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Register a new userOrigin of lifes building blocks in Carbon and Nitrogen rich surface hydrothermal vents
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There are two dominant and contrasting classes of origin of life scenarios: those predicting that life emerged in submarine hydrothermal systems, where chemical disequilibrium can provide an energy source for nascent life; and those predicting that life emerged within subaerial environments, where UV catalysis of reactions may occur to form the building blocks of life. Here, we describe a prebiotically plausible environment that draws on the strengths of both scenarios: surface hydrothermal vents. We show how key feedstock molecules for prebiotic chemistry can be produced in abundance in shallow and surficial hydrothermal systems. We calculate the chemistry of volcanic gases feeding these vents over a range of pressures and basalt C/N/O contents. If ultra-reducing carbon-rich nitrogen-rich gases interact with subsurface water at a volcanic vent they result in 1 mM to 1 M concentrations of diacetylene, acetylene, cyanoacetylene, hydrogen cyanide, bisulfite, hydrogen sulfide and soluble iron in vent water. One key feedstock molecule, cyanamide, is not formed in significant quantities within this scenario, suggesting that it may need to be delivered exogenously, or formed from hydrogen cyanide either via organometallic compounds, or by some as yet-unknown chemical synthesis. Given the likely ubiquity of surface hydrothermal vents on young, hot, terrestrial planets, these results identify a prebiotically plausible local geochemical environment, which is also amenable to future lab-based simulation.
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Chondrites are undifferentiated sediments of material left over from the earliest solar system and are widely considered as representatives of the unprocessed building blocks of the terrestrial planets. The chondrites, along with processed igneous meteorites, have been divided into two broad categories based upon their isotopic signatures; these have been termed the CC and NC groups and have been interpreted as reflecting their distinctive birth places within the solar system. The isotopic distinctiveness of NC and CC meteorites document limited radial-mixing in the accretionary disk. The enstatite and ordinary chondrites are NC-type and likely represent samples from inner solar system (likely $<$4 AU). Measurement and modeling of ratios of refractory lithophile elements (RLE) in enstatite chondrites establish these meteorites as the closest starting materials for the bulk of the silicate Earth and the core. Comparing chondritic and terrestrial RLE ratios demonstrate that the Bulk Silicate Earth, not the core, host the Earths inventory of Ti, Zr, Nb, and Ta, but not the full complement of V.
Geochemical and astronomical evidence demonstrate that planet formation occurred in two spatially and temporally separated reservoirs. The origin of this dichotomy is unknown. We use numerical models to investigate how the evolution of the solar protoplanetary disk influenced the timing of protoplanet formation and their internal evolution. Migration of the water snow line can generate two distinct bursts of planetesimal formation that sample different source regions. These reservoirs evolve in divergent geophysical modes and develop distinct volatile contents, consistent with constraints from accretion chronology, thermo-chemistry, and the mass divergence of inner and outer Solar System. Our simulations suggest that the compositional fractionation and isotopic dichotomy of the Solar System was initiated by the interplay between disk dynamics, heterogeneous accretion, and internal evolution of forming protoplanets.
Driven by the reduction of dangling bonds and the minimization of surface stress, reconstruction of silicon surfaces leads to a striking diversity of outcomes. Despite this variety even very elaborate structures are generally comprised of a small number of structural building blocks. We here identify important elementary building blocks and discuss their integration into the structural models as well as their impact on the electronic structure of the surface.
The inner regions of protoplanetary discs (from $sim$ 0.1 to 10 au) are the expected birthplace of planets, especially telluric. In those high temperature regions, solids can experience cyclical annealing, vaporisation and recondensation. Hot and warm dusty grains emits mostly in the infrared domain, notably in N-band (8 to 13~$mu$m). Studying their fine chemistry through mid-infrared spectro-interferometry with the new VLTI instrument MATISSE, which can spatially resolve these regions, requires detailed dust chemistry models. Using radiative transfer, we derived infrared spectra of a fiducial static protoplanetary disc model with different inner disc ($< 1$ au) dust compositions. The latter were derived from condensation sequences computed at LTE for three initial $C/O$ ratios: subsolar ($C/O=0.4$), solar ($C/O=0.54$), and supersolar ($C/O=1$). The three scenarios return very different N-band spectra, especially when considering the presence of sub-micron-sized dust grains. MATISSE should be able to detect these differences and trace the associated sub-au-scale radial changes. We propose a first interpretation of N-band `inner-disc spectra obtained with the former VLTI instrument MIDI on three Herbig stars (HD142527, HD144432, HD163296) and one T Tauri star (AS209). Notably, we could associate a supersolar (`carbon-rich) composition for HD142527 and a subsolar (`oxygen-rich) one for HD1444432. We show that the inner disc mineralogy can be very specific and not related to the dust composition derived from spatially unresolved mid-infrared spectroscopy. We highlight the need for including more complex chemistry when interpreting solid-state spectroscopic observations of the inner regions of discs, and for considering dynamical aspects for future studies.
Hydrogen cyanide (HCN) is a key feedstock molecule for the production of lifes building blocks. The formation of HCN in an N$_2$-rich atmospheres requires first that the triple bond between N$equiv$N be severed, and then that the atomic nitrogen find a carbon atom. These two tasks can be accomplished via photochemistry, lightning, impacts, or volcanism. The key requirements for producing appreciable amounts of HCN are the free availability of N$_2$ and a local carbon to oxygen ratio of C/O $geq 1$. We discuss the chemical mechanisms by which HCN can be formed and destroyed on rocky exoplanets with Earth-like N$_2$ content and surface water inventories, varying the oxidation state of the dominant carbon-containing atmospheric species. HCN is most readily produced in an atmosphere rich in methane (CH$_4$) or acetylene (C$_2$H$_2$), but can also be produced in significant amounts ($> 1$ ppm) within CO-dominated atmospheres. Methane is not necessary for the production of HCN. We show how destruction of HCN in a CO$_2$-rich atmosphere depends critically on the poorly-constrained energetic barrier for the reaction of HCN with atomic oxygen. We discuss the implications of our results for detecting photochemically produced HCN, for concentrating HCN on the planets surface, and its importance for prebiotic chemistry.
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