The Search for Underground Hydrothermal Activity Using Small Craters: an Example from the Nevada Test Site

Craters can create windows into subsurface geology. Hydrothermal alteration products that correlate with ejecta from small craters (<100 m deep) can flag possible near-surface hydrothermal activity. A region with such activity is a highly prized target for Mars exploration. Here we describe an airborne (satellite analog) study that identified mineral indicators of hydrothermal activity exposed by manmade explosion craters in a basalt flow. Abstract [1] presents the related ground-based study. This field development work draws mainly on operational expertise from outside NASA. One goal is to develop an operational foundation for routes to discovery for Mars. Introduction: Near-surface explosive tests at the Nevada Test Site (NTS) in the Mojave desert created numerous small craters (~25–400 m diameter) that are unique Mars analog sites. The NTS is a restricted access test facility, ~65 miles northwest of Las Vegas. The U.S. Department of Energy manages the NTS. The NTS sites are particularly valuable because controlled access preserved the craters relatively undisturbed. Infrared spectroscopy is the primary method used to explore the mineralogy of Mars remotely. The MarsLab project is the first thermal infrared, hyperspectral study of the mineralogy exposed by small, fresh terrestrial craters. The overarching project goals are: (1) define the types of materials that infrared airborne (satellite analog) and ground-based spectrometers identify and miss, and explain why; and (2) define implications for the exploration of Mars. Here we describe exploration of manmade craters on a basalt flow. The techniques used mimic those that are, or can be, used for Mars. Craters: We selected craters in Buckboard Mesa, a lava flow described as an olivine basalt, with a silica content of ~53% [2], and ~2.8 million years old [3]. The flow is ~70 m thick at the two crater sites discussed here [4]. Most of the basalt is gray to black, but local regions are oxidized to a reddish color. The flow overlays tuffaceous sandstone. The water table is ~240 m below the upper mesa [2]. Approximately 0.5– 4 m of aeolian silt and sand cover much of the flow [2]. Thus in most regions, aeolian deposits dominate the airborne observations. Thin carbonate [2] and opalline coatings occur on the ejecta basalt, especially at joints (Fig. 1). The coatings were formed by water that flowed along the joints [2,5]. During upheaval from the explosion (spalling), the rocks separated along the joints, preferentially exposing the coatings in the ejecta. Thus the ejecta can have notable coverage by the coatings, even though the volume of the hydrothermal alteration material may be small. The preferential exposure aids remote detection. Here we discuss studies of the craters Buckboard 12 and Danny Boy (Table 1). Other craters in basalt where we have airborne data include Buckboard 10, Pre-Schooner Bravo, Charlie, and Delta.