Interplanetary dust particles hit the surfaces of airless bodies in the Solar System, generating charged1 and neutral2 gas clouds, as well as secondary ejecta dust particles3. Gravitationally bound ejecta clouds that form dust exospheres were recognized by in situ dust instruments around the icy moons of Jupiter4 and Saturn5, but have hitherto not been observed near bodies with refractory regolith surfaces. High-altitude Apollo 15 and 17 observations of a ‘horizon glow’ indicated a putative population of high-density small dust particles near the lunar terminators6,7, although later orbital observations8,9 yielded upper limits on the abundance of such particles that were a factor of about 104 lower than that necessary to produce the Apollo results. Here we report observations of a permanent, asymmetric dust cloud around the Moon, caused by impacts of high-speed cometary dust particles on eccentric orbits, as opposed to particles of asteroidal origin following near-circular paths striking the Moon at lower speeds. The density of the lunar ejecta cloud increases during the annual meteor showers, especially the Geminids, because the lunar surface is exposed to the same stream of interplanetary dust particles. We expect all airless planetary objects to be immersed in similar tenuous clouds of dust.
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The LADEE/LDEX project was supported by NASA. Tests and calibrations were done at the dust accelerator facility of the University of Colorado, supported by NASA’s Solar System Exploration Research Virtual Institute (SSERVI). We are grateful for engineering and technical support from the Laboratory for Atmospheric and Space Physics (LASP), especially from M. Lankton (project manager), S. Gagnard and D. Gathright (mission operations), and D. James (calibration).
The authors declare no competing financial interests.
Extended data figures and tables
A particle of velocity v is recorded by a detector of sensitive area A. The surface normal of the detector area points along the velocity vector of the spacecraft vsc. The particle enters the instrument with an angle ω measured between the instrument boresight and the relative velocity vector of the particle vsc − v .
a, The calculated density for a standard set of parameters listed in Extended Data Table 1 for the model ejecta cloud12 as function of altitude (black line) normalized to the production rate N+. The density is recalculated using n = γ/(Avsc) (red line), the approach taken in this paper to infer the dust density from the measured impact rates γ, indicating an underestimate of <20% for altitudes below 100 km. b, Contour plot of the ratio of the ‘true’ model density over the recalculated density at the altitude h = 50 km, as a function of the opening cone angle of the ejecta plume ψ0 and the exponent of the power-law initial-speed distribution µ, appropriately setting the minimum speed u0, while keeping the maximum speed constant at 2vescape, maintaining a constant total kinetic energy of the ejecta particles.
a, The dust density n(h) of the lunar ejecta cloud as function of altitude and size (colour scale). The continuous black line shows the model prediction12 using the best-fit parameters listed in Extended Data Table 1. b, The cumulative dust mass in the lunar exosphere. The continuous blue line shows the ejecta model prediction (Extended Data Table 1). c, The initial normalized vertical velocity distribution f(u) calculated from n(h) using energy conservation. The continuous line shows f(u) ∝ u−3.4 ± 0.1 matched to the data at u ≥ 400 m s−1 (altitude h ≈ 50 km). Error bars were calculated by propagating the error through the various calculations, where N is the number of detected dust impacts.
a, The calculated flux of interplanetary dust particles Fimp reaching the lunar equatorial region as a function of lt and t (colour coded for monthly averages). b, The mass production rate, equation (9), calculated using a model for the spatial and velocity distributions of interplanetary dust particles near the Earth16, consistent with the observed asymmetric dust cloud.
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Horányi, M., Szalay, J., Kempf, S. et al. A permanent, asymmetric dust cloud around the Moon. Nature 522, 324–326 (2015). https://doi.org/10.1038/nature14479
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