HR: 0800h
AN: P21A-0207    [Abstracts]
TI: Salt Attack on Rocks and Expansion of Soils on Mars
AU: * Vaniman, D T
EM: vaniman@lanl.gov
AF: Los Alamos National Laboratory, MS D462, Los Alamos, NM 87545 United States
AU: Bish, D L
EM: bish@indiana.edu
AF: Indiana University, Dept. of Geological Sciences, 1001 E. 10th St., Bloomington, IN 47405 United States
AU: Chipera, S J
EM: chipera@lanl.gov
AF: Los Alamos National Laboratory, MS D462, Los Alamos, NM 87545 United States
AU: Carey, J W
EM: bcarey@lanl.gov
AF: Los Alamos National Laboratory, MS D462, Los Alamos, NM 87545 United States
AB: Salt-rich sediments observed by the MER rover Opportunity at Meridiani Planum show that brines have been present on Mars in the past, but a role for groundwater in widespread rock weathering and soil formation is uncertain. Experiments by several groups suggest instead the action of acid fog over long time spans, with episodic input of volcanic gases, as a more significant agent of Mars weathering. Salt minerals formed in these acid weathering experiments consistently include gypsum and alunogen, with epsomite or hexahydrite forming where olivine provides a source of Mg. Analogous to the martian acid fog scenario are terrestrial acid rain or acid fog attacks on building and monument stone by chemical action and mechanical wedging through growth of gypsum, anhydrite, epsomite, hexahydrite, kieserite, and other sulfate minerals. Physical effects can be aggressive, operating by both primary salt growth and hydration of anhydrous or less-hydrous primary salts. In contrast, soils evolve to states where chemical attack is lessened and salt mineral growth leads to expansion with cementation; in this situation the process becomes constructive rather than destructive. We have made synthetic salt-cemented soils (duricrusts) from clays, zeolites, palagonites and other media mixed with ultrapure Mg-sulfate solutions. Although near-neutral in pH, these solutions still exchange or leach Ca from the solids to form cements containing gypsum as well as hexahydrite. At low total P (1 torr) and low RH ($<$1%) hexahydrite becomes amorphous but gypsum does not. If allowed to rehydrate from vapor at higher RH, the Mg-sulfate component of the duricrust expands by formation of a complex mixture of Mg-sulfate phases with various hydration states. The expanded form is retained even if the duricrust is again dehydrated, suggesting that soil porosity thus formed is difficult to destroy. These processes can be considered in the context of Viking, Pathfinder, and MER evidence for differing salt components in the weathered surfaces of rocks versus duricrust-like materials in soils. The divergent chemical trends indicate that soil formation on Mars is not merely a result of enhanced weathering of locally comminuted rock but requires an eolian component. The resulting soils thus appear to be a three-component mixture of local detritus, a regional or global eolian component, and acid fog additions. In the absence of rainfall or groundwater action, expanded and salt-cemented soil horizons are likely to persist as a regolith component in soil-atmosphere interactions over long time spans.
DE: 6215 Extraterrestrial materials
DE: 6225 Mars
DE: 3672 Planetary mineralogy and petrology (5410)
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting