Mercury: Origin Theory
Subject: Origin of Planet Mercury
Brief Description: Smallest planet; closest to the Sun; fastest orbital speed.
Basic Synopsis: Mercury is the remnant of a gas giant.
A Few Basic Facts: Mercury is the second densest planet (Earth is first) with a metallic core that comprises approximately 75% of its total mass. The core is approximately 2,200 to 2,400 miles in diameter. The outer shell is only about 300 or 400 miles thick. See Figure 1.1
Detailed Synopsis: Mercury began in the same swirling gas cloud as the rest of the planets, moons and other objects in our solar system. It started with the same basic chemical composition as the other planets with the same general proportion of iron to rock to gaseous materials. When it formed, its core was a liquid iron dynamo1 and it generated a large enough magnetic field to protect its atmosphere from the solar winds. However, its rate of spin was not enough to maintain the dynamo and it quickly cooled. When this happened, the solar winds blew its atmosphere into space. Based on the size of the metallic core, it should be possible to approximate the original size of Mercury by using the solar system’s average mass/gas to iron ratio.
Starting as a gas giant accounts for the disproportionate size of its metallic core and for Mercury’s density. The planet has long puzzled scientists because the core and crust appear to have been pressured into its current compact condition. Scientists have theorized that the crumpled surface and lobe-shaped scarps (cliffs and ridges) were formed when the planet cooled and shrank. However, a large heavy atmosphere with strong wind currents and weather patterns under great pressure will account for these features.
Mercury’s magnetic field is still active. It is approximately 1% of the strength of Earth’s. Some believe this is because the core might still have some liquid properties with a functioning dynamo. However, when you consider the temperature, the spin, the density, and the size of Mercury, it is more likely that the iron in the planet has become a giant permanent magnet (ferromagnetic) with no active dynamo.
Conclusion: Mercury was a gas giant when the solar system started and it had an active magnetic dynamo that shielded its atmosphere. This was before the Sun gathered enough material to start its nuclear furnace and began generating a solar wind. Mercury’s heavy atmospheric pressure condensed the planet’s core and created its crumpled surface with ridges that extend hundreds of miles. When Mercury’s dynamo began to weaken, the solar winds began to blow its atmosphere into space. Once fully cooled, the dense metallic core was left in a weak but permanent magnet state. If scientists would like to study the core of a gas giant roughly the size of Neptune or Uranus, they need look no further than Mercury.
Figure 1.1This illustration shows the difference in core to crust ratio between the Earth and Mercury. When Mercury cooled, the liquid portion of its core became part of its solid core and the mantle became part of its crust.
Theory proposed by Shainathan