Analyses of volcanic ash are typically performed either by qualitatively classifying ash particles by eye or by quantitatively parameterizing its shape and texture. While complex shapes can be ...classified through qualitative analyses, the results are subjective due to the difficulty of categorizing complex shapes into a single class. Although quantitative analyses are objective, selection of shape parameters is required. Here, we applied a convolutional neural network (CNN) for the classification of volcanic ash. First, we defined four basal particle shapes (blocky, vesicular, elongated, rounded) generated by different eruption mechanisms (e.g., brittle fragmentation), and then trained the CNN using particles composed of only one basal shape. The CNN could recognize the basal shapes with over 90% accuracy. Using the trained network, we classified ash particles composed of multiple basal shapes based on the output of the network, which can be interpreted as a mixing ratio of the four basal shapes. Clustering of samples by the averaged probabilities and the intensity is consistent with the eruption type. The mixing ratio output by the CNN can be used to quantitatively classify complex shapes in nature without categorizing forcibly and without the need for shape parameters, which may lead to a new taxonomy.
As volcanic stratigraphy provides important information about volcanic activities, such as the eruption style, duration, magnitude, and their time sequences, its observation and description are ...fundamental tasks for volcanologists. Since outcrops are often obscured in nature, the first task would be identifying stratigraphic exposures in many cases. This identification/selection process has depended on humans and has difficulties in terms of time and effort consumption and in biases resulting from expertise levels. To address this issue, we present an approach that utilizes supervised machine learning with fine-tuning and forms the backbone to automatically extract the areas of stratigraphic exposures in visible images of volcanic outcrops. This study aimed to develop an automated method for identifying exposed stratigraphy. This method will aid in planning subsequent field investigations and quickly outputting results. We used U-Net and LinkNet, convolutional neural network architectures developed for image segmentation. Our dataset comprised 75 terrestrial outcrop images and their corresponding images with manually masked stratigraphic exposure areas. Aiming to recognize stratigraphic exposures in various situations, the original images include unnecessary objects such as sky and vegetation. Then, we compared 27 models with varying network architectures, hyperparameters, and training techniques. The highest validation accuracy was obtained by the model trained using the U-Net, fine-tuning, and ResNet50 backbone. Some of our trained U-Net and LinkNet models successfully excluded the sky and had difficulties in excluding vegetation, artifacts, and talus. Further surveys of reasonable training settings and network structures for obtaining higher prediction fidelities in lower time and effort costs are necessary. In this study, we demonstrated the usability of image segmentation algorithms in the observation and description of geological outcrops, which are often challenging for non-experts. Such approaches can contribute to passing accumulated knowledge on to future generations. The autonomous detection of stratigraphic exposures could enhance the output from the vast collection of remote sensing images obtained not only on Earth but also on other planetary bodies, such as Mars.
TIR, the thermal infrared imager on Hayabusa2, acquired high-resolution thermal images of the asteroid 162173 Ryugu for one asteroid rotation period on August 1, 2018 to investigate the ...thermophysical properties of the asteroid. The surface temperatures of Ryugu suggest that the surface has a low thermal inertia, indicating the presence of porous materials. Thermophysical models that neglect or oversimplify surface roughness cannot reproduce the flat diurnal temperature profiles observed during daytime. We performed numerical simulations of a thermophysical model, including the effects of roughness on the diurnal brightness temperature, the predictions of which successfully reproduced the observed diurnal variation of temperature. The global thermal inertia was obtained with a standard deviation of 225 ± 45 J m−2 s−0.5 K−1, which is relatively low but still within the range of the value estimated in our previous study (Okada et al., Nature 579, 518–522, 2020), confirming that the boulders on Ryugu are more porous in nature than typical carbonaceous chondrites. The global surface roughness (the ratio of the variance of the height relative to a local horizontal surface length) was determined as 0.41 ± 0.08, corresponding to a RMS surface slope of 47 ± 5°. We identified a slightly lower roughness distributed along the equatorial ridge, implying a mass movement of boulders from the equatorial ridge to the mid-latitudes.
•Thermophysical model using a shape model of rough surface reproduces observations.•We obtain a global thermal inertia of 225 ± 45 J m−2 s−0.5 K−1.•We obtain a global RMS surface slope of 47 ± 5°.•We confirm Ryugu's low global thermal inertia owing to porous constituent boulders.
One of the primary goals of Hayabusa2 is to land on the asteroid Ryugu to collect its surface materials. The key for a successful touchdown is to find a promising landing site that meets both ...scientific and engineering requirements. Due to the limited availability of pre-arrival information about Ryugu, the landing site selection (LSS) must be conducted based on proximity observations over a limited length of time. In addition, Ryugu was discovered to possess an unexpectedly high abundance of boulders with an absence of wide and flat areas, further complicating the LSS. To resolve these problems, we developed a systematic and stepwise LSS process with a focus on the surface topography of Ryugu and the associated touchdown safety. The proposed LSS scheme consists of two phases: Phase-I LSS, a comprehensive survey of potential landing areas at the 100-m scale based on the global mapping of Ryugu, and Phase-II LSS, a narrowing-down process of the candidate landing sites at the 10-m scale using high-resolution images and a local terrain model. To verify the feasibility of a precision landing at the target site, we also investigated the landing dispersion via a Monte Carlo simulation, which incorporates the effect of the irregular surface gravity field. One of the major characteristics of the Hayabusa2 LSS developed in this study is the iterative feedback between LSS analyses on the ground and actual spacecraft operations near the target asteroid. Using the newly developed method, we chose a landing site with a radius of 3 m, and Hayabusa2 successfully conducted its first touchdown on February 21, 2019. This paper reports the methodology and results of the stepwise iterative LSS for the first Hayabusa2 touchdown. The touchdown operation results reconstructed from flight data are also provided, demonstrating the validity of the adopted LSS strategy.
Martian magmatism within recent several hundreds of millions years is still controversial. Central Elysium Planitia (CEP) is suspected as a site of the latest magmatism on Mars, but hot debates have ...been caused as for the origin of this flat plain. Cones in CEP are expected to be a key to resolve this controversy. In previous works, there are 2 models proposed for the origin of CEP cones: volcanic rootless cone (e.g. Jaeger et al., 2007) and periglacial pingo (e.g. Burr et al., 2002; Page et al., 2009). In this study, we described detail morphology, distribution and size of CEP cones by using high-resolution images and topographic data. CEP cones are classified into 3 morphological types: Single Cone (SC), Double Cone (DC), and Lotus Fruit Cone (LC). DC has an inner cone in the summit crater of the outer cone, and LC has several inner cones in the summit crater of the outer cone. Several cones have moat structure around the edifice with peripheral rise. DCs and LCs are located in very flat areas of Athabasca Valles in the vicinity of Cerberus Fossae, while SCs distribute in the entire region of CEP. We compared CEP cones with terrestrial rootless cones and pingos in aerial photos. In Lake Myvatn, Iceland, there exist rootless cones which resemble DCs and LCs in CEP. Based on the similarities with terrestrial analogies, we concluded that the most feasible origin of CEP cones is rootless cones.
•In Central Elysium Planitia on Mars, we described cones morphology and compared with terrestrial cone morphologies to clarify its origin.•It is revealed that CEP cones have 4 unique characteristics; Double Cone (DC), Lotus Fruit Cone (LC), the moat structure, and the cone trail.•DCs and LCs distribute in vicinity of Cerberus Fossae mainly and some of them are aligned parallel to the young flow direction.•From result of comparison, we judged CEP cones are rootless cones, and indicated existence of magmatism on recent 10 Myr Mars.•Unique characteristics of cones suggest that Athabasca Valles was in the specific situation compared to other rootless cone fields on Mars.
Chryse and Acidalia Planitiae (CAP) are known as one of the areas which have abundant sites of recurring slope lineae (RSL) on Mars. We present a radar survey of shallow subsurface structures across ...the CAP regions using the Mars SHAllow RADar sounder (SHARAD) onboard the Mars Reconnaissance Orbiter (MRO). A total of 25 subsurface reflectors were identified. The detected reflectors do not constitute apparent subsurface structures larger than 30 km. Because those have no counterparts in nearby tracks, we could not suggest that those are real subsurface structures. This study suggests that the CAP region does not have special wide-spread subsurface features which could be linked to the RSL sites.
•We found subsurface reflectors at 25 locations across Chryse and Acidalia Planitiae, without structures larger than 30 km.•Because those have no counterparts in nearby tracks, we could not conclude that those are real subsurface structures.•Chryse and Acidalia Planitiae have abundant Recurring Slope Lineae, but the detected reflectors were not linked to them.
Explosive phreatomagmatic eruption is one of the most hazardous eruption styles, particularly in basaltic systems, as the instability of the conduit system can result in a sudden and unexpected shift ...of eruption style from a mild effusion of lavas to violently explosive activity. The geological investigations on the phreatomagmatic activities in the 7th Century, Suoana-Kazahaya eruption (SKE) of Miyakejima, reveal that the drop of magmatic overpressure in conduit and the distribution of groundwater controlled the occurrence of phreatomagmatic explosion. The “dry” magmatic eruption in the initial phase of the SKE indicates that the positive overpressure of magma in the propagating feeder dike prevents the invasion of external groundwater into the conduit. Explosive phreatomagmatic eruption occurred at the later phase of the SKE from the vents in the higher elevation. The drop of magmatic overpressure in the upper part of the feeder dike caused by the downslope propagation of the eruption fissure allowed groundwater inflow to the still hot and molten feeder dikes. The limited distribution of phreatomagmatic activities indicated the development of shallow groundwater, hosted in the porous pyroclastic deposits within a basin of less-permeable older edifice. As shifts of eruption style from initial magmatic to later phreatomagmatic explosive eruption style in the top of mafic volcanoes are globally observed in many mafic volcanic systems, such as Kilauea and Mt. Etna, this is probably a far more common eruption mechanism, than previously thought, and hence it needs to be considered in volcanic hazard scenario descriptions. The spatial distributions of phreatomagmatic activities in the SKE suggest that the detection of buried caldera structure in a volcanic edifice can indicate a potential site for phreatomagmatic explosion. The monitoring of the propagation of eruption fissure and drops of magmatic pressure and flux can indicate the potential of the phreatomagmatic explosion by the invasion of groundwater into the hot conduit.
•Downslope migration of magma caused the phreatomagmatic explosions in Miyakejima.•Buried caldera accommodates groundwater in the volcanic edifice for magma-water interaction.•Drop of magmatic overpressure caused the invasion of groundwater to the conduit.•Geology of edifice and dike-propagation pattern show the potential of phreatomagmatic explosion.
We surveyed the subsurface structure in eastern Coprates and Capri Chasmata in the equatorial region using high‐resolution visible images, digital terrain models, and radar sounding data. We ...identified subsurface reflectors in four areas of the chasmata. At the stratigraphic exposure on the chasmata walls, the corresponding depth of the reflector is ∼60 m. The bulk dielectric constants of the layers above the reflectors are calculated as 3.4–4.0, suggesting a rock‐air mixture with ∼39.3% and 46.1% porosity or a rock‐air‐ice mixture with <21.2% water ice fraction. This high porosity corresponds to nonwelded and unconsolidated sediments emplaced by aeolian, fluvial, and volcanic activities. If water ice actually exists, further studies and discussions are required for the mechanism to maintain it within low latitudes.
Plain Language Summary
To uncover the Martian subsurface world, we investigated eastern Coprates and Capri Chasmata in Valles Marineris, one of the largest exposures in the near‐equatorial region. We explored the subsurface structure using a combination of high‐resolution images, topographic data, and subsurface radar sounding data. In this section, we found four areas that have a subsurface reflector. Estimated dielectric constants constrain the possible bulk porosity (∼39.3% and 46.1%) and the upper limit of the possible volume fraction of water ice (∼21.2%) in the uppermost layer in this region.
Key Points
We identified subsurface reflectors in four areas in the eastern Coprates and Capri Chasmata based on SHARAD data
At the chasmata outcrop, the bulk dielectric constant of the uppermost layer was estimated to be 3.4–4.0
Estimated dielectric constants constrain the possible bulk porosity and the upper limit of the possible volume fraction of water ice
Asteroid 162173 Ryugu has numerous craters. The initial measurement of impact craters on Ryugu, by Sugita et al. (2019), is based on Hayabusa2 ONC images obtained during the first month after the ...arrival of Hayabusa2 in June 2018. Utilizing new images taken until February 2019, we constructed a global impact crater catalogue of Ryugu, which includes all craters larger than 20 m in diameter on the surface of Ryugu. As a result, we identified 77 craters on the surface of Ryugu. Ryugu shows variation in crater density which cannot be explained by the randomness of cratering; there are more craters at lower latitudes and fewer at higher latitudes, and fewer craters in the western bulge (160°E – 290°E) than in the region around the meridian (300°E – 30°E). This variation implies a complicated geologic history for Ryugu. It seems that the variation in crater density indicates that the equatorial ridge located in the western hemisphere is relatively young, while that located in the eastern hemisphere is a fossil structure formed during the short rotational period in the distant past.
•We examined the spatial distribution of impact craters on Ryugu.•We completed a global impact crater catalogue of Ryugu (D > 20 m).•Crater density variations cannot be explained by the randomness of cratering.•More craters are seen at lower latitudes and less at higher latitudes.•There are fewer craters in the western bulge and more around the meridian.
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