Electric Dust Devils and Dust Storms

1. Electric Dust Devils and Dust Storms Nilton O. Renno University of Michigan and Many Students & Collaborators

2. Contributors • Students Jacquelin Koch, UM Charles Yana, UM Aimee Covert, UM Dhruv Sarna, UM Lucas Fabian, UM • Collaborators Sushil Atreya, UM Ah San Wong, UM Bill Farrel, NASA/GSFC Gregory Delory, UC Berkeley Imke de Pater, UC Berkeley Christopher Ruf, UM Jay Wilson, Signal Research Corporation Maarten Roos Serrote, Observatorio Astronomico de Lisboa Stephen Rogacki, UM Vincent Abreu, UM AOSS PSG Meeting

3. Outline • Background • Heat Flux and Aerosol Budget on • Earth • Mars • Dust & the Search for Life on Mars • Electric Activity in Dust Devils & Storms • Observations • Laboratory Results • Theory • Evidence of Electrical Activity in Martian Dust Events • Conclusions AOSS PSG Meeting

4. Background AOSS PSG Meeting

5. Terrestrial Dust Devils Ubiquitous in arid regionsfrom spring to fall AOSS PSG Meeting

6. Dusty Convective Plumes Ubiquitous in arid regionsfrom spring to fall AOSS PSG Meeting

7. Electric Field in Terrestrial Dust Devils AOSS PSG Meeting

8. Large Terrestrial Dust Storms Relatively rare events AOSS PSG Meeting

9. Martian Dust Devil & Storms Ubiquitous Relatively rare Both Images are Courtesy of NASA/JPL/MSS. AOSS PSG Meeting

10. Electric Activity in Dust Devils and Dust Storms AOSS PSG Meeting

11. Charging Mechanism and Charge Separation • Asymmetric rubbing during collisions of sand/dust particles produce a net transfer of electrons to the smaller particles. • The smaller particles rise in the updraft, leading to charge separation. • Electric fields in excess of 10 kV/m have been measured at ~ 1.0 m above the surface. AOSS PSG Meeting

12. Recall the Electric Fields in Terrestrial Dust Devils AOSS PSG Meeting

13. Microdischarges Between Colliding Particles • Laboratory experiments suggest that an electric arc occurs after two particles collide, while they break away from each other (Bernhard, 1992). • The electric arcs (sparks) produce non-thermal electromagnetic radiation. • The bulk electric field observed in dust devils and dust storms is due to the residual charges left on the dust particles (Renno et al., 2004). AOSS PSG Meeting

14. Barometer Light(an example of picosecond microdischarges) • The length & therefore time-scale of the discharges depend on the activation energy of the gas. From Budakian et al. (1998) AOSS PSG Meeting

15. Laboratory Results of Electric Activity in Dust Devils & Storms AOSS PSG Meeting

16. Our Instrument KU detector head Tri-pod camera mount AOSS PSG Meeting

17. Hematite at 1000 hPa(195 mV) AOSS PSG Meeting

18. Hematite at 120 hPa(195 mV, but more peaks) AOSS PSG Meeting

19. Basalt at 120 hPa(same amplitude, but shorter tau) AOSS PSG Meeting

20. Millimeter Size Spheres(larger tau) AOSS PSG Meeting

21. A Scaling Theory for Electromagnetic Activity in Dust Devils & Storms AOSS PSG Meeting

22. A Simple Model for the Emission of Non-Thermal Radiation • Charging during collisions between sand and dust particles is limited by field emission. • Locally, a pair of colliding particles form a flat plate capacitor. • The total energy emitted during a micro-discharge comes from discharging the capacitor. • The emission’s spectral distribution depends on the time-scale to discharge the capacitor. AOSS PSG Meeting

23. Theoretical Results • Charge in the particles’ contact region q ~ A E ε • Energy dissipated per microdischarge is W ~ ½ C V2 ~ ½ q E d ~ ½ A E2ε d ~ 0.01 r2 (MKSI units) • Total Energy Flux • F ~ W / (τ x area over which W is distributed) ~ ½ V2 / Resistance ~ ½ E d / Resistance ~ 0.005 W/m2 (in our lab device) (it does not depend on the particles’ size!) Permitivity of air ~ 8.85 x 10-12 C2 N-1 m2 Electric fields: Max: Field emission; Res: Breakdown Particle contact area Length of the microdischarge ~15 μm. AOSS PSG Meeting

24. Theoretical Results • The time-scale of the microdischarge is τ = R C = R q/V ~ 2 R r2ε /d ~ 10 r2 (MKSI units), where is ε the permitivity of air, and d is the length of the microdischarge (from lab results). • Millimeter size particles: τ ~ 10-5 s → 1/τ ~ 0.1 MHz • 100 μm particles: τ ~ 10-7 s → 1/τ ~ 0.01 GHz • 10 μm particles: τ ~ 10-9 s → 1/τ ~ 1 GHz • 1 μm particles: τ ~ 10-11 s → 1/τ ~ 100 GHz (our current oscilloscope is limited to samplings at 1GHz) • These values are consistent with our laboratory results. ~107 Ω from Airton’s formula. AOSS PSG Meeting

25. Results for Mars (r ~ 10 - 100 μm)(see Renno et al., 2003) • Residual charge qres ~ A Eresε~ 10-18 to 10-16 C. • Maximum charge qMax ~ A Efieldε~ 10-13 to 10-11 C. • Number of collisions in a Mars DD ~1013 colisions/m2s. • Energy dissipated in a discharge ~ r2/100 ~ 10-12 to 10-10 J. • Energy flux in DD ~ 10 (small) and 103 W/m2 (large). (the energy flux could feedback on the vortex intensity!) • Time scale of a discharge τ~10-9 to 10-7 s. • The peak emission is at ω ~ 0.01 to 1 GHz. ~109 V/m AOSS PSG Meeting

26. Calculation of the Bulk Electric Fields in Dust Devils • It follows from the calculations of Renno et al. (2003) that the residual charge in terrestrial dust particles of ~10 μm radius is qres ~ 3 x 10-15 C. • This value is consistent with the results of laboratory experiments (Bernhard et al., 1992) and the observation of dust particles with charges of up to 10-12 C in terrestrial dust devils (Farrell et al., 2004). • Dust devils have dust concentrations np ~107 particles/m3 and dust fluxes Fd ~ 108 particles/m2s (Renno et al., 2003). • Thus, terrestrial dust devils have maximum charge densities of ~10-8 C/m3 and can produce vertical currents ~10-7 A/m2. AOSS PSG Meeting

27. Bulk Electric Fields in Dust Devils • Approximating a dust devil by a cylinder of radius R and height H, we find that it can produce near surface electric fields Er ~ (k π R2 np qres H) / [r (r2 + H2)1/2], where k = 9 x 109 N m2/C2 is Coulomb’s constant, and r (> R) is the distance from the center of the dust devil. • Thus, when H >> R we get Er ~ k π np qres R. AOSS PSG Meeting

28. Bulk Electric Fields in Terrestrial Dust Devils • Thus, for terrestrial dust devils Er ~ 850 R. • It follows from the above that the electric field near the boundary (r ~ R) of a typical terrestrial dust devil of R ~ 10 m is ER ~ 8.5 kV/m. • A large terrestrial dust devil of R ~ 100 m can produce electrical fields of ER ~ 85 kV/m. AOSS PSG Meeting

29. Bulk Electric Fields in Terrestrial Dust Storms • In large dust storms, R >> H and we get Er ~ 850 H. • It follows from the above that the electric field near the boundary (r ~ R) of a typical terrestrial dust storm of H ~ 100 m is ER ~ 85 kV/m. • The above value is consistent with observations. AOSS PSG Meeting

30. Bulk Electric Fields in Martian Dust Devils • The residual charges in Martian dust particles are between 1 and 3 orders of magnitude smaller than in terrestrial dust devils. • For Martian dust events Er ~ 30 R. Thus, the much larger Martian dust devils can produce electric fields larger than those observed in terrestrial dust devils. • A typical Martian dust devil of R ~ 100 m produces electric fields ER ~ 3 kV/m, while a big Martian dust devil of R ~ 1 km produces ER ~ 30 kV/m. • Since electric breakdown occurs at ~20 kV/m in the Martian atmosphere, it might occur in the largest Martian dust devils. AOSS PSG Meeting

31. Bulk Electric Fields in Martian Dust Storms • The larger dust storms can produce electric fields Er ~ 30 H much larger than those of dust devils. • A regional Martian dust storm of height H ~ 10 km could produce electric fields ER ~ 30 kV/m. • A large Martian dust storm of height H ~ 15 km might produce electric fields ER ~ 450 kV/m. • Thus, electric breakdown must occur in Martian dust storms and keep their electric fields around the breakdown value of ~ 20 kV/m. AOSS PSG Meeting

32. Chemical Implications of Predicted Bulk Electric Fields on Mars • Electric fields of ~20 kV/m leads to the production of more than two orders of magnitude H2O2 than that produced by solar UV radiation. • Hydrogen peroxide saturates the Martian atmosphere and perhaps H2O2 rains out mixed with dust (Atreya et al., 2005). Mars Electro-Photo Chemistry (Atreya et al., 2005). AOSS PSG Meeting

33. Evidence of Electromagnetic Activity in Martian Dust Events AOSS PSG Meeting

34. Fluctuations in Mars Disk Brightness Caused by Electric Dust Devils & Storms • We assume that ~ 1% of the energy dissipated is non-thermal radiation (a lower bound based on thermalized channels). • Dust storms with active regions covering ~10% of the Martian disk can produce brightness temperature perturbations of ~10 K (Renno et al., 2003). AOSS PSG Meeting

35. Observations of Mars Microwave Disk Brightness Doherty et al. (1979). AOSS PSG Meeting

36. Artistic Conception of a Glowing Martian Dust Devil AOSS PSG Meeting

37. Conclusions AOSS PSG Meeting

38. Conclusions • Microdischarges between colliding sand/dust particles produce non-thermal electromagnetic radiation. • Observations of Mars with the VLA can unambiguously test this idea. • Properly designed sensors can fingerprint the non-thermal emission. AOSS PSG Meeting

39. That’s All Folks! AOSS PSG Meeting