Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu

Grott, M.; Knollenberg, J.; Hamm, Maximilian; Ogawa, Kazunori; Jaumann, R.; Otto, Katharina A.; Delbò, M.; Michel, Patrick; Biele, Jens; Neumann, Wladimir; Knapmeyer, Martin; Kührt, Ekkehard; Senshu, Hiroki; Okada, T.; Helbert, J.; Maturilli, Alessandro; Müller, N.; Hagermann, A.; Sakatani, Naoya; Tanaka, Satoshi; Arai, Takehiko; Mottola, S.; Tachibana, Shogo; Pelivan, Ivanka; Drube, Line; Vincent, Jean‐Baptiste; Yano, Hajime; Pilorget, C.; Matz, K.-D.; Schmitz, Nicole; Koncz, Alexander; Schröder, Stefan; Trauthan, Frank; Schlotterer, Markus; Krause, Christian; Ho, Tra‐Mi; Moussi‐Soffys, Aurélie

doi:10.1038/s41550-019-0832-x

Cited by 144 publications

(169 citation statements)

References 41 publications

(37 reference statements)

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“…We can either take into account the bulk density as measured by Hayabusa2 (Watanabe et al, ) or assume 50% porosity yielding the density of the material reaching values that are close to carbonaceous chondrites densities. This value is also in a range of porosities (between 22% and 55%) derived from MARA instrument measurements of thermal inertia and thermal conductivity (Grott et al, ).…”

Section: Resultsmentioning

confidence: 72%

Magnetic Properties of Asteroid (162173) Ryugu

et al. 2020

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Observations of the magnetization state of asteroids indicate diverse properties. Values between 1.9 × 10 −6 Am 2 /kg (Eros) and 10 −2 Am 2 /kg (Braille) have been reported. A more detailed understanding of asteroidal magnetic properties allows far-reaching conclusions of the magnetization mechanism as well as the strength of the magnetic field of the solar system regions the asteroid formed in. The Hayabusa2 mission with its lander Mobile Asteroid Surface Scout is equipped with a magnetometer experiment, MasMag. MasMag is a state-of-the-art three-axis fluxgate magnetometer, successfully operated also on Philae, the Rosetta mission lander. MasMag has enabled, after Eros for the second time ever, to determine the magnetic field of an asteroid during descent and on-surface operations. The new observations show that Ryugu, a low-albedo C-type asteroid, has no detectable global magnetization, and any local magnetization is either small (< 10 −6 Am 2 /kg) or on very small (subcentimeter) scales. This implies, for example, that energetic solar wind particles could reach and alter the surface unimpeded by strong asteroidal magnetic fields, such as minimagnetospheres in case of the Moon.Plain Language Summary Magnetic measurements in space near and on solar system bodies such as asteroids can provide important information about their formation history and their material properties. Hayabusa2, a Japanese mission, visited asteroid Ryugu, a type of asteroid (carbon rich) not visited before. Ryugu is a rubble pile, that is, an agglomeration of rocks and boulders. It is not expected to have any global magnetic field, but it can be magnetized on smaller scales (boulders or pebbles). Hayabusa2 carried therefore a small lander called Mobile Asteroid Surface Scout equipped also with an instrument for magnetic field measurement (magnetometer). In this study, we present observations from the magnetometer that were collected during Mobile Asteroid Surface Scout's descent and landing on the surface of Ryugu. The magnetic measurements show that Ryugu is not magnetized on boulder (greater than centimeter) scales. This gives us indication on its origin and evolution. In particular, it shows that Ryugu and the bodies it was created from did not possess any magnetic field generation mechanism and that they were not created in an environment with strong background magnetic field. The results are important inputs for theories about the solar system evolution that work with magnetic field as one of the drivers for dust accretion and planetary formation.

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Section: Resultsmentioning

confidence: 72%

Magnetic Properties of Asteroid (162173) Ryugu

et al. 2020

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“…Close-up thermal images show the surface physical state and a variety of boulders at centimeter scale. As was seen by optical navigation cameras and the surface landers, it was evident that the surface of Ryugu is widely covered with several centimeter-to meter-sized rocks and boulders, but not with fine regolith [20,27,28]. The surroundings in the global and local thermal images correspond to the rough surface terrains that are covered with boulders down to the tens-of-centimeter scale in the close-up thermal images (see Figure 5).…”

Section: Close-up Thermal Images Of Ryugumentioning

confidence: 77%

“…This implies that the surface physical state of Ryugu is consistent with very porous and rough surface, where boulders have high porosity (30% to 50 %) and the rest of the surrounding surface is dominated by fragments of porous rocks several centimeters larger than the thermal skin depth [4]. It is important that the surface physical state predicted from the global thermal images was confirmed later by the close-up thermal imaging by TIR and also by the optical surface imaging and thermal radiation measurements from the surface lander Mobile Asteroid Surface Scout (MASCOT) [27,28].…”

Section: Global Thermal Images Of Ryugumentioning

confidence: 99%

Thermography of Asteroid and Future Applications in Space Missions

Okada

2020

Applied Sciences

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Featured Application: Activities in lunar, planetary, and asteroid science and exploration.Abstract: The Near-Earth Asteroid 162173 Ryugu is a C-type asteroid which preserves information about the ancient Solar System and is considered enriched in volatiles such as water and organics associated with the building blocks of life, and it is a potentially hazardous object that might impact Earth. Hayabusa2 is the asteroid explorer organized by the Japan Aerospace Exploration Agency to rendezvous with the asteroid and collect surface materials to return them to Earth. Thermography has been carried out from Hayabusa2 during the asteroid proximity phase, to unveil the thermophysical properties of the primitive Solar System small body, which offered a new insight for understanding the origin and evolution of the Solar System, and demonstrated the technology for future applications in space missions. Global, local, and close-up thermal images taken from various distances from the asteroid strongly contributed to the mission success, including suitable landing site selection, safe assessment during descents into the thermal environments and hazardous boulder abundance, and the detection of deployable devices against the sunlit asteroid surface. Potential applications of thermography in future planetary missions are introduced.

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“…Initial results from OSIRIS-REx and Hayabusa2 indicate that the rocks on both asteroids may have very low thermal conductivities. Recently, Grott et al (2019) determined that the boulder on the surface of Ryugu has a thermal . Λ s values were calculated using the effective particle diameter for each geometry.…”

Section: Journal Of Geophysical Research: Planetsmentioning

confidence: 99%

“…Two reference k m values are displayed with vertical lines/bars: the Cold Bokkeveld (CM2) meteorite (Opeil et al, 2010) and the Ryugu boulder examined by the MARA radiometer data . Model input values: macroporosity = 0.40; specific heat = 750 J kg −1 K −1 (a); particle density is a function of thermal conductivity, using the average of two models (b, c) for meteorite thermal conductivity versus microporosity and assuming an average CM grain density = 2,960 kg m The rotation period, P, for Bennu is 15,466 s , thermal inertia, TI, is assumed at 200 J m −2 K −1 s −1/2 , specific heat, c p , is 750 J kg −1 K −1 (Biele et al, 2019), and bulk density, ρ, is estimated as a function of k m , using the average of two models by Henke et al (2016) and Flynn et al (2018; see Methods section in Grott et al, 2019), assuming a grain density of 2,960 kg m −3 (Flynn et al, 2018) and a regolith macroporosity of 0.40.…”

Section: /2019je006100mentioning

confidence: 99%

Full‐Field Modeling of Heat Transfer in Asteroid Regolith: 1. Radiative Thermal Conductivity of Polydisperse Particulates

et al. 2020

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Characterizing the surface material of an asteroid is important for understanding its geology and for informing mission decisions, such as the selection of a sample site. Diurnal surface temperature amplitudes are directly related to the thermal properties of the materials on the surface. We describe a numerical model for studying the thermal conductivity of particulate regolith in vacuum. Heat diffusion and surface‐to‐surface radiation calculations are performed using the finite element (FE) method in three‐dimensional meshed geometries of randomly packed spherical particles. We validate the model for test cases where the total solid and radiative conductivity values of particulates with monodisperse particle size frequency distributions (SFDs) are determined at steady‐state thermal conditions. Then, we use the model to study the bulk radiative thermal conductivity of particulates with polydisperse, cumulative power law particle SFDs. We show that for each polydisperse particulate geometry tested, there is a corresponding monodisperse geometry with some effective particle diameter that has an identical radiative thermal conductivity. These effective diameters are found to correspond very well to the Sauter mean particle diameter, which is essentially the surface area‐weighted mean. Next, we show that the thermal conductivity of the particle material can have an important effect on the radiative component of the thermal conductivity of particulates, especially if the particle material conductivity is very low or the spheres are relatively large, owing to non‐isothermality in each particle. We provide an empirical correlation to predict the effects of non‐isothermality on radiative thermal conductivity in both monodisperse and polydisperse particulates.

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Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu

Cited by 144 publications

References 41 publications

Magnetic Properties of Asteroid (162173) Ryugu

Magnetic Properties of Asteroid (162173) Ryugu

Thermography of Asteroid and Future Applications in Space Missions

Full‐Field Modeling of Heat Transfer in Asteroid Regolith: 1. Radiative Thermal Conductivity of Polydisperse Particulates

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