Lunar Surface Atmosphere Spectrometer (LSAS)
Objectives: The instrument LSAS is designed to study the composition and structure of the Lunar atmosphere and surface by monitoring pickup ions derived from photoionization or ion sputtering on the surface. Sources of this surface bound atmosphere include interior release, particle sputtering, chemical sputtering, thermal desorption, and meteoritic bombardment. Because of the intimate connection of such an atmosphere with the surface and interior of the Moon, detailed studies of the composition, structure, and kinetic properties of the Lunar atmosphere will advance knowledge and understanding of the surface mineralogy and composition, all basic to understanding formation and evolution of this body. LSAS will:
• Map the composition of the Lunar surface.
• Monitor the response of the Lunar surface to energetic radiation.
• Observe sputtering from the permanently shadowed craters on the Moon
• Determine the composition of the lunar atmosphere.
• Search for lunar transient events.
• Study the plasma physics of the lunar environment including the presence and character of magnetic anomalies and mini magnetospheres, surface charging, properties of the dayside/nightside transition, the effects due to the Earth’s magnetotail, and effects due to the lunar wake (possible ambipolar E-field at terminator)
Mission Description: LSAS is an instrument proposed for the Lunar Reconnaissance Orbiter, the first robotic mission to the moon under the Exploration Initiative. However, along with the atmosphere of Earth's moon, a number of solar system bodies like Mercury, Io, Europa, Callisto, asteroids, etc. are collisionless exospheres whose inner boundaries are the surfaces of these objects. We anticipate that future orbital missions will explore these objects and will include a plasma package that will be enhanced with a near-source pickup ion measurement. We note that LSAS style instruments will help study the interaction of the Solar Wind with planetary bodies while satisfying many of the objectives of the Exploration Initiative.
Measurement Strategy: The Earth’s moon is the initial target of NASA’s Vision for Exploration, and at the moon the primary loss mechanism for ions heavier than helium in the atmosphere is ionization followed by pickup by the passing solar wind.This phenomenon is the key toward a sensitive measurement of the lunar atmosphere and its study made possible by the observational and theoretical methods of the SEC community. Ions created at or near the surface experience an acceleration due to the motional electric field derived from the streaming solar wind and the imbedded interplanetary magnetic field. This acceleration will deliver ions to an orbiting spacecraft where an ion-mass spectrometer can characterize the ion flux. With knowledge of the ion direction, energy, and the motional electric field, it will be possible to map the measured ion back to the lunar surface, and for sputtered ions, develop a map of the composition of the lunar surface.
LSAS is an ion composition mass spectrometer with heritage derived instruments on previous SEC missions including the ROSINA instrument of ROSETTA, the MESSENGER-FIPS instrument, as well as instruments on Wind and ACE.
The instrument, LSAS, is designed to study the composition and structure of the Lunar atmosphere and surface by monitoring pickup ions derived from photoionization or ion sputtering on the surface. In addition to these planetary objectives LSAS monitors the response of an exospheric body to the solar wind, the plasma physics of the lunar environment including the presence and character of magnetic anomalies and mini magnetospheres, surface charging, properties of the dayside/nightside transition, the effects due to the Earth’s magnetotail, and effects due to the lunar wake (possible ambipolar E-field at terminator). All of these topics are of interest to the SEC community.
LSAS is an instrument proposal to the Lunar Reconnaissance Orbiter the first robotic mission to the moon under the Exploration Initiative. As such it was important for us to address the Vision for Space Exploration, the requirements of LRO and the Exploration Initiative. The table to the right gives our responses to these requirements and while the success of our approach remains to be seen, we suggest this as one strategy for addressing this question in future SEC and Exploration Initiative Proposals.