Hurley, Dana - Propagation of Water in the Chang’e-3 Exhaust Plume from LADEE Observations

Abstract: 
Landing a vehicle on the Moon introduces gases from the exhaust plume into the normally sparse lunar exosphere.  A robotic lander such as the Chinese Chang’e-3 lander on the surface of the Moon would require burning an estimated 106 g of rocket fuel over ~12 minutes.  This rocket exhaust constitutes a 100 times temporary enhancement to the source rate to the lunar exosphere and an increase in the total mass of 10%.  It is important to understand the extent and duration of these effects if operations on the Moon become frequent. 
Whereas the native lunar exosphere is comprised primarily of helium and argon; the rocket exhaust comprises water, carbon dioxide, ammonia, and other HCNO products.  The distribution of particles in the lunar exosphere is largely controlled by the interactions between the particles and the lunar surface. Thus, if the propagation of the exhaust vapors can be monitored, it can reveal previously unknown properties of the gas-surface interaction with the lunar regolith. 
LADEE was in an elliptical, retrograde orbit with an inclination of 22° about the equator.  On the day of the Chang’e-3 landing, the Neutral Mass Spectrometer (NMS) acquired data for two orbits prior to the landing and on 4 of the orbits after the landing.  Relevant data were also acquired during one orbit on the day following the landing.  On each of these orbits, the NMS is turned on just after passing noon lunar local time and acquires in situ measurements of the neutral density about 2000 km from the landing site.   Water is detected at 2 sigma above the background level at this location for the three NMS orbits following the landing. 
We model the release and propagation of the exhaust gases on the Moon and compare to observations in orbit around the Moon from the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft. Model results are extremely sensitive to the assumed surface interactions.  In particular, because the exhaust gases have such a large initial velocity, the degree to which the molecules thermalize on contact with the regolith modulates the subsequent propagation of exhaust gases.  Without full thermalization, the bulk of the exhaust gases will escape the Moon on the first hop off of the lunar surface.  We present the comparisons between the models and the observations to constrain the amount of thermalization in the model required to agree with the observations.  Therefore, LADEE utilizes volatile constituents released during the Chang’e-3 landing on the Moon to determine surface-exosphere interactions of non-native species to the lunar environment.  This opportunistic observation adds to the planned scientific return of the LADEE mission.