Bacterial endospores are one of the hardiest life forms on Earth, renowned for their omnipresence in extreme environments, including polar ices, permafrost, and deserts. This project is devoted to the study of endospores in hyper-arid environments on Earth as terrestrial model to Mars. The Atacama Desert in Chile serves as the perfect model system because it is the driest desert on Earth. Previous studies have reported a near-sterile state in the soils. The Ponce Group has been on two sampling expeditions to the Atacama Desert to carry out a series of in situ and sample return analyses with the goal to study the microbial diversity, abundance and the response of the effect of water on the native microbes. New culture-independent methods, such as phospholipid fatty acid (PLFA) and total organic carbon (TOC) analyses were performed on the desert soils, along with terbium-dipicolinic acid (Tb-DPA) based assays. Significant differences in culturability and germinability were obtained along a depth profile in a soil pit corresponding to ages approximately equal to 25,000 and 35,000 years old based on radioactive carbon dating. A spike in microbial response was also observed when sterile water was added into otherwise-sterile desert soils. Endospore isolates obtained from the Atacama Desert were compared with laboratory strains and other environmental isolates in an effort to investigate the long-debated ecological role of endospores in a particular environment. Comparative studies were made on sporulation, outgrowth, germination kinetics, morphology, dipicolinic acid content, thermal and UV resistances. Environmental strains generally exhibited higher resistance and were less responsive to conventional chemical germinants. We are also working to assess the feasibility of using endospore abundance and viability as proxies to study the ecosystem in the Atacama.