Since early 2012, LRO/LAMP has carried out a series of limb observations from within lunar shadow to search for the presence of a high altitude dust exosphere, detected via forward scattering of sunlight from dust grains. Bright "horizon glow" was observed from orbit during several Apollo missions and interpreted in terms of dust at altitudes of several km and higher. However, no confirmation of such an exosphere has been made since that time. This raises basic questions about the source(s) of excess brightness in the early measurements and also the conditions for producing observable dust concentrations at km altitudes and higher. Ultraviolet measurements between ~170 nm and the LAMP long wavelength limit of ~200 nm are especially sensitive to scattering by small (0.1-0.2 µm radius) dust grains, since the scattering cross-section is near maximum, and the solar flux is rising rapidly with wavelength. Another advantage of UV measurements is the lack of interference by background zodiacal light which must be taken into account at longer wavelengths.
As of April 2013, LAMP has completed six limb observing sequences dedicated to the search for horizon glow, but no clear evidence of dust scattering has yet been detected. Upper limits for line-of-sight dust abundance have been estimated at less than a few hundred grains cm-2, by comparing measured noise-equivalent brightness with the results of Mie scattering simulations for the same observing geometries. We will review the results of the measurements and also present some derived upper-limits for one-dimensional exospheres based on the LAMP search.