1. ARES: An In Situ Sensor to Characterize Mars Atmospheric Electricity 1Montmessin, F., 1Godefroy, M., 1Hamelin, M., 1Berthelier, J.J., 1Yahi, S., 2Aplin, K., 1Simoes, F., Vieau, A.-J., and 3Szago, K. 1LATMOS, CNRS/UVSQ/IPSL, Paris, France 2Rutherford Appleton Laboratory, STFC, Didcot, United Kingdom 3KFKI, Budapest, Hungary
Executive Summary
The Atmospheric Relaxation and Electric field Sensor (ARES) experiment is devoted to the investigation of atmospheric electric phenomena. It will measure the ionization state of the atmosphere, the electric fields that result from various charging mechanisms and investigate the planet global electrical circuit. Atmospheric electrical phenomena are an important issue in dust transport and surface and atmospheric chemistry. Electrification processes may also affect a landed vehicle and could be an important issue for their safe and reliable operation and, more generally, for future landed systems in the context of the human exploration of Mars.
ARES is a double probe electric field instruments with two cylindrical sensors that can be installed on the meteorological mast. It measures the magnitude of the vertical component of the electric field and the potential of the lander with respect to background from DC to 2 kHz up to ~ 250 V/m which can be increased to ~20 kV/m in dedicated modes of operation. The vertical electric component of electromagnetic waves and the AC fluctuations of the potential of the rover will be measured in the frequency range from 8 Hz to 4 kHz. This channel may also be used to detect the impacts of charged dust particles and infer their fluxes and charge distribution. Operated in the relaxation probe mode, the instrument will also provide a measurement of the atmospheric conductivity separately for positive and negative ions. Knowledge of the wind velocity, dust loading and specific weather phenomena (e.g. strong local vortices leading to dust devils) observed by the meteorological experiments will enable to investigate in depth the physics of charging mechanisms and the generation of electric fields. Double probe electric field instruments have been flown on numerous balloon flights by members of our team, an initial version of ARES has been successful validated on three balloon flights in 2004, 2005 and The mass of ARES is 120g and its average power consumption 200 mW.
Electric fields from a few mV/m to several tens of V/m have been routinely measured as well as electric conductivities in the range to S/m (see for example Mozer, 1971; Berthelier et al., 1974, 2003; Holzworth and Bering, 1996; Grard, 1998; Berthelier et al., 2006).
A global atmospheric electrical circuit is likely to exist on Mars, between the surface and the ionosphere, with similarities and differences with the Earth’s circuit. Atmospheric ionization should be similar to that of the Earth’s stratosphere but impact charging through collisions between dust particles moved by the wind and the surface, or between dust particles themselves, is expected to be the dominant charging mechanism. Intense electric fields, possibly capable of producing electrical breakdown, are expected at the time of dust storms and in the vicinity of dust devils.
Atmospheric electricity is also involved in several processes that have a noticeable impact on the surface and atmosphere. At times of dust storms, electrostatic forces on fine electrically charged dust grains may become larger than aerodynamic forces due to the wind. They are expected to play a significant role in the dynamics of suspended dust particles and their interaction with the surface, thus on the processes that contribute to the erosion and long term evolution of the surface. By energizing the free electrons, the atmospheric electric fields control their interaction with both the surface and the atmospheric gases. They have a thus a definite role in the chain of physical and chemical processes that govern the chemical state of surface materials and the production of oxidized constituents in the atmosphere with far reaching consequences on the sustainability of proper conditions for life.
The ELF and VLF electric fields measured by ARES will help in investigating electrical breakdown phenomena in the atmosphere and detect wave emissions from the distant ionized environment of Mars.
By monitoring the charging of the lander, the ARES experiment will also provide engineering data of crucial importance to understand the electrical charging of landed vehicles. In the extremely dry and tenuous Martian atmosphere landed vehicles can easily become electrified a situation that may jeopardize the safety and reliability of instruments or sub-systems operations in future Mars missions and, in particular, human flights.
Simulation of impact electrification in a ‘dust devil’ (Melnik and Parrot, Zhai et al., 2006, Farrell et al., 2003 & 2006)
High voltages can be generated in dust devil vortex, but limited by electrical breakdown.
ARES SCIENTIFIC OBJECTIVES: 1. CHARACTERIZING THE MARS GLOBAL CIRCUIT
On Mars, a global electric circuit is expected to exist due to the presence of the following elements:
Dust devils and dust storms as generators
A conductive ionosphere > 1 S/m at 120 km altitude
A slightly conductive atmosphere in quiet weather regions: : to S/m (ionization by UV, Cosmic rays, micro-meteorites)
Ground conductivity ( S/m - TBC) ARES will provide new insights into the properties of each of the circuit component.
undisturbed weather
+ + + W
12W e-
CO3-(H2O)n
dust storm:
Triboelectric charging
Stormy
weather
- - -
H3O+(H2O)n
Solar UV radiation
ARES SCIENTIFIC OBJECTIVES: 2. PROVIDE NEW INSIGHTS INTO MARS DUST CYCLE
Dust lifting and transport are driven by the following forces:
Aerodynamic drag (FD)
Gravity
Cohesion between particle
….and electric forces (FE)
Simple computations indicate FD ~ FE (Berthelier et al., 2000)
Electric forces can concur with wind to enhance lifting, modulate wind influence on lifting, or can oppose/combine to wind motions.
Through its effects on dust, atmospheric electricity could play a major role in Mars climate !
Kock et al., 2007
- INSTRUMENT CONCEPT.
The very tight constraints imposed on the dimensions, mass and power of the instruments on Mars landers restrict the electric fields measurements to only one dimension, along the vertical direction. For obvious symmetry reasons, and as on Earth, the planetary large scale electric field of atmospheric origin is essentially vertical and this component is therefore of primary interest. The instrument makes
ARES SCIENTIFIC OBJECTIVES: 3. PROVIDE NEW INSIGHTS INTO MARS CHEMISTRY AND HABITABILITY
Electrostatic fields energize free electrons and enhance the rate of chemical reactions:
CO2 + e-  CO + O- & H2O + e-  OH + H
According to models (Atreya, 2006), H2O2 production of can be increased by a factor of 200.
Atreya et al., 2006
H2O2 efficiently removes organic material, which constitutes a challenge for life development and maintenance at the surface of Mars. As such, electric fields might partly dictate Martian habitability conditions and may also participate to explaining the recently discovered spatial heterogenity of Methane on Mars (Mumma et al., 2008).
Block diagram of the electronic layout of the instrument.
use of the so called “double probe” technique to measure the vertical DC and AC electric fields. It relies on measuring the potential difference between two identical sensors that float at the local potential of the atmosphere. The high impedance of the sensors with respect to the external medium, due to the small conductivity of the atmosphere, makes it necessary to use high input impedance (> 1014Ω), low leakage current (< A) preamplifiers. Space
qualified components with such performances are currently available. The electric conductivity is measured through the relaxation technique: the sensor potential is displaced from its normal level equal to the local floating potential and then let free to recover its
ARES SCIENTIFIC OBJECTIVES: 4. CHARACTERIZE EM WAVES AND CAVITY RESONANCES
Schumann resonances measurements give access to planetary scale electrical characteristics:
Generators (lightning)
Ionization state of atmosphere and ionosphere
Surface conductivity
equilibrium. The time constant τ = ε0/σ readily provides the electric conductivity σ of the atmosphere. The double probe technique has been used on numerous balloon flights over more than 30 years and has provided a wealth of excellent and reliable data over a large range of altitudes, from a few kilometres to ~ 35 km.