![]() If you want to calculate the radius of Mercury, you need to divide the diameter of Mercury in half. The Earth’s moon is only 3,474 km, so Mercury isn’t much bigger. The largest moon in the Solar System is Jupiter’s moon Ganymede, with a diameter of 5,268 km and the second largest moon is Saturn’s moon Titan, with a diameter of 5,152 km. In fact, there are two moons in the Solar System which actually have a larger diameter than Mercury. In other words, you could put almost 3 Mercurys side to side to match the diameter of Earth. Need some way to compare that to something more familiar? The diameter of Mercury is only 38% the Earth’s diameter. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington Diameter of Mercury (and the Radius) This WAC image showing a never-before-imaged area of Mercury’s surface was taken from an altitude of ~450 km (280 miles) above Mercury. Volcanic eruptions ceased on Mercury about 700-800 million years ago when the planet’s mantle had contracted enough to prevent lava flow. These features can be seen on top of other features, which is a clear indication that they are more recent. As the planet cooled and contracted cracks and ridges formed. These created smooth plains similar to those found on the Moon. Some regions would have been filled by magma eruptions from within the planet. The planet was most likely bombarded heavily by asteroids and comets during the Late Heavy Bombardment about 3.8 billion years ago. It is unlikely to change even after NASA’s MESSENGER spacecraft maps the entire surface. That knowledge is based on partial mapping of the planet(55%). The surface of Mercury is heavily cratered, much like the Moon, and the continued presence of those craters indicates that the planet has not been geologically active for billions of years. This cooling and shrinking is thought to be evidenced by the fracturing of the planet’s surface. The spin of the planet allows a small portion of the core to cool. The liquid core of the planet occupies about 42% of the planet’s volume. Some scientists think that Mercury is actually shrinking. The aforementioned are the three criteria that we will use to show the size of Mercury in relation to Earth. That is why older material refers to Mercury as the second smallest planet. ![]() ![]() It gained its ‘smallest’ title after Pluto was demoted. Surprisingly, it is also one of the most dense. How big is Mercury? Mercury is the smallest planet in the Solar System by surface area, volume, and equatorial diameter. ![]() Quick Mercury Stats Mass: 0.3302 x 10 24 kg Volume: 6.083 x 10 10 km 3 Average radius: 2439.7 km Average diameter: 4879.4 km Density: 5.427 g/cm 3 Escape velocity: 4.3 km/s Surface gravity: 3.7 m/s 2 Visual magnitude: -0.42 Natural satellites: 0 Rings? – No Semimajor axis: 57,910,000 km Orbit period: 87.969 days Perihelion: 46,000,000 km Aphelion: 69,820,000 km Mean orbital velocity: 47.87 km/s Maximum orbital velocity: 58.98 km/s Minimum orbital velocity: 38.86 km/s Orbit inclination: 7.00° Orbit eccentricity: 0.2056 Sidereal rotation period: 1407.6 hours Length of day: 4222.6 hours Discovery: Known since prehistoric times Minimum distance from Earth: 77,300,000 km Maximum distance from Earth: 221,900,000 km Maximum apparent diameter from Earth: 13 arc seconds Minimum apparent diameter from Earth: 4.5 arc seconds Maximum visual magnitude: -1.9 ![]()
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