Advanced Concept for the Detection of Weather

Hazards on Mars: Non-Thermal Microwave Emissions

by Colliding Dust/Sand Particles

NASA Institute for Advanced Concepts

Student Fellowship Prize Final Report

May 15, 2006

Aimee Covert

Mentor: Nilton O. Renno

Atmospheric, Oceanic and Space Sciences

Space Physics Research Laboratory

The University of Michigan

Ann Arbor, MI 48109

Abstract

Triboelectric charging occurs in dust devils and dust storms when small dust particles rub

against larger particles. In this process, electrons are transferred from large to small

particles. When charged particles separate after a collision, an electric discharge occurs

and non-thermal microwave radiation is emitted. We have detected these emissions in

laboratory experiments designed to simulate particle collisions in Martian dust events.

The high dust content and electrification of the ubiquitous Martian dust devils and dust

storms makes them dangerous to robotic and human missions. The non-thermal

microwave radiation provides an effective way to unambiguously detect the presence of

these electrified dust events. A sensor capable of identifying the non-thermal radiation

could successfully detect dust events even at night or during periods of low visibility.

The goal of this research has been to study these emissions using sensors developed by

our collaborators and us. We have been able to detect non-thermal microwave emissions

at close range and distinguish them from the background thermal radiation. This is an

important step towards our ultimate goal of developing an instrument capable of

unequivocally detecting dust events.

Introduction

Research Team

Two undergraduate students, Kevin Reed and Catalina Oaida, have been working with

me on this project. Kevin began working on the project as an REU fellow in the summer

of 2005, and Catalina as a UROP student (Undergraduate Research Opportunity

Program) this fall. They have been helping me run laboratory experiments and do data

analysis.

Background and Dust Electrification Theory

Dust devils and dust storms occur frequently on the Martian surface and are much larger

and stronger than their terrestrial counterparts. For example, terrestrial dust devils

typically have diameters of less than 10 meters, but on Mars dust devils frequently have

diameters between 100 m and 1 km and heights larger than 7 km. In addition, the dust

concentration within these dust devils is roughly 1000 times the background value. Dust

storms frequently grow and become global in extent [1]. This ubiquitous and forceful

weather phenomenon must be considered in mission planning and have been identified as

a serious hazard to robotic and human missions.

Weather hazards related to dust events pose a significant danger to future missions to

Mars. Electrical activity can cause discharge between components of equipment, or

ionize the air, causing potentially important chemical reactions. Dust can work its way

into spacesuits or visors and dust events can cause abrupt loss of visibility on the Martian

surface.

The large number of collisions between dust particles in Martian dust events produces

important electrical effects. Collisions in which a smaller particle is dragged across the

surface of a larger particle can cause large charge buildup on each particle (Figure 1).

The smaller particles are negatively charged and the larger particles are positively

charged. This process results in two phenomena of interest. First, when small,

negatively charged particles rise in the updraft and large, positively charged particles stay

near the ground, a bulk electric field is generated in the storm. In terrestrial dust events,

this field can sometimes be greater than 10 kV/m [2].

Additionally, when two charged particles separate after a collision, an electric micro-

discharge occurs and emits non-thermal microwave radiation [2]. In previous

experiments conducted between 2004 and 2005, particles of interest were collided in a

vortex generator designed to simulate dust devils. Microwave emissions were observed

in collisions between particles of aluminum, basalt, hematite, and chrome, all of which

contain materials present in the Martian regolith. These emissions were observed with

amplitudes significantly above the background value.

During the course of this fellowship, I continued and expanded on these experiments. I

improved the experimental design, giving a more accurate simulation of Martian dust

devils as well as providing better control over important experimental parameters such as

wind speed and pressure within the experimental chamber. In addition to the instruments

used to detect microwave emissions in the previous experiments, I also used an

instrument that can distinguish non-thermal from thermal emissions, helping to prove that

the emissions from dust phenomena are non-thermal microwaves and can be

distinguished from the thermal background noise.

The immediate goal of this research was to study the behavior of micro-discharges in

laboratory simulations of dust devils by quantifying the non-thermal microwave

emissions and its dependency on environmental parameters. The study of these

emissions allowed us to develop potential methods of distinguishing non-thermal

microwave emissions from background thermal radiation (noise). The development of

such a method would bring us close to our ultimate goal – the development of a sensor

capable of identifying potentially dangerous dust events even during periods of low

- - - - - - - - - - - - - - - - - -

- - -

- - - - -

- + +

+ + + + + + + + + + + + + +

Figure 1: As a small particle is rubbed across a large particle, it gains a net negative

charge, leaving the larger particle with a net positive charge.

visibility. This sensor could be used to detect the presence of dust events near a Martian

explorer.

Experiments

Experimental Overview and Purpose

The main purpose of our experiments is to use simple laboratory simulations of dust

devils to study the electric behavior outlined above. A vortex generator (Figure 2)

developed at the University of Michigan and designed to move particles as though they

were in a dust event, is used to collide particles of controlled type and size. Particles that

were chosen for this experiment are fairly representative of the Martian regolith. These

included aluminum, hematite (Fe2O3) and basalt. Particles were divided by size and

experiments were run using particles of three size classifications – small particles

(particles under 0.381 mm in diameter), large particles (particles over 0.381 mm in

diameter), and mixed particles (a mix of half small particles and half large particles by

volume). The purpose of simulations with particles of different sizes was to see if size

had an effect on the intensity of emissions or on the time-scale of the emission, two

parameters that could be used to “fingerprint” the emissions from dust events.

Experiments were also conducted at different pressures in order to investigate the effect

of the low Martian atmospheric pressure on emissions.

Figure 2: Vortex generator used to simulate dust devil activity in experiments.

We expected to see the most emissions in experiments run using mixed particles at low

pressure. Mixed particles would most likely produce collisions in which a smaller

particle rubs across a larger particle as described in the introduction to this paper.

Electric discharge occurs more easily in lower pressures, so we expected to observe more

emissions when experiments are conducted at lower pressure.

Experimental Setup

The measurement of microwave emissions was conducted using two setups. The first

setup (Figure 3) consists of a radiometer connected to a laptop via a data acquisition card

that measures emission amplitude vs. time. This setup allows us to quantify specific

emission peaks in the data. Emission readings are taken in packets of 6*105 data points

at a sampling rate of 20 MHz, which corresponds to 30 ms of uninterrupted data.

The second setup (Figure 4) used a different radiometer that allowed us to examine the

probability distribution function (pdf) of the emission amplitude. The goal of this setup

is to allow us to verify if the microwave emissions we are detecting are thermal or non-

thermal emissions. Since background microwave emissions are thermal blackbody

emissions, distinguishing non-thermal emissions from thermal emissions would allow us

Figure 3: Sensor used in equipment setup 1. This sensor provides amplitude vs. time

data for microwave emissions.

to be sure that we are detecting an alternate radiation source, such as a dust devil, and not

just background noise.

In all experiments the radiometer was aligned to look at the saltation layer of our

simulation. The saltation layer is the area near the bottom of the vortex where the most

collisions occur between large and small particles, and where microwave emission is

most likely to occur.

Results

In all experiments, we empirically observed static charging of the colliding particles.

After simulations, particles would cling to hands and clothing and plastic components in

the vortex generator. This behavior was present even for materials for which we did not

observe microwave emission. This indicates that our simulation was successful in

colliding particles so that they became charged. However, our basic experimental design

encountered other problems that affected our results. Most significantly, the fan used in

our vortex generator could not generate enough wind to lift as many particles at low

pressure as at high pressure. We were therefore unable to control the rate of collision

between particles during experiments conducted at different pressure.

Figure 4: Sensor used in equipment setup 2 (left). This sensor provides a probability

distribution function of the emission amplitude.

We also had problems with the loss of small particles from the experimental chamber.

Particles would escape through cracks between the edge of the bowl in the bottom of the

generator and the wall of the bell jar. Also, small particles would stick to the inside of

the bell jar wall. This resulted in fewer collisions between particles as experiments

progressed. We corrected this problem by modifying the vortex generator and sealing the

most prominent cracks as best we could. In addition, we ran analysis on data taken at the

beginning of the experiments, shortly after the fan had been turned on in order to reduce

this problem as much as possible.

Equipment Setup 1

Significant microwave emissions were only detected during experiments with aluminum

particles. Experimental data was analyzed using Matlab. A “peak” in the emissions was

defined as any value greater than an arbitrary threshold value significantly larger than the

observed background noise. We selected a threshold value of 0.650 V for these

experiments. The number of peaks per data set indicates the number of peaks in the

signal detected per 30 ms of data. We then took the average signal amplitude of data

values greater than the threshold value to quantify the amplitude of the peaks. Peak

width was measured by counting the number of data values above the threshold in a row.

The average peak width was then taken for each data set. These three statistics are

descriptive of the microwave emission behavior of the particles in our simulation. The

results from the aluminum experiments at high pressure are shown in Table 1 and

discussed in more detail below. Plots of data sets representative of typical experimental

results from tests with large, small, and mixed particles are shown in Figures 6-8, and are

compared with a plot of background microwave emissions in Figure 5.

Aluminum Large Small Mixed

Number of peaks per data set 7553.7 37.0 168.3

Average signal amplitude (V) 0.673 0.709 0.693

Average peak width (s) 5.395*10-8

9.01*10-8

7.92*10-8

Table 1: Results from aluminum experiments

Figure 6: Microwave emissions recorded during a dust devil simulation using large

aluminum particles.

Figure 5: Background microwave noise taken when vortex generator was running, but

no particles were placed inside the experimental chamber.

Figure 7: Microwave emissions recorded during a dust devil simulation using mixed

aluminum particles.

Figure 8: Microwave emissions recorded during a dust devil simulation using small

aluminum particles.

Large particles produced the most peaks per 30 ms data set, but discharge between the

small particles resulted in the strongest and longest-lasting peaks. However, what is truly

significant about these experiments is that we have shown that collisions between

particles followed by electric discharge produces microwave radiation that can be

measured remotely. However, we were only able to detect emissions for collisions

between aluminum particles. Although aluminum is present in the Martian soil, the

results would have been much more promising had we been able to detect emissions from

hematite and basalt.

There are several possible explanations for why we were unable to detect emissions from

other particles besides aluminum. First, we may not be taking measurements at the right

frequency to detect the emissions. Also, our sampling rate may not be fast enough to

detect the emissions. We are sampling at 20 MHz, which corresponds to recording the

signal amplitude every 5*10-8

s. Although this seems like a very short amount of time,

the peaks we detected from aluminum are already pushing this threshold of detection,

being on the order of 10-8

seconds long. Most peaks we detected exist for only one data

point, which means that if emission peaks from discharge between hematite and basalt

particles are even shorter, we would not be able to detect them. Finally, there may not be

enough emission in our small-scale simulation using hematite and basalt for them to be

detected.

Equipment Setup 2

We analyzed the data from these experiments in several ways. First, we plotted the

amplitude probability distribution function of the experimental data against the pdf

measured when the vortex generator was running normally, but no particles were placed

in the experimental chamber (this was used as a control pdf). This gave us a proper

background reference. In addition, we looked at the kurtosis of the experimental pdf of

the data. Kurtosis is the ratio of the fourth central moment of a curve to the second

moment squared. This is a good measure of the shape of a distribution. A kurtosis of 3

indicates a Gaussian distribution and any other value of the kurtosis would indicate a

non-Gaussian distribution. The kurtosis is significant because thermal microwave

emissions, like what is observed in background noise, have a Gaussian distribution. If we

have a non-Gaussian distribution in the experimental pdf, we know that we are observing

non-thermal emissions.

In all experiments, it was not possible to tell the experimental pdf from a Gaussian

distribution from simply looking at it. In some cases, though, there was a visible change

in the pdf even if it did still look Gaussian (Figure 8). However, looking at the kurtosis

of the data provided more useful results. We were able to measure differences in the

kurtosis of the control pdf for experiments run with mixed aluminum and large aluminum

particles (Figures 9 and 10).

Figure 8: Probability distribution function of experimental data using large

aluminum particles compared to the pdf of thermal background noise

Figure 9: Kurtosis of emissions from experiments using large aluminum particles at

1 ATM pressure. The experiment began at the 12-second mark.

These results are very important because we have identified a sensor that will allow us to

distinguish between non-thermal and thermal emissions. Such a sensor could be adapted

for use in the field.

Scientific Potential

The experiments run with the new radiometer used in setup 2 have allowed us to support

our theory that colliding particles in a dust devil produces non-thermal microwave

radiation that can be distinguished from background thermal emissions. In addition, the

equipment we have been using to conduct these recent experiments is flight-ready and

could potentially be used on Mars. These exciting results suggest great scientific

potential for this research. A sensor like the one used to measure kurtosis in our

experiments could be used to detect the presence of non-thermal microwave emissions

that would, in turn, detect the presence of a dust event.

In addition, JPL has approved our use of the Deep Space Network to monitor Mars for

the emissions we expect to see from dust activity. We also plan to propose that a sensor

like the one used in our second round of experiments be placed on NASA’s 2013 Mars

Science Orbiter.

Figure 10: Kurtosis of emissions from experiments using mixed aluminum particles at

1 ATM pressure. The experiment began at the 40-second mark.

Future Work

The work conducted under this student fellowship is an important first step towards the

ultimate goal of designing a sensor to unambiguously detect the presence of hazardous

dust events on Mars. However, much work remains before such a sensor could be put

into practice. Immediate goals include additional field work studying electrical activity

in terrestrial dust devils in Arizona. A research trip to Eloy, Arizona is scheduled for

May and June 2006 in which we will attempt to correlate non-thermal microwave

emissions from terrestrial dust devils with weather measurements.

Also, we plan to use the data that we obtained during the experiments conducted this year

to test a model of the emission of non-thermal radiation by dust events. We have a model

of the emissions from one discharge, but in order to identify these emissions from a dust

event we need a model of emissions from many discharges occurring at once.

Finally, our work with the Deep Space Network will contribute greatly towards our

understanding of this phenomenon and its application to Mars.

Acknowledgements

I would like to thank my mentor, Dr. Nilton Renno, for helping me with this project, as

well as my research team, Kevin Reed and Catalina Oaida. I would also like to thank

NIAC for this wonderful opportunity.

References

1. Renno, Nilton O., Ah-San Wong, Sushil K. Atreya. “Electrical discharges and

broadband radio emission by Martian dust devils and dust storms.” 19 November

2003.

2. Renno, Nilton O. et al. “MATADOR 2002: A pilot field experiment on convective

plumes and dust devils.” 7 July 2004.