Identifying Compositional Heterogeneity in Mars’ Nili

Introduction: Spectral anomalies in Nili Patera, a central caldera of the Syrtis Major shield volcano, were first observed by [1] using Thermal Emission Spectrometer (TES) data. With the advent of data from the Thermal Emission Imaging Spectrometer (THEMIS), it is now possible to explore in greater detail the nature of the volcanic materials that give rise to both spectral and spatial anomalies within and adjacent to the cal-dera. Observations: THEMIS. A pair of THEMIS images is shown as a mosaic in Figure 1. Bands 5, 7, and 8 where registered and converted to red, green, and blue using a decorre-lation stretch. The result is a dramatic display of the spectral and spatial heterogeneity of the caldera that was more crudely identified using TES and Viking data [1]. The region identified as Anomaly 1 in the figure shows such heterogeneity down to the limits of THEMIS resolution. Anomaly 2 is more homogenous but crops out in several places on the floor of the cal-dera and ~50 km to the south. TES. The apparent spectral similarity of the northern and southern occurrences of Anomaly 2 that is evident in the THEMIS image was confirmed using TES data. Because a single orbit track passed over both the southern Anomaly 2 and Anomaly 1, the spectra from these regions are shown in Figure 2a to facilitate the comparison between the two. Although the spectra show strong atmospheric features, the differences between the two are due to surface emissivity. Deconvolution: To investigate the compositional character of the two anomalies, a surface-atmosphere separation was performed using the technique of [2] with the 7 endmember spectra of [3]. Figure 2b shows the results following removal of the atmospheric constituents. The modeled surface spectra are also shown. Anomaly 1 is well modeled using only the Syrtis-type basalt spectrum. Anomaly 2 is modeled more poorly, using the Acidalia-type spectrum in a ratio of 2:1 relative to the Syrtis-type spectrum. Discussion: Anomaly 2 is poorly modeled using only the Mars-based spectral endmembers (Fig. 2b). This suggests that there are spectral components that are missing. A critical distinguishing spectral feature between the two anomalies occurs at ~470 cm-1 where a prominent narrow and relatively deep trough is present. This feature is absent from the Syrtis-type end-member but present in a less pronounced form in the Acidalia-type. Figure 2c shows that the feature at