| Popis: |
In March 2015, the NASA Dawn spacecraft entered orbit around Ceres, the largest object in the main asteroid belt. The Visible and InfraRed (VIR) mapping spectrometer onboard Dawn unveiled the mineralogy of Ceres at unprecedented spatial resolution. The VIR thermally-corrected average spectrum of Ceres revealed the presence of several absorption bands in the 2.5 to 4 µm region (at 2.7 µm, 3.1 µm, 3.3-3.4 µm, and 4 µm). The 2.7-µm band is diagnostic of hydrous minerals, the 3.1 µm band is associated with NH4-phyllosilicates, and the simultaneous occurrence of both the 3.3-3.4 and 4-µm bands is indicative of carbonates [1]. Although Ceres displays general homogeneity in surface composition, some exceptions can be found at the local scale. For example, Ahuna Mons is a unique isolated elliptical mountain (21x13 km), made up of a summit with sub-radial arcuate structures (ridges and troughs) and steep flanks formed by talus material [2]. Based on FC image data Ahuna Mons was interpreted to be a cryovolcano, formed by ascent of cryomagma and extrusion onto the surface followed by dome development (ibid). VIR spectroscopic analysis of Ahuna Mons highlights a lower abundance of hydrous minerals and NH4-phyllosilicates phases and a higher abundance of Na-carbonates than the surrounding areas observed at the same spatial resolution. Similarly, crater Haulani, characterized by widespread bright ejecta mixed with dark material, shows a reduction of the 2.7 and 3.1-µm bands in VIR data, probably due to lower amounts of hydrous material and NH4-phyllosilicates. Haulani's bright ejecta are characterized by a negative ("blue") spectral slope in the visible to near infrared spectral range. In contrast, the bright material unit in the center of crater Occator was discovered to be the most concentrated extraterrestrial deposit of Na-carbonates [3]. Even though each of these features has a distinct morphology, they may share some spectral characteristics. Here we discuss the mineralogy of these notable geologic features, with the goal of shedding light on their origin. |
