Bulk composition and early differentiation of Mars

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111,, doi: /2005je002645, 2006 [printed 112(E3), 2007] Bulk composition and early differentiation of Mars G. Jeffrey Taylor, 1 W. Boynton, 2 J. Brückner, 3 H. Wänke, 3 G. Dreibus, 3 K. Kerry, 2 J. Keller, 2 R. Reedy, 4 L. Evans, 5 R. Starr, 6 S. Squyres, 7 S. Karunatillake, 7 O. Gasnault, 8 S. Maurice, 8 C. d Uston, 8 P. Englert, 1 J. Dohm, 2,9 V. Baker, 2,9 D. Hamara, 2 D. Janes, 2 A. Sprague, 2 K. Kim, 2 and D. Drake 10 Received 23 November 2005; revised 12 April 2006; accepted 20 April 2006; published 19 December [1] We report the concentrations of K, Th, and Fe on the Martian surface, as determined by the gamma ray spectrometer onboard the 2001 Mars Odyssey spacecraft. K and Th are not uniformly distributed on Mars. K ranges from 2000 to 6000 ppm; Th ranges from 0.2 to 1 ppm. The K/Th ratio varies from 3000 to 9000, but over 95% of the surface has K/Th between 4000 and Concentrations of K and Th are generally higher than those in basaltic Martian meteorites (K = ppm; Th = ppm), indicating that Martian meteorites are not representative of the bulk crust. The average K/Th in the crust is 5300, consistent with the Wänke-Dreibus model composition for bulk silicate Mars. Fe concentrations support the idea that bulk Mars is enriched in FeO compared to Earth. The differences in K/Th and FeO between Earth and Mars are consistent with the planets accreting from narrow feeding zones. The concentration of Th on Mars does not vary as much as it does on the Moon (where it ranges from 0.1 to 12 ppm), suggesting that the primary differentiation of Mars differed from that of the Moon. If the average Th concentration (0.6 ppm) of the surface is equal to the average of the entire crust, the crust cannot be thicker than about 118 km. If the crust is about 57 km thick, as suggested by geophysical studies, then about half the Th is concentrated in the crust. Citation: Taylor, G. J., et al. (2006), Bulk composition and early differentiation of Mars, J. Geophys. Res., 111,, doi: / 2005JE [printed 112(E3), 2007] 1. Introduction 1 Hawaii Institute of Geophysics and Planetology, Honolulu, Hawaii, USA. 2 Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA. 3 Max-Planck-Institüt für Chemie, Mainz, Germany. 4 Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico, USA. 5 Computer Sciences Corporation, Lanham, Maryland, USA. 6 Department of Physics, Catholic University of America, Washington, DC, USA. 7 Center for Radiophysics and Space Research, Cornell University, Ithaca, New York, USA. 8 Centre d Etude Spatiale des Rayonnements, Centre National de la Recherche Scientifique/Université Paul Sabatier, Toulouse, France. 9 Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona, USA. 10 TechSource, Santa Fe, New Mexico, USA. Copyright 2006 by the American Geophysical Union /06/2005JE [2] The Mars Odyssey gamma ray spectrometer (GRS) provides the first direct determination of elemental concentrations of the entire Martian surface [Boynton et al., 2004], including materials at a depth of about one third meter from the surface. Although the spatial resolution is on the order of 500 km, the data allow us to address significant global problems concerning the geochemical and geological evolution of Mars. In this paper we focus on using the concentrations of K, Th, and Fe to test models for the bulk composition of Mars, which has implications for planetary accretion, and to investigate the formation and subsequent differentiation of the planet. 2. Methods [3] The Odyssey GRS and data reduction methods are described by W. V. Boynton et al. (Concentration of H, Si, Cl, K, Fe, and Th in the low and mid latitude regions of Mars, submitted to Journal of Geophysical Research, 2006, hereinafter referred to as Boynton et al., submitted manuscript, 2006). In this paper we present three types of data collected through March First, we provide maps of the concentrations of K, Th, K/Th, and Fe. These maps are derived from a 2 base map and smoothed with a 10 boxcar filter. They are useful for showing global variations in concentration. The maps are essentially global for K, Th, and K/Th, but are restricted to regions of relatively low hydrogen for Fe (see below). [4] Second, we use 5 5 binned data for x-y plots. The 5 5 data have been smoothed with a 10 filter around each point. (Filtering removes significant levels of noise in the initial data.) For Fe data, we report only those points in regions where H contents are low enough to not interfere in the determination of the Fe concentration. Hydrogen has 1of16

2 a high cross section for capturing thermal neutrons, so it can significantly affect the flux of thermal neutrons in the upper 30 cm of the Martian surface. To account for the effect of H, we use a correction procedure that involves both the measured fluxes of gamma rays from H, Fe, and Si, and the fluxes calculated from a neutron transport gamma ray production model (Boynton et al., submitted manuscript, 2006). We accomplished that by using the H mask described by Boynton et al. (submitted manuscript, 2006). This approach has resulted in reasonable values at equatorial latitudes. Because the approach results in uncertain values at higher polar latitudes where the influence of hydrogen dominates elemental signatures, the results presented here are constrained using a mask based upon both H concentration values and described by Boynton et al. (submitted manuscript, 2006). The H mask corresponds to roughly plus or minus 45 of latitude from the equator. We restrict our analysis of binned data for K and Th to between 75 south and 75 north latitude; at higher latitudes the concentrations of K and Th are diluted by very high water contents, increasing the uncertainty of those measurements. The typical uncertainties (relative percent) stemming from counting statistics for an average point are 5% for K, 10% for Th, and 5% for Fe. [5] Third, we use summed spectra to compute the concentrations in specific geologic regions (described below). These spectra involve large counting times (> s), and because statistical uncertainties vary as the square root of the counting time, they have correspondingly low statistical uncertainties. The uncertainty varies with the size of the region, hence with the total counting times in the summed spectra. Typical uncertainties are 1% for K, 3 5% for Th, and 2 4% for Fe. These are the uncertainties in the measurements and define the confidence to which we know the means. It does not reflect the variation in concentrations across the Martian surface. 3. Results [6] Maps of the distribution of K, Th, K/Th, and Fe are presented in Figures 1 3. K and Th are not uniformly distributed on Mars. The northern plains from about 60 to +180 E are rich in both, though the higher-than-average Th region extends much further south into the highlands. Both are generally medium to low over Tharsis. There is distinctly higher K and Th in the highlands in the region 15 to 45 S, and extending from 150 to +165 E. The region west of Hellas contains average K, but has relatively high Th. The K/Th ratio varies by somewhat over a factor of two, but most of the surface area is between 4500 and 6500 ppm. It is distinctly low west of Olympus Mons in Amazonis Planitia, in the region where Kasei Valles meets Chryse Planitia, in western Arabia Terra, and in Syrtis Major Planum. K/Th is high in Valles Marineris and surrounding region, Terra Cimmeria, and in the Hellas basin. These modest variations may reflect a combination of bulk Martian K and Th concentrations, igneous processes, and aqueous-related processes, including alteration, erosion, and deposition. However, the uncertainties in the K/Th measurements make these apparent differences statistically uncertain (Figure 2). For example, Hellas appears to have elevated K/Th with values up to 10,000, but the 1s uncertainty is over On the other hand, other areas appear to be distinctly higher or lower, such as the high region in and south of Valles Marinaris, at least at the 1s level. Details of the variations in K/Th are discussed by Taylor et al. [2006]. [7] Fe concentrations (Figure 3) in the northern plains are higher than in the southern highlands. Nevertheless, concentrations are almost everywhere higher than in typical terrestrial basalts and generally consistent with the high Fe contents of Martian meteorites (discussed further below) and with the inferred high FeO in bulk silicate Mars. Additional data are presented in the following sections, in which we compare our GRS data to the compositions of Martian meteorites, test models for the bulk composition of Mars, and explore the record of the planet s early differentiation. 4. Relation of Surface Compositions to the Entire Crust [8] The GRS measures the composition of the upper few tens of centimeters of the dusty Martian surface, yet we are trying to understand the formation of the entire crust and the bulk composition of the entire planet. Several factors allow us to extrapolate our surface measurements to great depth. One is that K and Th have very similar geochemical behavior in igneous systems, as shown by their similar, and very low (1), crystal-melt distribution coefficients [Beattie, 1993; Borg and Draper, 2003; Hauri et al., 1994]. Both elements are incompatible and their concentrations in magmas are not greatly affected by source rock composition or crystallizing phase, even when garnet is involved. There are interesting exceptions, however. Th is highly compatible in phosphate minerals [Jones, 1995]. Phosphates form late in the crystallization of a magma and are unlikely to be retained in a mantle source region, so probably do not play a role in fractioning K from Th during igneous processes. However, in principle, it could be significant if mantle regions were matasomatized by fluids that contained phosphate components. K is compatible in phlogopite [Halliday et al., 1995] and somewhat compatible in amphibole [Halliday et al., 1995], so if these phases were present in the Martian mantle, it could lead to fractionation of K from Th. Nevertheless, in general, K/Th in a lava flow reflects the ratio in its mantle source region. [9] Another reason for the upper few tens of centimeters reflecting the bulk composition of the crust is that the crust was constructed by intrusion of magma far below the surface and extrusion of lava on its surface. On Earth, the ratio of the intrusive to extrusive magma volumes is 5:1 [Crisp, 1984] in oceanic (basaltic) regions. If this holds for Mars, the abundant lava flows that decorate the surface (themselves forming thick sequences of lavas) are accompanied by five times as much magma of similar composition. Of course, we see only the uppermost, youngest lava flows, and it is possible that magma compositions changed with time. Basaltic SNC meteorites argue that there was no systematic change in magma composition with time. All are <600 Ma in age, but were produced from a range of mantle sources [e.g., Borg et al., 1997]. [10] A final reason for thinking that the upper few tens of centimeters reflects deeper material is that the crust has been 2of16

3 Figure 1. Maps of the distribution of K and Th on Mars, as measured by the Mars Odyssey gamma ray spectrometer. Data have been smoothed using a 10 boxcar filter. The data are displayed over a shaded relief map of Mars, with mission landing sites indicated: V1 and V2, Viking 1 and 2; PF, Pathfinder; M, Opportunity in Meridiani Planum; G, Spirit in Gusev Crater. churned by impacts. Hartmann et al. [2001] show that the surfaces formed before the early Amazonian would have been gardened to at least a few meters. Older surfaces would have been gardened even deeper. The oldest Noachian surfaces could have been gardened more or less uniformly to depths of 1 2 km [Hartmann and Neukum, 2001]. Ejecta from the many large basins on Mars would have excavated materials from depths of tens of kilometers. Thus impact 3of16

4 Figure 2. Maps of (top) the distribution of K/Th on Mars and (bottom) the one-sigma uncertainty in the measurements. Data have been smoothed using a 10 boxcar filter. The data are displayed over a shaded relief map of Mars, with mission landing sites indicated: V1 and V2, Viking 1 and 2; PF, Pathfinder; M, Opportunity in Meridiani Planum; G, Spirit in Gusev Crater. processes may provide us with an upper surface that reflects the entire crust. [11] The case for the surface representing the entire crust is not open and shut, however. Aqueous processes can fractionate K and Th. In fact, an important question is the extent to which the range in K/Th observed across the surface of Mars (Figure 2) is due to aqueous processes. Equally important, dust blankets much of the surface and 4of16

5 Figure 3. Map of the distribution of Fe on Mars, as measured by the Mars Odyssey gamma ray spectrometer. Data have been smoothed using a 10 boxcar filter. The mapped area encompasses only that portion of Mars in which H content does not dominate corrections. (Such corrections are not necessary for radioactive elements; Figures 1 2.) The data are displayed over a shaded relief map of Mars, with mission landing sites indicated: V1 and V2, Viking 1 and 2; PF, Pathfinder; M, Opportunity in Meridiani Planum; G, Spirit in Gusev Crater. The black line represents the 0-km contour, a reasonable separation between highlands and lowlands. the fine materials seem to have very similar compositions at all the Viking, Pathfinder, Spirit, and Opportunity landing sites [e.g., Gellert et al., 2004; Rieder et al., 2004; Yen et al., 2005]. The presence of these pervasive fines might blunt differences in bedrock composition from place to place on Mars. The strong evidence for the presence of layered chemical and siliciclastic sediments at the Opportunity site [Squyres et al., 2004] suggests that some regions within a GRS resolution element are sedimentary in origin, hence may not reflect the composition of the igneous crust. On the other hand, large areas are not dominated by fines or chemical sediments, as shown by TES spectra that indicate the presence of igneous minerals [Bandfield et al., 2000]. 5. Comparison to Martian Meteorites [12] Studies of Martian meteorites (also called SNC meteorites, for Shergottites, Nakhlites, and Chassignites) have shed light on Martian magmatic history, nature of the mantle reservoirs that gave rise to meteorite parent magmas, Martian bulk composition, and aqueous alteration processes on Mars [e.g., Bridges et al., 2001; McSween, 2003]. Thus the study of Martian meteorites plays an important role in understanding Mars. However, as Figure 4 shows, SNC meteorites in general have much lower K and Th contents than does the Martian surface. All but two SNC meteorites (shergottite Los Angeles and nakhlite NWA 817) have lower K than any GRS data points, and about half have lower Th. (Los Angeles and NWA 817 do not appear to be contaminated by weathering on Earth. Their Th/U ratios are 5.7 and 4.4 [Rubin et al., 2000; Sautter et al., 2002], which is within the range of most SNC meteorites. SNC meteorites contaminated by terrestrial groundwater, such as DaG 476 and SaU 005, have Th/U < 0.5.) The average surface is clearly higher in Th and K than the average SNC meteorite. Overall, it appears that the Martian meteorites are not representative of the Martian crust, as also reported by analysis of data from the Thermal Emission Spectrometer onboard Mars Global Surveyor [Hamilton et al., 2003]. The source areas for SNC meteorites must either be small relative to the GRS resolution or are covered with enough dust (altered surface material) to mask their chemical signatures. This suggests that the minimum concentrations of K and Th in surface rocks might be lower than the minimum values we observe in the GRS data set. [13] Recent publications have shown that basaltic SNC meteorites (shergottites and olivine-phyric shergottites) have distinct trends in their trace element abundance patterns such as La/Yb ratios, initial 87 Sr/ 86 Sr, 147 Sm/ 144 Nd, 180 Hf/ 183 W, and 176 Lu/ 177 Hf, and oxygen fugacity [Borg et al., 1997, 2003; Herd et al., 2002; Herd, 2003; Goodrich et al., 2003; Borg and Draper, 2003; McLennan, 2003]. These distinct trends were originally interpreted to reflect crustal assimilation accompanied by fractional crystallization. This interpretation fell out of favor because it does not explain the lack of correlation between indices of differentiation (e.g., Mg/Fe) and the concentrations of incompatible trace elements [e.g., Borg and Draper, 2003]. The alternative interpretation is that trends reflect the chemical properties of the mantle source regions for the basaltic SNC meteorites. 5of16

6 Figure 4. K and Th concentrations in 5 5 smoothed pixels from Mars Odyssey GRS data compared to basaltic (includes olivine phyric) SNC and other SNC meteorites. GRS data collected through March Uncertainty is shown for a typical data point. SNC data are from Lodders [1998, and references therein] and Meyer [2003, and references therein]. Note that all but two meteorites (basaltic shergottite Los Angeles and nahklite NWA 817) plot substantially below GRS data, implying that the meteorites are not representative of typical Martian surface materials. End-member cases are (1) an enriched source with high and approximately chondritic La/Yb, relatively high oxidation state (1.5 log units below the quartz-fayalite-magnetite [QFM] oxygen fugacity buffer), negative e Nd, and high initial 87 Sr/ 86 Sr, and (2) a depleted source with low La/Yb, reduced (3 4 log units below QFM), positive e Nd, and low Rb, leading to low initial 87 Sr/ 86 Sr. Basaltic SNC meteorites with intermediate properties formed by melting of sources intermediate to those two end-members. The nakhlites appear to derive from yet a third source region [Foley et al., 2005]. Isotopic data show conclusively that these distinctive source regions formed early in Martian history, approximately by 4.5 Ga [Borg et al., 2003; Foley et al., 2005], and only 20 Ma after calcium-aluminum-rich inclusions [Harper et al., 1995; Lee and Halliday, 1997; Blichert-Toft et al., 1999; Kleine et al., 2002; Yin et al., 2002], perhaps only 12 Ma after CAI formation [Foley et al., 2005], and that the mantle has remained unmixed since then. This implies no widespread homogenization of the Martian mantle or recycling of the crust. [14] GRS data place this story into a global context. La/Yb correlates with K and Th in basaltic shergottites (Figure 5), so K and Th concentrations also correlate with e Nd, initial 87 Sr/ 86 Sr, and oxygen fugacity. The average crust on Mars contains 3300 ppm K and 0.6 ppm Th (see below for a complete discussion of crustal averages). Comparing those to basaltic Martian meteorites (Figure 5) suggests that the average crust ought to have chondritic (or even slightly super chondritic) La/Yb and be as oxidized as Los Angeles and Shergotty (about QFM-1.5). It would have formed from undepleted mantle, and, given the isotopic data from the meteorites, the mantle source would have formed very early in Martian history. The lowest Th and K values (Figure 4) measured by GRS, if representative of the igneous rocks of the crust (see section 4), suggest that a portion of the crust formed from somewhat depleted mantle, but no large areas (larger than the 500-km GRS footprint) are composed of depleted basalts (those with 500 ppm K or less). If the crust is similar to basaltic shergottites, it appears that most of the crust formed from undepleted, possibly primitive mantle rock. This has implications for early differentiation, as discussed below. 6. Testing Models of Martian Bulk Composition 6.1. Models for the Bulk Composition of Mars [15] There are three prominent models for the bulk composition of Mars, each derived by a different method. Dreibus and Wänke [1984], Wänke and Dreibus [1988, 1994], and Longhi et al. [1992] estimate the bulk composition from element correlations in SNC meteorites, with the assumption that refractory elements are present in chondritic abundances. This model is directly tied to Mars through element abundances in SNC meteorites. The Wänke- Dreibus model predicts that K/Th in bulk Mars is As we show below, global K/Th is 5300, favoring the Wänke-Dreibus model. [16] Ganapathy and Anders [1974] and Morgan and Anders [1979] modeled Mars as a mixture of chondritic materials that had been modified by the same limited set of processes that affected chondrites, such as variations in condensation temperature and fractionation of metal from silicate. They propose that there are three primary condensates from the solar nebula: a high-temperature, refractoryrich condensate; Fe-Ni metal; and magnesian silicates. Morgan and Anders [1979] define a fourth component, FeS and FeO, which they postulate formed by reaction of Fe metal with H 2 S and H 2 O, respectively. Morgan and Anders point out that elements of similar volatility do not fractionate during nebular processes, allowing them to use four index elements (U, Fe, K, and Tl or 36 Ar) to calculate the abundances of 83 elements in the planet. They defined the concentrations of the index elements in Mars, using gamma ray data from the Soviet orbiter Mars 5, thermal models available at the time, the density of the mantle and the Martian moment of inertia, and volatile elements present in the atmosphere. Morgan and Anders 6of16

7 Figure 5. La/Yb variations versus K and Th in basaltic shergottites (SNC Martian meteorites). There is a clear trend of increasing K and Th with La/Yb. Chondritic La/Yb (1.5) is shown for comparison. The meteorite data suggest that if basaltic rock dominates the surface of Mars, the average Martian crust (as measured by GRS) has an approximately chondritic REE pattern, though it could be slightly enriched in light REE. Meteorite data sources are from Lodders [1998, and references therein] and Meyer [2003, and references therein]. [1979] predicted a value of 620 for K/Th in bulk Mars, much lower than our surface value of [17] Third, Lodders and Fegley [1997] focused on fitting the oxygen isotopic composition of Mars, known from SNC meteorites, to mixtures of chondritic meteorites. Their calculations led to the estimate that Mars was constructed from a mixture of 85% H-chondrites, 11% CV-chondrites, and 4% CI-chondrites. Lodders and Fegley [1997] predicted a value of 16,000 for K/Th in bulk Mars, much higher than our data indicate. (Sanloup et al. [1999] took a similar approach in estimating the composition of Mars but did not estimate the abundances of K and Th.) [18] In testing these models, we will focus on K/Th, the ratio of a moderately volatile element to a refractory element, and FeO, which varies among the terrestrial planets and among chondrite groups. We first summarize our data for a diverse set of geologic regions on Mars, and present a summary of bulk K/Th and FeO in Mars Regions Investigated and Their Compositions [19] We have divided Mars into numerous regions to investigate the relation of composition and geological province or surface properties. (For more information on geologic province designations, see Dohm et al. [2005].) The regions are usually large, so total counting times are large and analytical uncertainties in concentrations are small. To understand the global composition of Mars, we use K, Th, and Fe data from the following regions. [20] 1. The entire globe includes north and south polar regions, which are extremely rich in H (H 2 O), hence tend to dilute the measured concentrations of other elements, but is still useful for assessing the K/Th ratio. A modification of this is to include the entire area within the H mask. The results reported in Table 1 are global for K and Th, and within the H masked area for FeO. [21] 2. The ancient southern highlands are regions of the oldest crust, identified as places where magnetic anomalies 7of16

8 Table 1. K, Th, K/Th, and FeO in Selected Regions on Mars Region K a, ppm Th a, ppm K/Th b FeO a Global 3300 ± ± ± ± 0.1 Ancient Southern Highlands 3630 ± ± ± ± 0.2 Arabia Terra 3540 ± ± ± ± 0.3 Major volcanic provinces 3010 ± ± ± ± 0.1 Northern Plains 3860 ± ± ± ± 0.2 Rocky regions 3630 ± ± ± ± 0.2 Dusty regions 3290 ± ± ± ± 0.2 a Uncertainty represents 1-s counting statistics for global summed spectrum (total of s counting time for global average; > s for other regions). b Uncertainty calculated from (K/Th) [(s K /K) 2 +(s Th /Th) 2 ] 1/2, where K and Th are mean concentration of K and Th (columns 2 and 3). are visible. These highlands represent the oldest crust on Mars. [22] 3. Arabia Terra is a proposed ancient impact basin and possible site of extensive sedimentary deposition. The sediments would have been derived from the older highlands, and might represent an average composition of those highlands. [23] 4. Major volcanic provinces include Tharsis, Elysium, and Hadriaca/Tyrrhena. Basaltic lavas are probes of mantle composition, so the K/Th and Fe in them reflect the mantle composition at least since the Middle Noachian for Tharsis [Dohm et al., 2001], Hesperian for Elysium [e.g., Tanaka et al., 1992], and Late Noachian for Hadriaca/Tyrrhena [Crown et al., 1992]. [24] 5 Northern Plains include the entire northern plains, including the Vastitas Borealis Formation. This deposit makes an interesting contrast to ancient highlands deposits and allows an assessment of the extent to which aqueous processes might have affected K/Th. [25] 6. Rocky regions combine the places on Mars with highest rock abundance, determined on the basis of thermal inertia, TES mineralogy, and rock abundance models. [26] 7. Dusty regions are the dustiest regions on Mars, as quantitatively defined by thermal inertia and albedo. Dusty regions might represent homogenized surface material, hence may give a good estimate of the global abundance of K, Th, and Fe. This is supported by the similarity of elemental compositions of soils at the Pathfinder and Viking landing sites [e.g., Wänke et al., 2001]. It also follows a long tradition of using sediments to determine the composition of the terrestrial crust [McLennan et al., 1980]. However, we do not know the origin of the dusty material, so it might provide us with the composition of late stage volcanics and altered materials, rather than an average of crustal materials K/Th in Mars [27] The variation of K/Th on Mars is shown in Figures 1, 4, and 6. Most of the Martian surface has K/Th between about 4500 and 6500, with a sharp peak between 5000 and 6000, although the total range is from about 3200 to Note that there is a tail off to high K/Th. This may reflect fractionation of K from Th, perhaps by aqueous processes. [28] K and Th concentrations and the K/Th ratios for the regions are given in Table 1. Uncertainties are based solely on counting statistics, and so represent our confidence in the reported average value, not the range of values measured within each region. The range in the mean values is not large. K ranges from 3000 to 3600 ppm, Th from 0.55 to 0.71 ppm. Most important, the K/Th ratio is quite uniform, ranging from 4900 to Dusty and rocky regions are also indistinguishable in K/Th, suggesting that the presence of dusty materials does not affect our assessment of the average K/Th on the surface. The ancient highlands have K/Th essentially the same as the global averages. The K/Th ratio of the Martian surface and, we infer, for bulk silicate Mars, is clearly between 5000 and We adopt the global value of 5300 to represent the Martian K/Th ratio. [29] Our results are very different from those reported for the Phobos 2 mission [Surkov et al., 1994], which operated for only 12 days. K averages about 4000 ppm in the Phobos 2 data set, compared to our average of 3300 ppm (Table 1). Th is much higher in the Phobos 2 data, 2.3 ppm versus our crustal average of 0.6 ppm. This leads to a much lower K/ Th of 1740 in the Phobos 2 data, compared to our value of However, as discussed by Trombka et al. [1992] the peaks for Th in the Phobos 2 data are barely above background, hence have large statistical uncertainties. [30] In Table 2, we compare our measured K/Th for Mars to those of the three Martian bulk compositions described above, and to the Earth, Vesta, the Moon, and CI carbonaceous chondrites. Our inferred bulk K/Th is very close to that calculated in the Wänke-Dreibus model. In contrast, the model reported by Ganapathy and Anders [1974] and Morgan and Anders [1979] has a much lower K/Th (only 620), which is inconsistent with our value of However, this does not negate the Ganapathy and Anders [1974] approach used for estimating planetary bulk compositions. Updated compositional data for Martian surface might allow a more reasonable estimate using their approach. [31] The oxygen isotope model developed by Lodders and Fegley [1997] requires that K/Th be much higher than we find. Lodders and Fegley [1997] recognized that the abundance of alkalis was higher in their model than in the Wänke-Dreibus model and in Martian meteorites. They suggested that aqueous leaching in the mantle and hydrothermal activity led to the preferential deposition of K and other alkalis, as well as halogens, in the Martian crust. Several features of the GRS data argue against this interpretation. First, the process of alkali and halogen transport and crustal enrichment must operate surprisingly uniformly because K does not vary by more than about 50% (relative) across the surface (Figures 1 and 4). Second, and more importantly, the K/Th ratio does not vary greatly (Figure 6), so if K is enriched by aqueous transport, it is enriched everywhere on the surface by roughly the same amount. A third point comes from mass balances. We estimate in 8of16

9 Figure 6. Distribution of K/Th values on Mars. Data are for 5 smoothed bins between 75 south and 75 north latitude, using data through March There is a clear peak in the distribution at about section 6 the percentage of bulk planetary K and Th residing in the crust, using the Wänke-Dreibus K and Th concentrations for the primitive mantle. The percentage is the same for both elements, suggesting no special enrichment process for K. Finally, K does not correlate with Cl on the Martian surface [Keller et al., 2006]. If both elements were transported from the mantle together, one would expect that they would correlate, although subsequent aqueous processes might fractionate them. Lodders [2000] suggests that the lower volatile content of Mars derived from compositions of SNC meteorites is only apparent. Higher volatiles might be in an enriched reservoir not sampled among the SNC meteorites. This seems unlikely in light of our global data. The GRS data set shows that the entire crust has K/Th much lower than the oxygen isotope mixing model predicts. [32] We conclude that the Wänke-Dreibus model is a reasonable estimate of the K/Th concentration in bulk silicate Mars, and we adopt a K/Th value of 5300 for Mars. This is almost twice as high as reported for Earth (Table 2), confirming that Mars is enriched in moderately volatile elements compared to Earth. It is also clearly enriched compared to the Moon and the Howardite-Eucrite-Diogenite (HED) parent body (assumed to be asteroid 4 Vesta). Although Mars is enriched compared to those bodies, it is still substantially depleted compared to CI chondrites FeO in Mars [33] Our 5 5 Fe data are shown in Figure 7. We have converted the data from Fe to FeO for easier comparison with data from Martian meteorites and the Viking, Pathfinder, and MER landed missions. We present FeO data for specific regions in Table 1. There is a clear peak between FeO concentrations of 18 and 21 wt%. Higher values for Fe are confined to the northern lowlands and lower values occur in the southern highlands (Figure 3). This can also be seen in the averages for the regions we analyzed for this paper (Table 1). The ancient highlands are lower in FeO than are the northern plains, and the global average, sum of all rocky and dusty regions, and Arabia are intermediate, although not very different from the southern highlands. The global average (using the H mask) is 14.3 wt% Fe, corresponding to 18.4 wt% FeO. The GRS data are very slightly offset to higher values than those for SNC meteorites and surface samples (Figure 7), but the difference is not significant. Most importantly, our GRS analytical points are almost everywhere higher than common terrestrial basalts. Mid-ocean ridge basalts, for example, average 10.5 wt% FeO [Melson et al., 1976]. Thus the GRS results appear to confirm the inference from meteorites that Mars is enriched in FeO compared to Earth. The Wänke and Dreibus [1988, 1994] estimate of 17.9 wt % is close to our crustal average Table 2. Comparison of K, Th, and K/Th in Mars, Earth, Vesta, the Moon, and Carbonaceous Chondrites K, ppm Th, ppm K/Th Mars (Average Crust) a Mars Bulk Silicate-WD b Mars Bulk Silicate-L c ,400 Mars Bulk Silicate-MA d Earth Continental Crust e 11, Earth Bulk Silicate f Vesta (HED meteorites) g Moon (high-k KREEP) h CI Chondrites i ,000 a This work. b Dreibus and Wänke [1984] and Wänke and Dreibus [1988, 1994]. c Lodders and Fegley [1997]. d Morgan and Anders [1979]. e Taylor and McLennan [1985]. f Average of Taylor and McLennan [1985], Jagoutz et al. [1979], and McDonough and Sun [1995]. g Kitts and Lodders [1998] and Mittlefehldt and Lindstrom [1993]. h Warren and Wasson [1979] and Warren [1989]. i McDonough and Sun [1995]. 9of16

10 Figure 7. (a) GRS 5 5 smoothed data set for Fe (converted to FeO), which shows a broad peak between 18 and 21 wt %. (b) Data for Martian meteorites, Viking, Pathfinder, and Spirit landing sites, which show a similar result. The data are consistent with Mars having significantly more FeO in its mantle than Earth does. Data sources are as follows: SNC meteorites from Lodders [1998, and references therein] and Meyer [2003, and references therein]; Viking from Clark et al. [1982]; Pathfinder from Brückner et al. [2003]; Spirit data from Gellert et al. [2004]; and Opportunity data from Rieder et al. [2004]. For the MER sites, only samples abraded with the Rock Abrasion Tool are included for rock analyses. and consistent with the Martian moment of inertia [Bertka and Fei, 1998a, 1998b]. The geophysical data do, however, allow the value to be slightly lower (12 14 wt%), as suggested by high-pressure experiments on chondritic meteorites by Agee and Draper [2004] and supported by geochemical modeling [Borg and Draper, 2003; Draper et al., 2005]. [34] Petrologic processes might have affected the partitioning of FeO into the crust. Its partitioning depends on oxidation state, water content, and pressure. For example, hydrous partial melting tends to partition less FeO into melt than is in the original peridotite that was melting [Gaetani and Grove, 1998]. This effect is controlled by the temperature of melting rather than by the FeO content (in reality the activity of FeO) in the peridotite. Fractional crystallization of basaltic magma containing wt% water causes a decrease in FeO with crystallization at oxygen slightly below the QFM buffer [Minitti and Rutherford, 2000]. (In comparison, Minitti and Rutherford [2000] show that MgO decreases faster, resulting in a substantial increase in FeO/MgO.) Thus any magma generated by partial melting of a wet Martian mantle and subsequent fractionation of that hydrous magma would lead to lower FeO than in the original mantle peridotite. Nekvasil et al. [2003] used experimental data to estimate the type of magma that was parental to the Martian meteorite Chassigny. They suggested that the magma was alkaline and hydrous, and showed that fractionation of such a magma leads to a decrease in FeO with increasing crystallization and SiO 2. This interpretation suggests that, if anything, the interior of Mars is richer in FeO than the surface. Hydrous partial melting might be part of the cause of lower FeO in the highlands compared to the northern plains. Thus the difference in FeO might reflect lower H 2 O in the mantle source that melted to produce the highlands lavas and sediments derived from them that compose the northern plains. [35] Surface and near-surface aqueous processes might have caused transport of Fe from the highlands to the lowlands, thus accounting for the dichotomy in Fe concentrations (Figure 3), a possibility raised by Boynton et al. (submitted manuscript, 2006). This would be particularly likely at the low ph conditions that have prevailed on Mars [e.g., Burns, 1993; Hurowitz et al., 2005]. However, one would expect that the solutions that delivered large amounts of Fe to the northern plains would be brines, hence Cl ought to be enriched in the northern plains, which we do not observe [Keller et al., 2006] Implications for Planetary Accretion [36] There is a clear difference in the abundance of moderately volatile elements in Mars compared to Earth, as shown by K/Th being double that in Earth (Table 2). This implies that on average Mars accreted from materials that were richer in moderately volatile elements than the average of the materials that formed the Earth. In turn, this suggests that mixing among planetesimals was not extensive during the formation of the two planets, as argued on different grounds by Drake and Righter [2000]. They show that the Earth s composition is unique: It is different from known chondrites and other planets, for example, in Mg/Si and oxygen isotopic compositions. They conclude that there was not widespread mixing of material throughout the inner Solar System during accretion and that accretion zones were relatively narrow. This conclusion contrasts with calculations of the dynamics of planet formation, which suggest that there was considerable mixing of planetary embryos during the final stage of accretion [Wetherill, 1994; Wetherill and Stewart, 1993; Chambers, 2001]. [37] The Earth-Mars difference in abundances of moderately volatile elements does not necessarily imply a simple variation with heliocentric distance. Vesta, often viewed as a small terrestrial planet, resides in the inner asteroid belt yet has much lower K/Th than does Mars, accompanied by about the same concentration of FeO, 20 wt% [Warren, 1997]. Venus has roughly the same K/Th as does Earth, but the measurements have high uncertainty and substantial variation [Surkov et al., 1987]. K/Th is unknown for Mercury, and models for its composition vary widely [Taylor and Scott, 2004]; its K/Th will be determined by 10 of 16

11 the MESSENGER mission. The Moon is extremely low in volatiles (K/Th < 400, Table 2). This reflects either its formation by a giant impact or that the giant impactor was severely depleted in moderately volatile elements. [38] The much higher FeO abundance in Mars (18 wt%) shows that the planet is more oxidized than Earth (8 wt%). Robinson and Taylor [2001] argue that Venus has roughly the same FeO as Earth and that Mercury has only 2 3 wt%. The trend even includes Vesta in this case, although it would appear to be no more oxidized than Mars. The higher FeO in Mars may have been caused by oxidation of metallic iron by H 2 O during accretion of Mars, as hypothesized by Wänke and Dreibus [1994]. As discussed by Bertka and Fei [1998b], an oxidation event like this would increase the sulfur content of a metallic core. Whatever the cause of the variation of FeO in the inner solar system, comparison of the compositions of Mars and Vesta indicates that there is not a simple correlation of FeO and K/Th with heliocentric distance: both bodies have high FeO, but very different K/Th (Table 2). 7. Style of Differentiation [39] Taylor [1992] has defined three types of planetary crusts. Primary crust forms as the result of planet-wide, extensive early melting; i.e., from a magma ocean. Secondary crust forms by partial melting of the interior to produce basaltic rocks. This has clearly happened on all the terrestrial planets and the Moon; the lava plains and large volcanoes on Mars are clear evidence for the formation of secondary crust on Mars. Secondary crusts are basaltic. Tertiary crust forms by the partial melting and differentiation of secondary crustal materials and sediments derived from them. The presence of tertiary crust implies recycling of the crust. Taylor [1992] notes that the terrestrial continents may be the only example of tertiary crust. Global GRS data suggest that Mars is dominated by secondary crust, though it does not rule out formation of a primary crust from a global magma system. If present, tertiary crust is rare Did Mars Have A Magma Ocean (Primary Crust)? [40] It is likely that the Moon was surrounded by an ocean of magma when it formed; see reviews by Warren [1985, 2003]. This event produced the original (primary), anorthositic crust in the lunar highlands, a residual magma rich in potassium, rare earth elements, and phosphorus (KREEP) that seems to have concentrated in the Imbrium- Procellarum region of the nearside [Jolliff et al., 2000], and a density-unstable mantle whose overturn led to melting and formation of the Moon s secondary crust. To a great extent, the lunar magma ocean set the course for subsequent lunar magmatic evolution. Thus it is essential that we determine if Mars also had a magma ocean. K and Th concentrations show a much smaller range on Mars than they do on the Moon, suggesting that Mars did not have a magma ocean, that processes operated differently in it than on the Moon, or that the record of it has been erased. [41] We compare global GRS data for the Moon and Mars in Figure 8. The data clearly show the large depletion in K in the Moon compared to Mars. Equally striking is the very large range in Th on the Moon. It ranges from about 0.1 ppm to slightly more than 10 ppm, a factor of 100. K actually varies an equal amount but its low concentration makes it less obvious. This reflects the formation of anorthosite cumulates low in Th and K, and the concentration of residual, KREEP liquid that was available for excavation by large impacts and for incorporation into magmas. In contrast, Th on the Martian surface ranges from 0.2 to 1 ppm, a factor of only 5. K varies from roughly 2000 to 6000, a factor of only 3. It is possible that the pervasive presence of dust prevents us from measuring the true minimum values of crustal rocks. The SNC meteorite data (Figure 4) certainly show that some rocks have lower K than our global measurements. [42] It is possible that the striking difference between the lunar and Martian GRS data is due to the higher resolution of the lunar data. To test this, we summed the lunar data to have the same spatial resolution as do our 5 5 Odyssey GRS data (also shown in Figure 8). In this case the highest Th point in the lunar data is still over 8 ppm and there are several points in the range 4 8 ppm, showing that there are large areas of the Moon enriched in Th. There is therefore no evidence from the GRS data for large occurrences of primary cumulates on Mars or for extensively fractionated, KREEP-like materials. [43] Does this mean that there was not a magma ocean on Mars? Not necessarily. It may mean that the processes in the Martian magma ocean were so different from those in the lunar magma ocean that surface cumulates and highly evolved melts were either not produced or were not segregated from accompanying cumulus crystals. Theoretical considerations and detailed modeling [Hess, 2002; Elkins-Tanton et al., 2003, 2005; Borg and Draper, 2003] indicate that Al will be sequestered in the mantle, depleting the residual magma ocean in Al, and leading to delayed plagioclase nucleation. Plagioclase, even when it finally does crystallize, might have a higher density than the magma (especially if H 2 O is present), and hence sink [Hess, 2002]. This effectively rules out creation of a plagioclaserich crust as on the Moon. No other minerals are good candidates to form a floatation crust on Mars, so we do not expect to see regions with very low Th and K as we do in the lunar highlands. In comparing the Moon and Mars we should keep in mind that we do not yet understand the complex array of processes that operated in the lunar magma ocean, the fate of its products, and their final locations in the differentiated Moon. For example, Longhi [2003] proposed that there was a magma ocean, but that the feldspar-rich crust did not form directly from it. [44] The limited concentration ranges of K and Th on Mars could mean that there was less fractional crystallization and less crystal-melt segregation in a Martian magma ocean than in the lunar one. Elkins-Tanton and Parmentier [2004] point out that even in the lunar magma ocean most of the magma lies between the liquidus and solidus, hence will have a substantial crystal fraction. At crystal fractions above a critical value the melt-crystal system begins to behave as a solid, inhibiting fractionation. This happens at crystal fractions of about 50% [Van der Molen and Paterson, 1979; Wright and Okamura, 1977; Campbell et al., 1978; Marsh, 1988], although it can be as low as 25% if networks of lathshaped plagioclase crystals form [Philpotts et al., 1998]. This important effect led Elkins-Tanton and Parmentier 11 of 16

12 Figure 8. Comparison of GRS data for Mars (our data) and the Moon [Prettyman et al., 2002]. Lunar data are shown for both 60-km resolution elements and in degraded resolution equal to resolution elements on Mars (about 500 km). K/Th lines are for reference only, not fits to the data. The Moon clearly has much lower K/Th than Mars does. Nevertheless, it shows a much larger range in K and Th than GRS measurements of the Martian surface do. The lunar range (shown most readily by the Th data) is caused by the presence of large areas containing low-th rocks (the lunar highlands, composed of cumulate anorthosites) and very high Th (highly differentiated rocks called KREEP). Comparison of the high- and low-resolution lunar data shows that the high Th concentrations are not in small, localized locations. These differences in the distribution of trace elements may reflect very different differentiation histories for the two bodies. [2004] to propose that even the lunar magma ocean did not directly produce the anorthosite crust on the Moon, suggesting it formed shortly after crystallization as overturn occurs to remove strong density gradients. Longhi [2003] reaches the same conclusion from the perspective of phase equilibria in a lunar magma ocean. By analogy, the Martian magma ocean may not have segregated crystals and liquid efficiently enough to produce a distinctive layer of KREEP-like residual melt. The residual melt instead may have crystallized largely in place, perhaps in large part by local equilibrium crystallization. Overturn of the mantle shortly after the original Martian magma ocean had crystallized or almost crystallized, could have led to a variety of regions containing different amounts of depleted cumulates and trapped liquids, variable amounts of water (possibly leading to variations in oxygen fugacity in the source regions), and an initial crust composed of the lowest density materials. Calculations by Elkins-Tanton et al. [2003] suggest that this original crust would contain about 45 wt% SiO 2, low Al 2 O 3, and only 5 10 wt% FeO. Subsequent magma production would have led to higher FeO contents. Elkins-Tanton et al. [2003] predict that FeO increased with depth in Mars after overturn, but the upper mantle would still have low Al 2 O 3. In more detailed calculations, Elkins-Tanton et al. [2005] suggest that magmas produced by partial melting of overturned magma ocean cumulates would produce magmas with FeO contents of wt.%, in the range observed for the Martian surface. For a given percentage of melting (inferred from MgO content of the magma), deep partial melting produces magma with higher FeO than shallow melting. Perhaps the slightly lower FeO content of the southern highlands compared to the northern plains is the result of southern highland basalts being derived from, on average, shallower depths than those composing the northern plains Partitioning of K and Th Into the Crust [45] An important consideration in thermal evolution is how much of the planet s bulk K, Th, and U was partitioned into the crust versus remaining in the mantle. We show below that about half of the K and Th are fractionated into the crust, leaving enough in the mantle to allow for the young ages of basaltic SNC meteorites. Although the extent of melting during initial Martian differentiation is uncertain, isotopic data from SNC meteorites show unambiguously that Mars differentiated early. Their apparent depletion in Al is consistent with a melting event that involved depths of at least hundreds of kilometers so that garnet crystallized. This led to an early partitioning of K, Th, and U into the crust, which affected the subsequent magmatic evolution of Mars [McLennan, 2001; Hauck and Phillips, 2002; Kiefer, 2003]. [46] The average Th in the crust is 0.6 ppm (Table 2). The thickness of the crust has been estimated from geophysical data. Estimates place limits on the crustal thickness, with the broad limits of 29 to 115 km (summarized by Wieczorek and Zuber [2004]). A reasonable range is km [Wieczorek and Zuber, 2004], with a nominal value of 57 km. Assuming that the bulk Th in Mars is ppm (Table 2), crustal density of 2.9 g cm 3, mantle density of 3.5 g cm 3, and a core radius of 1634 km, then we can calculate the percentage of Th partitioned into the crust as a function of crustal thickness (Figure 9). The maximum thickness of the crust is 118 km, calculated by assuming that all the Th is in the crust. [47] For the nominal crustal thickness of 57 km, 50% of the Th is in the crust. If the thickness of the crust is closer to the upper limit of 81 km, then 70% of the Th is in the crust. A calculation for K concentrations gives the same answers. These are high values: the concentration of heat-producing elements in the terrestrial crust is <40% [McLennan, 2001], although the terrestrial crust composes only about 1% of the 12 of 16