Atmospheric oxidation drove climate change on Noachian Mars

Domain: earth_science
Published: 2024-07-05
Editorial outcome: minor_revision

Abstract
Modern Mars is bipolar, cold, and oxidizing, while early Mars was characterized by icy highlands, episodic warmth and reducing atmosphere. The timing and association of the climate and redox transitions remain inadequately understood. Here we examine the spatiotemporal distribution of the low surface iron abundance in the ancient Martian terrains, revealing that iron abundance decreases with elevation in the older Noachian terrains but with latitude in the younger Noachian terrains. These observations suggest: (a) low-temperature conditions contribute to surface iron depletion, likely facilitated by an oxic leaching through freeze-thaw cycles under a reducing atmosphere, and (b) temperature distribution modes shifted from elevation-dominant to latitude-dominant during the Noachian period. Additionally, we find iron leaching intensity decreases from the Early to Late Noachian epoch, suggesting a gradual atmospheric oxidation coupled with temperature mode transition during the Noachian period. We think atmospheric oxidation led to Mars becoming cold and bipolar in its early history.

Introduction
The modern-day Martian climate is characterized by extreme cold and aridity, with temperature varying according to latitude and water ice primarily accumulating in the polar regions. Conversely, modeling results suggest that temperature was predominantly elevation-dominant and water ice mainly accumulated in highlands and polar regions regardless of the mean annual temperature (MAT) on early Mars1–3. Several potential mechanisms4 have been proposed to drive the climate mode change, including the decrease in atmospheric CO2 or atmospheric H26,7, or a reduction in cloud radiative effect6–8. However, the exact process behind the climate mode change remains unclear. While significant cooling7 and drying9 have been suggested from Noachian Mars to Hesperian Mars, the timing of this climate mode transition remains poorly constrained10.

Although the present surface and atmosphere of Mars are oxidizing, emerging evidence points to the existence of reducing gases (e.g., H2) in a CO2-dominant atmosphere on early Mars11–13. A chemically reducing atmosphere on early Mars is supported by multiple sources of H2, including volcanoes14–16, serpentinization17,18, and impacts19,20. A reducing atmosphere can generate a strong greenhouse effect, which could have played a significant role in warming the early Martian climate14,21–27. The intensity of greenhouse warming is closely tied to the concentration of reducing gases in the atmosphere, therefore, the oxidation of the Martian atmosphere could theoretically result in cooling of Martian climate13.

The Mars Odyssey Gamma-ray spectroscopical (GRS) data reveal that surface iron (Fe) abundance in the Noachian terrains is relatively low compared to the Hesperian and the Amazonian terrains, as well as the global surface average Fe abundance28 (Fig. 1a). As there is no correlation with other elements that are expected to vary with Fe during normal igneous processes (such as the correlation between Fe and Si), planetary crustal evolution cannot explain the secular variation in Fe abundance28, especially the low Fe abundance in the Noachian terrains. Because GRS remote sensing technique is only sensitive to the uppermost ~30 cm of the surface, it is possible that the Fe depletion in the Noachian terrains could have been caused by surface aqueous processes28,29. Fe mobility can be affected by acidity, aqueous chemistry, redox state, and temperature during aqueous alteration28–31. Therefore, the redox and climate transitions could have been captured by the distribution of Fe abundance at the surface of Mars, especially when there was communication between the atmosphere, hydrosphere, and lithosphere and efficient top-down leaching during the Noachian period. The characteristics of surface water can be recorded through Fe mobility; however, the Hesperian and Amazonian terrains do not exhibit signs of Fe depletion (Fig. 1a). Consequently, the study predominantly focuses on investigating the Noachian terrains.

Although low pH levels (pH<3) can mobilize Fe, the acidic leaching scenario struggles to explain the Fe depletion across the extensive region of the surface (dark red area in Fig. 1b) from a mass balance perspective. This scenario is also inconsistent with the relatively high Th abundance found at the surface of the Noachian terrains28, the solubility of which increases significantly with decreasing pH32. Furthermore, the absence of a positive correlation between Fe and Cl does not support the leaching of Fe as FeCl4- complexes33, which can transfer Fe efficiently. On the other hand, water redox can greatly influence Fe mobility. Specifically, Fe is immobile and tend to precipitate as Fe (oxyhydro)oxides under oxidizing conditions but becomes soluble and mobile as Fe(II) under reducing conditions. For example, recent studies have demonstrated that Fe loss through leaching as mobile Fe(II) was a common process in paleosols rich in clay minerals under a reducing atmosphere11. Consequently, Fe depletion is highly likely a result of anoxic chemical weathering and leaching under reducing conditions.

Anoxic chemical weathering and leaching can happen with the formation of clay minerals at temperatures well above freezing11 or without the formation of clay minerals under temperatures near or slightly above 0°C34. Although overlaps existed, there is no clear correlation between distributions of the Fe depletion zones (dark red area in Fig. 1b) identified through GRS data and paleosols rich in clay minerals (pink cycles in Fig. 1b) detected via infrared remote sensing using Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data. The decoupling is possibly related to the limited clay minerals’ detections by the lack of areal coverage of CRISM data compared to the global coverage and larger footprint of GRS data. However, it is more likely that the Fe depletion was caused by reducing chemical weathering and leaching under temperatures around or just above 0°C, which inhibits the formation of clay minerals34.

This study aims to test whether the Fe depletion in the ancient Martian terrains resulted from icy weathering and leaching under reducing conditions by low-temperature processes, such as seasonal freeze-thaw cycles. A critical aspect of this research is the analysis of temporal and spatial distributions of Fe abundance to understand the timing of and the relationships among the redox and climate mode transitions. Our finding suggests that the surface temperature of Mars gradually evolved from an elevation-dominant to a latitude-dominant mode, coupled with atmospheric oxidation during the Noachian period.

Results
Spatiotemporal distribution of surface Fe abundance in the ancient Martian terrains
The global geologic map of Mars35 (Supplementary Fig. 1) was used to identify the apparent relative surface age of each 5°×5° bin of the GRS data28. In the Early and Middle Noachian terrains, the Fe abundance of regions with elevation above 3.0 km is approximately 14.0 wt.% (Fig. 2a and Supplementary Fig. 3a), which is close to global surface average (~14.3 wt.%). Between −3.0 to +3.0 km, Fe abundance shows a decreasing trend from ~15.4 wt.% to ~13.3 wt.% with increasing elevation (Fig. 2a). To be more specific, the decreasing trend is pronounced in tropics (20°S–20°N, Supplementary Figs. 4a, b and 5) and northern high latitudes (>20°N, Supplementary Fig. 4c, 5), both of which have elevations lower than 3 km. For southern high latitudes (>20°S), the average Fe abundance of regions with elevation above 3.0 km (~14.0 wt.%) is higher than that of regions between 0 and 3.0 km (~13.0 wt.%) (Supplementary Fig. 4d, 5). On the other hand, in the Late Noachian and Noachian-Hesperian transition (NHT) terrains, the surface Fe abundance is decorrelated with elevation (Supplementary Fig. 3a). It is worth noting that the correlation coefficient (r) between elevation and Fe abundance decreases gradually over time during the Noachian period (Fig. 3b).

Instead, there is a decreasing trend in Fe abundance with increasing latitude in the younger Noachian terrains (Fig. 2b and Supplementary Fig. 2b), especially on southern Mars with limited elevation variations. The correlation coefficient between Fe abundance and latitude is not strong in the Early Noachian terrains, but strong and positive in the Late Noachian (+0.52) and Noachian-Hesperian transition (+0.77) terrains of southern Mars (Supplementary Fig. 3b). It means that the correlation coefficient (r) increases gradually over time during the Noachian period (Fig. 3b).

The surface average Fe abundance in the Early, Middle, and Late Noachian terrains are all below the global surface average (Fig. 3a and Supplementary Fig. 6). Their values increase gradually with increasing amounts at ~0.58 wt.% from the Early to the Middle Noachian terrains, ~0.15 wt.% from the Middle to the Late Noachian terrains, and ~0.10 wt.% from the Late Noachian terrains to the global surface average, respectively (Fig. 3b). However, the Fe abundance at the surface of the NHT terrains is remarkably high (~15.4 wt.%), which is ~1.2 wt.% higher than that of the Late Noachian terrains and ~1.1 wt.% higher than the global surface average (Fig. 3a).

Statistical analyses
ANOVA analyses indicate a significant disparity in Fe abundance across elevations (F(5, 539) = 39.02, p < 0.001) in the Early and Middle Noachian terrains and latitudes (F(10, 61) = 6.67, p < 0.001) in the Late Noachian and NHT terrains (Supplementary Tables 1–5). The analyzing results for different latitude bands (>20°N, 20°S–20°N, <20°S) can be found in Supplementary Tables 2–4). Also, ANOVA revealed a significant difference in Fe abundance across different age categories (F(4, 810) = 41.37, p < 0.001; Supplementary Table 6). Furthermore, the Z-statistic tests indicated statistical significance (95% CI) in different age categories, except for Early Hesperian vs. NHT and Late Noachian vs. Middle Noachian comparisons (Supplementary Table 7).

Methods
Spatiotemporal analyses of surface Fe abundance
The global geologic map of Mars produced by Tanaka et al. (2014)35 was used to identify the apparent relative surface age of each 5°×5° bin of the GRS data28. Cells were then given a specific age value based on the assigned geologic unit: Early Noachian, Middle Noachian, Late Noachian, or Noachian-Hesperian transition (Supplementary Fig. 1). Most 5°×5° bins were not entirely age homogeneous. We assigned the most areally dominant apparent relative surface age for each bin. The dominant apparent surface age should correspond to at least half of the bin to avoid imprecise age assignment28. We excluded the cells without a specific geologic unit covering half of the bin. Although there is age uncertainty, the areal binning of our apparent surface age categories is considered robust28.

We used the average elevation of the gridded topographic data from NASA’s Mars Global Surveyor (MGS) Mars Orbiter Laser Altimeter (MOLA) (Smith et al., 2001) to represent the elevation of each bin. The correlation coefficient (r) and corresponding uncertainty (95% CI) between latitude, elevation, and Fe abundance at the surface of terrains of different epochs is analyzed (Supplementary Figs. 3 and 5). Three latitudinal bands (>20°N, 20°S–20°N, <20°S) were further divided to examine the correlation between Fe abundance and elevation of the Early and Middle Noachian terrains to reduce the latitude effect (Supplementary Figs. 4 and 5). Owing to a lack of sufficient data, we did not decompose the Late Noachian or NHT terrains. Concerning the relationship between Fe abundance and latitude, the analysis was primarily concentrated on the southern hemisphere (Fig. 2b and Supplementary Fig. 2b) in order to minimize the elevation effect because of the significant elevation variation in the northern hemisphere.

Statistical analyses. For the statistical approach, the aggregation of gamma-ray spectra yields mean elemental abundance and boxplot for each category. The Fe abundances approximately adhere to normal distribution. Therefore, we employed ANOVA and z-statistic tests to compare differences in elemental abundance averages. Comparisons that reject the null hypothesis are reported as “statistically significant” with the respective confidence interval for that determination.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors perform an analysis of the abundance of surface iron in ancient terrain on Mars. They\nfind that in the oldest terrain, iron abundances are lowest at high elevations, while in younger\nterrain, abundance is more closely correlated with latitude. Interpreting these results through the\nlens of chemical weathering, they argue that they are most consistent with a scenario where early\nMars was initially warmer with a more reducing atmosphere, and then cooled, oxidized and lost its\natmospheric mass over time. They also suggest that the chemical transition was coupled to a\nclimate transition whereby Mars’s surface ice migrated from highland equatorial regions to polar\nregions.\nThis is an interesting study with important conclusions that is certainly worthy of publication in\nNature Communications. I have a few comments for the authors to take into account, all of which\nare minor. After this, I’ll be happy to recommend publication.\nMinor comments:\nFor future reference, line numbers in the manuscript would have made it easier to write this\nreview.\n“the acidic leaching scenario is inconsistent with …”\nIt might also be added that achieving acidic conditions on the scales necessary to explain Fe\ndepletion across large regions of the surface is extremely challenging from a mass balance\nperspective.\n“Efficient Fe leaching under cooler conditions might have been related to elevated olivine\nweathering at cryogenic temperature. Freezing can increase the acidity of residual water, which\ncounterintuitively accelerates the kinetics of chemical weathering…”\nThis is an interesting idea. I think it’d be helpful here to specify the pH ranges we’re talking about,\nas acidic leaching is argued against as the main mechanism earlier in the text. Can the authors\nquantify the magnitude of the effect they’re talking about here? Seems like it matters for the\ninterpretation of their data.\n“Lower MAT can amplify the freezing, which can sustain chemical weathering and make the\nresidual water more acidic (Figure 4a). As a potentially counterintuitive result, more Fe can be\nreleased under colder conditions”\nThis text is a bit repetitive with the sentence I just quoted above. I suggest editing to improve the\nflow.\n“. The relatively limited distribution of the permanent ice deposits may explain why glacial features\nare not extensively identified in the southern highlands”\nIt’s worth citing and discussing some of the more recent literature on this topic here, e.g.:\nGrau Galofre, A., Jellinek, A.M. and Osinski, G.R., 2020. Valley formation on early Mars by\nsubglacial and fluvial erosion. Nature Geoscience, 13(10), pp.663-668.\nBoatwright, B.D. and Head, J.W., 2021. A Noachian proglacial paleolake on Mars: fluvial activity\nand lake formation within a closed-source drainage basin crater and implications for early Mars\nclimate. The Planetary Science Journal, 2(2), p.52.\n“Assuming the global average surface Fe abundance represents the original Fe abundance in the\nNoachian terrains”\nI didn’t quite follow the logic of this statement. The Fe content of e.g. Hesperian plains should be\nrepresentative of the starting Noachian value only if they are sourced from mantle material with\nthe same Fe content. Do we have independent constraints on this from mantle evolution modeling\nor other sources?\n\"A recent modeling result by Fan et al., (2022) indicates that the greenhouse warming effect could\nplay a more important role than atmospheric pressure in diabatic cooling\"\nIt’s worth noting that the Fan et al. preprint mischaracterizes past work on this topic, as neither\nWordsworth (2016) or the earlier Forget et al. (2013) claimed that sensible heat fluxes are solely\nresponsible for adiabatic cooling of surfaces under a thicker atmosphere (see e.g., Wordsworth\n2016; p. 396).\nFig. 1: The authors show sulfates on the plot but don’t discuss them much in the text. Are their\nresults consistent with observations of sulfur minerals on Noachian and Hesperian terrain?\nFig. 4: I strongly suggest changing “reducing greenhouse warming” to “decreasing greenhouse\nwarming” here to avoid confusion.", "reviewer_2": "Please check the attched comments.", "reviewer_3": "Please check the attached .docx file.", "reviewer_4": "Dear Jiacheng Liu et al.,\nThis was an exciting manuscript to read and some big impacts for the planetary community. In an\nattachment, I have provided a major comment with some more modest suggestions to help\nstrengthen the manuscript. Please reach out if you need any clarification and good luck with the\nprocess.\nSincerely,\nMike Thorpe\nReviewer #4 Attachment on the following page\n!"}, "author_response": "Dear Reviewers,\nWe are grateful for your thoughtful reviews, which have not only guided us in\nenhancing the manuscript but also recognized the potential impact of this work. In\nresponse to your valuable comments, we revised the manuscript thoroughly and\nconducted a self-check. Additionally, we prepared a detailed, point-by-point response\nto the reviewers’ feedback. In our response document, the green text represents our\nresponses, while the red text indicates the corresponding changes made in the revised\nmanuscript. We believe that this updated version of the manuscript addresses all\nconcerns raised from you.\nYou can find our replies to your comments in the following pages:\nReviewer #1: page 2-5\nReviewer #2&3: page 6-22\nReviewer #: page 23-26\nThank you very much for your reviewing efforts!\nSincerely,\nJiacheng Liu\n\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\n[...]\nReply: Thank you very much for your encouragement and useful comments. We\nmodified the manuscript carefully. We appreciate your guidance in enhancing our\nwork.\nMinor comments:\nFor future reference, line numbers in the manuscript would have made it easier to\nwrite this review.\nReply: We apologize for any inconvenience caused by the absence of line numbers in\nthe version of the manuscript you reviewed. It appears that other reviewers were able\nto see the line number of the original manuscript based on their review reports. We\nhope that the line number are clearly visible in the updated version for your reference.\n“the acidic leaching scenario is inconsistent with …”\nIt might also be added that achieving acidic conditions on the scales necessary to\nexplain Fe depletion across large regions of the surface is extremely challenging from\na mass balance perspective.\nReply: Thank you for raising this point. We incorporated the point in lines 57-59:\n“Although low pH levels (pH<3) can mobilize Fe, the acidic leaching scenario\nstruggle to explain the Fe depletion across the extensive region of the surface (dark\nred area in Fig. 1b) form a mass balance perspective.”\n“Efficient Fe leaching under cooler conditions might have been related to elevated\nolivine weathering at cryogenic temperature. Freezing can increase the acidity of\nresidual water, which counterintuitively accelerates the kinetics of chemical\nweathering…”\nThis is an interesting idea. I think it’d be helpful here to specify the pH ranges we’re\ntalking about, as acidic leaching is argued against as the main mechanism earlier in\nthe text. Can the authors quantify the magnitude of the effect they’re talking about\nhere? Seems like it matters for the interpretation of their data.\nReply: We agree you very much. In this revised version, the magnitude of the effect\nwas quantified, and the pH ranges were specified to address your concerns according\nto Nile’s work1. We added the point in lines 139-143: “Freezing can increase the\nacidity of residual water, potentially resulting in a pH level below 0, which\ncounterintuitively accelerates the kinetics of chemical weathering to alter minerals at\nsubfreezing temperatures2,3$4=:J:6G=:E>B< E6G: A:6FHE:9 6G #(%N>F 8CAD6E67@:GC\n;CEFG:E>G:9>FFC@HG>CBE6G:F >B6D0C;'$)FC@HG>CB6G ')N1.”\n“Lower MAT can amplify the freezing, which can sustain chemical weathering and\nmake the residual water more acidic (Figure 4a). As a potentially counterintuitive\nresult, more Fe can be released under colder conditions”\nThis text is a bit repetitive with the sentence I just quoted above. I suggest editing to\nimprove the flow.\nReply: We agree that the text was repetitive. We revised the sentence in lines 148-149\nto improve the flow: “Under lower MAT, freezing can be amplified therefore can\nfacilitate release of Fe and subsequent leaching.”\n“. The relatively limited distribution of the permanent ice deposits may explain why\nglacial features are not extensively identified in the southern highlands”\nIt’s worth citing and discussing some of the more recent literature on this topic here,\ne.g.:\nGrau Galofre, A., Jellinek, A.M. and Osinski, G.R., 2020. Valley formation on early\nMars by subglacial and fluvial erosion. Nature Geoscience, 13(10), pp.663-668.\nBoatwright, B.D. and Head, J.W., 2021. A Noachian proglacial paleolake on Mars:\nfluvial activity and lake formation within a closed-source drainage basin crater and\nimplications for early Mars climate. The Planetary Science Journal, 2(2), p.52.\nReply: Thank you for the suggestion to cite and discuss recent literature on this topic.\nThe two references were incorporated in lines 178-180: “Above 3.0 km, subglacial\nerosion4 can form channels and proglacial fluvial channels can be formed below 3.0\nkm5 during the Noachian period”.\n“Assuming the global average surface Fe abundance represents the original Fe\nabundance in the Noachian terrains”\nI didn’t quite follow the logic of this statement. The Fe content of e.g. Hesperian\nplains should be representative of the starting Noachian value only if they are sourced\nfrom mantle material with the same Fe content. Do we have independent constraints\non this from mantle evolution modeling or other sources?\nReply: We use the Fe abundance of ALH84001, the only Martian meteorite with a\nNoachian age, as independent constraint for estimating the Fe abundance of Mars\nNoachian crust. As the Fe abundance of meteorite ALH84001 (~14.3 wt.%)6,7 is\nsimilar to the global surface average Fe abundance bases on GRS result. It is\nreasonable to propose that 14.3 wt.% is a good estimate of the Fe abundance of\nNoachian crust. We made corresponding changes in lines 193-199:“Martian\nmeteorite ALH84001with an igneous age of 4.091 ± 0.030 billion years is the only\nmeteorite, which can be used to constrain the composition of Martian Noachian crust.\nThe Fe abundance of meteorite ALH84001 is ~14.3 wt.%6,7, which is similar to the\nglobal surface average Fe abundance bases on GRS result. Assuming ~14.3 wt. %\nrepresents the original Fe abundance in the Noachian terrains, ~0.58 wt.%, ~0.15\nwt.%, and ~0.10 wt.% Fe was leached during the Early, Middle, and Late Noachian\nepochs, respectively (Fig. 3b).”\n\"A recent modeling result by Fan et al., (2022) indicates that the greenhouse warming\neffect could play a more important role than atmospheric pressure in diabatic cooling\"\nIt’s worth noting that the Fan et al. preprint mischaracterizes past work on this topic,\nas neither Wordsworth (2016) or the earlier Forget et al. (2013) claimed that sensible\nheat fluxes are solely responsible for adiabatic cooling of surfaces under a thicker\natmosphere (see e.g., Wordsworth 2016; p. 396).\nReply: Thank you for the comment. Fan et al., (2022)’s finding was positively\ndiscussed and emphasized several times by reviewers 2&3. Wordsworth (2016)’s\nwork is well cited and discussed in the previous paragraph before the one we are\ntalking. In light of your comment and the suggestions from reviewers 2&3, we revised\nthe sentence in lines 248-250 to moderate the tone of the statement and provide a\nmore accurate representation of the cited works: “In addition to atmospheric pressure,\nrecent results by Fan et al., (2023)8 and Kite et al., (2022)9 suggest that the\ngreenhouse warming effect could also play an important role in diabatic cooling.”\nFig. 1: The authors show sulfates on the plot but don’t discuss them much in the text.\nAre their results consistent with observations of sulfur minerals on Noachian and\nHesperian terrain?\nReply: Thank you for pointing out the lack of discussion on sulfates in the\ntext. We recently published a study10 in Science Advances about the formation of\nsulfates by freeze-thaw cycle in low latitudes. We included a discussion about sulfates\nt sulfates in the final paragraph of main text in lines 284-301:“The seasonal freeze-\nthaw region has transitioned from high-elevation areas to high-latitude regions as the\nclimate mode has shifted with the gradual loss of H and H in the atmosphere. With\n2\nfurther cooling of Mars after NHT, seasonal freeze-thaw cycles could have migrated\nto low-latitude region during the Hesperian period. Owing to drying of Noachian\nMars by sequestration of water in the crust11, Hesperian Mars was arid with much less\ntop down-leaching. Cryosuction during freezing can induce accumulation of water\nand ions from groundwater in the near-surface environments12. During the thawing\nstage, strong evaporation under oxidizing conditions during the Hesperian period, can\nprecipitate salts and ferric oxides67. The enrichment of sulfates at Meridiani, the\nlanding site of the Opportunity rover, can be well explained by seasonal freeze-thaw\ncycles10 according to the geochemical trends and textural observations. The evidence\nincludes upwards enrichment of S but depletion of Cl, the upper bleaching and\nbrecciated zone and lower darker and less brecciated zone with a sudden horizontal\nboundary in a meter-scale section, and the platy structure with abundant coarse planar\nvoids but little fine voids. During the Hesperian period, the intersecting regions with\nhigh surface temperature (low latitude region) and a shallow water table promoted\nseasonal freeze-thaw with upward migration of groundwater via cryosuction, thus\nproviding favorable conditions for the formation of sulfates10. This mechanism may\nexplain why sulfates (blue squares in Fig. 1b) tend to be enriched in low latitude\nregions.”\nFig. 4: I strongly suggest changing “reducing greenhouse warming” to “decreasing\ngreenhouse warming” here to avoid confusion.\nReply: Thank you for the suggestion. To avoid any confusion, we deleted the phrase.\n\nReviewers 2 and 3 co-reviewed and have submitted identical reports.\nReviewer 2 and 3\n[...]\nReply: We appreciate your thorough review and suggestions regarding the citations to\nclimate papers in our manuscript. We revised the Introduction and Discussion to better\nincorporate the relevant literature, including Kite et al. (2022) and Fan et al. (2023). In\nlines 29-34, we now discuss the potential mechanisms and timing of the climate mode\ntransition: “Several potential mechanisms13 have been proposed to drive the climate\nmode change, including the decrease in atmospheric CO 3,14 or atmospheric H 8,9, or a\n2 2\nreduction in cloud radiative effect8,9,15. However, the exact process behind the climate\nmode change remains unclear. While significant cooling9 and drying11 have been\nsuggested from Noachian Mars to Hesperian Mars, the timing of this climate mode\ntransition remains poorly constrained16.”\n[...]\nReviewer 4\n[...]\nReply: Hello Mike, thank you for taking the time to review our manuscript and\nproviding valuable feedback. We are glad that you found the manuscript exciting and\nimpactful for the planetary community. We appreciate the suggestions you have\nprovided and have addressed them in a point-by-point manner."}]

Decision letter