Widespread exposure of Noachian phyllosilicates in the Margaritifer region of Mars: Implications for paleohydrology and astrobiological detection

Abstract

The best locations at which to detect evidence for early life on Mars are in materials formed in near‐surface aqueous environments, particularly where this resulted in the deposition of minerals such as clays that are favorable to preservation of organics. The geological history of the Margaritifer region has resulted in exceptional potential to preserve such deposits and to render them discoverable. Due to its topographic setting at the interface between highlands and lowlands, Margaritifer was a major sink for water and sediments in the early, Noachian, period, potentially creating environments that were habitable and conducive to clay formation. Subsequently, during the Late Hesperian to Amazonian, the ancient surface was extensively disrupted in association with the formation of multiple chaos regions. This activity had the potential to expose any astrobiological evidence from the earlier period. We used orbital image, spectral and topographic data to investigate the extent and means of exposure of Noachian clay‐bearing deposits across the region. We find that they are indeed exposed over a very wide area in Margaritifer and that their mineralogy is most consistent with clay formation in a low‐energy near‐neutral pH groundwater environment. We additionally find that evidence for subsequent acidic groundwater activity is absent, indicating that biosignature preservation in these units is favored, perhaps to a greater degree than for similar deposits in the surrounding region. Further, due to the intense Hesperian‐Amazonian geologic activity here, early clay‐bearing units are exposed to a greater degree than achievable in regions with more localized erosive mechanisms.

Large‐scale fluid‐deposited mineralization in Margaritifer Terra, Mars

“…The chaos and crater floor fracturing that dominated Hesperian geological activity here are generally attributed to surface collapse due to expulsion of fluid from the subsurface [e.g., Carr, 1979;Sato et al, 2010]. Indeed, recent work reports evidence for erosion adjacent to many Hesperian Margaritifer fractures, in many cases best explained by the action of upwelling subsurface fluid from the fractures [Thomas et al, 2017]. Indeed, recent work reports evidence for erosion adjacent to many Hesperian Margaritifer fractures, in many cases best explained by the action of upwelling subsurface fluid from the fractures [Thomas et al, 2017].…”

“…The majority of the crater fill has TI values (maximum 737 J m À2 K À1 s À1/2 ) consistent with an indurated surface [Putzig et al, 2005;Fergason et al, 2006] and is light-toned and finely fractured, but there are erosional remnants of darker-toned, lower TI material superposing it. However, the floor fill is crosscut by multiple fractures up to 650 m wide, and at many locations across Margaritifer such fractures have been shown to be the source of upwelling fluid that accomplished erosion [Thomas et al, 2017]. Some erosion can be attributed to wind action.…”

“…Margaritifer Terra (À23.6 to 19.7°N, 320.5 to 350.6°E) is a largely Noachian highland region just south of Mars' global dichotomy boundary [Tanaka et al, 2014] (Figure 1). It is surrounded and crosscut by the highest density of chaotically fractured regions on the planet and shows extensive fracturing in highland units [Thomas et al, 2017] and in the floors of impact craters peripheral to chaos [Bamberg et al, 2014]. There is considerable reason to expect Hesperian groundwater upwelling in Margaritifer.…”

“…This is supported by hydrological modeling showing that the topographic setting of Margaritifer favors high groundwater levels [Andrews-Hanna et al, 2010], indicating that if subsurface recharge continued after cryosphere formation, any underlying liquid aquifer would be pressurized and prone to upward migration if cryosphere fracturing occurred. Indeed, recent work reports evidence for erosion adjacent to many Hesperian Margaritifer fractures, in many cases best explained by the action of upwelling subsurface fluid from the fractures [Thomas et al, 2017]. The temperature, chemistry, and redox state of these fluids would have been dictated by conditions in their source region (and also the upwelling rate), and so there would have been a strong potential for precipitation of minerals as the fluids ascended fractures into significantly different surface conditions.…”

“…Some erosion can be attributed to wind action. However, the floor fill is crosscut by multiple fractures up to 650 m wide, and at many locations across Margaritifer such fractures have been shown to be the source of upwelling fluid that accomplished erosion [Thomas et al, 2017]. In light of the presence of channels in the rim of Ubud, indicative of overflow of fluid from the crater, it is probable that fluid upwelling occurred in Ubud also [Thomas et al, 2017, Figure 7].…”