An apparatus has been developed for measuring the low concentrations of liquid water and ice in relatively dry soil samples. Designed as a prototype of instruments for measuring the liquid-water and ice contents of Lunar and Martian soils, the apparatus could also be applied similarly to terrestrial desert soils and sands. The high sensitivity of this apparatus is best appreciated via a comparison: Whereas soil moisture contents of agricultural interest range between 3 and 30 weight percent, this apparatus is capable of measuring moisture contents from 0.01 to 10 weight percent (at room temperature). Moreover, it has been estimated that optimization of the design of the apparatus could enable measurement of moisture contents as low as 1 part per million by weight.

Figure 1. A Sample Chamber Containing a Four-Electrode Probe is mounted on a printed-circuit board that is plugged into a commercial impedance spectrometer: (a) top view of the soil moisture cup showing the four probes that are spaced 11.18 mm apart; (b) side view of soil moisture chamber inserted into printed wiring board that inserts into the LCR meter, and (c) LCR meter with soil-measuring cup.
The apparatus is a special-purpose impedance spectrometer: Its design is based on the fact that the electrical behavior of a typical soil sample is well approximated by a network of resistors and capacitors in which resistances decrease and capacitances increase (and, hence, the magnitude of impedance decreases) with increasing water content. The apparatus includes a commercial impedance spectrometer and a custom sample chamber. Four stainless-steel screws at the bottom of the jar are used as electrodes of a four-point impedance probe. The leads from the electrodes are routed to a 10-pin connector that is plugged into a printed-circuit board that, in turn, is plugged into the impedance spectrometer (see Figure 1). Special precautions were taken in constructing the printed-circuit board to shield the signal conductors to enable measurement of impedances as high as 3 GΩ, thereby enabling measurement of very low levels of moisture. The lower limit of impedance measurable by this apparatus is 100 Ω.

Figure 2. Three Regions measured by the impedance spectrometer that are explained by the soil moisture model. Measurements were obtained from fine silica sand and two samples of coarse silica sand with a diameter “d”. The soil water was doped with 100 mM KCl and measured at a frequency of 100 Hz. (Note: FSSUCR is fine silica sand from the University of California, Riverside; CSSMAL is coarse silica sand from Mallinckrodt Chemicals; and CSSUCR is coarse silica sand from the University of California, Riverside.)
For a typical measurement run, a sample of soil is placed in the jar and the magnitude and phase angle of impedance are measured at fixed frequencies of 100 Hz, 120 Hz, 1 kHz, 10 kHz, and 100 kHz, using applied AC potentials of 50 mV, 250 mV, and 1 V. The measurement data can then be plotted and analyzed to estimate water content, as illustrated by the example of Figure 2.

This work was done by Martin Buehler of Caltech for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management

JPL

Mail Stop 202-233

4800 Oak Grove Drive

Pasadena, CA 91109-8099

E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Refer to NPO-41822, volume and number of this NASA Tech Briefs issue, and the page number.

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Measuring Low Concentrations of Liquid Water in Soil

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This article first appeared in the February, 2009 issue of NASA Tech Briefs Magazine (Vol. 33 No. 2).

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Overview

The document discusses the measurement of dielectric constants to characterize lunar soils, particularly focusing on the detection of low concentrations of water in the lunar regolith. Authored by researchers from NASA's Jet Propulsion Laboratory and the University of California, Riverside, it highlights the significance of in-situ methods for analyzing lunar materials as part of future lunar exploration missions.

The introduction emphasizes the renewed interest in the Moon, necessitating reliable techniques to assess the lunar regolith's properties. The dielectric constant is a key parameter in this analysis, as it relates to the density and composition of lunar minerals, primarily silicates and oxides. The document presents preliminary results indicating that the dielectric constant can provide insights into the presence of specific minerals, such as titanium and iron, which affect the regolith's composition.

A significant portion of the document is dedicated to the detection of water ice on the Moon, particularly at the poles. Early data from the Lunar Prospector spacecraft suggested the presence of water ice mixed with the regolith at low concentrations (0.3 to 1 percent). However, subsequent research has not definitively proven the existence of water, raising questions about the detectability of water in lunar soils. The dielectric constants of ice and water are discussed, showing that at low frequencies, both have high dielectric constants, but the dielectric constant of ice decreases significantly at higher frequencies.

The document also details experimental results using impedance spectroscopy to measure moisture in coarse silica sand, demonstrating the ability to detect water concentrations as low as 0.001%. This capability is crucial for future lunar missions, where understanding soil moisture could inform resource utilization strategies.

In conclusion, the document advocates for the development of rapid survey techniques, including impedance spectroscopy, electrostatic measurements, and magnetic property assessments, to facilitate efficient in-situ measurements of lunar soils. These methods can be integrated into roving vehicles, allowing for comprehensive analysis during exploration missions. The research underscores the importance of characterizing lunar regolith to support sustainable human presence and resource utilization on the Moon.