Mercury Earliest Atmosphere

Mercury is a really peculiar planet. The tiniest in the solar system and the one nearest to the sun, it rotates slowly in a 3:2 spin resonance and experiences hot temperatures of up to 430 degrees Celsius during the day and cold temperatures of -170 degrees Celsius at night.

Its average density is just 1.5 percent lower than Earth’s and is the second highest in the solar system due to its greater iron-rich core than Earth. Surprisingly, it was discovered that Mercury’s surface is rich in volatile elements like sodium and sulphur despite being so close to the sun.

In particular, Mercury may have had a magma ocean early in its formation based on the planet’s division into an iron-rich core and rocky mantle (the geological region separating the core and the crust).

This ocean would have vaporised like any liquid, but on Mercury, the temperatures were probably so elevated that rock, not water, made up the vapour. In a recent study, Noah Jäggi and colleagues addressed the unresolved issue of why slightly volatile elements like sodium have built up on Mercury’s surface by modelling how the evaporation of the outermost layer of this magma ocean would create an atmosphere and determining whether declines from the atmosphere could alter Mercury’s composition. Jäggi, a graduate student at the University of Bern, stated to Phys.org that their findings were unexpected.

Lindy Elkins-Tanton

According to Lindy Elkins-Tanton, the director of the School of Earth and Space Exploration at Arizona State University, it is not uncommon for rocky planets during their early formation to have magma oceans.

This phenomenon is believed to occur in all rocky planets, where the intense impacts during the later stages of planet formation generate enough energy to melt the planets to varying depths.

In the early solar system, there were many flying rocks, significant collisions, and powerful bombardments. The planet’s surface and interior remained molten due to the heat created by these incidents, radioactive decay, and the gravitational setting of Mercury’s iron-rich core.

According to models, these activities raised the surface’s temperature to roughly 2,400 K (3,860 degrees Fahrenheit). Could Mercury’s composition alter as a result of atmospheric loss and evaporation?

Jäggi and his team made two initial assumptions about Mercury, one of which was bigger than it is now, as some scientists theorise, and four potential compositions for magma oceans. Volatile substances including water, hydrogen (H2), carbon dioxide, and carbon monoxide disintegrate in magma and can escape as a gas upon pressure release.

Only at the extremely high temperatures assumed to have occurred in the early magma ocean can relatively nonvolatile, rock-forming elements like silicon, sodium, or iron persist as gases such as silicon monoxide (SiO).

Volatile species have substantially higher equilibrium vapour pressures than nonvolatile species do for a specific temperature, which distinguishes them from each other in the world of gaseous species. This is the pressure that the atmosphere applies to the surface of the atmosphere and the magma when they coexist.

The research team used a coupled interior-atmospheric model to calculate the impact of ocean evaporation into the atmosphere and the subsequent mass loss from the atmosphere either to space or back to the earth after taking into account atmospheric chemical and physical processes. The Earth was cooling in the interim. Jäggi considered 1,500 K as a suitable approximation for the lifespan of the surface melt and as the end line for mass loss sourced from the magma ocean of Mercury since liquid magma starts to crystallise around 1,700 K (2,600 degrees Fahrenheit).

Volatile and nonvolatile scenarios

In both the cases of volatile and nonvolatile scenarios, the molten rock ocean undergoes evaporation to contribute to the formation of the atmosphere. There are four possible ways for molecules to leave the atmosphere: heating by the solar wind’s charged particles causes the plasma to escape; extremely energetic solar photons like X-rays and ultraviolet photons from the sun deep in the upper atmosphere lead to the evaporation of atmospheric substances, creating a gas outflow (also known as hydrodynamic escape); certain molecules with low mass, high altitude, and high velocity can swiftly exit the top of the atmosphere without colliding with other molecules, a process called Jeans escape; and high-energy photons generate ions that can escape through various means via a process known as photoionization.

According to the model developed by the team, they discovered that out of the four possible ways for escape, the significance of Jeans escape was minimal. Instead, the other mechanisms resulted in mass losses ranging from 1 million to 4 billion kilograms per second, depending on the timing of Mercury’s formation and assumptions regarding heating efficiency.

Among these mechanisms, hydrodynamic escape was found to have the highest impact, varying from being insignificant to being the predominant factor. Jäggi explained that this variation depended on the efficiency of heating atmospheric species and the amount of radiation generated and emitted by the early sun.

But more significantly, it was discovered that the overall mass loss from the two very different atmospheres tested—volatile and nonvolatile—was really relatively similar. Given the mass loss, the model’s estimated duration for effective interior-atmosphere chemical interaction was less than 10,000 years, suggesting atmospheric escape processes are only responsible for a small portion of Mercury’s initial mass—roughly 0.3 percent, or less than 2.3 kilometres of crust. (The radius of Mercury at this time is 2,440 km.)

Therefore, it appears that accumulated mass loss did not considerably alter the bulk mantle composition of Mercury throughout the magma ocean stage. The amount of material lost during the course of the magma ocean was thus defined by the cooling times, which rely upon the greenhouse effect created.

Jäggi expressed surprise at the overall unimportance of atmospheric mass loss from Mercury, disregarding hydrodynamic escape. This finding suggests that there must be additional factors contributing to the high sodium measurements observed on Mercury’s surface.

These measurements cannot be attributed to significant accumulation or loss based on the modeled rates of atmospheric loss and the estimated lifetimes of the molten rock ocean. The implications of these results could be extended to other celestial bodies such as the moon, exoplanets, or Earth-like planets that initiate in a hot magma phase and possess volatile elements derived from their constituent materials.

This article was originally published on Phys.Org

Reference: Noah Jäggi et al, Evolution of Mercury’s Earliest Atmosphere, The Planetary Science Journal (2021). DOI: 10.3847/PSJ/ac2dfb

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