Geologic mapping and characterization of Gale Crater and implications for its potential as a Mars Science Laboratory landing site

Background: Gale Crater is located at 5.3°S, 222.3°W (137.7°E) and has a diameter of ~155 km. It has b een a ta rget o f p articular i nterest d ue to the > 5 k m t all m ound o f layered material that occupies the center of the c rater. Gale Crater is currently one of four finalist landing s ites for the Mars Science Laboratory rover. Method: We used visible (CTX, HiRISE, MOC), infrared (THEMIS, CRISM, OMEGA) and topographic (MOLA, HRSC, CTX) datasets and data products to conduct a study of Gale Crater, with a particular focus on the region near the proposed Mars Science Laboratory (MSL) landing site and traverse. Conclusion: The rim of Gale Crater is dissected by fluvial channels, all of which flow into the crater with no obvious outlet. Sinuous ridges are common on the crater floor, including within the proposed MSL e llipse, and are interpreted to be inverted channels. Erosion-resistant polygonal ridges on the mound are common and are interpreted as fractures that have been a ltered or cemented by f luid. We identified key geomorphic units on the northwestern crater floor and mound, and present a simplified stratigraphy of these units, discussing their properties and potential origins. Some layers in the mound are traceable for >10 km, suggesting that a spring mound origin is unlikely. We were unable to rule out a lacustrine or aeolian origin for the lower mound using presently-available data. Pyroclastic processes likely have contributed to the layers of the Gale mound, but were probably not the dominant depositional processes. The upper part of the mound exhibits a pattern that could be cross-bedding, w hich w ould suggest a n a eolian dune-field origin f or tha t uni t. A eolian tr ansport appears to be the most plausible m echanism for removal of m aterial from the crater without breaching the rim; however, fluvial, mass-wasting, or periglacial processes could have contributed to the breakdown of material into f ine grains susceptible to aeolian transport. We have identified two potential traverses for MSL that provide access to the d iverse features on the crater f loor and the mound. We discuss the suitability of Gale Crater as a landing site for MSL in terms of diversity, context, ha bitability a nd b iomarker p reservation a nd conclude tha t Gale C rater would b e a scientifically rewarding and publicly engaging landing site. Introduction and previous work Gale Crater is located at 5.3°S, 222.3°W (137.7°E) and has a diameter of ~155 km. It is situated in the northeastern portion of the Aeolis quadrangle on the boundary between the southern cratered highlands and the lowlands of Elysium Planitia (Figure 1), and the crater has been estimated to be Noachian in age (~3.5–3.8 Ga) (Greeley and Guest 1987; Cabrol et al. 1999; Bridges 2001). Gale has been a target of particular interest due to the mound of material that occupies the center of the crater, standing ~6 km higher than the lowest point on the floor. The age of the mound has been loosely constrained to the late Noachian/early Hesperian (Milliken et al. Anderson and Bell III: Mars 5, 76-128, 2010 77 http://marsjournal.org 2010). Gale Crater was considered as a potential landing site for the Mars Exploration Rovers (MER; Golombek et al. 2003) and is currently one of four finalist landing sites for the Mars Science Laboratory (MSL) rover (Golombek et al. 2009). Early maps based on Viking data list a wide range of potential origins for the material in Gale Crater. Scott et al. (1978) interpreted the material as lava flows and aeolian deposits, Greeley and Guest (1987) suggested volcanic, aeolian or fluvial sedimentation, and Scott and Chapman (1995) invoked aeolian, pyroclastic, lava flow, fluvial and mass-wasted deposition. Cabrol et al. (1999) used Viking images, a Viking topographic map and several early Mars Orbiter Laser Altimeter (MOLA) profiles to suggest that Gale Crater may have hosted a lake intermittently from its formation in the Noachian until the early to middle Amazonian, and to speculate that it could have provided diverse environments for martian life, ranging from warm hydrothermal waters shortly after the crater-forming impact, to cold, ice-covered water at later times. Malin and Edgett (2000) identified Gale Crater as one of a class of partially filled impact craters on Mars. They cited the fact that the peak of the Gale mound is higher in elevation than some portions of the crater rim to suggest that the entire crater was filled with layered material that was subsequently eroded. Malin and Edgett (2000) also identified an erosional unconformity on the mound, suggesting at least two episodes of net deposition and a significant amount of erosion. Malin and Edgett (2000) also discussed a number of possible origins for the strata observed in Gale and other filled craters. Pyroclastic deposits were discussed but determined to be an unlikely source because terrestrial deposits thin very rapidly with distance from the source, and most of the layered rocks on Mars are far from potential volcanic vents. Impact ejecta was likewise ruled out because it rapidly thins with distance from the impact and therefore, to form thick deposits like the Gale Crater mound, would require "prodigious quantities" (Malin and Edgett 2000) of material. Aeolian deposition was considered a possible source if processes could be identified to explain the large volume of layered material and the apparent periodic nature of the layers in many deposits. Ultimately, Malin and Edgett (2000) favored a lacustrine origin for the layered material, citing the thickness and rhythmic nature of many layered deposits across the planet and their affinity for closed basins such as craters. Pelkey and Jakosky (2002) conducted a study of Gale Crater using data from the Mars Global Surveyor (MGS) MOLA and the Thermal Emission Spectrometer (TES), as well as other Viking Orbiter and MGS Mars Orbiter Camera (MOC) data. They found evidence for a thermally thick dust layer on the upper mound which thins to reveal darker, higher thermal inertia material. They interpreted the northern crater floor as a dust-covered, cemented mantle, while the southern crater floor had little dust cover and variable terrain. They also found that the sand sheet in Gale Crater had a higher than expected thermal inertia and suggested some combination of coarse grain size, induration or inhomogeneities in the field of view as an explanation. They suggested that dark-toned material may be transported from the southeast into the southern Figure 1. Global topographic map of Mars, based on MOLA data (Smith et al. 1999). The black arrow marks the location of Gale crater. (figure1.jpg) Anderson and Bell III: Mars 5, 76-128, 2010 78 http://marsjournal.org portion of Gale Crater and then northward around the mound. Pelkey and Jakosky (2002) concluded that interpreting the surface of Gale Crater is not straightforward, but that the surface layer varies considerably, likely due to multiple processes, and that aeolian processes have likely been important in shaping the surface. In a subsequent paper, Pelkey et al. (2004) added Mars Odyssey Thermal Emission Imaging System (THEMIS) thermal inertia and visible observations to their analysis. They confirmed the observations of Pelkey and Jakosky (2002) that dust cover increases with altitude on the Gale mound and that aeolian processes have played a significant role in shaping the current surface of the crater and mound. They also noted that the numerous valleys in the crater wall and mound support hypotheses for aqueous processes in Gale Crater, and that the valleys likely postdate any deep lake in the crater because they extend down to the crater floor. Thomson et al. (2008) interpreted ridges and fan-shaped mesas on the mound and crater rim as inverted fluvial channels and alluvial fans. They noted that there is no obvious change in slope to explain the transition from some inverted channels to fan-shaped features and suggested that this could be explained by a stream encountering a slower-moving body of water and depositing its sediment load as a fan. They also suggested that the upper mound material may be related to a widespread layered, yardang-forming unit known as the “Medusae Fossae Formation” (MFF). Recently, Zimbelman et al. (2010) have also mapped the Gale Crater mound as part of the MFF. Rossi et al. (2008), citing unconformities in the mound, a relatively young crater retention age, and claiming that there is "no or little evidence of fluvial activity in the immediate surroundings of the craters hosting bulges and within their rim" have hypothesized that the Gale Crater mound has a local origin as a large spring deposit. Rogers and Bandfield (2009) analyzed TES and THEMIS spectra of the dunes on the floor of Gale Crater and interpreted the results to indicate that they have a composition similar to olivine basalt, consistent with the Hamilton et al. (2007) decorrelation stretch mosaic of Gale Crater, in which mafic materials are displayed as magenta (Figure 2c). Analysis of OMEGA and CRISM observations confirm the presence of mafic minerals such as olivine and pyroxene in the dunes (Milliken et al. 2009). Although it was not chosen as a MER landing site, Gale Crater has remained a high-interest location for a landed mission. It was proposed as a landing site for MSL at the first landing site workshop (Bell et al. 2006; Bridges 2006). The proposed MSL landing site is located on top of a large fan-shaped feature (Bell et al. 2006) which extends to the southeast from the end of a valley at the base of the northwestern crater wall. Numerous Figure 2. (a) HRSC shaded relief map of Gale crater, based on observations H1916_0000, H1927_0000, and H1938_0000. The proposed MSL landing ellipse is located in the NW crater floor. The lowest elevation in the crater is marked with an arrow. (b) THEMIS thermal inertia map of Gale crater (Fergason et al. 2006). (c) THEMIS decorrelation stretch map of Gale crater, using bands 8, 7 , and 5 for red, green and blue, respectively. (Hamilton et al. 2007) The THEMIS maps do not cover the eastern and southern rim of the crater. (figure2.jpg) Anderson and Bell III: Mars 5, 76-128, 2010 79 http://marsjournal.org presentations at subsequent workshops have made a case for Gale Crater based on the exposure of a >5 km-thick sedimentary sequence, the numerous fluvial features on the mound and crater walls, and the detection of phyllosilicates and sulfates in the layered mound near the landing site (e.g., Thomson et al. 2007; Thomson and Bridges 2008; Milliken et al. 2008). Prior to the detection of hydrated minerals in Gale Crater, the site was interesting primarily for its geomorphology. However, the discovery of phyllosilicates and sulfates correlated to stratigraphic units in the northwestern mound, including the specific identification of the mineral nontronite (suggesting a moderate pH and possibly reducing conditions at the time of formation), have made Gale a more appealing site in terms of potential habitability and biomarker preservation (Milliken et al. 2009). In addition, the strata of the Gale mound appear to trend from phyllosilicate-bearing lower layers to sulfate and oxide-bearing middle layers to relatively unaltered upper layers (Milliken et al. 2010). Bibring et al. (2006) have proposed a global transition in climate and weathering on Mars that predicts a period of moderate pH and phyllosilicate production, followed by a period of acidic weathering with sulfate production and concluding with an era of superficial weathering to ferric oxides. It is possible that the layers of the Gale mound record this transition (Milliken et al. 2010) and can be used to test this hypothesis. Despite interest in Gale Crater as a potential landing site, the origin of the mound remains enigmatic. We have made observations from multiple datasets in an attempt to evaluate mound-origin hypotheses and to better describe the geomorphic units that a) appear to be significant in the stratigraphic sequence and b) that MSL would be likely to encounter if Gale were chosen as the landing site. As we will show, Gale Crater exposes a rich and diverse Martian history, and it is likely that a combination of depositional and erosional environments must be invoked to explain the features that are visible today.