ALL KNEES AND ELBOWS: HALITE'S GROWTH MORPHOLOGY AND THE ORIGIN OF LIFE

Among the reasons to investigate the properties of halite (NaCl) in the study of the origin of life on Earth are the apparent primitiveness of some halophilic (salt-loving) organisms, the hypothesis that evaporite deposits may have supported the development of the earliest biomolecules, the fact that many modern salt flats teem with unicellular life, and the recognition that the surface of Mars during its water-bearing stage may have hosted major evaporite basins. The question is by what specific mechanism(s) halite might have participated in the development and/or preservation of biotic or pre-biotic molecules. We carried out experiments that addressed the dynamics of actively crystallizing halite in the presence of polymer nanoparticles (organic amphiphilic macromolecules) that acted as analogs to proto-cellular material. Optical microscopy, atomic force microscopy, and laser scanning confocal fluorescence microscopy were used to trace the localization of the nanoparticles during and after halite crystallization. Our study revealed that the organic molecules are not regularly incorporated within the halite by way of specific molecular interactions with an atomic plane of the mineral. Instead, they are either concentrated on the mineral's surfaces or incorporated in fluid inclusions. Thus, the distribution of the organic molecules is controlled largely by the morphologic surface features (enhanced by the evaporitic growth forms) of halite. Rejection by (i.e., non-incorporation into) the crystallizing halite causes the organic molecules to increase in concentration in the evaporating brine. Ultimately the organics either adsorb in rectilinear patterns onto the hopper-enhanced surfaces and along discontinuities within the crystals, or they are encapsulated within fluid inclusions. Thus, our experiments showed that a mineral does not need to continuously incorporate organic molecules during its crystallization in order to preserve those molecules. Instead, the accumulation can be determined by morphologic/growth effects. Of additional importance in origin-of-life considerations is that halite in the natural environment rapidly can change its role from a protected repository (in the absence of water) to a source of organic particles (as soon as water is present) when the mineral dissolves.