the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Apr 2026
Morphological and Spectrophotometric exploitation of JANUS LEGA Dataset: Langrenus impact crater characterization and evolution of the highland "isthmus" between Mare Fecunditatis and Mare Tranquillitatis
Abstract. The European Space Agency’s (ESA) JUpiter ICy moons Explorer (JUICE) mission, launched in April 2023, performed the Lunar-Earth Gravity Assist (LEGA) maneuver in August 2024 as an instrument validation opportunity and science investigation. During this flyby, the Jovis, Amorum ac Natorum Undique Scrutator (JANUS) camera acquired a continuous multispectral imaging strip across the lunar surface, achieving spatial resolutions down to approximately 20 m/pixel. This paper presents a detailed morphological and spectrophotometric characterization of two regions of interest: Langrenus impact crater and the highland “isthmus” between Mare Fecunditatis and Mare Tranquillitatis.
Analysis of Langrenus reveals a structurally complex impact site located at the transition between mare and highland terrains. Spectral unmixing indicates that the crater interior is dominated by mafic components, with minor but spatially coherent olivine-compatible signatures identified within the northern hummocky floor terrain and portions of the crater rim. Although the limited spectral sampling prevents definitive mineralogical discrimination, these mafic signatures may reflect excavation of heterogeneous target materials during impact or redistribution of mare-derived components through post-impact mass wasting and regolith mixing processes. Furthermore, investigations of the highland “isthmus” bring evidence for distinct generations of volcanic activity. Spectral analysis demonstrates a long-lived, structurally modulated, low-Titanium volcanic system, distinct from large-scale mare volcanism, characterized by small-volume magmatic emplacements that exploited impact-generated fractures for ascent.
These results not only provide new insights into lunar geological processes but also serve as a successful end-to-end validation of the JANUS camera’s performance in an operational environment, confirming its readiness for the high-resolution characterization of the Galilean moons.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2026-1901', Anonymous Referee #1, 19 May 2026
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AC1: 'Reply on RC1', Alice Lucchetti, 26 Aug 2026
We thank the Referee for the positive and constructive assessment of our manuscript and for the helpful suggestions. We are pleased that the referee found the work interesting, well structured, and clearly written. We have revised the manuscript accordingly, providing a point-by-point response. Our responses are reported in bold.General comments
Please also mention how much was topographic dataset used during the photometric correction?
We have clarified this point in Section 2. The SLDEM2015 topographic dataset was used in the generation of the map-projected products and the associated geometry cubes, which provide the per-pixel incidence, emission, and phase angles used by the Hapke photometric correction.
Please also discuss any correlation between the possibility of spectral identification and optical maturity.
We agree and have added this discussion to Sections 3.4, 4.2.2, and the Conclusions. We now state that optical maturity influences the detectability of mineralogical absorptions because mature regolith tends to darken and redden spectra and suppress band contrast. Consequently, spectral identifications are more robust on optically fresher surfaces, while mature/low-contrast areas are interpreted more conservatively. In the isthmus region, the combined FeO and OMAT analysis helps distinguish true compositional variations from maturity-related spectral effects; for example, D2-D3 domes appear optically fresher than the older D1 domes and show a modest mafic enrichment.
This aspect as well as the domes also should receive more specific emphasis in the abstract also.
We revised the Abstract accordingly, below the revised version:
“The European Space Agency’s (ESA) JUpiter ICy moons Explorer (JUICE) mission, launched in April 2023, performed the Lunar-Earth Gravity Assist (LEGA) maneuver in August 2024 as an instrument validation opportunity and science investigation. During this flyby, the Jovis, Amorum ac Natorum Undique Scrutator (JANUS) camera acquired a continuous multispectral imaging strip across the lunar surface, achieving spatial resolutions down to approximately 20 m/pixel. This paper presents a detailed morphological and spectrophotometric characterization of two regions of interest: Langrenus impact crater and the highland “isthmus” between Mare Fecunditatis and Mare Tranquillitatis.
Analysis of Langrenus reveals a structurally complex impact site located at the transition between mare and highland terrains. Compared with the adjacent highlands and ejecta-dominated terrains, the crater interior shows a stronger mare-related mafic contribution. Spectral unmixing indicates that the crater interior is dominated by mafic components, with minor but spatially coherent olivine-compatible signatures identified within the northern hummocky floor terrain and portions of the crater rim. Although the limited multispectral sampling and the optical maturity state of the regolith prevent definitive mineralogical discrimination, these mafic signatures may reflect excavation of heterogeneous target materials during impact or redistribution of mare-derived components through post-impact mass wasting and regolith mixing processes.
Investigations of the highland “isthmus” provide evidence for distinct generations of dome-like volcanic or subvolcanic constructs that differ in preservation state, optical maturity, and subtle mafic enrichment. Spectral analysis indicates a localized, structurally influenced, low-titanium magmatic system, distinct from large-scale mare volcanism and characterized by small-volume emplacements whose distribution is consistent with control by impact-generated and regional fractures.
These results not only provide new insights into lunar geological processes but also serve as a successful end-to-end validation of the JANUS camera’s performance in an operational environment, confirming its readiness for the high-resolution characterization of the Galilean moons.”
Specific comments
Abstract: 'Spectral unmixing indicates that the crater interior is dominated by mafic components' — comparative aspects with surrounding terrain would help.
We revised the Abstract to make the comparison explicit. The text now states that, relative to the surrounding highland-dominated ejecta and adjacent mare plains, the Langrenus crater interior shows a mixed spectral character, with a significant mafic contribution and localized olivine-compatible signatures.
Around line 65: some information on the spectral coverage would be needed here.
We added the JANUS spectral range and filter information at the first instrument description: the 13-position filter wheel covers approximately 340-1080 nm with broadband and narrowband filters. We also refer the reader to Table 1 for the exact filter central wavelengths and bandwidths.
Line 91: 'Lunar_LRO_LROC-WAC_Mosaic_global_100m_June2013' — consider moving it to the references.
The dataset name has been removed from the figure caption and the LROC-WAC basemap product has been cited in the data/references section in a standard bibliographic form.
Line 111: 'declination' — not clear what it is relative to.
We revised this sentence to avoid ambiguity. Since declination is not used directly in the subsequent analysis, we now list only the geometric quantities used for photometric correction and mapping: latitude, longitude, spacecraft distance, phase angle, incidence angle, and emission angle.
Line 130: highland 'isthmus' region — an overview with marked sites/features would help; Fig. 3 could include site-identification text.
We updated Figure 3 to include labels for Mare Tranquillitatis and the highland isthmus. This should make the site identification clearer before the detailed map in Figure 13.
Line 179: 'distinctive herringbone pattern' might not be clear to all readers; cite the Springer Encyclopedia of Planetary Landforms.
We added a short explanation that herringbone patterns are diagnostic V-shaped arrangements of secondary craters formed by oblique ejecta emplacement and added the suggested reference to the Encyclopedia of Planetary Landforms.
Around line 210: Could melt ponds have been identified/separated in the spectral datasets?
Smooth plain terrain on terraces and parts of the crater floor is morphologically interpreted as impact melt or melt-bearing material. However, we did not separate small melt ponds as an independent spectral unit because the four JANUS bands over Langrenus, combined with shadowing, blocky textures, and possible regolith mixing, do not allow a robust diagnostic separation of melt ponds from other spectrally similar mafic/smooth materials.
Line 229: 'vertical throws' — many readers might not be familiar with this term.
We defined vertical throw at first use as the vertical component of displacement across a fault scarp, measured from the topographic offset between adjacent terrace surfaces. We added it in the text.
Line 247: The interpretation of weaker mare lithologies vs stronger highland lithologies fits better with explanation in the Discussion, not only in the Figure 6 caption.
We agree. The interpretive sentence has been shortened in the caption and the explanation has been moved/expanded in the Langrenus Discussion, where it is now presented as one possible interpretation supported by fault localization, mare-highland position, and structural asymmetry.
Figure 9: colour-based identification of measured locations is not easy; small numbers could help.
We improved Figure 9 by adding ROI numbers/labels in addition to colors and updated the caption accordingly.
Lines 364-366: important, might mention later in the summary also.
We agree, we emphasize that JANUS can retrieve internally consistent relative spectral trends, but that mineralogical identifications must remain conservative because of the limited number and spectral placement of the available filters.
Figure 10: scalebars are needed on the left image stripes.
Scale bars have been added to the left image panels of Figure 10.
Lines 405-407: limited number of spectral bands and degeneracies between mafic endmembers; the multicolour-based spectral resolution also matters.
We revised the sentence to explicitly mention both the limited number of bands and the broad/irregular spectral placement of the JANUS multicolour filters. The text now states that these factors limit band-center determination and increase degeneracies among mafic endmembers.
Line 427: numerical size of the spatial scale should be mentioned for the regional-local bridge.
We added the relevant numerical scale range: JANUS observations at 19-22 m/pixel bridge regional LROC-WAC mapping at ~75 m/pixel and ultra-high-resolution LROC-NAC observations at ~0.5-2 m/pixel, with Kaguya TC data at ~10 m/pixel providing an intermediate comparison.
Comment 15. Line 430: extensive terrace zone does not necessarily imply a heterogeneous target; terraced walls can form by collapse of the transient crater regardless of subsurface structure.
We agree and have softened the interpretation. The revised text no longer treats terrace width alone as proof of a heterogeneous target. Instead, we state that the asymmetry of the terrace zone, together with fault localization, mare-highland setting, and mapped pre-existing structures, is compatible with target heterogeneity and structural control during crater modification.
Lines 440-450: did you consider dating ejecta also or only the inner crater units?
We clarified that crater-count ages were derived only for inner crater units: the flat crater floor and the northern hummocky terrain. We did not date the ejecta blanket because ejecta surfaces are laterally heterogeneous and discontinuous, affected by secondary and self-secondary cratering, and partly mixed with surrounding mare and highland materials. These factors would make a reliable SFD age for ejecta difficult within the scope of the present work.
In addition, to assess the robustness of the crater chronology, we performed an independent Model Production Function (MPF) analysis of the same four crater size-frequency distributions, considering craters with diameters larger than 0.4 km. The MPF ages for the crater-floor unit are 3.4 ± 0.1 Ga for the JANUS count and 3.5 ± 0.1 Ga for the LROC-WAC count, closely reproducing the Imbrian ages obtained with the chronology function used in the main analysis. For the northern hummocky terrain, the corresponding MPF ages are 2.4 ± 0.2 Ga and 2.2 ± 0.3 Ga. Although its absolute age is partly model-dependent, both chronology approaches consistently indicate that the hummocky terrain is substantially younger than the adjacent crater floor.
We added the MPF methodology and results to Section 3.2, incorporated their implications into the Langrenus Discussion, and provide the individual MPF in Appendix B (Figures B1 and B2).
Lines 475-476: does this mean the mare/highland boundary was not well located previously?
We clarified it. We do not mean that the regional mare-highland boundary was incorrectly mapped. Rather, our point is that the effective compositional transition around Langrenus has been blurred at local scale by impact excavation, ejecta emplacement, regolith mixing, and post-impact mass wasting.
Around line 500 and elsewhere: domes are mentioned but not specifically discussed enough; include in the abstract also.
We expanded the Abstract and the isthmus Discussion to describe the dome generations explicitly. We now discuss D1 as an older, degraded dome-like population spectrally similar to the highland basement, and D2–D3 as better-preserved dome-like features with a modest mafic enrichment and a spatial association with pre-existing craters and fractures. We also revised the text to clarify that the available JANUS data do not uniquely distinguish between intrusive inflation, shallow subvolcanic uplift, or limited extrusive emplacement; these mechanisms are therefore discussed as possible interpretations rather than as definitive conclusions.
Line 538: D2 and D3 domes are spatially associated with pre-existing impact craters, suggesting magma rising induced by impact fracturing — not clear why such connection is expected.
We clarified this point in the revised Discussion. Impact cratering can generate fractured and brecciated zones, crater-floor weaknesses, and radial or concentric fracture networks that may reduce crustal strength and act as preferential pathways for later small-volume magma ascent. However, we now state more explicitly that this interpretation is based on spatial association and structural context only. JANUS images document surface morphology and spectral properties, but they do not directly image subsurface conduits. Therefore, the association between D2–D3 domes, old crater floors, and mapped fractures is described as consistent with structurally guided magma ascent, rather than as direct evidence for a unique ascent mechanism.
Line 536: 'The maria smooth plains locally infill the relatively recent craters' — not clear; basaltic emplacement happened long ago, would it be younger than recent craters?
We agree that the wording was misleading. We revised the sentence to avoid implying that mare basalts infilled relatively recent craters. The revised version states that mare smooth plains embay older/degraded crater floors and are themselves cut by younger, relatively recent craters.
Figure 13: the highland 'isthmus' could be indicated as text in the image.
We added a text label identifying the highland isthmus in Figure 13 and clarified in the caption that the label covers the broad highland block separating the two mare regions.
Line 579: 'Blue Channel' — unify the writing mode of channels, capitalize all or do not capitalize any.
We standardized the terminology throughout this section to lower-case form: red channel, green channel, and blue channel.
Line 679: 'structurally modulated volcanic regime' — does it mean differentiated or evolved magma appearance as time passed by?
We clarified this phrase. By “structurally modulated” we mean that the location and timing of emplacement were influenced by inherited impact- and tectonically generated fractures. We now discuss compositional evolution separately, noting that the modest mafic enrichment of D2–D3 relative to D1 may indicate slightly different source contributions, maturity state, or local mixing. We avoid implying that JANUS data resolve a single connected subsurface plumbing system or a specific source depth. Thus, the term refers primarily to structural control on emplacement sites, not necessarily to magma differentiation through time.
Citation: https://doi.org/10.5194/egusphere-2026-1901-AC1
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AC1: 'Reply on RC1', Alice Lucchetti, 26 Aug 2026
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RC2: 'Comment on egusphere-2026-1901', Anonymous Referee #2, 24 Jul 2026
To Whom It May Concern:
The manuscript is a very thorough and nicely written geological analysis of two regions of the Moon as observed by the JANUS camera, presenting a very interesting test of the imaging spectrometer. Overall, this paper presents new and novel research that will set a baseline for the analysis of JANUS data in the Jupiter system. I recommend publication after minor-to-moderate revisions. Please consider the following comments:
Figures
Due to the large amount of data presented, it is necessary to have a lot of figures.
Figure 1. To optimize space, consider just showing part A for figure 1. This would allow that part to be made bigger and more relevant to the reader. Parts B and C are seen later in the manuscript.
Figure 4. Suggest overlaying the geological map on a shaded relief product, so that the unit colors are no affected by the albedo contrast of the base image. Using a different mapping scale inside and outside of the crater is confusing. One suggestion would be to downgrade the map scale for this figure and show the higher resolution map in a later figure. Please also state the map scale in the caption.
Figure 5. Suggest also using a shaded relief product as the base map here. A and C are very similar such that A could potentially be replaced with the WAC data, while C is shown on shaded relief. Please also state the map scale in the caption. The details are hard to see on this size of figure - will the digitial version be high resolution so that it can be zoomed?
Figure 7. It is hard to see the count areas - perhaps use a white border for these? It is also hard to see the marked craters, but it may not be necessary to show these in the main figure. The data could be provided in the supplement. The details shown on the plots are hard to read because they are so small.
Figure 8. Again, the map will be easier to see if overlaid on a shaded relief product. Could these panels be made larger - or at least ensure that the resolution of the figure is high enough that it can be zoomed? Why are there two different plots for the boulder distribution? Suggest adding a label to the plots, so that the reader doesn't necessarily have to read the caption to understand what is being displayed.
Figure 9. Were the locations where the spectra were extracted small fresh craters or slopes or boulders? The geological setting of the exposures will help for understanding their relationship to the cratering process.
Figure 10. Given the the ROIs are somewhat rugged terrains, how good are the photometric corrections at removing topography effects? For ROI3, should the label be Cube #3?
Figure 11. The labels for the x axis should maybe be simplified, because it is not clear what they mean.
Figure 12. Based on the color-scale selected, Cube #4 seems to have a lot less Ol than the other areas, but the text remarks that the Ol composition is very uniform. Thus, does is the rainbow color-scale really the best one for representing the range of the data? Or does it make it seem larger than it really is?
Figure 13. This and the following figures are very dark when printed. Consider adjusting the contrast. For the geological map part, if the map is transparent, again consider overlaying on a shaded relief product, so that the details of the map units aren't confused by the underlying albedo variations. Are the units in stratigraphic order? Or is this just a feature map with no stratigraphic control? Please state the mapping scale.
Figure 14. See above. The caption could refer back to the key in 13. A label for D3 is missing.
Figure 15. The linework for where the different blue plains materials intersect doesn't seem to always make sense with what I assumed to be their stratigraphic position in the key shown in 13. Is there topographic data available that could better illustrate the position and extent of the domes?
Figure 16. The compositional data may be easier to understanding/interpret if overlayed on a shaded relief product.
Figure 17-19. Label the products directly; clarify ratios/proxies; order panels as described in the text; add the references for the data sets on which they are based.
Langrenus Regional Context and Geological Mapping
- Pages 9-10: improve the introduction to Langrenus at the mare-highland transition; use CSFD terminology; clarify rays and the relationship between Langrenus ejecta and Luna 16 samples.
- Page 10: consider referencing Oberbeck et al. because they discuss greater mixing of local material with increasing distance from the crater.
Crater chronology and block-size measurements
- Section 3.2: What do the randomness analyses for these count areas look like? Methods and data analysis should be described before reporting the result. Include relevant references such as van der Bogert et al. (2017), Prieur et al. (2017), and Krueger et al. (2016).
- Section 3.3 / Figure 8: clarify BSFD, dark/small figure, labels, why two plot versions are shown, and whether higher-resolution observations are planned or comparable.
Langrenus Spectrophotometry
- Page 22: would a mare-related spectral contribution extending across much of the crater interior be consistent with impact crater melt and breccia formation?
- Page 25: unmixing suggests a mare-related spectral contribution extending into highland units; is this consistent with mixing across boundaries such as Li and Mustard (2003)?
Langreus Discussion and Chronology
- Discussion: the regional mare hypothesis could support more than one Langrenus age scenario; older mare material could support hypothesis 1 as well. Mare thicknesses and regional context should be used more carefully.
- Discussion: extensive terrace zone does not by itself prove target heterogeneity; clarify whether target heterogeneity is lateral/compositional/structural.
Isthmus Mapping and Domes
- Section 4: some framework sentences should be moved to the Discussion; unit descriptions should come before interpretations; unit abbreviations should be consistent/italicized if used that way.
- Plains units: are these volcanic plains? basins? The interpretation may be too specific. RRC/OC crater labels: can they be assigned to lunar stratigraphic periods?
- D1/D2/D3 domes: do domes superpose earlier units or are they covered by plains? How can a plain cover a dome? Are D2 and D3 the same age? Are the mechanisms intrusive or extrusive?
- Dome composition: are these silicic domes or mafic domes? Avoid implying exposed silicic or intrusive bodies without evidence.
Isthmus Spectrophotometry and Composition
- Page 43: “which plumbing systems?” Earlier description of dome mechanisms is missing. Avoid implying direct connection to mare basalt plumbing systems.
- Page 44: In the Conclusion section, conclusion sentence is too long
Citation: https://doi.org/10.5194/egusphere-2026-1901-RC2 -
AC2: 'Reply on RC2', Alice Lucchetti, 26 Aug 2026
The manuscript is a very thorough and nicely written geological analysis of two regions of the Moon as observed by the JANUS camera, presenting a very interesting test of the imaging spectrometer. Overall, this paper presents new and novel research that will set a baseline for the analysis of JANUS data in the Jupiter system. I recommend publication after minor-to-moderate revisions. Please consider the following comments:
We thank the Referee for the very positive assessment of the manuscript. We revised the manuscript by improving the clarity of the geological mapping methods, figure readability, crater-count methodology, spectrophotometric caveats, and the interpretation of the volcanic domes in the highland isthmus. Several points were already addressed in the revision prepared for Referee 1, and we have further refined those sections where the annotated comments asked for additional clarification. Our responses are reported below in bold.
Figures
Due to the large amount of data presented, it is necessary to have a lot of figures.
Figure 1. To optimize space, consider just showing part A for figure 1. This would allow that part to be made bigger and more relevant to the reader. Parts B and C are seen later in the manuscript.
We retained Figure 1 because it provides the global JANUS ground-track context and allows readers to see where both regions of interest fall along the LEGA observation strip. To avoid redundancy, we simplified the caption and explicitly state that the detailed views are provided in Figures 2 and 3.
Figure 4. Suggest overlaying the geological map on a shaded relief product, so that the unit colors are no affected by the albedo contrast of the base image. Using a different mapping scale inside and outside of the crater is confusing. One suggestion would be to downgrade the map scale for this figure and show the higher resolution map in a later figure. Please also state the map scale in the caption.
We revised Figure 4 to improve readability and to clarify its role as a regional contextual figure. The map is now presented at a regional display scale, avoiding the combination of different mapping scales within the same figure. The higher-resolution and more detailed geological mapping of Langrenus crater is shown separately in Figure 5. The mapping/display scale for Figure 4 is approximately 1:1,500,000. A high-resolution version of the figure, approximately 600 dpi, has also been provided so that the digital version can be zoomed.
Figure 5. Suggest also using a shaded relief product as the base map here. A and C are very similar such that A could potentially be replaced with the WAC data, while C is shown on shaded relief. Please also state the map scale in the caption. The details are hard to see on this size of figure - will the digitial version be high resolution so that it can be zoomed?
We revised Figure 5 to improve the readability of the geological mapping and to provide a clearer comparison between the regional LROC-WAC-based mapping and the higher-resolution JANUS-based mapping. The previous panel C has been removed to avoid redundancy, and the figure now includes two panels: (A) the geological map derived from LROC-WAC data and (B) the geological map derived from JANUS camera data. This simplified layout makes the comparison between the two datasets clearer and allows the detailed JANUS mapping to be displayed more effectively. The mapping/display scale for Figure 5 is approximately 1:140,000. A high-resolution version of the figure, approximately 600 dpi, has been provided so that the mapped contacts, unit boundaries, and block distributions can be inspected by zooming in the digital version. Figure 4 now serves mainly as a regional contextual figure, whereas Figure 5 presents the detailed geological mapping of Langrenus crater in a clearer side-by-side format. The modified caption for Figure 5 is given below.
Figure 5 Caption. "Geological map of Langrenus crater within the crater rim. (A) Geological map, including tectonic faults, based on LROC-WAC data. Pre-impact faults dominantly strike NW–SE and are interpreted to radiate from the South Pole–Aitken basin, contributing to the partly polygonal outline of Langrenus crater. Faults mapped on the crater floor (Cf) and in the northern hummocky floor terrain (Hft) postdate crater formation. (B) Geological map, including tectonic faults, based on JANUS camera data. The higher spatial resolution of JANUS allows a more detailed mapping of the northern hummocky floor terrain, including a larger number of detected blocks than in the LROC-WAC-based map. The mapping/display scale is approximately 1:140,000."
We also improved the mapping methodology adding a methodological paragraph at the start of Section 3.1 explaining the basemaps, mapping criteria, working scale, and the distinction between descriptive mapping and interpretation, as follows: “Geological mapping was performed in a GIS environment using the LROC-WAC mosaic and GLD100/WAC topographic products as regional basemaps and JANUS images for the high-resolution swath. The regional LROC-WAC-based map shown in Figure 4 is displayed at an approximate scale of 1:1,500,000, whereas the detailed JANUS-based map shown in Figure 5 is displayed at an approximate scale of 1:140,000. Units were defined from morphology, texture, albedo, topographic expression, crater density, and cross-cutting or embayment relationships. Interpretations such as impact melt, terrace collapse, or hummocky mass-wasting deposits were assigned only after the descriptive mapping of unit boundaries and stratigraphic relationships.”
Figure 7. It is hard to see the count areas - perhaps use a white border for these? It is also hard to see the marked craters, but it may not be necessary to show these in the main figure. The data could be provided in the supplement. The details shown on the plots are hard to read because they are so small.
We revised Figure 7 to improve the readability of both the mapped count areas and the crater-counting plots. The figure resolution has been increased to 600 dpi so that the digital version can be inspected by zooming. The count areas are now highlighted with white borders, and the figure panels have been enlarged. We also increased the font size of the labels and plot details to make the CSFD results easier to read. The marked craters are retained in the main figure, but they are displayed in a subtle way so that they document the crater-counting procedure without overcrowding the figure. We also revised the caption to clarify the meaning of the count-area labels and to state that the excess of craters around ~1–1.5 km is interpreted cautiously because it may include secondary or self-secondary craters.
Figure 8. Again, the map will be easier to see if overlaid on a shaded relief product. Could these panels be made larger - or at least ensure that the resolution of the figure is high enough that it can be zoomed? Why are there two different plots for the boulder distribution? Suggest adding a label to the plots, so that the reader doesn't necessarily have to read the caption to understand what is being displayed.
We revised Figure 8 to improve the readability of both the map panels and the BSFD plots. The panels have been enlarged and rearranged with respect to the previous version to provide a clearer view of the mapped block populations and the corresponding boulder-size frequency distributions. The figure resolution has also been increased to 600 dpi so that the digital version can be inspected by zooming. We clarified in the caption why two different plot representations are shown: the double-logarithmic plot is used to evaluate approximate power-law behaviour at larger block sizes, whereas the semi-logarithmic plot emphasizes the resolution-dependent loss of small blocks and the different completeness limits of the datasets.
Figure 9. Were the locations where the spectra were extracted small fresh craters or slopes or boulders? The geological setting of the exposures will help for understanding their relationship to the cratering process.
We added geological context for the ROIs used in the spectral analysis. The selected sites are described in terms of mapped geomorphological setting rather than over-interpreted as specific lithologies, because the four-band JANUS data cannot uniquely distinguish megablocks, impact melt, breccia, or fresh small-crater ejecta. We added the following text in the Figure 9 caption: “The analyzed ROIs were selected to sample distinct mapped geomorphological settings rather than isolated pixels: rim-related rough material, block-rich northern hummocky floor terrain, relatively smooth crater-floor deposits, and local crater-wall or slope exposures. At JANUS spectral resolution these sites cannot be uniquely assigned to megablocks, impact-melt breccias, or small fresh-crater ejecta; however, their geological context allows coherent unit-scale trends to be distinguished from isolated bright or shadowed exposures.” The ROI numbering has been added after Referee 1 is retained.
Figure 10. Given the the ROIs are somewhat rugged terrains, how good are the photometric corrections at removing topography effects? For ROI3, should the label be Cube #3?
This point overlaps with Referee 1. Section 2 already explains that SLDEM2015 and JANUS geocubes provide per-pixel geometry for the Hapke correction. We now add a caveat that residual topographic effects may persist in steep, blocky, or shadowed terrain; therefore, interpretations are based only on coherent patterns repeated across multiple ROIs and cubes.
We also added to Section 3.4 the following text after the photometric-correction description: “Although the Hapke correction uses per-pixel illumination and viewing geometry, residual topographic effects may persist in rugged terrain, especially along steep crater walls, blocky hummocky surfaces, and shadow boundaries. For this reason, compositional interpretations are not based on single-pixel anomalies but on spectral behaviours that are spatially coherent, repeated in multiple ROIs, and consistent with mapped geological units.”
For ROI3 and ROI4, we modified the label to Cube #3.
Figure 11. The labels for the x axis should maybe be simplified, because it is not clear what they mean.
We retained the original FCLS endmember library, because the laboratory endmembers are part of the spectral unmixing model and need to remain traceable. To improve readability, we revised Figure 11 by replacing the long file-name labels on the x-axis with short endmember abbreviations. The full meaning of each abbreviation is now provided directly in the figure caption. No change was made to the endmember library or to the unmixing results.
Figure 12. Based on the color-scale selected, Cube #4 seems to have a lot less Ol than the other areas, but the text remarks that the Ol composition is very uniform. Thus, does is the rainbow color-scale really the best one for representing the range of the data? Or does it make it seem larger than it really is?
We revised the text, so that “homogeneity” refers to the overall dominance of broad mafic-related spectral components at cube scale, not to identical modeled olivine values everywhere. Cube #4 does show lower modeled olivine contribution, and this variation is now explicitly acknowledged. We retained the rainbow colour scale because the same colour scale is applied consistently to all four cubes, allowing direct visual comparison between panels. However, we revised the caption to clarify that the colour contrast emphasizes relative variations in modeled VNIR spectral contribution and should not be interpreted as quantitative modal olivine abundance.
We changed the caption of Figure 12, as follows: “Spatial abundance maps of olivine derived from the FCLS spectral unmixing applied to the four JANUS cubes covering Langrenus crater. The maps show a low but spatially coherent olivine-compatible contribution, with maximum modeled values of ~6%. The same rainbow colour scale is applied to all four cubes to allow direct comparison between panels. The colour contrast highlights relative variations in modeled VNIR spectral contribution and should not be interpreted as quantitative modal olivine abundance. The term homogeneous is used here in a relative sense: the four cubes show broadly similar endmember groups and no abrupt transition to a purely feldspathic spectral signature. Local differences nevertheless occur, and Cube #4 shows a lower modeled olivine contribution than the other cubes. These variations are small in absolute terms and should be interpreted as variations in modeled VNIR spectral contribution rather than as direct evidence for strong modal olivine variability.”
Figure 13. This and the following figures are very dark when printed. Consider adjusting the contrast. For the geological map part, if the map is transparent, again consider overlaying on a shaded relief product, so that the details of the map units aren't confused by the underlying albedo variations. Are the units in stratigraphic order? Or is this just a feature map with no stratigraphic control? Please state the mapping scale.
We revised Figure 13 to improve its readability in print and in the digital version. The basemap was lightened, the contrast was adjusted, and the transparency of the mapped units was increased so that the geological contacts and unit boundaries are easier to distinguish. We also tested shaded-relief/hillshade versions of the map; however, for the full-map view we selected the version that best preserves the visibility of both the basemap morphology and the geological units. The legend has been reorganized in stratigraphic order and is now consistent with the following close-up figures. We also clarified that Figure 13 is a geological map, not only a feature map, because it shows mapped geological units together with structural linework and stratigraphic relationships. The mapping scale is approximately 1:200,000–1:500,000, and the output/display scale of 1:2,000,000 has been added to the figure and caption.
Figure 14. See above. The caption could refer back to the key in 13. A label for D3 is missing.
We revised Figure 14 consistently with Figure 13. The contrast and transparency were adjusted to improve print readability, and the figure now uses the same unit colours and stratigraphic legend order as Figure 13. The missing D3 label has been added. The caption has also been revised to refer explicitly to the Figure 13 key, so that the unit colours and abbreviations do not need to be repeated in full. We also tested a hillshade-based version and selected the basemap that provides the clearest compromise between topographic/morphological visibility and readability of the mapped units. The D2-D3 features are described as positive-relief domical features spatially associated with old craters, not simply as crater-fill plains. We modified the text in section 4.1, as already stated in previous answer, adding the following text: “These features are mapped separately from surrounding plains based on their positive relief, arcuate or elliptical outlines, distinctive morphology, presence of fractures or cracks and stratigraphic contacts visible in JANUS images and supporting LROC/WAC topographic context (Figure 15).” And “The D2-D3 features are spatially associated with older crater floors, but their positive relief and fracture patterns distinguish them from simple crater-fill deposits.”
Figure 15. The linework for where the different blue plains materials intersect doesn't seem to always make sense with what I assumed to be their stratigraphic position in the key shown in 13. Is there topographic data available that could better illustrate the position and extent of the domes?
We revised Figure 15 to better illustrate the topographic and morphological context of the mapped dome-like features. A new hillshade/shaded-relief panel has been added to emphasize the position and extent of the domes and to help the reader distinguish positive-relief features from surrounding plains units. We also checked the contacts between the different plains units and the consistency of the linework with the stratigraphic order shown in the Figure 13 legend. The caption has been revised to clarify that the plains units are descriptive geomorphological units and that their stratigraphic relationships are based on mapped contacts, relative topographic expression, and embayment/cross-cutting relationships where visible. We did not add topographic profiles to the main figure because they would make the figure too crowded; instead, the added hillshade panel provides the requested topographic context in a more readable way.
Figure 16. The compositional data may be easier to understanding/interpret if overlayed on a shaded relief product.
We revised Figure 16 by displaying the FeO compositional layer over a subdued shaded-relief/morphological base. This improves the readability of the compositional variations and allows them to be interpreted together with the local topographic and geological context. The caption has also been revised to clarify that the FeO map is used as a relative compositional proxy and should be interpreted together with morphology and optical maturity, rather than as a standalone mineralogical map.
Figure 17-19. Label the products directly; clarify ratios/proxies; order panels as described in the text; add the references for the data sets on which they are based.
We added direct labels to the figure 17 panels for the maturity proxy, iron/mafic proxy, titanium proxy, Clementine-like RGB composite. We also revised the text to describe the isthmus as a FeO-poor feldspathic highland assemblage that has likely been brecciated and resurfaced, rather than as a single pristine anorthositic block. The dome units are described as low-Ti feldspathic to weakly mafic, not silicic.
We added to the Figure 17 caption: “These products are used as relative spectral proxies rather than direct mineralogical maps.”
We added to the Figure 19 caption: “The FeO and OMAT maps are interpreted together to distinguish compositional contrasts from maturity-related spectral effects. Brighter OMAT values indicate optically fresher surfaces, whereas lower OMAT values indicate more mature regolith.”
Langrenus Regional Context and Geological Mapping
Pages 9-10: improve the introduction to Langrenus at the mare-highland transition; use CSFD terminology; clarify rays and the relationship between Langrenus ejecta and Luna 16 samples.
We revised the beginning of Section 3 to better connect Langrenus to the mare-highland transition and to explain why the crater is relevant for assessing target heterogeneity, resurfacing, and spectral mixing, adding the following text: “This section combines regional geological context, geological mapping, crater size-frequency distribution (CSFD) measurements, block-size frequency distribution (BSFD) measurements, and multispectral observations to evaluate the formation and subsequent modification of Langrenus crater. Because Langrenus formed at the transition between Mare Fecunditatis and the surrounding highlands, the analysis aims to distinguish robust observations from alternative interpretations related to target heterogeneity, resurfacing, and surface composition.”
We also adopted the term crater size-frequency distribution (CSFD), clarified that the rays discussed are Langrenus rays, and refined the Luna 16/ejecta discussion.
Page 10: consider referencing Oberbeck et al. because they discuss greater mixing of local material with increasing distance from the crater.
We added this reference to support the interpretation that distal ejecta can be increasingly diluted by locally excavated or reworked substrate, complicating the identification of Langrenus material at the Luna 16 site. The following text has been added in Section 3: “This interpretation is also consistent with classical ejecta-mixing models, in which the proportion of locally excavated and reworked substrate material increases with distance from the parent crater, complicating the identification of distal primary ejecta components (Oberbeck et al., 1975).”
Crater chronology and block-size measurements
Section 3.2: What do the randomness analyses for these count areas look like? Methods and data analysis should be described before reporting the result. Include relevant references such as van der Bogert et al. (2017), Prieur et al. (2017), and Krueger et al. (2016).
We agree that a formal randomness analysis would provide an additional test for assessing crater clustering and possible secondary-crater contamination. In the present revision, we did not add a separate randomness-analysis plot because the crater-count areas are relatively limited, and the available JANUS/WAC comparison is primarily used to evaluate relative age differences between mapped interior units. However, we revised Section 3.2 to describe the crater-counting procedure before reporting the age results and to explicitly state how possible clustering, slope changes, and excess crater densities were evaluated qualitatively. In particular, the excess of craters at ~1–1.5 km is now discussed as a possible contribution from secondary or self-secondary craters.
We also made the interpretation of the younger hummocky-terrain age more cautious. We now state that, in addition to resurfacing or later mass wasting, crater degradation, surface roughness, and lower crater preservation on blocky or mass-wasted deposits may bias crater-retention ages toward younger values. The MPF (Marchi et al., 2009) chronology added after Referee 1 provides an independent check showing that the relative age relationship is robust, but the absolute age of the hummocky terrain remains model- and preservation-dependent. The MPF analysis has been added in Section 3.2 and Appendix B.
At the beginning of section 3.2, we added the following text: “Crater counts were performed on two interior units: the flat crater floor and the northern hummocky floor terrain. Count areas were selected to avoid obvious shadows, steep scarps, and boundaries between mapped units and were digitized as polygons shown in Figure 7. CSFDs were measured and fitted using Craterstats (Craterstats 3.0 python version via (https://github.com/ggmichael/craterstats) and the lunar chronology function adopted for the main analysis (Michael and Neukum, 2010; Michael et al., 2016). Craters smaller than the estimated completeness diameter were excluded from the age fits. Because secondary and self-secondary craters can affect complex-crater interiors, the CSFDs were inspected for local slope changes, clustering, and excess crater densities. The observed excess at ~1-1.5 km is therefore discussed as a possible secondary-crater contribution, rather than interpreted as a purely primary crater population.”
We discussed the younger hummocky-terrain age in Section 3.5 adding the following text: “However, the younger crater-retention ages of the hummocky terrain may also reflect poorer crater preservation on rough, blocky, or mass-wasted surfaces, where small craters can be degraded, buried, or rendered difficult to recognize more rapidly than on smoother crater-floor materials (e.g., Krueger et al., 2016; van der Bogert et al., 2017; Prieur et al., 2017). Thus, the hummocky-terrain ages are interpreted primarily as evidence for post-impact resurfacing and/or modification rather than as a direct age of the Langrenus-forming impact. If this terrain represents a later mass-wasting deposit, its crater-retention age cannot unambiguously date the primary Langrenus impact.”
We included the relevant references, as suggested.
Section 3.3 / Figure 8: clarify BSFD, dark/small figure, labels, why two plot versions are shown, and whether higher-resolution observations are planned or comparable.
We explained why both double-logarithmic and semi-logarithmic representations are shown adding the following text to the Figure 8 caption: “The double-logarithmic plot is used to evaluate approximate power-law behaviour at larger block sizes, whereas the semi-logarithmic representation emphasizes the resolution-dependent loss of small blocks and the different completeness limits of the datasets.”
We also clarified that the comparison uses LROC-NAC, Kaguya TC, JANUS, and LROC-WAC as a resolution sequence; while no additional higher-resolution lunar JANUS data are available from LEGA, the analysis demonstrates a workflow relevant to future high-resolution JANUS observations at Jupiter, adding the following text at the end of Section 3.3: “Although the LEGA lunar dataset does not include additional higher-resolution JANUS imaging of this area, the comparison with LROC-NAC and Kaguya TC provides a useful benchmark for assessing how JANUS bridges regional WAC-scale mapping and very-high-resolution targeted observations. This is directly relevant for future JANUS observations of blocky terrains on the Galilean moons.”
Langrenus Spectrophotometry
Page 22: would a mare-related spectral contribution extending across much of the crater interior be consistent with impact crater melt and breccia formation?
We clarified that this interpretation is consistent with the expected mixing of target materials during complex crater formation, melt production, brecciation, and post-impact mass wasting. The spectral data indicate a mare-related component but do not uniquely identify melt versus breccia.
We added the following text after the sentence on mare-related spectral contribution in Section 3.4.2: “Such a widespread mare-related spectral contribution is compatible with the expected formation of impact melt and breccia in a complex crater excavated at a mare-highland boundary. The JANUS spectra do not distinguish impact melt from brecciated target material, but the geological setting supports mixing and redistribution of mare-derived and highland-derived components during and after crater formation.”
Page 25: unmixing suggests a mare-related spectral contribution extending into highland units; is this consistent with mixing across boundaries such as Li and Mustard (2003)?
We revised this statement so it is not interpreted as evidence for extensive olivine-rich bedrock. The modest mafic contribution is consistent with local mixing across a mare-highland compositional boundary and with impact/regolith redistribution processes.
We replace the sentence with the following text “The modest mafic contribution inferred from unmixing is consistent with mixing across a mare-highland compositional boundary, as observed in other lunar transition zones (e.g., Li and Mustard, 2003), rather than requiring exposure of extensive olivine-rich bedrock. Conversely, flatter or more mature surfaces may show reduced spectral contrast due to space weathering and regolith gardening.”
Langrenus Discussion and Chronology
Discussion: the regional mare hypothesis could support more than one Langrenus age scenario; older mare material could support hypothesis 1 as well. Mare thicknesses and regional context should be used more carefully.
We revised the Discussion to avoid presenting regional mare age or thickness as uniquely supporting one scenario. The regional context is now used to explain possible target mixing and mare-derived components under both the Imbrian and Eratosthenian interpretations. We added the following text in Section 3.5 after the two age scenarios: “The regional mare stratigraphy does not uniquely discriminate between these scenarios. Older mare materials in the Fecunditatis region could provide mare-derived components available for excavation or mixing in either an Imbrian or an Eratosthenian Langrenus-forming event. Therefore, the regional mare context is used here primarily to constrain possible source materials and mixing processes, rather than as a standalone age diagnostic for the primary impact.”
Discussion: extensive terrace zone does not by itself prove target heterogeneity; clarify whether target heterogeneity is lateral/compositional/structural.
This was already softened after Referee 1, and we further specify that “heterogeneous target” means a laterally variable mare-highland target with structural weaknesses and compositional contrasts, not simply the existence of terraces. We added the following text: “Terraced crater walls can form by large-scale mass wasting during the collapse of the transient crater even in relatively homogeneous targets; therefore, the presence of terraces alone is not taken here as evidence for target heterogeneity. Instead, the interpretation of a heterogeneous target is based on the combination of terrace-zone asymmetry, fault localization, mare-highland setting, and compositional contrasts.”
Isthmus Mapping and Domes
Section 4: some framework sentences should be moved to the Discussion; unit descriptions should come before interpretations; unit abbreviations should be consistent/italicized if used that way.
We agree with the referee that the original Section 4.1 mixed unit descriptions and genetic interpretations too early. We therefore reorganized Section 4.1 so that the mapped units are first described using observable criteria only. Broader evolutionary and genetic interpretations were moved to Section 4.3. In the revised Discussion, we now discuss the possible significance of the D1 and D2–D3 dome-like features, plains resurfacing, crater-related weaknesses, and mapped fractures in a more cautious way. We also avoid presenting intrusive, extrusive, or diapiric emplacement as uniquely demonstrated by the data; instead, we describe these as possible interpretations consistent with the mapped stratigraphic and spectral relationships.
Plains units: are these volcanic plains? basins? The interpretation may be too specific. RRC/OC crater labels: can they be assigned to lunar stratigraphic periods?
We revised Section 4.1 to make clear that the mapped plains units are descriptive geomorphological units rather than automatically interpreted volcanic plains. They are now described using observable criteria, including surface roughness, albedo, crater density, degradation state, relative topography, and embayment relationships. Their possible origins are discussed separately in Section 4.3. We also clarified that the RRC, OC, and VOC labels are relative morphological/degradation classes, not formal lunar stratigraphic periods, because no independent CSFD ages were derived for these individual crater classes.
D1/D2/D3 domes: do domes superpose earlier units or are they covered by plains? How can a plain cover a dome? Are D2 and D3 the same age? Are the mechanisms intrusive or extrusive?
We revised Sections 4.1 and 4.3 to distinguish mapped dome-like positive-relief features from their genetic interpretation. D1, D2, and D3 are treated as mapped feature groups distinguished by preservation state, morphology, and stratigraphic context, not as precisely dated events. D2 and D3 are retained as morphologically distinct sub-units of a younger dome-like generation, but the current dataset does not establish independent absolute ages for D2 and D3. The available imagery and spectral data are consistent with localized small-volume volcanic or subvolcanic emplacement, but they do not uniquely discriminate between effusive dome construction, shallow intrusive inflation, volcano tectonic uplift, or a combination of these mechanisms.
Dome composition: are these silicic domes or mafic domes? Avoid implying exposed silicic or intrusive bodies without evidence.
We clarified that the dome-like features are not interpreted as silicic domes. Their spectral properties are described as low-Ti, feldspathic to weakly mafic relative to adjacent high-Ti mare basalts. The possible Mg-suite affinity remains tentative and is based on subtle mafic enrichment and feldspathic character, not on diagnostic mineralogical identification. We therefore removed wording that implied exposed diapiric or shallow intrusive bodies.
Isthmus Spectrophotometry and Composition
Page 43: “which plumbing systems?” Earlier description of dome mechanisms is missing. Avoid implying direct connection to mare basalt plumbing systems.
We replaced “plumbing system” with the expression “local structurally controlled magma-ascent pathways” and clarified that these are inferred from mapped fractures, crater-related weaknesses, and mechanically weak zones within the isthmus. We changed the sentence in the 4.3 Dicussion section in the following way: “The term local structurally controlled magma-ascent pathways is used here to refer to inferred fractures, crater-related weaknesses, and mechanically weak zones within the isthmus that may have favored small-volume magmatic or volcano tectonic emplacement. We do not imply that these pathways are directly imaged by JANUS, nor that they are connected to the large-scale mare basalt systems of Mare Tranquillitatis or Mare Fecunditatis.”
Page 44: In the Conclusion section, conclusion sentence is too long
We split the sentence and made the final paragraph more direct. We replaced the final sentence in the following way: “In particular, the ability of JANUS to resolve meter- to decameter-scale features and distinguish coherent colour and spectral units will be essential for interpreting the geological features and processes of Jovian icy satellites, along with their color characteristics. This work confirms that JANUS is fully prepared to achieve its ambitious scientific objectives at Jupiter, providing the high-fidelity data necessary to investigate the evolution and habitability of the icy worlds orbiting Jupiter.”
Citation: https://doi.org/10.5194/egusphere-2026-1901-AC2
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General aspects
The work is about the results of the JANUS camera based spectral observations of a selected Lunar area. The work is interesting partly as the calibration / validation of JANUS does matter and will influence many works in the future in the Jovian system, and the work also provides some new scientific results. The structure of the work is good, the language is perfect. The methods are described however few improvements would be useful. Minor revise is suggested. Please also mention how much was topographic dataset used during the photometric correction? Please also discuss any „correlation” between the possibility of spectral identification and optical maturity. This aspect as well as the domes also should receive more specific emphasis in the abstract also.
Specific aspects
Abstract
„Spectral unmixing indicates that the crater interior is dominated by mafic components,”
It is good and useful of course, but would make more sense if some comparative aspects have been mentioned on the crater’s interior properties compared to the surrounding terrain.
around line 65
some info on the spectral coverage would be needed here
91
„Lunar_LRO_LROC-WAC_Mosaic_global_100m_June2013”
consider moving it to the references
111
„declination”
not clear what is it here, relative to „something”?
130
„highland “isthmus” region”
would be good to have an overview with marked the mentioned sites / features, maybe on FIg.3 some texts for site identification could help
179
„distinctive herringbone pattern”
might be not clear for all readers, would help to cite the Springer based encyclopaedia of planetary landforms
around 210
Could have been melt ponds identified /separated in the spectral datasets?
229
„vertical throws”
many readers might not be familiar with this term
247
„The data suggest that crater modification is influenced by somewhat weaker mare lithologies in the western and stronger highland lithologies in the eastern terrace zone”
this might fit better with some explanation to the reasons to the Discussion section – this is a figure caption only
Fig 9
the colour based identification of measured locations is not easy, might be accompanied with small numbers in the panels – but I let this to be decided by the authors
364-366
important, might mention later in the summary also
Fig 10
scalebars are needed on the left image stripes
405-407
„However, given the limited number of spectral bands and expected degeneracies between mafic endmembers, these results should be interpreted as indicative of dominant spectral trends rather than precise modal mineralogy.”
yes and the multicolour based spectral resolution aslo matters
427
„to bridge regional and local scales and provide reliable data for geomorphological and crater-counting analyses”
the numerical size of this spatial scale might be also mentioned
430
„such as the extensive terrace zone in the western crater sector, indicate that the Langrenus impact event occurred into a heterogeneous target, where the western target portion already bears characteristics of the mare basin”
this might or need not be. Terraced wall might (and usually) form by large scale mass wasting during the collapse of transient crater regardless the subsurface structure
440-450
did you consider dating of ejecta also or only the inner crater units?
475-476
does this mean the mare/highland boundary was not well located previously?
around 500 and elsewhere
domes are mentioned but specifically not discussed enough, please include in the abstract also
538
„The D2 and D3 domes are spatially associated with pre-existing impact craters (OC) suggesting magma rising induced by impact fracturing”
not clear enough why such connection is expected
536
„The maria smooth plains locally infill the relatively recent craters (RRC).”
not clear, basaltic emplacement happened long time ago, would it be younger than recent craters?
Fig 13.
„the highland “isthmus”
could be indicated as text in the image – or does it cover large part of the image?
579
„Blue Channel”
please unify the writing mode of channels, capitalize all or do not capitalize any
679
„structurally modulated volcanic regime”
does it mean differentiated or evolved magma appearance as time passed by?