2:45 PM - 3:00 PM
[PPS03-16] Development of Albedo-based Measurement for Space Weathering Using Multi-Band Images of Itokawa
Introduction: ESA’s Hera Mission is under development for the launch in Oct. 2024 and is to investigate the S-type binary asteroid Didymos-Dimorphos system [1]. Dimorphos has been successfully impacted by spacecraft DART by NASA in Sep. 2022. Hera will investigate the resulting artificial crater and ejecta as well as pre-existing natural craters in detail. Because the artificial ejecta and crater are expected to be much less space weathered than other materials, a low degree of space weathering will be useful for identifying artificially excavated materials. The spatial distribution of different degrees of space weathering on an S-type asteroid was measured by JAXA’s Hayabusa [2, 3]. In particular, analysis of multi-band optical images has shown that color change on Itokawa is dominated by the first principal component (PC1), which is estimated to be due to space weathering [3].
However, because Hera’s asteroid framing camera (AFC), which has by far the highest spatial resolution among the Hera instrument suite, is panchromatic, we cannot use the multi-band approach taken by [2,3]. Nevertheless, Hayabusa’s optical observations have also shown that fresh craters on Itokawa exhibit higher albedo as well as lower spectral slope (i.e., bluer) [2–4](Fig. 1). This observation is consistent with reddening and darkening of ordinary chondrites and S-type asteroids [5]. Using this relation, in this study, we investigate if single-band albedo can be used as a quantitative measure for the degree of space weathering.
Correlation Analysis and Results: We explored this possibility by examining correlation between v-band albedo and PC1 by [3]. We used four bands (b, v, w, and p) of Itokawa images taken at low solar phase angles (<9.45 deg) and calibrated by [2,6] for analysis. The b- w- and p-band images were co-registered with the v-band image in 0.1 pixel increment to obtain the highest correlation coefficient. Regions with a reflectance of 0.1 or less were excluded from the analysis as shadow. We also binned 10x10 pixels before obtaining PC1 from multi-band images to reduce the effect of co-registration error.
The analysis results are shown in Fig. 2, showing a correlation coefficient between PC1 and v-band is 0.78 +/- 0.013. The analysis further shows that if one location on Itokawa has albedo higher than another by >0.022, the former location has higher PC1 than the other by 99%. In other words, we can distinguish fresher and older materials on an S-type asteroid if there is albedo difference more than 0.022 with 99% of confidence. Because the v-band albedo on the locations with the largest and average PC1s on Itokawa are 0.22 and 0.19, respectively, the two are distinguishable using v-band albedo with confidence >99%.
Applications for Itokawa Craters. Because space weathering increases with age, we should be able to identify young craters based on low degree of space weathering. If we can separate young populations of craters well, we should obtain a size frequency distribution (SFD) with a power law similar to that of all craters. Thus, following the analysis by [4], we examined if albedo can separate older and young craters on Itokawa. More specifically, we separated craters with significantly brighter craters as fresher craters and examined their SFD. We found that the SFD of the brighter craters exhibit a power-law slope similar to that of all craters as expected. This result supports that brighter craters may represent young crater population.
Conclusion: The above results suggest that space weathering on S-type asteroids could be estimated from albedo and that this approach may be applicable to Dimorphos.
References
[1] Michel et al. 2022, PSJ 3, 160.
[2] Ishiguro et al. 2007, MAPS 42, 1791.
[3] Koga et al. 2018, Icarus, 299, 386.
[4] Hirata et al. 2009, Icarus, 200,486.
[5] Chapman and Salisbury, 1973, Icarus, 19,507.
[6] Tatsumi et al. 2018, Icarus, 300,227.
However, because Hera’s asteroid framing camera (AFC), which has by far the highest spatial resolution among the Hera instrument suite, is panchromatic, we cannot use the multi-band approach taken by [2,3]. Nevertheless, Hayabusa’s optical observations have also shown that fresh craters on Itokawa exhibit higher albedo as well as lower spectral slope (i.e., bluer) [2–4](Fig. 1). This observation is consistent with reddening and darkening of ordinary chondrites and S-type asteroids [5]. Using this relation, in this study, we investigate if single-band albedo can be used as a quantitative measure for the degree of space weathering.
Correlation Analysis and Results: We explored this possibility by examining correlation between v-band albedo and PC1 by [3]. We used four bands (b, v, w, and p) of Itokawa images taken at low solar phase angles (<9.45 deg) and calibrated by [2,6] for analysis. The b- w- and p-band images were co-registered with the v-band image in 0.1 pixel increment to obtain the highest correlation coefficient. Regions with a reflectance of 0.1 or less were excluded from the analysis as shadow. We also binned 10x10 pixels before obtaining PC1 from multi-band images to reduce the effect of co-registration error.
The analysis results are shown in Fig. 2, showing a correlation coefficient between PC1 and v-band is 0.78 +/- 0.013. The analysis further shows that if one location on Itokawa has albedo higher than another by >0.022, the former location has higher PC1 than the other by 99%. In other words, we can distinguish fresher and older materials on an S-type asteroid if there is albedo difference more than 0.022 with 99% of confidence. Because the v-band albedo on the locations with the largest and average PC1s on Itokawa are 0.22 and 0.19, respectively, the two are distinguishable using v-band albedo with confidence >99%.
Applications for Itokawa Craters. Because space weathering increases with age, we should be able to identify young craters based on low degree of space weathering. If we can separate young populations of craters well, we should obtain a size frequency distribution (SFD) with a power law similar to that of all craters. Thus, following the analysis by [4], we examined if albedo can separate older and young craters on Itokawa. More specifically, we separated craters with significantly brighter craters as fresher craters and examined their SFD. We found that the SFD of the brighter craters exhibit a power-law slope similar to that of all craters as expected. This result supports that brighter craters may represent young crater population.
Conclusion: The above results suggest that space weathering on S-type asteroids could be estimated from albedo and that this approach may be applicable to Dimorphos.
References
[1] Michel et al. 2022, PSJ 3, 160.
[2] Ishiguro et al. 2007, MAPS 42, 1791.
[3] Koga et al. 2018, Icarus, 299, 386.
[4] Hirata et al. 2009, Icarus, 200,486.
[5] Chapman and Salisbury, 1973, Icarus, 19,507.
[6] Tatsumi et al. 2018, Icarus, 300,227.