NASA’s LADEE Mission (Lunar Atmosphere and Dust Environment Explorer) will address the following five core mission objectives:
- Determine the composition of the lunar exosphere and investigate the processes that control its distribution and variability including sources, sinks and surface interactions
- Characterize the lunar exospheric dust environment and measure any spatial and temporal variability and impacts on the lunar atmosphere
- Demonstrate the Lunar Laser Communication Demonstrations
- Create a low-cost reusable spacecraft architecture that can meet the needs of certain planetary science missions
- Demonstrate the capability of the Minotaur V as a launch vehicle for planetary missions
The first two of these objectives represent the science objectives of the LADEE mission while the remaining three objectives are associated with technical elements of the mission.
The big motivator behind the LADEE mission is to learn about the lunar exosphere before any significant human and robotic activity takes place on the Moon and change the exosphere irreversibly. An exosphere is a thin-atmosphere like volume surrounding a planetary body in which molecules are gravitationally bound to that body, but where the density is not sufficient to cause molecules to behave as a gas by colliding with each other. In planets with atmospheres, the uppermost portion (the thinnest layer of the atmosphere) is also called exosphere.
Several moons in our solar system have exospheres without having a dense atmosphere underneath. Large asteroids are also suspected of having exospheres. The Moon’s exosphere contains a varying composition of different molecular and atomic species as well as a varying dust content.
LADEE will determine the density, composition and temporal and spatial variability of the lunar exosphere to help understand the origin of the species that are present. LADEE is expected to refine previous measurements of species in the exosphere and also detect a range of other species – if present. The influence of the solar winds and meteorite impacts will be studied. The spacecraft is also expected to determine the density and temporal and spatial variability of exospheric dust particles to probe their origin and examine what type of processes allow them to remain in the exosphere.
These measurements will provide insight into basic exosphere dynamics that may also apply to other planets like Mercury, other moons such as the Galilean satellites and possibly large asteroids.
The lunar exosphere is produced by a range of processes that release particles from the lunar regolith. These processes include interaction with solar wind, impact vaporization, desorption due to solar UV radiation and magnetospheric plasmas as well as chemical and thermal release and internal activity. Sink processes include gravitational escape, photo-ionization, chemical loss and cold trapping.
Particles that are being released usually travel on a parabolic trajectory before being re-captured or lost into space.
The exact composition of the exosphere strongly depends on external factors such as meteoroid impacts, plasma conditions and solar illumination that drives surface temperature and UV abundance. The majority of release and sink mechanisms are poorly understood and will be studied by LADEE.
Earth based measurements and early in-situ investigations by the early lunar orbiters and Apollo science instruments have shown that helium, sodium, potassium and rubidium is present in the lunar exosphere. The density measured for these species is far less than the overall exospheric pressure measured by Apollo instruments leading to the conclusion that more species must be present. LADEE will make these new detections and refine the upper limits of the abundance of the already known species.
The second area of interest for the LADEE mission is exospheric dust. One of the question LADEE will answer is whether eyewitness accounts of Apollo astronauts seeing streaks of light in 10s of Kilometers in altitude above the Moon are related to exospheric dust or sodium glow.
Exospheric dust can be caused during impacts when fine-grained ejecta are generated and enter parabolic or even short-lived orbital trajectories around the Moon. Dust could also be transported through electrostatic forces that arise due to charging via photo-emission and plasma currents ranging from 10V to –4kV.
Dust levitations in the lowest few meters of the exosphere have been confirmed by imagery from the Surveyor landers. The reports from Apollo astronauts of glow in the upper region of the exosphere suggests the presence of dust at altitudes of up to 50 Kilometers and led to the hypothesis of dust fountains in which large surface potentials are exhibited near the terminator regions allowing small particles to be lofted to high altitudes.
Measurements by the Apollo Lunar Ejecta and Meteorites Experiments LEAM have shown a significant increase of dust activity around the time of sunrise and sunset (= near the terminator region). LADEE’s orbit has been designed in a way that allows the orbiter to pass the terminator region at low altitudes which will allow in-situ dust measurements by the LDEX instrument. Additionally, sun occultation and limb observations will return detailed information on dust distribution in the lunar exosphere.
With a mission duration of three lunar cycles, LADEE will be able to determine dust properties on a short scale and larger scale to examine the influence of solar events and geotail crossings on dust density within the exosphere.
Surveyor Images showing exospheric Glow
Limb Observations to model Exospheric Dust Densities
After the discovery of abundant water on the Moon by the LCROSS mission as well as Cassini, the Moon Mineralogy Mapper and Deep Impact, the question of volatiles on the Moon has been given much more attention. The big mystery is whether there is a dynamic volatile system with sources and losses as well as transport mechanisms through the exosphere. LADEE’s UVS and NMS instruments can detect exospheric water and hydroxyl and thus answer that question.
The presence of abundant water in permanently shadowed regions on the Moon has been confirmed by LCROSS, but whether that water is part of a static or dynamic system is not clear. Measurements by the Moon Mineralogy Mapper, Cassini and Deep Impact have shown the presence of a surfical water in a layer that might be just a few millimeters thick. Deep Impact observations also point to a diurnal variation in surface water content that indicates that this water is subject to thermally-driven surface exchange processes. It is possible that water is generated by the reaction of trapped hydrogen from solar wind with oxygen which is then transported to permanently shadowed regions where it accumulates.
LADEE takes a close look at present-day dynamic processes that are at play in volatile sources and dynamics. Measurements across the terminator will show the importance of thermal desorption of volatiles and regular passage through the solar wind and Earth’s magnetotail could demonstrate the importance of those phenomena. Should LADEE detect no water or OH, theories of immobilized water would be confirmed.