9:15 AM - 9:30 AM
[MZZ40-02] Space weathering distribution and resurfacing on asteroids
Hera is a ESA-JAXA joint mission to investigate the effect of DART impact on asteroid Dimorphos (Michel+, 2022). The cratering efficiency of DART impact was so large that a global deformation on Deimophos; a classical bowl shaped crater was unlikely formed (Raducan & Jutzi, 2023 :LPSC). Thus, we cannot use geomorphology for discriminating newly exposed materials from pre-existing surface materials. Thus, we need to use other methods for detecting freshly excavated material. An example is the degree of space weathering. Nakahara+(2023) showed that space weathering maps can be created based on albedo maps obtained from opposition observations. However, resurfacing due to seismic shaking occurs simultaneously with space weathering on the asteroid. In fact, a histogram of space weathering distribution on an S-type asteroid Itokawa has been obtained, which may reflect such combined effects (Koga+,2018). We also found that a similar histogram is found for albedo on Itokawa (Fig. 1A). Nevertheless, a method to quantitatively extract information on surface conditions from such histograms has not been established. Such a method would be useful for analyzing the data from the Hera mission, too. Thus, the goal of this study is to clarify how the space weathering distribution is related to resurfacing processes and space weathering progression.
We built a simple two-layer model that simulates competition between aging due to space weathering and rejuvenation due to resurfacing on the rubble pile asteroids. Based on the results of laser irradiation experiments on previous laboratory studies, we found that reflectance change due to space weathering can be expressed as an exponential function with an offset. Based on the recent observation (Noguchi+,2023) that the degree of space weathering is so small in the subsurface layer on a rubble pile, we assumed that only the surface layer accumulates space weathering and that the subsurface layer retains no space weathering. The spatial extent of resurfacing due to impact is examined for both an ideal case, where resurfacing area is constant, and the realistic case, where the size distribution follows a power-law distribution. The model parameters were adjusted to simulate Itokawa, for which space weathering distributions have been observed. Then, we examined statistical properties, such as the average and skewness of the distribution as a function of surface exposure time
The calculation results show that the space weathering distribution is controlled by two factors: the time scale ratio of resurfacing to space weathering and the ratio of camera resolution to the typical size of resurfacing. When the size of resurfacing area is always the same, the direction of histogram distortion (i.e., whether the histogram is distorted to the left or right) is determined by the time scale ratio. Furthermore, when resurfacing size is small, the histogram approaches a normal distribution. If resurfacing size follows a power law, the distribution is also controlled by the time scale ratio. When the power law index is large, histograms tend to have a sharp peak. Based on the constant size model, the time scale of resurfacing is estimated to be 1~3 Myr on Itokawa. This age is consistent with the cosmic ray exposure time (<9 Myr) based on Ne isotope measurements on Itokawa (Nagao+, 2011).
Regarding the applications to Hera data analysis, the above result suggests that ejecta smaller than the pixel size could contribute to change in the shape of albedo histogram. This opens up the possibility of detecting sub-pixel size ejecta on Dimorphos, suggesting that near-opposition observations of Dimorphos with AFC and Hyperscout-H might be useful for detecting thinly spread fresh ejecta in the distal locations.
Fig. 1: (A) Albedo histograms observed on S-type Asteroid Itokawa. It shows a distinctive distortion toward low albedo values, suggesting that the Itokawa surface has greater surface area for longer surface exposure age. (B) Dependence of albedo histogram shape on the ratio tau_sw/tau_rsf of space weathering timescale to resurfacing timescale as well as the ratio of camera resolution to resurfacing size.
We built a simple two-layer model that simulates competition between aging due to space weathering and rejuvenation due to resurfacing on the rubble pile asteroids. Based on the results of laser irradiation experiments on previous laboratory studies, we found that reflectance change due to space weathering can be expressed as an exponential function with an offset. Based on the recent observation (Noguchi+,2023) that the degree of space weathering is so small in the subsurface layer on a rubble pile, we assumed that only the surface layer accumulates space weathering and that the subsurface layer retains no space weathering. The spatial extent of resurfacing due to impact is examined for both an ideal case, where resurfacing area is constant, and the realistic case, where the size distribution follows a power-law distribution. The model parameters were adjusted to simulate Itokawa, for which space weathering distributions have been observed. Then, we examined statistical properties, such as the average and skewness of the distribution as a function of surface exposure time
The calculation results show that the space weathering distribution is controlled by two factors: the time scale ratio of resurfacing to space weathering and the ratio of camera resolution to the typical size of resurfacing. When the size of resurfacing area is always the same, the direction of histogram distortion (i.e., whether the histogram is distorted to the left or right) is determined by the time scale ratio. Furthermore, when resurfacing size is small, the histogram approaches a normal distribution. If resurfacing size follows a power law, the distribution is also controlled by the time scale ratio. When the power law index is large, histograms tend to have a sharp peak. Based on the constant size model, the time scale of resurfacing is estimated to be 1~3 Myr on Itokawa. This age is consistent with the cosmic ray exposure time (<9 Myr) based on Ne isotope measurements on Itokawa (Nagao+, 2011).
Regarding the applications to Hera data analysis, the above result suggests that ejecta smaller than the pixel size could contribute to change in the shape of albedo histogram. This opens up the possibility of detecting sub-pixel size ejecta on Dimorphos, suggesting that near-opposition observations of Dimorphos with AFC and Hyperscout-H might be useful for detecting thinly spread fresh ejecta in the distal locations.
Fig. 1: (A) Albedo histograms observed on S-type Asteroid Itokawa. It shows a distinctive distortion toward low albedo values, suggesting that the Itokawa surface has greater surface area for longer surface exposure age. (B) Dependence of albedo histogram shape on the ratio tau_sw/tau_rsf of space weathering timescale to resurfacing timescale as well as the ratio of camera resolution to resurfacing size.