compounds found in rocks (3 g/cm3). We believe that Earth has an interior core of iron making up about 16% of the Earth’s volume. Mercury has a density similar to Earth, 5.4 g/cm3. Because of Mercury’s small size and high density, scientists believe that about 70% of Mercury’s mass is composed of iron, mostly contained in its core.

Learn about the Core of Mercury

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Inside and Out
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Mercury is the second smallest planet, but it is also the second densest body in our solar system. Only Earth is denser than Mercury. Jupiter, the gas giant, is the most massive of our planets, but at 1.3 g/cm3 it is denser only than Saturn.