As humans venture back to the Moon, or onward to near-Earth objects and Mars, it is expected that the rigors of this exploration will far exceed those of Apollo . Terrestrial analogs can play a key role in our preparations for these complex voyages, since in addition to their scientific value, analogs afford the exploration community a means to safely prepare and test exploration strategies for future robotic and human planetary missions. Many relevant analog studies exist, and each is focused on a particular aspect of strategic development. Some analog programs such as the Pavilion Lake Research Project (PLRP) present the opportunity to investigate both real scientific and real exploration scenarios in tandem. The activities of this research program demand the use of techniques, tools, and strategies for underwater scientific exploration, and the challenges associated with the scientific exploration of Pavilion Lake are analogous to those human explorers will encounter on other planetary and small solar system bodies. The goal of this paper is to provide a historical synopsis of the PLRP's objectives, milestones, and contributions to both the scientific and exploration community. Here, we focus on detailing the development and deployment of an integrated science and exploration program with analog application to our understanding of early Earth systems and the preparation for future human space exploration. Over a decade of exploration and discovery is chronicled herein.

Thermal spring deposits are features of considerable interest to Mars scientists because of their potential as astrobiological oases and as records of the paleohydrology of the planet. Terrestrial counterparts can assist in recognizing such features on Mars and in developing technologies for their study and sampling. In this paper, we describe one such analog, the Dalhousie Springs complex in central Australia. The Dalhousie Springs complex is one of largest groundwater discharge landforms known on Earth. It is a carbonate-limited precipitation system due to the non-supersaturated ascending water. Spring carbonates are deposited as discrete mounds and outflow channels resting unconformably on older units. Although subject to postformation geomorphic modification, the spring deposits persist in the landscape and are recognizable long after the parental spring has shut down. We identify 14 specific microfacies belonging to seven facies, which form three environmental associations related to specific depositional environments. Diagenesis has occurred in several stages, as evidenced by distinctive textures on the deposits. Spring deposits on Mars would potentially be recognized by similar textures (although compositions may be quite different) and similar geomorphic relationships. However, in satellite images, spring deposits may be difficult to differentiate from deposits resulting from other processes that produce similar geomorphic features, including mud volcanoes, pingos, and rootless cones. Mineralogical data may assist, but ultimately ground truth will be required.

The Oligocene-Miocene Horse Spring Formation consists of sedimentary strata that record the onset and evolution of Miocene extensional tectonics in the Lake Mead region. The sedimentary basins of this formation hold critical clues for evaluating and testing competing models that attempt to explain the tectonic evolution of this important part of the Basin and Range. Detailed sedimentology, stratigraphy, isotope geochem istry, and new geochronology of carbonates of the Horse Spring Formation shed light on the details of middle Miocene depositional systems and provide important paleoclimatic and paleotopographic data that further our understanding of the geological evolution of this area. We investigated four carbonate sections in detail, two from the Bitter Ridge Limestone Member (Slot Canyon section, near the Gale Hills, and the West Longwell section, at the Bitter Ridge), one from the Thumb Member (East Longwell section, near the Bitter Ridge), and one from the Rainbow Gardens Member (Rainbow Gardens Recreation Area section), to understand the evolution of carbonate lake systems, to extricate paleoclimatic from tectonic signals in the sedimentary record, and to develop a more clear picture of the evolution of Horse Spring sedimentary basins. New 40 Ar/ 39 Ar dates from the Bitter Ridge Limestone, combined with dates in the published literature, suggest that the Bitter Ridge Lake may have evolved time-transgressively from the White Basin area in the east to the Rainbow Gardens area in the west. Possibly contemporaneous with this evolution, the lake gradually shifted from an open to a closed lake system, most likely due to tectonic partitioning of the basin or the creation of a tectonic sill that cut off the overflow for the lake. Stable isotope and lithofacies analyses provide one of the first detailed proxy records of paleoclimate for the Miocene of the Basin and Range and show strong evidence for an orbitally forced climate signal that represents changes in the precipitation/evaporation ratio for the Bitter Ridge Lake system. Because we can effectively show a climatic signal in the Bitter Ridge Limestone units over 100 k.y. and, likely, 40 k.y. time cycles, longer time-scale shifts in isotopic ratios are more likely due to tectonic processes. Based on a strong negative shift in oxygen isotopic ratios, previous researchers have suggested that the Lake Mead region experienced an increase in paleoelevation during Horse Spring time, while the remainder of the central Basin and Range to the north experienced a decrease in elevation for the same time period. Our data, when compared with data from the Pliocene Hualapai Limestone and those presented by previous researchers, appear to constrain the timing of this isotopic shift to between 15 and 13 Ma, coincident with the timing of the onset of rapid extension in this part of the Basin and Range. We hypothesize that this isotopic shift was due not to a change in paleoelevation due to magmatic activity alone, but was influenced by either (1) longer travel distances of air masses and the development of increased topographic corrugation as the Lake Mead region experienced accelerated rates of extension or (2) drainage reorganization of the early Colorado Plateau and the infusion of isotopically lighter waters from this emergent source.

Sediments deposited over the past 220,000 years in Bear Lake, Utah and Idaho, are predominantly calcareous silty clay, with calcite as the dominant carbonate mineral. The abundance of siliciclastic sediment indicates that the Bear River usually was connected to Bear Lake. However, three marl intervals containing more than 50% CaCO 3 were deposited during the Holocene and the last two interglacial intervals, equivalent to marine oxygen isotope stages (MIS) 5 and 7, indicating times when the Bear River was not connected to the lake. Aragonite is the dominant mineral in two of these three high-carbonate intervals. The high-carbonate, aragonitic intervals coincide with warm interglacial continental climates and warm Pacific sea-surface temperatures. Aragonite also is the dominant mineral in a carbonate-cemented microbialite mound that formed in the southwestern part of the lake over the last several thousand years. The history of carbonate sedimentation in Bear Lake is documented through the study of isotopic ratios of oxygen, carbon, and strontium, organic carbon content, CaCO 3 content, X-ray diffraction mineralogy, and HCl-leach chemistry on samples from sediment traps, gravity cores, piston cores, drill cores, and microbialites. Sediment-trap studies show that the carbonate mineral that precipitates in the surface waters of the lake today is high-Mg calcite. The lake began to precipitate high-Mg calcite sometime in the mid–twentieth century after the artificial diversion of Bear River into Bear Lake that began in 1911. This diversion drastically reduced the salinity and Mg 2+ :Ca 2+ of the lake water and changed the primary carbonate precipitate from aragonite to high-Mg calcite. However, sediment-trap and core studies show that aragonite is the dominant mineral accumulating on the lake floor today, even though it is not precipitating in surface waters. The isotopic studies show that this aragonite is derived from reworking and redistribution of shallow-water sediment that is at least 50 yr old, and probably older. Apparently, the microbialite mound also stopped forming aragonite cement sometime after Bear River diversion. Because of reworking of old aragonite, the bulk mineralogy of carbonate in bottom sediments has not changed very much since the diversion. However, the diversion is marked by very distinct changes in the chemical and isotopic composition of the bulk carbonate. After the last glacial interval (LGI), a large amount of endogenic carbonate began to precipitate in Bear Lake when the Pacific moisture that filled the large pluvial lakes of the Great Basin during the LGI diminished, and Bear River apparently abandoned Bear Lake. At first, the carbonate that formed was low-Mg calcite, but ~11,000 years ago, salinity and Mg 2+ :Ca 2+ thresholds must have been crossed because the amount of aragonite gradually increased. Aragonite is the dominant carbonate mineral that has accumulated in the lake for the past 7000 years, with the addition of high-Mg calcite after the diversion of Bear River into the lake at the beginning of the twentieth century.