The near-Earth asteroid (3200) Phaethon is the parent body of the Geminid meteor stream. Phaethon is also an active asteroid with a very blue spectrum. We conducted polarimetric observations of this ...asteroid over a wide range of solar phase angles α during its close approach to the Earth in autumn 2016. Our observation revealed that Phaethon exhibits extremely large linear polarization: P = 50.0 ± 1.1% at α = 106.5°, and its maximum is even larger. The strong polarization implies that Phaethon's geometric albedo is lower than the current estimate obtained through radiometric observation. This possibility stems from the potential uncertainty in Phaethon's absolute magnitude. An alternative possibility is that relatively large grains (~300 μm in diameter, presumably due to extensive heating near its perihelion) dominate this asteroid's surface. In addition, the asteroid's surface porosity, if it is substantially large, can also be an effective cause of this polarization.
The high-temperature (33–98 °C), highly acidic (pH 2.44–2.94) Goshogake mud volcano field in Akita Prefecture, northern Japan, is a mud volcano system associated with the Quaternary andesitic ...composite volcano Akita Yakeyama. The mud volcano features observed in Goshogake include salsa ponds, gryphons, and mud pots. This is a rare example of mud volcano systems linked with magmatic volcanism with the full range of landforms observed primarily in low-temperature mud volcanism in sedimentary settings. The Goshogake mud volcano field is probably not a simple hydrothermal system related to a magmatic volcano. Instead, it appears to be a hybrid system: sedimentary volcanism linked with deep mud sources and strongly influenced by its surrounding magmatic volcanism. The identified mineral species in the sampled mud include quartz group, hydrous silica, muscovite, kaolinite, pyrite, and sulfur. The presence of high-temperature silica polymorphs (tridymite and cristobalite) and/or microcrystalline opals (opal-C, opal-CT), and an amorphous form of silica (opal-A) in the mineralogy indicates that the mud volcano system likely involves conditions and processes such as high temperature, hydrothermal circulation, dehydration reaction at certain depths underneath the Goshogake mud volcano field. Although the source of the mud is not clearly determined, possible options include 1) hydrothermal alteration products of host rocks and sediments, 2) fine-grained sedimentary layer(s) at depth, or 3) a combination of 1) and 2). In-situ measurement of emitted gases from vents detected methane, which is possibly linked with the organic-rich sedimentary layer(s) aside from being a product of thermodynamic equilibrium with CO2 under the subsurface pressure-temperature conditions. The increases in δD and δ18O of water in Goshogake mud volcanoes with respect to the values of current meteoric water indicates that the water was derived not only from meteoric water, and deviation of the values may be explained by processes such as evaporation and mixing of other waters from deeper sources. Regarding the driving mechanisms at the Goshogake mud volcanoes, buoyancy of muddy sediment, high fluid pressure under overpressured or under-compacted conditions due to rapid sedimentation, together with the presence of a gas phase in the sediment, could be key factors. Although the Goshogake mud volcano field currently maintains a low-level activity resulting in only minor morphological changes of the mud volcano landforms, enhanced activities have been documented in the last half century. The historical records are inconclusive about the relationship between earthquakes and activity in the Goshogake mud volcano field.
•The Goshogake mud volcano field in Japan is an acidic, high-temperature system.•This active field exhibits a wide range of mud volcano morphology.•The mineralogy indicates high temperature, hydrothermal circulation, dehydration.•The mud originates by alteration, from deep sedimentary layer(s), and/or both.•This field appears to be a hybrid system of sedimentary and magmatic volcanism.
Abstract
Asteroid (99942) Apophis is a potentially hazardous asteroid. The Korea Astronomy and Space Science Institute (KASI) recently proposed a rendezvous mission with Apophis, with a planned ...asteroid observation campaign to help determine the mission strategy in advance. In this study, we performed multicolor photometric observations of Apophis using the 1.0 m telescope at Kawabe Cosmic Park on 2021 March 10. The visible colors (g′, r′, and i′) of Apophis were obtained. The average colors of Apophis during our observations were as follows: g′ − r′ = 0.618 ± 0.021 and r′ − i′ = 0.180 ± 0.021. No significant color variation was observed in r′ − i′, but a slight variation may have occurred in g′ − r′ during the observation period of 4.63 hr. The obtained colors appeared to match those of an Sq-class asteroid, which is the asteroid type previously determined for Apophis by Binzel et al. (2009, Icarus, 200, 480) using the visible to near-infrared (0.55–2.45 μm) reflectance spectrum.
We present the discovery of KELT-24 b, a massive hot Jupiter orbiting a bright (V = 8.3 mag, K = 7.2 mag) young F-star with a period of 5.6 days. The host star, KELT-24 (HD 93148), has a Teff = K, a ...mass of M* = M , a radius of R* = 1.506 0.022 R , and an age of Gyr. Its planetary companion (KELT-24 b) has a radius of RP = 1.272 0.021 RJ and a mass of MP = MJ, and from Doppler tomographic observations, we find that the planet's orbit is well-aligned to its host star's projected spin axis ( ). The young age estimated for KELT-24 suggests that it only recently started to evolve from the zero-age main sequence. KELT-24 is the brightest star known to host a transiting giant planet with a period between 5 and 10 days. Although the circularization timescale is much longer than the age of the system, we do not detect a large eccentricity or significant misalignment that is expected from dynamical migration. The brightness of its host star and its moderate surface gravity make KELT-24b an intriguing target for detailed atmospheric characterization through spectroscopic emission measurements since it would bridge the current literature results that have primarily focused on lower mass hot Jupiters and a few brown dwarfs.
We discuss in this paper possible roles of methane and carbon dioxide in geological processes on Mars. These volatiles in the martian crust may migrate upward from their sources either directly or ...via various traps (structural, sedimentary, ground ice, gas hydrates). They are then likely emitted to the atmosphere by seepage or through diverse vent structures. Though gas hydrates have never been directly detected on Mars, theoretical studies favor their presence in the crust and polar caps; they could have played an important role as significant gas reservoirs in the subsurface. The martian gas hydrates would possibly be a binary system of methane and carbon dioxide occupying clathrate cavities. Landforms such as mud volcanoes with well-known linkage to gas venting are extensively distributed on Earth, and methane is the primary gas involved. Thus, identification of these landforms on Mars could suggest that methane and possibly carbon dioxide have contributed to geological processes of the planet. For example, we present a newly identified field in Chryse Planitia where features closely resembling terrestrial mud volcanoes occur widely, though with no observable activity. We also present results of a preliminary search for possible recent or present-day, methane-emission zones in the regions over which enrichments of atmospheric methane have been reported.
Submarine mud volcanoes (MVs) have attracted significant interest in the scientific community for obtaining clues on the subsurface biosphere. On-land MVs, which are much less focused in this ...context, are equally important, and they may even provide insights also for astrobiology of extraterrestrial mud volcanism. Hereby, we characterized microbial communities of two active methane-seeping on-land MVs, Murono and Kamou, in central Japan. 16S rRNA gene profiling of those sites recovered the dominant archaeal sequences affiliated with methanogens. Anaerobic methanotrophs (ANME), with the subgroups ANME-1b and ANME-3, were recovered only from the Murono site albeit a greatly reduced relative abundance in the community compared to those of typical submarine MVs. The bacterial sequences affiliated to Caldatribacteriota JS1 were recovered from both sites; on the other hand, sulfate-reducing bacteria (SRB) of Desulfobulbaceae was recovered only from the Murono site. The major difference of on-land MVs from submarine MVs is that the high concentrations of sulfate are not always introduced to the subsurface from above. In addition, the XRD analysis of Murono shows the absence of sulfate-, sulfur-related mineral. Therefore, we hypothesize one scenario of ANME-1b and ANME-3 thriving at the depth of the Murono site independently from SRB, which is similar to the situations reported in some other methane-seeping sites with a sulfate-depleted condition. We note that previous investigations speculate that the erupted materials from Murono and Kamou originate from the Miocene marine strata. The fact that SRB (Desulfobulbaceae) capable of associating with ANME-3 was recovered from the Murono site presents an alternative scenario: the old sea-related juvenile water somehow worked as the source of additional sulfur-related components for the SRB-ANME syntrophic consortium forming at a deeper zone of the site. However, the reason for the differences between Murono and Kamou is still unknown, and this requires further investigation.
We are developing an instrument called Circum-Mars Dust Monitor (CMDM), which will be installed on the Mars Moon Explorer (MMX) being developed by JAXA, to observe dust particles in orbit around ...Mars. CMDM aims at in-situ observation of dust particles orbiting around Mars which has been theoretically predicted to exist ranging between tens of μ m and hundreds of μ m in size with impact speed ranging between about 0.2 km/s and about 0.8 km/s. The instrument must be a real-time dust instrument with high reliability to distinguish actual dust events from noise-induced trigger signals. In addition, a sensitive area of at least 1 m2 is required to study the undiscovered Martian dust ring particles to the background flux level of interplanetary dust particles. However, the resources required for the spacecraft system must be kept to a minimum. The sensor unit of the CMDM consists only of a polyimide film and a piezoelectric ceramic element. Therefore, the sensor has a sensitive area of 1 m2 but a mass of only a few hundred grams. In this paper, we describe the results of solving the technical problems in sensor development identified during the conceptual design and the results of sensitivity measurements using a sensor BBM made from materials actually used in-flight products.
The 3U CubeSat "ASTERISC" is a cosmic dust observation satellite developed by Chiba Institute of Technology and Tohoku University, and is planned to be launched in October 2021. ASTERISC is equipped ...with a new type of large-area dust sensor that uses piezoelectric elements to detect elastic waves induced by micron-sized cosmic dust or small space debris colliding with the polyimide film. Since the dust sensor measures only the magnitude of the momentum of the colliding particles, a system to measure and control the direction of the sensor membrane is required to identify the direction of particle flight. By using the classical spin stabilization method, which is rarely used in CubeSats, this satellite aims to maintain the sensor membrane direction toward the Sun constantly with low power consumption. In this paper, we propose the use of spin stabilization control law with magnetic torquers, a method for deriving the target direction vector, and the design of a cosine coarse sun sensor that is aimed to achieve a field of view about twice as large as the conventional method.
Titan is the only satellite that possesses a thick atmosphere, composed mainly of N2 and CH4. However, its origin and evolution remain largely unknown. Knowledge of the acquirement of a N2 atmosphere ...on Titan would provide insights into nitrogen evolution in planetary atmospheres as well as the formation of satellite systems around gas giants. Previous studies have proposed that the atmospheric N2 would have been converted from NH3 via shock heating by accreting satellitesimals in the highly reducing proto-atmosphere composed of NH3 and CH4. Nevertheless, the validity of this mechanism strongly depends on both the composition of the proto-atmosphere and kinetics of shock chemistry. Here, we show that a CO2-rich oxidizing proto-atmosphere is necessary to form N2 from NH3 efficiently by atmospheric shock heating. Efficient shock production of N2 is inhibited in a reducing proto-atmosphere composed of NH3 and CH4, because CH4 plays as the coolant gas owing to its large heat capacity. Our calculations show that the amount of N2 produced in a CO2-rich proto-atmosphere could have reached ~20 times that on the present Titan. Although further quantitative analysis are required (especially, the occurrence of catalytic reactions), our results imply that the chemical composition of satellitesimals that formed the Saturnian system is required to be oxidizing if the current atmospheric N2 is derived from the shock heating in the proto-atmosphere during accretion. This supports the formation of regular satellites in an actively supplied circumplanetary disk using CO2-rich materials originated from the solar nebula at the final stage of gas giant formation.
PDF
