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Rubble pile asteroids such as (162173) Ryugu have large bulk porosities, which are believed to result from void spaces in between the constituent boulders (macroporosity) as well as void spaces within the boulders themselves (microporosity). In general, both macroporosity and microporosity are estimated based on comparisons between the asteroid bulk density and both the bulk and grain density of meteorite analogs, and relatively large macroporosities are usually obtained. Here we use semiempirical models for the macroporosity of multicomponent mixtures to determine Ryugu's macroporosity based on the observed size‐frequency distribution (SFD) of boulders on the surface. We find that Ryugu's macroporosity can be significantly smaller than usually assumed, as the observed SFD allows for an efficient packing of boulders, resulting in a macroporosity of 16% ± 3%. Therefore, we confirm that Ryugu's high bulk porosity is a direct consequence of a very large boulder microporosity. Furthermore, using estimates of boulder microporosity of around 50% as derived from in situ measurements, the average grain density in boulders is 2,848 ± 152 kg m–3, similar to values obtained for CM and the Tagish lake meteorites. Ryugu's bulk porosity corresponding to the above values is 58%. Thus, the macroporosity of rubble pile asteroids may have been systematically overestimated in the past. Plain Language Summary: The carbonaceous asteroid (162173) Ryugu formed from fragments which reaccreted after its parent body was disrupted by a catastrophic collision. Asteroids of this type are also known as rubble piles and the reaccumulation process is thought to be one of the causes for their large bulk porosity. We have applied mixing models to determine the amount of interboulder porosity taking the observed abundance of large and small boulders on the surface into account. We find that the relative abundances of differently sized boulders allow for a very efficient packing, such that interboulder porosity in Ryugu is rather small and only 16% ± 3%. This implies that a large part of Ryugu's total porosity must reside inside the boulders themselves. Using estimates of boulder intrinsic porosity, we furthermore constrain the average density of the boulder's constituent minerals to 2,848 ± 152 kg m−3, which is consistent with values measured for carbonaceous meteorites as collected on Earth. Thus, interboulder porosity of rubble pile asteroids may have been systematically overestimated in the past. Key Points: Ryugu's large bulk porosity is distributed between intrinsic boulder microporosity and macroporosity due to void spaces in‐between bouldersWe use the boulder size‐frequency distribution as observed on the surface together with mixing models to estimate Ryugu's macroporosityWe find that macroporosity is 16% ± 3%, indicating that Ryugu's large bulk porosity of close to 50% is governed by microporosity [ABSTRACT FROM AUTHOR]

Both observations of C-type near-Earth asteroids and laboratory investigations of carbonaceous chondritic meteorites provide strong evidence for a high microporosity of C-type asteroids. Boulder microporosity values derived from in-situ measurements at the surface of the rubble-pile NEA (162173) Ryugu are as high as 55 %, which is substantially higher than for water-rich carbonaceous chondrite samples and could indicate distinct evolution paths for the parent body of Ryugu and parent bodies of carbonaceous chondrites, despite spectral similarities. In the present study, we calculate the evolution of the temperature and porosity for early solar system's planetesimals in order to constrain the range of parameters that result in microporosities compatible with Ryugu's high-porosity material and likely burial depths for the boulders observed at the surface. By varying key properties of the parent body, such as accretion time t 0 and radius R that have strong influence on temperature and porosity and by comparing the interior porosity distribution with the measured boulder microporosity, hydration, and partial dehydration of the material, we constrain a field within the (R , t 0)-diagram appropriate for bodies that are likely to have produced such material. Our calculations indicate a parent body size of only a few km and its early accretion within ≲2 − 3 Myr after the formation of Ca-Al-rich inclusions (CAIs). A gradual final porosity profile of best-fit bodies indicates production of both low- and high-density boulders from the parent body material. By contrast, parent body properties for CI and CM chondrites obtained by fitting carbonate formation data indicate a radius of ≈20 − 25 km and an accretion time of ≈3.75 Myr after CAIs. These results imply a population of km-sized early accreting highly porous planetesimals as parent bodies of the rubble-pile NEA Ryugu (and, potentially, other NEAs) and a population of larger and late accreting less porous planetesimals as parent bodies of water-rich carbonaceous chondrites. • We investigate the porosity for ice-rich planetesimals of the early solar system using numerical modeling. • Our models constrain the conditions which result in parent body porosities that agree with the Ryugu's boulder porosity. • We constrain Ryugu's parent body radius and accretion time in the range of a few km and ≲ 2–3 Myr after CAIs. • We examine a potential connection to water-rich carbonaceous chondrites and find a common parent body unlikely. • Our modeling suggests that Ryugu's parent body belonged to a different planetesimal population than parent bodies of CI and CM chondrites. [ABSTRACT FROM AUTHOR]

We investigated whether the MASCOT/MARA thermal radiometer can be used to determine regolith particle size and porosity on the asteroid Ryugu in the Hayabusa2 mission. We used a one-dimensional heat flow model and our own thermal conductivity model for regolith to compare MARA's performance and the expected Ryugu surface temperatures of various particle sizes and porosities. The results showed that MARA is capable of constraining particle size with ±30% uncertainty if particle size is near 4.25 mm and near-surface porosity is near 60% (a fraction of void spaces between regolith particles in a unit volume), assuming certain physical properties of regolith particles. The porosity cannot be constrained meaningfully through MARA observations alone; however, it is constrained with uncertainty of ±10% around 50% and ±5% around 70% with 2 σ probability if other supportive observations (by the MASCOT camera) provide the particle size. The results also show that nighttime temperature is more sensitive to the particle size and porosity variations and its observation is essential to achieve the small error ranges, while MARA is the only instrument to observe the nighttime temperature on Ryugu. In this study, the surface roughness effect in insolation and thermal radiations was neglected. The roughness effect is presumed to be less influential in nighttime, however, this effect must be carefully considered in actual data analysis of MARA. • Regolith particle size and porosity influenced surface temperature on virtual Ryugu. • MARA on Hayabusa2/MASCOT has a capability to constrain regolith particle size. • Cooperative observation of MARA and MasCam can constrain porosity. [ABSTRACT FROM AUTHOR]