Acidic volatiles and the Mars soil

Abstract

Abstract. Large portions of Mars' surface are covered with deposits of fine, homogeneous, weathered dusty-soil material. Nanophase iron oxides, silicate mineralolds, and salts prevail in the soil. The mode of formation of this somewhat peculiar type of soil is still far from being clear. One scenario suggests that weathering took place during early epochs when Mars may have been "warm and wet." The properties of the soil are not easily reconciled wi.'th this scenario. We propose another possible scenario that attributes, in part, the peculiar nature of the Martian dust and soft to a relatively "young" weathering product formed during the last few hundreds of millions of years in a process that involves acidic volatiles. We tested this hypothesis in an experimental study of the first step of acidolytic weathering of a partly palagonitized VOlcanic tephra of hawaiitic lava origin, using sulfi•c, hydrochloric and nitric acids and their mixtures. The tephra effectively "neutralize" the added acidity. The protonic acidity added to the tephra attacks the primary-minerals, releasing Fe, A1, and Mg, which control the pH, acting as Lewis-acid species of varying acid strengths. The full mount of acidity added to the tephra is stored in it, but only a very small fraction is preserved as the original protonic acidity. The majority of the added sulfate and chloride were present as salts and easily solubilized minerals. Well-crystallized sulfate salt minerals of aluminum and calcium were detected by powder X ray diffractomen, whereas secondary magnesium and iron minerals were not detected, due probably to lack of crystallinity. The presence of gyps m (CaSO4'2H20) and alunogen (A12(SO4)3'17H20) is probably responsible for the observed increased hygroscopicity of the acidified tephra and their tendency. to form hardened crusts. We suggest that if this mechanism is of importance on Mars, then the chemically weathered component of the Martian soil consists of a salt-rich mineral mixture containing the salts of the anionicligands SO4 and C1 resulting from volafiles emitted from volcanoes during more recent eruptions (up to 109 years B.P.). The lack of liquid water on Mars surface during that time slowed or halted mineralogical evolution into highly crystallized minerals having large mineral grains. The chemically weathered components are mixed wi.th _the products of physical weathering. The recently formed soil may cover and coat more evolved, hydrothermally modified, mineral deposits formed in earlier epochs Of Mars.

Stability of hydroxylated minerals on Mars: A study on the effects of exposure to ultraviolet radiation

“…In this paper we explore and reject the possibility that ultraviolet radiation incident upon the Martian surface can induce the dehydroxylation of FeOOH, thus accounting for its apparent absence now. Alternative, nonaqueous means of producing iron oxides and other nanocrystalline components in the Martian soil have been suggested elsewhere [ Yen and Murray, 1998;Banin et al, 1997]. Together these ideas support the possibility that Mars' surface was never warm and wet for a long enough period of time for Earth-like weathering to have occurred.…”

Mixed hydrothermal fluids and the origin of the Martian soil

“…The dust suspended in the fireball above the crater could experience penecontemporaneous alteration to form fine-grained clays. Such processes have been called upon by Blaney [1998] Other explanations for the formation of the basaltic rock component of the Martian soil include ambient alteration of surficial deposits, such as glass and shocked minerals, an acidfog mechanism involving acidic brine films on surface materials [Banin et al, 1997], or impact dust production [Blaney, 1998]. These mechanisms have the benefit of producing amorphous or microcrystalline material that is consistent with the spectroscopic properties of the soil.…”

Scattering matrices and expansion coefficients of martian analogue palagonite particles

“…The sample in this study is sample 91-16 that is described in detail by Banin et al (1997). Note that there is another Martian analogue palagonite sample described in the literature, namely sample 91-1 (see Roush and Bell, 1995 Before using sample 91-16 in our light scattering experiment, we removed the millimeter-sized particles by using a sieve with a 200-µm grid width, to avoid clogging the aerosol generator.…”

“…In this article, we present measurements of ratios of elements of the scattering matrix of irregularly shaped, randomly oriented Martian analogue palagonite particles, described by Banin et al (1997), as functions of the scattering angle. The material palagonite is believed to be a reasonable, but not perfect, analogue for the Martian surface and atmospheric dust particles.…”