Internal structure of Mercury

picture:
1. Crust—100–300 km thick
2. Mantle—600 km thick
3. Core—1,800 km radius

Mercury is one of four terrestrial planets in the solar system, and is a rocky body like the Earth. It is the smallest planet in the solar system, with an equatorial radius of 2439.7 km.Mercury is even smaller—albeit more massive—than the largest natural satellites in the solar system, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material.Mercury's density is the second highest in the Solar System at 5.427 g/cm³, only slightly less than Earth’s density of 5.515 g/cm³.If the effect of gravitational compression were to be factored out, the materials of which Mercury is made would be denser, with an uncompressed density of 5.3 g/cm³ versus Earth’s 4.4 g/cm³.Mercury’s density can be used to infer details of its inner structure. While the Earth’s high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not nearly as strongly compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.Geologists estimate that Mercury’s core occupies about 42% of its volume; for Earth this proportion is 17%. Recent research strongly suggests Mercury has a molten core.

Surrounding the core is a 600 km mantle consisting of silicates.Astronomers have postulated that, early in Mercury’s history, a giant impact with a body several hundred kilometers across stripped the planet of much of its original mantle material, resulting in the relatively thin mantle compared to the sizable core.

Based on data from the Mariner 10 mission and Earth-based observation, Mercury’s crust is believed to be 100–300 km thick.One distinctive feature of Mercury’s surface is the presence of numerous narrow ridges, some extending over several hundred kilometers. It is believed that these were formed as Mercury’s core and mantle cooled and contracted at a time when the crust had already solidified.

Mercury's core has a higher iron content than that of any other major planet in the Solar System, and several theories have been proposed to explain this. The most widely accepted theory is that Mercury originally had a metal-silicate ratio similar to common chondrite meteors, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass.However, early in the solar system’s history, Mercury may have been struck by a planetesimal of approximately 1/6 that mass.The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component.A similar process has been proposed to explain the formation of Earth’s Moon (see giant impact theory).

Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized. The planet would initially have had twice its present mass, but as the protosun contracted, temperatures near Mercury could have been between
2500 and 3500 K (Celsius equivalents about 273 degrees less), and possibly even as high as 10000 K.Much of Mercury’s surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" which could have been carried away by the solar wind.

A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost from the accreting material.Each of these hypotheses predicts a different surface composition, and two upcoming space missions, MESSENGER and BepiColombo, both aim to make observations to test them.