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In Situ Radiometric and Exposure Age Dating of The Research Articles using the instrument suite aboard the In Situ Radiometric and Exposure Age Curiosity rover exploring Gale crater. Geologic Setting Dating of the Martian Surface Gale is a ~150 km diameter impact crater that formed near the Late Noa- 1 2 2 1 chian - Early Hesperian boundary (5, , , , , K. A. Farley * C. Malespin P. Mahaffy J. P. Grotzinger P. M. 6), or around 3.7-3.5 Ga according to 3 4 5 5 2 Vasconcelos, R. E. Milliken, M. Malin, K. S. Edgett, A. A. Pavlov, impact crater models (3, 7). In addition J. A. Hurowitz,6 J. A. Grant,7 H. B. Miller,1 R. Arvidson,8 L. Beegle,9 to the ~5 km high central mountain of 9 2 10 2 stratified rock informally known as Mt. F. Calef, P. G. Conrad, W. E. Dietrich, J. Eigenbrode, R. Sharp, the crater is partially filled with 11 12 13 13 14 Gellert, S. Gupta, V. Hamilton, D. M. Hassler, K.W. Lewis, S. sedimentary rocks derived from the M. McLennan,6 D. Ming,15 R. Navarro-González,16 S. P. crater rim. Crater density distributions 17 18 1 19 imply a somewhat younger age (Early Schwenzer, A. Steele, E. M. Stolper, D. Y. Sumner, D. to Late Hesperian, or ~3.5-2.9 Ga) for 20 9 9 Vaniman, A. Vasavada, K. Williford, R. F. Wimmer- at least some of these deposits (5, 6). Schweingruber,21 and the MSL Science Team† While heading for its destination at Mt. Sharp, Curiosity traversed a gently 1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, sloping plain where local outcrops of USA. 2NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA. 3School of Earth Sciences, 4 pebble conglomerate were encountered, University of Queensland, Brisbane, Queensland, QLD 4072, Australia. Department of Geological recording the presence of an ancient Sciences, Brown University, Providence, RI 02912, USA. 5Malin Space Science Systems, San Diego, CA 6 7 stream bed (8). Most of this surface is 92121, USA. Department of Geosciences, Stony Brook University, Stony Brook, NY 11794, USA. Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, 6th at heavily cratered and covered with rock Independence SW, Washington, DC 20560, USA. 8Department of Earth and Planetary Sciences, and soil (Fig. 1). However, a substantial Washington University in St. Louis, St. Louis, MO, 63130, USA. 9Jet Propulsion Laboratory, California expanse of bare bedrock was encoun- Institute of Technology, Pasadena, CA 91109, USA. 10Earth and Planetary Science Department, University tered in an ~5 m deep topographic 11 of California, Berkeley, CA 94720, USA. Department of Physics, University of Guelph, Guelph, ON N1G trough (Yellowknife Bay) representing on December 9, 2013 2W1, Canada. 12Department of Earth Science and Engineering, Imperial College London, London SW7 13 14 an erosional window through a se- 2AZ, UK. Southwest Research Institute, Boulder, CO 80302, USA. Department of Geosciences, quence of stratified rocks known as the Princeton University, Princeton, NJ 08544, USA. 15NASA Johnson Space Center, Houston, TX 77058, USA. 16 17 Yellowknife Bay formation (Fig. S1 Universidad Nacional Autonoma de Mexico, Coyoacan, Mexico City 4510, Mexico. Department of Physical Sciences, CEPSAR, Walton Hall, Milton Keynes MK7 6AA, UK. 18Carnegie Institution, Geophysical and (9)). These rocks consist mostly of Laboratory, Washington, DC 20015, USA. 19Department of Geology, University of California, Davis, CA distal alluvial fan and lacustrine facies 95616, USA. 20Planetary Science Institute, Tucson, AZ 85719, USA. 21University of Kiel, Kiel D-24098, of basaltic bulk composition and were Germany. derived from the crater rim (9, 10). *Corresponding author. E-mail: [email protected] Compared to adjacent geological units, the Yellowknife Bay trough is notable www.sciencemag.org †MSL Science Team authors and affiliations are listed in the supplementary materials. for its lack of visible craters and its apparently greater degree of stripping We determined radiogenic and cosmogenic noble gases in a mudstone on the floor of impact ejecta and wind-blown soil. of Gale crater. A K-Ar age of 4.21 ± 0.35 Ga represents a mixture of detrital and This appearance suggests active ero- authigenic components, and confirms the expected antiquity of rocks comprising sion, distinctly different from the adja- 3 21 36 the crater rim. Cosmic-ray-produced He, Ne, and Ar yield concordant surface cent map units (9). However, the exposure ages of 78 ± 30 Ma. Surface exposure occurred mainly in the present depositional age of the Yellowknife geomorphic setting rather than during primary erosion and transport. Our Bay formation, its stratigraphic rela- Downloaded from observations are consistent with mudstone deposition shortly after the Gale impact, tionship to the adjacent alluvial fan and or possibly in a later event of rapid erosion and deposition. The mudstone remained to the strata of Mt. Sharp, and the caus- buried until recent exposure by wind-driven scarp retreat. Sedimentary rocks es and timing of its exposure are all exposed by this mechanism may thus offer the best potential for organic biomarker presently uncertain. preservation against destruction by cosmic radiation. As shown in Fig. 1, the Sheepbed mudstone and stratigraphically overly- ing Gillespie Lake sandstone of the Multiple orbiter and rover missions have documented a rich geologic Yellowknife Bay formation (9) form a rock couplet of apparently varia- record on the surface of Mars, likely spanning almost the entire history ble rock hardness, and their contact has eroded to form a decimeter-scale of the planet (e.g., 1). However, interpretation of this record is impeded topographic step. The Sheepbed-Gillespie Lake contact is sharp and by our limited understanding of the connection between the materials marked by scouring of the underlying mudstone, producing a small scarp observed and their absolute age. Impact crater densities are the primary and in some locations an overhang. Calved-off blocks of the sandstone means for establishing a chronology for geologic features on Mars and occur at the base and a few meters outboard of the scarp, but not beyond other solar system bodies (2–4), but crater-based dating methods suffer (see figure 3 of (9)). This suggests that residual fragments of the degrad- from multiple sources of uncertainty that obscure both absolute and rela- ing scarp are removed efficiently enough to leave only a very thin lag tive ages. In contrast, on Earth, isotopic dating methods provide a pow- deposit at locations where the Gillespie Lake member is now completely erful quantitative framework for defining geologic history and for absent. identifying the rates and causes of geologic phenomena. Here we report The distinctive erosional profile of the Sheepbed-Gillespie Lake con- results of an attempt to apply in-situ isotopic dating methods to Mars, / http://www.sciencemag.org/content/early/recent / 9 December 2013 / Page 1 / 10.1126/science.1247166 tact can be traced around the full width of Yellowknife Bay (9) with the energy neutrons into the Martian atmosphere (e.g., 19). Sheepbed unit making up the floor of the encircled trough. This trough is The concentration of a single cosmogenic isotope yields a non- elongated in the NE-SW direction, creating an amphitheater-shaped unique exposure history for the rock. The customary end-member inter- planimetric form open to the NE. Beyond the Gillespie Lake contact, the pretive models are to assume either no erosion, in which case a surface more resistant units higher in the section form similar scarps stepping exposure age is obtained, or steady erosion from great depth, in which upward a total of ~5 m to the uppermost unit of the Yellowknife Bay case a mean erosion rate is obtained (e.g., 17). Figure 3 shows that this formation (Fig. S1). Within the Sheepbed unit, mm- to cm-scale topo- non-uniqueness can be eliminated by combining spallogenic and neutron graphic protrusions form fields of small ridges that have been stream- capture isotope measurements. In particular, a rock of Sheepbed compo- lined in the NE-SW direction (Fig. 2). Along with the morphology of the sition initially at a depth of greater than a few meters that was instanta- Yellowknife Bay trough, this observations indicates a dominant role for neously exposed at the surface would have 36Ar/3He of ~1.5 and SW-directed wind erosion. These observations implicate eolian process- 36Ar/21Ne of ~13. In contrast, if steady erosion causes progressive es in eroding the mudstone, in efficiently wearing down any blocks that downward migration of the surface toward the sample, then the sample are delivered to its surface from the encircling scarps or from distal im- integrates the entire depth profile including the large subsurface 36Ar pacts, and in expanding the exposure of the Yellowknife Bay formation. peak. Such a rock would have 36Ar/3He~4 and 36Ar/21Ne~30. In both Scientific investigations at Yellowknife Bay focused on the Sheep- erosion scenarios the 3He/21Ne ratio is ~8; this isotope pair provides a bed mudstone. This fine-grained (<50 μm) rock was drilled twice for cross-check but no additional information on exposure history. mineralogical and evolved gas analyses (11, 12). In addition, multiple chemical analyses were obtained on outcropping mudstone as well as the Results and Discussion drill tailings. The two drilled samples, referred to as John Klein and Geochronology measurements were performed on the Cumberland Cumberland, were located ~3 m apart, and yielded very similar results. drilled powder (21). After drilling, the sample was sieved and an aliquot The mudstone is composed of detrital and authigenic components con- with estimated mass of 135 ± 18 mg (22) was introduced into a quartz sisting of both crystalline and x-ray amorphous phases (11). The detrital cup in the Sample Analysis at Mars (SAM) instrument.
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