The earliest atmosphere on Mercury

by David Appell

Mercury's early magma surface and atmospheric constituents are shown in a schematic. Credit: NASA.

Mercury is not normal. The smallest planet in the solar system is in a 3:2 spin resonance, which1-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-6556 It has the second-highest average density in the solar system, just 1.5 percent below Earth, due to its larger iron-rich core. Mercury's surface was rich in volatile elements such as sodium and sulfur, despite its proximity to the sun.

Mercury's separation into an iron-rich core and rocky mantle suggests it had a magma ocean early in its formation. The ocean would have evaporated, but in the case of Mercury, the temperatures were so high that the vapor was not composed of water, but rock. Noah Jggi and his colleagues modeled how the surface of the magma ocean would form an atmosphere and determined if the losses from the atmosphere could affect Mercury's composition. Jggi, a graduate student at the University of Bern, said their results were surprising.

The director of the School of Earth and Space Exploration at Arizona State University said that early planetary magma oceans are not unusual. All rocky planets have at least one or more magma oceans as they form. The impacts of accretion toward the end of planet formation will melt the planets to some depth.

The early solar system was full of flying rocks and large accidents. Mercury's surface and interior were molten because of the heat generated by the events and the radioactive decay that occurred. The models show that the temperature of the surface rose to 2,400 K. Is it possible that atmospheric loss could change the makeup of Mercury?

Jggi and his team assumed two initial sizes for Mercury, one larger than today's, and four possible ocean compositions. Carbon dioxide, carbon monoxide, hydrogen (H2), and water can escape when pressure is released. It is thought that the very high temperatures of the early magma ocean may have created the conditions for the creation of gases such as Silicon monoxide. For a given temperature, the equilibrium vapor pressures of the volatile and nonvolatile gaseous species are much different. When the two coexist, the atmosphere exerts pressure on the atmosphere-magma surface.

After accounting for atmospheric chemical and physical processes, the resulting mass loss from the atmosphere either to space or back to the planet was determined. The planet was cooling. The lifetime of the surface melt can be estimated by using 1,500 K of liquid magma, which is 1,700 degrees Fahrenheit.

The atmosphere is supplied by the magma ocean in both the volatile and nonvolatile case. Molecules can escape the atmosphere in four different ways, including photoevaporation of atmospheric species from the sun deep in the upper atmosphere,plasma heating from the solar wind of charged particles, and X-rays and ultraviolet photons from the sun deep in the upper atmosphere.

The team's model found that the four escape mechanisms were not very useful, with the others leading to mass losses from 1 million to 4 billion kilograms per second, depending on the timing of Mercury's formation and assumptions about heating efficiency.

The total loss of mass from the two different atmospheres was found to be the same. The model's timescale for efficient interior-atmosphere chemical exchange was less than 10,000 years, implying atmospheric escape processes only account for a small portion of Mercury's initial mass. The present-day Mercury's radius is 2,440 km.

Mercury's bulk mantle composition seems to have not been altered by cumulative mass loss. The cooling times, which are dependent on the greenhouse effect, determined how much material is lost over the lifespan of the magma ocean.

Jggi said the total atmospheric mass loss from Mercury was surprising. It tells us that the high sodium measurements on Mercury cannot be accumulated or lost in a significant amount, as they cannot be modeled and predicted. The results could be extended to the moon, an exoplanet or Earth-like planet that begins in a hot magma phase with a volatile budget delivered by its building blocks.

The Evolution of Mercury's Earliest Atmosphere was written by Noah Jggi. There is a DOI of 10.847/PSJ/ac2dfb.

The Planetary Science Journal has information.

The Science X Network will be launched in 2021.

The earliest atmosphere on Mercury was retrieved on December 24th, 2021.

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