Formation process of the Martian moons that can be revealed from the elemental composition observations: Evaluation of MMX MEGANE’s performance using Phobos’ elemental composition model

HIRATA Kaori / Dept. of Solar System Science, ISAS,
Dept. of Earth and Planetary Science, the University of Tokyo

“How were the two Martian moons, Phobos and Deimos, formed?” There are two leading hypotheses: the asteroid-capture origin, which is suggested by their surface color and texture, and the giant-impact origin, which reasonably explains their orbital properties. JAXA’s Martian Moons eXploration (MMX) mission aims to answer the question by comprehensive scientific observations and detailed ground analysis of samples returned from the surface of Phobos.

This study focuses on examinations of the elemental composition of Phobos to distinguish different formation processes. Phobos’ elemental composition is modeled assuming each of the two formation hypotheses to examine to what extent the resultant Phobos compositions are similar to or distinct from each other.

Using this model, this study reported that the formation hypothesis of Phobos can be discriminated based on the observations that will be captured by the MEGANE instrument onboard MMX, which will measure the surface elemental composition of Phobos, with a performance of ~70%. Furthermore, MEGANE observations might potentially determine the asteroid type of a captured body or an impactor. MEGANE observations, along with other science observations, are expected to make a significant contribution to achieving the science goal of MMX.

Research Summary

The two Martian moons, Phobos and Deimos, have been studied by observations from spacecrafts and with ground-based telescopes. However, the origin of the Martian moons still remains controversial with two leading hypotheses: the asteroid-capture origin, where an asteroid was gravitationally captured when it passed near Mars, and the giant-impact origin, where the Martian moons were formed from the accumulation of debris in a circum-Martian disk formed by a giant impact with Mars (Fig. 1). Elemental composition is the key to distinguishing these two origins. In the case of the asteroid-capture hypothesis, the composition of the Martian moons should correspond to the captured asteroid, while an impact-origin is thought to lead to an intermediate mix between that of Mars (BSM: Bulk Silicate Mars composition*1) and that of the impactor (Fig. 1). JAXA’s Martian Moons eXploration (MMX) aims to reveal the origin of Martian moons and plans to measure the average elemental composition of the upper 1 meter of the surface of Phobos using the gamma-ray neutron spectrometer MEGANE*2, which has been developed at the Johns Hopkins Applied Physics Laboratory on behalf of NASA. This study aims to discriminate the formation hypotheses based on the elemental composition of Phobos observed by MEGANE in the future, considering the realistic constraints such as the observation error of MEGANE and the uncertainty of type and composition of a captured/impacted body.

Fig1. Formation hypotheses of the Martian moons and their compositions. (Left) Capture origin: An asteroid that passed by Mars was gravitationally captured and became a Martian moon. Material originated from the captured asteroid constitute the Martian moons. (Right) Impact origin: The accumulation of debris in a circum-Martian disk formed in a giant impact with Mars formed the Martian moons. The mixture of material originating from the impactor and material excavated and ejected from Mars constitute the Martian moon.

We constructed a mixing model that represented the elemental composition of Phobos from a mixing of Martian and asteroidal compositions: 0% Martian component +100% asteroidal component for the capture origin, and 50% Martian component +50% asteroidal component for the impact origin (Fig. 2). Assuming the two formation hypotheses and compositions of 12 chondrite meteorites*3 for asteroidal components, 24 cases of different formation processes and the resultant elemental compositions of Phobos were modeled. Using 6 lithophile elements*4 measurable by MEGANE (Fe, Si, O, Ca, Mg, and Th), we investigated to what extent the compositions overlapped or differed from each other.

Fig2. A schematic diagram of the elemental composition model of Phobos. The elemental composition of Phobos is represented by a mixture of the compositions of Mars (red) and asteroids (blue or green). In the capture origin, Phobos’ composition is equivalent to that of chondrite meteorite associated with the captured asteroid. On the other hand, in the impact origin, Phobos has an intermediate composition between those of the chondrite corresponding to the impactor of Mars. Considering multiple types of chondritic compositions, some compositions (indicated by an arrow) can be explained either by the capture and/or impact origins.

This study quantitatively showed that the distinguishability between the formation hypotheses from the compositional observations by MEGANE depended on its observation error (Fig. 3). Assuming the errors from the initial instrument requirement (20-30%), the ability to distinguish the formation origins was calculated to be ~70%. Furthermore, when the formation origin is determined uniquely, the type of captured/impacted asteroid could also be determined uniquely from among 12 types with a probability of about 50%.

Fig3. The relationship between modeled Fe-Si compositions of Phobos and its possible origins for MEGANE observation errors of 0-30%. This study classifies Phobos compositions that will be measured by MEGANE in the future into four cases: Compositions that can be explained [1] only by the capture origin (yellow), [2] only by the impact origin (blue), [3] by either origin (gray), and [4] by neither origins (black). “MEGANE’s discrimination performances” were quantified by the proportion of compositions related to a unique origin, i.e., [1] or [2].

Our elemental composition model and the data analysis can be applied to the MEGANE observation data acquired by MMX in the future. For further advancements, the types of asteroidal compositions could be added or based on other scientific observations by MMX, progressing towards a better understanding of the formation process. The elemental composition observations of Phobos by MEGANE, together with other science observations by MMX, are expected to contribute greatly to the understanding of the origin of the Martian moons.


  • *1 Bulk Silicate Mars composition: The average composition of the silicate portion, i.e., the crust and mantle composed of rocks, of Mars. The giant impact origin of the Martian moons predicts that the crust and mantle materials of Mars are ejected to the space by the impact event and compose a part of the Martian moons .
  • *2 MEGANE: Mars-moon Exploration with GAmma rays and Neutrons, the gamma-ray and neutron spectrometer onboard the MMX spacecraft. MEGANE will measure the elemental composition by detecting gamma-rays or neutrons generated by atoms constituting surface materials when galactic cosmic rays are incident.
  • *3 Chondrite: Stony meteorites, which are composed mainly of silicate minerals rather than metals, that contains a granular texture called chondrules inside. Chondrules are thought to have been formed by the rapid cooling of silicate minerals molten like magma and are preserved without re-melting. Thus, parent bodies of chondrites are considered to be primitive bodies that have not experienced differentiation in high temperatures.
  • *4 Lithophile elements: Elements that tend to be concentrated in the silicate phase during differentiation a homogeneous melting state. For example, Na, Mg, Al, Si, etc. Incidentally, Siderophile elements include those tend to concentrate in the metallic phase along with iron and Atmophile elements include those tend to become gaseous.


Journal Title Icarus
Full title of the paper Mixing model of Phobos’ bulk elemental composition for the determination of its origin: Multivariate analysis of MMX/MEGANE data
Publish date 24 November 2023 (Available online), 1 March 2024 (Issue published)
Author(s) Kaori Hirata, Tomohiro Usui, Ryuki Hyodo, Hidenori Genda, Ryota Fukai, David J. Lawrence, Nancy L. Chabot, Patrick N. Peplowski, Hiroki Kusano
ISAS or JAXA member(s) among author(s) HIRATA Kaori (Dept. of Solar System Science, ISAS), USUI Tomohiro (Dept. of Solar System Science, ISAS), HYODO Ryuki (Dept. of Solar System Science, ISAS), FUKAI Ryota (Dept. of Solar System Science, ISAS)




PhD student in the Department of Earth and Planetary Science, Faculty of Science, the University of Tokyo, and in the Department of Solar System Science, Institute of Space and Astronautical Science. Since August 2023, she has been working as an International Visiting Expert at European Space Astronomy Centre (ESAC), European Space Agency (ESA).