Gypsum Gardens: Self-Assembled Tubular Structures of Calcium Sulfate with Relevance for the Detection of Extraterrestrial Life.

Form is a common and intuitive criterion to distinguish between the realm of living species and the inanimate nature. However, there are in fact no strict boundaries in terms of morphology, as exemplified by so-called chemical gardens, which form by self-assembly in purely inorganic systems and yet closely mimic the appearance of trees and other plants. While such structures have been reported for a broad range of compositions-most notably silicates of various types of metal cations as well as prominent (bio)minerals like calcium carbonate or phosphate-one important material has been missing in the comprehensive list of these chemobrionic systems: calcium sulfate. In the present work, we succeeded in preparing well-developed CaSO-based chemical gardens by addition of a concentrated solution of sodium sulfate to solid crystals of calcium chloride. The formed structures were characterized in detail with respect to their growth behavior, mineralogy, and texture. We find hollow tubular architectures consisting of oriented gypsum crystals and delineating smooth curvatures with multiple branching sites. Beyond the sheer beauty of these self-assembled mineral structures, the results of our study bear deep implications for the detection and interpretation of potential past life on Mars, where abundant deposits of calcium sulfate exist. In addition, the current picture of geochemical environments on the early red planet is fully consistent with the experimental conditions used in the present work, rendering the formation and presence of chemical gardens on Mars plausible.