Impact Ejecta Plumes at the Moon

The Lunar Dust Experiment, on‐board National Aeronautics and Space Administration's Lunar Atmosphere and Dust Environment Explorer, observed significant enhancements in impact rate measurements of lunar ejecta. These enhancements were attributed to the spacecraft crossing dense ejecta plumes generated by well‐timed and well‐placed interplanetary meteoroid impacts on the lunar surface. We have used a Monte Carlo approach to implement an initial speed distribution, derived from globally averaged Lunar Dust Experiment observations, in a 3‐D dynamical model describing the ballistic motion of ejecta particles. By matching this model to the observed enhancements, we constrained the initial ejecta angular distribution of these plumes. Our results indicate that lunar impact ejecta plumes reaching altitudes exceeding 24 km are far narrower than previously thought, with initial opening half angles of 8°±3°, suggesting that the high‐altitude lunar dust cloud may be dominated by reverse plumes. Plain Language Summary Earth's Moon is surrounded by an ever‐present dust cloud produced by meteoroids impacting its surface. By measuring this dust cloud and comparing to computer simulations, we can determine certain properties of the meteoroids producing it such as size, mass, and speed. This information, in turn, is important for determining how protected a spacecraft needs to be to survive the journey through our solar system. To derive these meteoroid properties, however, we must first determine the average shape of the ejecta dust produced by one impact. Objects impacting a powdery surface typically shoot a cone of dust upward. This article aims to determine how wide and hollow this cone is on average by comparing measurements of the lunar dust cloud acquired by the Lunar Dust Experiment on‐board National Aeronautics and Space Administration's Lunar Atmosphere and Dust Environment Explorer with a 3‐D simulation of a meteoroid impact. We find that the cone is far narrower that previously thought. Key Points The first in situ measurements of individual impact ejecta plumes at the Moon are further characterized Lunar impact ejecta plumes are far narrower than previously assumed The high‐altitude dust environment of a regolith body may be dominated by reverse plumes