Electrostatic Charing Hazards Originating from the Surface

International Workshop on Instrumentation for Planetary Missions (2012) 1060.pdf

ELECTROSTATIC CHARGING HAZARDS ORIGINATING FROM THE SURFACE (ECHOS) OF MARS WITH APPLICATIONS TO OTHER SURFACE/ATMOSPHERE INTERFACES. W. M. Farrell1, J. R. Mar- shall2, G. T Delory3, 1NASA/Goddard SFC, Greenbelt, MD ([email protected]), 2SETI institute, Mountain View, CA, 3 Univ. of California at Berkeley, Berkeley, CA.

Introduction: In 1999, a Human Exploration and Development of Space (HEDS) payload called ECHOS was included in the Mar03 Surveyor mission that was specifically designed to quantify the atmospheric electricity environment at the Martian surface. It was rec- ognized that mixing sand and dust in dust devils, storms, and saltating grains could be a source of electricity via contact electrification [1,2]. The ECHOS package was designed to quantify the amount of electrical energy in aeolain features, determine their effect on the environment and landed systems, and considers the electricity’s effect on atmospheric chemistry. Unfortunately, in 2002, after the loss of MPL, the mission was scrubbed. However, the idea that Mars’ near-surface atmosphere is electrically (and electro- Illustration of the proposed REDD instrument on Mars chemically) active continues to be a topic of great in- However, the MSL opportunity allowed the concept terest. We describe the evolving form of the ECHOS to be reconsidered as Mars Atmospheric Chemistry on science objectives, relevance, some interesting applica- Electrical Storms (MATCHES) with G. Delory as the tions, and finally conclude with future opportunities. PI. MATCHES would provide key information on the Evolution of the Instrument Concept. Early lab electrical environment in support of MSL’s Sample experiments involving mixing sand and dust in low Analysis of Mars (SAM) mass spectrometry/chemistry system. The electro-chemistry associated with electri- pressure CO2 (to mimic the Martian atmosphere) demonstrated that tribo-charged grains can excite both fied aeolian features [6, 7] was to be measured via the a visible corona and spark discharges [1]. Mills [3] was MATCHES-SAM combination. Unfortunately, the so impressed with the glow that it was suggested that added supporting instrument was rated very highly but the corona chemically-scavenges organics and was the not selected. source of the reactive regolith detected by Viking. Objectives. The objectives of the Mars03 ECHOS Sentman [4] suggested that a dedicated electrostatic instrument include the following: package be sent to the Martian surface to quantify this electrical activity, although it would be some years -Determine if saltation clouds are electrical in nature before the opportunity presented itself. In 1997, GSFC and determine if they are a realistic hazard submitted a proposal to the Mars 01 Surveyor AO to -Obtain the speed and direction of Martian winds place a low frequency radio and optical photometer on near/within dust devils and storms -Detect lightning, in whatever manifestation, from the Mars01 Surveyor Lander to sense RF and visible electrical dust clouds emission from dust-created corona. This instrument -Obtain the rate of dust devil occurrence was called Radio-optical Experiments for Dust Dis- -Derive reliable precursor acoustical, electrical, and charges (REDD) [5]. While not selected, the idea of chemical signatures for dust devils & storms charged dust, corona, and dust devil/storm electrical -Map the Lander potential under various environmental activity captured the imagination of the atmospheric situations community. -Determine the atmospheric breakdown potential as a In response to the Mars03 Surveyor AO, ECHOS function of environmental variables was proposed. The proposal was awarded and merged -Define the discharge hazard for objects with sharp with another proposal that included the system camera corners - the new instrument was renamed Mars Atmosphere and Dust in the Optical and Radio (MATADOR) with The MATCHES instrument to support MSL em- P. Smith at Univ. of Arizona as the PI. Unfortunately, phasized the emerging field of atmospheric electricity following the MPL loss, the Mars03 mission was can- and new chemistry, and thus has a stronger emphasis celed. on obtaining the trace species associated with electri- International Workshop on Instrumentation for Planetary Missions (2012) 1060.pdf

fied dust devils and storms. Specifically, given the rela- The later MATCHES concept had similar electrical tively low threshold for atmospheric breakdown, dust environmental sensing instruments (Radio, DC-E, and devils & storms were anticipated to create CO2+ ions electrometers) but then provided real-time DPU-to- and corona via impact ionization and CO, O- and OH DPU input to SAM, which derived the associated and H- via electron dissociative attachment with carbon chemical sensing of newly-formed trace species. If an dioxide and water [6]. The recombination of these electrical storm passed by MSL, the MATCHES elec- products could produce new reactive species like hy- drogen peroxide [7]. As surmised by Mills [3], the dust-created corona is expected to be reactive. Relevance. The astrobiological impact of Martian atmospheric electricity relates to the classic Urey-Miller experiments [8] where discharges can be considered an energy source to create more complex hydrocarbons. In the MEPAG goals #1 [Life], atmospheric processes and associated electricity is an energy sources that can alter chemistry, and possibly affect organic chemistry [3]. The exploration impact involves safety con- cerns that intense lightning could affect landing & take- off/ascent from the Martian surface (see MEPAG’s http://mepag.jpl.nasa.gov/reports/MHP_SSG_(06-02- 05).pdf). In MEPAG Goal #4 [Human Exploration], lightning that might affect human operations and take- off is specifically mentioned for further study. Instrument Suite. There has never been an experiment flown to Mars to quantify the electrification of the dusty atmosphere, its electrical induction effect on objects, and associated electron flow near the surface. ECHOS is specifically designed to obtain these measurements and to obtain a fundamental understanding of the system creating the electrical environment, new chemistry, and potential hazard. The sensors comprising the ECHOS suite are strongly interrelated. The ECHOS data sets are highly- complementary and will generate a detailed integrated trical sensors would detect it and trigger the fast- picture of the Martian electric dusty meteorological sampling laser spectrometer portion of SAM – specifi- system. ECHOS instruments emphasizing the electrically looking for peroxides. cal environment include a RADIO for measuring AC Electrometeorology.The name, electrometeorology, electric fields, a DC electric field sensor (DCE) for implies a strong connection between atmospheric detecting remote sources of quasi-electrostatic fields, phenomena and electricity, which is appropriate in and an electrometer system (DEC) for measuring local discussing Martian near-surface activity involving potentials at various locations on the lander (including charged dust. Laboratory experiments demonstrate that near corners/edges). The meteorological package dust under Martian pressure will generate and (MET) includes a pressure gauge, wind vector velocity exchange copious amounts of charge at the sensor, temperature sensor, and dust detector. The lat- microscopic level. Glow and filamentary-type ter three are placed on a main mast, and also on the soil discharges are observed in the lab [1,3], indicating the surface. A Paschen Breakdown Monitor measures the excitation of neutral atoms, breakdown of the gas, the electrical stability of the atmosphere by inducing elec- creation of ions, and the generation of unusual trical breakdown, and a chemical sensor is included to chemical species. These processes form hazards, since determine the abundance of oxidizing species in an charged (and discharging) grains adhere to surfaces, electric, dusty atmosphere. Data from these sensors are generate radio frequency “static” interference, and integrated into a single digital processing system possibly produce creactive chemical compounds like (IDPU) that acts as “one brain” for the suite, thereby peroxides. ECHOS has the ability to detect and maximizing the quality of data during targets of oppor- quantify these microphysical processes expected to tunity, such as dust storm passages. International Workshop on Instrumentation for Planetary Missions (2012) 1060.pdf

occur with Martian charged dust. the entire atmosphere both locally and remotely (see However, accumulations of dust in clouds (dust below). This complex electrified atmosphere has three devils or storms) also form a possible electrostatic components: (1) Meteorological Component. The discharge hazard due to the congreagation of charge electrified dust in suspension/saltation clouds is a [2,9]. Dust devils are convective structures with hot electrified system due to grain-grain contact cores, circular winds, and gusts in excess of 30 m/s (or electrification. (2) Geoelectric Component. An object immersed in the electrical atmosphere itself has an induced E. (3) Surface Component. The surface represents an electrical dielectric boundary that will also charge relative to the storm itself and the object. All three components are in continuous electrical interaction, with bi-directional current flow between the atmosphere and ground, and to any object in between. We expect the currents to be in the form of quasi-DC flows, conductivity-enhanced corona, and possibly even occasional fast discharges. Application #1: Atmospheric Chemistry. The transition from a neutral gas to an mildly- ionized gas under a stressing E-field is a collisional plasma physics problem that is describable via solutions to the Boltz- mann equation that includes electron vibration, excitation, and ionizing collisions with a CO2 gas at Martian- like pressures [6,15]. The essence of this process is the following: 1) The dust devil creates a dipolar-like E-field via dust tribo-charging and separation. 2) At Mars, for E fields > 10 kV/m, ambient electrons will be accelerated in the field but now with enhanced energy in their collisions with the CO2 gas molecules to both excite and ionize the molecules. This process of electron impact ionization releases 2 electrons for every one in colli- sion with a CO2 molecule to initiate the well-known electron avalanche process [6,15] that grows at a rate defined by Townsend’s first coefficient, α (defined as the number of electron impact ionizations per unit me- ter). The variable α is itself an exponentially growing 70 mph) [10]. They are believed to be generated most function with the E-field. 3) The free electrons can also in the post-storm “fair weather” period when static undergo electron dissociative attachment with CO2 and stability is decreased [11]. Terrestrial dust devils of H2O, to create CO, O-, OH and H- in the avalanche similar size can create 50-100 kiloV/m electric fields region. Given this energetic electron population [12]. By analogy, we anticipate Martian dust devils to ‘pulled’ from the atmospheric gas, there is then the have similar tribo-charging activity at 10-20 kV/m initiation of a corona, like that witnessed in the glowing levels [2] but less than those at Earth due to increased gas of Eden and Vonnegut [1] and Mills [3]. atmospheric dissipation. Such dust devils may have Figure 4 illustrates the plasma chemistry ex- electrostatic energy that could be a source for unusual pected to be created, with electron impact ionization new chemistry and may form a hazard via ESD, both occurring when electron energies exceed 15 eV, elec- directly to objects and between objects due to electric tron dissociative attachment of CO2 occurring with induction. An ECHOS priority is to monitor the electrons having energies near 4 and 8 eV, and electron electrostatic charge in the dust devil, to observe its dissociative attachment of water occurring with elec- discharges in the radio, optical, and audio spectrum, to trons having energies near 6.5 and 8.5 eV. We thus measure directly induced potentials on objects like the suspect that the same glowing gas formed by Eden and Lander and understand the new electrical-driven Vonnegut [1] and Mills [3] featuring electrons exciting chemistry. CO2 (with energies near and above 10 eV) also had Environmental System. From a system point of view, the dust in dust devils and storms can electrify International Workshop on Instrumentation for Planetary Missions (2012) 1060.pdf

large amounts of O- being created via electron dissoci- sponsible for the proposed electro-chemistry at Mars. ative attachment at the lower energy of 4 eV. Clearly, an instrument like ECHOS or MATCHES Delory et al. [6] and Atreya et al. [7] first would provide that validation. In essence, this instru- formulated the plasma chemistry by considering the ment should be flown once to assess the environment. electron avalanche process without electron losses. If it is more electrically-active than anticipated, target- Later studies considered electron losses from dust and ed electro-chemical instruments can be built and flown. Application #2: Discharges and Lightning. In 2009, Ruf et al. [17] presented the very exciting report of lightning detected from a Mars dust storm as observed from one of the DSN ground-based radio telescopes in the GHz emission band. These emissions were powerful, inferred to originate from a discharge 105 times more powerful than a nominal terrestrial lightning stroke. The sensitive kurtosis waveform analysis technique applied to the observations also detected a very strong modulation of the GHz signal at narrowband ultra-low frequency (ULF) tones near 9, 18, and 27 Hz. These tones were inferred to be a modulation from the surface-ionosphere cavity (called the Schumann Reso- nance (SR)) triggered by the lightning discharge itself. Such GHz emission from a lightning discharge has not been reported previously for Earth or the other planets. As such, this fast ‘super-bolt’ discharge represents an entirely new lightning emission type with no other planetary analog. The situation became complicated in Does Mars’ have the ca- 2010, when Gurnett et pacity to form large charging centers and lightning? Fig 4-Energetic electrons interact with the Martian al. [18] reported on the atmosphere [Delory et al, 2006] lack of any RF emission from lightning in the 4-5.5 electron attachment processes [13, 14, 15]. Jackson et MHz band of the MEx/MARSIS radio receiver in al. [15] developed the Dust Devil Electron Avalanche proximity about Mars. They used data collected over a Model (DDEAM) that includes the coupled interac- 5 year period that included multiple close passes over tions (coupled differential equations) from impact ioni- two major dust storm events. They placed a lower limit zation sources, dust losses, and dissociative attachment on the intensity of any lightning: They demonstrated losses. It also formulates the avalanche saturation pro- that a terrestrial-type discharge from the Martian sur- cesses in the ionized gas. face would be easily detectable – since its strength Atreya et al [7] found that the recombination would be 40 dB (10000 times) above the noise floor. of the electron dissociated Hs and Os could form the As such, the receiver had the sensitivity to detect a discharge with an RF emission strength that is 1/10000 reactive H2O2 in relatively large concentrations. This peroxide could then react with the surface and dust, weaker than that found in typical terrestrial stroke’s possibly cleansing the surface of organics. Farrell et al. energy. [16] found that there could be electron dissociation of Earlier this year, Anderson et al. [19] reported methane – thereby reducing the lifetime of this key bio- on the lack of Martian lightning detection at 3 & 8 marker. Peroxides might also alter the lifetime of this GHz in a ~3 month period in 2010 using the Earth- biomarker as well. As such, the new chemistry in elec- based Allen radio telescope. They unfortunately could trified corona is considered harsh. not reproduce the Ruf et al. result in a similar band While the lab and modeling studies described pass. new chemistry forming in the corona created by electri- How do we then reconcile these three very fied Martian dust storms, to date we do not have in situ disparate observations? Calculations in Ruf et al. [17] proof for the electric fields and currents that are re- indicate that the Martian atmosphere can collect and maintain large concentrations of storm charge –larger International Workshop on Instrumentation for Planetary Missions (2012) 1060.pdf

than those collected within terrestrial thunderstorms - in the creation of their reported Martian ‘super-bolt’. Yet, the sensitive receiver on the close-orbiting MEx orbiter did not detect a discharge above its noise threshold at sensitivity comparable to ~ 1/10000 of the terrestrial case. The two observations are philosophi- cally at odds. The best way to get ground-truth to settle this dilemma is the fly an ECHOS or MATCHES type instrument to the Martian surface. The Radio can detected the RF emission from a lightning event while the DC E-field system should detect both the large congre- Attenuation and dispersion of an impulsive RF signal gation of charges associated with the storm, itself, and in the surface-ionosphere waveguide. should also detect the Schumann Resonances which Future Applications. ECHOS is ideal for studing Ruf et al. infer to be unusually intense and of very high the meteorology and associated electricity, with a Q. Certainly, if the DC E-field system does not imme- wedded sensor suite to assess the fluid and electrical diately detect the SRs, then the Ruf et al. [17] super- envirionments associated with particulate transport. bolt may not exist (at least not as described therein). This package is not only ideal for Mars, but could be Corona emission still could exist without the super-bolt considered on a landed mission to Titan, where aeolian – in fact, copious corona like that observed in lab ex- effects are anticipated. The electrical portion of the periments by Eden and Vonnegut [1] and Mills [3] package could fly to Venus to search for the RF would act to dissipate large charging centers and re- signature from Venus lightning, especially that low duce the likelihood of a large charge buildup and su- frequency portion of the spectrum that may remained per-bolt. HEOMD is interested in the presence of electromagnetially-trapped between the surface- lightning discharges on Mars since such events repre- ionosphere. As we develop this package for Mars, sents clear hazards to avoid during landing and take-off there are obvious extensions to other solid bodies with from the planet surface. atmospheres. As describe above, ECHOS is a key part Application #3: Sub-surface Conductivity. of understandng forces that transport particualtes at the The energy in any impulsive electromagnetic event surface-atmosphere interface. (below ~ 0.1 MHz) on Mars becomes bounded and partially trapped in the surface/ionosphere waveguide. References: [1] Eden, H. F., and B. Vonnegut (1973), As these EM pulses propagate from their storm source, Nature, 280, 962–963. [2] Melnik, O., and M. Parrot (1998), they become attenuated and dispersed, and in doing so, J. Geophys. Res., 103, 29,107–29,117. [3] Mills, A. A. they are altered based on the properties of the wave- (1977), Nature, 268, 614. [4] Sentman, D. D. (1991) Elec- trostatic Fields in a dusty Martian environment, in Sand and guide itself – including the surface conductivity [5, 20, Dust on Mars, NASA CP 10074. [5] Farrell W. M. et al. 21]. In essence, as an electrified storm moves toward (2000), Acta Astronautica, 46, 25. [6] Delory, G. T., et al. or away from an observation point, the amount of at- (2006), Astrobiology, 6, 451–462. [7] Atreya, S. K., et al. tenuation and dispersion in the RF impulse varies, and (2006), Astrobiology, 6(3), 439–450. [8] Miller, S. L. this information can be inverted to understand the (1953), Science, 117, 528. [9] Farrell, W. M., et al. (1999). properties of the sub-surface (assuming a high conduc- J. Geophys. Res., 104, 3795–3801. [10] Ryan, J. A. and tivity in the ionosphere boundary). Lucich, R. D. (1983), J. Geophys. Res., 88, 11005. [11] A landed instrument like ECHOS could then Schofield, J. T., et al. (1997), Science, 278, 1752. [12] Jack- provide a regional survey of the sub-surface properties son, T. L., and W. M. Farrell (2006), IEEE Trans. Geosci. Remote Sens., 44(10), 2942– 2949. [13] Jackson, T. L., et al. via a lightning waveform inversion. This technique has (2008), Geophys. Res. Lett., 35, L16201. [14] Kok, J. F., and been mastered for terrestrial lightning and could be N. O. Renno (2009), Geophys. Res. Lett., 36, L05202. [15] applied at Mars – assuming a discharge that radiates Jackson, T. L., et al. (2010), J. Geophys. Res., 115, E05006. (preferably in the VLF) is present. Clearly, a landed [16] Farrell, W. M. , et al. (2006), Geophys. Res. Lett., 33, instrument is needed to assess the radiative properties L21203. [17] Ruf, C., et al. (2009), Geophys. Res. Lett., 36, of Mars dust features. L13202. [18] Gurnett, D. A.,et al. (2010), Geophys. Res. Lett., 37, L17802. [19] Anderson, M. M. , et al. (2012), Astrophys. J., 744, 15. [20] Cummer, S. A. and W. M. Far- rell (1999), J. Geophys. Res., 104, 14149. [21] Grimm, R. E. (2002), J. Geophys. Res., 107, 10.1029/2001JE001504.