University of Houston • University of Houston-Clear Lake • ISSO Annual Report Y2004 • 114-118
High-Frequency Dielectric Spectroscopy for Martian Soil Samples
Abstract--Researchers at the University of Houston and NASA-JSC seek to develop high frequency methods, such as broadband and narrowband (resonant) dielectric spectroscopy techniques, for the investigation of Martian soil simulants and live cell suspensions. Such methods hold great potential for use in high frequency characterization of the magnetite crystals found in the Martian meteorite Allan Hills 84001. Magnetite compounds found in the meteorite consist of a common inorganic rock and a biogenic product, which can be formed by a variety of organisms. It is clear that characterization and comparison of earth and Martian magnetofossils can provide significant information related to the evolution and history of Mars. In this initial study, we have developed high frequency dielectric spectroscopy probes and conducted test measurements of dielectric properties of live cell suspensions, yeast, and isopropyl alcohol. Currently, we are investigating JSC Mars-1 samples, a volcanic ash from Hawaii developed for the use as a Mars regolith simulant. Because these samples contain microorganisms and biomolecules, they are biologically active.
The question of whether life once existed on Mars,1 is still a subject of continued interest. Recent data from the robotic expedition of the rovers Opportunity and Spirit provide the most tantalizing evidence thus far that the landing sites of the rovers were once filled with water.2 Recent studies3 of the Martian meteorite Allan Hills 84001 (ALH84001) suggest that microbial life existed on Mars about four billion years ago.
The main mission of Opportunity and its twin Spirit, which have landed on the opposite sides of Mars, is to look for a geological evidence of Martian water. It is known that ice still exists on Mars near its poles. Most of the planet, however, including the equatorial region where Opportunity landed, appears to be devoid of any form of water. The Opportunity is equipped with a mini-thermal emission spectrometer and its first data have shown the presence of iron oxide hematite in the dark pebbles and gravel surrounding the landing area of the rover. NASA considers this as preliminary evidence of water presence in the past in this area. This conclusion is based on the fact that the observed hematite has a different infrared spectrum than that formed from hot lava. This hematite, newly discovered on Mars, appears to have formed at low temperatures in chemical reactions with water. Until now, scientists had been using data from Viking and Mariner missions (XRF analyses) and had developed several models of Martian soil composition, assuming that they are lava-like in composition due to volcanic features observed in Viking and Mariner images.4 The new rover discoveries will result in the development of new models of the Martian soil; they will also require the development of new methods for the characterization of such soil. Martian soil that requires not only dielectric but also magnetic susceptibility characterization.
In searching for the life evidence on Mars, scientists find that magnetite crystals found in the Martian meteorite Allan Hills 84001 (ALH84001) provided important clues when the existence of very small (magnetite (Fe3O4) crystals (tens of nanometers) were identified within carbonate globules together with their associated rims in the meteorite.5 Scientists believe that these magnetites are, in fact, magnetofossils, which can be the evidence of the oldest life forms known.6 There is a similarity between these magnetite particles and magnetic crystals produced by magnetotactic bacterias found on earth.
In order to analyze both electric and magnetic properties of Martial soil versus frequency, we have developed in our lab broadband and resonant techniques of the dielectric permittivity or permeability measurements in a very wide range of frequencies (0.3 MHz-20 GHz). Such high frequency dielectric characterization methods have already been successfully used in the past to characterize lunar samples.7
For broadband measurements a shielded open-circuited transmission line technique was adapted. This is one of the variants of the coaxial transmission line techniques used for many years, in which a sample cell is used for dielectric measurements of solids and liquids.8 For high sensitivity characterization of small soil samples, we have developed a resonant method,9 which also includes the split-post resonator technique (SPDR).
Currently, experiments carried out in the lab are designed to investigate Martian soil simulants provided by JSC PIs.
Coaxial Probe for Broadband Dielectric Response Measurements
Broadband dielectric measurements are commonly conducted using coaxial fixtures at high
frequencies and parallel plate capacitors at low frequencies. The probe implemented in our
work is sometimes called an open-circuited sample holder for permittivity measurements and
was used in the past mainly for liquids characterization. Such a technique can be used for
a wide range of frequencies starting around 1 MHz and extending up into the high GHz
region. We have made a few probes to perform broadband probe measurements of various
liquid and solid-state (in a form of powder) samples. Two final probes were selected for
the measurements: 7 mm with a APC-7 connector and 3.5 mm with a SMA connector. The probes
consist of a section of 7 mm and 3.5 mm inner diameter of transmission lines for the 7 mm
and 3.5 mm probes, respectively. The inner conductors in both probes are shorter than the
outer ones. The dielectric material to be measured fills in the coaxial section of the
holder and extends beyond the inner conductor into the tube formed by the outer conductor
of the transmission line. Such a holder is particularly useful for liquid and powder
measurements, because of very easy access to the cell through the open end. A sketch of
the probe is shown in Fig. 1.
Figure 1. A sketch shows the shielded open-circuited sample holder (dielectric probe) for dielectric measurements of liquids and powders. It consists of a section of coaxial transmission line designed and manufactured with the inner conductor shorter than the outer conductor. Dielectric material to be measured fills in the coaxial section of the holder and extends beyond the inner conductor into the tube formed by the outer conductor of the transmission line.
We have analyzed the probe using an analytical model of such a structure proposed by J. Baker-Jarvis et. al from NIST.10
Figure 2. A broadband probe (3.5 mm inner diameter) with a 3.5 SMA connector attached to network analyzer Agilent 8712ES (300 kHz-1.3 GHz) is shown. A Smith chart plot of the complex reflection coefficient of an empty probe can be seen on the displays of the analyzer.
Split-post dielectric resonator method (SPDR)
A very powerful and sensitive method for probing dielectric properties of materials at GHz
frequencies entails the use of a split-post dielectric resonator (SPDR). A split-post
dielectric resonator consists of a dielectric (e.g., sapphire) cylinder, surrounded by a
metal enclosure split in the middle. Figure 3 sketches a cross-section of an SPDR
developed in our lab. We have already used such technique to obtain images of the
dielectric and magnetic properties of a mall amount of yeast enclosed in a paper envelope.
Such resonators, when used for permittivity measurements, can provide very accurate data
of both dielectric constant and loss tangent. For the measurement of magnetic properties,
the sample has to be placed in the resonator in the maximum of magnetic field. For such
cases, we will also use a dielectric resonator technique with some modification to the
resonator.
Figure 3. Cross-Sectional Schematic of a Split-Post Dielectric Resonator Fixture is Shown
For small gaps and small or thin samples, such a resonator operates in the electromagnetic mode, which has only an azimuthal electric field component. As a result, the electric field remains continuous at the dielectric interfaces and it is also very uniform in the z-direction. An example of dielectric characterization of a small sample of dry yeast is shown in Fig. 4. Shift of resonant frequency of the split resonator was measured as a function of x-y scanning of the sample inside resonator dielectric disc gap. Both contour map and the image of the frequency shift demonstrate the highly sensitive nature of this characterization method.
Figure 4. Imaging of Very Small Volume of Dry Yeast Obtained Using a Single Dielectric-Split Resonator
The main problem in using an SPDR is its complicated geometry, which necessitates the use of advanced numerical methods for resonator analysis. To fix this problem, we have designed and assembled three split-post resonators working at 9.7, 10.5, and 11.3 GHz (Fig. 5). Measurements of the same sample in all three resonators allow us to simplify the calculation of complex dielectric permittivity numbers from experimental data. Resonators were designed to have the same gap (around 1mm, but different dimensions of both quartz rings and dielectric disks. We purchased some resonator parts from QWED Inc., which specializes in microwave characterization of dielectrics.
Figure 5. A Cluster of Three Split Resonators (10.5, 11.4 and 12.3 GHz)
In Fig. 5, details of the design and a picture of three split post dielectric resonators (SPDRs) are shown, each resonating at different frequency.
Such resonators, when used for permittivity measurements, can provide very accurate data of both dielectric constant and loss tangent. For a properly chosen sample volume, it is possible to resolve dielectric loss tangent to approximately 2 × 10-5 (assuming an accuracy of Q-factor measurements of about 1%). Resonant frequency values needed for the real permittivity component calculations can be measured with very high accuracy. The sensitivity of the method can be increased even further by replacing a copper enclosure (shown in Fig. 3) with a superconducting one.11
Results
The procedure for broadband complex permittivity measurements of liquid and/or solid state
samples consists of three steps: (1) measurements of reflection coefficient vs. frequency
of an empty probe, (2) measurement of reflection coefficient vs. frequency of a sample
filled probe, and (3) calculations of the complex permittivity from reflection coefficient
data using the theoretical model of the probes.
We use a vector network analyzer to measure the refection coefficient, G. Data for each sample characterized for complex perimittivity consist of two measurements of an empty probe and a probe filled with the sample. The calibration is done to the plane at the connector of the probe. This means that all losses due to the vector analyzer cables and connectors are removed up to the plane which is parallel to the bottom plane of the bead (see Fig. 1). The model discussed above allows one to calculate the refection coefficient G at the plane of the upper surface of bead, so the measured data has to be transform from the calibration plane to the upper bead. This is achieved by comparing experimental data (which includes the bead influence) of the reflection coefficient versus frequency of the empty probe, with the simulated response of the probe (without the bead included). This allows us to calculate effective thickness and complex permittivity of the bead. Knowing the effective parameters of the bead, we can correct experimental data of electromagnetic response of the empty probe to account for the presence of the bead.
The program to calculate complex permittivity from the reflection measurements was written using MathCAD software. Some of the calculations, for comparison purposes, were also done using a Fortran program obtained from NIST. Our two samples (yeast powder and isopropyl alcohol) were also tested by NIST, where in the framework of our collaboration, both probes were used and compared for the same samples. Results of the measurements carried out at NIST by Jim Baker-Jarvis and Mike Janezic are shown in Fig. 6. To validate our probes we have measured the same samples in our laboratory. Results of both measurements showed very close values.
Figure 6. Complex permittivity of dry yeast powder and isopropyl alcohol measured using National Institute for Standards and Technology 7 mm probe
In order to test the method performance for the characteriation of suspended cells, we measured complex permittivity of DI water and, next, the water solution of B. subtilis. Results are shown in Fig. 7. An excellent sensitivity was observed.
Figure 7. Complex permittivity of DI water and B-subtilis measured using our 3.5 mm probe. Dashed lines represent the plot for B-subtilis water solution; continuous lines, the plot for DI water. Real parts of the complex permittivity are shown on the left hand side of the plot; imaginary parts, on the right hand.
Summary
In summary, we have measured the complex permittivities of deionized (DI) water, a
suspension of B. subtilis in water, dry yeast, and a suspension of yeast in
water. Measurements were done over a very wide range of frequencies (1 MHz-20 GHz). The
same materials were also measured for the real part of the complex permittivity using a
split-dielectric resonator technique. We are currently analyzing this data and are
continuing to refine the software for complex permittivity calculations from the
reflection coefficient data.
Currently, we are investigating JSC Mars-1 samples, a volcanic ash from Hawaii developed for the use as a Mars regolith simulant. Such samples contain microorganisms and biomolecules; therefore, they are biologically active. A future objective will be determining whether our microwave methods can provide a rapid, easy-to-use method of detecting signatures of magnetic and dielectric components of Martian soil.
References
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the Early Earth, and Europa," J. Geophys. Res. 103 (1998): 19,359-64.
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Biosignatures," Proc., Nat. Acad. Sci. USA 98 (2001): 2164-69.
6K. L. Thomas-Keprta, S. J. Clemett, D. A. Bazylinksi, J. L. Kirschvink, D. S.
McKay, S. J. Wentworth, H. Vali, E. K. Gibson, Jr., and C. S. Romanek,
"Magnetofossils from Ancient Mars: A Robust Biosignature in the Martian Meteorite
ALH84001," Appl. & Environ. Microb. 68 (2002): 3663-72.
7H. E. Bussey, "Microwave Dielectric Measurements of Lunar Soil with
Coaxial Line Resonator Method," Proc., 10th Lunar Planet Science Conference
(1979): 2175-82.
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Technol. 1 (1990): 691; W. R. Scott and G. S. Smith, IEEE Trans. on
Instrumentation and Measurements 35 (1986): 130; J. Baker-Jarvis, M. D. Janezic, and C. A.
Jones, "Shielded Open-Circuited Sample Holders for Dielectric Measurements of Solids
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Presentations
Wosik, J. "Single and Multiple Cryogenic Coils for MRI Applications,"
Perspectives and Recent Developments, 7th Symp. on HTS in High Frequency Fields,
Barcelona, Spain, May 25-29, 2004.
Wosik, J., M. Kamel, L. Xue, L.-M. Xie, and J. Bankson. "HTS Array for Parallel
Imaging," Proc., Second Workshop on MRI Parallel Imaging, Zurich,
Switzerland, Oct. 15-17, 2004.
Wosik, J., K. Nesteruk, L.-M. Xie, J. A. Bankson, M. Kamel, and J. D. Hazle.
"Superconducting Phased Array," Proc., 12th Annual Meeting of the
International Society for Magnetic Resonance in Medicine, Kyoto, Japan, May 15-19, 2004.
Xue, L., L.-M. Xie, M. Kamel, and J. Wosik. "SNR Gain Dependence on Coil
Parameters," Proc., 12th Annual Meeting of the International Society for
Magnetic Resonance in Medicine, Kyoto, Japan, May 15-19, 2004.
Funding and Proposals
Miller, J. H., Jr. and J. Wosik. "Dielectric Spectroscopy of Biological Agents,"
DARPA/Naval Surface Warfare Center, Sept. 26, 2003-Sept. 25, 2005, $757,726 ($250,000 funded
to date).
Miller, J. H., Jr., D. S. McKay, G. E. Fox, J. Wosik, and D. Warmflash. "Biosensors
Based on Dielectric Response: A Non-Geocentric Approach for In Situ Life Detection,"
NASA-ASTEP (Astrobiology Science and Technology for Exploring Planets) Program, $895,277,
request for three years ($793,197 for UH, $102,080 for NASA-JSC) (pending).
Wosik, J. "Structural MRI of Trabecular Bone for Therapy Response Monitoring,"
NIH (solicitation PAR-04-023, Bioengineering Research Partnership, F. Wherli University of
Pennsylvania), from July 1, 2005-June 30, 2010, $498,000 for UH, total proposal $3,097,000
(submitted).
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