DEC (Dust Electrostatic Collector): an innovative QCM-device for the Lunar environment

The Moon is a prime location for space observations and for investigating fundamental questions about the origin and evolution of our Solar System.

One of the most important problems in view of future missions that will take place on the lunar surface (i.e., NASA Artemis program) is that of lunar dust, which can not only create interference with the instrumentation up to damage it (due to its small size and to its charge), but could also cause important problems for a possible human crew (i.e., vision obstruction, damage to spacesuits, obstruction of the respiratory tract). The study and characterization of lunar dust (physical properties, granulometry, electric charge) is therefore an aspect of fundamental importance and will be the main object of this research project. In order to study and characterize the Moon and its environment, several missions have been conducted over the years, among which we mainly remember the LEAM experiment [1], then LDEX [2,3,4] and finally the two missions Chang' E-3 (CE-3) and Chang' E-5 (CE-5) [5,6,7,8].

The lunar surface is exposed to solar radiation (which includes visible, ultraviolet and X radiation), as well as to the solar wind, charged particles, galactic cosmic rays and high-speed micrometeorites: this interaction can lead to various dust charging processes (electron/ion collisions, photoelectric emissions, contact charging with electron transfer).

 

Recent studies [9] have highlighted the presence of suspended dust of charged particles within 1 meter of the lunar surface. The interaction of the lunar surface with UV radiation and plasma causes the emission and re-absorption of photoelectrons and/or secondary electrons at the walls of microcavities formed between neighboring dust particles below the surface, which are responsible for generating unexpectedly large negative charges and intense particle-particle repulsive forces (Coulomb Force) to mobilize and lift off dust particles. The dynamics of charged dust in response to various electrical environments on airless bodies has been studied mainly theoretically and by computer simulations (e.g. the "Patched Charge Model" [10,11,12], a model that has been validated in the laboratory [Figure 1]), while in situ measurements are lacking.

 

Figure 1. Patched Charge Model representation [10]

 

To achieve this purpose, this work presents the development of the DEC (Dust Electrostatic Collector) instrument, whose basic concept mirrors that of the QCMs and which allows to pave the way for in situ applications in the near future. DEC is under development by an Italian team led by INAF-IAPS, in collaboration with CNR-IIA and Politecnico di Milano.  

The main goal of this activity is therefore to demonstrate the capability of an instrument based on QCM sensors to attract charged particles (of dimensions ≤10 μm), while evaluating their charge-to-mass ratio. The object of the research itself and its consequent practical applications are highly innovative elements since the study of these charged lunar particles has so far been theoretical and never faced experimentally.

The instrument core is a QCM (Quartz Crystal Microbalance) which oscillates at a resonant frequency depending on the mass deposited on its sensible area [13]. The deposition process of these charged particles on the instrument would be favored by a particular type of design for the breadboard (Figure 2), using an electron gun to simulate the charging and lofting processes and exploiting the effect of electric fields to attract the particles, a concept completely innovative and not yet applied in the field of microbalances. As matter of fact, the instrument will be capable of attracting charged dust grains by means of a variable Electric Field (EF), generated locally by the instrument itself. The application of this EF will break the equilibrium between the Electric and the Gravity Fields on the Moon, allowing the electrically charged dust grains to be attracted toward the microbalance and, in principle, by changing the local EF it will be possible to attract grain with different size and electric charge.

Figure 2. Setup design

It is assumed that the electrostatic lofting that occurs on the lunar surface is the same phenomenon that occurs on other bodies in the Solar System; for this reason, the study of the phenomena that occur on the lunar surface represents a first step towards the realization of a human outpost on the Moon and an excellent laboratory to characterize the processes that may govern the evolution of atmosphere-less planetary surfaces throughout the Solar System.

 

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[9] Dust charging and transport on airless planetary bodies. Wang, X., et al. 2016. Geophysical Research Letters 43.12: 6103-6110.

[10] Wang, X., et al., Geophysical Research Letters 43.12 (2016): 6103-6110.

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