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)
- RC1: 'Comment on egusphere-2026-1901', Anonymous Referee #1, 19 May 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
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- 1
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?