43rd Lunar and Planetary Science Conference (2012) 2516.pdf

EVIDENCES OF POSSIBLE HYDROTHERMAL ALTERATION IN XANTHE TERRA: IMPLICATIONS FOR SURFACE WATER ON EARLY MARS. C. Popa1, G. Di Achille1, F. Esposito1, V. Mennella1, L. Colange- li2 1OAC-INAF Salita Moiariello 16, 80131, Napoli Italy ([email protected]), 2ESA-ESTEC (SRE-SM) Po- stbus 299, 2200 AG Noordwijk, The Netherlands

Introduction: The western part of Xanthe Terra Results and discussion: We individuated 2 areas of (Mars) offers numerous examples of small to medium interest within the survey area tracking the presence of sized fluvial-like systems that interact with a geologic alteration products identified via MRO-CRISM obser- bedrock characterized by old Martian material mostly vations. The alteration acted upon Nplh and Hpl3 units of Noachian age and compositionally defined by py- in target area 1 and is characterized by the presence of roxene-rich units [1]. The units are consistent with the phyllosilicates, while target area 2 is characterized by extension of the previously defined Nplh unit [2]. Hes- minor phyllosilicates with abundant silica in sinter like perian units (Hpl3) consist of olivine-rich lava flows deposits in the distal part of a fan delta (Figure 1 and [1,2] which outcrop along Ganges Chasma upper walls 2) already identified from its morphology by [4]. ’s b and culminate at the Morella crater floor [1]. These a geologic units are carved by channels that form hydro- #2

graphic systems represented by delta and/or fan delta #3 deposits [2, 3]. Mineral alterations are associated with #4 #5 the presence of mafic-rich rocks along these channels, dissected crater rims, and deltaic deposits. Our study #6 suggests that the alterations are most likely a result of #7 hydrothermal processes most likely related to impact crater formation. We use these mineral assemblages to put constraints on the duration and extent of water- rock interactions during early Noachian-Hesperian [1]. Data and methods: The geomorphologic map was digitized using THEMIS day global mosaic. The com- positional units in Figure 1 were identified via OMEGA mapping. MRO-CRISM observations were used for detailed studies of the target areas. Laboratory measurements with analogue (natural and/or synthetic opal A and opal CT) materials were done to record the spectral response in NIR for comparison reasons.

Figure 1 Geomorphologic map of the studied area. 50°0’0"W [? c c re re re c c c re c c re och c cof cw ch c c c re c ch re cv cre c c [? Delta or alluvial fansc cof re ch cof re c c c c cof Craterre c youngc (smooth) floor c c re cof c chch re c c c c Crater c c c re ch c ch re c c c cof Crater brecciated floor cof cof ch cof ls cw Crater Central Peak(s) c c c rerec c cof c c c ch c c cofcof cof Chaoticc c Terrain cof re c re cch c c c cof cofre ch c c c cofChannelch c cofre c c cc c ch ch cof re c re cpm c ch ch cw c c Rampart ejecta cw och cw re c re c cw c c cof c c ch c Cratercr Wallc c ch c c c re cr c re c cof cof c ch ch c ch re re c re re c c cch c re cof chCavus cof rech c c cc re c re cof c cc cof ch c c Buried crater c re c c Figure 2 Alteration recorded at target area 1 (a) [5] and cof c c ch cof Ridged materialc (Npl ) ch c c [? ch cof cof re c cre c c c c c Outflowls Channelch cof cre c re re c re bc cofcof c c c c c cr cof ch c c c c cCof c cre c Secondarych och craters cw c c cof cof rec cof cof cc re cc c sc c re c cch cre c ls c c re Collapsed terrain (pits) area 2, with corresponding locations below each spectra c c c bc bc c c c c c cr re cof c Chasmata,ch chre Npl ,Npl ,Hrand re cw c cavCofc c c crre cof c c c re re c re re c c c Cof Hpl unitsc undifferentiatedcof c c cr ch [?ch cof c re cw Delta ch c cof c cdtf ls re cw ch cre collection. c re c [? cof ch ch Olivine enriched unit (Hpl ) c ch Cof cavch c cr c cof cof Dd c cr ch re re c re c c cw Buried channel c ch Cof cp Ch rec c cw c ls chCsf c c ch bc re c sc c c crre bc Pyroxene enriched (Npl +Npl ) c cw bc c c re ccw cre c c c re c re c c c re re cof c c c cr c ls Given the current data coverage, we were unable to re c c Cav c c c cr cht c c recr cre re c re re c c re c bc cof c c c c c Target areach 2 re cr c c ch c cof c c bc bc c c identify obvious phyllosilicate alteration outside the ch c c c c c c c re c re c re c c c ch ch cw c c cw c c cw c re cof rec cof re cc c ct c c och cch cw re channel areas, where the bulk of the mafic rich Noa- re ch cw cw ch c cht re Rom c re cof cc re cof rec re c ch ch ch cw re c re ch cr re ch ch re re re c ch c Cof re bc c chian units outcrop. The alteration recorded on target ch c c bc cre recre bc crre bc c c bc cof ch c c c re bc cht cof c bc ch cre c cof recr bc bcbc cof c re recr bc bc cc cre bc bc sc sc ch c recof cr sc area 1 is Hesperian. In fact, the channel cuts through re sc c ch c c Cof scsc cc c re c sc sc sc c c ch bc re c c cof re c sc re c c cof re re the Hpl unit constraining the oldest possible age for cw re scsc rec c c 3 rec cof cof rec ch c cw sc c bc cw cof bc c bc rec cof cr re c cw bc [? cr c r c bc c c the alteration to the Hesperian (Figure 2c). This fact bc re c rom c c bcc bc Targetbc area 1 c c cc c re cr cr re re c c c cof cre re cr bc re bc c c re re cr c cof c c c re suggests that dry atmospheric conditions were domi- bc c re c re cw re cr c bc c re c re cre rec c cr ch c c c c c c re cw rec c ch c c peu c c cc c bc rec c bc re c c nant from the Npl unit deposition up to the channel c c rec h bc c c re c c re cre peu cr rec chcof c re pea Chasma forming event, implying the lack of enough alteration bc bc c re re c cw c cp re cr peupeu c ch re re re cr during the early Noachian to support the idea of an bc cre peu c cre c rom c mu mu c cre peu peu re och bc ch ch bcc peu cw c recc Hau rom cav c de och re bc cof [? c c c c c c re cw cav cre c bc peu cw c ch re extensive presence of liquid water. bc c re c re bcre re c rec ccav c cre rom c cof re re re 50°0’0"W 43rd Lunar and Planetary Science Conference (2012) 2516.pdf

hosts a fan delta deposit (Figure 3). The water that fed this deposit is generated via crater impact melt ex- pressed by rampart ejecta morphology north of the target. We suggest that this rampart generating event fed the western tributary of Nanedi Valles, as well as the small channel that provided the silica rich water for the sinter formation in the fan delta distal part (Figure 3). Similar systems are present to the north, although there are no alteration traces (e.g. Tyras, Drillon). Laboratory analysis was done in order to explain the observed mineralogy. Synthetic opal was precipitated from silica rich water, then the material was measured in the NIR spectral range in a purged atmosphere (Fig- ure 4). These measurements show that after deposition in the distal part the water was soon lost from the de- posit, suggesting a water poor atmosphere. Conclusions: The distribution of mineral assemblages produced by water interacting with mafic Noachian outcrops in western Xanthe Terra suggests the occur- rence of episodic alteration events most likely related to underground water releases associated to rampart impact craters [6, 7, 8]. Most of the alterations seems concentrated between the late Noachian and during the Figure 3 (above) Extent of siliceous sinter deposited in the Hesperian. This is not consistent with the hypothesis of distal part of the fan delta. The amorphous silica extends an extended period of widespread surface aqueous further from the outcrop area in the area below the sediment alteration during the Noachian (i.e. the Phyllosian) [9]. cover. The target area 2 can be used to place constraints on the time span of such events. In fact, a 50-km diameter crater could sustain hydrological activity for as long as 104 years [10,11]. The siliceous deposit in target area 2 is dehydrated (Fig. 3-4), which points to immediate exposure to dry atmosphere (with composition close to the present one), suggesting a different process of siliceous sinter deposition with respect to those in terrestrial atmosphere [12]. This process suggests the presence of a silica rich source material which could be possibly represented by the presence of granophyre (eutectic between feldspar and quartz) as suggested by the study of terrestrial analogues (e.g. Sudbury basin) [13].

References: [1] Popa C. et al. submitted. [2] Witbeck, N. E. . Figure 4 Time series spectral measurements depicting vari- et al. (1991) Atlas of Mars 1:2k Geologic Series [3] Di . ous water content in amorphous opal. Spectra: blue is the Achille G. et al. (2007) J. Geophys. Res. 112 [4] Hauber E. most hydrated phase after the generation of synthetic silica; et al. (2009). Planetary and Space Science 57(8-9): 944-957. green is the range of water content in most opal A and opal [5] Komatsu G. et al. (2009) Icarus 201(2): 474-491. [6] CT samples from Earth, either sedimentary or hydrothermal Newsom H. E. et al. (1990) J. Geophys. Res. [7] Brakenridge (e.g. chalcedony, flint, agate, etc); red – synthetic Martian et al. (1985) Geology 13, 859-862 [8] Ehlmann B. L. et al. sinter; black CRISM measurement over silica outcrop in the (2011) Nature 479, 53-60 [9] Bibring et al. (2006) Science . distal part of fan delta from Figure 3 312 [10] Marzo, G. A. et al. (2010) Icarus 208(2): 667-683. [11] Rathbun J.A. and Squyres S. W. (2002) Icarus 157(2), In target area 2 there is a distinct water related event 362-372 [12] Day and Jones (2008) J. of Sed. Research connected to an unnamed 60 km diameter crater that 78(4): 301-315. [13] Therriault A. M. et al. (2002) Economic Geology, 97(7): 1521-1540.