2:00 PM - 2:15 PM
[PPS04-02] Constraints on the origin of Phobos by the multivariate analysis of MMX MEGANE data using a compositional database of astromaterials
★Invited Papers
Keywords:Martian moons, formation hypothesis, elemental composition, multivariate analysis, gamma-ray and neutron spectroscopy
The formation process of the two Martian moons (Phobos and Deimos) is still in debate between two competing hypotheses: The capture of an asteroid by Mars gravity [e.g., 1] and the in-situ formation from the circum-Martian disk produced by a giant impact [e.g., 2]. Martian Moons eXploration (MMX) mission aims to reveal the formation and evolution processes of the Martian moons and the Mars system by remote sensing and returned sample analysis [e.g., 3,4,5]. MEGANE (Mars-moon Exploration with GAmma rays and NEutrons) onboard MMX is a gamma-ray and neutron spectrometer that will measure the elemental composition of Phobos [6]. The goal of this study is to provide the method of MEGANE data analysis to determine the formation hypothesis using the multivariate analysis of the elemental composition database of astromaterials.
We construct the mixing model to express the bulk composition of Phobos as the linear mixture of two components, materials from Mars and those from an asteroid. The end-member compositions are assumed as the bulk silicate Mars [7] and 12 chondrite subgroups [8,9]. The calculation uses the elemental abundances (wt. %) of six lithophile elements (Fe, Si, O, Ca, Mg, and Th) measurable by MEGANE. The mixing ratio range is calculated for a given Phobos composition and a given chondrite composition, assuming relative errors for MEGANE observation (EP) and the compositional variation. The yielded mixing ratio judges the formation hypotheses for a given Phobos composition. The discrimination performance (Dhypothesis) is defined as the indicator of to what extent the formation hypothesis is discriminated and quantifies the parameter dependency in our model.
Our model visualizes the relationship between MEGANE data and the formation hypotheses in the 6-element compositional space. When we assume that Phobos consists of the materials from Mars (50%) and the impactor (50%) in the impact hypothesis [10], at least 65% of the compositional area determines the hypothesis uniquely (EP = 30%) when all 6 elements are considered. The discrimination performance increases from 74 to 87% as EP decreases from 20 to 10%. Even if we conservatively assume the mixing ratio criteria of 30-70% for the impact hypothesis, the behavior of the discrimination performance does not change largely. The discrimination performance depends on EP, which is influenced by the characteristics of the MEGANE operation plan such as accumulation time and trajectory altitude. The MEGANE instrument performance and the MMX initial operation plan suggest EP of 20–30% [6], resulting in ~70% discrimination performance. As an application of this model, the chondrite subgroup for a captured or impacted asteroid can be determined by our analysis flow. Assuming the expected MEGANE errors [6], our results suggest that approximately 30% of the compositional area uniquely yields the asteroid type.
MEGANE also measures the abundance of some moderately volatile elements. If we add the elemental abundance of K (wt. %) to our calculation, the discrimination performance is improved by 5–10%. However, in the impact hypothesis, it is inferred that volatiles have preferentially been lost from the impact-induced disk [11], making it difficult to use the volatile abundance for the determination of the formation hypothesis. Nevertheless, the volatile abundance measured by MEGANE may enable the estimate of degassing rate from disk materials.
[1] Burns, 1992, Mars, 1283-1301. [2] Rosenblatt+, 2016, Nat. Geosci., 9(8), 581-583. [3] Kuramoto+, 2021, Earth Planet. Space, 74, 12. [4] Nakamura+, 2021, Earth Planet. Space, 73, 227. [5] Usui+, 2020, Space Sci. Rev., 216, 49. [6] Lawrence+, 2019, Earth Space Sci., 6, 2605-2623. [7] Taylor, 2013, Chemie der Erde, 73, 401-420. [8] Alexander, 2019b, Geochim. Cosmochim. Acta, 254, 277-309. [9] Alexander, 2019a, Geochim. Cosmochim. Acta, 254, 246-276. [10] Hyodo+, 2017, Astrophys. J., 845(2), 125. [11] Hyodo+, 2018, Astrophys. J., 860(2), 150.
We construct the mixing model to express the bulk composition of Phobos as the linear mixture of two components, materials from Mars and those from an asteroid. The end-member compositions are assumed as the bulk silicate Mars [7] and 12 chondrite subgroups [8,9]. The calculation uses the elemental abundances (wt. %) of six lithophile elements (Fe, Si, O, Ca, Mg, and Th) measurable by MEGANE. The mixing ratio range is calculated for a given Phobos composition and a given chondrite composition, assuming relative errors for MEGANE observation (EP) and the compositional variation. The yielded mixing ratio judges the formation hypotheses for a given Phobos composition. The discrimination performance (Dhypothesis) is defined as the indicator of to what extent the formation hypothesis is discriminated and quantifies the parameter dependency in our model.
Our model visualizes the relationship between MEGANE data and the formation hypotheses in the 6-element compositional space. When we assume that Phobos consists of the materials from Mars (50%) and the impactor (50%) in the impact hypothesis [10], at least 65% of the compositional area determines the hypothesis uniquely (EP = 30%) when all 6 elements are considered. The discrimination performance increases from 74 to 87% as EP decreases from 20 to 10%. Even if we conservatively assume the mixing ratio criteria of 30-70% for the impact hypothesis, the behavior of the discrimination performance does not change largely. The discrimination performance depends on EP, which is influenced by the characteristics of the MEGANE operation plan such as accumulation time and trajectory altitude. The MEGANE instrument performance and the MMX initial operation plan suggest EP of 20–30% [6], resulting in ~70% discrimination performance. As an application of this model, the chondrite subgroup for a captured or impacted asteroid can be determined by our analysis flow. Assuming the expected MEGANE errors [6], our results suggest that approximately 30% of the compositional area uniquely yields the asteroid type.
MEGANE also measures the abundance of some moderately volatile elements. If we add the elemental abundance of K (wt. %) to our calculation, the discrimination performance is improved by 5–10%. However, in the impact hypothesis, it is inferred that volatiles have preferentially been lost from the impact-induced disk [11], making it difficult to use the volatile abundance for the determination of the formation hypothesis. Nevertheless, the volatile abundance measured by MEGANE may enable the estimate of degassing rate from disk materials.
[1] Burns, 1992, Mars, 1283-1301. [2] Rosenblatt+, 2016, Nat. Geosci., 9(8), 581-583. [3] Kuramoto+, 2021, Earth Planet. Space, 74, 12. [4] Nakamura+, 2021, Earth Planet. Space, 73, 227. [5] Usui+, 2020, Space Sci. Rev., 216, 49. [6] Lawrence+, 2019, Earth Space Sci., 6, 2605-2623. [7] Taylor, 2013, Chemie der Erde, 73, 401-420. [8] Alexander, 2019b, Geochim. Cosmochim. Acta, 254, 277-309. [9] Alexander, 2019a, Geochim. Cosmochim. Acta, 254, 246-276. [10] Hyodo+, 2017, Astrophys. J., 845(2), 125. [11] Hyodo+, 2018, Astrophys. J., 860(2), 150.