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GEOCHEMICAL EVOLUTION of the CRUST of MARS.. James L Workshop on Early Mars (1997) 3069.pdf GEOCHEMICAL EVOLUTION OF THE CRUST OF MARS.. James L. Gooding, SN2/Planetary Missions and Materials Branch, NASA Johnson Space Center, Houston, TX 77058-3696 USA (E-mail: jgood- [email protected]). Introduction. Crust is defined here as the solid Igneous Evolution. Thermal models suggest that outer portion of Mars that includes crystallized Mars experienced substantial internal melting and magmas, produced by igneous differentiation of the differentiation shortly after accretion [2]. It is in- planet, as well as other volcanic deposits and any ferred from compositions of Martian meteorites that clastic or chemical sedimentary rocks formed by internal heating of Mars was enabled by abundances weathering and diagenesis. Implicit in this defini- of natural radionuclides (K, Th, and U) that are tion is the rock rubble known as the regolith, includ- broadly similar to those in the mantle of Earth. Al- ing unlithified evaporites. lowing for uncertainties in the rate of convection in In the context of the search for life, the evolution the early Martian mantle, the resultant igneous crust of the Mars crust is important for the thermal and of Mars is predicted from thermal models to be about chemical environmental controls that it imposed 50-150 km thick. upon the approach of biogenic chemical elements The radioisotope systematics of Martian meteor- toward molecular self-replication. Prematurely ar- ites, all of which are mafic igneous rocks, indicate rested crustal evolution would be bad news for the that the Martian igneous crust is rather highly emergence and evolution of life. On the other hand, evolved in the geochemical sense. Based on the Nd- extensive crustal evolution probably would foster the Sm and Rb-Sr systems, the Martian meteorites repre- mature evolution of biogenic compounds. sent rocks that are more highly evolved than basalts on the Moon. Based on our understanding of Earth, The Big Picture. From photogeologic evidence, measurable radioisotope indicators such as the pa- 87 86 resurfacing of Mars -- including all endogenous and rameters e(Nd) and ( Sr/ Sr)o numerically increase exogenous processes recognized thus far -- occurred as differentiation and igneous crustal evolution pro- most rapidly during the first ~1 Gyr of planetary ceed. Therefore, taken at face value, the evidence history (Fig. 1). The abundance of asteroid impacts, from available samples indicates comparable degrees and the extent of volcanism, during Noachian time of igneous evolution on Mars and Earth (Fig. 2). indicates early crustal evolution characterized by hot, high-energy environments. Thereafter, the rates of Shergottite meteorites Modern Earth Crust all endogenous processes apparently declined until a Series5 resurgence during Hesperian time. It was not until 20 the latter rekindling that water- and ice-related sedimentation appear correlated in the record. 10 0 Resurfacing History of Mars (Tanaka et al., 1988) (Nd) Area -10 (106 km2) 100 -20 80 -30 60 0.69 0.70 0.71 0.72 0.73 TOTAL 87 86 40 Volcanic ( Sr/ Sr)0 Impact 20 Periglacial Figure 2. Sm-Nd and Rb-Sr data that indicate com- Fluvial 0 Eolian parable degrees of radioisotopic evolution for the crusts of Mars [3] and Earth [4]. In the absence of rock cycling through plate tec- tonics, which has dominated the last 0.2-0.6 Gyr of Figure 1. Summary of processes that affected evolu- Earth history but which appears to be absent from tion of the Martian crust through geologic time, Mars, the mechanism for accomplishing radioisotope based on photogeologic interpretations [1]. maturation of Martian lavas remains unidentified. Nonetheless, the isotopic records demonstrate that Workshop on Early Mars (1997) 3069.pdf GEOCHEMICAL EVOLUTION OF THE MARS CRUST: J. L. Gooding igneous heat sources extensively drove the chemical reactions to form carbonate minerals. Adsorbed CO2 refinement of the crust to Earth-like degrees of ma- is effectively invisible by remote sensing, leaving the turity. In fact, the shergottite meteorites yield ra- remote search for carbonate minerals as the main diometric ages, which can be interpreted as lava-flow method for testing the CO2 transmigration hypothe- ages, on the order of < 0.2 Gyr – indicating a con- ses. Among the various spacecraft- or ground-based, tinuing Martian internal heat engine through Late remote-sensing experiments that have been con- Amazonian time. ducted, all but one have failed to find definitive evi- dence for carbonate minerals [7]. The single posi- Sedimentary Evolution. None of the Martian tive identification is interpreted as 1-3% by volume meteorites recognized to date are sedimentary rocks. of unidentified carbonate mineral(s) in Martian at- Therefore, we lack direct evidence, in the form of mospheric dust [8]. The Viking landers were ill- samples, to enlighten us with regard to the sedimen- equipped to identify minerals and, therefore, shed no tary geology of Mars. light on the possible occurrence of carbonates in From photogeologic evidence, water-related Mars soils [9]. sedimentation peaked twice during Martian history – Even though they are igneous rocks, the Martian neither time in clear correlation with volcanism (Fig. meteorites contain trace amounts of carbonate min- 1). Therefore, it is not obvious whether the episodes erals [10] that prove the viability, if not the actual of fluvial dominance involved hot or cold water. capacity, of chemical reactions to fix CO2 into the Furthermore, photogeologic evidence for sedimen- crust. At least some of the carbonates found in the tary basins, which might have contained standing Martian meteorites can be understood as products of bodies of water for prolonged periods of time, is ground-water chemistry. What remains to be de- limited. termined is whether there exist massive, bedded car- Hypotheses for the transmigration of an early bonates that would have required for their formation dense atmosphere into the crust, possibly mediated substantial and long-lived bodies of surface water. by surface water, are intuitively appealing but not yet supportable with observational evidence. The first- Conclusions. The igneous portion of the Mars order problem is to account for a putative inventory crust appears to be geochemically highly evolved. of hundreds to thousands of millibars of CO2 that Unfortunately, the maturity of the sedimentary evo- presumably was outgassed during igneous differen- lution of the crust remains largely unknown. Indi- tiation (Fig. 3; [5]). The same volcanic outgassing is cations from the igneous record suggest that geo- predicted to have released water equivalent to a chemical evolution included biologically favorable globally condensed layer of tens of meters [6]. energy sources during at least the first ~1 Gyr of history. But given the current lack of information, almost nothing can be said about whether sustained space bodies of liquid water occurred at the surface or left escape (< 10 mb / 4.5 Gyr) behind rock records of chemical sedimentation. Atmosphere th (~ 7 mb) References. [1] Tanaka K. et al. (1988) Proc. 18 LPSC, p. 665. [2] Schubert G. et al. (1992) In degassing Liquid Water (140-3000 mb) MARS (H. H. Kieffer et al., eds.), Univ. Arizona adsorption & condensation & weathering & Press, Tucson, p. 147. [3] Jones J. H. (1989) Proc. desorption sublimation diagenesis 19th LPSC, p. 465. [4] McCulloch M. T. and Ben- Planetary Adsorbed in Permanent Carbonate nett V. C. (1994) Geochim. Cosmochim. Acta, 58, p. Interior Regolith CO2 Cap Minerals & 4717. [5] Kahn R. (1985) Icarus, 62, p. 175. [6] (< 280 mb) (< 1 mb) Rocks Greeley R. (1987) Science, 236, p. 1653. [7] Soder- (? mb) blom L. A. (1992) ) In MARS (H. H. Kieffer et al., eds.), Univ. Arizona Press, Tucson, p. 557. [8] Pol- Figure 3. Box model for the transmigration of CO2 lack J. B. et al. (1990) J. Geophys. Res., 95, p. into the Mars crust, as modified after [5]. 14595. [9] Arvidson R. E. et al. (1989) Rev. Geo- phys., 27, p. 39. [10] Gooding J. L. (1992) Icarus, Because only a tiny fraction of the CO2 inventory 99, p. 28. can be attributed to icy condensate, model mecha- nisms for sequestering the CO2 have included ad- sorption on a thick, fine-grained regolith or chemical.
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