sign in sign up
<<
Home , Olivine

Lunar and Planetary Science XXXVII (2006) 1645.pdf

DOES OLIVINE INDICATES DRY CONDITIONS ON ? Alberto G. Fairén. Centro de Biología Molecular, Universidad Autónoma de Madrid, 28049-Cantoblanco, Madrid, Spain. ([email protected]).

The existence of liquid , the soil still contain 19% to has been hotly debated, especially following the 26% of the in olivine, but local rock outcrops discovery of the olivine [(Mg,Fe)2SiO4] in have a maximum of only 1% olivine [8]. This several regions of the planet by the latest Mars means that local soils result from a combination of missions. Olivine, a greenish / iron secondary originating from the intense orthosilicate common in rocks, has been chemical alteration of the ancient bedrock, mixed claimed to indicate spatially limited chemical with olivine, silicates, , hematite and [1-3] because it readily evolves to magnetite-rich young dust which sources from , goethite, serpentine, chlorite, smectite, distant soils by physical processes, a manifestation maghemite, and hematite in the presence of water of billions of years of wind redistribution. As [1]. Here is argued that olivine distribution may in such, the presence of olivine in soils indicates that fact indicate the presence of localized and the atmospheric dust has been largely originated significant amounts of liquid water on the surface by mechanical weathering of basaltic rocks in of Mars in different times over the history of the places where chemical alteration by liquid water planet. has not been playing a dominant role recently On a broad scale, olivine is widely (probably the greatest part of the surface), but led distributed in the Martian southern highlands, and no information at all about places where water did the Martian regolith as a global unit has up to 26% play a significative role during the Amazonian. of the iron in olivine [4], but is remarkably absent Along with the , the chemistry in the northern lowlands, in the greatest southern also shows close relations between weathered and impact craters, in the Tharsis and Elysium unweathered materials in the MER landing sites. volcanic domes, and in Arabia Terra [3]. These In , the positive correlation between ferric are exactly the places where significant amounts oxides and sulphur, the negative correlation of liquid water are hypothesized to have been between ferric oxides and iron in olivine, and the sourced and/or ponded in different times: water is negative correlation between iron in olivine and expected to source from the highest tops of the sulphur [4] indicates that the ferrous iron Tharsis and Elysium volcanoes and from a giant component decreases as the amount of ferric iron aquifer in Arabia [5], and to pond in the major and sulphur increases: the source of ferric iron is plains of Hellas, Argyre, Isidis and the northern best explained by the oxidation of ferrous iron in lowlands. olivine by liquid water; and the source of Ca- and On a local scale, some places are Mg- sulfates, iron oxides and oxyhydroxides, by indicative of the actual reliability of olivine as a the reaction of soluble from the host rock qualifier for water-related chemical weathering on (e.g., olivine) with S derived from SO2-rich water. Mars. , a joint of grabens in the west In Meridiani, waters rich in sulfuric acid flank of Isidis Planitia and northeast of the Syrtis percolated through the basaltic sediment, Major volcanic shield, is the place where olivine- promoting the chemical weathering of olivine- bearing material was first described on Mars, and bearing and forming fine grained the largest spatially contiguous exposure of the siliciclastics and evaporites (jarosite, melanterite, mineral [1,2]. When analyzing specific small- other Ca-, Mg-, and Fe-sulfates, and iron oxides) scale geomorphologic features of the region, Nili that ultimately derived in the exposed Fossae Hesperian-Amazonian (≤3.5 Gy) [1] in craters and fissures through an extended olivine show crosscutting relations diagenetic history [9]. with previous (>3.5 Gy) [6] valley The evolutive sequence which network systems forming distributary fans and immediately emerges includes an ancient extended crater lakes. In addition, phyllosillicates in the period of time of pervasive olivine-rich basaltic form of (Fe/Mg)-rich clays, typical aqueous , accompanied with flowing alteration products of igneous minerals, have been and ponding of water across the surface in Nili detected in Noachian terrains northern Syrtis Fossae, Gusev and Meridiani (and probably many Major [7], in close relation with olivine outcrops. other sites during the earliest times of Martian All this implies an extended period of time of history), the subsequent desertification of these flowing and ponding water across the surface in locations, the formation of modern olivine-rich the same location where olivine is described (see via more recent volcanism, and the wide figure). eolian distribution of olivine-rich dust over the Also in the two landing sites of the Mars Martian surface. Variable amounts of water may Exploration Rovers (MER), Gusev crater and still have been flowing recently in different Lunar and Planetary Science XXXVII (2006) 1645.pdf

locations, altering previous olivine [10]. In conclusion, olivine-bearing both at local (e.g., MER sites) and at global (e.g., materials can not longer qualify as reliable the lowlands) scales. The chemical alteration of indicators for dry surface conditions on Mars. olivine is also supported by evidence in Martian

References:

[1] T. M. Hoefen et al. Science 302, 627-630 [6] C. I. Fassett & J. W. Head III. Geophys. Res. (2003). Lett., doi:10.1029/2005GL023456 (2005). [2] V. E. Hamilton & P. R. Christensen. [7] F. Poulet et al. Nature 438, 623-627 (2005). Geology 33, 433-436 (2005). [8] G. Klingelhöfer et al. Science 306, 1740- [3] H. Y. McSween et al. J. Geophys. Res., in 1745 (2004). press. [9] S. W. Squyres & A. H. Knoll. Planet. [4] A. S. Yen et al. Nature 436, 49-54 (2005). Sci. Lett. 240, 1-10 (2005). [5] J. M. Dohm et al. J. Geophys. Res., [10] S. R. Sutton et al. LPSC XXXIII, #1278 submitted. (2002).

Figure:

Geomorphological and mineralogical analysis of the Nili Fossae/West Isidis Planitia region. Left: Coloured MOLA map covering the area 72-86º E and 10-30º N, composed from JPL/NASA data. The location of major patches of olivine-rich materials [1] are outlined in violet. Box 1 marks the situation of the crater in which fluvial sedimentary deposits, fans and deltas have been described [6]. Box 2 indicates the location of the Mars Express mineralogical analyses [7]. Box 1: THEMIS infrared daytime mosaic of the crater with false color MOLA topography. Inset indicates the location of the composite MOC narrow angle image below, where pervasive layering (A) and crosscuting ridges (B) are apparent and strongly suggest episodic water activity. Box 2: HRSC/Mars Express image with olivine-rich mineral mapped by OMEGA in green, hydrated minerals in purple, and clays in red.