Abstract | Impact ejecta flows are found on rocky planets, and some icy moons. In contrast to ballistically emplaced ejecta, which thins exponentially from crater rims, ejecta flows have a layered morphology with steeper margins, often lobate in shape. This fluidized morphology is thought to result from deposition by ground-hugging flows. On Mercury, seven ejecta flow deposits were reported by Xiao and Komatsu (2013), described as single layer deposits, extending into adjacent, older craters. These flows could have occurred during the impact process, or afterwards by mass-wasting. Hokusai crater also has an apparent ejecta flow (Barnouin et al., 2015), but by contrast it occurs on relatively flat ground and does not flow into an adjacent crater.
Here we present a global survey of Mercury identifying 36 craters with ejecta flows and a further 27 probable examples. The majority occur around craters ~30–80 km in diameter and are widespread across the planet. All but two of the ejecta flows appear to extend into adjacent craters or other topographic lows. In addition to Hokusai, an unnamed crater, also on the northern plains, has an ejecta flow on relatively flat ground (<2° slope) and not extending into an adjacent crater. These two flows look distinct from other examples on Mercury, resembling single-layer ejecta craters on Mars, with distal ramparts and a ropey texture. The prevalence of Mercurian ejecta flows shown by this survey is also of note, representing at least a fourfold increase in the number of known flows.
For ejecta flows on Mars, Earth, and some icy moons, volatiles are proposed to drive the fluidization of the flow. As Mercury has volatile-bearing materials at the surface, these could be a factor in ejecta fluidization. However, we find no evidence for volatile involvement, and features indicative of local volatile concentration (e.g. hollows) don’t occur preferentially near to ejecta flows. Slope of local terrain is clearly a major factor in influencing ejecta flow development on Mercury, since most flows extend downslope into adjacent craters. Perhaps some flows are post-impact mass movements, rather than forming during the impact process. However, other work shows that for at least one example, the time of emplacement was contemporaneous with impact (Lennox et al., this meeting).
Of the flows on low slopes, Hokusai crater exhibits evidence of excess impact melt: a possible fluidizing agent (Barnouin et al., 2015). However, the other crater with a flow on relatively flat ground has no identifiable impact melt present outside the crater rim. The crater is also considerably smaller than Hokusai (~37 km vs 95 km diameter), and smaller craters tend to have proportionally less impact melt. Understanding how this crater’s ejecta flow formed would give insight into the factors influencing ejecta fluidization on Mercury, as for this crater the slope of local terrain, volatile presence, and impact melt excess do not appear to be obvious factors.
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