Horgan, and 4 co-authors, reported in 2019 that Noachian-aged Jezero crater is the only known location on Mars where clear orbital detections of carbonates are found in proximity to clear fluvio-lacustrine features indicating the past presence of a paleolake; however, it is unclear whether or not the carbonates in Jezero are related to the lacustrine activity. This distinction is critical for evaluating the astrobiological potential of the site, as lacustrine carbonates on Earth are capable of preserving biosignatures at scales that may be detectable by a landed mission like the Mars 2020 rover, which is planned to land in Jezero in February 2021.
In this study, the authors conducted a detailed investigation of the mineralogical and morphological properties of geological units within Jezero crater in order to better constrain the origin of carbonates in the basin and their timing relative to fluvio-lacustrine activity. Using orbital visible/near-infrared hyperspectral images from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) along with high resolution imagery and digital elevation models, the authors identified a distinct carbonate-bearing unit, the “Marginal Carbonates,” located along the inner margin of the crater, near the largest inlet valley and the western delta. Based on their strong carbonate signatures, topographic properties, and location in the crater, the authors proposed that this unit may preserve authigenic lacustrine carbonates, precipitated in the near-shore environment of the Jezero paleolake. Comparison to carbonate deposits from terrestrial closed basin lakes suggests that if the Marginal Carbonates are lacustrine in origin, they could preserve macro- and microscopic biosignatures in microbialite rocks like stromatolites, some of which would likely be detectable by Mars 2020.
The Marginal Carbonates may represent just one phase of a complex fluvio-lacustrine history in Jezero crater, as the authors found that the spectral diversity of the fluvio-lacustrine deposits in the crater is consistent with a long-lived lake system cataloging the deposition and erosion of regional geologic units. Thus, Jezero crater may contain a unique record of the evolution of surface environments, climates, and habitability on early Mars.
The authors' detailed spectral and morphological analyses of the major geologic units in Jezero crater, combined with a synthesis of previous studies, shows that the geological history of the basin has the potential to be much more complex than previously hypothesized. In particular, it was unclear from previous studies whether or not the carbonate-bearing terrains in the crater had any genetic relationship to the past lake activity that is indicated by the presence of the western delta. The authors found that the carbonate-bearing terrains in Jezero are all generally spectrally and morphologically similar, in line with previous mapping studies that largely grouped these units together. However, the authors also found that topographic properties, variations in the relative strength of key spectral signatures, erosional surface textures, and geologic context varies sufficiently across the crater to suggest different emplacement and/or modification histories for the carbonate-bearing units, some of which may be related to fluvio-lacustrine activity.
The olivine and carbonate-bearing Mottled Terrain along the northern rim and interior of Jezero exhibits spectral and morphological characteristics that are all comparable to similar properties of the regional olivine/carbonate-bearing unit on the plains to the west and south of Jezero, but the presence of localized Fe/Mg-smectite signatures within this unit in Jezero suggests that this portion of the unit may have undergone fluvial and/or lacustrine modification. The Light-toned Floor may also be part of the regional olivine/carbonate-bearing unit, as we find that the significant olivine sand component is unlikely to have accumulated via fluvial transport. Recent studies of the regional olivine-bearing unit have suggested that it has properties consistent with an ultramafic tephra deposit, which, in Jezero, is consistent with the draping relationship between the Mottled Terrain and the eroded northern rim. Alteration of the regional unit to form carbonates may have occurred through a number of different processes, but the widespread distribution and local variability of alteration across the unit along with the alteration assemblage is consistent with low to moderate temperature alteration of fresh ultramafic tephra by rain or snow melt.
The primary characteristic that distinguishes the Marginal Carbonates from the Mottled Terrain and Light-toned Floor is that they are located on the western inner margin of the crater, close to the western inlet, and within a narrow range of elevations that would correspond to the highest shorelines during the closed-basin phase of the Jezero lake sequence. In contrast, the regional olivine/carbonate unit drapes and mantles topography over a large range of elevations. The Marginal Carbonates exhibit much stronger carbonate spectral signatures throughout the unit than the other carbonate-bearing terrains, with olivine and hydration spectral signatures that are strong near the inlet but fade to the south, terminating in relatively uncontaminated hydrated carbonate spectral signatures in terraced deposits along the inner rim and around local knobs. Their surface texture is also somewhat distinct from the other carbonate terrains in the crater. While these spectral and textural properties could be attributed to the variability that has been observed across the regional olivine/carbonate-bearing unit, together with the topographic distribution of the Marginal Carbonates they are also consistent with fluvial transport of the regional olivine/carbonate-bearing unit into the crater and lacustrine precipitation of carbonate in the near-shore environment of a closed lake basin. Based on comparison to terrestrial analogs, the Marginal Carbonates are consistent with a combination of fluvial and beach or strandline carbonate deposits, which can include microbialites, tufas, and oolitic sediments.
All three carbonate units would make excellent targets for in situ investigation by Mars 2020 or other future landed missions. As geological targets, both the Mottled Terrain/Light-toned Floor and the Marginal Carbonates contain carbonates that were most likely produced directly or indirectly through weathering by carbonic acid derived from the atmosphere. These units all thus represent key chemical and isotopic records of the late Noachian atmosphere, which could be used to better constrain the composition, thickness, and loss rates of the atmosphere during the peak of martian surface aqueous activity. The chemistry and sedimentology of the Marginal Carbonates and of the sedimentary facies preserved in the western delta could also help to constrain the chemistry and hydrogeology of Noachian fluvio-lacustrine systems, and their links to Noachian paleoclimate. However, perhaps the most compelling reason for detailed future in situ investigations in Jezero crater is the astrobiological potential of these units. If the Marginal Carbonates are indeed lacustrine shoreline deposits, they would be a particularly high priority target for future in situ investigations and sample return, as near-shore carbonate deposits on Earth are commonly biologically mediated (e.g., carbonate reefs, microbialites, and perhaps tufa), and have high biosignature preservation potential. In particular, lacustrine carbonates are capable of preserving morphological and textural biosignatures at scales that may be detectable by a landed mission.
Carbonates are just one part of the fluvio-lacustrine history in Jezero, as revealed by the mineral stratigraphy of the western delta and northern fans. The different sedimentological facies within the western delta exhibit distinct mineral assemblages that suggest changes in the source units in the watershed. The northern fans exhibit a similar mineral stratigraphy, suggesting that they either record the same regional trends in watershed mineralogy or were once part of the western delta. Based on these observations, there may be as many as four distinct lacustrine phases preserved in Jezero. Lacustrine activity in Jezero may have started early, with fluvial erosion of the regional basement unit and the northern rim of Jezero to form the lowermost units of the delta and fans. The second lacustrine phase may have begun either after or contemporaneous with deposition of the regional olivine/carbonate-bearing unit, resulting in erosion of this unit on the plains to form the current uppermost units on the northern fans, the point bar facies of the western delta, and the Marginal Carbonates. The third lacustrine phase is represented by the inverted channels that cap much of the western delta, and corresponds to additional transport of the basement unit, likely via incision through overlying units. The timing of the breach to form the outlet valley is only constrained to sometime after deposition of the Marginal Carbonates, but may have occurred during this third phase. The last lacustrine phase would then have formed the incised valley on the western delta, corresponding to a final shallow lake level well below the breach. This proposed history may be consistent with the relatively continuous scenario put forward by Goudge et al. (2018), in which the western delta represents gradually increasing lake levels and shoreline transgression, followed by the breach to form the outlet valley, and potentially later deposition in a shallower lake. However, the mineral diversity in the fans, delta, and Marginal Carbonates could also be consistent with a much more complex and long-lived fluvio-lacustrine history in Jezero, with distinct lacustrine sequences separated in time by fluctuating climatic conditions, resulting in fluctuating or even periodic lacustrine activity.
Managed & Built by I2M Consulting, LLC | © 2010-
