Lunar and Planetary Science XXXVII (2006)
2401.pdf
ORIGIN OF ROCKS AND COBBLES ON THE MERIDIANI PLAINS AS SEEN BY OPPORTUNITY. B. L. Jolliff1, W. H. Farrand2, J. R. Johnson3, C. Schröder4, C. M. Weitz5, and the Athena Science Team. 1 Department of Earth and Planetary Sciences and the McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130 (
[email protected]), 2Space Science Institute, 4750 Walnut St. #205, Boulder, CO 80301, 3 U.S. Geological Survey, 2255 N. Gemini Dr., Flagstaff, AZ 86001, 4Institut für Anorganische und Analytische Chemie, Johannes Gutenberg-Universität, Mainz, Germany; 5Planetary Science Institute, Tucson, AZ 85719. Introduction: At the Meridiani landing site and areas thus far investigated by Opportunity, numerous cobbles (e.g., rock fragments >1 cm) have been observed. Such rock fragments tend to cluster in troughs between ripples on the plains and are notably abundant on rocky surfaces in and around impact craters [1], especially north of Erebus, a ~300 m diameter, highly degraded crater, where Opportunity has been working since about sol 550. Cobbles are an important component of the surface materials and they represent potential source rocks for finer soil components as well as potential representatives of overlying or buried stratigraphy. To date, only about a half dozen such rocks have been analyzed with the in-situ instruments; however, through a combination of information from these analyses and corresponding Pancam multispectral analyses [1,2], lithologic types in sight of the rover can be inferred. Thus far, the cobbles represent a variety of materials including basalt, two meteorites, Meridiani sulfate-rich outcrop material, and possible impact breccias. Data Set: Cobbles and rocks discussed here include Bounce Rock (sols B066-B070), Lion Stone (sols B106-B108), Barberton (sol B122), Heat Shield Rock (sol B349), Russet (sol B381), Arkansas (sol B551), Perseverance (sol B554), and Antistasi (sol B641). Bounce Rock is a pyroxene-rich rock of basaltic mineralogy and composition. Its composition and mafic mineralogy are very similar to those of EETA79001 lithology B and Zagami [3,4]. It occurs outside of Eagle crater, isolated on the very flat Meridiani plains (Fig. 1a). No other rock similar to Bounce Rock has been anaBounce Rock
(a)
(b)
~35 cm
Lion Stone Sol 104
Sol 065 Barberton
McDonnell
3 cm
Sol 121
(c)
(d)
Russet
Sol 381
Figure 1. Composite of rocks and cobbles at Meridiani. Note airbag mark below Bounce Rock and Mössbauer face-plate impression at soil target McDonnell.
lyzed with the in-situ instruments, and compositional data for all Meridiani materials to date indicate that Bounce Rock is exotic to the site, delivered as secondary impact ejecta [4]. Its composition is not suitable as a source for the basaltic component of the Meridiani sands, e.g., Millstone_Dahlia (sol B166) and Auk_RAT (sol B237), especially its high Ca and Na, and low K contents relative to the basaltic sands. Lion Stone (Fig. 1b), which sits on the rim of Endurance crater, and Russet (Fig. 1d), which lies on the plains ~800 m south of Endurance and near the 10-m crater Jason, are outcrop rocks, delivered to their present locations as crater ejecta. Their compositions are essentially those of sulfate-rich outcrop rocks measured in place in locations near where they occur. Barberton was found just beyond the southern rim of Endurance crater (Fig. 1c). Its composition is unlike any other materials analyzed by Opportunity. It is rich in Mg and Ni and it is especially poor in Al and Ca. Analysis of this small cobble likely included some dust and surface soil. Subtraction of soil/dust yields a composition for Barberton that is similar to chondritic meteorites. Heat Shield Rock is clearly an iron meteorite with kamacite in its Mössbauer spectrum [5]. Cobbles are relatively abundant on outcrop “pavement” surfaces approaching Erebus from the north and northwest. Three cobbles have been analyzed in this area, including Arkansas, Perseverance, and Antistasi (Fig. 2). These cobbles drew attention because their Pancam spectra indicated lithologic differences from typical sulfate-rich outcrop materials. In-situ measurements revealed that each of these has substantially less S than the outcrop lithology (Fig. 3). Arkansas has ~9 wt% S as SO3, Perseverance, ~6, and Antistasi, ~3. Although moderately rich in S, Arkansas has a composition that sets it apart from weathered surfaces or dustcovered outcrop rocks, which typically have compositions intermediate to soil and freshly exposed (RATground) outcrop counterparts. Perseverance has a soillike composition, but with an unusual enrichment in P relative to Meridiani soils (Fig 3d). Antistasi has a composition that lies between soil and basalt, and for many elements, it is similar to the natural surface of Bounce Rock; however, it has significantly higher Al and Na, higher Mg, and lower Fe and Ca. Compared to soils, it also has higher P, a characteristic shared with both Arkansas and Perseverance (Fig. 3). Microscopic images of the three cobbles appear similar and reveal areas that may have clast-in-matrix breccia texture (Fig. 2c). Pan-
Lunar and Planetary Science XXXVII (2006)
2401.pdf
Soil Bright soil Spherule-rich Fract fill/rind Rk w/soil Rk nat surf Rk brushed Rk RAT'd Cobble Barberton Bounce Rock
40
Spherulerich materials
35 30
Fe2O3
25
Bright Soil
20
Bounce 15
Lion Stone
Nat Rk surface
Soil on Rock
Barberton 14.0 12.0
Perseverance Arkansas
Bounce
Lion Stone
2.0
0
5
10
15
20
25
0
5
10
15
Antistasi Cobbles Bounce
Al2O3
Soil 8
Spherulerich materials
6
Soil on Rock
Nat Rk surface
Barberton
RAT'd Rocks
4
30
Cobbles
1.2
Soil on Rock
1.0
Soil Bounce
0.8
Bright Soil
Nat Rk surface
RAT'd Rocks Soil Bright soil Spherule-rich Fract fill/rind Rk w/soil Rk nat surf Rk brushed Rk RAT'd Cobble Barberton Bounce Rock
Barberton
0.6
0.4
2
0.2
0
0.0 0
5
10
15
SO3
20
25
30
0
5
10
15
20
25
30
SO3
Figure 3. Compositional plots of APXS data for Meridiani materials.
(c)
Sol 641 Antistasi
25
1.4
P2O5
Soil Bright soil Spherule-rich Fract fill/rind Rk w/soil Rk nat surf Rk brushed Rk RAT'd Cobble Barberton Bounce Rock
14
10
20
SO3
Sol 551 Arkansas 12
Soil Bright soil Spherule-rich Fract fill/rind Rk w/soil Rk nat surf Rk brushed Rk RAT'd Cobble Barberton Bounce Rock
0.0
30
SO3
Sol 641 Antistasi
RAT'd Rocks
4.0
0
(b)
Nat Rk surface
Soil on Rock
Soil
8.0
6.0
Russet (Cobble Nat surf)
5
Russet
Cobbles 10.0
RAT'd Rocks
Antistasi
10
(a)
Soil
Barberton
16.0
MgO
Sol 554 Perseverance
Sol 554 Perseverance
Figure 2. Cobbles near Erebus crater.
cam spectral analysis also suggests that these cobbles may be variably coated. They exhibit spectral variations ranging from basalt-like to sulfate-rich outcrop. Discussion: The observations to date suggest diverse hypotheses for the origins for cobbles at Meridiani [see 1], including the following: (1) Cobbles may be fragments of meteorites (Barberton, Heat Shield Rock). (2) They may be fragments of secondary impact ejecta delivered from outside the local region (Bounce Rock). (3) They may be locally derived ejecta (Lion Stone and Russet). (4) They may be erosional remnants of a layer that once lay above the presently exposed outcrops. (5) They may be erosional remnants of a deeper layer within the underlying but local stratigraphy, brought to the surface as impact ejecta. (6) They may be impact melts of the outcrop lithology or a mixture of the outcrop lithology and underlying strata. (7) They may be resistant material eroded from the sulfate-rich outcrop, e.g., rinds and fracture fillings. The possibility that meteorites may be common at the surface includes the finding of the large iron meteorite, Heat Shield Rock, and the abundance of Ni in surface soils [6]. Barberton appears to be an example of a chondrite. Although, it is the only one thus far analyzed in-situ, cobbles with similar Pancam spectra have been seen. Bounce Rock is a clear case of impact ejecta delivered from a far distant source crater. From its compositional and mineralogical differences from other materials, it appears to have no local relations. Lion Stone and Russet are obvious cases of impact-ejected outcrop material of the Burns formation [7].
The origins of the cobbles near Erebus are not yet clear and could involve (4) through (7) above. If derived from now eroded overlying strata (4), cobbles should be present in similar abundance everywhere on the plains; instead, they increase in abundance near Erebus. This observation supports (5) because such materials should be more abundant near craters large enough to excavate the layer, especially if the materials were more resistant to erosion than the more sulfate-rich strata. This case would be important because these materials would represent the erosional remnant of “inverted” stratigraphy (5). At Erebus, such a layer may or may not be exposed in the present rim scarp, which has not been seen up close as of this writing. Likewise, impact breccia (6), which could be a mixture of underlying strata, should also be most abundant in proximal crater ejecta. Textures of the three cobbles investigated near the rim of Erebus suggest that they could be impact breccias, and their compositions suggest they would be more resistant to weathering than the sulfate-rich outcrop rocks. Untangling their compositions and precursor rock components may provide clues to the lithology of underlying strata that are not otherwise exposed. Finally, Pancam imaging supports the origin of some cobbles as bits of weathering rinds and fracture-fill material (7); however, to date, these materials have not been fully characterized with the in-situ instruments. Acknowledgments: The MER instrument and engineering teams are thanked for their dedication to the rovers and tireless efforts to generate and deliver the data. Funding for this work is through NASA support of the MER Athena science team. References: [1] Weitz et al., JGR submitted, 2006; [2] Farrand et al., LPSC 37, This Vol., 2006; [3] Klingelhöfer et al., Science 306, 2004; [4] Zipfel et al., MAPS 39, A118, 2004; [5] Rodionov et al., Geophys. Res. Abs. 7, EGU, 2005; [6] Yen et al., Nature 436, 2005. [7] Grotzinger et al., EPSL 240, 2005.
