Visible/near Infrared Spectral Trends between Meridiani Planum Surface Materials: Comparisons between Spherules, Basaltic Sands, Outcrop, Rinds, and Cobbles

Introduction: The Mars Exploration Rover Oppo-tunity has been exploring a portion of Meridiani Planum and has encountered a number of surface materials. These include light-toned sulfate-rich outcrop (the Burns Formation), basaltic sand, hematite-rich spherules, and, less commonly, dark-toned cobbles and dark-toned rind and fracture fill materials. The nature of the outcrop has been extensively examined [e.g., 1,2,3,4,]. While addressed by [3], the nature of the rind/fracture fill material remains enigmatic. The cobbles encountered at various places on the Meridiani plains are also enigmatic and appear compositionally diverse [5]. Multispectral visible and near infrared images from Opportunity's Pancam [6] can be used to look for common color properties between these surface materials. The Pancam Instrument and Data: Pancam consists of two digital cameras mounted on a mast over the deck of the rover. The two cameras enable stereo imaging. Each Pancam " eye " utilizes a 1024x1024 active imaging area frame transfer CCD detector array and has an 8-position filter wheel allowing multispectral imaging in the 400 to 1000 nm VNIR spectral range. Operational multispectral observations typically consisted of observations in 13 " geology " filters with spectral overlap between the eyes near 435 and 750 nm. Further details on the Pancam instrument are provided in [6]. Processing of Multispectral Data: Pancam data are calibrated to absolute radiance using pre-flight radiance coefficients derived from integrating sphere observations and corrected for detector and electronic temperature variations. The data are calibrated to radiance factor (I/F) by reference to measurements made of a calibration target mounted on the deck of the rover. An empirical correction for dust accumulation on the calibration target has been applied to the data [7]. By dividing by the solar incidence angle at the time of data acquisition, the data are converted to relative reflectance (R*). The R* spectra of materials of interest were decomposed to a number of spectral parameters discussed below. Images and Spectra of Meridiani Surface Materials: As discussed in [8], there are two broad color classes that describe Meridiani outcrop. In Pancam L3,5,7 (673, 535, 432 nm) color composites these classes appear buff and red to purple colored and are referenced as such here. In true color renditions of the Meridiani outcrop [9], these color differences are more muted. We also draw a distinction between " rinds " which are oriented coplanar, or nearly so, to bedding (Fig. 1a) and " fracture fill " which are …