Hydrothermal activity generated by impact melt emplacement on the rim of Ritchey crater, Mars

Domain: earth_science
Published: 2025-04-04
Editorial outcome: major_revision

Abstract
Impact-induced hydrothermal systems have the potential to sustain long-lived aqueous environments throughout the history of Mars, yet their nature and distribution are not well-understood. While post-impact hydrothermal alteration on Mars has traditionally been studied at central peaks, we reported similar processes within a well-preserved impactite stratigraphy across the inner rim of Ritchey crater. This stratigraphy comprises a sheet unit overlying fragmented breccia, consistent with the emplacement of impact melt rocks on ballistic ejecta deposits, similar to features observed in complex craters on Earth. Analysis of CRISM hyperspectral data revealed alteration minerals including serpentine, chlorite, Mg-carbonate in fractured bedrock, veins, and erosional windows underneath the sheet unit. These alteration minerals are unrelated to post-impact fluvio-lacustrine facies or pre-impact target bedrock. Instead, their formation is most plausibly attributed to the emplacement of hot impact melt and subsequent groundwater percolation through fractured bedrock at the crater rim. The widespread distribution of the alteration minerals on the inner rim suggests that impact cratering can create extensive habitable environments. Our findings underscore the potential for alteration minerals at crater rims to preserve biosignatures, a key objective of the Mars 2020 mission at Jezero crater.

Introduction
In hypervelocity impact events, the substantial kinetic energy of the impactor undergoes conversion into internal energy, where shock-induced high pressures and temperatures generate impact melt1,2. On water-bearing or icy planets3,4, and even on asteroids5, the ensuing propagation of heat and water after meteoric impact culminates in the development of impact-induced hydrothermal systems, which have the potential to support diverse forms of life6.

Numerical simulations have predicted impact-generated hydrothermal systems on Mars driven by heat from impact melt, triggering basin-wide groundwater percolation7–9 and subsequent chemical alteration10, consistent with observations from terrestrial impact systems11. These hydrothermally altered sites potentially fulfill important requirements for habitability, i.e., liquid water, nutrients, and energy3,12,13. Moreover, potential biosignatures on Mars, if present, were likely to be preserved in chemically precipitated minerals (e.g., carbonates and silica) from various settings, particularly in association with hydrothermal activity3,14,15. However, the extent, mechanism, and distribution of such hydrothermal systems in impact craters on Mars are unclear. Most previous research has searched for signatures of hydrothermal alteration within central peaks, where a series of alteration minerals associated with vein-16,17 or mound-like features18,19 have been identified in orbital datasets. Less commonly, alteration minerals at crater rims were detected in many craters20–22, including Jezero23, but no clear relationship with the impact or impact-related hydrothermal alteration was established. As the Mars 2020 Perseverance rover is exploring and sampling the crater rim of Jezero, it is crucial to scrutinize the origin of alteration minerals for potential links to post-impact hydrothermal systems. One challenge in making this link is that ancient Martian craters like Jezero are often heavily eroded, so lacking convincing geological evidence for heat sources. A sufficiently large, young, and well-preserved impact crater would reveal more about the nature of impact-induced hydrothermal alteration within crater rims.

Ritchey crater is located ~200 km south of Valles Marineris and is a complex crater 78 km in diameter, with exceptionally well-preserved central uplift, terraces, rim, and ejecta (Fig. 1A). Crater counting on the Ritchey ejecta yields a retention age of 3.46 Ga, suggesting an Early Hesperian impact age21. At the crater center, a smooth, coherent layer and a dark, rough unit were speculated to be a preserved impact melt deposit21. Although the crater floor is covered by aeolian deposits, the inner rim exhibits well-exposed and intact bedrock. Fluvial channels and related fan deposits21 emanating from the wall indicate at least some post-impact aqueous activity occurred.

The excellent preservation of Ritchey crater offers a unique opportunity to study impactite-alteration stratigraphy on the rim for large, complex craters on Mars. Here we use Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) hyperspectral visible/near-infrared24 and High-Resolution Imaging Science Experiment (HiRISE25) images to identify alteration minerals, impactite stratigraphy, and post-impact geological processes on the rim area, in order to investigate their relationship to impact-induced hydrothermal activity. Through these observations, we seek to provide an important framework to understand the alteration processes and astrobiological potential of martian crater rims.

Results
Geomorphology and stratigraphy
On the inner rim of Ritchey crater, the most common stratigraphic sequence presents as fractured light-toned bedrock overlain by a fragmented breccia unit (Fig. 1C, D), and capped by a darker-toned sheet-like unit (Fig. 1C, D). Outcrops of light-toned massive bedrock at the base of the sequence exhibit a ridged texture, with smooth escarpments along their slope (Fig. 1C, D). The fragmented breccia unit contains large, rounded to unsorted boulders (up to ~70 m) embedded in a medium-gray toned matrix (Fig. 1C, inset 2). On the top, the sheet unit forms a ten- to decameters-thick coherent layer that can be traced laterally across several km (Fig. 1B). With a homogeneous and smooth surface, the sheet unit is increasingly brecciated toward the margin (although the brecciated margin may be covered by dust), embedded with smaller blocks (usually <10 m) than the underlying fragmented breccia unit (Figs. 1C, D, 2A, C). Some light-toned, elongated (up to 40–50 m long), and vertical pipe-like structures are observed inside the sheet unit (Fig. 1C).

At its marginal slope/escarpment, the sheet unit sometimes grades to a rough-surfaced pitted unit (Fig. 3B and S1E) or a ridged unit (Figs. 1C, 3F, G) that overlies the light-toned basement rocks. The pitted unit is a less regular and coherent, patchy material (Fig. 3B), comprising closely-spaced, decameter- to hundred-meter-sized pits that form “soap froth” networks with negative topography whose rims do not appear to be raised above the surrounding terrain. Blocks and brecciated fabrics are embedded at the interior walls of the pits (Fig. 3B inset). The rims of the pits are not surrounded by ejecta, so unlikely to be impact craters. Between the pitted unit and the basement, a ridged unit is present, showing elongated, near-linear ridge features (Fig. 3C, F) and brecciated fabrics at the side slopes (Fig. 2A, C).

Spectral data
CRISM spectra of typical basement rocks in the Ritchey crater rim show strong pyroxene absorption bands (0.9–1.0 μm, 1.9–2.0 μm) most consistent with low-Ca pyroxene (LCP), with no associated alteration mineral detections21. The sheet-like unit also shows broad absorptions centered near 0.9 and 1.9 μm consistent with LCP, but with a shoulder near 1.2 μm potentially due to mixing with olivine or glass26 (Fig. 3I, spectra a and g). The sheet unit appears to be largely unaltered, as no hydration (e.g., 1.9 or 1.4 μm) bands and only a weak, possible Al/Si-OH (2.21 μm) band is consistently detected (Fig. 3I, spectrum a; Fig. S1C, D). The pitted unit has similar spectral features (Fig. S1B, D) as the sheet unit. In contrast, the fragmented breccia (Fig. 3E) does not show pyroxene bands, and instead resembles olivine or glass with a broad band centered >1.0 μm, and a weak possible band at 1.9 μm, suggesting minimal hydration (Fig. 3I, spectrum e).

Strong signatures of alteration are detected in the ridged unit and altered bedrock. A clear 2.29–2.31 μm band and weak 1.39–1.42 μm band in the ridged units suggest the presence of Fe/Mg-bearing phyllosilicates such as nontronite and saponite (Fig. 3I, spectra c and d). Many outcrops also show evidence for Mg-carbonate based on an additional 2.51–2.53 μm band (e.g., Fig. 3I, spectra c–f) and 3.4–3.5 μm band, 3.9 μm band drop related to CO2 overtones and combinations (see Supplementary Data for the full-length spectrum).

Localized signatures of other alteration minerals are also present in the crater rim. Serpentine was identified in the ridged network at the southern rim (Fig. 3F) according to a diagnostic 2.12 μm band, along with 2.3 and 2.5 μm Fe/Mg-OH bands (Fig. 3I, spectrum f). Furthermore, chlorite was detected based on 2.26, 2.00, and 2.3 μm bands (Fig. 3I, spectrum h) at the southern rim (Fig. 1A). At the northern rim, possible prehnite was identified adjacent to a carbonate outcrop (ROI I in Fig S1), based on the diagnostic 1.48 μm absorption and a 2.35 μm Mg-OH band drop.

Methods
Visible images and topography
Landform investigation was mainly based on the enhanced false-color greyscale (RED) and IRB (infrared-blue) HiRISE (High-Resolution Imaging Science Experiment25) visible images up to 25 cm/pixel. The 20 m/pixel Context Camera mosaic71 (see also https://murray-lab.caltech.edu/CTX/) was also used as a lower-resolution basemap to substantiate the areas where HiRISE are not covered. Digital elevation model (DEM) data up to 20 m/pixel were acquired from HRSC (High-Resolution Stereo Camera72), supplemented by the ~300 m/pixel-resolution MOLA (Mars Orbiter Laser Altimeter73) DEM data. A three-dimensional terrain model was generated using the commercial software ArcGIS Pro (Esri).

CRISM data
Determination of mineralogy was carried out using CRISM visible/near-infrared (~0.3–2.6 μm) hyperspectral image data (Compact Reconnaissance Imaging Spectrometer for Mars24,74) at the most recently calibrated MTRDR level (Mapped Targeted Reduced Data Records75,76). Eight FRT (full resolution targeted, 18 m/pixel) images and two HRL (half resolution long, 36 m/pixel) images were analysed using the CAT (CRISM analysis toolkit) in ENVI. A set of spectral parameters containing spectral reflectance at diagnostic wavelengths is compiled in the delivered MTRDR data, and these parameters are combined as RGB components to highlight particular minerals75 such as carbonates (CR2 browse product) and phyllosilicates with Fe and Mg (PFM browse product), as shown in Fig. S2. All parameters in selected browse products were reviewed using a linear stretch of 50-98% to enhance spectral features and suppress noises. Average reflectance spectra were extracted from each Region of Interest (ROI) containing several tens to thousands of pixels. These spectra were then ratioed (Supplementary Data 1) against spectrally neutral, bland areas (Supplementary Data 2) at similar elevations to further suppress atmospheric and instrumental effects. Spectra were compared to standard laboratory spectra (Supplementary Data 3) from NASA RELAB facility77 at Brown University: magnesite (F1CC06B by Jack Mustard), saponite (CASA58, Edward A. Cloutis), serpentine (LALZ01, Takahiro Hiroi), chlorite (LACL14, Takahiro Hiroi), and prehnite (LAZE03, Edward A. Cloutis), and published results (glass-pyroxene mixtures from ref. 78).

Data availability
All original data used in this study are publicly available on the NASA Planetary Data System (PDS) at https://pds.nasa.gov/. The selected and processed CRISM spectral data used for analysis are present in the Supplementary Data files.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of Hydrothermal activity generated by impact melt emplacement on the rim of Ritchey crater, Mars by Zeng and Horgan\nThis manuscript presents evidence of impact-generated hydrothermal activity at Ritchey Crater Mars. The authors describe the impactite stratigraphy and mineralogy of the inner rim of Ritchey Crater. They describe the stratigraphic sequence which consists of light-toned bedrock overlain by a fragmented breccia unit and capped by a darker-toned sheet-like unit. For this work, the authors used spectral data from CRISM to detect the mineralogy at the inner rim of the crater, which suggests the presence of Fe/Mg-bearing phyllosilicates and Mg-carbonates. They hypothesise that these phyllosilicate-carbonate assemblages are indicative of hydrothermal alteration driven by contact with hot melt-rich breccias, leading to the percolation of groundwater which reacted with the bedrock at the crater rim to form these alteration minerals. They suggest that the environment formed by impact cratering at the inner rim, created hydrothermal conditions conducive to life, thus potentially creating an area in which biosignatures may be preserved. This is well argued and justified.\nGeneral questions:\n- What are the noteworthy results?\nThe paper presents evidence for the existence of a post.impact hydrothermal system on Mars, which is especially important as this type of evidence is hard to access from orbit in most places, because of dust coverage. Thus, the authors present a rare and extremely important example.\n- Will the work be of significance to the field and related fields? How does it compare to the established literature? If the work is not original, please provide relevant references.\nThis is original work, which will provide a very important reference point for the exploration of Jezero crater – and other craters in the future. It also relates to the Curiosity mission, as this mission has not found any non-sedimentary units within its 12 year mission, which is, however, up the central mound which is in itself sediment covered. This work could make Richey crater a future mission target to look for life on Mars.\n- Does the work support the conclusions and claims, or is additional evidence needed?\nYes\n- Are there any flaws in the data analysis, interpretation and conclusions? - Do these prohibit publication or require revision?\nNo\n- Is the methodology sound? Does the work meet the expected standards in your field?\nYes.\n- Is there enough detail provided in the methods for the work to be reproduced?\nYes.\nIn detail:\nLine 30: We wonder if it is worth pointing out that the paper by Naumov is a collection of terrestrial ground truth observations? Those are usually highly regarded alongside numerical models (citation 11) and orbital observations such as the work presented in this manuscript.\nLine 42: There is at least one attempt to overcome this problem through a series of models and ‘mapping’ the different depth layers of a hypothetical crater. See https://www.sciencedirect.com/science/article/pii/S0019103513002558#f0020. It might be worth discussing this either here or in the light of your own findings.\nLine 60: In Figure 1A there are references to Fig. 2E and 2F, which we suspect are pointing to the supporting material. Please indicate this accordingly.\nLine 198: It would be worthwhile to get back to the numerical modelling work, where p-T-x conditions were also defined for a variety of impact-generated hydrothermal situations.\nLine 215: To further aid understanding, adding an estimate of extend of the hydrothermal system in time and space, guided by the Abramov and Kring paper (Ref 8) would add to the calculations of impact melt volumes etc. There also has been hydrothermal modelling work beyond the Schwenzer and Kring 2009 paper (REF 11) by the same authors (2013, Icarus: https://www.sciencedirect.com/science/article/abs/pii/S0019103513002558) detailing on the mineralogical and temperature variability as well as depth extension of the mineral assemblages. All this could complete this picture.\nLine 240-242: The fossil evidence that has recently emerged is a good proof of potential microbial life in those systems. However, it could aid the understanding to look at what colonises active hydrothermal systems on Earth, even if that means to look at volcanic systems as they are the only ones available to study.\nSupplementary materials: It is not always clear what belongs where in the supplementary materials. We suggest to place figure captions to the figures and add relevant Figure numbers at the top of each figure. It would also be good to add one sentence to the main text about the content of the supplementary materials. In the version provided for review, the pdf does not contain the tables, the datasets have generic headers. While I assume this will be sorted out in production, it would be good to have a clear understanding of those things at all times.", "reviewer_2": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_3": "My comments are in the attached file titled 'Review comments NCOMMS paper'.", "reviewer_4": "The manuscript “Hydrothermal activity generated by impact melt emplacement on the rim of Ritchey crater, Mars” by Zeng and Horgan describes a posited impactite stratigraphy on the rim of Ritchey crater and hypothesizes that the alteration minerals within it were formed in an impact-induced hydrothermal system, due to their association with fractured bedrock. Serpentine, chlorite, Mg carbonate, and clays are found within bedrock that are underneath the unaltered sheet unit, and, if indicative of a hydrothermal environment, would indicate a habitable environment.\nThe paper is well-written and has a very nice discussion of a hydrothermal model and figure at the end of the manuscript. However, I have two major concerns about the results and their impact to the Mars literature that may preclude publication in the target journal.\n1) I did not find the arguments compelling for why this crater rim environment was uniquely indicative of an impact-induced hydrothermal system, instead of simply representing the exposure of pre-existing altered bedrock by the impact. The alteration minerals are detected in crater wall bedrock, over which an inferred impact melt unit is emplaced. The paper did not justify why the altered bedrock could not simply just be altered Noachian bedrock that existed before the Ritchey impact and then was exposed along Ritchey’s crater walls. In this case, hydrothermal alteration need not be invoked to explain the observed minerals. This is a more conservative interpretation that has been used by several studies including Mustard et al., 2008; Sun and Milliken, 2015; Quantin et al., 2012, etc. In addition to these concerns, I also had questions about how the authors distinguished clays from carbonate in the CRISM spectra, and if alteration minerals like chlorite were uniquely isolated to veins, or just generally associated with vein-bearing bedrock. Please see the detailed comments for more information.\n2) Even if the alteration minerals detected could be appropriately attributed to a hydrothermal origin, the paper did not justify why these findings would represent a significant advancement or finding for Mars literature. Impact-induced alteration or hydrothermal alteration has already been invoked for alteration minerals found in many martian craters, including the same Ritchey crater as in this study, at its central peak (Sun and Milliken, 2014). Other examples of similar findings include Toro crater (Marzo et al., 2010), Majuro crater (Mangold et al., 2012), Holden crater (Osinski et al., 2013), in addition to more global surveys of craters (Sun and Milliken, 2015; Ehmann et al., 2009; Carter et al., 2013). This paper did not explain how these results would be substantially different from existing literature to warrant urgent and prompt publication.\nDetailed comments:\nFigure 1:\n- It would be helpful to relate the units identified in this work to the map units from Sun and Milliken 2014, as that work had identified fan deposits lining the edge of the crater rim – are these the same as the inferred sheet units here?\n- Please fix the labels in Figure 1A – I could not locate Figure 2E or 2F in the paper. Please also see comments for Figure 2, as I was not sure what the Figure 2A and 2C boxes referred to.\n- 1C: The breccia textures are not clear where “fragmented breccia” is indicated in the image.\n- 1C1: Alternatively, could these light-toned linear features not be small (fluvial) channels, or a trail left from e.g., a boulder rolling down the slope, or the tops of local ridges?\n- 1C2: What does the carbonate-bearing vein in C2 look like in the CRISM spectra? Is this spectrum h in Figure 3?\n- 1D: the brecciated slope (line 123) is difficult to see at this image resolution\nFigure 2: The caption identifies these images as being from the eastern rim of Ritchey crater, but in Figure 1A, Figures 2A and 2C are from the northern and western crater rim respectively.\nLines 119-125: Sun and Milliken (2014) identified numerous fan deposits lining the edge of the crater rim – are those distinct from this dark unit?\nFigure 3:\n- Where are the images in Figure 3 located in Ritchey crater? Could they be included in the Figure 1 overview?\n- Clays and carbonates have similar absorption features, and many published clays on Mars have a 2.5 micron feature in addition to the usual 2.3 micron feature. How does this work distinguish between these two spectrally similar mineral groups?\n- As with Figure 1, the brecciated textures that are referred to in the caption are very difficult to see at this resolution (Figure 1E seems ok though)\n- Is there a reason why the h chlorite spectrum location is not displayed in Figure 3 but in the supplemental figures? Again this would be nice to include in the Figure 1 overview\n- 3B: Is the clay/carbonate identified in the actual veins running through the bedrock, or does “altered bedrock with vein” just mean that the clay/carbonate could not be isolated to the veins uniquely? Also, the veins are not very visible in this image, perhaps point them out?\nLines 192-198: Why could the clays and carbonates in the bedrock not be attributed to pre-existing clays/carbonates that formed in the bedrock before the impact even happened? Why do they have to be hydrothermal? Is the carbonate vein in Figure 1C specifically localized to that dark vein, and if so, what does that spectrum look like? And in Figure 3G, I assume this citation refers to spectrum b (altered bedrock with vein), but it is not clear if the carbonate signatures are localized to the veins (which are difficult to see in Figure 3B), or if it is just generally associated with bedrock that happens to have veins in it (but the carbonate cannot be distinctly localized to the veins).\n- As an aside, Figure 11 in Osinski et al. (2013) (also Tornabene’s 2009 abstract https://www.lpi.usra.edu/meetings/lpsc2009/pdf/1766.pdf) has a neat example of an altered vein in bedrock at Holden crater, with the signatures isolated to the vein portion, that could be a helpful citation.\nLines 199-202: Sun and Milliken (2014) identified fan deposits along the crater rim, and also reported on the detection of clays and hydrated silica in association with those fan deposits. It should again be clarified either any of the inferred impact melt deposits could actually represent fluvial landforms, and the statement “there is no morphological evidence of fluvio-lacustrine origin for phyllosilicates or carbonates at the crater wall” re-assessed.\nLine 202: It is not clear how it is possible to make this statement “Nor in the ejecta of Ritchey is there an clear indication of the same phyllosilicates-carbonate signatures detected in the rim.” At least, based on Figure S2, it does not appear that there is actual CRISM coverage covering the crater ejecta, so how would one be able to determine if there were or weren’t alteration minerals present in the ejecta?\nLine 216: Again, I am not convinced that the presence of alteration minerals in bedrock adjacent to an inferred impact melt must require that they formed from hydrothermal alteration. Couldn’t this bedrock have been already altered (from non-Ritchey processes) and was simply exposed by the impact along the crater wall? This more conservative interpretation was taken for numerous clay/alteration detections found in impact across Mars (e.g., Mustard et al., 2008; Sun and Milliken, 2015; Quantin et al., 2012).", "reviewer_5": "Review comments on the paper by Zeng and Horgan, submitted to NCOMMS for possible publication.\nIn this study, the authors report potential hydrothermal activity induced by impact-driven alteration of fractured and permeable bedrock at the rim of Ritchey crater (79 km diameter; 28.5 S, 51 W). They propose that the emplacement of hot impact melt over the crater rim facilitated groundwater percolation and subsequent alteration of the crater rim bedrocks. This hypothesis is based on the observation that there is no correlation between the alteration minerals (serpentine, chlorite, and Mg carbonate) and both post-impact and pre-impact morphological facies. The study implies that a potentially habitable environment existed in this region. This is based on the distribution of alteration minerals on the inner rim, which might have preserved biosignatures.\nThe authors utilized HiRISE images to identify impactite stratigraphy and infer post-geological processes on the rim of Ritchey crater. I have several concerns regarding their morphological interpretations and encourage the authors to provide their perspective on these issues. These comments and suggestions can be discussed in more detail during the next review. The manuscript contains extensive information about the impact-triggered hydrothermal activity on Mars. With appropriate revisions, I believe it can become a significant contribution to the field. Below, I offer my comments and suggestions for the morphologic and geologic observations in the paper.\n(1) Vertical pipe/Channels: In Figure 1C, authors have marked vertical pipe/channels in inset 1. They compare these features as degassing pipes observed in The Ries suevite and hydrothermal veins in the Chicxulub suevite.\nI have prepared another version of the figure (large scale colored version) for my review purpose to give my comments. Please see below.\nThe features marked in Ritchey crater show distinct brightness (compare all the features marked by black and white arrows) and bear different shapes (compare shapes of features marked by black arrows with the white ones). Additionally, the orientations of features marked by white arrows are different as compared to the downslope orientation of the features marked by black arrows. Maybe these are simply the dust clean surfaces revealing a variety of tones and shapes, and not essentially vertical pipe/channels. Please see below examples for reference. If they are flow structures, are they dried up flows? If shape is the original criteria of inference, I think authors should review it.\nFigure sources:\n(a) https://www.jpl.nasa.gov/images/pia14454-hues-in-a-crater-slope\n(b) https://www.esa.int/ESA_Multimedia/Images/2021/02/At_the_rim_of_a_crater\n(c) https://www.jpl.nasa.gov/images/pia23854-keeping-a-watchful-eye\n(d) https://hirise.lpl.arizona.edu/PSP_007767_1970\n(2) In the inset 2 in Figure 1C, it is not clear which feature is being marked as vein/vein fillings. The red arrow is not placed accurately. Same comment for yellow arrows in Figure 1D. They are not pointed accurately to ridges. I suggest please reviewing them and pointing the arrows precisely toward the features.\nWith respect to inferring that the features are veins, I suggest authors to please review it. Please see below references for veins observed on Mars using HiRISE, please compare with your observations with these.\nFirst, I would request authors to confirm whether I have marked the veins correctly?\nMy main concern is that if these are carbonate-rich vein fillings, why they are appearing dark with respect to the surrounding area. As such, mineral filled veins appear bright. Please see below.\n(3) I am unable to find out Figures 2A-F. The actual Figure 2 in the paper is different than what is marked on Figure 1. I am sorry, but I struggled to correlate. Is there a mistake from authors side regarding figure numbering, or are these Figures from supplementary. However, supplementary section has no such figures.\n(4) What is the basis of interpreting ‘sheet unit’ on the basis of morphology in Figures 1B and 1D? Morphology wise they have a relatively smooth texture and appear dark than the surrounding. However, there are other smooth and dark toned units present in this figure, which are located adjacent to the inferred ‘sheet unit’. How do you differentiate between them? Please see the below figure.\nThe inferred ‘sheet unit’ marked in the paper is shown here as ‘sheet unit’. Authors should clarify how the inferred sheet unit is different in morphology with the other units marked as ‘sheet’. In one of the ‘sheet’ units, brecciated units are evident.\nI do not find mention of ‘sheet unit’ or an example figure to compare in both the papers (references 29 and 30) that authors have suggested in line no. 92-93.\nAuthors mention in line 130-131 to see Fig. S1B and S1D for more lobate textures for fluidized evidence of sheet unit. Unfortunately, Fig. S1B and S1D do not such any such evidence. Please clarify. I cannot find lobate features.\n(5) Authors have identified pits (Figure 3B) in support of inferring sheet unit as a fluidized melt sheet. They also relate ‘vertical pipes’ as degassing pipes with the same goal. They have also suggested that the pits are not surrounded by ejecta, so they are unlikely to be impact craters. Please see the figure.\nI have marked the pits identified in the paper for reference. Please review this inference. There are so many similar circular features in this small area and none of them is surrounded by ejecta. Morphologically all circular features seem identical and cannot be distinguished from each other. So how do the authors use the evidence of circular features to support their inference of the sheet unit as the impact melt-rich breccia?"}, "author_response": "We thank the reviewer for the supportive and positive comments!\nIn detail:\nLine 30: We wonder if it is worth pointing out that the paper by Naumov is a collection of terrestrial ground truth observations? Those are usually highly regarded alongside numerical models (citation 11) and orbital observations such as the work presented in this manuscript.\nThank you for your suggestions. It would be indeed better, as the reviewer suggested, to point out that these are terrestrial observations to support similar processes on Mars. Please see modifications at: Line 30 in clean revised ms version/ Line 30 in the revised track-change ms file:\n“consistent with observations from terrestrial impact systems”\nLine 42: There is at least one attempt to overcome this problem through a series of models and ‘mapping’ the different depth layers of a hypothetical crater. See https://www.sciencedirect.com/science/article/pii/S0019103513002558#f0020. It might be worth discussing this either here or in the light of your own findings.\nThank you for this helpful reference! We cited this reference in later chapters. Here we slightly reworded the sentence. Please see modifications at: Line 41 in clean revised ms version/ Line 43 in the revised track-change ms file:\n“One challenge of making this link is that ancient Martian craters like Jezero are often heavily eroded, so lacking convincing geological evidence for heat sources.”\nLine 60: In Figure 1A there are references to Fig. 2E and 2F, which we suspect are pointing to the supporting material. Please indicate this accordingly.\nResponse: We apologize for the errors in labeling and many thanks for the reminder! Indeed, in Fig 1A the labeled “Fig 2E and 2F” actually refer to Fig 3E and Fig 3F. Please see modifications at: the updated Fig 1A and figure captions accordingly.\nLine 198: It would be worthwhile to get back to the numerical modelling work, where p-T-x conditions were also defined for a variety of impact-generated hydrothermal situations.\nResponse: thank you for your suggestion! We added a short discussion of numerical modeling to further suggest the subcrater permeability. However, we do not find systematic p-T change in our case and thus have not expanded this discussion. Please see modifications at: Line 184-196, 223-238 in the clean revised ms version/ Line 236-252 and Line 287-303 in the revised track-change ms file:\n“We therefore hypothesize that phyllosilicate-carbonate assemblages beneath the impactite stratigraphy formed via post-impact hydrothermal alteration driven by the contact with the hot melt-rich breccia. Some carbonate-rich bedrock outcrops exhibit dark fractures (Fig 1C). These features could be consistent with mineral filled veins or impact melt dikes. Spectral signatures indicate that the area of interest is rich in carbonate and phyllosilicate (Fig 3I), resembling carbonate veins in the hydrothermally altered ultramafic rocks on Earth (e.g., listwanite in Oman42, 43, 44). Alternatively, the dark material could be impact melt injected into basement fractures (dike suevite), as the materials appear to be affiliated with the sheet unit (Fig 1C, green arrow). This morphology is similar to an inferred injection dike within a megabreccia block in Holden crater on Mars45. Finally, it is possible that low albedo of the features are shadows or sand infilling topographic lows. Regardless, the production of serpentine, saponite, chlorite, carbonate, and prehnite) by hydrothermal alteration of alkaline fluid at fractured mafic/ultramafic areas is in line with hydrothermal modeling10, 46 and has been verified by remote sensing data on both Earth42 and Mars47, 48. ”\nLine 215: To further aid understanding, adding an estimate of extend of the hydrothermal system in time and space, guided by the Abramov and Kring paper (Ref 8) would add to the calculations of impact melt volumes etc. There also has been hydrothermal modelling work beyond the Schwenzer and Kring 2009 paper (REF 11) by the same authors (2013, Icarus: https://www.sciencedirect.com/science/article/abs/pii/S0019103513002558) detailing on the mineralogical and temperature variability as well as depth extension of the mineral assemblages. All this could complete this picture.\nResponse: thank you for your suggestions and providing the Schwenzer paper for reference!\nPlease see modifications at: Line 223-238 in the clean revised ms version/Line 287-303 in the revised track-change ms file:\n“The emplaced melt is thicker in the crater basin than on the wall-to-rim area, to retain a hot crater floor over time. Furthermore, uplifted, hot materials at the central peak provide additional heat to the area. Together, these effects result in a decreasing gradient in temperature8, 45 and alteration strength46 from crater center to crater rim. Indeed, the variation in temperature and water availability between the crater center and crater rim profoundly changes the diversity in alteration minerals46. As the melt is cooling down, hydrothermal alteration would have driven a series of low-temperature alteration reactions below 400 °C: (1) Chlorite-related alteration commonly between 300 to 400 °C (e.g, chlorite-bearing assemblage at Sudbury impact structure on Earth58); (2) Serpentinization (<150 to 400 °C), one of the most important reactions for astrobiological interests because it mobilizes critical elements and delivers chemicals (such as hydrogen and methane) to support critical microbial metabolism similar to the earliest biogeochemical systems on Earth59; (3) low temperature (<100 °C) carbonation to form saponite and carbonate44. Apart from being controlled by temperature, at low temperature as 150 °C, water-rock ratio is essential for the affected mineral diversity: serpentine and chlorite form at low water availability whereas the formation of Fe/Mg phyllosilicate requires more water10, 46. The implied water-rock ratio variability at Ritchey may suggest permeability variation46 subcrater bedrocks due to different degrees of impact fracturing as well as the change in melt thickness across the crater.\nHydrothermal modeling of a complex crater comparable to Ritchey predicts that the hydrothermal alteration below 200 ℃ occurs on the crater rim for at least 20000 years8.”\nLine 240-242: The fossil evidence that has recently emerged is a good proof of potential microbial life in those systems. However, it could aid the understanding to look at what colonises active hydrothermal systems on Earth, even if that means to look at volcanic systems as they are the only ones available to study.\nSupplementary materials: It is not always clear what belongs where in the supplementary materials. We suggest to place figure captions to the figures and add relevant Figure numbers at the top of each figure. It would also be good to add one sentence to the main text about the content of the supplementary materials. In the version provided for review, the pdf does not contain the tables, the datasets have generic headers. While I assume this will be sorted out in production, it would be good to have a clear understanding of those things at all times.\nResponse:\n(1) microbial life: thank you for your suggestion. We added the discussion of microbial colonization in hydrothermal systems in both acid (lava flow) and alkaline hydrothermal settings (deep subsurface) on Earth.\nPlease see modifications at: Line 243 in the clean revised ms version/Line 309 in the revised track-change ms file:\n“On Earth, a diverse microbial community including thermophiles/hyperthermophiles3 and chemoautotrophs61 is found in the modern hydrothermal settings related to basaltic lava flows62, hydrothermal vent63 and deep alkaline ultramafic environments64, 65.”\n(2) Supplementary files: We apologize for the wrong labels in Fig 1A, as they refer to Fig 3A to 3F. As the journal requires, we uploaded the supplementary files as separate pdf with figure captions. The supplementary table is uploaded under the name of Dataset 1. I will address this issue to the editor in the letter so hopefully it will be solved in the revised version!\n[... Author responses to Reviewer 3 and 4 omitted for brevity in this JSON representation ...]"}]

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