Low-Frequency Electrical Properties for Subsurface Exploration of the Earth and Planets, 15-R9709

Principal Investigators
Robert E. Grimm
David E. Stillman

Inclusive Dates:  03/01/07 – 07/01/08

Background - The focus of this project was to make low-frequency electrical-properties measurements of saline ice, salt hydrates, and ice/hydrate/soil mixtures to determine the geophysical detection limits and properties of ices on the Moon, Mars and outer-planet satellites. The project further sought to assess bioelectrical signatures from soil-microbe mixtures for both terrestrial and extraterrestrial assays.

Approach - SwRI's Planetary Electrical Properties and Geochemistry Laboratory is equipped to measure complex dielectric constant over a wide range of frequency and temperature. From these broadband dielectric spectra, individual mechanisms of storage and dissipation of electrical energy can be modeled and identified. The project team performed dozens of individual experiments, varying ice-impurity composition and content, ice content, and soil type.

Accomplishments - During the course of the investigation, the team discovered that much more detailed study of saline ice and salt hydrates was necessary to provide the proper foundation for later analysis of mixtures. This work resulted in new fundamental contributions in structural chemistry that were not originally anticipated, namely:

• When impurity ions such as Cl^- are incorporated into the ice lattice, counterions such as Ca²⁺ are not similarly accommodated but instead reside interstitially
   
• The dielectric relaxation frequency in ice increases with increasing impurity content until a quantifiable saturation limit is reached
   
• Ionic defects in salt hydrates are much more abundant than in doped ice, indicating that disruption of hydrogen bonding is common
   
• Below the eutectic temperature of an ice-salt mixture, the salt hydrates always form an electrically connected phase among disconnected ice
   
• Above the eutectic temperature, a threshold of approximately 3 mM salt is necessary for micron-sized brine channels to form a connected network.

For ice-soil mixtures:

• Ice properties are recoverable from the mixture, with a detection limit around one percent
   
• The proximity of the silicates causes additional defects to form in the ice, beyond the nominal limits
   
• Unfrozen water between the ice and silicate is evident
   
• There is an interfacial polarization between the ice and silicate
   
• Above the eutectic temperature, pore sizes must be micron-sized or larger to form electrically connected fluid channels; in agreement with the ice-only measurements
   
• The cation-exchange capacity of the silicates affects the brine properties.

The team did not complete the microbe study. Initial tests were not successful and were not followed up because of additional effort spent early in the project on saline ice and salt hydrates. The team did conclude, however, that unfrozen water at subfreezing temperatures cannot support microbial life – in contrast to speculation in the astrobiology literature – because the electrical conductivity is not high enough to indicate useful transport of nutrients and wastes.

This project resulted in six conference presentations (one invited), and two journal articles. SwRI won three NASA grants based on work in progress. The work has led to several additional pending and planned proposals, including flight instruments. The work performed for this project is the foundation for many years of electrical-properties studies relevant to ices throughout the Solar System.

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