Comment on “Mars as the parent body of the CI carbonaceous chondrites” by J.E. Brandenburg

Abstract

Listen

Translate article

Geological and chemical data refute a martian origin for the CI carbonaceous chondrites.I. IntroductionBrandenburg [1996] hypothesizes that the CI carbonaceouschondrites originated as water-deposited sediments on Mars,and seeks to show that CIs and recognized martian materials areso similar that a common origin is likely. However, that paperdoes not reconcile its hypothesis with available chemical andgeological data on the CIs, on Mars, and on martian materials.Most available data are inconsistent with a martian origin forthe CIs.In the scientific method, knowledge progresses by theproposal and testing of new hypotheses. A hypothesis must berejected if it and its predictions are inconsistent with theavailable data. Modified hypotheses can be proposed to explaininconsistent data, but such modifications can grow uponthemselves, yielding Gordian tangles of exceptions and specialcases. Occam's Razor cuts through such tangles - one shouldchoose the simplest hypothesis that fits the data. The simplesthypothesis consistent with all available data is that the CIcarbonaceous chondrites did not originate on Mars.Here, l will first consider Brandenburg's [1996] proposal thatthe Cls formed as water-deposited sediments on Mars, and thatthese sediments had limited chemical interactions with theirmartian environment. Finally, I will address oxygen isotoperatios, the strongest link between the CIs and the martianmeteorites.II. 'Martian Geology' of CIsBrandenburg [1996] proposes that the CI ehondrites formedas water-deposited sediments early in Mars' history, with thesediment derived from asteroidal material accreting onto Mars.This hypothesis is inconsistent with the petrography of the CIsthemselves, and inconsistent with the geology and geochemistryof early Mars.Above all, the CI meteorites are not sedimentary rocksdeposited from water. The Cls contain no petrographic featuresthat would be expected in a water.laid sediment, e.g., layeringin composition or grainsize, cross-bedding, or evidence of soft-sediment deformation [Dodd, 1981]. The Cls consist ofchemically distinct domains rich in clay and serpentine withsmall proportions of magnetite and anhydrous silicate mineralssuch as olivine and pyroxene [Dodd, 1981]. This structuresuggests that the Cls are breccias, and the ranges in O isotopiccompositions of their constituents [Rowe et al., 1994] suggestCopyright 1996 by the American Geophysical Union.Paper number 96GL03075.0094-8534/96/96GL-03075505.00that the Cls are polymict. In this light, it seems probable that theCIs are impact breccias of chondritic asteroidal materials,extensively altered in place.Further, it seems highly unlikely that a sediment formed ollMars during its early history could contain only extra-martianmaterial. The CI meteorites formed at ~ 4.5 Ga [MacDougall etaL, 1984; Endress et al., 1996], a time when Mars' surface wasbeing struck repeatedly by large meteorites and asteroids. Impactcrater scars from this time are abundant on the martian southernhighlands, and impacts would have distributed ejecta widelyacross Mars. It seems unlikely that a water-filled basin on Mars,putative site of CI sedimentation, could have avoided collectingsome of this impact ejecta. All known martian materials arederived from basaltic magmas, and the southern highlands crust(of which we have one meteoritic sample) also appears to bebasaltic [Singer and McSween, 1993; McSween, 1994; Treiman,1995]. These martian basaltic materials are quite distinct fromCls [Dreibus and Wiinke, 1985] and ought to be readilyrecognized within Cls. Yet Cls contain no known physical orchemical traces of this martian material (anhydrous silicates inCls are not martian, as their solar flare tracks suggest extendedinterplanetary exposure [Dodd, 1981 ]).III. Chemical ClosureBrandenburg [1996] hypothesizes further that some elementabundances and isotopic ratios in the 'CI sediments' are likethose of Mars, while others retain primordial compositions: CImaterial would be 'leftovers' of Mars accretion with solarcomposition and isotopics. It is not clear fromBrandenburg [1996] whether the C1 material had 'Martianisotopics' (presumably meaning of C, H, N, and O) after orbefore it landed on Mars, but neither option is reasonable.'Martian isotopics' after MarsIn the first option, with the CIs' Martian isotopics arisingthrough chemical interactions on Mars, the CIs must somehowbe prevented from exchanging elements other than C, H, O, andN with their martian environment. Alteration of hypothetical CIprecursors on Mars could have had little effect on mostelemental abundances - such 'isochemical' alteration is, in fact,the standard model for fbrmation of CIs [Dodd, 1981 ]. However,noble gases are readily exchanged during aqueous alteration[e.g., Drake et al., 1994], and one should expect that noble gasabundances and isotopic composition in 'martian Cls' wouldresemble those of Mars.In fact, the noble gases in CIs are quite distinct from those inany known martian source, and cannot have exchanged3275