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TRANSCRIPT
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
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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.