Dust Environment Explorer Research Articles

The Lunar Dust Experiment (LDEX) Onboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) Mission

The Lunar Dust Experiment (LDEX) is an in situ dust detector onboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission. It is designed to characterize the variability of the dust in the lunar exosphere by mapping the size and spatial distributions of dust grains in the lunar environment as a function of local time and the position of the Moon with respect to the magnetosphere of the Earth. LDEX gauged the relative contributions of the two competing dust sources: (a) ejecta production due to the continual bombardment of the Moon by interplanetary micrometeoroids, and (b) lofting of small grains from the lunar surface due to plasma-induced near-surface electric fields.

The Lunar Atmosphere and Dust Environment Explorer Mission

The Lunar Atmosphere and Dust Environment Explorer (LADEE) mission was designed to address long-standing scientific questions about the Moon’s environment, including the assessment of the composition of the lunar atmosphere, and characterization of the lunar dust environment at low orbital altitudes. LADEE was derived from the Modular Common Spacecraft Bus design that was developed at NASA Ames Research Center; it used modularized subassemblies and existing commercial spaceflight hardware to reduce cost. LADEE was launched on the very first Minotaur V, and was also the first deep space mission launched from Wallops Flight Facility in Virginia. LADEE was equipped with two in situ instruments and a remote sensing instrument to address the atmosphere and dust measurement requirements. LADEE also carried the first deep-space optical communications demonstration, the Lunar Laser Communications Demonstration. LADEE was launched in early September, 2013, took science data for over 140 days in low lunar orbit, and impacted the surface on April 18, 2014.

Lunar exospheric argon modeling

Argon is one of the few known constituents of the lunar exosphere. The surface-based mass spectrometer Lunar Atmosphere Composition Experiment (LACE) deployed during the Apollo 17 mission first detected argon, and its study is among the subjects of the Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP) and Lunar Atmospheric and Dust Environment Explorer (LADEE) mission investigations. We performed a detailed Monte Carlo simulation of neutral atomic argon that we use to better understand its transport and storage across the lunar surface. We took into account several loss processes: ionization by solar photons, charge-exchange with solar protons, and cold trapping as computed by recent LRO/Lunar Orbiter Laser Altimeter (LOLA) mapping of Permanently Shaded Regions (PSRs). Recycling of photo-ions and solar radiation acceleration are also considered. We report that (i) contrary to previous assumptions, charge exchange is a loss process as efficient as photo-ionization, (ii) the PSR cold-trapping flux is comparable to the ionization flux (photo-ionization and charge-exchange), and (iii) solar radiation pressure has negligible effect on the argon density, as expected. We determine that the release of 2.6×1028atoms on top of a pre-existing argon exosphere is required to explain the maximum amount of argon measured by LACE. The total number of atoms (1.0×1029) corresponds to ∼6700kg of argon, 30% of which (∼1900kg) may be stored in the cold traps after 120days in the absence of space weathering processes. The required population is consistent with the amount of argon that can be released during a High Frequency Teleseismic (HFT) Event, i.e. a big, rare and localized moonquake, although we show that LACE could not distinguish between a localized and a global event. The density of argon measured at the time of LACE appears to have originated from no less than four such episodic events. Finally, we show that the extent of the PSRs that trap argon, 0.007% of the total lunar surface, is consistent with the presence of adsorbed water in such PSRs.

The Neutral Mass Spectrometer on the Lunar Atmosphere and Dust Environment Explorer Mission

The Neutral Mass Spectrometer (NMS) of the Lunar Atmosphere and Dust Environment Explorer (LADEE) Mission is designed to measure the composition and variability of the tenuous lunar atmosphere. The NMS complements two other instruments on the LADEE spacecraft designed to secure spectroscopic measurements of lunar composition and in situ measurement of lunar dust over the course of a 100-day mission in order to sample multiple lunation periods. The NMS utilizes a dual ion source designed to measure both surface reactive and inert species and a quadrupole analyzer. The NMS is expected to secure time resolved measurements of helium and argon and determine abundance or upper limits for many other species either sputtered or thermally evolved from the lunar surface.

Preliminary Results From Probe of the Moon's Dust Environment

NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE), a robotic mission with the primary goal of gathering detailed information about the composition and structure of the thin lunar atmosphere, was launched on 6 September 2013. Since the spacecraft reached the Moon on 6 October, LADEE has returned streams of data important to understanding how dust is distributed above the lunar surface; where sources and sinks of lunar atmospheric dust can be found; and how dust distribution, sources, and sinks change through time.

Upper limits for a lunar dust exosphere from far-ultraviolet spectroscopy by LRO/LAMP

Since early 2012, the Lyman-Alpha Mapping Project (LAMP) far-ultraviolet spectrograph on the Lunar Reconnaissance Orbiter (LRO) spacecraft has carried out a series of limb observations from within lunar shadow to search for the presence of a high altitude dust exosphere 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. Far-ultraviolet measurements between 170 and 190nm, near the LAMP long wavelength cutoff, are especially sensitive to scattering by small (0.1–0.2μm radius) dust grains, since the scattering cross-section is near-maximum, and the solar flux is rising rapidly with wavelength. An additional advantage of ultraviolet measurements is the lack of interference by background zodiacal light which must be taken into account at longer wavelengths. As of July 2013, LAMP has completed several limb-observing sequences dedicated to the search for horizon glow, but no clear evidence of dust scattering has yet been obtained. Upper limits for vertical dust column abundance have been estimated at less than 10 grains cm−2 (0.1μm grain radius), by comparing the measured noise-equivalent brightness with the results of Mie scattering simulations for the same observing geometries. These results indicate that Lunar Atmosphere Dust Environment Explorer (LADEE) UVS lunar dust observations will be considerably more challenging than planned.

Scattering properties of lunar dust analogs

The Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft is designed to characterize the exospheric dust environment using an on-board suite of specialized sensors. The objective of this paper is to present results from scattering experiments using an aqueous suspension of lunar simulants that contains a population of dust grains ranging in size from ~0.1μm to 10μm. The intensity of scattered light is measured with a commercial version of the ultraviolet–visible spectrometer (UVS) used in the LADEE mission. We show that our data is consistent with the fact that micron-sized particles tend to form agglomerates rather than remaining isolated entities and that certain characteristics of the target particles can be predicted from intensity measurements alone. These results can be used directly to assess general features of the lunar exosphere. Further analysis of particle properties from such remote sensing data will require more refined measurements such as polarization features or other components of the Stokes vector.

Production of neutral gas by micrometeoroid impacts

We present the first direct laboratory measurement of vapor produced by simulated micrometeoroid bombardment. New in situ observations from the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft, and the anticipation of results from the Lunar Atmosphere and Dust Environment Explorer (LADEE), have highlighted the uncertainty surrounding the role of micrometeoroid impacts in sustaining planetary exospheres. In a recent series of experiments, the quantity of neutral molecules generated by impacts of simulated micrometeorids of 0.1–1μm radius was measured using a fast ion gauge, over a speed range of 1–10km/s. The quantity of neutrals released per unit projectile mass, N/m, is consistent with a power law N/m=vβ in the projectile speed v, with β∼2.4. At the highest speeds tested, the number of neutrals liberated is equivalent to 5% of the atoms in the projectile; complete vaporization is projected at speeds exceeding 20km/s.

ARTEMIS observations of lunar pickup ions: Mass constraints on ion species

Observations of heavy ions of lunar origin give important information regarding lunar exospheric processes, especially with respect to exospheric particle abundance and composition. Electrostatic analyzers without a time‐of‐flight section provide highly sensitive, absolute density detection but without mass discrimination. Here we place constraints on lunar ion species through inference of the average ion mass using such instruments. The technique is based on the plasma quasi‐neutrality requirement, an independent electron density measurement, and the fact that electrostatic analyzers underestimate the ion density by the square root of the ion mass. Applied to a case of such observations by ARTEMIS in the terrestrial lobe reported by Poppe et al. (2012), our technique suggests an average mass of 28 amu for lunar pickup ions. This result, consistent with the lower limit of 24 amu derived in the Poppe et al. model, suggests that the observed ions were most likely Al+ and Si+. The technique is also refined and applied to a more complicated event with a series of heavy ion surges in the plasma sheet, to show the spatial and/or temporal dependence of the observed lunar ion species. The technique is particularly timely given the planned conjunctions and coordinated lunar studies by NASA's Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) and Lunar Atmosphere and Dust Environment Explorer missions.

Metallic species, oxygen and silicon in the lunar exosphere: Upper limits and prospects for LADEE measurements

The only species that have been so far detected in the lunar exosphere are Na, K, Ar, and He. However, models for the production and loss of species derived from the lunar regolith through micrometeoroid impact vaporization, sputtering, and photon‐stimulated desorption, predict that a host of other species should exist in the lunar exosphere. Assuming that loss processes are limited to ballistic escape, photoionization, and recycling to the surface, we have computed column abundances and compared them to published upper limits for the Moon. Only for Ca do modeled abundances clearly exceed the available measurements. This result suggests the relevance of some loss processes that were not included in the model, such as the possibility of gas‐to‐solid phase condensation during micrometeoroid impacts or the formation of stable metallic oxides. Our simulations and the recalculation of efficiencies for resonant light scattering show that models for other species studied are not well constrained by existing measurements. This fact underlines the need for improved remote and in situ measurements of the lunar exosphere such as those planned by the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft. Our simulations of the LADEE neutral mass spectrometer and visible/ultraviolet spectrometer indicate that LADEE measurements promise to provide definitive observations or set stringent upper limits for all regolith‐driven exospheric species. We predict that observations by LADEE will constrain assumed model parameters for the exosphere of the Moon.

A reanalysis of the Apollo light scattering observations, and implications for lunar exospheric dust

Conspicuous excess brightness, exceeding that expected from coronal and zodiacal light (CZL), was observed above the lunar horizon in the Apollo 15 coronal photographic sequence acquired immediately after orbital sunset (surface sunrise). This excess brightness systematically faded as the Command Module moved farther into shadow, eventually becoming indistinguishable from the CZL background. These observations have previously been attributed to scattering by ultrafine dust grains (radius ∼0.1 microns) in the lunar exosphere, and used to obtain coarse estimates of dust concentration at several altitudes and an order-of-magnitude estimate of ∼10 −9 g cm −2 for the column mass of dust near the terminator, collectively referred to as model “0”. We have reanalyzed the Apollo 15 orbital sunset sequence by incorporating the known sightline geometries in a Mie-scattering simulation code, and then inverting the measured intensities to retrieve exospheric dust concentration as a function of altitude and distance from the terminator. Results are presented in terms of monodisperse (single grain size) dust distributions. For a grain radius of 0.10 microns, our retrieved dust concentration near the terminator (∼0.010 cm −3) is in agreement with model “0” at z=10 km, as is the dust column mass (∼3–6×10 −10 g cm −2), but the present results indicate generally larger dust scale heights, and much lower concentrations near 1 km (<0.08 cm −3 vs. a few times 0.1 cm −3 for model “0"). The concentration of dust at high altitudes ( z>50 km) is virtually unconstrained by the measurements. The dust exosphere extends into shadow a distance somewhere between 100 and 200 km from the terminator, depending on the uncertain contribution of CZL to the total brightness. These refined estimates of the distribution and concentration of exospheric dust above the lunar sunrise terminator should place new and more rigorous constraints on exospheric dust transport models, as well as provide valuable support for upcoming missions such as the Lunar Atmosphere and Dust Environment Explorer (LADEE).

Optical scattering processes observed at the Moon: Predictions for the LADEE Ultraviolet Spectrometer

The Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft will orbit the Moon at an altitude of ≈50 km with a payload that includes the Ultraviolet Spectrometer (UVS) instrument, which will obtain high spectral resolution measurements at near-ultraviolet and visible wavelengths (≈231–826 nm). When LADEE/UVS observes the lunar limb from within the shadow of the Moon it is anticipated that it will detect a lunar horizon glow (LHG) due to sunlight scattered from submicron exospheric dust, as well as emission lines from exospheric gases (particularly sodium), in the presence of the bright coronal and zodiacal light (CZL) background. A modularized code has been developed at NMSU for simulations of scattered light sources as observed by orbiting instruments in lunar shadow. Predictions for the LADEE UVS and star tracker cameras indicate that LHG, sodium (Na) emission lines, and CZL can be distinguished based on spatial morphology and spectral characteristics, with LHG dominant at blue wavelengths (∼250–450 nm) and small tangent heights. If present, LHG should be readily detected by LADEE/UVS and distinguishable from other sources of optical scattering. Observations from UVS and the other instruments aboard LADEE will significantly advance our understanding of how the Moon interacts with the surrounding space environment; these new insights will be applicable to the many other airless bodies in the solar system.