Researchers Study Mars Phoenix Data for Salt, Water

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Vincent Chevrier, research professor in the Arkansas Center for Space and Planetary Sciences, is analyzing data from the Mars Phoenix mission to get a better grasp on the relationship between the ground and the atmosphere and determine whether liquid water is possible under the conditions found at the Phoenix site.

FAYETTEVILLE, Ark. – A University of Arkansas researcher has received funding from NASA to study data from the Mars Phoenix mission and develop a better understanding of how the soil on Mars interacts with the atmosphere, and whether these interactions ever produce liquid water.

Vincent Chevrier, a research professor in the Arkansas Center for Space and Planetary Sciences at the University of Arkansas, will work with graduate student Jennifer Hanley to analyze data from the Phoenix mission, which originally determined the presence of perchlorates, compounds containing the ion ClO₄^-, on the planet’s surface. The data, which were taken over the course of about six months, include information on temperature, humidity, electrical conductivity, heat parameters and permittivity, which relates to a material’s ability to transmit an electric field.

These tens of thousands of measurements contain information on how the Martian soil interacts with the atmosphere. Chevrier will examine how the soil affects the stability of ice, the humidity and the formation of liquid brine solutions, which contain liquid water.

 “Our group has shown that it is thermodynamically possible to have a stable liquid in the soil for a few hours a day under certain conditions,” Chevrier said. “The effect of the regolith, or soil, on the water cycle is poorly understood and the Phoenix data provide a unique insight into these processes.” Changes in some of the electrical data from Phoenix may indicate the presence of liquid water, but such changes may be extremely subtle, or even nonexistent.

“You need a continuous layer of fluid in order to detect changes in these parameters,” Chevrier said. “A drop of water won’t do it.”

In addition to examining the activity of water at the Mars Phoenix site, Chevrier and Hanley will examine the composition and nature of the salts in the soil at the site, including the perchlorates. Perchlorates attract water, which means that they may help control humidity in the soil and atmosphere, said Chevrier. The current meteorological models for Mars make use of constants to fit the data, and these models work well for predicting where a Mars explorer should land.

“However, these models are not good for understanding what is going on beyond the constants,” Chevrier said.

The researchers will look at how the soil interacts with the atmosphere by studying data on three processes. First, they will examine the exchange of water vapor between salts so they can find out how the salts affect the atmosphere. They also will look at the kinetics, or speed, of adsorption, where water molecules collect around grains in the soil. Third, they will examine the ice layer under the top layer of soil, looking for signs of sublimation, where the ice becomes gas and diffuses through the soil.

After studying the soil, they will focus on liquid water.

“If the salts can exchange, maybe they will form a brine solution,” Chevrier said. This will require a detailed examination of the data, as Chevrier’s previous study showed that liquid water might be stable for only two or three hours on a given day.

Next they will reinvestigate the chemical data to see if there might be chlorate, the ClO₃^- ion, present as well as perchlorate. The chemical data don’t completely add up – the number of cations, or ions of positive charge, found by Phoenix does not fit scientists’ current understanding of the chemical composition of the soil. Chevrier and his team believe the explanation may involve the presence of chlorates as well as perchlorate. These two molecules look similar to the instruments found on Phoenix and have about the same stability, but the presence of chlorates in addition to perchlorates might help scientists determine the composition of the original salt assemblage in the soil.