Meteoritics & Planetary Science 38, Nr 2, 225–241 (2003) Abstract available online at http://meteoritics.org Precursors of Mars: Constraints from nitrogen and oxygen isotopic compositions of martian meteorites Ratan K. MOHAPATRA1, 2 and S. V. S. MURTY1* 1Physical Research Laboratory, Ahmedabad-380009, India 2Max-Planck-Institut für Chemie, Postfach 3060, D55020 Mainz, Germany *Corresponding author. E-mail: [email protected] (Received 11 January 2002; revision accepted 10 January 2003) Abstract–We present an approach to assess the nature of materials involved in the accretion of Mars by the planet’s nitrogen (δ15N) and oxygen (∆17O) isotopic compositions as derived from data on martian meteorites. δ15N for Mars has been derived from nitrogen and xenon systematics, while ∆17O has been taken from the literature data. These signatures indicate that Mars has most probably accreted from enstatite and ordinary chondritic materials in a ratio of 74:26 and may not have a significant contribution from the carbonaceous (CI, CM, or CV) chondrites. This is consistent with the chromium isotopic (ε53Cr) signatures of martian meteorites and the bulk planet Fe/Si ratio for Mars as suggested by the moment of inertia factor (I/MR2) obtained from the Mars Pathfinder data. Further, a simple homogeneous accretion from the above two types of materials is found to be consistent with the planet’s moment of inertia factor and the bulk composition of the mantle. But, it requires a core with 6.7 wt% Si, which is consistent with the new results from the high pressure and temperature melting experiments and chemical data on the opaque minerals in enstatite chondrites. INTRODUCTION The chemical and isotopic compositions of the martian meteorites, obtained over the years, have been helpful in Terrestrial planets are believed (Taylor 1994) to have deriving a reasonable working model of the martian mantle formed by the accretion of early solar system objects that are (Wänke and Dreibus 1988). By using this and various represented by primitive meteorites (such as chondrites). The geophysical constraints, a number of models have been bulk chemical, isotopic, and geophysical properties of a proposed in recent years for the accretion of Mars. Thus, planet, to a large extent, depend on the nature of materials Wänke and Dreibus (1988, WD henceforth), have proposed in involved in its accretion. Thus, attempts have been made by a number of papers (Dreibus and Wänke 1984, 1985, and various researchers to evaluate the nature of precursor 1987; Wänke and Dreibus 1994) that Mars probably accreted materials for the accretion of Mars based on the above from two types (A and B) of materials in a ratio of 60:40. parameters, as reviewed in a number of recent articles These two precursors were presumed to be similar to CI (Longhi et al. 1992; Lodders and Fegley 1997; Sanloup, chondrites in their major (nonvolatile) element compositions. Jambon, and Gillet 1999; Lodders 2000). Two important Component ‘A’ was considered to be reduced and volatile- developments have affected the course of such studies. free, while component ‘B’ was considered to be oxidized and Although the geophysical parameters (such as mass, radius, volatile-rich. Although a number of drawbacks of the WD and moment of inertia factor) of Mars were the basis for most model have been pointed out (e.g., Bertka and Fei 1998a; of the earlier models, the discovery of martian meteorites (or Sanloup, Jambon, and Gillet 1999), this model of the martian SNC meteorites, McSween 1994) has provided a new set of mantle is still favored by many workers. constraints that helped to refine our understanding of planet A somewhat different approach using martian meteorites Mars. Similarly, the uncertainties associated with the moment is based on their oxygen isotopic compositions. Oxygen of inertia factor used in the earlier models (0.345 to 0.377, see forms the bulk of Mars (~40% by weight, Wänke and Dreibus Longhi et al. 1992 for a review) could be minimized (e.g., 1988). It serves as a means for classifying different types of 0.366 ± 0.002, Folkner et al. 1997) by data from the recent objects in the solar system (Clayton, Onuma, and Mayeda Mars Pathfinder mission. 1976), and thus is a potential tracer in the search for the 225 © Meteoritical Society, 2003. Printed in USA. 226 R. K. Mohapatra and S. V. S. Murty planet’s precursors. Oxygen isotopic data for martian chondrites. Using the mean oxygen isotopic compositions meteorites (Clayton and Mayeda 1983, 1996; Franchi et al. (δ17O = 2.51 ± 0.21 and δ18O = 4.31 ± 0.31) of SNC 1999), on a three-isotope (δ17O versus δ18O) plot, define a meteorites, Lodders and Fegley (1997, henceforth LF) have line parallel to the Terrestrial Fractionation Line (TFL), which proposed a mixture of three precursors comprising H, CV, and in turn, is a trend defined by high precision measurements on CI chondrites in a ratio of 85:11:4 for Mars. On the other a wide range of samples from Earth (see Clayton 1993 for a hand, a similar approach by Sanloup, Jambon, and Gillet review). The martian (meteorite) fractionation line, as it is (1999), based on the mean δ18O of 4.4 ± 0.3, has proposed a interpreted, is defined by an offset (∆17O = δ17O − 0.52 × mixture (45:55) of EH and H chondrites for Mars. An δ18O) of 0.321 ± 0.013 (Franchi et al. 1999) from the TFL, interesting point arises here: although these three models are and points to an oxygen reservoir different from those in other apparently based on a similar database of martian meteorites meteorite classes (Fig. 1). But the oxygen isotopic signature (Clayton and Mayeda 1983, 1996) and on similar (within of Mars derived from such data has been shown to be uncertainties) ∆17O and mean oxygen isotopic ratios for Mars, consistent with mixtures of various types of chondritic their results on the nature of precursors for Mars are different. signatures. Delaney (1994) has invoked a precursor for Mars Although each of them requires a variable contribution from that is a mixture of 90% H and 10% bulk Murchison (CM) H chondrites, the derivation of the other component(s) is Fig. 1. Oxygen three-isotope plot showing the fields of carbonaceous (CI and CV; Clayton and Mayeda 1999), enstatite (Newton, Franchi, and Pillinger 2000), and ordinary (Clayton et al. 1991) chondrites. Also shown are the Terrestrial Fractionation Line (TFL, e.g., Clayton and Mayeda 1996) and the data for Mars (Clayton and Mayeda 1983, 1996; Franchi et al. 1999). The field of the CM chondrites, which exhibits a large scatter in the δ17O and δ18O but is consistent with a ∆17O of −4.28 (Clayton and Mayeda 1999), has not been included to avoid clutter. Precursors of Mars 227 somewhat ambiguous. The inconsistencies between the A closer look at Fig. 1 suggests that it is very difficult to models of Delaney (1994) and Sanloup, Jambon, and Gillet unambiguously assess the nature of precursor materials for (1999) can be explained by the fact that the oxygen isotopic Mars based only on the oxygen isotopic signatures of martian signatures of Mars (∆17O, δ17O, and δ18O) do not have a meteorites. Clearly, a number of mixing relations involving unique solution in terms of possible two-component mixing two or more chondritic end-members can be invoked, as can relations (as can be realized in Fig. 1). However, it is not clear be realized from the above short review of the literature. why contributions from CM and EH chondrites are essential Therefore, the results suggested by oxygen isotopic modeling (respectively) in the models of Delaney (1994) and Sanloup, have further been evaluated for each of the models by Jambon, and Gillet (1999), and why these have been including other bulk features of Mars (see Delaney 1994; discarded in the Lodders and Fegley (1997) model. It should Lodders and Fegley 1997; Sanloup, Jambon, and Gillet 1999). be noted here that the solar type xenon isotopic signatures of As shown in Table 1, the density and moment of inertia of the martian mantle, as inferred from the martian meteorite Mars, which are based on geophysical observations and are Chassigny (e.g., Ott 1988; Mathew and Marti 2001), are not perhaps more rigid constraints than martian meteorites, also consistent with the omission of a contribution from enstatite do not help us in minimizing the ambiguity presented by the chondrites (which have solar-like noble gas isotopic earlier models. Therefore, the nature of precursor materials compositions, Crabb and Anders 1981) to Mars. Further, for Mars is far from being resolved. Clearly, other sources of xenon in the carbonaceous chondrites (which commonly have constraints or information should be involved in this study. been invoked as a dominant precursor for Mars) is clearly In this paper, we present one such possible approach different from solar in its isotopic compositions (e.g., Ozima using the isotopic composition of nitrogen (δ15N) in Mars, and Podosek 1983). Similarly, the recent, more precise laser which is based on a preliminary study presented as an abstract fluorination oxygen isotopic data on martian meteorites earlier (Mohapatra and Murty 2001). Nitrogen (like oxygen) (Franchi et al. 1999) and enstatite chondrites (Newton, has also been recognized as a tracer for distinguishing the Franchi, and Pillinger 2000) also appear inconsistent with the different types of meteorites in the solar system (e.g., omission of an enstatite chondritic component in Mars. This Kerridge 1993). Although it occurs in trace quantities, the can be easily realized from Fig. 1, where about 30% of the isotopic composition of nitrogen in terrestrial planets has martian meteorite data actually plot inside the compositional been shown to be a useful parameter that can be exploited in field of enstatite chondrites.