Radar-based observation of a lava tube on Venus

Domain: neuroscience
Published: 2026-02-09
Editorial outcome: major_revision

Abstract
Intense volcanism has played a significant role in shaping Venus’s surface and geology. The existence of lava tubes (i.e., pyroducts) on Venus has been largely hypothesized but never confirmed. Being a subsurface structure, the presence of a lava tube can be revealed by a localized collapse of the roof denoted as skylight. Between 1990 and 1992, the Synthetic Aperture Radar (SAR) instrument onboard the Magellan spacecraft mapped the Venusian surface. By leveraging a SAR imaging technique developed for detecting and characterizing accessible subsurface conduits in the proximity of skylights, we analysed the Magellan radar images in locations where there is evidence of localized surface collapses. Our analyses reveal the existence of a large and open subsurface conduit in the Nyx Mons region. This feature is hypothesized to be a pyroduct, characterized by a diameter of about 1 km, a roof thickness of at least 150 m and an empty void height of no less than 375 m. The conduit extends in the subsurface for at least 300 meters from the skylight.

Introduction
Lava tubes, natural underground tunnels formed by volcanic activity, have emerged as scientifically significant features for planetary exploration1,2. They typically originate in basaltic lava flows, where low-viscosity lava is either entrenched and crusted over or inflated in-between preexisting lava layers3. Multiple modalities have been observed for the formation of a lava tube through overcrusting4–6. These modalities include (i) a solid crust is formed from the levees toward the center of a channelized lava flow; (ii) overflows grow the stream levees until an arched roof is formed over the lava flow and (iii) floating mobile pieces of crust of solidified lava coalesce into a solid roof. Another lava tube formation process is incremental inflation of cooled lava sheets by flowing hot lava7. Once a lava tube is formed, either by overcrusting or inflation, thermal and physical erosion contribute to further shaping the conduit. Then, if the lava supply is discontinued, a partially- or fully-drained conduit may be left. Lava tubes can be modeled as quasi-cylindrical structures2, with their lower and upper sections referred to as the floor and roof, respectively. By examining the formations, structures, and mineralogical compositions within lava tubes, it is possible to gain insights into volcanic activity, lava flow dynamics, and the geological processes shaping the planetary surfaces8.

Beside Earth, evidence of lava tubes has been identified on other celestial bodies such as Mars9–11 and the Moon12,13. For example, recent research provides compelling evidence of a subsurface cave conduit beneath the Mare Tranquillitatis Pit (a lunar skylight) on the Moon14. The main evidence for postulating their existence are localized collapses of the near surface15 (i.e., skylights). Skylights are typically associated with the collapse of a portion of a lava tube roof due to gravitational and erosional forces2. As a result, they enable opportunities for observing and accessing a subterranean cave system.

The existence of lava tubes on Venus is uncertain and the focus of ongoing research. While no direct evidence of lava tubes on Venus has been discovered so far, there are several clues of indirect evidence that suggest their possible widespread presence such as long sinuous features, fractures, and collapse chains16,17 that are often associated with them. Collapse pit chains are typically observed by optical imaging sensors. Optical images (e.g., by photoclinometry) are of great aid in locating and analyzing skylights. However, the use of satellite optical remote sensing instruments is precluded on Venus because of its thick CO2 atmosphere. NASA’s Magellan18 spacecraft was equipped with a radar system able to acquire data in different modes. One of them is a side-looking SAR mode operating in S band (2.385 GHz) that was specifically designed to study Venus’ surface. SAR works by transmitting radio waves toward the surface and measuring the time it takes for the waves to bounce back after interacting with the terrain. By processing the returned radar signals, a detailed backscattering map of the Venusian surface has been produced.

Magellan detected the presence of several pit chains on Venus19,20, thus revealing one of the main indicators for the possible presence of lava tubes. A recent morphometric analysis of Venusian pits using radar-derived parameters from Magellan SAR data21 found that these pits exhibit extremely large collapse volumes and display striking morphological similarities to lunar pit craters. They appear as linear to curvilinear assemblages of individual craters. These chains extend from tens to several thousands of kilometers in length and are distributed across diverse Venusian landscapes. Their prevalence is particularly notable in connection with chasmata and coronae. The distinctive characteristics of these formations strongly suggest their origin in collapses induced by the removal of underlying support. This loss of support could be attributed to various causes such as the collapse of a lava tube roof, extensional faulting, or receding magma chambers. Another hypothesis proposed for Venus is that these pit chains were produced in association with underlying dykes swarms22. In this scenario, they may represent collapse features formed above laterally propagating subsurface dykes. However, the specific mechanism responsible for the generation of collapse pits on Venus remains an unresolved question16. In fact, early analyses of Magellan data suggested that a number of pit craters may be associated with collapsed lava tubes23. This hypothesis was mainly dictated by the pits’ locations and the types of terrain surrounding them, as it was not possible to discern the collapse origin by only available data on surface shape and dimension of the pits. Hence, even if plausible, the hypothesis remains unconfirmed.

Here, we investigate whether Venusian pits provide access to an empty subsurface conduit and thus could potentially be collapsed sections of lava tubes (i.e., skylights). To achieve this goal, we analyze the Magellan dataset by leveraging a recent data processing methodology for hidden cavities and lava tubes characterization and accessibility assessment from radar images14,24. These recent studies demonstrate that, under suitable assumptions, the use of Synthetic Aperture Radar images acquired from spaceborne platforms enables the determination of lava tubes subsurface morphometry and the related accessibility near collapse pits on Earth24 and other celestial bodies14. This is possible by taking advantage of the side-looking acquisition geometry of the sensor resulting in radar waves traveling within the observed skylight and being reflected by the lava tube interior.

Results
Study area and Magellan radar image analyses
The study area is located on the western flank of the Nyx Mons (Supplementary Fig. 1a). The Nyx Mons is a 362 km-diameter shield volcano25. The investigated region is well-documented for its large occurrence of collapse pit chains26–28. Sawford et al.27 noted the existence of graben systems in the region, with some near the pit crater chains, yet not fully bounding them. However, no evidence of lava tubes has been provided.

Within this region, we focused our attention on one pit denoted as A (Fig. 1), imaged by the Magellan radar from West to East and with incidence and look angles equal to about 42.4° and 40°, respectively. The pit exhibits radar backscattering characteristics which notably differ from the other ones in the Magellan radar images21 (see examples shown in Fig. 2). In greater detail, the Magellan SAR responses reveal distinct morphological contrasts in the presence of pits as shown in the three example pits displayed in Fig. 2. The pit near Idunn Mons (Fig. 2a) exhibits a pronounced radar shadow coupled with a high-intensity bright return along its eastern margin, consistent with steep, vertically walled topography21. In contrast, the pit in Ganiki Planitia (Fig. 2b) shows a weaker and less sharply defined radar shadow, suggesting that it is either shallower or contains a non-uniform infill distribution21 compared to the pit near Idunn Mons. The candidate skylight denoted as pit A near Nyx Mons (Fig. 2c and Fig. 1) presents a significantly different response characterized by a well-defined shadow and an asymmetric bright radar return that extends far beyond the pit margin. This asymmetry is identical to that observed in previous radar studies24 in the presence of a collapsed lava tube roof, linking to subsurface voids. The anomalous increase in backscattering can be attributed to the lava tube roof and interior24. According to this interpretation, the pit would result from the collapse of the conduit’s roof section, providing access to the subsurface (i.e., empty void).

We estimated the geometric characteristics of pit A (see Fig. 3) by inverting the Magellan radar signals originating from the skylight interior (see Methods). All the hereby reported measurements are associated with an uncertainty of ~±75 m, corresponding to the pixel size in the slant range direction of the Magellan radar dataset (see Methods for details on uncertainty determination). In detail, the skylight measures 1545 m × 1070 m (major-axis by minor-axis). The estimated collapse depth is about 450 m. The radar signal propagates inside the cave for at least 300 m. This value represents the conduit length for which the radar signal can propagate inside the lava tube before extinguishing. The subsurface conduit has a width that varies between 1350 m near the skylight and 525 m when reaching the maximum radar visibility point. The average width is equal to 937.5 m. Note that this conduit width may underestimate the true value as it depends on the points of intersection between the conduit’s curvature and the oriented electromagnetic waves transmitted from the radar’s acquisition position14. The lava tube roof thickness is estimated to be about 150 m. The cavity height measured inside the hypothesized conduit is ~375 m. As represented in Fig. 3, we hypothesize the presence of a rock pile on the pit floor, originating from the roof collapse. The rock pile slopes down inside the conduit with an estimated angle of ~14°. The cavity height measured from the top of the rock pile is about 300 m.

The East-to-West radar data was not acquired over the target area. Therefore, it is not possible to determine the characteristics of the lava tube below the West wall.

We interpret the pit A to belong to a winding (sinuous) collapse chain originating northwest and progressively descending southeast (see Supplementary Fig. 1b). The Magellan altimetry data29 indicate that the surface slopes toward the southeast from pit A (see Supplementary Fig. 1c). This trend supports the idea that, if associated with a sinuous collapse chain, pit A orientation aligns with downhill topography. This is an additional indicator of a lava tube system4. It is worth noting that the Magellan data altimetric vertical uncertainty and low horizontal spatial resolution limit our ability to resolve small topographic variations. As lava tubes typically form sinuous chains of collapses predominantly oriented along the direction of the tube2, we can infer that the identified potential lava tube is likely to span a length of at least 45 km below the surface. We have evidence that there is at least one section of the conduit that does not show any collapse. It has a length of 13 km corresponding to the paths from A to B (see Supplementary Fig. 1). The other pits belonging to the chain denoted as B, C, and D show no evidence of direct subsurface access as they have a radar response very similar to the one attributable to skylights with blocked conduit entrance as observed on Earth analogs24. The access to the conduit in these pits is likely to be occluded by the deposit of collapse material.

Morphometric comparison of planetary lava tube collapses
Pit craters that likely originated from the collapse of a lava tube roof have been identified on Mars2,10,11,30 and the Moon2,12,14,15,31. Sauro et al.2 conducted a morphometric analysis on potential planetary lava tubes, by selecting candidate collapse chains in the lunar regions of Marius Hills and Gruithuisen, and in the martian regions of Arsia, Olympus and Hadriaca volcanoes; the analysis also included a set of skylights located in the known terrestrial lava tube systems of Corona (Spain), Kazamura (Hawai’i) and Undara (Australia). In comparison to the results of Sauro et al., the size of Venusian pit A (major-axis by minor-axis) is in line with what has been observed on the Moon2, but larger than what has been identified on Mars, which, in turn, exceeds the size observed for terrestrial cases2. The estimated collapse linear volume for pit A (see “Methods”) is equal to 7.56 × 105 m3. This value is in agreement with lunar observations2, while it is larger than the linear volumes measured on Mars2 (which are in the order of 104 m3) and on the Earth (which are in the order of 101–102 m3). The asymmetry ratio for pit A (see “Methods”) is 2.38. Interestingly, well-known lunar skylights, that may provide access to the subsurface, such as Marius Hill Pit, have combined values2 of asymmetry ratio and linear volume similar to the ones we measured for Venus’ pit A.

Comparison between Venusian and terrestrial lava tubes radar response
To further validate our results, we compare the radar backscattering response of A with the one of a previously studied lava tube’s skylights on Earth. Situated in the northern part of Lanzarote, Spain, the Corona volcano has created one of the Earth’s most extensive lava tube systems that displays several collapse pits32. The Corona lava tube spans a length of ~7.6 km (with a total cave development of about 9.7 km, including side branches and upper levels) and exhibits cave sections that reach widths32 of up to ~28 m. This remarkable network comprises multiple skylights and large collapses along the path of the lava tube. With an estimated minimum length of 45 km and a width that is greater than 1 km, the hypothesized Venusian lava tube notably exceeds the dimensions of the Corona system. Nonetheless, the largest lava tubes on Earth, like the one at Corona, are considered as highly valuable analogs to their extraterrestrial counterparts32.

The Corona lava tube system has been subject of an extensive study with spaceborne SAR systems to infer the lava tubes characteristics near collapse pits24. Figure 4a, b reports the SAR and optical images of a previously studied24 collapsed lava tube’s roof (known as Jameo Agujerado) providing access to the subsurface. In Fig. 4a, the radar image is superimposed on the 3D Lidar scans and drone photogrammetry of the surface and the subsurface24. The radar image of Jameo Agujerado shows backscattering features that are strikingly similar to the radar responses observed for pit A on Venus (see Fig. 1 and Fig. 4a). This comparison shows that both cases are characterized by a typical lava tube response characterized by a well-defined shadow and an asymmetric bright radar return that extends far beyond the pit margin (see also Fig. 2). This further provides evidence that A is a collapsed lava tube section (i.e., skylight) providing access to the subsurface. Figure 4c shows an image of the eastern entrance (the one being imaged by the radar) of Jameo Agujerado. We hypothesize that the entrance to the lava tube conduit departing from pit A resembles the one of Jameo Agujerado although at a very different spatial scale (hundreds of meters on Venus versus tens of meters on Earth).

Methods
Radar model and conditions for the detection of the interior of lava tubes
We consider a SAR sensor operating from orbit. We assume that after focusing the system range and azimuth resolutions are better than the cavity width w. The radar received power PR is directly proportional to the radar backscattering coefficient σ0. The value of PR is given by24:

PR(θ) = Kσ0(θ) / R4 (1)

The system constant K in the equation depends on factors such as the transmitted power, the antenna gain, and other relevant parameters. The variable R denotes the slant range. The term clutter refers to all unwanted radar echoes that have the potential to obscure the desired signal. In the SAR image, the cavity interior (which is the signal of interest) near a collapse point can be detected if the radar echo power originating from the cavity is larger than the echo power arising from the surface (i.e., clutter) for the same image pixel. This is considering the cavity interior and surface scattering points at the same slant range that results in the superimposition of two radar echoes. Therefore, the cavity can be detected only when the radar cross section of the cavity interior σ0c is greater than that of the surface σ0s (i.e., σ0c(θc) > σ0s(θs)) for any given pixel. Here, θc and θs represent the cavity and over the surface, respectively.

Radar model for the determination of skylight and lava tubes geometric parameters
The lava tube interior is illuminated by the radar if the look angle θL satisfies21,24:

θL ≤ atan(D / h) (2)

where D and h are the pit diameter (range direction) and the cavity roof thickness, respectively. This implies that θL should be sufficiently low to allow the radar signal to propagate inside the lava tube, rather than being solely reflected by the cavity roof exposed by the skylight.

The collapse depth h = ht + hc is estimated as (see Supplementary Fig. 5):

h = ht + hc = ΔRg,1 sinθI cosθL (3)

where ΔRg,1 is the ground range interval of the radar shadow cast from the pit’s edge, hc is the cavity height measured from the top of the rock pile and θI is the radar incidence angle. The radar signal propagation P1 inside the lava tube is equal to24:

P1 = (ΔRg,2 − ΔRg,3) sinθI sinθL (4)

where ΔRg,2 is the ground range extent of the radar anomaly originating from the lava tube interior measured from the relevant pit edge and ΔRg,3 is the ground range interval measured between the reflection from the pit’s edge and the first reflection from the roof (see Supplementary Fig. 5). The radar signal propagation inside the lava tube can be also measured with reference to the pit’s edge. In this case, the measurement is denoted as P2 and it is estimated to be:

P2 ≤ ΔRg,2 sinθI sinθL (5)

The maximum depth Ht based on radar signal propagation inside the cavity is equal to:

Ht = hc + hpile + ht ≥ ΔRg,2 sinθI cosθL (6)

A radar measurement taken at a slant range resolution equal to Rs = Rr sin(θ), where Rr is the ground range resolution, has a root-mean-square error δR equal to21:

δR = Rr = 2SNR (16)

Given that, for Magellan’s system, Rr = 88 m and SNR = 8 dB, a value of 24.77 m is obtained for δR. Thus, we have that the uncertainty due to SNR is lower than the one given by the pixel resolution, which is equal to p = 75 m (δR < p). Knowing the values of the incidence angle, θI, and look angle, θL, corresponding to the analyzed SAR measurement, we can bound the errors for the different estimated quantities. The pit examined in this paper, namely pit A (see Fig. 3), was imaged by Magellan’s SAR system with an incidence angle of 42.4° and a look angle of 40°. Consequently, the following uncertainties can be evaluated for the estimated geometric parameters (see Methods and Supplementary Fig. 5). The estimated collapse depth uncertainty is evaluated as p sinθI cosθL = 38.74 m. The radar signal propagation uncertainty is equal to p sinθI sinθL = 32.75 m. The lava tube roof thickness (worst-case overestimation model) uncertainty is equal to p sinθI / cosθL = 66.02 m. The cave height uncertainty is computed as p sinθI cosθL = 38.74 m.

Magellan radar system specifications and image data format
Magellan is a SAR system operating in S-band (2.385 GHz). The transmitted peak power is 325 W and the swath width is 25 km (variable). The Full Resolution Radar Mosaicked images (FMAP) used in this study are full-resolution (75 m/pixel) global mosaics produced by the U.S. Geological Survey from Magellan Full-resolution Basic Image Data Record (F-BIDR) data. FMAP products are reprojected from the radar coordinates (delay-doppler) onto the surface of Venus by using information about the spacecraft location based on a priori predictions and earth-based tracking. For this study, the left-look FMAP mosaic has been employed. The radar resolution ranges from 120 m × 120 m (range × azimuth) to 280 m × 120 m, depending on the spacecraft altitude.

Sentinel 1 radar system specifications and image data format
The SAR images used in this paper were acquired by Sentinel 1. Sentinel 1 is a SAR system operating in C-band (5.405 GHz). The Sentinel 1 Ground Range Detected (GRD) products used in this work encompass focused SAR data that have undergone detection, multi-looking, and projection onto the ground range utilizing an Earth ellipsoid model. The ellipsoid projection of the GRD products is adjusted based on the terrain height. Ground range coordinates refer to the slant range coordinates that have been projected onto the Earth’s ellipsoid. The pixel values represent the detected magnitude. The resultant product exhibits an approximately square spatial resolution and square pixel spacing of 10 m × 10 m, with reduced speckle owing to the multi-look processing. The spatial resolution is about 20.5 m × 22.5 m in range and azimuth, respectively.

Capella radar system specifications and image data format
The VHR SAR spotlight image of Lanzarote of size 5 km × 5 km used in this paper was acquired with Capella Space X-band radar systems. The radar central frequency and bandwidth are 9.65 GHz and 500 MHz, respectively. Data has been acquired in HH polarization with a pulse repetition frequency of 10 KHz. After the geocoding and orthorectification, the ground range resolution can vary between 0.5 and 0.8 m depending on the look angle. The azimuth resolution is 0.5 m without sidelobe filtering. The standard look angle range for Capella Space X-Band data is between 25° and 40°. Given the small size (5 km × 5 km) of the considered image, the look angle does not appreciably vary across the imaged scene. No post-processing has been applied to the data. We exploited the GEO (Geocoded Terrain Corrected) data products, in which pixel values contain the radiometrically calibrated intensity in linear scale. The image is multi-looked nine times in the azimuth direction to enhance their radiometric resolution. GEO data products are geocoded and terrain-corrected using a Digital Elevation Model to improve the geolocation accuracy beyond what is achievable with only considering the ellipsoid.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript details the very exciting confirmation of a lava tube entrance on Venus. Over the last few decades, orbital data has strongly suggested (or even confirmed) the existence and nature of lava tubes on the moon and Mars, and this new confirmation for Venus just goes to show that these are ubiquitous features of planetary surfaces with histories of basaltic volcanism. It uses an approach developed and previously applied by the authors for characterizing lunar lava tube entrances using radar, and includes a comparison of radar data for a terrestrial analog lava tube, and the manuscript convincingly rules out other potential features. As such it is worthy of publication.\nHowever, there is definitely room for improvement. My main criticism of this manuscript is its repeated exaggeration- of the novelty of this work (e.g. there being no confirmed Martian detections) and its importance (e.g. relevance to future robotic missions, access to layered deposits), however this could be remedied by more measured language, better referencing, and more realistic (but still important!) implications. The core of the manuscript is strong enough to support publication without the hyperbole.\nHere are some specific recommendations:\nThe manuscript misses out on some early studies of crater chains (and prospective lava tubes) from the early post-Magellan mission days. Citations of those works would help show how long this question has been out there, and that this work is using a novel SAR interpretation approach (and new analog comparisons) to answer a question where only speculation existed before.\nThis one is only a thesis but has extensive coverage of the topic:\nMelville, Graeme P (1994). Lava tubes and channels of the Earth, Venus, Moon and Mars. University of Wollongong. Thesis.\nhttps://hdl.handle.net/10779/uow.27649461.v1\nThis one is shorter but mentions Venus lava tubes (co-authored by the author of the thesis listed above):\nPublications of the Astronomical Society of Australia, Volume 10, Issue 3, 1993, pp. 200 – 202. DOI:\nhttps://doi.org/10.1017/S1323358000025662\nThe introduction says that “…from optical images (e.g., by photoclinometry) it is very difficult to assess whether a subterranean cave extends from the skylight.” I will agree that this is true, but it also undersells some of the quality work that has been done trying to do just that- e.g. in Haruyama et al. (2009), a paper you’re already citing elsewhere. You make the stronger case for your work in the next sentence (e.g. you couldn’t use imagery for Venus anyways).\nA sentence later in the Intro, you say that “Magellan is a spaceborne side-looking SAR…” Do you mean that Magellan HAD a spaceborne side-looking SAR? Or more precisely that it had a radar system (the RDRS) that could operate in a SAR mode?\nPage 7, last sentence of the first paragraph states “These deposits tend to increase the radar backscattering uniformly around the feature” in reference to impact craters. The reference cited relates to radar imaging of craters on the Moon- could you cite a Venus-specific source? Venus craters have much different ejecta patterns than lunar ones, often involving more impact melt, and not as “rough” in radar as lunar craters (and can even be brighter than their surroundings in radar images).\nSee https://www.hou.usra.edu/meetings/lpsc2023/pdf/1052.pdf and other refs regarding Venus ejecta.\nPage 7, second paragraph, second sentence: “Given that there are no confirmed detections of intact lava tubes for the Martian case…” Are you sure you want to say this? Do you mean confirmed using the SAR method? I’m sure there are members of the HiRISE team (MRO) who would contend that their lava tube entrances on Alba and Arsia Mons are “confirmed.” Sauro et al. (2020), which you are already citing, discusses this. With no other discussion in this manuscript about what you mean by “confirmed,” this statement is unsupported.\nSame paragraph, in reference to the critical factors influencing lava tube stability- the reference cited is great but makes no mention of Venus. You should make it clearer that this reference does not mention Venus (or Venus-specific conditions). I don’t know but I’d be curious if factors such as high atmospheric pressure, or prolonged cooling times (given high surface temperatures) could affect their stability, or the tensional strength of the lava material. As you’ve just stated ahead of this sentence, there is currently no info about the structural stability of lava tubes on Venus. While it is clearly beyond the scope of this paper to model this, it might be good to acknowledge that Venus surface conditions could potentially influence this (and perhaps that further work is needed to assess whether the general conditions for lava tube stability developed in the paper cited can be applied to Venus).\nFinal paragraph, page 8. Some of the applications seem wildly fanciful to me and should be scaled back. That this skylight “could also be of great interest as targets for a possible lander robotic mission” with a citation to a DAVINCI mission paper is way off- that is not a robotic mission and will at best only return data for minutes if it as a soft landing, and it already has a target area. The next sentence talks about “preserved layers within the conduit walls could serve as a detailed record of the planet’s volcanic history, enabling the reconstruction of geological timelines.” I’ve spent many weeks in lava tubes on multiple continents (and islands) and I have never seen layered deposits, except for maybe one flow covering another flow or two (but likely separated in time by years at most). It’s just not the kind of environment that preserves a long record.\nI’ve made multiple minor corrections and suggestions for more concise language, for example replacing “in the proximity of” with “near” throughout. I’m attaching an annotated PDF with my suggestions.\nGood luck!", "reviewer_2": "The authors present an argument that a geologic feature on the western flank of Nyx Mons, Venus, is a lava tube. This would be a new identification of a lava tube on Venus. Lava tubes have been discussed briefly in the Venus volcanology literature but little to no evidence has been presented identifying one. The primary evidence that the feature is a lava tube is the unique radar return pattern and a qualitative comparison to a terrestrial example at Lanzarote, Spain. The authors do an excellent job using radar remote sensing to present an argument that the feature on Venus is a lava tube. However, the manuscript is lacking in several essential areas that I explain below. Identifying a lava tube on Venus is an exciting possibility, however as this manuscript is currently written I cannot recommend it be published. The majority of my comments can be addressed with text edits. I recommend major revisions and look forward to seeing the revised manuscript. -Ian T.W. Flynn\nMajor Comments:\n• A general major comment is that the authors rely on the planetary cave literature rather than the planetary volcanology literature. This by itself would not be an issue, but the authors do not include accurate information about lava tube formation and similar volcanology processes. I make several line edits and other major comments related to this issue. As the manuscript focuses on the identification of a lava tube, a volcanic feature, I encourage the authors to revisit some of their citations to make sure they are appropriate.\n• The authors do not cover the full spectrum of volcanology processes that could form a lava tube. I made a note of this in the introduction (see the attached PDF) and below are some references that could be used to help the authors expand the introduction. Only a few additional sentences would be needed to explain the other lava tube formation processes in the introduction. As written, the authors give the impression lava tubes can only form under one mechanism, and this is not correct.\no Peterson, D. W., Holcomb, R. T., Tilling, R. I., & Christiansen, R. L. (1994). Development of lava tubes in the light of observations at Mauna Ulu, Kilauea Volcano, Hawaii. Bulletin of Volcanology, 56, 343-360.\no Valerio, A., Tallarico, A., & Dragoni, M. (2008). Mechanisms of formation of lava tubes. Journal of Geophysical Research: Solid Earth, 113(B8).\no Calvari, S., & Pinkerton, H. (1999). Lava tube morphology on Etna and evidence for lava flow emplacement mechanisms. Journal of Volcanology and Geothermal Research, 90(3-4), 263-280.\n• Similar to the above comments, the authors should add some discussion either in the introduction or discussion as to if the formation of lava tubes on Venus is expected. Given the extreme environmental conditions on Venus, it is unclear on how lava tubes will form on the planet. Head and Wilson, (1988) briefly describe that lava tube formation on Venus should be more common than on Earth. Flynn et al., (2023) discusses how the reduced gravity on Venus should promote the increased formation of crust cover, which could promote the formation of lava tubes. There needs to be some discussion in the manuscript about whether the formation of lava tubes is expected or could occur on Venus.\no Head III, J. W., & Wilson, L. (1986). Volcanic processes and landforms on Venus: Theory, predictions, and observations. Journal of Geophysical Research: Solid Earth, 91(B9), 9407-9446.\no Flynn, I. T., Chevrel, M. O., & Ramsey, M. S. (2023). Adaptation of a thermorheological lava flow model for Venus conditions. Journal of Geophysical Research: Planets, 128(7), e2022JE007710.\n• Page 5 last paragraph: The authors do an adequate job comparing their results to those on the Moons and Mars. However, there are two issues with the comparison. (1) There should be a comparison to an equivalent Earth lava tube included. This is necessary to contextualize the results between the other planetary bodies. (2) The Mars reference is for pit craters and not all pit craters are formed via lava tube collapse (Wyrick et al., 2004). The authors need to be consistent and make sure they are comparing the correct volcanic features to each other. Furthermore, we do not have good spatial constraints (lengths, widths, volumes) on the reported extents of planetary lava tubes. As this text is currently written it makes it sound like these values are 100% accurate and does not provide the necessary context.\no Wyrick, D., Ferrill, D. A., Morris, A. P., Colton, S. L., & Sims, D. W. (2004). Distribution, morphology, and origins of Martian pit crater chains. Journal of Geophysical Research: Planets, 109(6). https://doi.org/10.1029/2004JE002240\n• The main text of the manuscript mentions the “unique radar backscatter” associated with lava tube skylights. This is the primary result used in the manuscript to indicate the Venusian feature is a lava tube. However, the specific radar return pattern is not described in quantitative detail in the main text. There is a minor description of the unique radar return pattern in a figure caption, but the supposed diagnostic radar return pattern for a lava tube is not described in the main text. The “unique radar backscatter” used to identify a Venusian lava tube needs to be described in the main text in detail so that readers can evaluate the identification criteria. At present this description lacks significant detail and needs to be greatly expanded.\nMinor Comments:\n• All my line edits are made within the PDF. I made comments over specific locations where sentences needed revisions. Text with lines through it indicate it should be removed or substantially rephrased.\n• Line numbers in a manuscript are standard formatting practice for Nature articles and most journals. It was frustrating to not have them and made making specific comments more difficult. Please make sure to include line numbers in the future.\n• A rille is not a lava tube. A rille is believed to form through a primarily deconstructive process (thermos- or mechanical-erosion of the substrate during a high-volume effusive eruption). Lava tubes are primarily a constructive process. Please update this detail throughout the manuscript.\n• I agree with the authors that the feature may be a lava tube. Simila features are observed at Api Mons, Uti Hiata Mons, and Kali Mons. It would be useful in the discussion section or a conclusion paragraph to highlight the possibility of identifying lava tubes elsewhere on Venus.\n• The manuscript switches between refereeing to the feature as a cave and skylight. This is confusing at times, as caves can have different formation processes and geologic implications. Please use consistent terminology throughout the manuscript. Since you are focused on volcanic features I suggest using volcanology terms.", "reviewer_3": "Review by Ernst of Carrer et al. “Radar-Based Observation of a Large Subsurface Lava Tube on Venus”\nThe absence of line numbers makes reviewing this paper more difficult!\nThe authors make the interesting proposal that a pit (labelled A in Fig.1) in the area of Nyx Mons, Bell Regio, Venus could be an example of a “skylight” on the top of a lava tube.\nI like their idea that such pits could be “skylights”, and think that there their geometric analysis is thoughtful and comprehensive\nHowever, for multiple reasons detailed below, I think this example of this pit (labelled A in Fig. 1) and other pits visible in Fig. 1 represent collapse features located above laterally propagating subsurface dykes—particularly given that pit chains are widespread on Venus and that everywhere I have looked (in at least 40 different regions in connection with the detailed mapping by our student members of our International Venus Research Group), the pit chains tend to be parallel to sets of grabens, which are typically interpreted to be underlain by mafic dyke swarms.\nAnd for these reasons (elaborated upon below), I must suggestion rejection of this manuscript.\n1) First of all the particular feature that they have chosen, labelled A in figure 1 is not particularly unique and is clearly aligned with other pits along a NW-SE trend---- and another line of smaller pits about 15 km to the NE (by the way, Figure 1 needs a scale bar). There is also another NW-SE trend of small pits near their white arrow.\nIn Davey et al. (2013 Can. J. Earth Sci) and in Patterson et al. (2016, LPSC, extended abstr. #2097) it is proposed that pit chains are typically aligned with sets of grabens, and also underlain by dyke swarms---- since there is now abundant literature to suggest that graben-fissure fracture systems (with linear, radiating and circumferential patterns) on Venus are typically underlain by mafic dyke swarms (e.g. Grosfils and Head, 1994 EMP; Grosfils and Head, 1994 GRL; Ernst et al. 2001 Ann Rev Eand Planetary Rev; Buchan and Ernst, 2021 Gondwan. Res.; El Bilali et al. 2023 Comm. E and Environ; El Bilali and Ernst, 2024 Nat. Comm.; Ben Marzoug et al. 2024 Icarus; Hannour et al. 2024 Icarus; Tessier et al. 2024 Icarus).\nIn fact Riaz et al. (2023; LPSC, extended abstract #1510) did detailed mapping of graben fissure systems and pit chains in the Bell Regio region and have interpreted a number of distinct dyke swarms. So I think that your pit chains (including the one labelled A) are likely part of one of her dyke swarms (haven’t had a chance to double check)\nI should also point out that Davey et al. (2013) in particular suggest that whether dykes produce overlying grabens or pit chains, depends on the surface material— specifically that less cohesive surface units (e.g. poorly welded volcaniclastic material) would be more likely to produce pits, while more massive mechanically competent flows have surface failure in graben mode.\nIt also true that the pit chains on Mars are generally linked to subsurface dykes: e.g. Mege and Masson (1996 Planet. Space Sci.), and the recent El Yazidi et al (2024 Icarus). Although I should note that the recent paper of Wyrick and Buczkowski (2002, JGR Planets) is less favouring of the dyke swarm interpretation)\n2) A second concern of the skylight interpretation is the following: that “skylight A to belong to a winding (sinuous) collapse chain originating NW and progressively descending SE (see Supplementary Fig. 2).” But if this is the path of lava tube, then the downhill topography direction should be to the SE. Therefore available Magellan topography data needs to be discussed to reveal whether the downhill direction is to the SE (unfortunately, this may be difficult to do with the large uncertainty footprint of the Magellan altimetry data).\n3) A third concern is that while their geometric analysis of SAR data seems rigorous, I think represents an overinterpretation based on the poor resolution of the available Magellan data. There are large uncertainties in their analysis given the 75 m/ pixel resolution and potentially additional uncertainties such as a “layover effect” in SAR image interpretation ---\nI think the uncertainties in their analysis are large in comparison with the values they determined \"a roof thickness of at least 150 m and an empty void height of no less than 375 m. The conduit extends in the subsurface for at least 300 meters from the skylight.\"\nI suspect if they included uncertainties and error propagation in their analysis that the uncertainties would be of the sclae of their preferred values.\nI should point out that their analysis approach may be useful for higher resolution data from future missions to Venus\nAnother comment on this text--- near the end of manuscript “The preserved layers within the conduit walls could serve as a detailed record of the planet's volcanic history, enabling the reconstruction of geological timelines and aiding in the understanding of geological processes on Venus and other celestial bodies.” But the lava tube represents a single flow---\nand ledges on the walls of the tube will simply reveal changes partial crystallization of the flow at different stages of the flow history of this particular flow--- NOT anything more grand about the “detailed record of the planet's volcanic history”\nFigure 1: needs a scale bar"}, "author_response": "Radar-Based Observation of a Large Subsurface Lava Tube on\nVenus\nLeonardo Carrer, Elena Diana & Lorenzo Bruzzone\nWe would like to thank the Editor and the Reviewers for their valuable suggestions. We\nperformed the following major modifications to the paper:\n• Added early Venus lava tube studies and planetary volcanology references; expanded\ndiscussion on lava tube formation processes and expected occurrence on Venus under\nextreme environmental conditions.\n• Corrected Magellan radar description, clarified limitations of optical imagery, updated\nVenus specific references for radar backscatter, and refined terminology for\nconsistency.\n• Expanded description of radar backscatter signatures, added terrestrial and Venusian\npit comparisons, included uncertainty analysis for geometric estimates from Magellan\ndata, improved the radar signatures description of the various terrestrial analogues in\nthe Discussion section.\n• Added morphometric comparison with planetary and terrestrial lava tubes; discussed\nalternative interpretations of Pit A (dyke-related collapse) while highlighting the\ndistinctive radar signature supporting the skylight and horizontal conduit hypothesis;\nconfirmed downhill orientation using topographic data, removed speculative claims\nregarding preserved layered deposits.\n• Reformulated exaggerated claims, integrated minor stylistic edits, added Figure 1\nscale bar, and included line numbers per journal standards.\n• Implemented editorial suggestions provided by the Reviewers in the pdf file.\nIn the following pages the point-by-point answers to the Reviewers comments are provided.\nThis manuscript details the very exciting confirmation of a lava tube entrance on Venus. Over\nthe last few decades, orbital data has strongly suggested (or even confirmed) the existence\nand nature of lava tubes on the moon and Mars, and this new confirmation for Venus just goes\nto show that these are ubiquitous features of planetary surfaces with histories of basaltic\nvolcanism. It uses an approach developed and previously applied by the authors for\ncharacterizing lunar lava tube entrances using radar, and includes a comparison of radar data\nfor a terrestrial analog lava tube, and the manuscript convincingly rules out other potential\nfeatures. As such it is worthy of publication.\nHowever, there is definitely room for improvement. My main criticism of this manuscript is its\nrepeated exaggeration- of the novelty of this work (e.g. there being no confirmed Martian\ndetections) and its importance (e.g. relevance to future robotic missions, access to layered\ndeposits), however this could be remedied by more measured language, better referencing,\nand more realistic (but still important!) implications. The core of the manuscript is strong\nenough to support publication without the hyperbole.\nHere are some specific recommendations:\n[RC-1-1]The manuscript misses out on some early studies of crater chains (and prospective\nlava tubes) from the early post-Magellan mission days. Citations of those works would help\nshow how long this question has been out there, and that this work is using a novel SAR\ninterpretation approach (and new analog comparisons) to answer a question where only\nspeculation existed before. This one is only a thesis but has extensive coverage of the topic:\nMelville, Graeme P (1994). Lava tubes and channels of the Earth, Venus, Moon and Mars.\nUniversity of Wollongong. Thesis. https://hdl.handle.net/10779/uow.27649461.v1 This one is\nshorter but mentions Venus lava tubes (co-authored by the author of the thesis listed above):\nPublications of the Astronomical Society of Australia, Volume 10, Issue 3, 1993, pp. 200 –\n202. DOI: https://doi.org/10.1017/S1323358000025662\nAuthor Reply: Thank you for the useful suggestion. We have included the citation to the\nproposed thesis work with relation to the hypothesis of lava tubes on Venus.\n[RC-1-2]The introduction says that “…from optical images (e.g., by photoclinometry) it is very\ndifficult to assess whether a subterranean cave extends from the skylight.” I will agree that this\nis true, but it also undersells some of the quality work that has been done trying to do just that-\ne.g. in Haruyama et al. (2009), a paper you’re already citing elsewhere. You make the\nstronger case for your work in the next sentence (e.g. you couldn’t use imagery for Venus\nanyways).\nAuthor Reply: We agree. We reformulated it by stating that optical images are of great aid in\nlocating and analyzing skylights, but the use of satellite optical remote sensing instruments is\nprecluded on Venus because of its thick atmosphere.\n2\n[RC-1-3] A sentence later in the Intro, you say that “Magellan is a spaceborne side-looking\nSAR…” Do you mean that Magellan HAD a spaceborne side-looking SAR? Or more precisely\nthat it had a radar system (the RDRS) that could operate in a SAR mode?\nAuthor Reply: Thank you for pointing out the misleading sentence. Magellan is actually the\nname of the mission and spacecraft. Its primary instrument was a radar system capable of\noperating in three different acquisition modes, (a SAR mode, an altimeter mode and a\nradiometer mode). We have corrected the mentioned sentence accordingly.\n[RC-1-4] Page 7, last sentence of the first paragraph states “These deposits tend to increase\nthe radar backscattering uniformly around the feature” in reference to impact craters. The\nreference cited relates to radar imaging of craters on the Moon- could you cite a Venus-\nspecific source? Venus craters have much different ejecta patterns than lunar ones, often\ninvolving more impact melt, and not as “rough” in radar as lunar craters (and can even be\nbrighter than their surroundings in radar images). See\nhttps://www.hou.usra.edu/meetings/lpsc2023/pdf/1052.pdf and other refs regarding Venus\nejecta.\nAuthor Reply: Thank you for your suggestion. We have added citations to the following\npapers:\n- The unusual surface texture of Venusian impact ejecta (Neish et al. 2023), as suggested\n- Geology and Distribution of Impact Craters on Venus: what are they telling us? (Schaber et\nal., 1992)\nFrom the two papers it is possible to observe that, despite the peculiar aspect of some\nimpact craters, (e.g., due to outflow deposits), bright areas related to ejecta surround in a\nrelatively uniform manner the central crater rather than concentrating exclusively on a single\ndirection. Moreover, according to Schaber et al., it appears that extensive bright outflows are\ngenerally related to bigger impact craters.\n[RC-1-5] Page 7, second paragraph, second sentence: “Given that there are no confirmed\ndetections of intact lava tubes for the Martian case…” Are you sure you want to say this? Do\nyou mean confirmed using the SAR method? I’m sure there are members of the HiRISE team\n(MRO) who would contend that their lava tube entrances on Alba and Arsia Mons are\n“confirmed.” Sauro et al. (2020), which you are already citing, discusses this. With no other\ndiscussion in this manuscript about what you mean by “confirmed,” this statement is\nunsupported.\nAuthor Reply: We thank the reviewer for this helpful clarification. We have removed the\nsentence to avoid ambiguity. In our original wording, by “confirmed” we intended to refer\nspecifically to skylights that demonstrably provide access to an open, laterally extensive\nsubsurface conduit (“an intact lava tube“), rather than simply the surface expression of a\ncollapse feature.\n3\n[RC-1-6] Same paragraph, in reference to the critical factors influencing lava tube stability- the\nreference cited is great but makes no mention of Venus. You should make it clearer that this\nreference does not mention Venus (or Venus-specific conditions). I don’t know but I’d be\ncurious if factors such as high atmospheric pressure, or prolonged cooling times (given high\nsurface temperatures) could affect their stability, or the tensional strength of the lava material.\nAs you’ve just stated ahead of this sentence, there is currently no info about the structural\nstability of lava tubes on Venus. While it is clearly beyond the scope of this paper to model\nthis, it might be good to acknowledge that Venus surface conditions could potentially influence\nthis (and perhaps that further work is needed to assess whether the general conditions for\nlava tube stability developed in the paper cited can be applied to Venus).\nAuthor Reply: Agreed. We modified this sentence in the manuscript: “The critical factors\ninfluencing their stability are the thickness of the tube's ceiling, the lava tube cross section and\nthe tensile strength of the lava material” → “The critical factors influencing their stability are\nthe thickness of the tube's ceiling, the lava tube cross section and the tensile strength of the\nlava material.The surface of Venus experiences significantly higher temperatures and\npressures than Mars or the Moon. These extreme conditions may influence the morphology\nand stability of the lava tube.”\n[RC-1-7] Final paragraph, page 8. Some of the applications seem wildly fanciful to me and\nshould be scaled back. That this skylight “could also be of great interest as targets for a\npossible lander robotic mission” with a citation to a DAVINCI mission paper is way off- that is\nnot a robotic mission and will at best only return data for minutes if it as a soft landing, and it\nalready has a target area. The next sentence talks about “preserved layers within the conduit\nwalls could serve as a detailed record of the planet’s volcanic history, enabling the\nreconstruction of geological timelines.” I’ve spent many weeks in lava tubes on multiple\ncontinents (and islands) and I have never seen layered deposits, except for maybe one flow\ncovering another flow or two (but likely separated in time by years at most). It’s just not the\nkind of environment that preserves a long record.\nAuthor Reply: Thank you for the suggestion. We decided to remove the paragraph.\n[RC-1-8] I’ve made multiple minor corrections and suggestions for more concise language,\nfor example replacing “in the proximity of” with “near” throughout. I’m attaching an annotated\nPDF with my suggestions.\nAuthor Reply: Thank you. The corrections have been integrated in the revised paper.\nThe authors present an argument that a geologic feature on the western flank of Nyx Mons,\nVenus, is a lava tube. This would be a new identification of a lava tube on Venus. Lava tubes\nhave been discussed briefly in the Venus volcanology literature but little to no evidence has\nbeen presented identifying one. The primary evidence that the feature is a lava tube is the\nunique radar return pattern and a qualitative comparison to a terrestrial example at Lanzarote,\nSpain. The authors do an excellent job using radar remote sensing to present an argument\nthat the feature on Venus is a lava tube. However, the manuscript is lacking in several\nessential areas that I explain below. Identifying a lava tube on Venus is an exciting possibility,\nhowever as this manuscript is currently written I cannot recommend it be published. The\nmajority of my comments can be addressed with text edits. I recommend major revisions and\nlook forward to seeing the revised manuscript.\nMajor Comments:\n[RC-2-1] A general major comment is that the authors rely on the planetary cave literature\nrather than the planetary volcanology literature. This by itself would not be an issue, but the\nauthors do not include accurate information about lava tube formation and similar volcanology\nprocesses. I make several line edits and other major comments related to this issue. As the\nmanuscript focuses on the identification of a lava tube, a volcanic feature, I encourage the\nauthors to revisit some of their citations to make sure they are appropriate.\nAuthor Reply: Thank you. We greatly revised the paper by including planetary volcanology\nliterature to complement the one on planetary caves.\n[RC-2-2] The authors do not cover the full spectrum of volcanology processes that could form\na lava tube. I made a note of this in the introduction (see the attached PDF) and below are\nsome references that could be used to help the authors expand the introduction. Only a few\nadditional sentences would be needed to explain the other lava tube formation processes in\nthe introduction. As written, the authors give the impression lava tubes can only form under\none mechanism, and this is not correct.\no Peterson, D. W., Holcomb, R. T., Tilling, R. I., & Christiansen, R. L. (1994).\nDevelopment of lava tubes in the light of observations at Mauna Ulu, Kilauea Volcano,\nHawaii. Bulletin of Volcanology, 56, 343-360.\no Valerio, A., Tallarico, A., & Dragoni, M. (2008). Mechanisms of formation of lava\ntubes. Journal of Geophysical Research: Solid Earth, 113(B8).\no Calvari, S., & Pinkerton, H. (1999). Lava tube morphology on Etna and evidence for\nlava flow emplacement mechanisms. Journal of Volcanology and Geothermal\nResearch, 90(3-4), 263-280.\nAuthor Reply: Thank you for pointing out the confusion that can arise from the limited\nexplanation about lava tube formation. As suggested, we have extended the introduction to\ninclude a more detailed explanation.\n5\n[RC-2-3] Similar to the above comments, the authors should add some discussion either in\nthe introduction or discussion as to if the formation of lava tubes on Venus is expected. Given\nthe extreme environmental conditions on Venus, it is unclear on how lava tubes will form on\nthe planet. Head and Wilson, (1988) briefly describe that lava tube formation on Venus should\nbe more common than on Earth. Flynn et al., (2023) discusses how the reduced gravity on\nVenus should promote the increased formation of crust cover, which could promote the\nformation of lava tubes. There needs to be some discussion in the manuscript about whether\nthe formation of lava tubes is expected or could occur on Venus.\no Head III, J. W., & Wilson, L. (1986). Volcanic processes and landforms on Venus:\nTheory, predictions, and observations. Journal of Geophysical Research: Solid Earth,\n91(B9), 9407-9446.\no Flynn, I. T., Chevrel, M. O., & Ramsey, M. S. (2023). Adaptation of a\nthermorheological lava flow model for Venus conditions. Journal of Geophysical\nResearch: Planets, 128(7), e2022JE007710.\nAuthor Reply: Thank you for the suggestion. In the Discussion section, we have added a\ndiscussion about why lava tubes are expected to form also on Venus, including reference to\nthe suggested papers.\n[RC-2-4] Page 5 last paragraph: The authors do an adequate job comparing their results to\nthose on the Moons and Mars. However, there are two issues with the comparison. (1) There\nshould be a comparison to an equivalent Earth lava tube included. This is necessary to\ncontextualize the results between the other planetary bodies. (2) The Mars reference is for pit\ncraters and not all pit craters are formed via lava tube collapse (Wyrick et al., 2004). The\nauthors need to be consistent and make sure they are comparing the correct volcanic features\nto each other. Furthermore, we do not have good spatial constraints (lengths, widths, volumes)\non the reported extents of planetary lava tubes. As this text is currently written it makes it\nsound like these values are 100% accurate and does not provide the necessary context.\no Wyrick, D., Ferrill, D. A., Morris, A. P., Colton, S. L., & Sims, D. W. (2004).\nDistribution, morphology, and origins of Martian pit crater chains. Journal of\nGeophysical Research: Planets, 109(6). https://doi.org/10.1029/2004JE002240\nAuthor Reply: We agree with the comment. We have decided to expand the discussion, by\nadding a new subsection in the Results section to better address the morphometric\ncomparison of planetary lava tube collapses. Now, we explicitly cite the paper from which we\nextracted the measures for martian, lunar and terrestrial lava tube collapse candidates (Sauro\net al. 2020). Finally, we have included a brief size comparison between the hypothesized lava\ntube on Venus and the Corona lava tube.\n[RC-2-5] The main text of the manuscript mentions the “unique radar backscatter” associated\nwith lava tube skylights. This is the primary result used in the manuscript to indicate the\nVenusian feature is a lava tube. However, the specific radar return pattern is not described in\nquantitative detail in the main text. There is a minor description of the unique radar return\npattern in a figure caption, but the supposed diagnostic radar return pattern for a lava tube is\nnot described in the main text. The “unique radar backscatter” used to identify a Venusian lava\ntube needs to be described in the main text in detail so that readers can evaluate the\nidentification criteria. At present this description lacks significant detail and needs to be greatly\nexpanded.\nexpanded the main text (see revised Study Area and Magellan Radar Image Analyses\nsection) to provide a more detailed description of the radar backscatter pattern that we\nassociate with lava tube skylights. Furthermore, we have added additional radar images to the\ndiscussion (see Supplementary Fig.2), illustrating more clearly the contrasts between the\nMagellan backscattering response of a skylight connected to a lateral subsurface conduit and\nthat of other types of pits (e.g., vertical collapse pits).\n● All my line edits are made within the PDF. I made comments over specific locations\nwhere sentences needed revisions. Text with lines through it indicate it should be\nremoved or substantially rephrased.\nAuthor Reply: Thank you. We have revised the paper according to the comments\nwithin the PDF.\n● Line numbers in a manuscript are standard formatting practice for Nature articles and\nmost journals. It was frustrating to not have them and made making specific\ncomments more difficult. Please make sure to include line numbers in the future.\nAuthor Reply: We included the line numbers. We apologize for having forgotten\nthem. Thank you for the detailed suggestions.\n● A rille is not a lava tube. A rille is believed to form through a primarily deconstructive\nprocess (thermos- or mechanical-erosion of the substrate during a high-volume\neffusive eruption). Lava tubes are primarily a constructive process. Please update\nthis detail throughout the manuscript.\nAuthor Reply: Thank you for pointing out this. We have fixed this in the paper.\n● I agree with the authors that the feature may be a lava tube. Simila features are\nobserved at Api Mons, Uti Hiata Mons, and Kali Mons. It would be useful in the\ndiscussion section or a conclusion paragraph to highlight the possibility of identifying\nlava tubes elsewhere on Venus.\nAuthor Reply: The possibility of detecting similar feature across other regions on\nVenus has been included in the conclusions section.\n● The manuscript switches between refereeing to the feature as a cave and skylight.\nThis is confusing at times, as caves can have different formation processes and\ngeologic implications. Please use consistent terminology throughout the manuscript.\nSince you are focused on volcanic features I suggest using volcanology terms.\nAuthor Reply: This has been fixed in the revised paper.\nThe authors make the interesting proposal that a pit (labelled A in Fig.1) in the area of Nyx\nMons, Bell Regio, Venus could be an example of a “skylight” on the top of a lava tube.\nI like their idea that such pits could be “skylights”, and think that their geometric analysis is\nthoughtful and comprehensive.\nHowever, for multiple reasons detailed below, I think this example of this pit (labelled A in Fig.\n1) and other pits visible in Fig. 1 represent collapse features located above laterally\npropagating subsurface dykes—particularly given that pit chains are widespread on Venus\nand that everywhere I have looked (in at least 40 different regions in connection with the\ndetailed mapping by our student members of our International Venus Research Group), the\npit chains tend to be parallel to sets of grabens, which are typically interpreted to be underlain\nby mafic dyke swarms.\nAnd for these reasons (elaborated upon below), I must suggest rejection of this manuscript.\n[RC-3-1] First of all the particular feature that they have chosen, labelled A in figure 1 is not\nparticularly unique and is clearly aligned with other pits along a NW-SE trend---- and another\nline of smaller pits about 15 km to the NE (by the way, Figure 1 needs a scale bar). There is\nalso another NW-SE trend of small pits near their white arrow.\nIn Davey et al. (2013 Can. J. Earth Sci) and in Patterson et al. (2016, LPSC, extended abstr.\n#2097) it is proposed that pit chains are typically aligned with sets of grabens, and also\nunderlain by dyke swarms---- since there is now abundant literature to suggest that graben-\nfissure fracture systems (with linear, radiating and circumferential patterns) on Venus are\ntypically underlain by mafic dyke swarms (e.g. Grosfils and Head, 1994 EMP; Grosfils and\nHead, 1994 GRL; Ernst et al. 2001 Ann Rev Eand Planetary Rev; Buchan and Ernst, 2021\nGondwan. Res.; El Bilali et al. 2023 Comm. E and Environ; El Bilali and Ernst, 2024 Nat.\nComm.; Ben Marzoug et al. 2024 Icarus; Hannour et al. 2024 Icarus; Tessier et al. 2024\nIcarus).\nIn fact Riaz et al. (2023; LPSC, extended abstract #1510) did detailed mapping of graben\nfissure systems and pit chains in the Bell Regio region and have interpreted a number of\ndistinct dyke swarms. So I think that your pit chains (including the one labelled A) are likely\npart of one of her dyke swarms (haven’t had a chance to double check)\nI should also point out that Davey et al. (2013) in particular suggest that whether dykes\nproduce overlying grabens or pit chains, depends on the surface material— specifically that\nless cohesive surface units (e.g. poorly welded volcaniclastic material) would be more likely\nto produce pits, while more massive mechanically competent flows have surface failure in\ngraben mode.\nIt also true that the pit chains on Mars are generally linked to subsurface dykes: e.g. Mege\nand Masson (1996 Planet. Space Sci.), and the recent El Yazidi et al (2024 Icarus). Although\nI should note that the recent paper of Wyrick and Buczkowski (2002, JGR Planets) is less\nfavouring of the dyke swarm interpretation)\nhave originated from different geologic processes, such as tectonic or volcanic activity such\nas dyke intrusion (Davey et al,.2013, Sawford et al. 2015,Wyrick and D. L. Buczkowski 2022).\nIn fact, we included a discussion on this in our previous paper (Carrer et al., 2024) where we\nassess the morphometry of pit craters on Venus from SAR data. In that paper, we have not\nidentified any discernible differences in the analyzed pit’s morphometric radar-derived\nparameters that would suggest variations in the underlying genetic processes. In this context,\nit is worth highlighting recent research suggesting that the shape and dimensions of pit craters\nmay not provide substantial insights into the specific processes responsible for their formation\nor the characteristics of the surrounding materials (Magee et al., 2022).\nHowever, the pit analyzed in this study exhibits a distinctive radar signature that sets it apart\nfrom all other venusian pits we examined in Carrer et al. (2024). Its radar response closely\nresembles that of a lava tube skylight, consistent with the terrestrial analogs presented in\nCarrer et al. (2022), which we also reference in this paper (see also the additional analysis we\nadded in the new Supplementary Fig. 2). Taking into account the comment of the reviewer,\nwe decided to add another example of SAR of collapse pits, produced by dyke swarms in\nIceland.\nThe pits belong to a 1.7 km long chain located in the Kollóttadyngja lava shield, in the Northern\nVolcanic Zone of Iceland, and is considered to be part of the Askja fissure swarm (Hjartardóttir\n2008). The chain includes five pits crater, with a maximum width of about 150 m and a\nmaximum depth of about 50 m. Besides, two shallower pits can be observed east to the main\nchain. Analyses have been conducted that associate the pit chain with the intrusion of one or\nmore dikes under the Kollóttadyngja lava shield. In particular, Hjartardóttir (2008) suggests\nthat the chain results from the collapse of a subsurface pipe, originated by an horizontal\ncomponent of a dike under the mountain.\nThe radar response of the dyke induced pit resemble the response of a typical Venusian pit\nwith no horizontal continuation. Thus based on the radar signature the most plausible\nexplanation is that pit A is a surface collapse into an open void. We acknowledge the dyke\nswarm hypothesis and the revisioned paper explicitly discuss both these possibilities. We also\nadded text noting that pit chains on Venus are often interpreted as dyke-swarm collapse\nfeatures, and we cite relevant literature (Davey et al., Riaz et al., etc.) for completeness. We\nalso state in the paper that dike intrusion accompanied by roof collapse remains a possible\nalternative. Laterally propagating magmatic dikes can separate overlying strata and create\nvoid spaces, and if the overlying material is sufficiently thin, localized collapse may occur,\nforming a pit that connects to a horizontal magmatic pathway.\nAnother relevant hypothesis is the one of pit chains produced in association with dyke\nswarms, as these chains on Venus are frequently interpreted in this latter context 35,36. A\nrelevant terrestrial analogue example is from the Kollóttadyngja lava shield in the Northern\nVolcanic Zone of Iceland, where a 1.7 km-long chain of collapse pits, up to about 150 m wide\nand about 50 m deep, has been linked to the Askja fissure swarm37. The chain is interpreted\nas having formed by collapse into a subsurface pipe generated by the horizontal component\nof a dyke intrusion beneath the shield. The radar response of these dyke-induced pits\nresembles that of typical Venusian pits with no horizontal continuation (Supplementary Fig. 5).\nTherefore, the observed radar signature is most consistent with the interpretation that pit A\nrepresents a lava tube roof collapse into an underlying void with horizontal continuation. In\ngeneral, laterally propagating magmatic dykes can separate overlying strata and create void\nspaces. If the overlying material is sufficiently thin, localized collapse may occur, forming a pit\nthat connects to an horizontal magmatic pathway. The analysed terrestrial SAR data favours\nthe lava tube hypothesis but we cannot completely rule out the possibility that pit A formed\nthrough dyke intrusion.\nreported in Supplementary Fig. 4 in the paper.\nReferences for this answer\nCarrer, L., Diana, E., & Bruzzone, L. (2024). Assessing the Morphometry of Pit Craters on Venus from\nOrbital Synthetic Aperture Radar Data. IEEE Transactions on Geoscience and Remote Sensing.\nCarrer, L., Castelletti, D., Pozzobon, R., Sauro, F., & Bruzzone, L. (2022). A novel method for hidden\nnatural caves characterization and accessibility assessment from spaceborne VHR SAR images. IEEE\nTransactions on Geoscience and Remote Sensing, 61, 1-11.\nMagee, C., Kling, C., Byrne, P., & A‐L Jackson, C. (2022). Seismic reflection data reveal the 3D\nsubsurface structure of pit craters. Journal of Geophysical Research: Planets, 127(8), e2021JE007155.\nS. C. Davey, R. E. Ernst, C. Samson, and E. B. Grosfils, “Hierarchical clustering of pit Crater chains on\nVenus,” Can. J. Earth Sci., vol. 50, no. 1, pp. 109–126, Jan. 2013.\nW. Sawford, R. Ernst, C. Samson, and S. Davey, “Pit Crater chains in the NYX mons region, Venus,” in\nProc. 46th Lunar Planet. Sci. Conf., 2015, pp. 1–2.\nD. Y. Wyrick and D. L. Buczkowski, “Pit Crater chains across the solar system: Evidence for subterranean\ntectonic caves, porosity and permeability pathways on planetary bodies,” J. Geophys. Res. Planets, vol.\n127, no. 12, Dec. 2022, Art. no. e2022JE007281.\nHjartardóttir Á.R., “The fissure swarm of the Askja central volcano”, M.Sc thesis. University of Iceland,\nReykjavík, 2008, pp 1-113.\nErnst, R.E., Grosfils, E.B., and Mège, D., “Giant dyke swarms: Earth, Venus, Mars”, Annual Review of\nEarth and Planetary Sciences, 2001 ,29(1): 489–534.\nRiaz, M., Ernst, R.E., El Bilali, H., “Dyke Swarms of Bell Regio, Venus”, 54th Lunar and Planetary Science\nConference 2023, Abstract 1510.\n12\n[RC-3-2] A second concern of the skylight interpretation is the following: that “skylight A to\nbelong to a winding (sinuous) collapse chain originating NW and progressively descending SE\n(see Supplementary Fig. 2).” But if this is the path of lava tube, then the downhill topography\ndirection should be to the SE. Therefore available Magellan topography data needs to be\ndiscussed to reveal whether the downhill direction is to the SE (unfortunately, this may be\ndifficult to do with the large uncertainty footprint of the Magellan altimetry data).\nAuthor Reply: Thank you for raising this important point. We have conducted a\ncomprehensive topographic analysis—now incorporated in the revised manuscript—using\nthree independent datasets to assess whether Pit A lies downhill toward the SE (i) Magellan\nAltimetry (standard processing) (ii) Magellan Altimetry (Ford reprocessing) (iii)Stereo-derived\ntopography from Magellan SAR stereo models. All the dataset confirm that the downhill\ndirection is toward the South-East (see Figures). It is worth noting that we used JMARS for\nVenus (Herrick et al.,2017) to perform the analysis (https://jmars.asu.edu/).\nDespite the relatively coarse resolution of Magellan altimetry all three datasets consistently\nindicate that the surface slopes toward the southeast from Pit A. This trend supports the idea\nthat, if associated with a sinuous collapse chain, its orientation aligns with downhill topography\nas expected for a lava tube system. However, we agree that interpreting this result requires\ncaution. The altimetric uncertainty limits our ability to resolve small topographic variations. We\nexplicitly acknowledge this limitation in the manuscript revision.\nHerrick, R. R., & Wren, P. (2017, November). JMARS: Collecting and ingesting data\nto create a useful scientific analysis tool. In 15th Meeting of the Venus Exploration\nand Analysis Group (VEXAG) (Vol. 15, No. 2061, p. 8021).\n[RC-3-3]A third concern is that while their geometric analysis of SAR data seems rigorous, I\nthink represents an overinterpretation based on the poor resolution of the available Magellan\ndata. There are large uncertainties in their analysis given the 75 m/ pixel resolution and\npotentially additional uncertainties such as a “layover effect” in SAR image interpretation ---\nI think the uncertainties in their analysis are large in comparison with the values they\ndetermined \"a roof thickness of at least 150 m and an empty void height of no less than 375\nm. The conduit extends in the subsurface for at least 300 meters from the skylight.\"\nI suspect if they included uncertainties and error propagation in their analysis that the\nuncertainties would be of the scale of their preferred values.\nI should point out that their analysis approach may be useful for higher resolution data from\nfuture missions to Venus.\nAuthor Reply: Thank you for your comment. We already developed an uncertainty model in\nCarrer et al.,2024. In lights of the reviewer comment, we added the ‘’Analysis of the\nuncertainties in the Determination of Skylight and Lava Tubes Geometric Parameters from\nMagellan Data’’ section to Methods.\n𝛿𝑅 = 𝑅 / √2𝑆𝑁𝑅 = 24.77 m by assuming 𝑅 = 88 m and SNR = 8 dB. Thus for Magellan\n𝑟 𝑟\nwe have that the uncertainty due to SNR is lower than the one given by the pixel resolution p:\n𝛿𝑅 < 𝑝\nGiven the incidence and look angle equal to 42.4 and 40 degrees , respectively, we can bound\nthe errors to for the different estimated quantities. Estimated collapse depth uncertainty:\n𝑝 𝑠𝑖𝑛𝜃 ⋅ 𝑐𝑜𝑠(𝜃 ) = 38.74 m\n𝑖 𝑙\nRadar Signal Propagation uncertainty:\n𝑝 𝑠𝑖𝑛𝜃 ⋅𝑠𝑖𝑛(𝜃 ) = 32.75 m\n𝑖 𝑙\nLava tube roof thickness (worst-case overestimation model) uncertainty:\n𝑝 𝑠𝑖𝑛𝜃 / 𝑐𝑜𝑠(𝜃 ) = 66.02 m\n𝑖 𝑖\nCave height uncertainty:\n𝑝 𝑠𝑖𝑛𝜃 ⋅ 𝑐𝑜𝑠(𝜃 ) = 38.74 m\n𝑖 𝑙\nThus according to the above, the uncertainty is a fraction of the pixel resolution equal to 75 m\nand does not affect the conclusion of our analysis.\n[RC-3-4]Another comment on this text--- near the end of manuscript “The preserved layers\nwithin the conduit walls could serve as a detailed record of the planet's volcanic history,\nenabling the reconstruction of geological timelines and aiding in the understanding of\ngeological processes on Venus and other celestial bodies.” But the lava tube represents a\nsingle flow--- and ledges on the walls of the tube will simply reveal changes partial\ncrystallization of the flow at different stages of the flow history of this particular flow--- NOT\nanything more grand about the “detailed record of the planet's volcanic history”\nAuthor Reply: We decided to remove this sentence as suggested also by other Reviewers.\n[RC-3-5]Figure 1: needs a scale bar\nAuthor Reply: We added the scale bar\nLeonardo Carrer, Elena Diana & Lorenzo Bruzzone\nWe would like to thank the Reviewers for their valuable suggestions. We implemented all the\nrequested modifications to the paper (see tracked highlights in yellow).\nWe address Reviewer #3’s remaining concerns. The clarification on SAR uncertainties\nexplains that the reviewer’s 100–200 m values reflect ground-range resolution, whereas our\nmeasurements rely on the finer slant-range resolution appropriate for lava tube geometry. We\nalso explain that the radar dark signature of the collapse material results from geometric\nshadowing within the pit (this was already stated in the Results section of the paper),\nconsistent with previous Venus SAR studies. The alternative dyke-swarm interpretation is now\nintroduced early in the Introduction for balance, and we added explicit discussion showing that\nthe method is not sensitive to layover under the modest topography of the study area.\nDetailed replies to all comments are provided below.\nReviewer 1\nI was very happy to read this revised manuscript and the response to reviewers, and was\nimpressed with how thoroughly they addressed my concerns and recommendations. The new\nreferences provide a stronger basis for their interpretations and better integrate this study into\nthe existing body of work on planetary lava flows (and orbital detections of potential lava tubes\non other planets), and introduced me to a few great new papers. The more over-the-top and\nless substantiated, generalized claims from the last version are gone, and we are left with a\nfocused, well-supported, but still very exciting application of a novel radar processing method\non Magellan data, yielding the first (to my knowledge) confirmed detection of a lava tube on\nVenus.\nAuthor Reply: Thank you for your comment and your useful suggestion. We promoted the\nfigure to the main text as Fig. 2 and included all the Reviewer editorial comments in the\nmanuscript.\nReviewer 2\n1. I recommend a minor title change by removing the words “large” and “subsurface”. The new\ntitle would be “Radar-Based Observations of a Lava Tube on Venus”. My logic behind this is\nthat we don’t have constraints on how large Venusian lava tubes are. This one is large when\ncompared to Earth lava tubes, but we don’t know how it compares to other Venusian lava\ntubes, thus having “large” in the title may be misleading. As for “subsurface”, lava tubes,\nby definition, are a structure that is below the surface. Saying a “subsurface lava tube” is\nredundant. Please consider this title change.\nAuthor Reply: We modified the title according to the suggestion.\n1\n2. There are basic volcanology terms that are consistent throughout the peer-reviewed\npublished literature. I have made note of these issues in my line edits. Please use established\nterms for describing volcanic features and processes.\nAuthor Reply: We implemented all the Reviewer suggestions. Thank you.\nReviewer 3\nRegarding the uncertainties: they have calculated the uncertainties given a SAR pixel size of\n75 m. But actually the SAR uncertainty is 100-200 m, which larger than the pixel size--- I am\nnot sure of the specific SAR uncertainty for the Nyx Mons area.\nAlso in their modelling they infer that they are seeing into the interior of a lava tube >300 m\ndeep. So the I think the \"layover effect\" (where variations in elevation may cause lateral offsets\nin the positioning of the returned radar signal) may be relevant and should be discussed.\nAuthor Reply: We believe that the discrepancy between our calculations and the values\nreported by the Reviewer (i.e., 100-200 m) arises from how ground resolution is measured in\nradar systems. In general, Synthetic Aperture Radar (SAR) images measure distances in the\nslant-range plane, meaning that pixel spacing is referenced to the radar line of sight (see Fig.\n1). When the radar beam intersects the surface at an incidence angle, the slant-range pixel\nprojects onto a larger footprint in ground range. As a result, the effective ground-range\nresolution becomes coarser depending on the incidence angle. This is summarized in the\nfollowing well-known radar theory equations. The radar slant range resolution is equal to:\nΔ𝑅 = 𝑐 /2𝐵\n𝑠𝑙𝑎𝑛𝑡 0\nwhere 𝑐 is the speed of light and 𝐵 is the bandwidth. The ground range resolution is equal to:\n0\n𝑐 Δ𝑅\n0 𝑠𝑙𝑎𝑛𝑡\nΔ𝑅 = =\n𝑠𝑙𝑎𝑛𝑡 2𝐵𝑠𝑖𝑛𝜃 𝑠𝑖𝑛𝜃\n𝑖 𝑖\nGiven the Magellan radar bandwidth B=2.26 MHz (see [Ref-1] and [Ref-2]), we obtain a slant-\nrange resolution of approximately 66 m. Some studies suggest using an effective slant-range\nvalue of 88 m (see [Ref-3]), which accounts for non-idealities. This is the value we adopt in\nthis paper, consistent with Ref. 3. Assuming typical values of the Magellan radar incidence\nangle (see [Ref-4] and the figure reproduced from that paper), we obtain a ground range\nresolution that aligns with the values provided by the Reviewer (i.e., 100-200 m). Thus, the\nslant-range resolution is notably finer than the ground range resolution. In our radar\nmethodology we convert the ground-range resolution to slant-range resolution before\nperforming any geometric measurements related to lava tubes [Ref-4] [Ref-5]. This can always\nbe performed on that by taking into account the geometric factor described by 𝑠𝑖𝑛𝜃 [Ref-4]\n𝑖\n[Ref-5]. The additional uncertainty is computed from the slant range using the model provided\nin [Ref-4] and reported in ‘Analysis of the Uncertainties in the Determination of Skylight and\nLava Tubes Geometric Parameters from Magellan Data’ method applied specifically to Nyx\nMons in this manuscript. As a result, considering our specific geometry that is pertinent for\n2\nlava tubes geometric analysis [Ref-4] [Ref-5], the measurement uncertainties are below the\npixel resolution of 75 m as reported in the manuscript under revision.\nAs a note, when measuring the distance between two features in a Magellan SAR (or any\nSAR) image, the uncertainty in relative measurements differs from the absolute geolocation\nuncertainty of individual features. Relative distances are generally more accurate, as many\nsystematic errors cancel out.\nDiscussion on the Layover effect:\nThe methodology is independent of surface layover effects especially with the modest\ntopography present in the study area, which is notably smooth (see Supplementary Fig. 1c).\nIndeed, it explicitly accounts for the layover of the tube interior when computing its depth.\nVariations in surface elevation do not appreciably affect the results as surface roughness\ndoes, as the measurement specifically targets the interior of the tube (analyzing the echo\nreturns from those specific features) rather than the surface above it [Ref-4], [Ref-5], [Ref-6],\n[Ref-7]. To make this explicit, we added the following statement to the discussion section:\n“Previous studies have demonstrated that the methodology is not adversely affected by\nsurface layover for small to moderate topographic variations, such as those present in this\ncase” (refs. 14, 21, 24, 33, see Supplementary Fig. 1c).\nFigure 1. Typical geometry definition for a Synthetic Aperture Radar. The radar measures distances in\nthe slant range plane.\ncoarser than the slant range resolution.\nReferences for this Answer\n[Ref-1] Saunders, R. S., Spear, A. J., Allin, P. C., Austin, R. S., Berman, A. L., Chandlee, R. C., ... &\nWall, S. D. (1992). Magellan mission summary. Journal of Geophysical Research: Planets, 97(E8),\n13067-13090.\n[Ref-2] Ford, J. P. (1993). Guide to Magellan image interpretation. National Aeronautics and Space\nAdministration, Jet Propulsion Laboratory, California Institute of Technology.\n[Ref-3] S. Hensley and S. Shaffer, “Automatic DEM generation using Magellan stereo data,” in Proc.\nIEEE Int. Geosci. Remote Sens. Symp., vol. 3, Sep. 1994, pp. 1470–147\n[Ref-4] Carrer, L., Diana, E., & Bruzzone, L. (2024). Assessing the Morphometry of Pit Craters on Venus\nfrom Orbital Synthetic Aperture Radar Data. IEEE Transactions on Geoscience and Remote Sensing.\n[Ref-5] Carrer, L., Castelletti, D., Pozzobon, R., Sauro, F., & Bruzzone, L. (2022). A novel method for\nhidden natural caves characterization and accessibility assessment from spaceborne VHR SAR\nimages. IEEE Transactions on Geoscience and Remote Sensing, 61, 1-11.\n[Ref-6] Carrer, L., Pozzobon, R., Sauro, F., Castelletti, D., Patterson, G. W., & Bruzzone, L. (2024).\nRadar evidence of an accessible cave conduit on the Moon below the Mare Tranquillitatis pit. Nature\nAstronomy, 8(9), 1119-1126.\n4\n[Ref-7] Carrer, L., Castelletti, D., Pozzobon, R., Sauro, F., & Bruzzone, L. (2024). A method for the\ncharacterization of the interior of pits from single spaceborne SAR images. IEEE Geoscience and\nRemote Sensing Letters, 21, 1-5.\nThe link between figure 1 and Figure 2 (with their model) is illustrated in Supplementary Figure\n6. However, how do we know that the radar bright material on the east side (labelled Delta R\ng2 and Delta R g3) is the interior of a lava tube and not some rough surface lava flow material.\nAlso the supposed rock pile (Delta R g1) is radar dark which would seem to indicate that it is\nsmooth material--- That would NOT be consistent with representing collapse material from the\ntop of the lava tube.\nReply: These concerns were comprehensively addressed in [Ref-4], which also utilizes\nMagellan SAR data to study pit crater depths on Venus. The study demonstrates that it is\nhighly unlikely that the radar-bright material represents rough lava flows, as the observed\nbackscattering characteristics align with analogous features in terrestrial, lunar, and Venusian\nSAR datasets. Additionally, [Ref-4] includes 3D radar simulations based on laser scans of\nterrestrial analogues that specifically support this interpretation. The radar-dark appearance\nof Delta R g1 is attributed to shadowing effects within the pit rather than the intrinsic\nbackscatter of the rock pile, explaining its low backscattering coefficient.\n[Ref-4] Carrer, L., Diana, E., & Bruzzone, L. (2024). Assessing the Morphometry of Pit Craters on Venus\nfrom Orbital Synthetic Aperture Radar Data. IEEE Transactions on Geoscience and Remote Sensing.\nWhen I look at the Figure 1, is it clear that their chosen pit /skylight is part of trend of at\nleast 5 pits, and so if their chose pit (A) is a skylight then the 5 other pits should also be. So you\nshould comment on the characteristics of these other pits and any evidence for or against\nthem as skylights. But we know lines of pits associated with underlying dykes are common on\nVenus only; but the present text is focussed on the lava tube Skylight story. It still seems that\nthe authors are pushing the skylight interpretation too hard, and should more fully address the\nunderlying dyke swarm---in the revised manuscript, the dyke swarm interpretation is only\naddressed at the end of the paper.\nReply: Taking into account the suggestion of the reviewer, in the revised paper we added\nsince the introduction the possible interpretation of pits associated with underlying dykes\namong those plausible. Then later, as already done in the previous version, we analyze this\ninterpretation and explain the reason for which in our opinion this is much less convincing than\nthe lava tube one also using terrestrial analogous. Note that it is not our intention to push the\nskylight interpretation too hard (we do not completely rule out the dyke swarm interpretation\nin the text) but to align all the evidence obtained by our analysis that indicate the lava tube\nhypothesis as the most plausible.\nThe characteristics of 5 pits in Figure 1 are consistent with skylights whose entrances to the\nunderlying lava tubes are obstructed by collapse material. Their radar and morphological\nsignatures closely match those of analogous skylight features observed on Earth and the\nMoon [Ref-1[Ref-2]]. It is worth mentioning that there was already a sentence in the Results\nsection addressing this point: “The other pits belonging to the chain denoted as B, C, and D\n5\nshow no evidence of direct subsurface access as they have a radar response very similar to\nthe one attributable to skylights with blocked conduit entrance as observed on Earth\nanalogues24”\n[Ref-1] Carrer, L., Castelletti, D., Pozzobon, R., Sauro, F., & Bruzzone, L. (2022). A novel method for\nhidden natural caves characterization and accessibility assessment from spaceborne VHR SAR\nimages. IEEE Transactions on Geoscience and Remote Sensing, 61, 1-11.\n[Ref-2] Carrer, L., Pozzobon, R., Sauro, F., Castelletti, D., Patterson, G. W., & Bruzzone, L. (2024).\nRadar evidence of an accessible cave conduit on the Moon below the Mare Tranquillitatis pit. Nature\nAstronomy, 8(9), 1119-1126.\n6"}]

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