Bottom Of Crater Research Articles

Three‐Dimensional Electrostatic Hybrid Particle‐In‐Cell Simulations of the Plasma Mini‐Wake Near a Lunar Polar Crater

AbstractLunar polar region has become the focus of future explorations due to the possible ice reservoir in the permanently shadowed craters. However, the space environment near the polar crater is quite complicated, and a plasma mini‐wake can be caused by the topographic obstruction. So far, three‐dimensional (3D) numerical simulations of the mini‐wake around a crater far larger than the Debye length are still limited. Here we present a 3D electrostatic hybrid particle‐in‐cell model to study the plasma mini‐wake of a polar crater on scale of about 1 km. It is found that the mini‐wake can begin upstream from the crater with a cone angle of about 8.8°. There is a plasma void with extra electrons near the leeward crater wall, where the electric potential can be as low as −60 V. A part of solar wind ions can be diverted into the crater, and the ratio of the diverted flux is about 4% on the crater bottom and about 18% on the windward crater wall, which provide an important source for the surface sputtering. Further studies show that the mini‐wake can change with the solar wind parameters and the crater shapes. Our results are helpful to assess the space environment and the water loss rate of a polar crater, and have general implications in studying the plasma mini‐wake caused by a crater on the other airless bodies.

The damage characteristics and formation mechanism of ultrahigh strength 7055 aluminum alloy under hypervelocity impact

This paper studies the damage morphology evolution of ultrahigh strength 7055 aluminum alloy plate under the hypervelocity impact of aluminum projectile, and analyzes the formation mechanism of crater morphology at different impact velocities. It is found that the impact crater evolves from spherical crown shape to spherical-conical composite shape, and eventually transforms into hemispherical shape with the increase of impact velocity. The evolution of crater shape is highly associated with the ratio of steady-state shock pressure and the strength of the alloy. The variation of crater diameter as a function of impact velocity follows the hypothesis of hemispherical theory, while the crater depth shows a great deviation due to the formation of conical region at the crater bottom and spall fracture at the back of the target. It is demonstrated that the formation of conical crater is related to the poor resistance to shear localization of 7055 aluminum alloy due to its low strain hardening capacity under dynamic loading. Hence, adiabatic shear bands are generated and evolve into shear cracks, thereby causing the formation of conical region at the crater bottom. It is suggested that the resistance to shear localization should be highly considered for the design, evaluation and selection of protection materials for spacecraft.

Influence of UO2 crystal orientation on laser ablation performance

Crystallographic orientation dependence deteriorates the performance of surface analysis methods such as secondary ion mass spectrometry (SIMS) and focused ion beam (FIB). This study explores the corresponding potential challenges of laser ablation (LA) as a powerful sampling tool for inductively coupled plasma-mass spectrometry (ICP-MS). To this end, three UO2 single crystals of different orientation as well as polycrystalline UO2 were produced and characterized. Subsequently, a ns-laser ablation system was employed to study laser-matter interaction in detail. Firing the laser continuously at 1Hz with various single shot fluence (2, 4, 6, 8, 12Jcm-2) for diverse periods created LA craters impacted by cumulative fluence between 50 and 650Jcm-2. Repeated LA experiments on the (100) plane of a UO2 single crystal at the beginning and end of the entire study revealed highly reproducible (<3%) LA rates, only limited by the fluctuation of the laser energy output of the ns-LA system. After thorough cleaning of the ablated samples, surface roughness and average depth of LA craters were determined using confocal laser scanning profilometry. Both LA rate and average depth of craters decreased exponentially with increasing single shot fluence independently of the crystal orientation. Surface roughness increased linearly with increasing cumulative fluence having largest intensification for lowest single shot fluence. Scanning electron microscope (SEM) images not only revealed the conical silhouette of LA craters, but also identified a convex meniscus at its bottom. This particular shape of the crater bottom with a deeper ring surrounding the central region is a result of melted and re-solidified UO2 generated during the LA process and the main limiting factor for the achievable depth resolution. The rapid re-solidification of the liquid phase after each single laser shot created tiles of different shape and orientation, depending on UO2 crystal orientation. Three different types of ejected particles radially distributed around the LA craters were identified by SEM, providing profound insights into laser-UO2 interaction.

Ballistic penetration of high-entropy CrMnFeCoNi alloy: Experiments and modelling

Ballistic impact experiments are conducted on a CrMnFeCoNi high-entropy alloy (HEA) using spherical steel projectiles, to investigate the penetration behavior in a wide range of impact velocities (∼800 to 2300 m s−1). Penetration processes are captured by high-speed camera. Postmortem samples are characterized via three-dimensional laser scanning, scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy and microhardness tests. Both depth and diameter of impact crater show a nearly linear increase with increasing impact velocity. Penetration induces increased hardness; hardness is the same near the crater bottom and the sidewall, and the peak hardness (∼400 Hv1.0) is independent of impact velocity. The penetration-induced hardening is caused by multiple deformation defects including dislocations, twins and kink bands (KBs). Quantitative analysis further reveals that KB boundaries have a 5–50° misorientation angle, and their density decreases with increasing misorientation angle. KB boundary density is higher near the crater bottom than near the crater sidewall, while this trend is inverse for twinning boundary density. A finite element method model based on measured static and dynamic mechanical properties reproduces experimental observations and is used for interpreting deformation mechanisms.

Conical focusing: mechanism for singular jetting from collapsing drop-impact craters

Fast microjets can emerge out of liquid pools from the rebounding of drop-impact craters, or when a bubble bursts at its surface. The fastest jets are the narrowest and are a source of aerosols both from the ocean and from a glass of champagne, of importance to climate and the olfactory senses. The most singular jets, which we observe experimentally at a maximum velocity of $137\pm 4\ {\rm m}\ {\rm s}^{-1}$ and a diameter of $12\ \mathrm {\mu }{\rm m}$ , under reduced ambient pressure, are produced when a small dimple forms at the crater bottom and rebounds without pinching off a small bubble. The radial collapse and rebounding of this dimple is purely inertial, but highly sensitive to initial conditions. High-resolution numerical simulations reveal a new focusing mechanism, which drives the fastest jet within a converging conical channel, where an entrained air sheet provides effective slip at the outer boundary of the conically converging flow into the jet. This configuration bypasses any viscous cutoff of the jetting speed and explains the extreme sensitivity to initial conditions observed in detailed experiments of the phenomenon.

Effect of surface stress concentration control and surface material strengthening on the fatigue performance of shot-peened single-crystal superalloy

This study analyzes the effect of shot peening on the surface integrity of nickel-based single crystal superalloys (SX) and the evolution of stress concentration factors caused by surface finishing and casting micropores near the surface before and after shot peening by numerical simulation. On one hand, SX forms a surface gradient plastic deformation structure after shot peening. The surface yield strength is 82% higher than that of the matrix by nanoindentation fitting. The deformation of the surrounding materials of the near-surface casting micropores is significantly greater than that of the materials at the same depth by EBSD. On the other hand, the series parameters of surface roughness after shot peening are higher than those of grinding. However, the stress concentration Kt1.32 at the bottom of the smooth crater is significantly lower than that of grinding "furrow" Kt1.94. The surface gradient deformation caused by shot peening flattens the surface casting micropores so that the stress concentration at the upper end of the micropores is significantly smaller than that in the grinding state. Based on the above two effects, the rotary bending fatigue limit of the single crystal alloy after shot peening at 650 °C is increased by 13.0% compared with the ground state. The peened sample no longer originates from the casting micropores.

Improving the accuracy of high-repetition-rate LIBS based on laser ablation and scanning parameters optimization.

Laser-induced breakdown spectroscopy system based on high-repetition-rate microchip laser (HR-LIBS) has been widely used in elemental analysis due to its high energy stability, good portability and fast spectral acquisition speed. However, repeated ablation on powder pellets like soil and coal using HR-LIBS system encounters the problem of serious decline in measurement accuracy. In this work, the relationship between laser ablation and scanning parameters, their correlation with spectral intensity, as well as the optimization approach were fundamentally studied. The correlations among the crater overlapping rate, crater depth and spectral intensity were obtained. An HR-LIBS system with microchip laser (4 kHz repetition rate, 100 µJ laser pulse energy) to perform repeated scanning ablation was established. A theoretical model of the ablation crater morphology for repeated scanning ablation was developed. By taking soil pellets as the experimental samples, the linear fitting curves of crater depth and the spectral intensity ratio were established with the R2 of 0.90∼0.99. The experimental results showed that as the crater depth developed during repeated ablation, the Si-normalized spectral intensity decreased, and thus the spectral repeatability decreased. It was found that by optimizing the overlapping rate to form a flat crater bottom, the confinement effect of the crater on the plasma could be avoided. As a result, the spectral repeatability was significantly improved. The relative standard deviation (RSD) of Si-normalized spectral intensity was improved from 5% to 0.6%. Finally, repeated ablation was performed with the optimized overlapping rate on soil pellets. The R2 of calibration curves of Fe, Mg, Ca, and Al were all above 0.993, and the average RSDs were between 0.5% and 1%. This study provides a fast, accurate, and stable method for the analysis of the samples consisting of various materials with high heterogeneity.

Shockwave and spallation in silver and other materials by sub‐ns laser pulse at 1016 W/cm2 intensity

AbstractThe laser spallation effect due to intense shockwaves caused by a brief and intense laser pulse irradiating a target surface, 2 mm thick, has been investigated for silver and other materials. For 300 ps IR laser pulse, at intensities of the order of 1016 W/cm2, the shockwave may produce deformations of the back‐face in ductile materials, such as Ag, Cu and Al. In heavy materials with high tensile strength, such as Ta, the shockwave produces cracks in the bottom of the laser crater but not deformation in the back‐face, while in brittle materials, such as monocrystalline Ge, it produces only superficial cracks and flaking, but not deformation and spallation of the back‐face. In thick polymeric materials, such as high‐density polyethylene, the ablated crater shape is well defined and the shockwave is strongly damped, and no deformation has been observed in the back‐face. The laser ablation yield and the ion acceleration in the backward direction have been measured by mass lost and time‐of‐flight measurements. SEM microscopy of the different irradiated targets, showing details of the crater size, edges, flaking and deformation in the back‐face, useful for a discussion on the shockwave propagation and shock pressure calculation, is presented.

The new lava dome growth of Nevado del Ruiz (2015–2021)

The morphology of the summit of Nevado del Ruiz volcano (Colombia) and its active Arenas crater is the product of complex interactions between effusive and explosive eruptions, and the dynamics of the summit glacier. Here, we document the morphologic evolution of the summit of Nevado del Ruiz, and the growth of its dome, from a variety of methods: monitoring data (2010 to 2021), photogrammetry, remote sensing, and quantitative modeling. The present morphology of Arenas crater, with small terraces limited by the walls of the crater, various vents of ash emission, and zones of fumarolic activity, has been shaped by the activity following the eruptions of 1845, 1985, 1989, 2012 and the volcanic unrest of the last 10 years. The latest emplacement of a lava dome at the bottom of the main crater began in 2015. The dome grew, with fluctuations in its extrusion rate between ~0.19 m3/s (November 2015) and 0.02 m3/s (February 2018), until December 2019, reaching a diameter of ~130 m, a maximum height of ~60 m, and a volume of 1.7 ± 0.2 × 106 m3.

Reactive jet density distribution effect on its penetration behavior

In this paper, the penetration mechanism of reactive jet with non-uniform density distribution is studied. The simulations show that the density deficit occurs in the whole reactive jet, and the density increases from the jet tip to tail. The density of jet tip is approximately 1.5 g/cm3, which is lower than that of the reactive liner materials. The X-ray experiments show similar results with the simulations. The density decreasing effect of jet tip has a significant influence on the penetration behavior when the reactive jet impacts steel plate. According to the simulation results, this paper assumes that the density gradient in the jet section has linear distribution. Then, the deflagration pressure generated by each jet element at the bottom of crater is introduced into the Bernoulli equation. Based on the virtual origin model and Szendrei-Held equation, the analytical models for penetration depth and radial cratering of reactive jet with the density reduction are obtained. Moreover, to further prove the validity of analytical models, the penetration experiments of the reactive liner shaped charge against steel plate under different standoffs are carried out. There is a convergence between the analytical crater profiles and experimental results when reactive jets penetrate steel plates under different standoffs, especially at standoff of 1.5 and 2.0 charge diameters.

Expected life-time of an optical silica fiber intended for open space applications: Probabilistic predictive model

The recently suggested probabilistic design-for-reliability (PDfR) concept in microelectronics and photonics reliability engineering enables quantifying the static fatigue (delayed fracture) lifetime and the corresponding (in effect, never-zero) probability of failure of optical silica fibers intended for open space (outside the spacecraft) applications. In the author's opinion, an adequate operational reliability of space photonics cannot be assured, if it is not quantified, and, because of numerous inevitable intervening uncertainties, such a quantification should be done on the probabilistic basis. Multi-parametric Boltzmann–Arrhenius–Zhurkov (BAZ) constitutive equation is used to design and interpret the results of a highly-cost-effective and highly focused failure-oriented accelerated testing (FOAT). Such testing is both the experimental basis of the PDfR and a physically meaningful proof-test for optical fibers. The combined action of a moderately low temperature, tensile loading, ionizing radiation and random vibrations is considered. Although random vibrations might not be an actual stressor in the application in question, it is nonetheless included in the analysis as a critical stimulus: fracture mechanics studies indicate that fatigue and brittle cracks propagate particularly rapidly, when the material under test is subjected to a low-temperature/vibration bias. The general concept is illustrated by a detailed numerical example. It should be pointed out, however, that the calculated data, although confirm the viability of the suggested methodology, use hypothetical input information and should be viewed just as a suitable illustration, not more. The developed approach is rather broad, and could be applied to any type of optical fibers, including laser fibers, and, perhaps, with some modifications, also to other types of optical, opto-electronic and even non-optical materials and vulnerable structural elements (such as, e.g., solder joint interconnections) intended for highly demanding space applications. The approach could be applied also, as has been indicated by one of the reviewers, in other astrophysics related conditions and locations, where optical fiber instrumentation is intended to be employed in low temperature conditions, such as, e.g., those that take place at the bottom of the Shackleton impact crater at the lunar south pole. The important restriction of our model has to do with the fact that the BAZ equation, which is based on classical thermodynamics, assumes that no failure could possibly occur at absolute zero. Because of that, the suggested model should be used at temperatures, sufficiently remote from the absolute zero. Future work should focus, first of all, on the experimental verification of the suggested approach and model.

Uplift of the Crater Bottom of the Sarychev Peak Volcano (Matua Island, Kurils) in 2017–2019 Inferred from Satellite Images As a Precursor of an Upcoming Strong Eruption

Uplift of the Crater Bottom of the Sarychev Peak Volcano (Matua Island, Kurils) in 2017–2019 Inferred from Satellite Images As a Precursor of an Upcoming Strong Eruption

Near-Infrared Femtosecond Laser Ablation of Au-Coated Ni: Effect of Organic Fluids and Water on Crater Morphology, Ablation Efficiency and Hydrodynamic Properties of NiAu Nanoparticles.

Scanning electron microscopy (SEM) and profilometry of the crater morphology and ablation efficiency upon femtosecond laser ablation of Au-coated Ni targets in various fluids revealed a pronounced dependence on the ablation medium. For ethanol, a sufficient ablation efficiency was obtained, whereas for 2-butanol a higher efficiency indicated stronger laser–target interaction. Hierarchical features in the crater periphery pointed to asymmetrical energy deposition or a residual effect of the Coulomb-explosion-initiating ablation. Significant beam deviation in 2-butanol caused maximum multiple scattering at the crater bottom. The highest values of microstrain and increased grain size, obtained from Williamson–Hall plots, indicated the superposition of mechanical stress, defect formation and propagation of fatigue cracks in the crater circumference. For n-hexane, deposition of frozen droplets in the outer crater region suggested a femtosecond-laser-induced phase explosion. A maximum ablation depth occurred in water, likely due to its high cooling efficiency. Grazing incidence micro X-ray diffraction (GIXRD) of the used target showed residual carbon and partial surface oxidation. The produced nanoparticle colloids were examined by multiangle dynamic light scattering (DLS), employing larger scattering angles for higher sensitivity toward smaller nanoparticles. The smallest nanoparticles were obtained in 2-butanol and ethanol. In n-hexane, floating carbon flakes originated from femtosecond-laser-induced solvent decomposition.

Foreign object damage tolerance and fatigue analysis of induction hardened S38C axles

Flying ballast impact damage on railway axles poses a threat to structural integrity and safety of high-speed trains. A comprehensive evaluation on the fatigue strength debit of axles with foreign object damage (FOD) is vital for their safe operation and maintenance under damage tolerance philosophy. In this study, FODs were simulated by shooting tungsten steel balls to the specimens extracted from surface induction hardened S38C axles. Specifically, tests were performed under three impact velocities, i.e. 200, 300 and 400 m/s, and five incident angles for each impact velocity. Morphologies of impact damage were observed by scanning electron microscope (SEM) as well as the cross-sections. Fatigue strength of impacted specimens were then evaluated. Results show that depth and volume of damaged zone increases with the incident angle at the velocity of 200 m/s. Fatigue strength losses of specimens impacted at velocities of 300 and 400 m/s is around 50% compared with that of smooth ones. Fatigue crack originated from the exit rim for 200 m/s glanced impacted specimens, while multiple cracks propagated from both exit rim and bottom of crater impacted at 300 and 400 m/s. Finally, fatigue strength of damaged specimens was evaluated based on the defect depth concept and Kitagawa–Takahashi (K–T) diagram.

Experimental Investigation on Failure Behaviors of G50 Ultra-High Strength Steel Targets Struck by Tungsten Alloy Spherical Fragments at High Velocity

An experimental investigation is presented into the failure behaviors of the G50 ultra-high strength steel targets struck by tungsten alloy spherical fragments at high velocity. The depth of penetration and the crater volume of G50 steel targets at velocities ranging from 923 to 1,807 m/s are obtained by ballistic gun experiments. A conic-like crater is observed in the G50 steel target after impact by a tungsten alloy spherical fragment, which is different from that in the experiments of low strength steel targets. It is believed that the eroding and fragmentation of the tungsten fragment during the penetration process give rise to this phenomenon. In addition, several tensile cracks are found both at the crater surface and the crater bottom, which are considered to be caused by tensile stress induced by the superposition of rarefaction waves at some local areas of the impacted interface. Numerical simulations of the penetration of tungsten alloy fragments into G50 steel targets are performed to predict failure features of the targets. It is shown that the numerical results are in good agreement with available experimental results.

Self-focusing of UV radiation in 1 mm scale plasma in a deep ablative crater produced by 100 ns, 1 GW KrF laser pulse in the context of ICF

Experiments at the GARPUN KrF laser facility and 2D simulations using the NUTCY code were performed to study the irradiation of metal and polymethyl methacrylate (PMMA) targets by 100 ns UV pulses at intensities up to 5 × 1012 W cm−2. In both targets, a deep crater of length 1 mm was produced owing to the 2D geometry of the supersonic propagation of the ablation front in condensed matter that was pushed sideways by a conical shock wave. Small-scale filamentation of the laser beam caused by thermal self-focusing of radiation in the crater-confined plasma was evidenced by the presence of a microcrater relief on the bottom of the main crater. In translucent PMMA, with a penetration depth for UV light of several hundred micrometers, a long narrow channel of length 1 mm and diameter 30 μm was observed emerging from the crater vertex. Similar channels with a length-to-diameter aspect ratio of ∼1000 were produced by a repeated-pulse KrF laser in PMMA and fused silica glass at an intensity of ∼109 W cm−2. This channel formation is attributed to the effects of radiation self-focusing in the plasma and Kerr self-focusing in a partially transparent target material after shallow-angle reflection by the crater wall. Experimental modeling of the initial stage of inertial confinement fusion-scale direct-drive KrF laser interaction with subcritical coronal plasmas from spherical and cone-type targets using crater-confined plasmas seems to be feasible with increased laser intensity above 1014 W cm−2.

Influence of polycrystalline material on crater shape optimization and roughness using low-power/low-pressure direct-current glow discharge mass spectrometry

It is likely that observation of roughness at crater bottom upon GDMS sputtering is due to differential sputtering of grains.

A multi-species modeling framework for describing supersonic jet-induced cratering in a granular bed: Cratering on Titan case study

The dynamics of cratering caused by a multi-component supersonic jet due to its interaction with the granular soil on the surface of Titan is investigated using a two-phase model and numerical simulations. Both fluid and particles are mathematically described in an Eulerian framework. The fluid is modeled using multi-species Large Eddy Simulation accounting for the details of diffusion among species, and the solid phase is described by a Kinetic-Theory-based model. The general framework is that of a recent model that successfully predicted specific details of Mars/Earth cratering observed by the Mars Science Laboratory (Balakrishnan and Bellan, International Journal of Multiphase Flow, 99, 1–29, 2018), however the distinctive differences between the present and previous model allows new physics to be uncovered that is peculiar to cratering in the presence of a dense atmosphere. Unsteady, three-dimensional simulations are performed to elucidate the morphological features of the crater and the dynamics of its evolution with time. Parametric variations of the initial conditions are conducted to understand the effect of the surface evenness, jet Mach number, particle size in the granular bed, jet composition, and the inter-granular stress coefficient. The new observation of crater-in-a-crater formation at early times is discussed and explained; this phenomenon is transient and the subsequent consumption of the inner crater by the surrounding crater walls is explained in detail. The jet exhibits complex morphological features revealed by vorticity and helicity analysis. The primary-crater walls display ripples along their surface that are traced to the interaction with the dense jet fluid which, having reached the crater bottom, changes direction to exit the crater using the spaces unoccupied by the jet and rubs the crater walls. Detailed analyses of the particle volume fraction and the particle momentum revealed the detailed morphology of the craters and the ejections from the craters, and related them to the initial conditions. A larger jet Mach number results in increased erosion due to the higher momentum content of the jet. A smaller increase in the jet momentum obtained by increasing the jet-fluid molar mass jet results in a crater that is more angled at the top, this feature being due to the increased jet expansion rate. The inter-granular stress coefficient does not affect the large-scale features of the crater, but does affect the peak compaction values. Species diffusion is investigated and it is found that both regular and uphill diffusion occur, the latter being primarily concentrated in the vicinity of the jet and the depths of the crater. The presented detailed formulation and numerical methodology for investigating supersonic jet-induced cratering on granular soil are sufficiently robust to accommodate plume-induced cratering on a variety of planetary bodies having an atmosphere.

Shaped charge penetration into high- and ultrahigh-strength Steel–Fiber reactive powder concrete targets

Abstract Experiments on shaped charge penetration into high and ultrahigh strength steel-fiber reactive powder concrete (RPC) targets were performed in this paper. Results show that the variation of penetration depth and crater diameter with concrete strength is different from that of shaped charge penetration into normal strength concrete (NSC). The crater diameter of RPC is smaller than that of NSC penetrated by the shaped charge. The jet particles are strongly disturbed and hardly reach the crater bottom because they pass through the narrow channel formed by jet penetration into the RPC. The effects of radial drift velocity and gap effects of jet particles for a shaped charge penetration into RPC target are discussed. Moreover, a theoretical model is presented to describe the penetration of shaped charge into RPC target. As the concrete strength increases, the penetration resistance increases and the entrance crater diameter decreases. Given the drift velocity and narrow crater channel, the low-velocity jet particles can hardly reach the crater bottom to increase the penetration depth. Moreover, the narrow channel has a stronger interference to the jet particles with increasing concrete strength; hence, the gap effects must be considered. The drift velocity and gap effects, which are the same as penetration resistance, also have significant effects during the process of shaped charge penetration into ultrahigh-strength concrete. The crater profiles are calculated through a theoretical model, and the results are in good agreement with the experiments.

The buried caldera boundary of the Vesuvius 1631 eruption revealed by present-day soil CO2 concentration

Volcanic risk at Vesuvius is one of the highest in the world due to the ~670,000 inhabitants living in the Red Zone, the area exposed to both pyroclastic flows and tephra fallout, to be evacuated before renewal of any eruptive activity. The national emergency plan for Vesuvius builds its risk zonation on a scenario similar to the last sub-Plinian eruption, which occurred in 1631. This study aims at providing new insights on the geometry of the caldera associated with this historical eruption. The impact of past Vesuvius eruptions on present-day soil CO2 concentration has been investigated by means of an extended geochemical survey carried out for identifying the circulation pathways of hydrothermal fluids inside the volcano. We performed 4018 soil CO2 concentration measurements over the whole Somma-Vesuvius volcanic complex, covering an area of 50 km2. Besides relatively low values, the results show a significant spatial CO2 concentration heterogeneity over Somma-Vesuvius ranging from the atmospheric value (~400 ppm) up to ~24,140 ppm. The summit of Vesuvius shows an area with anomalous CO2 concentrations well matching the crater rim of the 1906 eruption. Along the cone flanks, secondary CO2 anomalies highlight a roughly circular preferential pathway detected along 8 radial profiles at distances between ~840 m and ~1150 m from the bottom of the present-day crater resulting from the last eruption in 1944. In depth review of the available literature highlights an agreement between this circle-like shaped anomaly and the 1631 sub-Plinian eruption caldera boundary. Indeed, based on the historical chronicles the depression produced by the 1631 eruption had a diameter of 1686 m, whereas the CO2 circular anomaly indicates a diameter of 1956 m. Finally, the results were compared with a 3-D density model obtained from a recent gravity survey that corroborates both the literature and the CO2 data in terms of potential buried structure at the base of the Vesuvius cone.