HR: 0800h
AN: P21A-0205    [Abstracts]
TI: Hydrated Salts: Dehydration, Dissolution, and Incongruent Melting In Terrestrial Evaporites and at Meridiani Planum, Mars
AU: * Kargel, J S
EM: jkargel@usgs.gov
AF: U.S. Geol. Surv., 3226 N. Fourth St., Flagstaff, AZ 86004 United States
AU: Dougherty, A
EM: doughera@lafayette.edu
AF: Lafayette College, 800 West High St., Easton, PA 18042 United States
AU: Feldman, W
EM: wfeldman@lanl.gov
AF: Los Alamos National Lab., M/S D466/Space&Atmospheric Sci., Los Alamos, NM 87545 United States
AU: Hogenboom, D
EM: hogenbod@lafayette.edu
AF: Lafayette College, 800 West High St., Easton, PA 18042 United States
AU: Marion, G
EM: gmarion@dri.edu
AF: Desert Research Inst., 2215 Raggio Parkway, Reno, NV 89512 United States
AU: McCarthy, C
EM: christine_mccarthy@brown.edu
AF: Brown Univ., 324 Brook St., Providence, RI 02912 United States
AU: Prieto-Ballesteros, O
EM: prietobo@inta.es
AF: Centro de Astrobiologia, INTA-CSIC, Ctra. Ajalvir km. 4, TorrejĒn de ArdĒz, Madrid, 28850 Spain
AB: The Opportunity rover and orbital observations of Meridiani Planum (Mars) have revealed much about the region's stratigraphy, chemistry, sedimentology, and mineralogy of what appears to be a layered lacustrine sedimentary sequence, including chemical sediments deposited by evaporative or freezing processes in a large, saline lake or sea. The roles of evaporation versus freezing are not clearly elucidated in the data, but both freezing and evaporation are likely on Mars for any paleoclimate scenario that minimizes excursions from current climate. The rock sequences reveal many interesting features reported in press accounts, NASA press releases, and in conference presentations by the MER science teams. The topic dealt with here concerns indirect sedimentologic indications of phase changes that seem to have generated negative volume changes due to dehydration, dissolution, and/or annealing. These indicators include microkarstic and polygonal structures in the laminated chemical sediments. These processes have operated on a small scale at Meridiani Planum, and serve as possible analogs for processes operative on mega scales elsewhere on Mars. Comparable processes are common in terrestrial evaporite basins and in sequences of evaporitic rocks. Fluctuations of mineral water content drive large changes in volume and pore fluid pressure, and these exert stresses that can drive extensional fracturing and faulting, folding, thrusting, and diapirism. These processes may be even more important on Mars than on Earth, because on Mars solid salts may be more abundant, more widespread, and subject to larger ranges of hydration states; the effects of these processes may be better preserved for lack of erasure by fluvial erosional processes and other degradational processes on Earth's more active surface. Specific processes and reactions proposed here can account for the sedimentologic structures observed at Meridiani Planum based on aqueous chemical phase equilibria and phase-volume data applied to the mineralogies and chemistries observed or inferred from spacecraft data. Dehydration and/or open-system incongruent dissolution of Mg-Fe-Ca-sulfate hydrates can explain both the microkarstic and decimeter-scale polygonal structures observed by the Opportunity rover. Close analogs of these inferred processes and observed features are common in terrestrial evaporite sequences. Considering scenarios for minimized excursions from current climate, we attribute the structures either to dehydration or dissolution etching by cryogenic acid brines-- or both operating in tandem or in sequence. These processes operating at low rates may remain active even as Opportunity observes the layered/laminated rock sequence. Inclusion on future spacecraft of simple soil pH measurements would do much to resolve questions of possible present-day activity of acidic brine films. Because many salt dehydration steps occur at temperatures far below the melting point of ice, future differential scanning calorimetry/thermal analysis must consider very small increments of heating in the 200-400 K temperature range if we are to understand adequately the composition and hydration states of Martian salts.
DE: 5415 Erosion and weathering
DE: 5470 Surface materials and properties
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting