The 8 Planets – Part 1: Mercury



What comes to mind when you think about Mercury? Perhaps the poisonous silvery liquid in old thermometers or the Roman counterpart of the Greek messenger god Hermes? The first planet of the solar system? The Moon’s look-alike? Or maybe… all of the above!

The innermost and terrestrial planet, Mercury is closest to the Sun and feels more of the Sun’s gravity than any other planet. Discovered as early as the 14th century BC, Mercury is one of the ancient planets. Mercury, like the speedy Roman messenger god, is also the speediest planet, traveling on the most elliptical (eccentric) orbit of the 8 planets. As the smallest planet, Mercury has the weakest gravity and no moons. With its heavily cratered surface, Mercury easily looks similar to Earth’s Moon and has been geologically inactive for billions of years. Its negligible atmosphere offers little resistance and protections against onslaughts of meteors and asteroids. Surprisingly, Mercury is not the hottest planet of the solar system even though it is closest to the Sun, because its very thin atmosphere cannot trap much heat. Moreover, Mercury has a low albedo (not very reflective) since its atmosphere mostly absorbs rather than reflects light from the Sun. Consequently, the Sun’s light obscures the already dim Mercury from our view, but individuals usually observe Mercury at dawn or dusk during minimal sunlight— hence, the terms morning star and evening star. Mercury’s largest surface feature is the 4,000-mile Caloris Basin, one of ~15 impact craters. Near the Caloris Basin is a region of hilly terrain called the “Weird Terrain.” Mercury has large ridges up to several hundred kilometers high on its surface. Other features are smooth plains and compression folds (rupes). Mercury’s core is large and rich in iron, since its gravity is not as strong as larger planets to compress it. It is 70% metallic and 30% silicate material. Around the now-believed-to-be molten core is the 500-700 km thick mantle and a 100-300 km think crust. In addition, due to its slow rotational period and small size, Mercury has a significant magnetic field, about 1.1% as strong as Earth’s. Like, Venus, Mercury appears in phases when observing from Earth. On average, Mercury is the closest planet to Earth at 1.08 AU (Earth to Venus is 1.3 AU).

HOW DID MERCURY FORM? – 3 Hypotheses

  1. Mercury was struck by a planetesimal 1/6 the mass of the planet and several kilometers across. The collision destroyed most of the crust and mantle, leaving a large core.
  2. Mercury was formed by the solar nebula (a gaseous cloud from which the Sun and planets formed by condensation) before the Sun’s energy output stabilized. Temperatures during the formation could have been as low as 2,500 K – 3,000 K or as high as 10,000 K. Originally twice the size as it is now, the protosun’s contraction high temperatures vaporized the outer layers of rock on Mercury.
  3. The solar nebula caused drag on materials accreting into Mercury, so that light elements left and heavier elements remained.

* MESSENGER found higher levels of potassium and sulfur than previously thought on Mercury’s surface. This means hypotheses 1 and 2 are unlikely, thus favoring the third hypothesis.

MISSIONS: MESSENGER, Mariner 10, BepiColombo


  • Order in Solar System: #1
  • Number of Moons: 0
  • Orbital Period: 88 days
  • Rotational Period: 59 days
  • Mass: 3.3 x 10^23 kg (0.055 Earths)
  • Volume: 6.083 x 10^10 km³ (0.056 Earths)
  • Radius: 2,439 km (0.3829 Earths)
  • Surface Area: 7.48 x 10^7 km² (0.147 Earths)
  • Density: 5.427 g/cm
  • Surface Pressure: trace
  • Eccentricity of Orbit: 0.2
  • Surface Temperature (Average): 340 K
  • Escape Velocity: 4.25 km/s
  • Apparent Magnitude: -2.6 to 5.7