The elastic property of asteroids is one of the paramount parameters for understanding their physical nature. For example, the rigidity enables us to discuss the asteroid’s shape and surface features such as craters and boulders, leading to a better understanding of geomorphological and geological features on small celestial bodies. The sound velocity allows us to construct an equation of state that is the most fundamental step to simulate the formation of small bodies numerically. Moreover, seismic wave velocities and attenuation factors are useful to account for resurfacing caused by impact-induced seismic shaking. The elastic property of asteroids thus plays an important role in elucidating the asteroid’s evolution and current geological processes. The Hayabusa2 spacecraft brought back the rock samples from C-type asteroid (162173) Ryugu in December 2020. As a part of the initial analysis of returned samples, we measured the seismic wave velocity of the Ryugu samples using the pulse transmission method. We found that P- and S-wave velocities of the Ryugu samples were about 2.1 km/s and 1.2 km/s, respectively. We also estimated Young’s modulus of 6.0 – 8.0 GPa. A comparison of the derived parameters with those of carbonaceous chondrites showed that the Ryugu samples have a similar elastic property to the Tagish Lake meteorite, which may have come from a D-type asteroid. Both Ryugu and Tagish Lake show a high degree of aqueous alteration and few high-temperature components such as chondrules, indicating that they formed in the outer region of the solar system.

We surveyed the subsurface structure of Chryse and Acidalia Planitiae using data from the Mars SHAllow RADar sounder (SHARAD) onboard the Mars Reconnaissance Orbiter (MRO). Several subsurface reflectors were identified in these regions, but these reflectors do not constitute apparent subsurface structures larger than 30 km. The bulk dielectric constants of the uppermost layers at two locations were estimated as 5.3 and 5.9 from the combination of high-resolution images and topographic data. These values were used to constrain the possible bulk porosity (approximately 28 % and 25 %) and the upper limit for the volume fraction of water ice (~43 % and ~50 %). The estimated porosities of the shallow subsurface layers in these locations can be explained by the emplacement of basaltic rock or till that may contain some ice-cemented regolith.

Asteroid 162173 (Ryugu) is a carbonaceous asteroid that was visited by Japan’s Hayabusa2 spacecraft in 2018. The formation mechanism of spinning-top shape of Ryugu is an essential clue to the dynamical history of the near-Earth asteroid. In this study, we address the spin-state evolution of Ryugu induced by the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect, i.e., the thermal recoil torque that changes the rotation period and spin-pole direction. Given the current orbit, spin state, and three-dimensional shape observed by Hayabusa2, we computed the YORP torque exerted on Ryugu using a simplified thermal model approximating zero thermal conductivity. Despite differences in meter-scaled topography, all 20 shape models that we examined indicate that the spin velocity of Ryugu is currently decreasing at a rate of (-0.42—6.3)*10-6 deg/day2. Our findings also suggest that the thermal torque on the asteroid is responsible for maintaining the spin pole upright with respect to the orbital plane. Therefore, the YORP effect could explain the significant spin-down from a period of 3.5 h initially to 7.6 h currently. The corresponding time scale of the rotational deceleration is estimated to be 0.58–8.7 million years, depending on the input shape models. This time scale is comparable to e.g., the formation period of the largest crater, Urashima (5–12 Ma) or the western bulge (2–9 Ma) as derived from previous studies on crater statistics in Ryugu. It is considered that the rotation of the asteroid started to decelerate in the wake of the major crater formation or the resurfacing event on the western hemisphere.

All material that is collected from Mars (gases, dust, rock, regolith) will need to be carefully handled, stored, and analyzed following Earth return to minimize the alteration or contamination that could occur, and to maximize the scientific information that can be extracted from the samples, now and into the future. A Sample Receiving Facility (SRF) would be where the Earth Entry System is opened, and the sample tubes opened and processed after they land on Earth. The Mars Sample Return (MSR) Science Planning Group Phase 2 (MSPG2) was tasked with identifying the steps that encompass the curation activities that would happen within an MSR SRF and any anticipated curation-related requirements. To make the samples accessible for scientific investigation, a series of observations and preliminary analytical measurements would need to be completed to produce a sample catalog for the scientific community. The sample catalog would provide data to make informed requests for samples for scientific investigations and for the approval of allocations of appropriate samples to satisfy these requests. The catalog would include data and information generated during all phases of activity, including data derived from the landed Mars 2020 mission, during sample retrieval and transport to Earth, and upon receipt within the SRF, as well as through the initial sample characterization process, sterilization- and time-sensitive and science investigations. The Initial sample characterization process can be divided into three phases, with increasing complexity and invasiveness: Pre-Basic Characterization (Pre-BC), Basic Characterization (BC), and Preliminary Examination (PE). A significant portion of the Curation Focus Group’s efforts was determining which analyzes and thus instrumentation would be required to produce the sample catalog and how and when certain instrumentation should be used. The goal is to provide enough information for the PIs to request material for their studies but to avoid doing targeted scientific research better left to peer-reviewed competitive processes. Disclaimer: The decision to implement Mars Sample Return will not be finalized until NASA’s completion of the National Environmental Policy Act (NEPA) process. This document is being made available for planning and information purposes only.

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 ~46.1% porosity, or a rock-air-ice mixture with ~21.2% water-ice fraction. Recent climate models suggest that water-ice is unstable on the surface around the equatorial regions. However, considering the recent high obliquity that occurred ~0.4 Ma and a slow diffusivity of water-ice, the existence of subsurface water-ice deeper than a few meters cannot be ruled out. If water-ice is actually contained in the layer, our results show the maximum volume of putative water-ice in the chasmata is 16.6 km.