Analytical protocols for Phobos regolith samples returned by the Martian Moons eXploration (MMX) mission

Abstract

Japan Aerospace Exploration Agency (JAXA) will launch a spacecraft in 2024 for a sample return mission from Phobos (Martian Moons eXploration: MMX). Touchdown operations are planned to be performed twice at different landing sites on the Phobos surface to collect > 10 g of the Phobos surface materials with coring and pneumatic sampling systems on board. The Sample Analysis Working Team (SAWT) of MMX is now designing analytical protocols of the returned Phobos samples to shed light on the origin of the Martian moons as well as the evolution of the Mars–moon system. Observations of petrology and mineralogy, and measurements of bulk chemical compositions and stable isotopic ratios of, e.g., O, Cr, Ti, and Zn can provide crucial information about the origin of Phobos. If Phobos is a captured asteroid composed of primitive chondritic materials, as inferred from its reflectance spectra, geochemical data including the nature of organic matter as well as bulk H and N isotopic compositions characterize the volatile materials in the samples and constrain the type of the captured asteroid. Cosmogenic and solar wind components, most pronounced in noble gas isotopic compositions, can reveal surface processes on Phobos. Long- and short-lived radionuclide chronometry such as 53Mn–53Cr and 87Rb–87Sr systematics can date pivotal events like impacts, thermal metamorphism, and aqueous alteration on Phobos. It should be noted that the Phobos regolith is expected to contain a small amount of materials delivered from Mars, which may be physically and chemically different from any Martian meteorites in our collection and thus are particularly precious. The analysis plan will be designed to detect such Martian materials, if any, from the returned samples dominated by the endogenous Phobos materials in curation procedures at JAXA before they are processed for further analyses.

Martian moons exploration MMX: sample return mission to Phobos elucidating formation processes of habitable planets

“…Measurement of 53 Cr/ 54 Cr ratio with a precision of 10 -6 enough to resolve source body type (Trinquier et al 2008) requires a sample grain of a few mg or size ~ 1 mm if the Cr concentration is ~ 3000 ppm equivalent to carbonaceous chondrites (Fujiya et al 2021). Here, a silicate grain with a density of 3 g/cm 3 is supposed.…”

“…For Itokawa samples with tens μm, oxygen isotope analysis was achieved with precision sufficient to resolve source body type (e.g., Yamamoto et al 2011;Yurimoto et al 2011;Nakashima et al 2013). Other isotope systems such as 50 Ti/ 47 Ti are also valuable (Warren 2011) and measurable with sufficient precision for ~ 1 mm grains (Fujiya et al 2021). The majority of Phobos sample grains are expected to concentrate in a certain compositional range which shows the Phobos indigenous material and source body type.…”

“…Both methods that use extinct radionuclides for model age determination require refractory grain and altered grain with ~ 10 μm, respectively (e.g., Kita et al 2013;Fujiya et al 2012), which are probably available if Phobos is similar to the carbonaceous chondrites in its composition. A high-precision absolute age determination using U-Pb systematics requires a refractory grain of several mm in size (Fujiya et al 2021); the availability of such a large grain might be limited considering the expected typical grain size.…”

“…Since the current strategy of MMX sample analyses is reported in detail elsewhere (Usui et al 2020;Fujiya et al 2021), here we briefly summarize its essence.…”

“…For organics-bearing grains, which is highly expected for the case of capture origin of Phobos, a series of volatile analyses including cavity ring-down laser absorption spectroscopy (CRDS) and gas chromatography/mass spectrometry will be applied depending on the gain mass and volatile concentration. See Fujiya et al (2021) for details including other analysis techniques to be applied.…”