51st Lunar and Planetary Science Conference (2020) 1367.pdf
SCIENTIFIC VALUES OF MARTIAN IMPACT EJECTA ON ITS MOONS. R. Hyodo1,2, K. Kurosawa3, H. Genda2, T. Usui1, and K. Fujita1, 1ISAS, JAXA ([email protected]), 2Earth-Life Science Institute, Tokyo Institute of Technology, 3Planetary Exploration Research Center, Chiba Institute of Technology.
Introduction: Mars has two small moons, Phobos debris, to assess the quantitative likelihood of delivery and Deimos. Their origin is debated. Historically, the to the Martian moon(s). moons are thought to be gravitationally captured aster- The impact location on the surface of Mars, orbital oids [1,2]. However, explaining their orbital properties phase of the moons were fully randomized. Other im- – almost circular and almost coplanar orbits – is chal- pact parameters, such as impact velocity and impactor lenging due to the isotropic nature of capture process. mass, were chosen from the expected impactor distri- Recently, more and more studies investigated the giant bution within the last 500 Myr [13]. impact scenario where the moons accrete within the In addition, the delivery from the five largest recent debris produced by a large impact on Mars [3,4,5,6,7]. (<10 Myr) craters, Mojave, Tooting, McMurdo, Corin- The endogenous bulk material properties of the moons to, and Zunil were also investigated selectively by strongly depend on its origin. If the moons are the cap- 10,000 Monte Carlo runs for each crater. In these spe- tured objects, the bulk materials would be analogous to cific cases, the impact location on the Martian surface a type of chondrite [8]. If a giant impact has produced and the impact energy were fixed as that of the current the moons, the bulk materials of the moons would be a crater position and that capable of producing the ob- mixture of Martian and impactor materials that experi- served crater diameter, respectively. enced a high temperature phase [6]. In addition, – re- In the calculations, Phobos is fixed at the current gardless of its formation scenarios – materials deliv- orbital distance to Mars. During the last 500 Myr, Pho- ered by natural influx of background asteroids and bos orbit has shrunken and its cross section increased impact-ejecta of Mars produced by numerous small about ×5. Thus, we multiplied the conservative value impacts are expected to be mixed in the surface rego- of 1/5 to the results obtained from the Monte Carlo lith of the moons [9,10]. simulations to derive values within the last 500 Myr. Here, we re-visited the amount and condition of a recent delivery of impact ejecta from Mars to its 1x1010 moons by using state-of-the-art numerical approaches 1000 [11], whereas previous studies used a simple analytical 1x109 models [9,10]. We report about 10-100 times increase 100 of the mass transferred from Mars to its moons than 1x108 the previous estimations due to mainly better numeri- 10 7 cal approach to this problem [11]. 1x10 1 Martian moons are the target bodies of the Japan 1x106
Aerospace eXploration Agency (JAXA) sample return 0.1 Transferred mass to Phobos [kg]
mission Martian Moons eXploration (MMX). MMX 100000 Martian material in the regolith [ppm] plans to collect surface material from Phobos and re- Zunil Corinto McMurdo Tooting Mojave Random 260km Total turn samples to Earth (launch in 2024 scheduled) [12]. Fig. 1: The mass transferred from Mars to Phobos at Therefore, a detailed study of the surface materials on different impact events on Mars [11]. “Random” and Phobos (potential sample material) is required to max- “260km” are those at the sum of numerous impacts that imize the scientific results of the MMX mission. formed D=2-100 km craters and a single impact that formed D=260 km crater during the last 500 Myr, re- Numerical Methods: In order to study the amount spectively. “Zunil”, “Corinto”, “McMurdo”, “Tooting” and condition of mass transfer from Mars to its moons, and “Mojave” are those at the five largest recent crater- we proceeded with the following two steps. Firstly, forming events (D>10 km and <10 Myr). “Total” is the impact simulations under a variety of impact condi- sum of all the cases considered here. Y-axis on the right tions expected for asteroidal impacts on Mars were side of the panel shows the corresponding fraction of performed to obtain datasets of positions and velocities Martian materials assuming they are mixed homogene- of the impact debris immediately after impact. The ously within a 1 m-depth of Phobos regolith. Further obtained datasets were then used in 30,000 Monte Car- details are found in Hyodo et al. (2019) [11]. lo runs of impact bombardment on Mars, which in- clude the long-term orbital evolution of the impact 51st Lunar and Planetary Science Conference (2020) 1367.pdf
Mass Transfer from Mars to Phobos: Together Scientific Values of Martian Materials on its with the Martian crater isochron [13], our Monte Carlo Moons: MMX plans to collect samples of >10 g of simulations provide the median values of net mass Phobos regolith. Assuming a typical cubic grain diam- transfer from Mars to Phobos (Figure 1; see also Hy- eter for the Mars fraction of 300 μm and a density of 3 odo et al. (2019) [11]). g/cm, our results indicate that at least ~34 Martian During the last 500 Myr, the total number of craters grains (~340 ppm from the “Random” case) would in Hartmann isochrons is more than 10 for their diame- exist in the returned sample. In addition, another ~76 ters of 2