16:15 〜 16:30
[PCG17-02] MMX/MEGANEの元素組成データを用いたPhobosの起源の制約について
キーワード：元素組成、火星衛星探査、衝突説、捕獲説
Two leading hypotheses for the origin of Phobos and Deimos have been proposed: the asteroid capture [e.g., Burns et al., 1992; Murchie et al., 1991] and the giant impact [e.g., Rosenblatt et al., 2016; Hyodo et al., 2017] hypotheses. The orbital properties of Phobos and Deimos suggest the giant impact scenario, whereas their reflectance spectra, similar to D-type asteroids, support the capture origin. This controversy partly arises from the surface reflectance spectra that would have been modified by space weathering and late accretions. In order to constrain Phobos’ origin, we focus on the bulk elemental composition of Phobos which is expected to preserve the original building blocks, either a captured asteroid or a mixture of the impactor and the Martian crustal materials [Usui et al., 2020].
The Mars-moon Exploration with GAmma rays and NEutrons (MEGANE), which is a science payload of Martian Moons eXploration (MMX) mission, enables measurement of the bulk elemental abundances of Phobos using gamma-ray and neutron spectroscopy [Lawrence et al., 2019]. In this study, we constructed a multivariate mixing model of Phobos’ bulk elemental composition and examined the model’s performance to discriminate the Phobos’ origin hypotheses.
Our mixing model assumes that Phobos consists of a mixture of materials from Mars and chondrite parent bodies. We employed a set of variables that consists of elemental abundances of O, Mg, Si, Ca, Fe, and Th and assumed 13 types of compositions as end-components of the starting building blocks. The Phobos’ composition (P) including a relative error (EP) of 0, 10, and 20%, and the matrix of the Mars and chondrite compositions (M) yield the mixing ratio (R) as a solution of the mixing equation, P=MR. R ranges from 50% in the impact scenario or 100% in the capture scenario. Based on the acquired R, we classified the P into 4 cases; (1) only the capture origin, (2) only the impact origin, (3) both origins, or (4) neither origins reasonably explain the P. Cases-1 and -2 yield the unique solution for the possible origin of Phobos, whereas the cases-3 and -4 do not. Conducting the mixing calculation with the 13 end-member compositions, we determined the case at all P in 6-dimension compositional space. We evaluate our model’s performance to determine the origin of Phobos using the discrimination performance (D), a ratio of data points relating to the unique origin (cases -1 and -2) to all data points (cases -1, -2, and -3).
Discrimination performances calculated with 0, 10, and 20% of EP are 96, 87, and 74%, respectively, suggesting that reducing the observation error of MEGANE improves our model’s performance to determine the Phobos’ origin only from the elemental composition. Also, discrimination performances vary depending on the measured Fe-Si composition of Phobos. For example, some end-components (e.g., L, EL, R) have similar Fe-Si compositions to those of the mixture of Mars and other end-components (e.g., H, EH). These overlaps result in 80, 60, and 40% of the smallest discrimination performances on Fe-Si diagrams calculated with 0, 10, and 20% of EP. If Phobos has a bulk composition similar to these types of chondrites (e.g., L, EL, R), these six elements data from MEGANE may not provide unique constraints on Phobos’ origin without additional MEGANE element measurements and the mineralogical and petrologic information obtained by other onboard instruments and the analysis of the returned sample [Usui et al., 2020].
The relative precision of MEGANE is strongly dependent on the total acquired measurement time and the altitude of the measurements [Peplowski, 2016]. If the MMX mission is able to obtain MEGANE measurements beyond 10 days of accumulated time at an altitude lower than 1 body radius used to provide the current sensitivity estimates [Lawrence et al., 2019], the relative precision of MEGANE’s measurements can be improved.
The Mars-moon Exploration with GAmma rays and NEutrons (MEGANE), which is a science payload of Martian Moons eXploration (MMX) mission, enables measurement of the bulk elemental abundances of Phobos using gamma-ray and neutron spectroscopy [Lawrence et al., 2019]. In this study, we constructed a multivariate mixing model of Phobos’ bulk elemental composition and examined the model’s performance to discriminate the Phobos’ origin hypotheses.
Our mixing model assumes that Phobos consists of a mixture of materials from Mars and chondrite parent bodies. We employed a set of variables that consists of elemental abundances of O, Mg, Si, Ca, Fe, and Th and assumed 13 types of compositions as end-components of the starting building blocks. The Phobos’ composition (P) including a relative error (EP) of 0, 10, and 20%, and the matrix of the Mars and chondrite compositions (M) yield the mixing ratio (R) as a solution of the mixing equation, P=MR. R ranges from 50% in the impact scenario or 100% in the capture scenario. Based on the acquired R, we classified the P into 4 cases; (1) only the capture origin, (2) only the impact origin, (3) both origins, or (4) neither origins reasonably explain the P. Cases-1 and -2 yield the unique solution for the possible origin of Phobos, whereas the cases-3 and -4 do not. Conducting the mixing calculation with the 13 end-member compositions, we determined the case at all P in 6-dimension compositional space. We evaluate our model’s performance to determine the origin of Phobos using the discrimination performance (D), a ratio of data points relating to the unique origin (cases -1 and -2) to all data points (cases -1, -2, and -3).
Discrimination performances calculated with 0, 10, and 20% of EP are 96, 87, and 74%, respectively, suggesting that reducing the observation error of MEGANE improves our model’s performance to determine the Phobos’ origin only from the elemental composition. Also, discrimination performances vary depending on the measured Fe-Si composition of Phobos. For example, some end-components (e.g., L, EL, R) have similar Fe-Si compositions to those of the mixture of Mars and other end-components (e.g., H, EH). These overlaps result in 80, 60, and 40% of the smallest discrimination performances on Fe-Si diagrams calculated with 0, 10, and 20% of EP. If Phobos has a bulk composition similar to these types of chondrites (e.g., L, EL, R), these six elements data from MEGANE may not provide unique constraints on Phobos’ origin without additional MEGANE element measurements and the mineralogical and petrologic information obtained by other onboard instruments and the analysis of the returned sample [Usui et al., 2020].
The relative precision of MEGANE is strongly dependent on the total acquired measurement time and the altitude of the measurements [Peplowski, 2016]. If the MMX mission is able to obtain MEGANE measurements beyond 10 days of accumulated time at an altitude lower than 1 body radius used to provide the current sensitivity estimates [Lawrence et al., 2019], the relative precision of MEGANE’s measurements can be improved.