Transport of impact ejecta from Mars to its moons as a means to reveal Martian history

Abstract

Throughout the history of the solar system, Mars has experienced continuous asteroidal impacts. These impacts have produced impact-generated Mars ejecta, and a fraction of this debris is delivered to Earth as Martian meteorites. Another fraction of the ejecta is delivered to the moons of Mars, Phobos and Deimos. Here, we studied the amount and condition of recent delivery of impact ejecta from Mars to its moons. Using state-of-the-art numerical approaches, we report, for the first time, that materials delivered from Mars to its moons are physically and chemically different from the Martian meteorites, which are all igneous rocks with a limited range of ages. We show that Mars ejecta mixed in the regolith of its moons potentially covers all its geological eras and consists of all types of rocks, from sedimentary to igneous. A Martian moons sample-return mission will bring such materials back to Earth, and the samples will provide a wealth of “time-resolved” geochemical information about the evolution of Martian surface environments.

The Importance of Phobos Sample Return for Understanding the Mars-Moon System

“…Samples collected by MMX are expected to represent a mixture of endogenous and exogenous materials in Phobos' regolith. The former represents Phobos' building blocks that record information of the moon's origin, while the latter is expected to contain solar system projectiles and ejecta derived from Mars and Deimos (Ramsley and Head 2013;Nayak et al 2016;Hyodo et al 2019). Although the depth profile of Phobos regolith regarding material distribution is unknown, the ratio of [exogenous /endogenous] abundances is expected to be highest at the top-most regolith layer, which is where sampling will occur.…”

“…The regolith of Phobos is likely to contain ejecta derived from Mars and Deimos due to Phobos' location in Mars' orbit (Ramsley and Head 2013;Nayak et al 2016;Hyodo et al 2019). Though the exact nature of Deimos-originating material is unknown, the spectral similarity between Deimos and the dominant red unit of Phobos suggests that Deimosoriginating materials resemble the endogenous Phobos materials (Fraeman et al 2012); note that Phobos' surface shows two distinct units named "blue unit" and "red unit" after the relative slopes of their reflectance spectra (Murchie and Erard 1996).…”

“…On the other hand, Mars-originating materials would differ from the endogenous Phobos materials even if Phobos was formed by a giant impact. The endogenous Phobos material formed by a giant impact should have sampled both crust and mantle reservoirs of Mars (Hyodo et al 2017), whereas the later Mars-originating materials would have mostly derived from the near-surface of Mars (Hyodo et al 2019). The Martian near-surface materials have oxygen ( 17 O), hydrogen (D/H), and carbon (δ 13 C) isotopic compositions that are distinct from those of the Martian crust and mantle reservoirs (Leshin et al 1996;Farquhar and Thiemens 2000;Usui et al 2012;Agee et al 2013;Webster et al 2013;Usui et al 2015).…”

“…The young Amazonian terranes are dominated by anhydrous ferric oxides such as hematite, suggestive of acidic and drier conditions. Meteorite impacts would have randomly sampled these Martian surficial materials, some of which would have subsequently been deposited on the surface of Phobos as late accreting materials in the regolith (Hyodo et al 2019).…”

“…7 Timeline of the evolution of magnetosphere, atmosphere, hydrosphere, cryosphere, and surface mineralogy of Mars. Age distribution of Martian meteorites are also plotted (Hyodo et al 2019) timing of secondary aqueous alteration events (Fujiya et al 2012). Temperature of aqueous alteration would be estimated by C-O clumped isotope thermometry (e.g., Halevy et al 2011).…”