The oldest atmosphere on Mercury


A diagram of the surface of the primitive magma of Mercury and atmospheric constituents in its lower atmosphere and its non-homogeneous exosphere where the upper atmosphere merges with space. Credit: NASA

Mercury is a most unusual planet. The smallest planet in the solar system and the planet closest to the sun, it is in a 3: 2 rotational resonance, slowly spinning and experiencing scorching heat up to 430 degrees Celsius, and the freezing night side, up to -170 degrees Celsius. Due to its iron-rich core much larger than that of Earth, it has the second highest average density in the solar system, just 1.5% below that of Earth. Despite its proximity to the sun, the surface of Mercury was surprisingly found to be rich in volatile elements such as sodium and sulfur.

Notably, the separation of the planet into an iron-rich core and rock mantle (the geological region between the core and the crust) suggests that Mercury had an ocean of magma early in its formation. Like any liquid, this ocean would have evaporated, but in the case of Mercury, the temperatures would probably have been so high that the vapor was not made of water, but of rock. In a new study published in The Journal of Planetary Sciences, Noah Jäggi and his colleagues modeled how evaporation from the surface of this magma ocean would form an atmosphere and determined whether losses from the atmosphere could alter the composition of Mercury, addressing the open question of why moderately volatile elements. like sodium have accumulated on the surface of Mercury. Their results were surprising, Jäggi, a graduate student at the University of Bern, told

The first oceans of planetary magma are not unusual, explained Lindy Elkins-Tanton, director of Arizona State University’s School of Earth and Space Exploration. “We believe that all rocky planets have one or more, if not more, magmatic oceans as they form. The impacts of accretion towards the end of the planet’s formation are only energetic; they will melt the planets to a certain depth.

The early solar system was a rough, active place, full of flying rocks, massive collisions, and heavy bombardment. The heat generated by these events, in addition to radioactive decay and heat produced by the gravitational adjustment of the iron-rich core of Mercury, kept the surface and interior of the planet molten. Models indicate that these processes caused the surface temperature to rise to about 2,400 K (3,860 degrees Fahrenheit). Could evaporation then atmospheric loss change the composition of Mercury?

Jäggi and his team assumed two initial sizes for Mercury, one larger than it is today, as some scientists assume, and four possible magmatic ocean compositions. Volatile species like carbon dioxide, carbon monoxide, hydrogen (H2), and water dissolve in magma and can escape as a gas when pressure is released. Comparable non-volatile rock elements such as silicon, sodium or iron can only exist as gases such as silicon monoxide (SiO) at very high temperatures that are believed to have existed in the ocean. primitive magma. The difference between volatile and non-volatile gas species is that, for a given temperature, volatile species have much higher equilibrium vapor pressures than non-volatiles. It is the pressure that the atmosphere exerts on the atmosphere-magma surface when the two coexist.

The research team ran a coupled interior-atmosphere model to determine the effect of ocean evaporation from the atmosphere, and after taking into account atmospheric chemical and physical processes, the resulting mass loss of the atmosphere towards space or towards the planet. During this time, the planet was cooling. Liquid magma begins to crystallize at 1,700 K (2,600 degrees Fahrenheit), which makes the 1,500 K used by Jäggi a good approximation of surface melt life and sets the end point of mass loss. from the magmatic ocean of Mercury.

In both the volatile and non-volatile case, the ocean of magma evaporates to fuel the atmosphere. Molecules can escape from the atmosphere in one of four ways: heating the plasma by the solar wind of charged particles; photoevaporation of atmospheric species from very high energy solar photons such as x-rays and ultraviolet photons from the sun deep in the upper atmosphere creating a gas flow (also called hydrodynamic exhaust); Jeans Escape, where high-altitude, high-speed, low-mass molecules escape from the top of the atmosphere before encountering another molecular collision; and photoionization, where high energy photons produce ions that escape by various means.

The team’s model revealed that of the four potential escape mechanisms, Jean’s escapement was negligible, with the others resulting in mass losses of 1 million to 4 billion kilograms per second, depending on the timing of the training. of Mercury and assumptions about heating efficiency, with the upper range coming from the hydrodynamic exhaust – “from insignificant to predominant,” Jäggi said, depending on how efficiently the atmospheric species are heated and of the amount of radiation produced and delivered by the early sun.

Importantly, the total mass loss of the two very different atmospheres tested – volatile and non-volatile – turned out to be quite similar. Considering the mass loss, the resulting model timescale for efficient chemical exchange between the interior and the atmosphere was less than 10,000 years, implying that atmospheric exhaust processes represent only about 0.3% of the initial mass of Mercury, or less than 2.3 kilometers of crust. (The current radius of Mercury is 2,440 km.)

Thus, the cumulative mass loss does not appear to have significantly altered the composition of the bulk mantle of Mercury during the oceanic phase of magma. Thus, the cooling times, which depend on the induced greenhouse effect, determined the amount of material lost during the lifetime of the magmatic ocean.

The insignificance of Mercury’s total atmospheric mass loss, hydrodynamic exhaust aside, was surprising, Jäggi said. “This tells us that there must be more to the measurements of high sodium content on the surface of Mercury, as they cannot be accumulated or lost in significant amounts given our modeled loss rates and the duration of life of the magma oceans. ” The findings could be extended to the moon, an exoplanet, or an Earth-like planet that begins in a hot magma phase “with a volatile budget provided by its building blocks.”

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More information:

Noah Jäggi et al, Evolution of the oldest atmosphere of Mercury, The Journal of Planetary Sciences (2021). DOI: 10.3847 / PSJ / ac2dfb

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