Stubbs, Timothy - The Effects of Meteoroid Streams on the Lunar Environment: Observations from the LADEE Mission

Abstract: 
Impacts on the lunar surface from meteoroid streams encountered by the Earth-Moon system can result in measurable enhancements in both the lunar atmosphere and dust environment. Here we describe the annual meteoroid streams incident at the Moon during the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission, and discuss their effects on the lunar environment. The LADEE science payload consisted of three instruments: the Ultraviolet/Visible Spectrometer (UVS); the Lunar Dust Experiment (LDEX); and the Neutral Mass Spectrometer (NMS). All three instruments were capable of detecting the effects of an encounter with a meteoroid stream.

The Earth-Moon system frequently encounters debris trails from comets and asteroids, which are referred to as meteoroid streams. The meteoroids in these streams have similar velocities and are on near-parallel trajectories, so when they enter the Earth’s atmosphere the resulting shower of meteors appears to be emanating from a virtual point on the sky called the radiant. Meteor (and meteoroid) rates vary as a function of the Earth’s position in its orbit, with an activity curve that increases to a peak and then decreases. During its time in lunar orbit, the LADEE mission coincided with 18 out of 35 IAU established annual streams. These streams are relatively well characterized and are broad enough that both the Earth and Moon pass through them. Unlike at the Earth, all of the stream meteoroids incident at the Moon will impact its surface and create ejecta clouds and release neutral and ionized species into the exosphere. As stream meteoroids move on near-parallel trajectories we expect to observe asymmetries in their effects on the lunar environment. Therefore, it is necessary to know where the streams are normally incident on the lunar surface (i.e., the locations of the stream radiants in the Moon frame).

Based on the Zenith Hourly Rates (ZHR) derived from meteors observed at Earth, one might expect the Geminids (ZHR peak ≈92–120 on 13/14 December 2013) and Quadrantids (ZHR peak of ≈120 or greater on 3 January 2014) to have had the most significant effect on the lunar environment during the mission. (For comparison, the hourly rate for sporadic background meteors is ≈9.5.) While a substantial exospheric response was clearly observed by the LADEE instruments during the Geminids, the apparent response during the Quadrantids was more subdued. One contribution to this difference may have been due to the location of the stream radiant on the lunar surface relative to LADEE’s orbit. The Geminids radiant was predicted to be at Selenocentric Solar Ecliptic (SSE) latitude of 10.2°N, which was very close to LADEE’s equatorial orbit. Whereas the Quadrantids radiant was much farther poleward of LADEE at SSE latitude of 64.8°N, which could explain the diminished exospheric activity and suggests that the response of the lunar environment to meteoroid streams could be relatively localized.