arXiv subjects

Daigo Shoji

Publications and source records attributed to Daigo Shoji.

16 recordsLinked to original sources

arXiv2025

Raman Spectroscopy of Salt Deposits from the Simulated Subsurface Ocean of Enceladus

Saturn's ice-covered moon Enceladus may host a subsurface ocean with biologically relevant chemistry. Plumes released from this ocean preserve information on its chemical state, and previous analyses suggest weakly to strongly alkaline pH (approximately pH 8--12). Constraining the pH requires identification of pH-sensitive minerals in plume deposits. Several analytical techniques could provide such mineralogical information, but few are practical for deployment on planetary missions. Raman spectrometers, which have recently advanced for \textit{in situ} exploration and have been incorporated into flight instruments, offer a feasible approach for mineral identification on icy moons. However, their applicability to pH estimation from plume-derived minerals has not been investigated. In this study, we evaluate whether Raman measurements of plume particles deposited on the surface of Enceladus can be used to distinguish between weakly and strongly alkaline subsurface ocean models. Fluids with pH values of 9 and 11 were frozen under vacuum conditions analogous to those on Enceladus. The resulting salt deposits were then analyzed using a flight-like Raman spectrometer. The Raman spectra show pH-dependent carbonate precipitation: NaHCO$_3$ and Na$_2$CO$_3$ peaks were detected at pH 9, whereas only Na$_2$CO$_3$ peaks were detected at pH 11. These findings demonstrate that Raman spectroscopy can distinguish pH-dependent carbonate phases. This capability allows us to constrain whether the pH of the subsurface ocean is weakly alkaline or strongly alkaline, which is a key parameter for assessing its chemical evolution and potential habitability.

astro-ph.EP↗

arXiv2025

Exploring Impact Vapor Plume Reactions from Asteroidal Impacts: Monte Carlo Simulations and Implications for Biomolecules Synthesis

During a hypervelocity impact, both the impactor and target materials evaporate, generating an impact vapor plume with temperatures reaching several thousand K. As the plume cools through adiabatic expansion, chemical reactions are predicted to quench, leading to a non-equilibrium composition. However, it is still unclear how chemical reactions proceed during the cooling impact vapor plume and lead to the synthesis of organic molecules. In this study, to investigate the evolution of chemical composition within impact vapor plumes, we conducted a Monte Carlo chemical reaction simulation for complex organic synthesis, developed in our previous work. Our model does not rely on a predefined reaction network; instead, it utilizes imposed conditions for chemical changes and an approximate method for calculating reaction rates suited to our objectives. Additionally, we developed a new approach to couple these chemical reaction calculations with the rapid temperature and pressure decay in the vapor plume. Results show diverse organic molecule production depending on the impactor materials assumed in this study. These products include important precursors to biomolecules such as amino acids, sugars, and nucleobases. On the other hand, for all impactor compositions, the abundance of biomolecules themselves remains extremely low throughout the reactions from an impact to quenching. Therefore, our results suggest that biomolecules are not directly produced in impact vapor plumes but rather synthesized through reactions of these precursor molecules in aqueous solutions, following H2O condensation as the vapor plume cools. Many of the detected organic compounds, including the precursor molecules such as imine compounds and formamide, are not included in the reaction networks of previous kinetic model simulations, and their formation has not been predicted.

astro-ph.EP↗

arXiv2024

Reactive molecular dynamics simulations of micrometeoroid bombardment for space weathering of asteroid (162173) Ryugu

Remote sensing observations by Hayabusa2 and laboratory measurements have revealed that the phyllosilicates on asteroid (162173) Ryugu are dehydrated/dehydroxylated due to space weathering. Reactive molecular dynamics simulations were performed to evaluate the magnitude of the dehydroxylation of Mg-rich serpentine by micrometeoroid impacts. When micrometeoroids were not coupled with interplanetary magnetic fields, serpentine could be dehydroxylated by micrometeoroids as small as 2 nm in size. In particular, ~200 O-H bonds dissociated when the meteoroids were derived from cometary activity (the impact velocity was ~20 km s$^{-1}$). When nano-sized dust particles were accelerated to ~300 km s$^{-1}$ by the magnetic fields of solar wind plasma, the number of dissociated O-H bonds increased by one order of magnitude. Consequently even 1 nm-sized dust particles can contribute to the space weathering of Ryugu. In all cases, Si-OH, H2O, and free OH were generated from the hydroxyls initially connected to Mg, which could partially offset dehydration. Despite the limitations of our computational resources, which restricted the simulation time scale to 1 ps, reactive molecular dynamic simulations demonstrated that micrometeoroid bombardment could influence the space weathering of asteroids.

astro-ph.EP↗

arXiv2024

Monte Carlo simulation of UV-driven synthesis of complex organic molecules on icy grain surfaces

Complex organic molecules (COMs) have been widely observed in molecular clouds and protostellar environments. One of the formation mechanisms of COMs is radical reactions on the icy grain surface driven by UV irradiation. While many experiments have reported that various COMs can be synthesized under such ice conditions, the majority of the reaction processes are unclear. Complementary numerical simulations are necessary to unveil the synthetic process behind the formation of COMs. In this study, we develop a chemical reaction simulation using a Monte Carlo method. To explore the complex reaction network of COM synthesis, the model was designed to eliminate the need to prepare reaction pathways and to keep computational costs low. With this simulation, we investigate the chemical reactions occurring on icy dust surfaces during and after UV irradiation, assuming a protoplanetary disk environment. We aim to reveal the types of organic molecules produced in a disk and the formation mechanisms of COMs, in particular, amino acids and sugars. The results show that photodissociation and subsequent radical-radical reactions cause random rearrangement of the covalent bonds in the initial molecules composed of methanol, formaldehyde, ammonia, and water. Consequently, highly complex molecules such as amino acids and sugars were produced in a wide range of the initial conditions. We found that the final abundances of amino acids and sugars have extremely similar dependence on the atomic ratios of the initial molecules, which peak at C/H~0.1-0.3 and O/H~0.3-0.5, although the amino acids abundance is usually more than ten times higher than that of sugars. To understand this dependence, a semi-analytical formula was derived. Additionally, parameter surveys have suggested that the decomposition reactions of amino acids and sugars undergo a rapid transition within the threshold of a given parameter.

astro-ph.EP↗

arXiv2023

Classification of the lunar surface pattern by AI architectures: Does AI see a rabbit in the Moon?

In Asian countries, there is a tradition that a rabbit, known as the Moon rabbit, lives on the Moon. Typically, two reasons are mentioned for the origin of this tradition. The first reason is that the color pattern of the lunar surface resembles the shape of a rabbit. The second reason is that both the Moon and rabbits are symbols of fertility, as the Moon appears and disappears (i.e., waxing and waning) cyclically and rabbits are known for their high fertility. Considering the latter reason, is the color pattern of the lunar surface not similar to a rabbit? Here, the similarity between rabbit and the lunar surface pattern was evaluated using seven AI architectures. In the test conducted with Contrastive Language-Image Pre-Training (CLIP), which can classify images based on given words, it was assumed that people frequently observe the Moon in the early evening. Under this condition, the lunar surface pattern was found to be more similar to a rabbit than a face in low-latitude regions, while it could also be classified as a face as the latitude increases. This result is consistent with that the oldest literatures about the Moon rabbit were written in India and that a tradition of seeing a human face in the Moon exists in Europe. In a 1000-class test using seven AI architectures, ConvNeXt and CLIP sometimes classified the lunar surface pattern as a rabbit with relatively high probabilities. Cultures are generated by our attitude to the environment. Both dynamic and static similarities may be essential to induce our imagination.

cs.CV↗

arXiv2022

Rabbit, toad, and the Moon: Can machine categorize them into one class?

Recent machine learning algorithms such as neural networks can classify objects and actions in video frames with high accuracy. Here, I discuss a classification of objects based on basal dynamic patterns referencing one tradition, the link between rabbit, toad, and the Moon, which can be seen in several cultures. In order for them to be classified into one class, a basic pattern of behavior (cyclic appearance and disappearance) works as a feature point. A static character such as the shape and time scale of the behavior are not essential for this classification. In cognitive semantics, image schemas are introduced to describe basal patterns of events. If learning of these image schemas is attained, a machine may be able to categorize rabbit, toad, and the Moon as the same class. For learning, video frames that show boundary boxes or segmentation may be helpful. Although this discussion is preliminary and many tasks remain to be solved, the classification based on basal behaviors can be an important topic for cognitive processes and computer science.

cs.CV↗

arXiv2022

Monte Carlo Simulation of Sugar Synthesis on Icy Dust Particles Intermittently Irradiated by UV in a Protoplanetary Disk

Context. While synthesis of organic molecules in molecular clouds or protoplanetary disks is complex, observations of interstellar grains, analyses of carbonaceous chondrites, and UV photochemistry experiments are rapidly developing and providing constraints on and clues to the complex organic molecule synthesis in space. It motivates us to construct a theoretical synthesis model. Aims. We develop a new code to simulate global reaction sequences of organic molecules to apply it for sugar synthesis by intermittent UV irradiation on the surface of icy particles in a protoplanetary disk. Here we show the first results of our new simulation. Methods. We apply a Monte Carlo method to select reaction sequences from all possible reactions, using the graph-theoretic matrix model for chemical reactions and modeling reactions on the icy particles during UV irradiation. Results. We here obtain the results consistent with the organic molecules in carbonaceous chondrites and obtained by the experiments, however, through a different pathway from the conventional formose reactions previously suggested. During UV irradiation, loosely-bonded O-rich large molecules are continuously created and destroyed. After UV irradiation is turned off, the ribose abundance rapidly increases, through the decomposition of the large molecules with break-ups of O-O bonds and replacements of C-OH by C-H to reach O/C = 1 for sugars. The sugar abundance is regulated mostly by the total atomic ratio H/O of starting materials, but not by their specific molecule forms. Deoxyribose is simultaneously synthesized, and most of the molecules end up with complex C-rich molecules.

astro-ph.EP↗

arXiv2021

Tidal evolution of the eccentric moon around dwarf planet (225088) Gonggong

Recent astronomical observations revealed that (225088) Gonggong, a 1000-km-sized trans-Neptunian dwarf planet, hosts an eccentric satellite, Xiangliu, with an eccentricity of approximately 0.3. As the majority of known satellite systems around trans-Neptunian dwarf planets have circular orbits, the observed eccentricity of Gonggong--Xiangliu system may reflect the singular properties of the system. In this study, we assumed that Gonggong--Xiangliu system formed via a giant impact and investigated the following secular tidal evolution of Gonggong--Xiangliu system under the simplifying assumption of homogeneous bodies and of zero orbital inclination. We conducted coupled thermal--orbital evolution simulations using the Andrade viscoelastic model and included higher-order eccentricity functions. The distribution of the final eccentricity from a large number of simulations with different initial conditions revealed that the radius of Xiangliu is not larger than 100 km. We also derived the analytical solution of the semilatus rectum evolution, a function of the radius of Xiangliu. From the point of view of the final semilatus rectum, the radius of Xiangliu was estimated to be close to 100 km. Together with the results of the Hubble Space Telescope observations, our findings suggest Gonggong and Xiangliu have similar albedos.

astro-ph.EP↗

arXiv2020

Enceladus Farm: Can plants grow with Enceladus' water? (Preliminary Report)

Enceladus is a saturnian satellite that should have liquid water inside of it (subsurface ocean). Measurements and experiments on water plume from Enceladus have revealed that Enceladus' ocean contains several salts such as NaCl. On the Earth, salt in soil has become a serious problem for agriculture, and importance of salt-tolerant plants are indicated. In order to test the effect of Enceladus' water to terrestrial plant, by hydroponic, we tried to grow three salt-tolerant plants (ice plant, swiss chard and salicornia) simulating Enceladus' water (Enceladus Farm project). Using water with 0.33% NaCl and 0.4% NaHCO$_3$, which is consistent with the observations of Enceladus, all plants could grow if they were germinated and grown with pure water until each plant had a few leaves. However, growth rate can be suppressed compared with the plants cultivated with pure water. Because our first test was performed with loose conditions, more works are needed to evaluate the effect of Enceladus' water to plant growth. However, in addition to the works to grow plants on lunar and martian grounds, Enceladus' water may be used to consider properties of plant from wider environment.

astro-ph.EP↗

arXiv2019

Angle of repose of Martian wet sand using discrete element method: Implication for the seasonal cycle of recurring slope lineae(RSL) by relative humidity

Seasonal cycle of Martian relative humidity shows that, in the southern and the northern RSL sites, humidity decreases when RSL increment while humidity is high on the fading season. We suggest that the recurrence of RSL is caused by the different angle of response between wet and dry grains. 2D DEM simulation indicates that, when the ratio of a liquid bridge volume to a particle volume is the order of 10^{-7}, the repose angle of Martian sand can increase to ~40 degrees due to cohesion, which is near the slope around RSL starting area. Thus, if this amount of liquid bridge is generated by deliquescence or capillary condensation of vapor and if the water evaporates following the humidity and temperature cycles, Martian sand can induce avalanche on RSL slope. When grains are wet again, sand and dust deposit on RSL, which may be the mechanism of RSL disappearance and recharge of grains for the next season. In our model, because liquid induces RSL indirectly (RSL itself is the granular flow), the amount of liquid can be drastically reduced compared with the wet mechanism. In this case, as well as temperature, the seasonal cycle of relative humidity plays an important role in the recurrence of RSL.

astro-ph.EP↗

arXiv2018

Condition for the deflection of vertical cracks at dissimilar ice interfaces on Europa

The surface of Europa contains many quasi-circular morphologies called lenticulae. Although the formation mechanism of lenticulae is not understood, sill intrusion from the subsurface ocean is one promising hypothesis. However, it remains unclear how vertical cracks from the ocean deflect horizontally to allow sill intrusion in Europa. In this study, the critical stress intensity factor of Europan ice required for deflection was evaluated by considering crack theory at the interface between dissimilar materials and experimental results on ice. For deflection to occur at the interface between two dissimilar ices, the ratio of the critical stress intensity factor of the interface to that of the upper layer should be at most 0.45--0.5. This critical ratio may be attained if the interface is caused by brine-containing ice with a volume fraction of >30 ppt (3%) and pure (no-brine) ice. Thus, a region with a temperature equal to the eutectic point (e.g., an area of approximately 240 K in the convective layer) is a candidate for the region in which the deflection occurs.

astro-ph.EP↗

arXiv2018

Classification of volcanic ash particles using a convolutional neural network and probability

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.

physics.geo-ph↗

arXiv2017

What does a convolutional neural network recognize in the moon?

Many people see a human face or animals in the pattern of the maria on the moon. Although the pattern corresponds to the actual variation in composition of the lunar surface, the culture and environment of each society influence the recognition of these objects (i.e., symbols) as specific entities. In contrast, a convolutional neural network (CNN) recognizes objects from characteristic shapes in a training data set. Using CNN, this study evaluates the probabilities of the pattern of lunar maria categorized into the shape of a crab, a lion and a hare. If Mare Frigoris (a dark band on the moon) is included in the lunar image, the lion is recognized. However, in an image without Mare Frigoris, the hare has the highest probability of recognition. Thus, the recognition of objects similar to the lunar pattern depends on which part of the lunar maria is taken into account. In human recognition, before we find similarities between the lunar maria and objects such as animals, we may be persuaded in advance to see a particular image from our culture and environment and then adjust the lunar pattern to the shape of the imagined object.

cs.CV↗

arXiv2017

Shape recognition of volcanic ash by simple convolutional neural network

Shape analyses of tephra grains result in understanding eruption mechanism of volcanoes. However, we have to define and select parameter set such as convexity for the precise discrimination of tephra grains. Selection of the best parameter set for the recognition of tephra shapes is complicated. Actually, many shape parameters have been suggested. Recently, neural network has made a great success in the field of machine learning. Convolutional neural network can recognize the shape of images without human bias and shape parameters. We applied the simple convolutional neural network developed for the handwritten digits to the recognition of tephra shapes. The network was trained by Morphologi tephra images, and it can recognize the tephra shapes with approximately 90% of accuracy.

physics.geo-ph↗

arXiv2017

Frequency-dependent tidal dissipation in a viscoelastic Saturnian core and expansion of Mimas' semi-major axis

Regarding tidal dissipation in Saturn, usually parameterized by Saturn's quality factor Q, there remains a discrepancy between conventional estimates and the latest determination that has been derived from astrometric observations of Saturn's inner satellites. If dissipation in Saturn is as large as the astrometric observations suggest, conventional models predict that Mimas' initial orbit should be located inside Saturn's synchronous orbit or even inside its Roche limit. Using simple structure models and assuming Saturn's core to be viscoelastic, we look for dissipation models which are consistent with both the latest astrometric observations and with Mimas' orbital migration. Firstly, using a two-layer model of Saturn's interior structure, we constrain the ranges of rigidity and viscosity of Saturn's core which are consistent with Saturn's dissipation derived from astrometric observations at the tidal frequencies of Enceladus, Tethys, and Dione. Next, within the constrained viscosity and rigidity ranges, we calculate Mimas' semi-major axis considering the frequency dependence of viscoelastic dissipation in Saturn's core. By the two calculations, we evaluate (1) Saturnian models which can explain the astrometrically determined Saturnian dissipation, and (2) whether Mimas' initial semi-major axis is larger than the synchronous orbit. We show that if the core is assumed to be solid with a viscosity of 10^{13}-10^{14} Pa s, the lower boundary of the observed Saturnian dissipation at tidal frequencies of Enceladus, Tethys, and Dione can be explained by our model. In this viscosity range, Mimas can stay outside the synchronous orbit and the Roche limit for 4.5 billion years of evolution. In the case of a frequency dependent viscoelastic dissipative core, the lower boundary of the observed Saturnian dissipation can be consistent with the orbital expansion of Mimas.

astro-ph.EP↗

arXiv2014

Thermal-orbital coupled tidal heating and habitability of Martian-sized extrasolar planets around M stars

M type stars are good targets in the search for habitable extrasolar planets. Because of their low effective temperatures, the habitable zone of M stars is very close to the star itself. For planets close to their stars, tidal heating plays an important role in thermal and orbital evolutions, especially when the planet orbit has a relatively large eccentricity. Although tidal heating interacts with the thermal state and orbit of the planet, such coupled calculations for extrasolar planets around M star have not been conducted. We perform coupled calculations using simple structural and orbital models, and analyze the thermal state and habitability of a terrestrial planet. Considering this planet to be Martian sized, the tide heats up and partially melts the mantle, maintaining an equilibrium state if the mass of the star is less than 0.2 times the mass of the Sun and the initial eccentricity of the orbit is more than 0.2. The reduction of heat dissipation due to the melted mantle allows the planet to stay in the habitable zone for more than 10 Gyr even though the orbital distance is small. The surface heat flux at the equilibrium state is between that of Mars and Io. The thermal state of the planet mainly depends on the initial value of the eccentricity and the mass of the star.

astro-ph.EP↗