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Impact Melt Flows on the Moon: Observations and Models EPSC Abstracts Vol. 13, EPSC-DPS2019-1067-1, 2019 EPSC-DPS Joint Meeting 2019 c Author(s) 2019. CC Attribution 4.0 license. Impact melt flows on the Moon: observations and models Natalia Artemieva (1,2), Veronica Bray (3) (1) Planetary Science Institute, Arizona, USA, (2) Institute for Dynamics of Geospheres ([email protected]) , (3) Lunar and Planetary Laboratory, University of Arizona, USA Abstract topography. Our numerical modeling explores this supposition. We compare modeling results with observed melt flows in the extended ejecta blanket of the Pierazzo crater on the Moon. Surface flow dynamics of ballistically deposited melts depends on its viscosity, deposition velocity (i.e., distance from the parent crater), local topography, and the amount of entrained cold materials. 1. Introduction This work concentrates on the analysis of Pierazzo crater (9.2 km in diameter) located on the far side of the Moon (259.7E, 3.25N) [1]. This crater has extensive LRO Narrow Angle Camera (NAC) coverage and a visible ray system extending beyond 450 km from the crater rim - suggesting a relatively young age. We analyse and model melt flows within the discontinuous ejecta at distances 11-40 km from the crater rim. 1.1 Mapping LROC images extending ~ 40km from the Pierazzo crater rim were mosaicked and melt flows and ponds mapped (Fig 1A). The largest flows show clear melt- like morphology: cooling cracks, channelling, lobate toe and pooling in low topography (Fig 1B/C). Fig. 1: A) Simplified mapping of [1] to show only the Fractal dimensions of several flows were recorded (D lobate flows present around Pierazzo crater (black). = 1.05-1.17) and support melt-like rheology, rather These are over-laid on the Global Lunar DTM 100 m. than dry granular flow [1]. Melt flows are noted in The approximate edge of the continuous ejecta is 1.5% (~50 km2) of the mapping area, most marked with dashed white line. The location of the commonly on 6-18˚ crater-facing slopes, and also flow shown in B and C is marked with a white box. B emerging from confined depressions such as pre- and C show the flow channelling and lobate toes of a existing craters. Flows originate in amphitheater- 1.5km flow. Images B and C are 300 m across. headed depressions, or occur without any obvious starting point, and reach up to 2.56 km in length. 1.2 Numerical model Bray et al. [1] suggest a mix of solid and molten ejecta is deposited, after ballistic ejection, in a We use the SOVA code [3] to solve the Navier- turbulent ground flow. Melt is quickly quenched as Stokes equation. Deposition velocity of ejecta ranges the ejecta travels along the ground, unless ‘stalled’ by from 100 to 300 m/s; mixing with lunar surface materials is minor in this case. The surface slope varies from -15 (outward-facing slope) to +15 at 40 km from the crater), only highly viscous melts (crater-facing slope). Melt viscosity ranges from 104 survive and form flows; melts with low and medium to 107 Pa·s, corresponding to anorthosite at a viscosity are highly turbulent and are dispersed into temperature of 1430-1250 K [2]. We start small droplets. simulations at the moment when ejected blob of melt touches the lunar surface, which is treated as a solid non-penetrable plane with either slip or no-slip boundary conditions. 1.3 Preliminary estimates Combining ejecta scaling with melt production scaling, we can estimate that the melt fraction in ejecta is within a few percent at most. However, for Figure 2: Melt flow with medium viscosity on the the purpose of our current model we assume that a slope (the parent crater is on the left at a distance of “melt blob” is not mixed with ejected large solid 11 km): A: 15° outward-facing slope; B: no slope; C: 15° crater-facing slope; D: same as C with a large blocks whereas small fragments with radius r (m- sized and less) are already “locked” within melt and solid fragment inside. move with the same velocity. Characteristic time of 2 momentum exchange is m=2r /9, where is melt 3. Summary and Conclusions viscosity and is its density. At the same time temperature of melt may differ from the temperature We successfully reproduced the behavior of ejected of entrained solid fragments as a characteristic time impact melts at distances of a few crater radii: first, they are deposited ballistically, then, depending of heat exchange is much larger: =r2C /K, where h p mainly on their viscosity and deposition velocity, C is the heat capacity and K is the heat transfer p form a thin blanket consisting of solidified melt coefficient. Whereas mm-sized fragments are droplets (low viscosity), a melt pond (high viscosity, dissolved in melt within a few seconds, cm-sized Fig 2A/B), or a melt flow (medium viscosity, fragments can survive a few minutes (which is a Fig2C/D). Superheated impact melt within a ground- typical time of a ballistic flight and melt flow hugging flow of ejecta does not exist: it is mixed emplacement); larger fragments remain at least with either solid ejecta or with local materials upon partially solid. landing and, hence, cools to sub-liquidus temperatures. This relatively viscous material may 2. Modelling Results form ponds or flows depending on the local topography. Figure 1 shows melt distribution on a flat surface and on slopes one minute after landing with a velocity away from the crater of 100 m/s. Independent of Acknowledgements topography, highly viscous melt behaves like a solid This work was supported by the LRO project and by material - its shape changes only slightly and most probably its deposit looks like a melt pond, not a NASA LDAP Grant NNX15AP93G flow. Low-viscosity melt is intensively dispersed upon landing, forming a thin blanket of melt droplets. References The most interesting case is for a simulated melt 6 [1] Bray, V. et al: Lobate impact melt flows within the ‘blob’ of medium viscosity (10 Pa·s): Melt landing extended ejecta blanket of Pierazzo crater, Icarus, Vol. 301, on an outward-facing slope and on the flat surface pp. 26-36, 2018. forms a melt pond (larger in diameter and thinner in the latter case). For the same melt properties, a melt [2] Morrison, A. et al.: Rheological investigation of lunar ‘blob’ landing on crater-facing slope moves first highland and mare impact melt simulants, Icarus, Vol. 317, upward and then downward forming a pronounced pp. 307-323, 2019. flow. Its characteristics are comparable with observations: length of 1.2 km, thickness of ~10m. If [3] Shuvalov, V: : Multi-dimensional hydrodynamic code the deposition velocity is higher (~300 m/s, deposited SOVA for interfacial flows: Application to the thermal layer effect, Shock waves, Vol. 9, pp. 381-390, 1999. .
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