FIELDWORK AT AN IMPACT CRATER IN BASALT AS A MEANS TO RECONCILE REMOTE SENSING AND LABORATORY ANALYSES

Save for rare rovers, fieldwork and sample analyses are generally rare for planetary surfaces other than Earth, though remote data of those surfaces are abundant. Similarly, whereas basalt and impact craters are abundant on planetary surfaces, an impact site into basalt is rare on Earth. Thus there is little ground-truthing of basaltic impactites and the ejecta lobes where they are located. This paper will discuss a three-headed approach to ground-truth remote visual and infrared data with laboratory data of samples and detailed fieldwork of ejecta lobes.

Lonar Crater, India is the best-preserved terrestrial impact site in basalt as ejecta is still present. Lonar Crater is a young (~570 ka), ~1.8 km impact site emplaced in ~65 Ma Deccan basalt, which is an excellent analog material for Mars with ~45-50% labradorite and ~35% augite/pigeonite *before* lower flows were altered and then shocked. Lab/sample data of shocked and altered basalts provide analogs for Martian meteorites and rover instrumentation. Whereas distinctive impact glasses of the five Classes of shocked basalt [Kieffer et al., 1976] will be discussed, lobes in the ejecta are dominated by at least 10 types of altered, unshocked basalt containing calcite, zeolite, chlorite, hematite, smectite, and palagonite. These alteration types in the field and corresponding petrographic/mineralogic/geochemical data will be shown.

The goal is for field measurements and laboratory data to guide the interpretation of remote data of the analog impact crater. Pre-impact stratigraphy was not complex: 3 flows of fresh basalt overlying 3 flows of aqueously-altered basalt, and both are found as impact breccia clasts in a ~8 m thick lithic (unshocked, “throw out”) and ~1 m suevite (all ranges of shock pressure, “fall out”) ejecta. Fieldwork demonstrates that underlying, altered basalt (by groundwater/aqueous alteration) is only exposed in the ejecta due to impact. Otherwise it would be at depth. Sample analyses of shocked basalt are measured from a wide range of instrumentation and compared to unshocked (both fresh and altered) basalt. Remote data from ASTER and others will be examined to determine if key outcrops exhibit spectral features corresponding to the analyses of samples collected from that outrcrop.