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Origin of Mercury






The origin of Mercury has strongly influenced the development of the crust, so that it is relevant to consider the problem at this stage. Various models have been proposed to account for the high density of Mercury. Early explanations involved a physical fractionation of iron from silicate based on magnetic properties or density.

These essentially rely on Mercury accumulating from small particles in contrast to the planetesimal hypothesis in which hierarchical accretion of such bodies occurs from an assortment of planetesimals of varying sizes. The success of the planetesi-mal model has led to the abandonment of these earlier ideas. The concept of a condensation sequence extending outwards from this dense iron-rich planet through the less dense terrestrial planets out to the gas giants has been a seductive trap for modelers. But the discovery of a partially liquid core, implying the presence of volatile sulfur as FeS makes such hypotheses less viable. One popular model has relied on a chemical separation in the nebula of iron from silicates due to volatility differences. This model builds Mercury from small particles but relies on the small differences in volatility between iron and ma gnesi um – irons ilicates that were in volved. It suffers from the major de fec t th at the c ond en sa tion te mper atu r e of i ro n (13 60 K) is ve ry close to that of other refractory elements, not ably magnesium (1340 K) and silicon (1300 K) that form the other major mantle components of the terrestrial planets. Other models have invoked vaporization of the silicate shell, but have been largely abandoned.

The current explanation for the high iron/silicate ratio of Mercury lies within the framework of the planetesimal hypothesis. Bodies the size of Mercury formed within the first few million years after T zero and slowly accreted into the Earth and Venus. Many collisions occurred. One such is postulated to be with a body of 0.20 Mercury masses, withan impact velocity of 20 km/sec that disrupted a proto-Mercury of about twice the present mass of the planet. This giant collision removed much of an ear li er more massive silicate mantle, reducing it to centimeter-size fragments. The tougher iron core survived in this scenario and re-accreted a thin coat ing o f the dispersed s ili cate, the remaindere it her falling into the Sun or possibly being accreted onto proto-Venus or proto-Earth. Althou gh this model also reassembles the mantle (but not the core) o f Mercury from small particles, this is a differents cenario from accreting the entire planet from dustabinitio, with fund a-mentally different consequences for the composition of the planet.

Thus the high density of Mercury is an accidental consequence of its origin, not part of a grand design of the Solar System, that at a first glance extends from the dense inner planets outwards to the low-density giant planets. Although collisions were ubiquitous during the accretion of the inner planets, this large-impact hypothesis that disrupted proto-Mercury might affect obliquities and rotation rates, or form a moon if the collisional parameters are right. The impact energy of the Moon-forming event on the Earth was only about 20% of that needed to disrupt the planet but smaller analogues of Mercury and the Moon probably suffered break-ups.

Among many questions waiting to be answered are whether there was a preferential recondensation of refractory elements following the collision, or whether the reassembly of the planet was isochemical? Our judgement is that a more refractory mantle resulted, perhaps condensing from a vapor phase, as seems to have occurred with the Moon. The K/Th and/or K/U ratio, hope-fully to be established by the Messenger mission, will inform us of the relative amounts of volatile and refractory elements present in the planet and so test this idea.

Presumably some of the volatile elements survived the traumatic collision as shown by the presence of the alkali ions sodium and potassium in the tenuous mercurian atmosphere, but we require new data before we can even begin to have an adequate understanding of the composition of the planet.

Once the planet was finally assembled, differentiation must have occurred rapidly. Several principal observations support this conclusion: the high density of Mercury, the presence of silicate material and a lunar-like composi-tion and topography at the surface and the absence of younger geological activity. These observations lead to the conclusion that the planet has a high iron content, which must be segregated into a core about 0.75 – 0.80 of mercurian radius, overlain by a thin silicate mantle and crust. The heavy cratering of the crust must have been early, by analogy with the Moon, and the crust must have been thick enough and cold enough to preserve the record of this bombardment from before 4.0 Gyr.






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