The latitude dependence of dielectric breakdown on the Moon

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Garry Harris
5 years ago
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Exploration Research Virtual Institute 1

Propulsion Laboratory, California Institute

1 The latitude dependence of dielectric breakdown on the Moon Andrew Jordan1,2, T. J. Stubbs3,2, J. K. Wilson1,2, P. O. Hayne4, N. A. Schwadron1,2, H. E. Spence1,2 and N. R. Izenberg5 EOS Space Science Center, University of New Hampshire 2 Solar System Exploration Research Virtual Institute 1 NASA Goddard Space Flight Center 4 Jet Propulsion Laboratory, California Institute of Technology 3 5 The Johns Hopkins University Applied Physics Laboratory

2 Breakdown: explosive process that rapidly creates conducting channels in a dielectric via melting and vaporizing

3 Breakdown: explosive process that rapidly creates conducting channels in a dielectric via melting and vaporizing Examples: Spacecraft anomalies (Koons et al., 1998) Io regolith (Campins and Krider, 1989) Lunar regolith (Kirkici et al., 1996) Shusterman et al. (#5056) (Campins and Krider, 1989)

4 Breakdown: explosive process that rapidly creates conducting channels in a dielectric via melting and vaporizing Examples: Spacecraft anomalies (Koons et al., 1998) Io regolith (Campins and Krider, 1989) Lunar regolith (Kirkici et al., 1996) Shusterman et al. (#5056) Things to remember: Discharging timescale: characteristic timescale to dissipate charge build-up ~1010 particles cm-2 must be deposited within discharging timescale (Campins and Krider, 1989)

5 Colder regolith: less electrically conductive slower to dissipate charge more likely to undergo breakdown So PSRs are a prime location for breakdown

6 (Jordan et al., 2016) Colder regolith: less electrically conductive slower to dissipate charge more likely to undergo breakdown So PSRs are a prime location for breakdown

7 (Jordan et al., 2016) Colder regolith: less electrically conductive slower to dissipate charge more likely to undergo breakdown So PSRs are a prime location for breakdown In PSRs, breakdown weathering predicted to be comparable to impact weathering: 10-25% melted/vaporized

Colder regolith: less electrically conductive slower to dissipate charge more likely to undergo breakdown So PSRs are a prime location for breakdown (Jordan et al.

8 Colder regolith: less electrically conductive slower to dissipate charge more likely to undergo breakdown So PSRs are a prime location for breakdown (Jordan et al., 2016) Goal: Determine whether dielectric breakdown may affect the evolution of lunar regolith at all latitudes In PSRs, breakdown weathering predicted to be comparable to impact weathering: 10-25% melted/vaporized

9 (results from model by Hayne and Aharonson, 2015; Jordan et al., under review)

10 Permittivity (~2ε0) Discharging timescale = Conductivity (fcn of temp) (Jordan et al., under review)

11 Permittivity (~2ε0) Discharging timescale = Conductivity (fcn of temp) Discharging timescale is inversely related to temperature (Jordan et al., under review)

12 CRaTER SEP data (>10 MeV protons) (Jordan et al., under review) SEP events deposit ~95% of their fluence in 3 days (Kecskeméty et al., 2009)

13 Breakdown predicted to occur for all events >1010 cm-2 du rati o n Breakdown predicted to occur only for larger events (Jordan et al., under review) SEP

14 Breakdown weathering may be comparable to impact weathering in PSRs

15 Energy density weathering needed to Breakdown may be comparable melt/vaporize all regolith to impact weathering in PSRs (Cintala, 1992)

16 Flux of breakdown energy density into top 1 mm of regolith Energy density weathering needed to Breakdown may be comparable melt/vaporize all regolith to impact weathering in PSRs (Cintala, 1992)

17 Flux of breakdown energy density into top 1 mm of regolith Exposure time = time until protected from SEPs by ~1 mm of regolith (~1 Myr) Energy density weathering needed to Breakdown may be comparable melt/vaporize all regolith to impact weathering in PSRs (Cintala, 1992)

18 Flux of breakdown energy density into top 1 mm of regolith Energy density from breakdown Exposure time = time until protected from SEPs by ~1 mm of regolith (~1 Myr) Energy density weathering needed to Breakdown may be comparable melt/vaporize all regolith to impact weathering in PSRs (Cintala, 1992)

19 Flux of breakdown energy density into top 1 mm of regolith Energy density from breakdown Exposure time = time until protected from SEPs by ~1 mm of regolith (~1 Myr) Energy density needed to melt/vaporize regolith Energy density weathering needed to Breakdown may be comparable melt/vaporize all regolith to impact weathering in PSRs (Cintala, 1992)

20 Flux of breakdown energy density into top 1 mm of regolith Energy density from breakdown Exposure time = time until protected from SEPs by ~1 mm of regolith (~1 Myr) Energy density needed to melt/vaporize regolith = Energy density weathering needed to Breakdown may be comparable melt/vaporize all regolith to impact weathering in PSRs (Cintala, 1992) Regolith fraction affected by breakdown

21 Flux of breakdown energy into top 1 mm of regolith Folds in both event rates and fluences from Feynman et al. (1993) ~2.5x (Jordan et al., under review) For comparison: ~109 J m-3 needed to melt/vaporize all regolith (Cintala, 1992)

22 Energy density from breakdown Energy density needed to melt/vaporize regolith = Fraction of regolith affected by breakdown

23 = Fraction of regolith affected by breakdown (assuming exposure time of 1 Myr and SEP penetration of 1 mm) (Jordan et al., under review) Energy density from breakdown Energy density needed to melt/vaporize regolith

24 = Fraction of regolith affected X 100 = 4-7% (global average) by breakdown (assuming exposure time of 1 Myr and SEP penetration of 1 mm) (Jordan et al., under review) Energy density from breakdown Energy density needed to melt/vaporize regolith

25 Dielectric breakdown weathering may play an important role in regolith evolution outside PSRs PSRs: 10-25% 85 : 6-12% 45 : 4-8% 0 : 2-5% 45 : 4-8% 85 : 6-12% PSRs: 10-25%

26 Dielectric breakdown weathering may play an important role in regolith evolution outside PSRs PSRs: 10-25% 85 : 6-12% 45 : 4-8% 0 : 2-5% Future Work More modeling needed to understand charge deposition as function of depth during SEP events Experiments (e.g., Shusterman et al.) needed to... Determine if breakdown weathering could create an observable latitudinal dependence (like that in Thomson et al., 2016, LPSC?) Discover if sparked material is hiding in Apollo samples 45 : 4-8% 85 : 6-12% PSRs: 10-25%

27 Backup Slides

30 Solar energetic particles have gyroradii larger than the Moon (1 MeV proton in 5 nt magnetic field: ~30,000 km [~17 RMoon]). (Joyce et al., 2013)

31 Events must have fluence > 1010 cm-2 to cause breakdown Some events with fluence 1010 cm-2 cause breakdown All events with fluence 1010 cm-2 cause breakdown

32 Energy density needed to affect all regolith (Cintala, 1992) Vaporize: 7.3 x 109 J m-3 90% melt + 10% vaporize: 3.8 x 109 J m-3

33 Melt Jordan et al. (2015)

34 Vesta Mercury High-obliquity asteroids (Vasavada et al., 1999) (Stubbs and Wang, 2012) PSRs: <100 K (Paige et al., 2013) Nightside: Similar to Moon Poles: In shadow for long time Polar craters: Avg. temp. <100 K

35 Galileo data Satellite Thebe 1.29 ± 0.12 Amalthea 1.26 ± 0.10 Metis 1.28 ± 0.09 (Simonelli et al., 2000) (Fischer et al., 1996) Voyager experienced breakdown (Leung et al., 1986) Leading/Trailing Albedo Ratio

36 Spacecraft anomalies caused by the space environment (Koons et al., 1998) Series Number of anomalies % Category 1 Electrostatic discharge Category 2 Single event upset Category 3 Radiation damage Category 4 Miscellaneous (impacts, etc.)

37 As large SEP events continually cause breakdown, they increase the fraction of regolith affected by breakdown. Event 1 Event 2 Event 3

38 Impact ejecta buries the old regolith, exposing new regolith to SEP events that cause breakdown. Event 4 Event 1 Event 5 Event 2 Event 6 Event 3

39 Impact ejecta buries the old regolith, exposing new regolith to SEP events that cause breakdown. Event 4 Event 1 Event 5 Event 2 Event 6 Event 3 Gardening enables breakdown weathering to affect more than just the current top 1 mm of regolith.

40 Previous slides show gardening as burial, not mixing. But this is still a good approximation. If the gardened zone is thoroughly mixed, then on average, each grain has spent equal times at all depths from the surface to the bottom of the gardened zone. This agrees with the burial version to within a factor of two.

N. A. Schwadron, J. K. Wilson, M. D. Looper, A. P. Jordan, H. E. Spence, J. B. Blake, A. W. Case, Y. Iwata, J. C. Kasper, W. M. Farrell, D. J.

Signatures of Volatiles in the CRaTER Proton Albedo N. A. Schwadron, J. K. Wilson, M. D. Looper, A. P. Jordan, H. E. Spence, J. B. Blake, A. W. Case, Y. Iwata, J. C. Kasper, W. M. Farrell, D. J. Lawrence,