Opportunity Mars Rover mission: Overview and selected results from Purgatory ripple to traverses to Endeavour crater

Opportunity has been traversing the Meridiani plains since 25 January 2004 (sol 1), acquiring numerous observations of the atmosphere, soils, and rocks. This paper provides an overview of key disco ...


Introduction 24
The Mars Exploration Rover (MER) Opportunity touched down on the Meridiani 25 plains on January 25, 2004 (Figs. [1][2]. Since landing, Opportunity has conducted 26 numerous traverses and made extensive measurements with its Athena science payload 27 (Table 1), including examination of impact crater ejecta deposits, rims, and walls to 28 access and characterize stratigraphic rock sections within the Burns formation [e.g., 29 Squyres and Knoll, 2005], detailed examination of a variety of cobbles and boulders 30 exposed on the surface, and characterization of the aeolian ripples that partially cover 31 plains outcrops. In addition, numerous atmospheric opacity and cloud measurements have 32 been acquired using Pancam and Navcam, and the Alpha Particle X-Ray Spectrometer 33 (APXS) has been used to monitor atmospheric argon mixing ratios. 34 The purpose of this paper is to summarize operations and present selected 35 scientific highlights from the time Opportunity left the Purgatory* aeolian ripple on sol 36 511 (July 1,2005) to the first relatively high spatial resolution views of the Endeavour 37 crater rim on sol 2300 (July 13, 2010; Fig.1 and Table 2). The paper also includes a 38 synthesis of orbital and rover-based data for areas along and close to Opportunity's 39 traverses for interpretations of material properties, morphology, and geologic histories on 40 both local and regional scales. This overview is meant to complement papers that provide 41 detailed findings from Opportunity's measurements that are included as the fourth set of that underlie the ripples were found to be largely ancient aeolian sandstone deposits, with 68 local reworking within ephemeral lakes [Squyres et al., 2004;Grotzinger et al., 2005, 69 2006; Metz et al., 2009]. Post depositional aqueous processes have altered the deposits as 70 evidenced by the presence of hematitic concretions and fracture-filling deposits 71 [McLennan et al., 2005;Knoll et al., 2008]. Opportunity has continued to search for the 72 mud-rich rock facies that would help confirm the hypothesis that the sands formed from 73 precursor evaporates in a playa lake environment. 74 The Burns formation rocks examined by Opportunity are part of a regional-scale 75 deposit that covers several hundred thousand square kilometers and is best explained by 76 accumulation during one or more periods of rising ground water [e.g., Andrews-Hanna et 77 al., 2010] (Fig. 1). These deposits are draped unconformably onto dissected cratered 78 terrain and exhibit an impact crater size frequency distribution indicative of a preservation 79 age of Noachian or Early Hesperian [Arvidson et al., 2006]. Pre-existing craters are also 80 evident and show partial burial by the sedimentary deposits, including the ~20 km 81 diameter Endeavour crater toward which Opportunity is traversing (Fig. 1). Bopolu is a 82 ~19 km diameter crater located to the southwest of Opportunity (Fig. 1). This crater 83 clearly post-dates the deposition of the sulfate-rich sedimentary deposits, given that the 84 floor and rim of the crater, along with its ejecta deposits, exhibit basaltic signatures 85 [Christensen et al., 2001] and the ejecta deposits extend over the sedimentary deposits 86 ( Fig. 1). Bopolu, and other rayed craters on Meridiani Planum, lack hematite signatures on 87 their ejecta deposits, implying that these craters formed after the hematite was 88 concentrated on the surface   Detailed measurements conducted by Opportunity in Eagle and Endurance craters 90 showed the utility of impact craters for assessing the stratigraphy of the layered 91 to escape during launch. Consequently, useful MI images can no longer be acquired with  138   the dust cover closed, and images taken with the dust cover open are visibly affected by  139   dust contamination. This contamination has reduced the signal/noise in Opportunity MI  140 images, but has not affected the ability to retrieve useful textural information from the 141 data. Unfortunately, dust accumulation on the Mini-TES exterior mirror compromised the 142 ability to retrieve quantitative information about mineralogy from data acquired after ~sol 143 1217. The instrument ceased responding to commands from the rover on sol 2257. 144 The Alpha Particle X-Ray Spectrometer (APXS) has continued to operate 145 nominally, acquiring compositional information for soils and rocks and making 146 measurements of atmospheric argon. The Mössbauer Spectrometer (MB) has also 147 continued to acquire data for soils and rocks, although significant decay of the cobalt-57 148 radioactive source (271.79 day half life) eventually required measurements extending 149 over several sols to achieve appropriately high spectral signal to noise values. The Rock 150 Abrasion Tool (RAT) has ground into 38 rocks over the course of the mission. During the 151 period covered by this paper 18 rocks were brushed and 15 were ground. By sol 2300 the 152 grinding bit pads were worn to ~20 to 30% of their original thicknesses and the brush was 153 slightly bent, no longer sweeping out a complete circle. All three RAT encoder motors 154 have stopped operating, leading to step-by-step manual approaches for commanding the 155 RAT to avoid a brush or grind failure or damage to the instrument. 156 Overview of Mission Activities: Table 2 provides a sol-by-sol description of  157 Opportunity's activities. Fig. 3 shows a timeline of major activities with available solar 158 panel energy and atmospheric opacity on a sol-by-sol basis. Traverses have been mainly 159 from north to south, with stops in or near craters to examine rock strata and other "jogs" 160 to examine important science targets or avoid large ripples (informally termed 161 "purgatoids" after the Purgatory ripple, where Opportunity was embedded between sols 162 446 and 484) (Fig. 2). By ~sol 2250, after bypassing several fields of purgatoids, 163 Opportunity was able to start traversing southeast, directly toward the rim of Noachian-164 aged Endeavour crater, with the intent of exploring the ancient rocks exposed on the 165 crater's rim. 166 The initial primary science target for Opportunity, after leaving Purgatory, was 167 Erebus, a ~220 m wide, degraded impact crater with ripples surrounding the crater and 168 occupying most of the crater floor (Fig. 2  . This led to a southern to southwestern 194 path south from Victoria to avoid the purgatoid fields, and then turning to the southeast on 195 a more direct route to the rim of Endeavour (Fig. 2). 196

Modern Atmospheric Processes 197
In addition to addressing the primary mission themes of characterizing past 198 environmental conditions, and the role of water in formation and alteration of crustal 199 materials, Opportunity has continued to make periodic measurements that pertain to Winter atmospheric measurements for science have focused on sky imaging to 216 detect the well-known winter aphelion water ice cloud "belt" [e.g., Clancy et al., 1996]. 217 Fewer clouds were observed during the three periods near aphelion than would have been 218 predicted from Earth-based and orbital observations [i.e., Wolff et al., 1999]. The paucity 219 of clouds suggests that the meteorology of the Meridiani region may be more complex 220 (and thus more interesting) than previously understood. 221 When not in use to measure compositions of rocks and soils, and when rover 222 energy permitted, APXS has been used to monitor seasonal and inter-annual variations in 223 atmospheric argon contents (Fig. 4). The argon mixing ratio is a tracer for atmospheric 224 transport because it is a non-condensable gas under Martian conditions. On the other 225 hand, the carbon dioxide content of the atmosphere varies significantly as a function of 226 season because it condenses over the winter pole to form the seasonal ice cap. The 227 southern winter is longer and colder than the northern winter season because the southern 228 winter season occurs near aphelion. The southern pole is also topographically higher than 229 the northern pole. Consequently, the southern winter carbon dioxide cap is more 230 extensive than the northern winter cap. These dynamics are evident in global pressure 231 variations recorded by the Viking Landers, with lowest surface pressures associated with 232 the southern winter [Tillman et al., 1993]. 233 Sprague et al. [2007] reported global variations in argon mixing ratios using 234 Odyssey gamma ray spectrometer data, focusing on the six-fold argon enhancement 235 during the winter over the southern polar region (-75 to 90° latitude). Argon mixing ratios 236 were found to peak over the south pole at L s =90 degrees and then undergo a rapid decline 237 to lowest values by the southern summer (L s =270 degrees). Argon mixing ratios for an 238 adjacent latitude band (-60 to 75 degrees latitude) showed a similar pattern, but shifted to 239 a slightly later time. Argon was not found to concentrate above the north polar winter 240 cap. These patterns were interpreted as evidence for meridional transport of argon with 241 carbon dioxide as part of the global atmospheric circulation system, particularly 242 combined with relatively weak south pole to equator transient eddies that cause build-up 243 of argon over the winter cap [Nelli et al., 2007]. cluster, a group of small impact craters spread over an area of ~ 120 m by 80 m, and is 301 located to the south of Victoria (Table 2). Rayleigh (and others within the cluster) must 302 have formed after the last major phase ripple migration ceased, since the crater cuts 303 across the ripples and exposes layers perpendicular to the ripple crest ( shear stresses between the wheels and soil were modeled using the classical Bekker-344 Wong terramechanics expressions that describe relationships among normal pressure, 345 applied wheel torque, wheel slip, and wheel sinkage as a function of soil properties [e.g., 346 Wong et al., 2003]: 347 where: σ is the normal stress and τ is the shear stress between the wheel and soil; k c /b is 350 the ratio of soil cohesion moduli to wheel width; k φ is the internal friction moduli, z is the 351 depth of wheel sinkage, n is a scaling exponent; c is the soil cohesion, φ is the soil angle 352 of internal friction, j is the slip value between the wheel and soil, and k x is the shear 353 deformation modulus in the longitudinal or drive direction. The value for j is determined 354 based on the magnitude of wheel sinkage into soil. 355 Increased wheel sinkage due to increased weight over a given wheel generally 356 leads to increased contact area between the wheel and soil and increased compaction 357 resistance, thereby increasing the amount of slippage, S, for a driven wheel as motor 358 torques are increased to compensate: 359 Where: V is the longitudinal velocity of the wheel, R is the wheel radius, and ω is the 361 wheel angular velocity. As slippage increases, additional sinkage generally occurs as soil 362 is moved in the direction of the spinning wheel. This further increases motion resistance 363 as the wheel comes in contact with additional soil during sinkage. At some point the 364 maximum soil shear stress before failure is reached and slippage becomes effectively 365 100%, causing longitudinal motion to cease. 366 The sol 2220 drive ended when visual odometry [Maimone et al., 2007] showed 367 ~58% wheel slip, which was above the limit set for continuing the drive. This was a 368 fortuitous event for Opportunity since on-board use of the imaging systems to track slip 369 was done every ~20 m or so during a traverse. The high slip occurred when Opportunity 370 was driving backwards and scaling the western side of the ripple at an acute angle (~25°) 371 relative to the ripple crest azimuth, with a rover tilt magnitude of ~8° (Figs. 12-13). The 372 ~58% slippage values occurred when six wheels were on the ripple. Wheel sinkage 373 measured from Navcam data taken after extrication from the ripple was ~5 cm. The front 374 wheel (i.e., on the downslope side) tracks show evidence of slip sinkage, based on 375 disruption of the cleat imprints (Fig. 13). found that the middle and rear wheels bore most of the weight and thus underwent the 393 most sinkage, thereby significantly increasing compaction resistance. Slippage increased 394 as torque was increased to maintain constant wheel angular velocity, leading to slip 395 sinkage, and exceeding the 58% slippage limit for continuing the drive. Opportunity was 396 able to back out of the ripple with one drive and was then commanded to continue to 397 drive south along an inter-ripple zone until a smaller ripple system was encountered to 398 cross over toward the east. 399

Cobble and Boulder-Sized Rock Fragments 400
During the mission period covered by this paper Opportunity has characterized a 401 number of individual rock fragments with a variety of sizes. These rocks have been found 402 near craters, in isolated clusters covering small to moderate (10s to 100s m 2 ) areas, and 403 sometimes as isolated pebbles, cobbles, or boulders separated by hundreds of meters. For 404 reference, Table 3 Table 3. 408 Five basic types of rock fragments were found and characterized in detail during 409 Opportunity operations: 1. Local impact ejecta that consist of sulfate-rich sedimentary 410 material (e.g., Chocolate hills, Figs. 14-15); 2. Basaltic materials that are likely impact 411 ejecta fragments (e.g., Bounce Rock) from distant sources; 3. Rock fragments that are a 412 mix of sulfate and basaltic materials that are likely impact melt products (e.g., Arkansas); 413 4. Stony-iron meteorites (e.g., Barberton); and 5. Iron-nickel meteorites (e.g., Block 414 Island, Fig. 16). 415 Chocolate Hills is an ejecta fragment from Concepción crater, a ~10 m wide, 416 relatively fresh impact crater located on the plains to the south of Victoria (Figs. 2, 14- Pancam false-color images (e.g., Fig. 16). For Heat Shield Rock, another Fe-Ni 456 meteorite located to the south of Endurance crater, most RAT-brushed surfaces exhibit 457 Pancam-derived spectra similar to laboratory spectra of the Canyon Diablo IAB meteorite 458 (Fig. 17). On the other hand, areas covered with purple coatings exhibit enhanced 535 nm 459 band depths, and more negative spectral slopes between 753 nm and 934 nm, as 460 compared to more typical natural or brushed surfaces on these meteorites (Fig. 17) paper Opportunity explored outcrops on the plains and ventured into Erebus and Victoria 501 craters to continue stratigraphic measurements designed to understand in more detail the 502 origin and environments of deposition that produced the layered sulfate rocks that 503 underlie the Meridiani plains (Fig. 1). Particular emphasis was placed on the search for 504 evidence of a sulfate-rich mud facies that might have been the source of the sandstones 505 encountered by Opportunity. Finding those deposits would allow confirmation or 506 rejection of the hypothesis that the sands were sourced in an evaporitic lake environment. 507 The first set of very detailed measurements focused on the Olympia outcrops 508 confirming that these features originated as clastic infillings of partings, later cemented to 519 provide differential resistance to erosion. The fill is chemically similar to bedrock 520 materials and not to modern soils. Taken together, the evidence suggests that these 521 features formed after the primary phases of deposition and diagenesis, but prior to 522 deposition of the modern soils (Knoll et al., 2008). 523 Victoria is the largest crater examined by Opportunity to date, ~750 m wide and 524 ~75 m deep. It was a primary target for exploration during the sols covered by this paper 525 because of the extensive Burns formation stratigraphic exposures on its walls (Figs. 2 and 526 20) [Squyres et al., 2009]. The approach to Victoria from the northwest allowed 527 traversing across the annulus surrounding Victoria, a planar region that was found to 528 consist of aeolian basaltic sands and hematitic concretions that partially cover the tops of 529 beveled ejecta blocks (Fig. 21). The ejecta deposit consists of relatively soft sulfate-rich 530 rocks evenly eroded by wind to form the planar annulus that surrounds the crater [Grant 531 et al., 2008]. Remote sensing of the crater wall from various promontories on the rim of 532 Victoria showed that blocky ejecta deposits dominate the upper few meters of wall rock 533 (Fig. 22). The ejecta blocks are layered, contain hematitic concretions, and have 534 coloration consistent with an origin as sulfate-rich bedrock. There is no evidence from 535 Victoria's wall rocks or ejecta that the impact event penetrated into the underlying 536 Noachian crust. 537 Duck Bay was chosen for entry into Victoria for detailed measurements because of 538 the extensive Burns formation outcrops and the relatively easy ingress and exit paths 539 (Fig. 23). Beneath the ejecta deposit exposed at Duck Bay are four discrete layers that 540 were examined using both remote sensing and in-situ instrumentation [Squyres et al., 541 2009]. Steno is the layer in contact with the ejecta and is underlain by a relatively bright 542 layer, Smith. Lyell and Gilbert are the next two layers examined during the Duck Bay 543 campaign. Steno consists of a fine to medium-grained sandstone with well-defined 544 laminae. Cross-bedding is evident, as are hematitic concretions. Steno is separated from 545 Smith by an unconformity. Smith is a relatively bright sandstone and exhibits fine-scale 546 laminations. Lyell is transitional with Smith and exhibits tabular, prismatic vugs, cross-547 bedding, and an abundance of hematitic concretions. Gilbert was only measured in one 548 location and the contact with Lyell is gradational. Lyell and Smith are also sandstones. 549 Pancam observations show that the Smith unit has an abrupt spectral down-turn at 1000 550 nm, consistent with the presence of the molecular water vibrational mode 2v1 + v3 and 551 3vOH for OH-bearing minerals (Fig. 23) [see: Rice et al., 2010]. No evidence was found 552 in any of the layers for the mud facies that might have been the source for the sulfate-rich 553 sandstones. In fact, the rocks examined in Victoria, both using remote sensing from 554 capes, and detailed measurements in Duck Bay, are best interpreted as sulfate-rich 555 aeolian sands altered and cemented by ground water infiltration [Squyres et al., 2009]. 556 In-situ data were acquired for undisturbed, brushed, and ratted rock targets within 557 each of the four stratigraphic layers in Duck Bay. Correspondence analysis shows the 558 importance of removing aeolian sand and dust covers and any coatings from these rocks 559 to understand their intrinsic characteristics (Fig. 24). In particular, the first factor in 560 APXS data, accounting for 92% of the variance of the data set, shows a trend from 561 basaltic to more sulfate-rich materials for natural, brushed, as opposed to ratted targets. 562 Ratted targets have the highest sulfur content and least contamination by coatings or 563 basaltic sands. The second factor, accounting for 4% of the variance, shows that the 564 ratted rock targets can be discriminated from one another on the basis of chlorine content, 565 with Gilbert showing the highest value, and Steno the lowest. A trend of increasing 566 chlorine content with increasing depth is also evident in a scatter plot of chlorine to silica 567 content as a function of depth (Fig. 25). On the other hand, the sulfur and magnesium 568 contents, relative to silica, both decrease as a function of depth beneath the surface (Fig.  569 26). These compositional patterns correlate well with the hydration index computed from 570 the depth of the 1000 nm band evident in the Smith unit (Fig. 24). Further, this bright, 571 upper unit appears to continue around the entire crater and can be fit with a horizontal 572 plane . 573 The rocks examined in Duck Bay, although still part of the Burns formation, are 574 separated laterally and topographically from the Karatepe section outcrops examined in 575 Endurance crater. Even so, the Karatepe section also shows a bright upper layer (above 576 the Whatanga contact) that is depleted in chlorine relative to silica and enhanced in 577 magnesium and sulfur relative to silica as compared to rocks exposed at greater depths 578

The Rim of Endeavour Crater and Adjacent Layered Sedimentary Rocks 588
Opportunity has thus far been exploring sedimentary rocks and soils that 589 unconformably overlie the Noachian crust (Fig. 1). Endeavour crater predates the 590 sedimentary deposits and the crater rim exposes materials of Noachian age (Figs. 1, 27-591 28). This is clear from geologic mapping and also initial spectral analysis of CRISM 592 hyperspectral data [Murchie et al., 2007] covering the rim and surrounding areas. 593 Specifically, Wray et al. [2009] showed from analysis of CRISM spectra covering 0.4 to 594 2.5 µm in wavelength that portions of the rim expose iron and magnesium-rich smectite 595 clay minerals. In addition, these authors showed that the sedimentary rocks adjacent to 596 the rim have spectra that are indicative of hydrated sulfates. 597 The long term objectives for the Opportunity extended mission are to drive to the 598 hydrated sulfate deposits and Noachian-aged rim materials of Endeavour. By sol 2239 the 599 rover was within ~11 km of the rim (Fig. 27). Pancam color and super-resolution imaging 600 (Fig. 28), combined with periodic collection of imaging data from the HiRISE, CTX, and 601 CRISM instruments, are helping to define traverses to locations where the hydrated 602 sulfate sedimentary rocks and altered rim materials are best exposed and accessible to 603   Emission spectra (5 to 29 µm, 10 cm -1 resolution) with 8 or 20 mrad FOV. Internal and external blackbody calibration targets. Instrument put in "stand down" mode on sol 2257 after failing to respond to commands.

Instrument Deployment Device (IDD)-Mounted In-Situ Package
APXS: Alpha Particle X-Ray Spectrometer 244 Cm alpha particle sources, and x-ray detectors, 3.8 cm FOV.
MB: Mössbauer Spectrometer 57 Fe spectrometer in backscatter mode; 57 Co/Rh source and Si-PIN diode detectors; field of view approximately 1.5 cm.

RAT: Rock Abrasion Tool
Tool capable of brushing surfaces and grinding 5 mm deep by 4.5 cm wide surface on rocks.