Chemical and sedimentological data indicate that reduced iron and sulfur were generated, mobilized and precipitated following the deposition of fine-grained oxidized iron- and phosphorous-bearing sediment. Except when found in authigenic nodules and reaction front rims, phosphate is not associated with a mineral phase (for example, there is no indication that apatite or merrillite are present; Fig. 4c). Accordingly, we suggest that during deposition, phosphate was adsorbed on Fe3+-, Al- and Si-rich sediment grains21. In the Bright Angel area, iron and phosphate have been redistributed into authigenic nodules and reaction front rims; mass balance calculations suggest closed-system reorganization of these chemical components into vivianite (Supplementary Text). Fe-phosphate-enriched masses are not associated with Al 2 O 3 (Supplementary Fig. 21h), which might otherwise suggest the co-existence of Al-phosphate minerals typical of transport of Al3+ and Fe3+ under oxidizing, low-pH conditions, such as variscite (AlPO 4 ·2H 2 O) and strengite (FePO 4 ·2H 2 O)22,23 (Fig. 4c). Instead, transport of Fe2+, Zn2+ and PO 4 3− probably occurred under non-oxidizing conditions, which, combined with moderate pH, prevented mobilization of Al3+. Such conditions favour the precipitation of vivianite23. The apparent absence of Fe-phosphate nodules and reaction fronts in most of the conglomerate-bearing Masonic Temple area suggests a depositional facies control on the development of these specific features.
In the Bright Angel area, Fe-phosphate minerals are associated with organic matter (Fig. 3d). A pathway to the formation of vivianite is via the oxidation of this organic matter, which would have been coupled to the reductive dissolution of Fe3+ in sediment grains. This process would have liberated Fe2+ and PO 4 3− to solution and precipitated Fe2+-phosphate. Similar precipitation and redox reactions have been considered for an occurrence of Mn–P-rich nodules in Gale Crater24, and for submillimetre-scale mixed valence Fe-phosphate grains in a conglomerate outcrop in the Jezero Western Fan25,26. Sulfate reduction coupled to organic matter oxidation could also be responsible for precipitation of Fe-sulfide in the Apollo Temple target and in the cores of reaction fronts in Cheyava Falls. As reduced Fe- and S-bearing phases formed, the mudstone colour properties were modified by iron reduction, bleaching it of its red colour in proportion to the abundance of available organic matter (Fig. 5c,d).
Here we consider the null hypothesis: that within the low-temperature sedimentary-diagenetic setting we have proposed for the Bright Angel formation, abiotic reactions produced ferrous Fe and reduced S and concentrated them in authigenic nodules and reaction fronts. The null hypothesis predicts that abiotic reactions can reduce sedimentary Fe3+ to aqueous Fe2+, which is then incorporated in the Fe-phosphate and Fe-sulfide minerals we have identified. A wide variety of organic carbon compounds are known to promote the abiotic reductive dissolution of ferric iron oxide minerals at temperatures between 10 °C and 80 °C (refs. 27,28,29). The presence of organic matter in Bright Angel formation mudstone (Fig. 3d), which could have been produced on Mars through abiotic synthesis30,31 or delivered from non-biological exogenic sources30,32, suggests that such reactions could have occurred. Further analysis is required to determine whether the specific organic compounds present in the Bright Angel formation can drive the reduction of mineral-hosted sedimentary Fe3+ at low temperature. Another possible pathway to the production of Fe2+ is through the abiotic oxidation of pyrite by Fe3+ (aq)33. This process would require both the presence of detrital pyrite and low solution pH, which would permit Fe3+ (aq) to be present. As previously discussed, neither condition appears to be met in the Bright Angel formation.
The null hypothesis also predicts that an abiotic source of dissolved sulfide was available to be incorporated in authigenic Fe-sulfide. Dissolved sulfide facilitates the reductive dissolution of ferric iron oxides, with half-lives ranging from years to hours depending on Fe-oxide mineralogy, crystallinity and pH34,35, providing another potential pathway to the production of Fe2+ (aq). Magmatic degassing of reduced sulfur-bearing gases (for example, ref. 36) to local groundwater could provide a potential source of dissolved sulfide during diagenesis. However, geological constraints demand that this sulfide migrate in from a distal, high-temperature sulfide-gas-producing system, to the low-temperature depositional-diagenetic environment of the Bright Angel formation. No evidence for sulfide-producing hydrothermal or magmatic systems was observed in the Crater Floor, Western Fan or Margin Unit before investigation of the Bright Angel formation. Abiotic reduction of sulfate to sulfide by organic matter is another possible source of dissolved sulfide that could both reduce Fe3+-bearing sediment and provide the reduced sulfur required to form Fe-sulfide minerals37. However, sulfate reduction by reduced carbon compounds is energetically demanding and kinetically inhibited by the symmetry of the SO 4 2− ion38, so abiotic reaction rates are exceedingly slow at temperatures <150–200 °C (refs. 37,38). As discussed previously, the Bright Angel formation shows no unambiguous evidence that it was heated in contact with adjacent geologic units, and burial to depths in excess of about 5 km would be required to achieve temperatures >150 °C during the Noachian39.
Given the potential challenges to the null hypothesis, we consider here an alternative biological pathway for the formation of authigenic nodules and reaction fronts. On Earth, vivianite nodules are known to form in fresh water23,40,41 and marine42,43 settings as a by-product of low-temperature microbially mediated Fe-reduction reactions. Fe-sulfide minerals, such as greigite, pyrite and mackinawite, can also be formed as products of microbial sulfate reduction44,45, and have been observed in close spatial association with vivianite42. Greigite precipitation, from precursor mackinawite, is favoured by a high dissolved Fe2+/SO 4 2− ratio, typical of diagenetic fluids in freshwater and ferruginous marine environments44. Repeated sulfidation of Fe2+ in vivianite followed by sulfide oxidation promotes stable incorporation of Zn and other heavy metals into vivianite46. Thus, Zn enrichment in nodules is consistent with a hybrid iron- and sulfate-reducing mechanism. Minerals like these, produced by Fe- and S-based metabolisms, provide some of the earliest chemical evidence for life on Earth47,48, and are thought to represent potential biosignatures in the search for life on Mars49. The fact that the reaction fronts observed in the Cheyava Falls target are defined by small, spot-shaped, bleached zones in an overall Fe-oxide-bearing, red-coloured rock invites comparison to terrestrial ‘reduction halos’ in modern marine sediments50 and ‘reduction spots’, which are concentrically zoned features found in rocks of Precambrian and younger age on Earth51. A biologically influenced origin has been proposed by some for reduction spots51, although this is not a universally held perspective52.
Under a biological scenario, the mixture of reactants available in the Bright Angel formation at the time of deposition could have provided raw ingredients for a set of biological redox reactions that drove Fe and S reduction, organic matter oxidation, and precipitation of Fe2+-phosphate and Fe-sulfide minerals. In this scenario, oxidized iron and sulfate would be used as terminal electron acceptors for organic matter consumption, promoting the formation of minerals through the release of chemical by-products: Fe-phosphate minerals in the case of iron reduction and Fe-sulfide minerals in the case of sulfate reduction. Where authigenic nodules were formed, the reaction would have shut off before additional reductive processes occurred. In the places where larger reaction fronts formed, the presence of sulfide-bearing cores suggests that sulfate-reducing metabolisms with lower energy yields could have taken hold once those regions of the rock had been depleted of available Fe3+, but had not yet exhausted organic carbon in the reaction front core.
In summary, our analysis leads us to conclude that the Bright Angel formation contains textures, chemical and mineral characteristics, and organic signatures that warrant consideration as ‘potential biosignatures’53,54,55, that is, “a feature that is consistent with biological processes and that, when encountered, challenges the researcher to attribute it either to inanimate or to biological processes, compelling them to gather more data before reaching a conclusion as to the presence or absence of life53”. This assessment is further supported by the geological context of the Bright Angel formation, which indicates that it is sedimentary in origin and deposited from water under habitable conditions. Many significant questions remain about the origin of the nodules and reaction fronts encountered by Perseverance. We suggest that further in situ, laboratory, modelling and field analogue research into both abiotic and biological processes that give rise to the suite of mineral and organic phases observed in the Bright Angel formation will improve our understanding of the conditions under which they formed. Ultimately, the return of samples from Mars for study on Earth, including the Sapphire Canyon sample collected from the Bright Angel formation, would provide the best opportunity to understand the processes that gave rise to the unique features described here.