Comprehensive analysis of the alteration of Tyrrhena Terra: Implications for source-to-sink processes on Mars

Abstract

Tyrrhena Terra, a region located in the cratered Noachian highlands between Hellas and Isidis Planitia on Mars, is distinguished by its extensive presence of hydrated minerals. As a key region for understanding the early Martian aqueous environment, various evidence for hydrated alteration has been reported by previous studies (Loizeau et al., 2012; Rogers, 2011; Carter et al., 2013; Bultel et al, 2015; Viviano et al., 2023). However, the effects of varying crater scales on mineral formation and distribution have not yet been well understood.In this comprehensive investigation, we identified hydrated minerals across Tyrrhena Terra (2°N-22°S, 64°E-104°E), using near-infrared hyperspectral data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Along with high-resolution imaging from the Context Camera (CTX) and the High-Resolution Imaging Science Experiment (HiRISE), we explored the relationship between impact crater size and degradation rate and the diversity of hydrated minerals. Based on crater diameter, the observations were systematically categorized into five groups: away from craters, 20 km. Additionally, craters larger than 20 km were subdivided based on degradation morphology into three types (Mangold et al., 2012): type I (highly degraded), type II, and type III (least degraded).Our findings showed a widespread distribution of phyllosilicates, including Fe/Mg-smectites, chlorite, kaolinite, mica, and their mixtures, as well as other hydrated minerals such as prehnite, zeolite, carbonate, and hydrated silica (Figure 1). We observed a significant increase in mineral diversity with larger crater diameters, even when excluding minerals located on central peaks (Figure 2). Such trend indicates that the subsurface compositions vary significantly with depth, since the post-date impact generated hydrothermal deposit mostly found in the central peak of craters. By excluding minerals on central peaks, we reduced the influence from impact-driven hydrothermalism for hydrated minerals formation, thus maximized the role of impact events to be the excavation of subsurface minerals.Figure 1. The distribution of hydrated minerals in Tyrrhena Terra.Figure 2. The variation of the hydrated minerals with the diameter of impact crater.Notably, for craters larger than 20 km, the mineral diversity also shows a strong correlation with the degradation rate of craters (Figure 3). Only Fe/Mg-smectites and chlorite were observed in highly degraded type I craters, likely dating back to the Noachian period, while prehnite, zeolite, carbonate, and hydrated silica were more common in less degraded type II and type III craters. Moreover, the context of Fe/Mg-smectites exposures in Tyrrhena Terra is particularly intriguing. Fe/Mg-smectites were prevalent not only in type I craters, but also in ancient sediments that are incised by valley networks, isolated from impacts, and with small amount of Al-smectite mixtures observed (Figure 4c); as well as in exposures of small impacts (less than 5 km in diameter) on ancient sediments (Figure 4b, CRISM image FRT00014005 and FRT00024086). In contrast, the interiors of large, well-preserved craters exhibit a complex diversity of minerals, including those formed under relatively high temperatures, such as prehnite and zeolite. It appears that minerals formation involves processes beyond only impact events. Such difference revealed clear evidence for active surface alteration in Noachian period, resulting in the transformation of hydrated minerals.Figure 3. The variation of the hydrated minerals with the degradation rate of impact crater larger than 20 km.Figure 4. Key examples for the context of Fe/Mg-smectites and Al-smectites (mixed with Fe/Mg-smectite) in Tyrrhena Terra. (a) CRISM spectral ratios (top and middle frame) compared to Fe/Mg-smectites and Al-smectites laboratory spectra from USGS and RELAB (bottom frame). (b) Fe/Mg-smectites exposed by small craters on ancient sediments. (c) Fe/Mg-smectites on ancient sediments and isolated from impacts (in magenta), as well as small exposures of Al-smectites mixed with Fe/Mg-smectites (in cyan, extracted from Carter et al., 2023). The RGB channels are Band 233, Band 78, and Band 13 respectively for all CRISM images.Our findings highlight a regional source-to-sink geological process where minerals were transformed and redistributed. These minerals are excavated by local impacts, subsequently altered by surface weathering processes, and transported through fluvial activity to their present locations. This suggests that more than one period of hydrated alteration was active in early Martian history, likely predating the formation of widespread valley networks, with each period lasting long enough for hydrated minerals to be formed or altered. This research offers new insights into the early Martian aqueous history and its past environmental conditions.