Mercury: the last survivor?

The planet closest to the Sun, Mercury, has a series of peculiarities that make it difficult to deduce its exact origin. A new study suggests that Mercury may have been the sole survivor of a group of planets that were “born” too close to our star.

Anomalous formation?
As we have discussed elsewhere in this section, our solar system was formed from an interstellar cloud of gas and dust. Most of the mass of this cloud ended up concentrated in the Sun and the rest swirled around our star, giving rise to the disk of gas and dust in which the planets grew.

The dust on this disk was made of grains of rock and metal that, when collided, clumped together and produced larger and larger objects. If the diameter of these masses of rock and metal exceeded hundreds of kilometers, their own gravity plunged the densest elements toward the center of the object. This is the reason why the interior of today’s rocky planets (Mercury, Venus, Earth and Mars) is divided into a dense metallic core composed mainly of iron and nickel and a lighter rock mantle.

Taking this into account, Mercury is a particular planet because its metallic core is inordinately large in proportion to its small size. The simplest explanation at first glance would be that Mercury simply absorbed more metal from its environment than other planets during its formation, but this idea does not fit with what we know today about the mechanisms of planet formation. In fact, the properties of Mercury indicate that its past was much more turbulent.

Planetary collisions
In addition to the large size of its core, Mercury is characterized by containing fewer volatile elements than would be expected. Volatile elements are those that vaporize easily at high temperatures and, in this context, not only include those found in the form of gas in the Earth’s atmosphere (such as oxygen or nitrogen), but also some metals with boiling points. relatively low as potassium, cesium and rubidium.

These characteristics can be explained if Mercury had a thick rocky mantle in the past, in such a way that its mass was more or less twice that of today. During the first million years of the solar system’s history, another celestial body would have collided with Mercury and thrown much of its mantle into space. Although a fraction of the ejected rocky material would have fallen back towards the planet, being so close to the Sun, the solar wind and the influence of other celestial bodies would have removed much of this debris from Mercury, preventing the planet from recovering all its mass. original.

This hypothesis explains not only why Mercury has such a large nucleus, but also its lack of volatile substances, since the very high temperatures generated during the impact would have vaporized all those elements with low boiling points and would have allowed them to escape into space .

The last survivor
A new study has carried out computer simulations in order to emulate what the solar system might have looked like during its first million years of existence and to investigate its possible evolution. To achieve this, its authors have modeled many solar systems with different numbers of protoplanets and different masses, and then simulate the evolution of their orbits over millions of years. The objective was to find what combinations ended up producing a solar system more similar to the current one after that time.

The results of this study suggest that, in its early days, the solar system could contain between 3 and 6 protoplanets the size of Mars at a distance of between 30 and 100 million kilometers from the Sun (the strip that today is occupied by Mercury and Venus). In many of the simulated scenarios, several of these primordial celestial bodies end up colliding and giving rise to planets with characteristics similar to those of Mercury.

Matthew S. Clement et al. “Dynamical Avenues for Mercury’s Origin. I. The Lone Survivor of a Primordial Generation of Short-period Protoplanets ”. The Astronomical Journal, Volume 161, Number 5 (2021)