Lunar Volatiles Network: A Ring Around The Poles
The lunar surface is constantly being weathered by the solar wind and bombarded by meteoric infall. Solar wind implants protons and an array of multi-charged ion species into the surface. These are re-emitted as reflected protons, thermal and energetic atomic hydrogen, molecular hydrogen, and methane. However, once emitted, it remains unclear how this exospheric material migrates and deposits back onto the surface and to the poles. Micrometeoroids deliver volatiles, promptly releasing some fraction to escape the Moon’s gravity. However, the lower energy portion of the released products returns to the lunar surface. Again, it remains unclear how this meteoric-released material migrates and deposits itself over the lunar surface. An overarching question driving the network: How does the dynamic modern volatile system connect to the (possibly) ancient buried reservoirs of hydrogen located in the polar cold traps? If H and H-bearing molecule migration is not as potent as we believe, then the modern environmental processes operating at mid-latitudes may not impact the reservoirs in the cold traps. The two systems would be mostly isolated. We are learning now that migration depends on the specific form of the released hydrogen-bearing compound and the nature of the surface adsorption encountered along the migration pathway. For example, recent lab work by Poston et al. [2015, Icarus, 255, 24] suggest that the sorbent properties of lunar regolith are complex, with mature surfaces containing some fraction of high energy binding sites capable of trapping water molecules even in warm temperatures, thereby limiting migration. Conversely, this effect would increase the likelihood of finding H trapped at mid- latitudes (possibly like that recently reported via IR observations [Li and Milliken, 2017, Sci. Adv., 3:e1701471 ]). The understanding of the volatile migration is not only of basic science interest, but will also lead to greater understanding of the renewability of the polar cold trap resources for use in exploration. The lunar volatile system may be also representative of similar dynamic processes at Mercury or Ceres. We present a preliminary concept for a network mission to advance the understanding of the lunar volatile system, drawing analogies from how we study Earth’s weather. We will use a combination of remote sensing orbital assets to provide global context in combination with a set of landed stations providing ground-level in situ observations of the local volatile influx. Taken together, these observations will feed forward into new-generation volatile transport models that fully-integrate the observational sets to provide back- and forward- history of the species migration. The network will also make use of periods of extreme space environmental ‘weather’, monitoring volatile migration changes during passing CMEs and meteor showers. Such extremes that alter the driving source can provide key insights into the overall volatile exosphere-surface system dynamics. The orbital asset would be instrumented with a combination of UV, IR and in situ exospheric instruments. Since the lunar surface is a site of implantation and sorption, each lander would have the capability to examine the local surface in the IR and UV to detect surface-trapped species and their variation with local time, solar wind exposure, and temperature. A neutral mass spectrometer would also be included to obtain in situ measurements of the local near-surface gas environment. The landed stations could each also have an exospheric UV system looking above or out just above the horizon to obtain a column measurement of local exospheric content, although a development effort might be to consider other methods to examine the exosphere from a surface station. A plasma ion mass spectrometer, electron spectrometer, and dust detector could also be onboard to obtain information on the drivers and exo-ionosphere formation. We propose that a set of landed stations would ring both poles. A set would also be located at specific latitudinal locations (i.e., every 30o in latitude) along two or more pre-defined longitudes. One or more of the landed stations could also release water, D2O, or some other tracer gas which then could be traced by the other stations in the network. Development efforts include an evolved definition of the lander instruments, lander nighttime survivability, integration of the landed measurements to the contextual orbital measurements, integration of the landed and orbital measurements into volatile transport models, and cost estimation.