42nd Lunar and Planetary Science Conference (2011) 2507.pdf IGNEOUS COMPOSITIONS IN ARES VALLIS: TIMING AND IMPORTANCE OF VOLCANICS AND FLUVIAL PROCESSES AT THE NOACHIAN-HESPERIAN BOUNDARY. J. H. Wilson1 and J. F. Mustard1, 1Department of Geological Sciences, Brown University, Providence, RI 02912 ([email protected]) Introduction: Ares Vallis is one of many outflow mented as described in [22]. Both the L and S detector channels on Mars generally thought to have formed by data were used. We will be using the much improved the catastrophic outflow of subsurface water e.g. [1, 2, TRR3 data formation as they becom available but this 3]. Yet, the presence of diagnostic signatures of igne- research is focused on the starndard TRR2 data. Spec- ous compositions indicates a complex history. Olivine tra were extracted from regions of interest on the basis is one of the more readily detected igneous mineral of spectral parameter maps [23] using a 5 X 5 pixel signatures and has been detected globally with various average. Residual artifacts after calibration and atmos- instruments including TES, e.g. [4, 5, 6, 7, 8], pheric removal are suppressed by ratioing the spectra THEMIS, e.g. [4, 8, 9], OMEGA, e.g. [10, 11, 12], and to a spectrally neutral region from the same column by the MER rovers in situ, e.g. [13, 14, 15]. Deposits because of the nature of 2-D detectors. with igneous signatures in Ares Vallis can be detected CTX and HiRISE observations, P07_003762_1861 with the high resolution Compact Reconnaissance Im- and ESP_013216_1870 and PSP_008166_1885, re- aging Spectrometer for Mars (CRISM) [16] and their spectively, were used to document morphologies. An morphologic and geologic context assessed with the ArcGIS-ready MOLA [24] raster from the USGS was Context Camera (CTX) [17], the High Resolution Im- used to aid in the interpretation of the morphologies. aging Science Experiment (HiRISE) [18] data, as well Mineral Detections: Mafic minerals are recog- as other imaging sensors, allowing for a more detailed nized and mapped on the basis of crystal field transi- characterization of the deposits. tion absorptions in the 1-2.5 µm region and procedures Ares Vallis has been proposed to have been formed for this recognition and mapping have been presented by multiple outflow events; crater counting in the area previously [12, 22]. Olivine and pyroxene have been has dated the exposed surface of Ares Vallis at an age detected on the floor of Ares Vallis, whereas only oli- of mid-Hesperian e.g. [19] or Hesperian to early Ama- vine detections have been made on the plateau to the zonian e.g. [20]. In addition, data from instruments on east of the channel and on the edges of the channel, the Mars Reconnaissance Orbiter (MRO) and Mars near the outflow source (Fig. 1). Olivine is recognized Express allow the characterization of the nature of the by its broad 1 µm absorption, while pyroxene is recog- Noachian plains units between Ares Vallis and Meri- nized by its broad 1 and 2 µm absorptions. The pyrox- diani Planum. In light of these findings, it is of interest ene absorptions indicate a composition intermediate to constrain the timing of igneous, volcanic and fluvial between low-Ca (LCP) and high-Ca (HCP) and resem- activity in the area. In particular, Ares Valles formed bles spectra of Hawaiian basalt, from the CRISM spec- at the critical period near the stratigraphic boundary tral library [25]. Since LCP-rich compositions are more between the Noachian and Hesperian as global-scale common in old Noachian terrains and more HCP-rich processes transitioned from the era of phyllosilicate in Hesperian terrains [12, 26], it is possible that the formation to sulfate formation [21]. lava flow in the floor of Ares Vallis has captured a shift Datasets and Methods: We use data from the Ob- in the evolution of magmas. Olivines on the plains servatoire pour la Minéralogie, l'Eau, les Glaces, et units are similar in spectral character to those of Noa- l'Activité [21]. Basic reduction of OMEGA data from chian olivines in the Nili Fossae region and have no radiance to reflectance including atmospheric removal detectable pyroxene absoprtions. The deposits on the is performed as described by [12]. We also analyze full channel floor are clearly of a different nature than those resolution CRISM TRR2 observations that cover parts found on the surrounding, geologically older plateaus. of the floor of Ares Vallis in the vicinity of 6.7°N, Morphology: HiRISE shows that the deposits on 19.5W, as well as some of the surrounding terrains. the floor of Ares Vallis, where mafic detections are These are full resolution targeted hyperspectral obser- observed with CRISM, have a texture analagous to vations with spatial resolutions of 18-36 meters per cooling fractures in lava (Figs. 1a,b,c). [27] has re- pixel in 544 separate wavelengths between 0.362 μm ported olivine signatures in floor deposits and ejecta and 3.92 μm in hyperspectral mode with a spectral res- associated with Taytay crater on the northeastern edge olution of 6.55 nm/channel. CRISM multispectral data of Ares Vallis. The spectra from the central peaks ex- available over the plains units consist of a subset of 72 hibit weak Fe/Mg phyllosilicate absorptions suggesting wavelengths binned to 200 meters per pixel. The at- excavation of Noachian basement; Taytay is large mospheric and photometric corrections were imple- enough to excavate to a depth [28] comparable to that 42nd Lunar and Planetary Science Conference (2011) 2507.pdf of the channel floor’s current elevation. Furthermore, rian-aged lava flow on the floor of Ares Vallis may the ejecta of Taytay crater cover this mafic deposit in capture a compositional evolution in this region. Since the channel. These observations, together, are indica- the detections herein are similar to other Hesperian tive that the mafic unit, dated to the early Hesperian volcanics, this regional compositional evolution may [29], that forms the floor of the channel is a post- be important for interpreting igneous materials from channel fill that occurred sometime before or during other regions on Mars. the early Hesperian (Fig. 1c). Future Work: Crater counts will be performed on MOLA topography profiles indicate that the floor the plateau units that appear to be resurfacing events, in of Ares Vallis is very flat and that it slopes downward order to determine their relationship to the Noachian from both ends to a point near the bend in the channel basement and for. The derived ages will then be relata- at 10°N, 23.9°W. HiRISE and CTX show that the ble to the ages given by other authors for Ares Vallis. floor deposit in this region covers previously carved References: [1] Carr, M. H. 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[22] Mustard J. F. plains given the present topography (Fig. 1d). The et al. (2008) Nature, 454, 305. [23] Pelkey S.M. et al. (2007) JGR, morphologic relationships regarding the ejecta from 112, E08S14 [24] Smith D. E. et al. (2001) JGR, 106, 23,689– Taytay crater and the mafic floor deposit confirm the 23,722. [25] Murchie, S. et al. (2006) NASA PDS MRO-M-CRISM- current hypothesis that most outflow occurred early in 4-SPECLIB-v1.0. [26] Skok J.R. et al. (2010) JGR, 115, E00D14. Mars’ history, by the end of the Noachian, and that [27] Wilson J.H. and Mustard J.F. (2010) LPSC XLI,abstract 2516.