Did Mars' Soil Salt Come From Evaporation or Freezing?

Modeling salt precipitation from brines on Mars: Evaporation versus freezing origin for soil salts

Authors:

Toner et al

Abstract:

Perchlorates, in mixture with sulfates, chlorides, and carbonates, have been found in relatively high concentrations in martian soils. To determine probable soil salt assemblages from aqueous chemical data, equilibrium models have been developed to predict salt precipitation sequences during either freezing or evaporation of brines. However, these models have not been validated for multicomponent systems and some model predictions are clearly in error. In this study, we built a Pitzer model in the Na–K–Ca–Mg–Cl–SO4–ClO4–H2O system at 298.15 K using compilations of solubility data in ternary and quaternary perchlorate systems. The model is a significant improvement over FREZCHEM, particularly for Na–Mg–Cl–ClO4, Ca–Cl–ClO4, and Na–SO4–ClO4 mixtures. We applied our model to the evaporation of a nominal Phoenix Lander Wet Chemistry Laboratory (WCL) solution at 298.15 K and compare our results to FREZCHEM. Both models predict the early precipitation of KClO4, hydromagnesite (3MgCO3·Mg(OH)2·3H2O), gypsum (CaSO4·2H2O), and epsomite (MgSO4·7H2O), followed by dehydration of epsomite and gypsum to kieserite (MgSO4·H2O) and anhydrite (CaSO4) respectively. At low residual water contents, our model predicts the precipitation of halite (NaCl), NaClO4·H2O, and Mg(ClO4)2·6H2O, whereas halite and NaClO4·H2O never precipitate in FREZCHEM. Our model predicts that calcite does not precipitate from evaporating WCL solutions at 298.15 K, which conflicts with other evidence for calcite in Phoenix soils. Previous studies that modeled freezing of WCL solutions found that calcite does form. Furthermore, our model predicts that ∼0.3 wt.% H2O is held in hydrated salts after the WCL solution has completely evaporated at 298.15 K, whereas previous studies have found that ∼1.3 wt.% H2O is held in hydrated salts if WCL solutions freeze. Given minimum water contents in Mars soils of 1.5–2 wt.% H2O measured from orbital spectra and in situ measurements, our modeling results suggest that salts at the Phoenix site were not formed during evaporation near 298.15 K, whereas formation during freezing remains possible.

How Vulnerable is the Carbon dioxide Stored in Soil to Global Warming?

The response of soil microbial communities to changes in temperature increases the potential for more carbon dioxide to be released from the world's soils as global temperatures rise, scientists have revealed.

The potential for global warming to stimulate decomposition rates in soils, and thus release large quantities of carbon dioxide to the atmosphere, has long been considered to be one of the potentially most important positive feedbacks to climate change. However, the results from more recent studies have suggested that responses within microbial communities could greatly reduce, or even eliminate, the potential for soil carbon losses under global warming.

This key idea was tested using soils collected from a thermal gradient from the Arctic to the Amazon rainforest. The results, published in Nature, show that, contrary to expectations, microbial community responses resulted in an overall increase, rather than a decrease, in the effects of temperature on rates of carbon dioxide release from soils.

Dr Kristiina Karhu of the University of Helsinki, and lead author of the paper, said: "Because soils store more than twice as much carbon than the atmosphere, changes in rates of decomposition and carbon dioxide release from soil could be very important. Our findings suggest that warming will increase the activity of soil microbes to a greater extent than was previously expected, which could have implications for future rates of climate change."

Responses were not found to be equal in all soils, and differed between geographical regions and ecosystem types. Managed agricultural soils stood out, being the only soils in which microbial community responses reduced the effects of a temperature change on rates of carbon dioxide release. On the other hand, the greatest stimulation was observed in the soils with the greatest carbon content and from boreal and arctic ecosystems, which are warming most rapidly. This indicates that the observed responses could increase the vulnerability of some of the world's most important soil carbon stocks to climate change.

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Stanford Researchers Create Equations to Describe Weathering's Role in the Carbon Cycle

Favorable conditions for life on Earth are enabled in part by the natural shuttling of carbon dioxide from the planet's atmosphere to its rocky interior and back again. Now Stanford scientists have devised a pair of math equations that better describe how topography, rock compositions and the movement of water through a landscape affects this vital recycling process.

Scientists have long suspected that the so-called the geologic carbon cycle is responsible for Earth's clement and life-friendly conditions because it helps regulate atmospheric concentrations of carbon dioxide, a greenhouse gas that acts to trap the sun's heat. This cycle is also thought to have played an important role in slowly thawing the planet during those rare times in the past when temperatures dipped so low that the globe was plunged into a "snowball-Earth" scenario and glaciers blanketed the equator.

"Our equations suggest that different landscapes have different potentials for regulating the transfer of carbon dioxide," said Kate Maher, an assistant professor of geological and environmental sciences who developed the equations along with her colleague, Environmental Earth System Science Professor Page Chamberlain. The research, which was supported by the National Research Foundation, is described in the March 14 issue of the journal Science.

The geologic carbon cycle begins when volcanoes release carbon dioxide into the atmosphere. Some of the carbon dioxide (CO2) mixes with rainwater and falls back to Earth as carbonic acid. On land, the carbonic acid chemically erodes, or "weathers," silicate rocks exposed at the Earth's surface to produce bicarbonate and release elements such as calcium and magnesium that eventually wash into the ocean.

Over millions of years, these elements are transformed into rocks such as limestone. When plate tectonics push the carbonate-loaded seafloor down into the Earth's mantle, the carbon is released again as CO2, which is vented back into the atmosphere through volcanic eruptions, thereby completing the cycle.

The equations developed by Maher and Chamberlain address the weathering component of the geologic carbon cycle. The amount of weathering that occurs depends on several factors. One is the makeup of the soil: older soils that have already been weathered dissolve more slowly compared to soils made of fresh rock. "As you weather soil and sediment over time, they become less and less chemically reactive," Maher said. "Physical erosion, which is often associated with mountainous regions, replenishes the soil with reactive minerals."

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Perchlorates in Northern Mars may not be Present in Equitorial Latitudes

Aqueous and Non-Aqueous Soil Processes on the Northern Plains of Modern Mars: Insights from the Distribution of Perchlorate Salts at the Phoenix Landing Site and in Earth Analog Environments

Authors:

Cull et al

Abstract:

In 2008, the Phoenix lander returned chemical evidence of perchlorate salts in the soils at its landing site on the northern plains of Mars. Subsequent spectral mapping of the perchlorate using Phoenix's multispectral Surface Stereo Imager (SSI) showed that concentrated patches of the salt exist in the subsurface. Because atmospheric formation of Martian perchlorate should form a highly-concentrated layer of salt on the surface, subsurface concentrated patches have been interpreted as evidence that Phoenix soils experienced minor amounts of aqueous reworking after perchlorate formation. Here, we present results from a wide-scale mapping of the Phoenix landing site using SSI multispectral data. We report that, contrary to preliminary case studies, limited occurrences of rocks and soil clods with perchlorate coatings are also found on the undisturbed surface. The discovery of these patches on undisturbed surfaces points to more complex processes operating on modern-day polar soils, perhaps including a combination of aqueous and mechanical processes, such as cryoturbation. Finally, we combine results from this study with an analysis of perchlorate redistribution mechanisms on Earth to illustrate the mechanisms likely responsible for modern processing of soils on the northern plains of Mars.