Elsevier

Planetary and Space Science

Volume 59, Issue 14, November 2011, Pages 1758-1768
Planetary and Space Science

The role of cohesive forces in particle launching on the Moon and asteroids

https://doi.org/10.1016/j.pss.2011.04.017Get rights and content

Abstract

The movement of small dust particles due to electrostatic forces, seismic activity and micrometeoroid bombardment has been hypothesized to occur on the Moon and asteroids. There currently exists significant uncertainty in the method of launching these small dust particles, which in turn makes the selection of accurate initial conditions for numerical simulations difficult. We evaluate the electric field strength required to launch small particles given surface gravitation, cohesion and seismic shaking. We find that the electric field strength required for dust particle launching is dominated by the cohesive force for micron-sized dust particles. There exists an intermediate dust particle size that requires the least electric field strength to launch. We see that the inclusion of the cohesive force significantly influences our understanding of dust lofting.

Highlights

► Modeled electrostatic force required to loft dust particles on Moon and asteroids. ► Including cohesion significantly increases the electrostatic force required. ► Charge amplification beyond Gauss' law prediction is required to loft particles. ► Particles requiring smallest electric field for lofting may be larger than 10μm.

Section snippets

Particle equations of motion

A dust particle resting on the surface of an airless body is subject to four forces: gravity (Fgrav), cohesion (Fco), electrostatics (Fes), and the normal force (N). A diagram of a dust particle resting on a planar surface can be seen in Fig. 1.

As a first order approximation for seismic activity, we model every point on the planar surface to have both vertical and horizontal sinusoidal acceleration:a=Atsin(Ωtt)x^+Ansin(Ωnt)y^where At is the amplitude of the transverse oscillations, An is the

Particle launching and separation

For the purposes of this work, a dust particle will be said to be launched if its initial inertial acceleration is in the positive direction (up) and has a magnitude greater than the positive acceleration rate of the surface (considering vertical seismic acceleration). For instance, a particle with a negative inertial acceleration (y¨i<0) will not be considered to be launched although it may no longer be in contact with the planar surface (y¨rel>0). A particle is said to be separated from the

Effects of cohesion neglecting seismic shaking

Given Eq. (19) and assuming no seismic acceleration (An=0), we can plot the required electric field strength for particles of different sizes to be launched (Fig. 2), given varying surface gravitation (see Table 1) and varying surface cleanliness (S). Essentially, Fig. 2 provides the necessary electric field strength and charging as a function of particle size in order for electrostatic dust lofting to occur, if the charge on a dust particle can be approximated by Gauss' law. From Fig. 2, we

Experimental agreement

There are two classes of experiments that are relevant to the discussion of cohesion and dust motion that has been presented here. First, there have been a series of experiments looking at the motion of dust particles initially resting on a surface and subjected to a plasma sheath. The second class of experiments supplies evidence for the preferential motion of larger sized dust particles (as opposed to submicron particles) due cohesion in a powder. We will discuss the agreement of our

Discussion

We have shown that cohesive forces are significant when considering the particle sizes that could be launched off the surface of the Moon and asteroids. The particles that will be launched from the Moon at the lowest electric field strength will be 100–1000μm in radius. The tendency of large conglomerates of particles rather than the individual submicron particles to be lofted has been observed by Marshall et al. (in press). Additionally, the difficulty in clearing films of particles less than

Conclusion

The movement of small dust particles due to electrostatic forces has been hypothesized to occur on the Moon and asteroids. However, there exists significant uncertainty in the method of launching these particles off the surfaces of these bodies. By taking an inventory of the forces affecting dust particles resting on stationary and seismically active surfaces, we have shown that the electric field strength required to launch a particle from the surface is at least an order of magnitude larger

Acknowledgments

C.M.H. wishes to acknowledge Z. Sternovsky and the two reviewers for their constructive criticisms. C.M.H. acknowledges support from NASA Headquarters under the NASA Earth and Space Science Fellowship Program: Grant NNX09AR51H. D.J.S. acknowledges support from the NASA Planetary Geology and Geophysics Program.

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