Remote Sensing of Geologic Materials at Man-made Craters

Introduction: Craters made by nuclear detonations at the Nevada Test Site (NTS) provide unique test beds for cratering processes and remote detection of the geologic materials present [1,2,3,4]. Here we present results of a remote sensing study that detected an unusual infrared spectral signature at Schooner crater. The signature matches an unusually fine tectosilicate. The deposits are present as large beds. Laboratory analysis shows a composition of feldspar and unexpectedly, tridymite and cristobalite. The finding is significant because: (1) Most remote sensing searches of Mars use laboratory spectra measured of coarse silicates (greater than ~75 μm). This study shows that interesting material at a cratering site may remain undetected in searches that do not include spectra of fine silicates. (2) The detection illustrates that it is important to detect a material even when the material is not included in the database used for the search. However, a typical “linear unmixing” approach does not accomplish that type of search well. Schooner Crater: Schooner has been long recognized as a good analog to craters on the moon and Mars [2,3]. Schooner is part of a series of large craters made at the NTS to study excavation effects [5]. The craters are particularly valuable because the controlled site access preserved them relatively undisturbed, and for the extensive geologic and drilling studies. Schooner was made in 1968 using a 31 kt nuclear detonation with a charge depth of 108 m [3,6]. The resulting crater has an apparent depth of 63 m [6]. Schooner is an interesting potential analog to craters in layered terrain on Mars. The Schooner site was selected to combine strong and weak layering with a water table [6]. The detonation was at approximately the boundary between underlying densely welded tuff, and overlying non-welded or weakly welded tuff (Table 1). The perched water table at this boundary caused a significant gas acceleration phase [2]. Airborne Data: In June 2005, “SEBASS” imaged Schooner, covering the 3–5 and 7.5–12.5 μm ranges in 256 bands, at 2 m/pixel spatial resolution. SEBASS is the only airborne imager used for thermal-infrared, hyperspectral Mars and lunar analog studies [7]. Ground Data: In Feb 2006 we imaged Schooner using RamVan, which is a ground-based imaging spectrometer. RamVan covers the 7–13 μm range in 181 bands. The Mars 2003 rover MiniTES is similar to the RamVan instrument. Both are raster-scanning, thermal infrared spectrometers [8]. RamVan has scanned several other craters at the NTS [9,10]. Spectral behavior of fine silicates: Coarse silicates typically have a broad emissivity trough centered in the ~9–10 μm spectral range, called a reststrahlen band [11]. However, as the particles become small enough to become translucent, radiance begins to survive passage through the grains, and that alters the spectral shape. That effect is called “volume scattering” [12,14,16,17]. For fine silicates, volume scattering begins to invert the ~9–10 μm emissivity trough, and for tectosilicates causes a broad spectral trough centered at ~11.5 μm [12, 13]. The trough occurs in the ~11 μm range for these silicates because that is where their transmission is high. Salisbury and Wald [14] refer to this broad emissivity trough in very fine silicates as a “transparency” feature [12,14]. They also note that the transparency feature can provide compositional information. Fig. 1 illustrates this spectral behavior for an example rhyolite from the ASTER spectral library [15].