Water from a Stone

Categories: Interview Mars
This high-resolution color photo of the surface of Mars was taken by Viking Lander 2 at its Utopia Planitia landing site on May 18, 1979, and relayed to Earth by Orbiter 1 on June 7. It shows a thin coating of water ice on the rocks and soil.
Image Credit: NASA/JPL

Since the earliest planetary landers, much attention has been paid to the biological implications of returning extraterrestrial samples back to Earth. The moon rocks led to the creation of NASA’s Planetary Protection Office to oversee any possible cross-contamination issues. But less attention has been paid to the mineralogical implications of sample returns: what happens if the rocks themselves change during their trip to Earth?

The changing between different mineral types is something that can happen spontaneously with temperature, pressure or humidity. A salt is not just a mineral block, but can store lots of water in its chemical structure–fifty percent by weight in some cases. So if the samples are subject to environmental changes, that water can change its state at the same time as the salt’s crystal structure rearranges. One case study is being investigated by a team of mineralogists at Indiana University and Los Alamos National Laboratory. Their research question centers on what happens to Epsom salts, or magnesium sulfate, when exposed to heat, pressure or moisture? Also do such salty rocks, particularly near the martian equator, keep water from evaporating into the thin martian atmosphere?

The existence of magnesium sulfate salts on Mars was first suggested by the 1976 Viking missions and has since been confirmed by the Mars Exploration Rovers (MER) as well as the Pathfinder mission. The subject of sample changes en route back to Earth has been given new focus, given the recent finding of high Epsom salt concentrations both at Opportunity’s crater sites (up to 40% locally by weight Epsom salt) and Gusev’s subsurface (up to 15% locally in rover trenching experiments below 6 inches of topsoil). The Indiana University and Los Alamos scientists are testing various salts to record changes under conditions that were slightly less than Mars atmospheric pressure, as well as other experiments conducted at temperatures from minus 280 degrees Fahrenheit to 77 degrees Fahrenheit and pressures from less than 1/1000 of Earth’s atmosphere to ambient pressure.

Water-linked concretions, or blueberries at Meridiani
Image Credit: NASA

Astrobiology Magazine had the opportunity to talk with David Bish, Haydn Murray Chair of Applied Clay Mineralogy at Indiana University and a co-author of the study. "We were able to show that under Mars-like conditions, magnesium sulfate salts can contain a great deal of water," said Bish. He and his Los Alamos colleagues have proposed that the proportion and distribution of related mineral types–called hexahydrite, kieserite and other magnesium sulfate salts– on Mars may hold a record of past changes in climate and whether or not water once flowed there. The results also showed that because of the ease with which water transformations take place in the magnesium sulfate salts, the mineralogy might be more accurately characterized in situ before samples are removed from the Mars surface. Observing environmentally sensitive salts in their pristine state may only be possible when looked at on Mars.

Astrobiology Magazine (AM): Data from the Mars Global Surveyor and Mars Odyssey spacecraft show that the region surrounding the Opportunity rover’s landing site probably had a body of water at least 330,000 square kilometers, or 127,000 square miles. That would make the ancient sea larger in surface area than all the Great Lakes combined, or comparable to Europe’s Baltic Sea. Are your mineralogical results mainly motivated by such orbital evidence for near-surface Martian water, up to ten percent by weight?

David Bish (DB): The results were motivated by a combination of Odyssey data that provided evidence for heterogeneously distributed hydrogen on the martian surface and chemical data from Viking, Pathfinder, and MER, all showing correlations between magnesium (Mg) and sulfur (S). The Odyssey results strongly suggested that water was present in near-equatorial regions on Mars in regions where water ice should not be stable, arguing for the presence of water in some other form, perhaps hydrated minerals.

An artist's rendition of 2001 Mars Odyssey as it enters orbit
An artist’s rendition of 2001 Mars Odyssey as it entered orbit. On Oct. 28, 2003, during a period of intense solar activity, the radiation-detecting instrument stopped working properly.
Credit: JPL

AM: The various sulfate-rich regions seen on the surface at the Opportunity site have been attributed to mainly magnesium sulfates. Why has your team chosen to investigate the temperature, pressure and humidity features of such Epsom salts as their model system?

DB: The Viking, Pathfinder, and MER data all suggest the presence of Mg sulfates, usually indirectly through correlations between Mg and S. Therefore we chose to investigate the hydration/dehydration behavior of a complete suite of hydrated Mg sulfates. Our studies attempted to quantify which hydrated Mg sulfates would be stable under current martian surface conditions. Our studies also allowed us to eliminate some minerals from further consideration.

AM: Do such salts generally hold more water but also lose it with small changes in environmental conditions?

DB: The hydrated Mg sulfates can hold up to ~50% H2O, and the more-hydrated Mg sulfates lose some/much of their H2O with small changes in conditions. The less-hydrated sulfates, such as kieserite, hold their small amount of H2O tightly and do not readily lose this upon changes in environmental condition. In our experiments we also formed a hydrated but amorphous Mg sulfate that holds more water than kieserite under Mars-like conditions but unlike kieserite loses this water slowly. We are trying to determine the kinetics of this water loss and the possible role that such amorphous salts may play.

In general, hydrated salts (and many other hydrated minerals) have a hydration state that is a reflection of the particular temperature and water vapor pressure conditions under which they exist. If temperature and/or water vapor pressure change, the hydration state of the mineral or even the particular mineral may change.

Trenching experiments where five wheels are locked and the one free wheel digs up to its radius, about six inches, then turns around to image the trench. See Spirit’s images and slideshow

AM: The only digging currently going on near the surface is limited to about 6 inches, or half a wheel radius. Can you estimate about what depth a near-surface salt deposit might introduce water inventories for Mars?

DB: Although little is known about the distribution of water (or hydrogen) as a function of depth in the martian near surface, Odyssey’s neutron spectrometer provides information on water-equivalent hydrogen within about a meter of the surface.

At present, kieserite is almost definitely stable on the surface, so no digging is necessary. Hexahydrite may be stable on the present surface but existing thermodynamic data appear inadequate to allow accurate predictions of stability as a function of temperature and partial pressure of water. We do not know the variation in partial pressure of water as a function of depth, but it is likely that greater depths will stabilize increasingly hydrated Mg sulfates. The more hydrous salts will also be favored at higher latitudes. Epsomite does not appear to be stable at the martian surface at lower altitudes.

AM: The transformation of mineral types into kieserite particularly was often referred to among the Mars’ mission geologists as a crystal type that would be compelling evidence for surface water history, yet kieserite stores about 5 times less water than the alternative sulfates (epsomite and hexahydrite). Why is kieserite an important part of the water puzzle to geologists?

DB: Although kieserite holds considerably less water in its crystal structure than do hexahydrite or epsomite, it nevertheless contains one molecule of water (H2O) for each molecule of magnesium sulfate (MgSO4) and requires water to form. Indeed, under some conditions it could be the only hydrated Mg sulfate to form (as it is in some deposits on Earth). Kieserite is much more stable to environmental change than the other hydrated Mg sulfates. It can be hydrated to hexahydrite and epsomite but these two minerals form an amorphous hydrated Mg sulfate on dehydration. The existence of kieserite thus may suggest that the more hydrated forms of Mg sulfate have not existed in the past and kieserite formed directly. Alternatively, given sufficient time, the amorphous form of hydrated Mg sulfate may eventually crystallize to kieserite at low partial pressures of H2O, although we do not have data to support such a transformation.

AM: One limit to near-surface and surface water seems to be the one percent of terrestrial pressure on Mars. Is your impression that mineral storage might reveal a mechanism for chemically binding enough water to hinder its sublimation from ice?

Magnesium Sulfate is found in many mineral waters, and in sea water. It is also known as Epsom Salt. Credit: scribecom.com

DB: Definitely. We believe that our data on hydrous minerals, including zeolites, smectites, and at least kieserite, show firmly that "mineral storage" is a viable mechanism for binding water indefinitely in a form that will not evaporate or sublimate under current martian surface conditions. This is possible because the water molecules in these minerals are very strongly bound to the mineral structures and are, in some cases such as some of the Mg sulfates, an integral part of the crystal structures. In these cases, the hydration state is not variable with time as it represents the "stable" state of the mineral.

AM: What future plans do you have for extending the current studies?

DB: We are continuing to study hydrated sulfates, including calcium (Ca) and iron (Fe) sulfates, with a goal of measuring quantitative thermodynamic data on these minerals that would allow us to predict with some certainty their behavior under martian surface conditions. We are also continuing studies on possible martian zeolite and clay minerals, both under simulated martian conditions and also at higher temperatures and partial pressures of water. Accurate data measured under the latter conditions should allow us to extrapolate to martian surface conditions.

Los Alamos scientists David Vaniman, Steve Chipera, Claire Fialips, J. William Carey and William Feldman are co-authors with D. Bish. Their article was published in the October 7 issue of Nature.

Related Web Pages

Mars Bathing in Epsom Salts
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Living on Mars
The Martian Future
Mars Exploration Rover Mission: Home
NASA Mars Exploration Program