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Chemistry of Rocks and Soils at Meridiani Planum from the Alpha Particle X-ray Spectrometer

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Science  03 Dec 2004:
Vol. 306, Issue 5702, pp. 1746-1749
DOI: 10.1126/science.1104358

Abstract

The Alpha Particle X-ray Spectrometer on the Opportunity rover determined major and minor elements of soils and rocks in Meridiani Planum. Chemical compositions differentiate between basaltic rocks, evaporite-rich rocks, basaltic soils, and hematite-rich soils. Although soils are compositionally similar to those at previous landing sites, differences in iron and some minor element concentrations signify the addition of local components. Rocky outcrops are rich in sulfur and variably enriched in bromine relative to chlorine. The interaction with water in the past is indicated by the chemical features in rocks and soils at this site.

Opportunity landed on Mars at Meridiani Planum on 25 January 2004 coordinated universal time (UTC). The landing site was selected because in this region orbital instruments had identified hematite (1), a mineral possibly indicative of the past presence of water on Mars. Opportunity came to rest within a small crater, named Eagle crater (2), surrounded by intriguing outcrops (3).

The Alpha Particle X-ray Spectrometer (APXS) (4) measured the composition of soils (5) and rocks. Results obtained during the first 90 sols (6) are presented in Tables 1 and 2. These data were obtained under low-temperature conditions, yielding a resolution of <165 eV at 5.9 keV, and were collected when measurement times were adequate for good counting statistics. As a rule, these data were taken during the early morning hours, typically between 4:00 and 9:00 local solar time. Several sites have only been measured in the so-called “touch-and-go” mode (3) at elevated temperatures and with short measurement durations. Because of the low quality of these data, only two of these outcrop surfaces are included in the tables.

Table 1.

Chemical composition of soils at Meridiani Planum, in weight percent (21). Abs. stat. error, absolute statistical error, which is a 2σ error for a typical measurement of 4 hours duration. This number can also be considered as detection limit.

Feature First soil Hematite slope Big Dig Mont Blanc DogPark PHOTIDO Plains
Target Abs. stat. error Tarmac Hema2 Hema trench1 Hema trenchwall2 Les Hauches Jack Russell Beagle Burrow Nougat
Note Surface Surface Floor Side wall Surface Surface Floor Plains, scuff disturbed
Na2O 0.3 1.4 1.6 1.5 1.5 1.7 1.6 1.7 1.8
MgO 0.2 7.2 7.0 7.0 7.0 7.2 6.4 7.2 7.4
Al2O3 0.2 8.8 8.1 8.7 8.6 8.8 7.2 9.4 8.8
SiO2 0.4 45.5 41.9 45.0 44.3 44.7 37.8 46.3 45.0
P2O5 0.04 0.82 0.83 0.83 0.78 0.94 0.75 0.79 0.89
SO3 0.05 4.93 4.68 6.92 5.62 7.29 4.79 4.52 5.77
Cl 0.03 0.43 0.46 0.47 0.40 0.54 0.46 0.33 0.41
K2O 0.1 0.48 0.43 0.49 0.45 0.48 0.37 0.40 0.45
CaO 0.05 7.52 6.27 6.90 6.88 6.82 5.15 6.94 6.90
TiO2 0.05 1.09 0.83 1.17 1.30 1.07 0.70 1.29 1.15
Cr2O3 0.02 0.52 0.37 0.45 0.52 0.37 0.36 0.54 0.51
MnO 0.02 0.40 0.32 0.38 0.38 0.37 0.28 0.38 0.40
FeO 0.2 20.1 26.5 19.1 21.3 19.0 33.4 19.3 19.9
Ni ppm 40 640 820 800 840 640 1090 750 590
Zn ppm 40 280 350 490 400 470 340 290 370
Br ppm 30 30 30 190 160 20 30 40 290
Table 2.

Chemical composition of rocks at Meridiani Planum, in weight percent (21). Abs. stat. error, absolute statistical error, which is a 2σ error for a typical measurement of 4 hours duration. ND, not detected.

Feature Stone Mountain McKittrick Guadalupe Last Chance Dells Flat rock Berry Bowl Real Sharks Tooth Bounce rock Pilbara
Target Abs. stat. error Robert E. middleRAT Guadalupe RAT King3 Makar Hi-Ho Mojo2 Rubel Empty Enamel 1 Glanz 2 Case RedHerring Maggie Golf
Note As isView inline As is RAT As is RAT As is As isView inline As is RAT As is, full As is, empty As is As is RAT As is RAT
Na2O 0.3 1.1 1.7 1.1 1.3 1.0 1.5 1.8 1.7 1.2 1.7 1.6 1.6 1.5 1.3 1.7 1.1
MgO 0.2 7.9 7.7 7.4 7.4 7.8 7.1 7.6 7.3 7.8 6.4 7.4 7.6 6.1 6.4 7.2 8.0
Al2O3 0.2 7.2 8.1 6.0 7.0 5.7 6.8 8.4 8.1 6.0 7.6 7.8 6.7 9.3 10.1 10.3 5.6
SiO2 0.4 39.5 42.7 38.1 40.1 36.3 38.1 43.5 42.7 36.2 38.9 42.4 38.1 47.7 50.8 47.0 34.7
P2O5 0.04 0.99 1.01 1.00 1.04 0.99 1.03 1.04 1.02 1.03 0.78 1.00 1.02 1.00 0.95 0.92 0.98
SO3 0.3 19.2 12.7 21.0 18.6 24.6 18.7 11.4 12.9 23.3 5.50 14.0 18.5 3.59 0.52 4.61 24.7
Cl 0.03 0.54 0.60 0.39 0.59 0.33 0.61 0.59 0.59 0.36 0.49 0.68 0.57 0.38 0.06 0.66 0.44
K2O 0.1 0.58 0.54 0.56 0.58 0.54 0.56 0.56 0.57 0.59 0.38 0.57 0.52 0.26 0.10 0.29 0.50
CaO 0.05 5.11 5.92 4.49 5.02 5.02 4.95 5.87 6.17 5.28 5.34 5.65 4.46 11.3 12.5 10.2 4.90
TiO2 0.05 0.75 0.91 0.85 0.88 0.67 0.72 0.94 0.92 0.77 0.73 0.88 0.71 0.89 0.78 0.74 0.78
Cr2O3 0.02 0.20 0.29 0.22 0.20 0.19 0.19 0.36 0.26 0.23 0.32 0.22 0.19 0.14 0.12 0.14 0.23
MnO 0.02 0.26 0.32 0.32 0.31 0.32 0.30 0.35 0.31 0.27 0.29 0.36 0.25 0.46 0.43 0.40 0.37
FeO 0.2 16.0 16.9 17.6 16.2 15.8 18.6 16.9 16.8 16.3 31.0 16.8 19.0 16.7 15.6 15.4 16.7
Ni ppm 40 780 710 880 810 690 800 720 760 790 950 730 770 350 180 260 750
Zn ppm 40 670 340 320 430 370 440 330 480 490 380 490 440 100 50 180 580
Br ppm 30 ND 270 430 50 30 30 440 110 120 40 120 120 30 30 30 30
  • View inline* Errors are higher by a factor of 2 because of short measurement time at elevated temperature.

The first measurement with the APXS was performed on a stretch of soil named Tarmac within the base of Eagle crater. Its composition (Table 1) is similar to soils measured by the Spirit rover at Gusev crater and to soils analyzed by Pathfinder at the mouth of Ares Vallis (7). Except for Fe, major elements in Meridiani soils differ only slightly from soils at other landing sites. Larger differences exist for some minor elements—such as Na, Ti, Cr, Mn, and Ni—suggesting that there has been some admixture of debris from local rocks at Meridiani Planum to an otherwise globally homogenized soil (Fig. 1). Specifically, Fe, Ni, and Cr concentrations are higher in Meridiani soils than in Gusev soil. Larger variations of Fe are likely due to variable amounts of hematite-rich spherules, also found at Meridiani Planum.

Fig. 1.

Chemical composition of martian soils in Meridani Planum, compared with a Gusev crater soil (7).

On sol 15, without any brushing or grinding (surface as is), the APXS was placed against a rocky outcrop called Stone Mountain, located near the base of the Eagle crater wall. The analysis of this area showed a sulfur concentration of 7.7 weight % (wt %) (or 19.2 wt % SO3) (Fig. 2), much higher than observed on Mars before. However, a corresponding increase in Cl, which is typical for all soils (79), was not observed in this outcrop. During the next weeks, several measurements were performed on various outcrops. Typically, the same spot was analyzed before (surface as is) and after application of the Rock Abrasion Tool (RAT) (10). At a location referred to as McKittrick on the outcrop El Capitan, the observed S concentration increased from 5.1 wt % in the as-is analysis to 8.4 wt % (or 21 wt % SO3) in the RAT analysis (Fig. 2 and Table 2). An increase in S after surface removal was a common observation on these outcrops.

Fig. 2.

Sulfate (yellow) and iron (red) concentrations in soils, spherules, and outcrops in Meridiani Planum.

Analyses before and after abrading of McKittrick showed Br concentrations of 270 and 430 parts per million (ppm), respectively. When compared with the observed Cl concentrations, these concentrations are unusually high, because in most martian meteorites the Cl/Br ratio is close to the ratio in carbonaceous chondrite type 1 (CI) of 270 (11) (Fig. 3). Thus, the ratio of 9.1 in the McKittrick RAT measurement corresponds to an enrichment of Br over Cl relative to CI chondrite by a factor of 30. Another analysis area, called Guadalupe, about 20 cm higher on the El Capitan outcrop, was even richer in S than McKittrick but did not exhibit any detectable enrichment of Br relative to Cl (Figs. 2 and 3).

Fig. 3.

Chlorine versus bromine concentrations in samples of Meridiani Planum are compared with martian meteorites (19), seawater, and terrestrial salts (20).

The wheels of Opportunity were used to excavate a trench in the soil. A difference in chemical composition, especially of Fe and Br, was observed between a location on undisturbed soil (Hematite Slope) and two locations inside the trench Big Dig, one on the wall and one on the floor (Table 1 and Fig. 2). Although the concentration of Br in the undisturbed soil was at the detection limit of the APXS (about 30 ppm), it was detectable on the wall and the floor of the trench Big Dig (Fig. 3) with Br enrichment factors of about 11 relative to the Cl/Br ratio of CI chondrites.

The landing site is covered with spherical grains nicknamed blueberries. APXS analyses were made on an outcrop called Berry Bowl. One half of this rock had a flat surface (Berry Bowl empty), and the other showed a depression filled with a large number of spherules (Berry Bowl full, see Table 2 and Fig. 2). Berry Bowl empty revealed concentrations similar to the outcrop McKittrick before grinding (McKittrick as is in Fig. 2) with a high SO3 content of 14.0 wt %. The APXS analysis of Berry Bowl full shows an unusually high Fe content of 24.1 wt % (31.0 wt % FeO equivalent) and a low SO3 content of 5.5 wt %, which is similar to the soil on Hematite Slope (Fig. 2). However, part of the spectrum originated from the Berry Bowl outcrop material and a detritus of dark sand (possibly basaltic grains) because the spherules do not completely fill the area of the bowl. The Fe/Mn ratios of Berry Bowl full (Fe/Mn = 107) and of the soil on Hematite Slope (Fe/Mn = 82) are higher than the ratios measured in other soils and outcrops (Fe/Mn ∼ 50). Because Mn2+ has chemical characteristics similar to Fe2+, a high Fe/Mn ratio can result from the presence of oxidized Fe3+-bearing minerals. Mössbauer spectra of the spherules indicate that hematite (Fe2O3) is the major Fe-bearing mineral (12). Because the high Fe/Mn ratios are present only in undisturbed hematite-rich soils (Fe/Mn ∼ 80 to 120) overlain with spherules but are absent in the excavated trench (Fe/Mn ∼ 50), where spherules are rare, the APXS results support the findings from other instruments (3) that the spherules are the carrier of hematite.

Except for Fe, S, and Br, the soils and outcrops show similar chemical compositions (Figs. 1 and 2). However, because of a difference in the Fe2+ and Fe3+ concentrations, the Mössbauer results (12) indicate a different mineralogy between hematite-free soils (high Fe2+) and outcrops (high Fe3+). On the basis of the similar Fe/Mn ratios in rocks and these soils, the difference in mineralogy can be explained by an isochemical change resulting from a change in the oxidation state of iron.

The compositional variations between soils and outcrops reflect the enrichment of S in the outcrops. However, the Mg/Si, K/Si, and P/Si ratios increase with increasing S concentrations, whereas Cr/Si decreases (Fig. 4). Overall, therefore, the chemical evidence suggests the deposition of sulfate salts such as MgSO4. Hydrated Mg sulfates have been assumed (13) to explain low-latitude hydrogen-rich deposits on Mars. In addition, all K can be attributed to Jarosite (12). The enrichment of P points toward the presence of soluble phosphates.

Fig. 4.

Element/Si ratios in outcrops (normalized to their ratios in soil Tarmac) versus concentration of SO3 (equivalent to the relative amount of sulfate salts in the samples). The graph shows all outcrop targets of Table 2 except Berry Bowl full.

Although more detailed studies would be necessary to unravel the origin of the outcrop rocks, with the available data we visualize the following scenario: Water, containing sulfuric (and hydrochloric) acid derived from volcanic exhalations reacted with rocky material to form brines. According to (14), such brines could even exist today. Olivine dissolved readily under these acidic and oxidizing conditions and Mg and Fe sulfates and also silica would have been formed. In contrast to olivine, pyroxene and feldspar dissolved to a much lesser extent. Because Cr is low in olivine, it is also low in the outcrops. Phosphates dissolved even more easily as reflected by the high P content of the outcrops (Fig. 4). Hence, we would expect high concentrations of large-ion lithophile elements such as U, Th, and the rare earth elements. Thus, in general, elements mainly occurring in soluble phases should be enriched and elements mainly contained in insoluble phases should be depleted in the outcrops relative to soil (Fig. 4). Because of evaporation, the brines gradually became richer in sulfates that eventually started to precipitate. Sulfur and Cl are decoupled in outcrop rocks, which can be explained by the higher solubility of chlorides in brines. The most soluble components, the rare bromides, became enriched in residual brines. Evaporitic materials were intermingled during the deposition process with possibly airborne siliciclastic materials (3) to produce the bulk chemistry observed by the APXS. This set of events, alternating with accumulation of fresh windblown soil, occurred in episodes over an extended period of time, explaining the large-scale multiple layering observed in these rocks.

The chemical processes described above are not limited to the small Eagle crater where Opportunity landed. A few hundred meters away from Eagle crater, in a smaller crater called Fram, the rock Pilbara was abraded with the RAT and analyzed with the APXS. It has a composition similar to the outcrop Guadalupe in Eagle crater (Table 2).

After leaving Eagle crater, Opportunity examined a nearby rock, named Bounce rock, with a textural appearance different from the outcrops in Eagle crater. Two areas on the untreated rock surface and one area that had been abraded with the RAT were analyzed by all Athena instruments (15). The APXS analyses show that Bounce rock has a chemical composition of basaltic rocks more evolved than Humphrey, a primitive basaltic rock typical of rocks found in Gusev crater (7, 16). It is characterized by high P2O5 of 0.95 wt %, a S concentration of 0.2 wt %, an Fe/Mn ratio of 36 [consistent with that of shergottites (17) (Fe/Mn = 36 to 44)], a low Mg number [molar MgO/(MgO+FeO)] of 0.42, and a high Ca/Al ratio of 1.7 (18). Although variable in shergottites, the Al2O3 concentration and Mg number of Bounce rock are identical with lithology B of the shergottite EETA79001 (19) within error limits.

Bounce rock, however, has a lower FeO concentration (15.6 wt %) and a higher CaO concentration (12.5 wt %) than those of shergottites. It has a calculated mineral norm that is pyroxene normative: (in wt %) magnetite 0.25, ilmenite 1.4, chromite 0.2, apatite 2.0, albite 12.7, anorthite 22.0, diopside 31.1, and hypersthene 30.5. This is the first instance in which a sample measured on Mars has a chemical composition that is close to a group of meteorites that has long been believed to have originated from Mars.

References and Notes

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