Scientific importance of the sample analyses of Phobos regolith and the analytical protocols ofreturned samples by the MMX mission
Wataru Fujiya1, Yoshihiro Furukawa2, Haruna Sugahara3, Mizuho Koike4, Ken-ichi Bajo5, Nancy L. Chabot6, Yayoi N. Miura7,Frederic Moynier8, Sara S. Russell9, Shogo Tachibana10, Yoshinori Takano11, Tomohiro Usui3, and Michael E. Zolensky12
1
Ibaraki University, 2Tohoku University, 3Institute of Space and Astronautical Science, JAXA, 4Hiroshima University,
5
Hokkaido University, 6Johns Hopkins University Applied Physics Laboratory, 7Earthquake Research Institute, University ofTokyo, 8Institut de Physique du Globe de Paris, CNRS, 9Natural History Museum, 10UTOPS, University of Tokyo, 11Japan
Agency for Marine-Earth Science and Technology, 12NASA Johnson Space Center
The Martian Moons eXploration (MMX) mission by JAXA is a sample return mission from a Martian moon, Phobos, aiming
at collecting >10 g of the regolith materials on Phobos. The touchdown operations are planned to be performed twice at
different landing sites. The regolith materials will be collected using coring (C-) and pneumatic (P-) sampling systems [1]. We,Sample Analysis Working Team (SAWT) members, are now designing the analytical protocols of returned Phobos samples [2].
The origin of the Martian moons is unclear, but there are currently two favored formation scenarios: (i) the in-situ formation(giant impact) scenario [e.g., 3], and (ii) the captured asteroid scenario [e.g., 4]. If Phobos formed by giant impact, then thePhobos building blocks were likely heated to high temperatures (ca., ~2000 K), and the returned samples will consist of
igneous and/or glassy materials produced by the solidification of melt or the condensation of gas [5]. On the other hand, If
Phobos is a captured asteroid, then the returned samples would be primitive materials like carbonaceous chondrites as inferredfrom Phobos’ surface spectra resembling D-type asteroids [e.g., 6]. Observations of the returned samples under an optical
microscope, quantitative analysis of their chemical compositions, and isotope measurements of, e.g., O, Ti, and Cr, candistinguish between the above formation scenarios, and characterize the Phobos endogenous materials [e.g., 7].
In the case of the giant impact scenario, the volatile loss accompanied by the giant impact can be evaluated using the isotopic
compositions of moderately volatile elements like Zn [8]. Radiometric dating by, e.g., Pb-Pb and Rb-Sr systematics, will
provide chronological information about the giant impact event [9]. In the case of the captured asteroid scenario, analyses oforganic matter, bulk H and N isotopic compositions, and presolar grain abundance will provide insights into the primitivenessof the Phobos endogenous materials. The timing when the Martian gravity captured Phobos can be constrained by thecombination of Ar-Ar dating of the returned samples and the crater counting of the Phobos’ surface [10]. In either formation
scenario, the sample analyses mentioned above can shed light on the material transport in the solar system and the delivery ofvolatiles to the terrestrial planets.
It should be noted that the Phobos’ regolith may contain materials ejected from Mars by impact throughout the Martian history[11]. The Martian materials on Phobos may include fragile ones like sedimentary rock, which cannot be found in Martian
meteorites commonly shocked by >5 GPa [12]. These Martian materials will provide crucial information about the surface
evolution of Mars. Furthermore, biomarkers or even potential microorganisms could be detected in the Martian materials on
Phobos, although MMX has no concern about viable Phobos organisms to be returned [13]. We plan to design the analytical
protocols of the returned samples to detect such Martian materials in curation procedures before they are processed for further
analyses.
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2014. Planet. Space Sci. 102:152-163. [11] Hyodo R. et al. (2019) Sci. Rep. 9:19833. [12] Fritz J. et al. 2005. Meteorit. Planet.
Sci. 40:1393-1411. [13] Hyodo R. & Usui T. 2020. Science 373:742.