TheThe LunarLunar DustDust --PlasmaPlasma EnvironmentEnvironment

Timothy J. Stubbs 1,2 , William M. Farrell 2, Jasper S. Halekas 3, Michael R. Collier 2, Richard R. Vondrak 2, & Gregory T. Delory 3

Timothy.J.Stubbs@nasa.gov

Lunar X-ray Observatory(LXO)/ Magnetosheath Explorer (MagEX) meeting, Hilton Garden Inn, October 25, 2007.

1 University of Maryland, Baltimore County 2 NASA Goddard Space Flight Center 3 University of California, Berkeley TheThe ApolloApollo AstronautAstronaut ExperienceExperience ofof thethe LunarLunar DustDust --PlasmaPlasma EnvironmentEnvironment

“… one of the most aggravating, restricting facets of lunar surface exploration is the dust and its adherence to everything no matter what kind of material, whether it be skin, suit material, metal, no matter what it be and it’s restrictive friction-like action to everything it gets on. ”

Eugene Cernan, Commander Apollo 17. EvidenceEvidence forfor DustDust AboveAbove thethe LunarLunar SurfaceSurface

Horizon glow from forward scattered sunlight

• Dust grains with radius of 5 – 6 m at about 10 to 30 cm from the surface, where electrostatic and gravitational forces balance. • Horizon glow ~10 7 too bright to be explained by micro-meteoroid- generated ejecta [Zook et al., 1995].

Composite image of morning and evening of local western horizon [Criswell, 1973]. DustDust ObservedObserved atat HighHigh AltitudesAltitudes fromfrom OrbitOrbit

Schematic of situation consistent with Apollo 17 observations [McCoy, 1976]. Lunar dust at high altitudes (up to ~100 km).

0.1 m-scale dust present Gene Cernan sketches sporadically (~minutes). [McCoy and Criswell, 1974]. InIn --SituSitu EvidenceEvidence forfor DustDust TransportTransport

Terminators

Berg et al. [1976]

Apollo 17 Lunar Ejecta and Meteorites (LEAM) experiment. PossiblePossible DustyDusty HorizonHorizon GlowGlow seenseen byby ClementineClementine StarStar Tracker?Tracker?

Above: image of possible horizon glow above the lunar surface. The bright crescent Earth partially visible at bottom. Right: the dark Moon bathed in earth- shine, with a glow above the lunar limb. Venus at top appears larger due to CCD saturation. HowHow comecome thethe RetroreflectorsRetroreflectors areare stillstill working?working?

Retroreflectors, or corner-cube prisms, return any incident light back to where it came from. By measuring the time taken for laser pulses to travel from a telescope on the Earth to a retroreflector on the Moon and back again, it is possible to determine the distance between them, and test “Einstein’s Apollo 11 experiment. Equivalence Principle”.

Laser Ranging Retroreflector (LR 3) experiments were deployed on the Moon by Apollo 11, 14 & 15. Retroreflectors also on Lunokhod 1 & 2.

APOLLO – Tom Murphy (UCSD) et al. – designed to get much better counting statistics and accuracy than earlier experiments, which typically achieved <<1 return photons per pulse. Return rate predicted to be ≈7±3 photons per pulse.

They only get ≈0.5 photons per pulse! IT GOT DUSTY? WhatWhat aboutabout thethe SurveyorSurveyor 33 partsparts fromfrom ApolloApollo 12?12?

Pete Conrad (Cmdr The TV mirror was Apollo 12) collecting parts protected from this from Surveyor 3. sandblasting. Dust, a few microns and less Areas of Surveyor 3 in diameter, adhering to the facing Apollo 12 as it “… areas stripped and TV mirror. landed nearby were rubbed free of Mechanism …? effectively “sandblasted” contaminating dust.” by dust kicked up by its thrusters. IT GOT DUSTY! Images: Mandeville and Lem [1972] TheThe LunarLunar DustDust --PlasmaPlasma EnvironmentEnvironment RecentRecent Progress:Progress: LunarLunar ProspectorProspector

Lunar Prospector Electron Reflectometry Objective of LP/ER was to map the lunar magnetic anomalies. However, adiabatically reflected electrons give information about both ER magnetic and electric MAG fields at the lunar surface.

ER : Electron Reflectometer measured See Lin et al., Science, [1998] for 3D distributions (~10 eV – 20 keV). more details about this experiment. MAG : Magnetometer. LunarLunar SurfaceSurface ChargingCharging inin thethe SolarSolar WindWind

LUNAR WAKE Lunar Prospector Lunar Electron Observations Electron

~ +4 V ~ −200 V

~ −40 V

~ −4 V m–1 Predictions

Surface Charging Charging Surface Stubbs et al. [2007a] MoonMoon inin thethe MagnetosphereMagnetosphere

Stubbs et al. [2007c] LunarLunar SurfaceSurface ChargingCharging inin thethe EarthEarth ’’ss MagnetosphericMagnetospheric PlasmaPlasma SheetSheet

The plasma sheet is hotter and more tenuous than the solar wind

~ +10 V

~ −600 V

~ +12 V m–1 ~ −1 V m–1 DirectDirect ObservationsObservations ofof ExtremeExtreme SurfaceSurface ChargingCharging

4 keV beam of secondary electrons accelerated upward by negative surface potential. Energy-dependent loss cone indicative of repulsive surface potential. • High energy electrons hit the surface and are lost • Low energy electrons are reflected by the potential. Halekas et al. [2006]

Surface potentials of up to several kilovolts (negative) found: • In the terrestrial plasma sheet (high plasma temperature), • In the solar wind during Solar Energetic Particle events. DustDust TransportTransport RegimesRegimes

Lofting: ZMAX >> D Dust accelerated rapidly away from the surface and follows ballistic φ trajectories. Negligible grain charging in the photoelectron ( S > 0) or φ plasma ( S < 0) sheaths [Stubbs et al., 2006; 2007b]

Levitation: ZMAX ~ D φ Occurs above a photoelectron sheath ( S > 0), where dust grains can both maintain a positive charge and be in a location where Fq ≈ φ Fg, e.g. Colwell et al. [2005]. Could also occur for S < 0 if dust grains remain within the lunar shadow [Sickafoose et al. 2001].

Limited: ZMAX < D φ In a photoelectron sheath ( S > 0), the relatively slow moving positively charged grains leaving the surface are charged negative by the electrons in the sheath. Therefore, they are attracted back toward the φ surface [Colwell et al., 2005]. Again, could occur for S < 0. DynamicDynamic DustDust ““FountainFountain ”” ModelModel

Stubbs et al. [2006] PredictionsPredictions ofof DustDust DynamicDynamic FountainFountain ModelModel

Lofted dust

Levitating dust

Stubbs et al., 2007b MurphyMurphy andand VondrakVondrak [1993][1993] ModelModel DistributionDistribution atat thethe TerminatorTerminator

LRO orbit

Model “0”

9 −2 ρcolumn ≈ 1.4 × 10 m (Lunokhod calibration) Scattering in Visible, Ultraviolet and Infrared (+ Thermal Emission) PredictedPredicted SkySky BrightnessBrightness inin thethe VisibleVisible

Murphy and Vondrak, unpublished

UV and IR predicted to be less bright. Significant IR thermal emissions predicted. Comparative Planetology Particulates in Solid-Body Exospheres

Earth Mars Moon • Mostly gas, particulates • Mostly gas, but larger • “Dusty” Exosphere – small fraction of mass. fraction of particulates. particulates dominant mass component? • Particles: Aerosols, • Particles: Dust dust and ice. aerodynamically lifted • Particles: Dust from surface. electrostatically lifted • Active surface- from surface (>100 km). atmosphere interface. • Active surface- atmosphere interface. • Active surface- • Many ice-laden exosphere interface. thunderstorms driven by • Dust storms can trigger local temperature a global meteorological • Dust accelerated up to inversions. instability. 1 km/s at terminator. AnthropogenicAnthropogenic DustDust

- In addition to “natural” transport, dust will get kicked-up by astronauts. - Astronauts and equipment moving around on the lunar surface will create & exchange charge via contact electrification – V triboelectricity. - In an “equivalent circuit”, the astronaut can be treated as a “capacitor” which charges as it moves across the lunar surface. The Human Capacitor - Input voltages to this capacitor include triboelectric, photoelectric & plasma The astronaut is electrically currents, & induction in high E-field regions. connected to the lunar dust- - These processes will result in the charged plasma environment via dust being attracted toward, and adhering the voltage sources of the to, the charged astronaut. Capacitor-Electrical System - The attached dust will subsequently be carried into the habitat. DustDust FromFrom thethe SurfaceSurface toto thethe Habitat:Habitat: WhatWhat isis thethe ““SelectionSelection ”” Process?Process?

Surface Dust Distribution Dust coated EVA suit Dust in the Habitat

Probability of dust getting from the Probability of dust getting from suit/ surface to the EVA suit depends on: equipment into the habitat depends on: - Grain size, charge & “perturbation” - Grain-suit adhesion - Surface/astronaut electric potential - Effectiveness of mitigation efforts In turn, this depends on: In turn, this depends on: - Grain size distribution & composition - Grain shape, composition & charge - Ambient plasma, illumination & - EVA suit material. thermal conditions - Airlock design SomeSome FutureFuture NecessaryNecessary inin --situsitu MeasurementsMeasurements

Measurement Instrumentation

Exospheric dust Photometer (passive) concentrations LIDAR (active)

Surface electric potentials E-fields instrument with boom (and maybe magnetometer)

Dust strikes Impact sensor (dust mass, velocity and (more sensitive and charge distributions) discriminating than LEAM).

Plasma characteristics Electron spectrometers Ion spectrometers

Measurements required on the surface (local) and from orbit (global), preferably at the same time. LLunarunar EEmissions,missions, EElectrons,lectrons, andand DDustust (LEED)(LEED) SurfaceSurface InstrumentInstrument Objectives: - Identify the processes that create the dusty exosphere. - Discover the forces that accelerate charged dust near the terminator. - Detect the charging of astronauts and equipment in this environment.

RF charged dust DC E-field detection system, system with similar to heritage from techniques used POLAR and at the planets Cluster. [Gurnett, 1983].

Electron and Patch plate to measure dust adhesion on Ion plasma various materials. spectrometers, Integrated IDPU to obtain well-correlated like those built high rate data during the most active for Triana. periods. Lunar Explorer for Elements And Hazards (LEEAH)

• Magnetic & Electric Fields • Ion composition CUTTING EDGE SCIENCE with PROVEN SYSTEMS • Electron/ion spectra & • In-situ and remote • Finalist for LRO Secondary Payload; funded Phase A direction dust analysis development in 2006 • High-heritage instruments and spacecraft (TRL 7-9) from THEMIS & Lunar Prospector • Science, operations, management teams in place; ~2.25 yr development schedule • Low cost secondary (<$60M) or primary (<$100M) mission options on EELV, Minotaur, Delta-II

SCIENCE • Lunar surface charging in response to solar and plasma environment • Dust transport and dusty plasmas/exosphere • Map surface composition and volatiles • Fundamental space plasma physics and lunar- solar interactions EXPLORATION

• Identify resources, including H 2O • Quantify dust electrification and motion • Correlate with environmental drivers for prediction and mitigation SummarySummary andand ConclusionsConclusions

There is much compelling evidence from the Apollo era for the electrostatic transport of lunar dust. Fundamental Solar System process. Understanding how astronauts and equipment couple to the lunar dust-plasma environment is crucial. Important for determining how dust enters the habitat and in what state – how has it been “selected”. Require targeted observations of this environment as soon as possible, in order to develop effective dust mitigation strategies for robotic and human exploration.