Search for a high‐altitude lunar dust exosphere using Clementine navigational star tracker measurements

Abstract

During the 1994 Clementine lunar mapping mission, portions of 25 orbits were dedicated to a search for lunar horizon glow (LHG) using the spacecraft star tracker navigation cameras. Previous putative detections of LHG were believed to result from forward scattering of sunlight by exospheric dust grains with radii ≈ 0.1 µm, observable above the limb from within the shadow of the Moon near orbital sunrise or sunset. We have examined star tracker image sequences from five Clementine orbits in which the limb occulted the Sun, and was at least partially shadowed from earthshine, minimizing the chance of stray light contamination. No LHG appears in the image data, or in any of the net brightness images, after subtraction of a reference zodiacal light model. However, some of the images display faint excess limb brightness that appears to be solar streamer structure. Therefore, we derive upper limits for the amount of dust in the lunar exosphere that could be hidden by these brightness fluctuations using a dust‐scattering simulation code and simple exponential dust profiles defined by surface concentration n0 and scale height H. Simulations using grains of radius 0.1 µm show that fluctuations in the observed excess brightness can be matched by a dust exosphere with a vertical column abundance n0H of 5–30 cm−2 and overlying mass <10−12 g cm−2. These dust upper limit estimates are highly dependent on assumed grain size due to the rapid increase in per‐grain brightness with grain radius.

Solar wind‐generated current in the Lunar Dust Experiment

“…However, the reanalysis of LEAM data indicates that the dust activity can be much weaker [ Grün and Horányi , ]. In addition, the electrostatic force seems insufficient to overcome the cohesive force [ Marshall et al ., ], there is no correlation between LHG and solar UV [ Glenar et al ., ], and the dust abundance can be about 4 orders of magnitude smaller than the Apollo measurements according to the Clementine [ Glenar et al ., ] and Lunar Reconnaissance Orbiter (LRO) [ Feldman et al ., ] observations. These results suggest that the tenuous dust exosphere is more likely caused by micrometeoroid impacts.…”

A permanent, asymmetric dust cloud around the Moon

“…Gravitationally bound ejecta clouds that form dust exospheres were recognized by in situ dust instruments around the icy moons of Jupiter 4 and Saturn 5 , 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 terminators 6,7 , although later orbital observations 8,9 yielded upper limits on the abundance of such particles that were a factor of about 10 4 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 accumulated charge collected by LDEX in dt 5 0.1 s intervals (J dust ), averaged over the sunrise terminator between 5:30 and 6:30 LT. The coloured lines show the predicted value of J dust based on the impacts of small particles alone using the upper limits of the dust densities estimated by remote sensing visible6,8,20 and ultraviolet 9 observations. Error bars show 1s on logJ dust as the current measurements are log-normally distributed.…”

In Situ Measurements of Lunar Dust at the Chang'E‐3 Landing Site in the Northern Mare Imbrium

“…It was revealed by scientific payloads onboard CE‐3 mission that the CE‐3 landing site is a relatively young high‐titanium mare basalt region characterized by thin regolith and numerous small craters, which is compositionally different from the several Apollo landing sites (Xiao et al, ; Zhang et al, ). Although the dust transport mechanisms are closely correlated to the regolith composition (Glenar et al, ), similar to Apollo landing cases, the CE‐3 7500 N lander engine exhausted large amounts of propellant during descent stage, which also, as a result, kicked up a lot of dust (as demonstrated in Figure S3), posing a promising challenge to photo‐sensitive and thermo‐sensitive devices.…”

“…Thus, in order to mitigate the lunar dust‐related problems, provide substantial guidance for the planning of future lunar exploration even extraterrestrial settlement constructions, and offer valuable information for understanding lunar atmospheric evolution, in situ measurement of dust deposition on the lunar surface is the first step for achieving these purposes. To date, however, the reports about in situ measurements of dust on the lunar near surface are comparatively few (Berg et al, ; Grün & Horányi, ; O'Brien, ) even though some studies were executed from lunar orbits (Feldman et al, ; Glenar et al, ; Grava et al, ; Horányi et al, ; Szalay & Horányi, ; Wooden et al, ). On 14 December 2013 at 21:11 (UTC + 8), Chang'E‐3 (CE‐3) spacecraft, the first visitor from China soft‐landing on the Moon surface, successfully touched down at 44.12°N, 19.51°W on the Moon (Fa et al, )—a region that was neglected by previous expeditions, which carried, among other instruments, one lunar dust detector (LDD), providing an unique opportunity to learn about the lunar dust deposition in the northern Mare Imbrium.…”