New upper limits on the lunar nanodust exosphere

The Lyman-Alpha Mapping Project (LAMP) FUV spectrograph onboard the Lunar Reconnaissance Orbiter (LRO) carried out a campaign to search for backscattering of sunlight from lunar exospheric nanodust grains, and investigate its dependence on meteoroid stream activity. This campaign was spurred by the detection of a broadband signal recently reported [Wooden et al., Nat. Geosc. 2016, accepted] from the Ultraviolet and Visible Spectrometer (UVS) onboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) during the Quadrantids - a major annual meteoroid stream encountered by the Earth-Moon system. The LADEE/UVS signal was found to be consistent with an extended exosphere of very small ( 20-30 nm radius) dust grains. Using Mie scattering models, we found that the scattered sunlight from such a dust cloud should be easily detectable by LRO/LAMP near its long wavelength limit of operation (170-190 nm) . LRO performed a series of inertial pointing stares as close as possible to the anti-sunward direction to maximize the backscattering efficiency or intensity from any exospheric dust as seen from LAMP. These stares also pointed at a region of the sky devoid of bright stars in order to minimize any background signal. The observations were targeted to coincide with the peak in activity of two major meteoroid streams: the Geminids (December 14th 2015) and the Quadrantids (January 4th 2016). The upper limit on the lunar nanodust concentration along LAMP line of sight that we derive for the Quadrantids meteoroid stream is 3x105 grains/cm2 assuming a grain size of 20 nm, equivalent to a dust mass concentration of 10-11 grams/cm2 (assuming spherical grains). These upper limits are at least 100 times smaller than the LADEE/UVS inferred column densities. We discuss the possible explanations and the implications of this discrepancy and we present analysis of the Geminid meteoroid stream campaign.