10:00 〜 10:15
[PPS03-23] Density Distribution within Small Solar System Bodies Based on Smooth Terrain Shape: Asteroid 25143 Itokawa and Comet 67P/Churyumov-Gerasimenko
キーワード：太陽系小天体、小惑星25143イトカワ、チュリモフ・ゲラシメンコ彗星、内部密度分布構造、重力場
Small solar system bodies are the remnants of the early stage of the solar system formation. Shapes and interior structures of small bodies tell us about how they formed and evolved. Particularly, the interior density distribution of small bodies is an important clue for better understanding of planetesimal collisions and subsequent accumulation processes. However, we have much less information about their interiors than their surface properties, partly because it is difficult to measure the exterior gravity precisely in a small body mission. We have investigated a method to estimate density distribution within a small body based on combining spacecraft imagery and the simulated gravity field on the surface of the small body (Kanamaru and Sasaki, 2019; Kanamaru, Sasaki and Wieczorek, in prep.).
If a small body possesses loose regolith sufficiently, over geologic time, impact-induced seismic shaking or cometary activity may have triggered mass movement from a high standing region to a low standing region (Richardson and Bowling, 2014). Our inversion of density distribution assumes that a very flat region on a small body, which is covered with fine gravels and associated with the low standing area in gravitational potential, might approximate an equi-potential surface. Therefore, the inversion processes include (1) mapping a smooth terrain on a 3D shape model of the small body using the Small Body Mapping Tool, and (2) computing dynamic elevations above a reference potential surface given ununiform density distribution (as shown in the upper left panel in Figure). As a criteria for mapping the smooth terrain, we also used a surface roughness defined as the standard deviation of the dynamic elevations within a circle of a specific scale (upper right panel in Figure). The misfit between the smooth terrains and the equi-potential surface is characterized by the standard deviation of the surface elevations within the flat region. On the above assumption of the topographic erosion process, the minimum misfit gives a best fitting density. We applied this method to asteroid Itokawa and comet 67P/Churyumov-Gerasimenko to investigate whether each of their lobes has a different density from the other or not.
There exists three major smooth terrains on Itokawa, named MUSES-C Regio, Sagamihara Regio and Uchinoura Regio. Given different densities to the two lobes of Itokawa (the head and body), the misfit for the smooth terrains is minimized where the head of Itokawa has a higher density than the mean bulk density. Based on the consideration of the systematic errors that arise from the uncertainty of the shape model itself and the calculation error of the gravity field, the allowable range of the head density is estimated to be 2,450 (2,220-2,670) kg/m3 in contrast to the body density of 1,930 (1,976-1,879) kg/m3. Such a great contrast in density between the two lobes is consistent with the independent estimate of the center-of-mass/center-of-figure offset of Itokawa based on the YORP spin-up observation (Lowry et al., 2014).
The comet nucleus of 67P also has a smooth terrain called Hapi, which is likely to be covered with particles falling from other regions. In the density inversion for the two lobes of 67P, the head and body (see the middle left panel in Figure), the best fitting density of the head is similar to the mean bulk density unlike the case of Itokawa (lower panel in Figure). This result indicates a global scaled homogeneity of 67P, which is consistent with the gravity measurement by Patzold et al. (2016). If the bi-lobed shape of the comet was formed by a sub-catastrophic disruption of its parent body and merger of two main fragments (Jutzi and Benz, 2017), it is possible for the two lobes to have the same density.
If a small body possesses loose regolith sufficiently, over geologic time, impact-induced seismic shaking or cometary activity may have triggered mass movement from a high standing region to a low standing region (Richardson and Bowling, 2014). Our inversion of density distribution assumes that a very flat region on a small body, which is covered with fine gravels and associated with the low standing area in gravitational potential, might approximate an equi-potential surface. Therefore, the inversion processes include (1) mapping a smooth terrain on a 3D shape model of the small body using the Small Body Mapping Tool, and (2) computing dynamic elevations above a reference potential surface given ununiform density distribution (as shown in the upper left panel in Figure). As a criteria for mapping the smooth terrain, we also used a surface roughness defined as the standard deviation of the dynamic elevations within a circle of a specific scale (upper right panel in Figure). The misfit between the smooth terrains and the equi-potential surface is characterized by the standard deviation of the surface elevations within the flat region. On the above assumption of the topographic erosion process, the minimum misfit gives a best fitting density. We applied this method to asteroid Itokawa and comet 67P/Churyumov-Gerasimenko to investigate whether each of their lobes has a different density from the other or not.
There exists three major smooth terrains on Itokawa, named MUSES-C Regio, Sagamihara Regio and Uchinoura Regio. Given different densities to the two lobes of Itokawa (the head and body), the misfit for the smooth terrains is minimized where the head of Itokawa has a higher density than the mean bulk density. Based on the consideration of the systematic errors that arise from the uncertainty of the shape model itself and the calculation error of the gravity field, the allowable range of the head density is estimated to be 2,450 (2,220-2,670) kg/m3 in contrast to the body density of 1,930 (1,976-1,879) kg/m3. Such a great contrast in density between the two lobes is consistent with the independent estimate of the center-of-mass/center-of-figure offset of Itokawa based on the YORP spin-up observation (Lowry et al., 2014).
The comet nucleus of 67P also has a smooth terrain called Hapi, which is likely to be covered with particles falling from other regions. In the density inversion for the two lobes of 67P, the head and body (see the middle left panel in Figure), the best fitting density of the head is similar to the mean bulk density unlike the case of Itokawa (lower panel in Figure). This result indicates a global scaled homogeneity of 67P, which is consistent with the gravity measurement by Patzold et al. (2016). If the bi-lobed shape of the comet was formed by a sub-catastrophic disruption of its parent body and merger of two main fragments (Jutzi and Benz, 2017), it is possible for the two lobes to have the same density.