A new theory on Mercury's origin

Of all the terrestrial planets in the Solar System Mercury stands out with a remarkably high core-mantle ratio, with its core occupying about 85% of the planetary radius. Several different theories tried to explain its high Fe/Si-ratio; the giant impact theory (e.g. [1]) for instance argues that one or more giant impacts stripped away most of the Hermean mantle, while the core remained and formed the smallest of the terrestrial planets. Another theory explains the high density of Mercury through a partial volatilization during the time of the solar nebula (e.g. [2]). Here, proto-Mercury is assumed to be substantially more massive than at present-day with a composition close to those of the other terrestrial planets. When the planet was surrounded by the hot solar nebula, however, most of the mantle evaporated, ending up with present-day Mercury. Other theories argue with the particular primordial conditions of its orbital location that might have favored the accretion of dense and volatile poor building-blocks such as enstatite chondrites (e.g. [3,4]). Messenger, however, revealed a surface composition that is surprisingly rich in volatile and moderately volatile elements [4]. This is hardly compatible with the giant impact and vaporization theory but supports hypothesis that connect Mercury’s high core-mantle ratio to the particular conditions of its orbital location.

Within this talk, we will for the first time present a new model that connects these conditions with accretion and partial planetary evaporation. We will argue that Mercury (in contrast to old evaporation theories) was released out of the nebula as a small planetary embryo, comparable in size to the moon, that was covered with a global magma ocean. While the embryo proceeds to grow through frequent impactors, (moderately) volatile elements evaporate from the magma ocean and are lost into space due to the high surface temperature, the low gravity of the body and the high XUV flux from the young Sun. Here, lighter and more volatile elements are preferentially lost from the embryo, while the heavier and less volatile elements escape less efficient. Due to the continuous growth of proto-Mercury, however, the gravitational energy will start to dominate over the thermal energy of the evaporated particles, making them harder and harder to escape, which ultimately halts the loss of moderately volatile elements. Mercury subsequently finalizes its accretion with relatively volatile rich material and evolves to the body we can observe at present-day. We simulated the escape of (moderately) volatile elements with an adopted version of a 1D hydrodynamic upper atmosphere model (e.g. [5]) and will present our results here for the first time.

References: [1] Benz, W. et al., Space Sciences Series of ISSI, Volume 26, p. 7., 2008. [2] Cameron, A.G.W., Icarus, Volume 64, Issue 2, p. 285-294., 1985. [3] Charlier, B. and Namur, O., Elements, Volume 15, p. 9-14, 2019. [4] Nittler, Larry R. et al., Science, Volume 333, Issue 6051, pp. 1847, 2011. []5 Erkaev, et al., MNRAS, Volume 460, Issue 2, p.1300-1309, 2016.