Samuel P. Kounaves

Detection of Perchlorate and the Soluble Chemistry of Martian Soil at the Phoenix Lander Site

Phoenix Ascending The Phoenix mission landed on Mars in March 2008 with the goal of studying the ice-rich soil of the planets northern arctic region. Phoenix included a robotic arm, with a camera attached to it, with the capacity to excavate through the soil to the ice layer beneath it, scoop up soil and water ice samples, and deliver them to a combination of other instruments—including a wet chemistry lab and a high-temperature oven combined with a mass spectrometer—for chemical and geological analysis. Using this setup, Smith et al. (p. 58) found a layer of ice at depths of 5 to 15 centimeters, Boynton et al. (p. 61) found evidence for the presence of calcium carbonate in the soil, and Hecht et al. (p. 64) found that most of the soluble chlorine at the surface is in the form of perchlorate. Together these results suggest that the soil at the Phoenix landing site must have suffered alteration through the action of liquid water in geologically the recent past. The analysis revealed an alkaline environment, in contrast to that found by the Mars Exploration Rovers, indicating that many different environments have existed on Mars. Phoenix also carried a lidar, an instrument that sends laser light upward into the atmosphere and detects the light scattered back by clouds and dust. An analysis of the data by Whiteway et al. (p. 68) showed that clouds of ice crystals that precipitated back to the surface formed on a daily basis, providing a mechanism to place ice at the surface. Most of the chlorine at the Phoenix landing site is in the form of perchlorate, a salt that is highly soluble in water. The Wet Chemistry Laboratory on the Phoenix Mars Lander performed aqueous chemical analyses of martian soil from the polygon-patterned northern plains of the Vastitas Borealis. The solutions contained ~10 mM of dissolved salts with 0.4 to 0.6% perchlorate (ClO4) by mass leached from each sample. The remaining anions included small concentrations of chloride, bicarbonate, and possibly sulfate. Cations were dominated by Mg2+ and Na+, with small contributions from K+ and Ca2+. A moderately alkaline pH of 7.7 ± 0.5 was measured, consistent with a carbonate-buffered solution. Samples analyzed from the surface and the excavated boundary of the ~5-centimeter-deep ice table showed no significant difference in soluble chemistry.

H2O at the Phoenix Landing Site

Phoenix Ascending The Phoenix mission landed on Mars in March 2008 with the goal of studying the ice-rich soil of the planets northern arctic region. Phoenix included a robotic arm, with a camera attached to it, with the capacity to excavate through the soil to the ice layer beneath it, scoop up soil and water ice samples, and deliver them to a combination of other instruments—including a wet chemistry lab and a high-temperature oven combined with a mass spectrometer—for chemical and geological analysis. Using this setup, Smith et al. (p. 58) found a layer of ice at depths of 5 to 15 centimeters, Boynton et al. (p. 61) found evidence for the presence of calcium carbonate in the soil, and Hecht et al. (p. 64) found that most of the soluble chlorine at the surface is in the form of perchlorate. Together these results suggest that the soil at the Phoenix landing site must have suffered alteration through the action of liquid water in geologically the recent past. The analysis revealed an alkaline environment, in contrast to that found by the Mars Exploration Rovers, indicating that many different environments have existed on Mars. Phoenix also carried a lidar, an instrument that sends laser light upward into the atmosphere and detects the light scattered back by clouds and dust. An analysis of the data by Whiteway et al. (p. 68) showed that clouds of ice crystals that precipitated back to the surface formed on a daily basis, providing a mechanism to place ice at the surface. A water ice layer was found 5 to 15 centimeters beneath the soil of the north polar region of Mars. The Phoenix mission investigated patterned ground and weather in the northern arctic region of Mars for 5 months starting 25 May 2008 (solar longitude between 76.5° and 148°). A shallow ice table was uncovered by the robotic arm in the center and edge of a nearby polygon at depths of 5 to 18 centimeters. In late summer, snowfall and frost blanketed the surface at night; H2O ice and vapor constantly interacted with the soil. The soil was alkaline (pH = 7.7) and contained CaCO3, aqueous minerals, and salts up to several weight percent in the indurated surface soil. Their formation likely required the presence of water.

Evidence for Calcium Carbonate at the Mars Phoenix Landing Site

Phoenix Ascending The Phoenix mission landed on Mars in March 2008 with the goal of studying the ice-rich soil of the planets northern arctic region. Phoenix included a robotic arm, with a camera attached to it, with the capacity to excavate through the soil to the ice layer beneath it, scoop up soil and water ice samples, and deliver them to a combination of other instruments—including a wet chemistry lab and a high-temperature oven combined with a mass spectrometer—for chemical and geological analysis. Using this setup, Smith et al. (p. 58) found a layer of ice at depths of 5 to 15 centimeters, Boynton et al. (p. 61) found evidence for the presence of calcium carbonate in the soil, and Hecht et al. (p. 64) found that most of the soluble chlorine at the surface is in the form of perchlorate. Together these results suggest that the soil at the Phoenix landing site must have suffered alteration through the action of liquid water in geologically the recent past. The analysis revealed an alkaline environment, in contrast to that found by the Mars Exploration Rovers, indicating that many different environments have existed on Mars. Phoenix also carried a lidar, an instrument that sends laser light upward into the atmosphere and detects the light scattered back by clouds and dust. An analysis of the data by Whiteway et al. (p. 68) showed that clouds of ice crystals that precipitated back to the surface formed on a daily basis, providing a mechanism to place ice at the surface. The action of liquid water may have helped to form the calcium carbonate found in the soils around the Phoenix landing site. Carbonates are generally products of aqueous processes and may hold important clues about the history of liquid water on the surface of Mars. Calcium carbonate (approximately 3 to 5 weight percent) has been identified in the soils around the Phoenix landing site by scanning calorimetry showing an endothermic transition beginning around 725°C accompanied by evolution of carbon dioxide and by the ability of the soil to buffer pH against acid addition. Based on empirical kinetics, the amount of calcium carbonate is most consistent with formation in the past by the interaction of atmospheric carbon dioxide with liquid water films on particle surfaces.

Microfabricated Ultramicroelectrode Arrays: Developments, Advances, and Applications in Environmental Analysis

The wider availability of microlithographic techniques for the fabrication of electrochemical devices has led to a signi®cant increase in the development of microfabricated arrays of microelectrodes and their use in a wide variety of analytical problems. The major microfabrication steps and the capabilities and limitations of this microsensor technology are reviewed in this article. Several examples are summarized to illustrate the breadth of work with silicon-based microelectrode arrays, with special emphasis on their use for environmental analysis in a range of diverse settings including remote electroanalysis on Mars.

Discovery of Natural Perchlorate in the Antarctic Dry Valleys and Its Global Implications

In the past few years, it has become increasingly apparent that perchlorate (ClO(4)(-)) is present on all continents, except the polar regions where it had not yet been assessed, and that it may have a significant natural source. Here, we report on the discovery of perchlorate in soil and ice from several Antarctic Dry Valleys (ADVs) where concentrations reach up to 1100 microg/kg. In the driest ADV, perchlorate correlates with atmospherically deposited nitrate. Far from anthropogenic activity, ADV perchlorate provides unambiguous evidence that natural perchlorate is ubiquitous on Earth. The discovery has significant implications for the origin of perchlorate, its global biogeochemical interactions, and possible interactions with the polar ice sheets. The results support the hypotheses that perchlorate is produced globally and continuously in the Earths atmosphere, that it typically accumulates in hyperarid areas, and that it does not build up in oceans or other wet environments most likely because of microbial reduction on a global scale.

Voltammetric measurement of arsenic in natural waters.

There are several U.S. EPA approved methodologies for the determination of arsenic in ground water. Such technologies are lab-based, time intensive and can lead to a large capital cost for multi-sample analysis. In light of the number of sites found to contain arsenic at levels higher than the maximum contaminant level (MCL), on-site screening and monitoring systems are an attractive alternative. This review article summarizes several examples in the recent literature to illustrate the breadth of work in voltammetric analysis of arsenic in environmental samples. Also, included are recent voltammetric results, obtained with a microfabricated gold array and a field portable potentiostat, at an arsenic contaminated site in southern New Jersey.

An iridium-based mercury-film electrode: Part I. Selection of substrate and preparation

A mercury-film electrode with iridium as the substrate has been developed. Various metals were considered as potential electrode substrates, but only iridium was found to possess the desirable properties as a Hg-film substrate. After testing several pretreatment procedures the recommendation is to polish with 1 μm diamond, rinse with chromic acid and cathodize at −2.0 V vs. SCE. Different deposition conditions and solutions were tested for optimizing the conditions of film formation. The use of a square-wave deposition potential and 0.1 M HClO4 as electrolyte resulted in a dramatic improvement in the formation of a stable Hg film. Finally a complete procedure is given for the formation of a stable Hg film on iridium.

Microfabricated heavy metal ion sensor

Abstract A novel microfabricated electrochemical sensor has been developed for the detection and measurement of heavy metal ions in aqueous media. The sensor consists of a silicon substrate, on which is fabricated an electrically interconnected, but diffusionally isolated, array of thin-film iridium microelectrodes upon which a thin film of mercury is electrodeposited. Square-wave anodic-stripping voltammetry is used for quantitative analysis. This method involves an initial preconcentration phase in which the array is held at a cathodic potential such that the metal ions are reduced and amalgamated into the mercury, followed by anodic stripping (re-oxidation) of the metal ions. The charge required to strip a given ionic species completely is proportional to its initial concentration in the test solution. Sensitivity in the parts per billion range has been demonstrated without the addition of supporting electrolytes, deoxygenation, agitation, or any other alterations to the water samples.

The importance of concentration effects at the electrode surface in anodic stripping voltammetric measurements of complexation of metal ions at natural water concentrations

Abstract The influence of the ligand/metal ion concentration ratio on the shape, peak current and peak potential of curves obtained by anodic stripping voltammetry (a.s.v.) at the hanging mercury drop electrode is described, particularly with respect to the use of a.s.v. for speciation of metal ions at very low concentrations as is often found in natural waters. The lead(II)/triethylenetetramine system is used as a model of a fully labile reversible system. It is shown that the total metal ion concentration at the electrode surface ( C o M ) during the stripping step may be much larger (30–300 times in typical conditions) than that in the bulk solution ( C M ), the exact value depending on the deposition time t d . Consequently, changes in the peak characteristics are observed when the ligand/metal concentration ratio in the bulk of the solution, C L / C M , is less than 1000. Semi-empirical equations, experimentally tested, are given, which enable C o M / C M to be estimated for a specified solution and a.s.v. conditions, which correct for the “surface concentration effect” when a.s.v. is used to measure complexation, and which describe the influence of the parameters such as stirring efficiency, radius of the mercury drop and C L / C M . The implications of the results are discussed for determinations of total metal ion in complex media, of speciation based on peak-potential shifts or stripping voltammetric curves, and of complexation capacity.

Microfabricated electrochemical analysis system for heavy metal detection

A low power, hand-held system has been developed for the measurement of heavy metal ions in aqueous solutions. The system consists of an electrode array sensor, a high performance single chip potentiostat and a microcontroller circuit. The sensor is a microfabricated array of iridium electrodes, onto which a thin film of mercury is electroplated. Quantitative heavy metal analysis is performed using square-wave anodic stripping voltammetry. Measured results show a one part-per-billion sensitivity and multiple use capability.