Sprecher
Beschreibung
The physical chemical steps leading to life on Earth took place at a time when endogenous water/rock interactions were strongly influenced by exogenous processes such as impact bombardments. At present, there is no direct measure of the influx of extraterrestrial matter in the epoch of late accretion approximately 4.48-4.0 Ga. Nevertheless, it is within that time span that life emerged on Hadean Earth, and some specific constraints on the physical and chemical regimes that modulated its emergence are now coming to light. Geochemical evidence shows that the last time the terrestrial silicate reservoirs were separated (4.48 Ga) coincides with the onset of giant planet migration (GPM) and re-set ages of asteroidal meteorites. The appearance of relatively evolved (felsic) crust and a hydrosphere on Earth before about 4.38 Ga also shows that the necessary pre-requisites for life: (i) liquid water, (ii) chemical disequilibria, (iii) organic chemical building blocks, and (iv) adequate time for biogenesis to occur were already in place a mere 100 Myr after the formation of the solar system. That does not mean that life appeared at that time, only that there were no obvious obstacles to militate the chemistry. On that basis, it is interesting to explore the degree to which extraterrestrial infall of sulphur, carbon and phosphorus fed the prebiotic planetary organic-chemistry reactor. Calculations based on monotonic decline of impactors pinned to the cratering record, dynamical simulations and geochronology for the time following GPM and benchmarked with the age of Orientale basin on the Moon at ca. 3.8 Ga, show that the total mass delivered was 21023 g. Using bulk S, C and P contents of carbonaceous (CC), ordinary (OC) and enstatite chondrites (EC) as a guide for the range of possible compositions of late accretion material, conservative ranges for the cumulative exogenous sources of these bio-essential elements to the surface zone of the Hadean Earth yield: Sulfur (5.4-11.21021g), C (7.8-691020g) and P (1.9-4.31020g). At face value, these sources are significant when compared to the contemporary surface inventories of these elements, but obviously sensitive to the chosen source (CC vs. OC vs. EC).
Bio-essential element augmentations from GPMs are likely a shared feature of some rocky differentiated exoplanets around F-, G and K-stars (M-stars are a special case that deserve separate consideration). Intriguingly, spectral data of moderately volatile vs. moderately refractory elements collected from polluted white dwarfs (WDs) suggest that rocky differentiated planets are not restricted to Sun-like stars, but are instead probably commonplace in the galaxy. The progenitors of the WDs are of stellar spectral classes (e.g. B) not under consideration for hosting worlds with biological potential.
