MERCURY vs. MARS Mercury, the planet with the closest orbit to the sun, is only about 0.7 the size of Mars, and Mars is twice as heavy. Here are some comparisons: Mercury's Radius: 2440 km, 70% as wide as Mars Mercury's Mass: 3.30 x 1023 kg, half of Mars mass. Mercury's Density: 5.43, 138% of Mars' density