• Share
  • Email
  • Embed
  • Like
  • Save
  • Private Content
In situ radiometric_and_exposure_age_dating_of_the_martian_surface
 

In situ radiometric_and_exposure_age_dating_of_the_martian_surface

on

  • 1,189 views

 

Statistics

Views

Total Views
1,189
Views on SlideShare
770
Embed Views
419

Actions

Likes
0
Downloads
2
Comments
0

4 Embeds 419

http://blog.cienctec.com.br 223
http://goasa2013.blogspot.com.br 180
http://goasa2013.blogspot.com 13
http://feedly.com 3

Accessibility

Categories

Upload Details

Uploaded via as Adobe PDF

Usage Rights

© All Rights Reserved

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
    Processing…
Post Comment
Edit your comment

    In situ radiometric_and_exposure_age_dating_of_the_martian_surface In situ radiometric_and_exposure_age_dating_of_the_martian_surface Document Transcript

    • Research Articles using the instrument suite aboard the Curiosity rover exploring Gale crater. Geologic Setting Gale is a ~150 km diameter impact crater that formed near the Late Noachian - Early Hesperian boundary (5, K. A. Farley,1* C. Malespin,2 P. Mahaffy,2 J. P. Grotzinger,1 P. M. 6), or around 3.7-3.5 Ga according to Vasconcelos,3 R. E. Milliken,4 M. Malin,5 K. S. Edgett,5 A. A. Pavlov,2 impact crater models (3, 7). In addition to the ~5 km high central mountain of J. A. Hurowitz,6 J. A. Grant,7 H. B. Miller,1 R. Arvidson,8 L. Beegle,9 stratified rock informally known as Mt. 9 2 10 2 F. Calef, P. G. Conrad, W. E. Dietrich, J. Eigenbrode, R. Sharp, the crater is partially filled with Gellert,11 S. Gupta,12 V. Hamilton,13 D. M. Hassler,13 K.W. Lewis,14 S. sedimentary rocks derived from the crater rim. Crater density distributions M. McLennan,6 D. Ming,15 R. Navarro-González,16 S. P. imply a somewhat younger age (Early 17 18 1 19 Schwenzer, A. Steele, E. M. Stolper, D. Y. Sumner, D. to Late Hesperian, or ~3.5-2.9 Ga) for at least some of these deposits (5, 6). Vaniman,20 A. Vasavada,9 K. Williford,9 R. F. WimmerWhile heading for its destination at Mt. Schweingruber,21 and the MSL Science Team† Sharp, Curiosity traversed a gently 1 Division 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, pebble conglomerate were encountered, 4 University of Queensland, Brisbane, Queensland, QLD 4072, Australia. Department of Geological recording the presence of an ancient 5 Sciences, Brown University, Providence, RI 02912, USA. Malin Space Science Systems, San Diego, CA stream bed (8). Most of this surface is 6 7 92121, USA. Department of Geosciences, Stony Brook University, Stony Brook, NY 11794, USA. Center heavily cratered and covered with rock for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, 6th at and soil (Fig. 1). However, a substantial Independence SW, Washington, DC 20560, USA. 8Department of Earth and Planetary Sciences, expanse of bare bedrock was encounWashington University in St. Louis, St. Louis, MO, 63130, USA. 9Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA. 10Earth and Planetary Science Department, University tered in an ~5 m deep topographic of California, Berkeley, CA 94720, USA. 11Department of Physics, University of Guelph, Guelph, ON N1G trough (Yellowknife Bay) representing 12 2W1, Canada. Department of Earth Science and Engineering, Imperial College London, London SW7 an erosional window through a se2AZ, UK. 13Southwest Research Institute, Boulder, CO 80302, USA. 14Department of Geosciences, quence of stratified rocks known as the 15 Princeton University, Princeton, NJ 08544, USA. NASA Johnson Space Center, Houston, TX 77058, USA. Yellowknife Bay formation (Fig. S1 16 Universidad Nacional Autonoma de Mexico, Coyoacan, Mexico City 4510, Mexico. 17Department of 18 and (9)). These rocks consist mostly of Physical Sciences, CEPSAR, Walton Hall, Milton Keynes MK7 6AA, UK. Carnegie Institution, Geophysical distal alluvial fan and lacustrine facies Laboratory, Washington, DC 20015, USA. 19Department of Geology, University of California, Davis, CA of basaltic bulk composition and were 95616, USA. 20Planetary Science Institute, Tucson, AZ 85719, USA. 21University of Kiel, Kiel D-24098, Germany. derived from the crater rim (9, 10). Compared to adjacent geological units, *Corresponding author. E-mail: farley@gps.caltech.edu the Yellowknife Bay trough is notable †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 eroauthigenic components, and confirms the expected antiquity of rocks comprising sion, distinctly different from the adjathe crater rim. Cosmic-ray-produced 3He, 21Ne, and 36Ar 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 relaobservations 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 causburied 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 overlying 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 variarecord 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 degradfrom multiple sources of uncertainty that obscure both absolute and relaing scarp are removed efficiently enough to leave only a very thin lag tive ages. In contrast, on Earth, isotopic dating methods provide a powdeposit 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 conresults 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 Downloaded from www.sciencemag.org on December 9, 2013 In Situ Radiometric and Exposure Age Dating of the Martian Surface
    • tact can be traced around the full width of Yellowknife Bay (9) with the Sheepbed unit making up the floor of the encircled trough. This trough is elongated in the NE-SW direction, creating an amphitheater-shaped planimetric form open to the NE. Beyond the Gillespie Lake contact, the more resistant units higher in the section form similar scarps stepping upward a total of ~5 m to the uppermost unit of the Yellowknife Bay formation (Fig. S1). Within the Sheepbed unit, mm- to cm-scale topographic protrusions form fields of small ridges that have been streamlined in the NE-SW direction (Fig. 2). Along with the morphology of the Yellowknife Bay trough, this observations indicates a dominant role for SW-directed wind erosion. These observations implicate eolian processes in eroding the mudstone, in efficiently wearing down any blocks that are delivered to its surface from the encircling scarps or from distal impacts, and in expanding the exposure of the Yellowknife Bay formation. Scientific investigations at Yellowknife Bay focused on the Sheepbed mudstone. This fine-grained (<50 μm) rock was drilled twice for mineralogical and evolved gas analyses (11, 12). In addition, multiple chemical analyses were obtained on outcropping mudstone as well as the drill tailings. The two drilled samples, referred to as John Klein and Cumberland, were located ~3 m apart, and yielded very similar results. The mudstone is composed of detrital and authigenic components consisting of both crystalline and x-ray amorphous phases (11). The detrital fraction includes minerals typical of volcanic rocks: plagioclase, pyroxenes, and minor olivine, sanidine and ilmenite. Crystalline authigenic phases include smectitic clay (possibly saponite) and magnetite, thought to have formed as a result of chemical alteration of detrital olivine, and minor akaganeite, pyrrhotite, bassanite, and hematite. Quartz and halite were reported, but very near the CheMin detection limit. Chlorate or perchlorate was also tentatively identified (12). A model for the amorphous component, comprising about one third of the sample, indicates it is composed mainly of Si, Fe, Ca, S, and Cl (11). It likely includes poorly crystalline or finely crystalline materials and may include detrital volcanic and impact-derived glass. The presence of smectite, magnetite, and akaganeite suggests that the mudstone has not experienced burial heating above ~200°C, (11, 13). It is possible that the sample experienced no burial heating at all. Geochronology Overview: K/Ar and Cosmogenic Isotopes Noble gas isotopes can quantitatively constrain the age and erosion history of the Sheepbed mudstone. 40Ar from 40K decay will record a potassium-weighted average of the formation or cooling ages of the multiple components of the mudstone. 36Ar, 21Ne, and 3He are produced by irradiation of the uppermost ~2-3 m of the Martian surface by galactic cosmic rays (GCRs) (14). These cosmogenic isotopes are commonly used to assess exposure histories of meteorites (e.g., 15, 16) and erosion rates and styles of terrestrial rocks (e.g., 17). Because the Martian atmosphere is very thin and the magnetic field very weak (18), the Martian surface is only weakly shielded from GCRs. Thus cosmogenic isotope production rates are much higher than on Earth (14, 19). The chemistry of the Sheepbed mudstone (10) is such that the most abundant cosmogenic isotope is likely to be 36Ar produced from the capture of cosmogenic thermal neutrons by Cl, followed by 3He produced by spallation mainly of O, Si, and Mg, followed by 21Ne from spallation of Mg, Si, and Al (20). The mudstone’s exposure history is thus recorded by multiple isotopic systems. The depth-dependence of the production functions of the two spallation isotopes are very similar: a small maximum at ~15 cm below the surface reflecting development of the nuclear cascade in the uppermost layers of rock, followed by an exponential decay with an e-folding depth of ~1 m as the energetic particles attenuate (Fig. 3). In contrast, the production rate of 36Ar from neutron capture has a larger subsurface maximum at greater depth (~60 cm) arising from the combined effects of the production of neutrons within the descending cascade and the loss of low energy neutrons into the Martian atmosphere (e.g., 19). The concentration of a single cosmogenic isotope yields a nonunique exposure history for the rock. The customary end-member interpretive models are to assume either no erosion, in which case a surface exposure age is obtained, or steady erosion from great depth, in which case a mean erosion rate is obtained (e.g., 17). Figure 3 shows that this non-uniqueness can be eliminated by combining spallogenic and neutron capture isotope measurements. In particular, a rock of Sheepbed composition initially at a depth of greater than a few meters that was instantaneously exposed at the surface would have 36Ar/3He of ~1.5 and 36 Ar/21Ne of ~13. In contrast, if steady erosion causes progressive downward migration of the surface toward the sample, then the sample integrates the entire depth profile including the large subsurface 36Ar peak. Such a rock would have 36Ar/3He~4 and 36Ar/21Ne~30. In both erosion scenarios the 3He/21Ne ratio is ~8; this isotope pair provides a cross-check but no additional information on exposure history. Results and Discussion Geochronology measurements were performed on the Cumberland drilled powder (21). After drilling, the sample was sieved and an aliquot with estimated mass of 135 ± 18 mg (22) was introduced into a quartz cup in the Sample Analysis at Mars (SAM) instrument. The cup was moved into position in an oven, sealed against Mars atmosphere, and evacuated. The sample was then heated to a maximum temperature of ~890°C for 25 min, and the evolved gases were purified and admitted into the quadrupole mass spectrometer for analysis. All reported measurements used the high sensitivity semi-static analysis mode. K-Ar Age The Ar concentration and the APXS-determined K2O concentration yield a K-Ar age of 4.21 ± 0.35 Ga (1σ) for the Cumberland sample (Table 1). This age calculation assumes all measured 40Ar is radiogenic (23). Although a fairly uniform level of excess Ar is thought to be present in some young Martian meteorites (24), that same level of excess would contribute <0.03 Ga to the age of this high K2O, high Ar concentration rock. One important implication of this very high K-Ar age is that processing in the SAM oven has extracted essentially all radiogenic 40Ar despite a fairly low extraction temperature. This surprisingly high yield (25) may result from rapid diffusive loss from the inherently fine grain size of the mudstone (probably enhanced by the powdering process of the drill (26)). Alternatively, the volatile constituents in the mudstone may promote loss via flux-induced melting. Since fine grain size and flux-melting will affect the noble gas isotopes similarly, effective extraction of 36Ar, 21Ne and 3He might also be expected. The high age also implies that a very large fraction of the radiogenic Ar was retained in the mudstone over geologic time. The interpretation of the K-Ar age depends critically on where potassium is located in the rock. If a fraction FD of the potassium is in the detrital phases and the remainder in authigenic phases, then the bulk age (TB) is a potassium-weighted average of the age of the detritus (TD) and the age of authigenesis (TA): TB=(1-FD)TA+FDTD. If all of the potassium is carried in the detrital components (mainly sanidine, plagioclase and possibly basalt glass), FD=1. In this case the K-Ar system records a mixture of the ages of components present in the crater rim, principally bedrock ranging from minimally to completely reset by the Gale-forming impact. Crater rim lithologies as well as the crater itself are thought to range from Noachian to early-Hesperian in age, or about 4.1 to 3.5 Ga (5, 6, 27–29), in agreement with the K-Ar age of 4.21 ± 0.35 Ga we measured. Because formation of K-bearing authigenic phases can only lower the mudstone age below that of the detrital component, we conclude with 95% confidence (30) that the potassium-weighted mean age of the materials in the Gale crater wall exceeds 3.6 Ga. Alternatively, if the potassium is entirely hosted within authigenic components (phyllo- / http://www.sciencemag.org/content/early/recent / 13 December 2013 / Page 2 / 10.1126/science.1247166
    • silicates or amorphous phases other than basalt glass), FD=0 and the age records the formation age of those phases. In this case 4.21 Ga would represent a minimum age of mudstone deposition. At present we have no definitive way to determine the potassium distribution in the mudstone, but we can make a reasonable estimate. Kbearing phases determined by CheMin include sanidine (1.8%), andesine feldspar (21%) and an unknown fraction of basaltic or impact glass (11). CheMin cannot determine the K2O content of any of these phases, so we assume representative values of 13%, 0.3%, and 1%, respectively (31). By assuming the mudstone contains 10% glass, we conclude that the detrital component accounts for 80% of the measured K2O. The remainder could then be in the authigenic clay (18% of the mudstone) with 0.6% K2O, in agreement with K2O concentrations in saponite in a Martian meteorite (32). If we assume TD<4.1 Ga based on crater-count ages of the Gale impact and host terrains, and use the 95% confidence lower limit on the K-Ar age of 3.6 Ga, we obtain a current best estimate that the mudstone was deposited prior to 1.6 Ga. Assuming either a younger age for the detrital component or a smaller fraction of it in the mudstone would cause this estimate to rise. Cosmogenic 36Ar, 21Ne, and 3He 36 Ar, 21Ne, and 3He were all detected at levels that cannot be attributed to sources other than cosmic ray irradiation (33). In the mudstone, 36Ar/3He and 36Ar/21Ne are 1.7 ± 0.5 and 12 ± 5, respectively (34). These ratios are within error of predictions of the no-erosion scenario (1.5 and 13) and very different from the prediction of the steady erosion scenario (4 and 30). Thus we cast our results in terms of surface exposure age, which for 3He, 21Ne, and 36Ar are 72 ± 15, 84 ± 28 and 79 ± 24 Ma, respectively. These ages could be the sum of multiple shorter exposure intervals separated by burial events, e.g., by exposure during initial deposition and then again by recent re-exposure, or alternatively by burial and exhumation associated with migrating eolian deposits. However, these events would have to involve rapid burial and removal of at least a few meters of cover to maintain the good match of measured nuclide concentration ratios to those of production at the surface. The agreement among these results argues for complete extraction of all three noble gases in the SAM oven, and against substantial loss of noble gases over the ~78 Myr period of cosmic ray exposure. For example, diffusive loss of He from amorphous phases, plagioclase and phyllosilicates might occur at Mars ambient temperature (35), but the 3He/21Ne ratio of ~7.5 ± 2.6 matches the expected production ratio of ~8. Similarly, dissolution and reprecipitation of a water-soluble Cl-rich phase by occasional wetting of the mudstone might release accumulated 36Ar. If this occurred to a significant extent, then the agreement between the 36Ar exposure age and the 21Ne and 3He exposure ages would have to be fortuitous. While not impossible, we see no evidence for it in these data. A significant portion of the 3He and 21Ne must be carried by detrital minerals including plagioclase and pyroxene because these are important host phases for the major target elements (36). In contrast, the enrichment of Cl compared to a typical Martian basalt suggests much of this element was added to the mudstone from solution (9, 10). This distinction implies that, while 3He and 21Ne might record cosmic ray irradiation that occurred during primary exposure and transport in addition to that acquired during modern exposure at Yellowknife Bay, the same is not true of 36Ar. The logical interpretation of the good agreement among all three exposure ages is that all three reflect exposure only in the modern setting. The apparent absence of a detrital cosmogenic signal favors deposition of the mudstone in an environment in which erosion and transport were fairly rapid and/or in which the Martian surface was shielded from cosmic rays by a reasonably dense atmosphere. The transition from comparatively wet conditions and a thick atmosphere to cold and dry conditions with a thin atmosphere is thought to have occurred around the Noachian-Hesperian boundary (37). Because wet conditions favor rapid erosion, in either scenario the cosmogenic isotope observations would support deposition of the Sheepbed mudstone shortly after Gale formation. Alternatively, deposition may have occurred in association with a later event in which material was eroded and transported rapidly enough to prevent detectable cosmogenic production. A late erosional event has also been suggested for Amazonian-age fan formation in some other Martian craters (38–40). Topography, stratigraphy and surface exposure dating suggest that the Yellowknife Bay trough is currently a focus of erosion, and that erosion is occurring by scarp retreat. The Sheepbed mudstone lies below a sequence of relatively more resistant units that define a series of topographic steps (A-A’ in Fig. S1). The pace of wind erosion for the entire escarpment will be set by the retreat rate of these more resistant units. Deflation of the softer units, such as the mudstone, however, undercuts and contributes to collapse of the resistant layers (Fig. 1), such that both direct wind abrasion of resistant units and removal of the softer units contribute to slope retreat. Resistant units and corresponding local steps will thicken or thin as erosion sweeps into units with variable thicknesses and resistance properties. This predominately lateral erosion has caused scarp migration to the southwestward and possibly other directions, and produced several meters of surface lowering. The general flattening of the topographic profile (Fig. S1) as it extends into the Yellowknife Bay trough may indicate that a more resistant unit beneath the Sheepbed mudstone has slowed continued removal of this unit. In such a model, the scarp retreat rate can be estimated from the surface exposure ages. To prevent cosmogenic nuclide accumulation requires at least 2-3 m of overburden. Southwestward from the drill site (and downwind, according to Fig. 3) this amount of overburden is first encountered about 60 m away (Fig. S1). Thus, in a model in which the mudstone is rapidly exposed from >2-3 m depth to the surface, the scarp retreat rate over the last ~80 Ma averages ~0.75 m/Ma. At this rate, the full extent of the Yellowknife Bay exposure could have been produced over several hundred million years of steady lateral erosion. Alternatively, wind erosion and associated scarp retreat may be highly episodic, perhaps in response to climatic variations tied to Mars’ obliquity cycle (41, 42). Either way, surface exposure ages may vary substantially around the Yellowknife Bay outcrop and other similar exposures, in principle offering a test of the scarp retreat hypothesis. Implications for Organic Preservation Cosmic rays penetrating the uppermost several meters of rock create a cascade of atomic and subatomic particles, electrons, and photons that ionize molecules and atoms in their path (e.g., 43, 44). Complex organic molecules are particularly susceptible to degradation by such particles, and because these particles also produce the cosmogenic nuclides, we have a way to assess the likely magnitude of this degradation in the Cumberland sample. Degradation of organic molecules in Martian surface rocks subjected to cosmic ray irradiation has been modeled (44). When scaled to the long-term cosmic ray dose implied by the cosmogenic nuclides in the Cumberland sample, these rates predict reductions of between 2 and 3 orders of magnitude in the concentration of organic molecules in the 100-400 AMU range. The scarp retreat hypothesis offers a possible strategy for minimizing the degree of organic degradation in samples obtained during the further course of exploration by Curiosity and possible future missions. Because exposure ages and thus cosmic ray doses should decrease toward a bounding scarp (at least in the downwind direction), such a location may offer the best potential for organic preservation. Given our estimated scarp retreat rate, locations within a few tens of cm of a fewmeter-scale scarp may have been exposed to cosmic rays for less than a few Ma. Such a short exposure compares favorably to what can reasonably be expected from alternative sampling strategies relying on recent / http://www.sciencemag.org/content/early/recent / 13 December 2013 / Page 3 / 10.1126/science.1247166
    • impact craters or drilling to obtain fresh strata. References and Notes 1. M. H. Carr, J. W. Head III, Geologic History of Mars. Earth Planet. Sci. Lett. 294, 185–203 (2010). doi:10.1016/j.epsl.2009.06.042 2. E. M. Shoemaker, R. J. Hackman, R. E. Eggleton, “Interplanetary correlation of geologic time,” in Advances in the Astronautical Sciences (Plenum, New York, 1962), vol. 8, pp. 70–89. 3. W. K. Hartmann, G. Neukum, Cratering chronology and the evolution of Mars. Space Sci. Rev. 96, 165–194 (2001). doi:10.1023/A:1011945222010 4. W. K. Hartmann, Martian cratering 8: Isochron refinement and the chronology of Mars. Icarus 174, 294–320 (2005). doi:10.1016/j.icarus.2004.11.023 5. L. Le Deit, E. Hauber, F. Fueten, M. Pondrelli, A. P. Rossi, R. Jaumann, Sequence of infilling events in Gale crater, Mars: Results from morphology, stratigraphy, and mineralogy. J. Geophys. Res. Planets n/a (2013). doi:10.1002/2012JE004322 6. B. J. Thomson, N. T. Bridges, R. Milliken, A. Baldridge, S. J. Hook, J. K. Crowley, G. M. Marion, C. R. de Souza Filho, A. J. Brown, C. M. Weitz, Constraints on the origin and evolution of the layered mound in Gale Crater, Mars using Mars Reconnaissance Orbiter data. Icarus 214, 413–432 (2011). doi:10.1016/j.icarus.2011.05.002 7. B. A. Ivanov, Mars/Moon cratering rate ratio estimates. Space Sci. Rev. 96, 87– 104 (2001). doi:10.1023/A:1011941121102 8. R. M. E. Williams, J. P. Grotzinger, W. E. Dietrich, S. Gupta, D. Y. Sumner, R. C. Wiens, N. Mangold, M. C. Malin, K. S. Edgett, S. Maurice, O. Forni, O. Gasnault, A. Ollila, H. E. Newsom, G. Dromart, M. C. Palucis, R. A. Yingst, R. B. Anderson, K. E. Herkenhoff, S. Le Mouélic, W. Goetz, M. B. Madsen, A. Koefoed, J. K. Jensen, J. C. Bridges, S. P. Schwenzer, K. W. Lewis, K. M. Stack, D. Rubin, L. C. Kah, J. F. Bell, 3rd, J. D. Farmer, R. Sullivan, T. Van Beek, D. L. Blaney, O. Pariser, R. G. Deen, O. Kemppinen, N. Bridges, J. R. Johnson, M. Minitti, D. Cremers, L. Edgar, A. Godber, M. Wadhwa, D. Wellington, I. McEwan, C. Newman, M. Richardson, A. Charpentier, L. Peret, P. King, J. Blank, G. Weigle, M. Schmidt, S. Li, R. Milliken, K. Robertson, V. Sun, M. Baker, C. Edwards, B. Ehlmann, K. Farley, J. Griffes, H. Miller, M. Newcombe, C. Pilorget, M. Rice, K. Siebach, E. Stolper, C. Brunet, V. Hipkin, R. Leveille, G. Marchand, P. Sobron Sanchez, L. Favot, G. Cody, A. Steele, L. Fluckiger, D. Lees, A. Nefian, M. Martin, M. Gailhanou, F. Westall, G. Israel, C. Agard, J. Baroukh, C. Donny, A. Gaboriaud, P. Guillemot, V. Lafaille, E. Lorigny, A. Paillet, R. Perez, M. Saccoccio, C. Yana, C. A. Aparicio, J. Caride Rodriguez, I. Carrasco Blazquez, F. Gomez Gomez, J. G. Elvira, S. Hettrich, A. Lepinette Malvitte, M. Marin Jimenez, J. M. Frias, J. M. Soler, F. J. M. Torres, A. Molina Jurado, L. M. Sotomayor, G. Munoz Caro, S. Navarro Lopez, V. P. Gonzalez, J. P. Garcia, J. A. Rodriguez Manfredi, J. J. R. Planello, S. Alejandra Sans Fuentes, E. Sebastian Martinez, J. Torres Redondo, R. U. O’Callaghan, M.-P. Zorzano Mier, S. Chipera, J.-L. Lacour, P. Mauchien, J.-B. Sirven, H. Manning, A. Fairen, A. Hayes, J. Joseph, S. Squyres, P. Thomas, A. Dupont, A. Lundberg, N. Melikechi, A. Mezzacappa, J. DeMarines, D. Grinspoon, G. Reitz, B. Prats, E. Atlaskin, M. Genzer, A.-M. Harri, H. Haukka, H. Kahanpaa, J. Kauhanen, M. Paton, J. Polkko, W. Schmidt, T. Siili, C. Fabre, J. Wray, M. B. Wilhelm, F. Poitrasson, K. Patel, S. Gorevan, S. Indyk, G. Paulsen, D. Bish, J. Schieber, B. Gondet, Y. Langevin, C. Geffroy, D. Baratoux, G. Berger, A. Cros, C. Uston, J. Lasue, Q.-M. Lee, P.-Y. Meslin, E. Pallier, Y. Parot, P. Pinet, S. Schroder, M. Toplis, E. Lewin, W. Brunner, E. Heydari, C. Achilles, D. Oehler, B. Sutter, M. Cabane, D. Coscia, C. Szopa, F. Robert, V. Sautter, M. Nachon, A. Buch, F. Stalport, P. Coll, P. Francois, F. Raulin, S. Teinturier, J. Cameron, S. Clegg, A. Cousin, D. DeLapp, R. Dingler, R. S. Jackson, S. Johnstone, N. Lanza, C. Little, T. Nelson, R. B. Williams, A. Jones, L. Kirkland, A. Treiman, B. Baker, B. Cantor, M. Caplinger, S. Davis, B. Duston, D. Fay, C. Hardgrove, D. Harker, P. Herrera, E. Jensen, M. R. Kennedy, G. Krezoski, D. Krysak, L. Lipkaman, E. McCartney, S. McNair, B. Nixon, L. Posiolova, M. Ravine, A. Salamon, L. Saper, K. Stoiber, K. Supulver, J. Van Beek, R. Zimdar, K. L. French, K. Iagnemma, K. Miller, R. Summons, F. Goesmann, S. Hviid, M. Johnson, M. Lefavor, E. Lyness, E. Breves, M. D. Dyar, C. Fassett, D. F. Blake, T. Bristow, D. DesMarais, L. Edwards, R. Haberle, T. Hoehler, J. Hollingsworth, M. Kahre, L. Keely, C. McKay, L. Bleacher, W. Brinckerhoff, D. Choi, P. Conrad, J. P. Dworkin, J. Eigenbrode, M. Floyd, C. Freissinet, J. Garvin, D. Glavin, D. Harpold, P. Mahaffy, D. K. Martin, A. McAdam, A. Pavlov, E. Raaen, M. D. Smith, J. Stern, F. Tan, M. Trainer, M. Meyer, A. Posner, M. Voytek, R. C. Anderson, A. Aubrey, L. W. Beegle, A. Behar, D. Brinza, F. Calef, L. Christensen, J. A. Crisp, L. DeFlores, J. Feldman, S. Feldman, G. Flesch, J. Hurowitz, I. Jun, D. Keymeulen, J. Maki, M. Mischna, J. M. Morookian, T. Parker, B. Pavri, M. Schoppers, A. Sengstacken, J. J. Simmonds, N. Spanovich, M. de la Torre Juarez, A. R. Vasavada, C. R. Webster, A. Yen, P. D. Archer, F. Cucinotta, J. H. Jones, D. Ming, R. V. Morris, P. Niles, E. Rampe, T. Nolan, M. Fisk, L. Radziemski, B. Barraclough, S. Bender, D. Berman, E. N. Dobrea, R. Tokar, D. Vaniman, L. Leshin, T. Cleghorn, W. Huntress, G. Manhes, J. Hudgins, T. Olson, N. Stewart, P. Sarrazin, J. Grant, E. Vicenzi, S. A. Wilson, M. Bullock, B. Ehresmann, V. Hamilton, D. Hassler, J. Peterson, S. Rafkin, C. Zeitlin, F. Fedosov, D. Golovin, N. Karpushkina, A. Kozyrev, M. Litvak, A. Malakhov, I. Mitrofanov, M. Mokrousov, S. Nikiforov, V. Prokhorov, A. Sanin, V. Tretyakov, A. Varenikov, A. Vostrukhin, R. Kuzmin, B. Clark, M. Wolff, S. McLennan, O. Botta, D. Drake, K. Bean, M. Lemmon, E. M. Lee, R. Sucharski, M. A. P. Hernandez, J. J. Blanco Avalos, M. Ramos, M.-H. Kim, C. Malespin, I. Plante, J.-P. Muller, R. N. Gonzalez, R. Ewing, W. Boynton, R. Downs, M. Fitzgibbon, K. Harshman, S. Morrison, O. Kortmann, A. Williams, G. Lugmair, M. A. Wilson, B. Jakosky, T. B. Zunic, J. Frydenvang, K. Kinch, S. L. S. Stipp, N. Boyd, J. L. Campbell, R. Gellert, G. Perrett, I. Pradler, S. VanBommel, S. Jacob, T. Owen, S. Rowland, H. Savijarvi, E. Boehm, S. Bottcher, S. Burmeister, J. Guo, J. Kohler, C. M. Garcia, R. M. Mellin, R. W. Schweingruber, T. McConnochie, M. Benna, H. Franz, H. Bower, A. Brunner, H. Blau, T. Boucher, M. Carmosino, S. Atreya, H. Elliott, D. Halleaux, N. Renno, M. Wong, R. Pepin, B. Elliott, J. Spray, L. Thompson, S. Gordon, J. Williams, P. Vasconcelos, J. Bentz, K. Nealson, R. Popa, J. Moersch, C. Tate, M. Day, G. Kocurek, B. Hallet, R. Sletten, R. Francis, E. McCullough, E. Cloutis, I. L. ten Kate, R. Arvidson, A. Fraeman, D. Scholes, S. Slavney, T. Stein, J. Ward, J. Berger, J. E. Moores, MSL Science Team, Martian fluvial conglomerates at Gale crater. Science 340, 1068–1072 (2013). doi:10.1126/science.1237317 Medline 9. J. P. Grotzinger et al., A habitable fluvio-lacustrine environment at Yellowknife Bay, Gale Crater, Mars. Science, published online 9 December 2013 (10.1126/science.1242777). 10. S. M. McLennan et al., Elemental geochemistry of sedimentary rocks in Yellowknife Bay, Gale Crater, Mars. Science, published online 9 December 2013 (10.1126/science.1244734). 11. D. Vaniman et al., Mineralogy of a mudstone on Mars. Science, published online 9 December 2013 (10.1126/science.1243480). 12. D. W. Ming et al., Organic, volatile, and isotopic compositions of a sedimentary rock in Yellowknife Bay, Gale Crater, Mars. Science, published online 9 December 2013 (10.1126/science.1245267). 13. L. P. Keller, K. L. Thomas, R. N. Clayton, T. K. Mayeda, J. M. DeHart, D. S. McKay, Aqueous alteration of the Bali CV3 chondrite: evidence from mineralogy, mineral chemistry, and oxygen isotopic compositions. Geochim. Cosmochim. Acta 58, 5589–5598 (1994). doi:10.1016/0016-7037(94)90252-6 Medline 14. D. Lal, Cosmogenic and nucleogenic isotopic changes in Mars: their rates and implications to the evolutionary history of Martian surface. Geochim. Cosmochim. Acta 57, 4627–4637 (1993). doi:10.1016/0016-7037(93)90188-3 Medline 15. O. Eugster, Cosmic-ray exposure ages of meteorites and lunar rocks and their significance. Chemie Der Erde-Geochemistry 63, 3–30 (2003). doi:10.1078/0009-2819-00021 16. K. Marti, T. Graf, Cosmic-ray exposure history of ordinary chondrites. Annu. Rev. Earth Planet. Sci. 20, 221–243 (1992). doi:10.1146/annurev.ea.20.050192.001253 17. T. Cerling, H. Craig, Geomorphology and in-situ cosmogenic isotopes. Annu. Rev. Earth Planet. Sci. 22, 273–317 (1994). doi:10.1146/annurev.ea.22.050194.001421 18. M. H. Acuña, J. E. P. Connerney, P. Wasilewski, R. P. Lin, K. A. Anderson, C. W. Carlson, J. McFadden, D. W. Curtis, D. Mitchell, H. Reme, C. Mazelle, J. A. Sauvaud, C. d’Uston, A. Cros, J. L. Medale, S. J. Bauer, P. Cloutier, M. Mayhew, D. Winterhalter, N. F. Ness, Magnetic field and plasma observations at Mars: Initial results of the Mars global surveyor mission. Science 279, 1676–1680 (1998). doi:10.1126/science.279.5357.1676 Medline 19. M. N. Rao, D. D. Bogard, L. E. Nyquist, D. S. McKay, J. Masarik, Neutron capture isotopes in the martian regolith and implications for Martian atmospheric noble gases. Icarus 156, 352–372 (2002). doi:10.1006/icar.2001.6809 20. Cosmogenic production rate calculations are given in the supplementary materials. 21. Full method details are provided in the supplementary materials. 22. Details of mass estimate and uncertainty are given in the supplementary / http://www.sciencemag.org/content/early/recent / 13 December 2013 / Page 4 / 10.1126/science.1247166
    • materials. 23. Atmospheric and excess argon are assumed negligible, as described in the supplementary materials. 24. D. Bogard, J. Park, D. Garrison, 39Ar-40Ar “ages” and origin of excess 40Ar in Martian shergottites. Meteorit. Planet. Sci. 44, 905–923 (2009). doi:10.1111/j.1945-5100.2009.tb00777.x 25. Full release of Ar for Ar geochronology often requires much higher temperatures. See supplementary materials for a discussion of this issue. 26. R. C. Anderson, L. Jandura, A. B. Okon, D. Sunshine, C. Roumeliotis, L. W. Beegle, J. Hurowitz, B. Kennedy, D. Limonadi, S. McCloskey, M. Robinson, C. Seybold, K. Brown, Collecting samples in Gale Crater, Mars; an overview of the Mars Science Laboratory sample acquisition, sample processing and handling system. Space Sci. Rev. 170, 57–75 (2012). doi:10.1007/s11214-0129898-9 27. D. H. Scott, M. G. Chapman, Geologic and topographic maps of the Elysium paleolake basin, Mars. U.S. Geol. Surv. Misc. Invest. Map, I-2397 (1995). 28. R. Greeley, J. E. Guest, Geologic map of the eastern equatorial region of Mars. U.S. Geol. Surv. Misc. Invest. Map I-1802-B (1987). 29. J. J. Wray, Gale crater: the Mars Science Laboratory/Curiosity rover landing site. Int. J. Astrobiol. 12, 25–38 (2013). doi:10.1017/S1473550412000328 30. Using the cumlative normal distribution. 31. Potassium concentrations were estimated from terrestrial examples of volcanic sanidine and andesine [e.g., (45, 46)]. The relatively high K2O content assumed for the basalt glass is consistent with ongoing analyses of basalts in Gale crater. The K2O concentration and glass abundance can be traded off to yield the same overall mass balance. 32. J. C. Bridges, S. P. Schwenzer, The nakhlite hydrothermal brine on Mars. Earth Planet. Sci. Lett. 359-360, 117–123 (2012). doi:10.1016/j.epsl.2012.09.044 33. Possible alternative sources and why they can be beglected are described in the supplementary materials. 34. Mass errors cancel in these ratios. 35. Laboratory data bearing on the question of diffusive noble gas loss are given in the supplementary materials. 36. Likely host minerals for each of the noble gases are given in the supplemntary materials. 37. J.-P. Bibring, Y. Langevin, J. F. Mustard, F. Poulet, R. Arvidson, A. Gendrin, B. Gondet, N. Mangold, P. Pinet, F. Forget, M. Berthé, J. P. Bibring, A. Gendrin, C. Gomez, B. Gondet, D. Jouglet, F. Poulet, A. Soufflot, M. Vincendon, M. Combes, P. Drossart, T. Encrenaz, T. Fouchet, R. Merchiorri, G. Belluci, F. Altieri, V. Formisano, F. Capaccioni, P. Cerroni, A. Coradini, S. Fonti, O. Korablev, V. Kottsov, N. Ignatiev, V. Moroz, D. Titov, L. Zasova, D. Loiseau, N. Mangold, P. Pinet, S. Douté, B. Schmitt, C. Sotin, E. Hauber, H. Hoffmann, R. Jaumann, U. Keller, R. Arvidson, J. F. Mustard, T. Duxbury, F. Forget, G. Neukum, Global mineralogical and aqueous mars history derived from OMEGA/Mars Express data. Science 312, 400–404 (2006). doi:10.1126/science.1122659 Medline 38. J. A. Grant, S. A. Wilson, Late alluvial fan formation in southern Margaritifer Terra, Mars. Geophys. Res. Lett. 38, L08201 (2011). doi:10.1029/2011GL046844 39. J. A. Grant, S. A. Wilson, A possible synoptic source of water for alluvial fan formation in southern Margaritifer Terra, Mars. Planet. Space Sci. 72, 44–52 (2012). doi:10.1016/j.pss.2012.05.020 40. N. Mangold, E. S. Kite, M. G. Kleinhans, H. Newsom, V. Ansan, E. Hauber, E. Kraal, C. Quantin, K. Tanaka, The origin and timing of fluvial activity at Eberswalde crater, Mars. Icarus 220, 530–551 (2012). doi:10.1016/j.icarus.2012.05.026 41. J. Laskar, A. C. M. Correia, M. Gastineau, F. Joutel, B. Levrard, P. Robutel, Long term evolution and chaotic diffusion of the insolation quantities of Mars. Icarus 170, 343–364 (2004). doi:10.1016/j.icarus.2004.04.005 42. W. R. Ward, Large-scale variations in the obliquity of Mars. Science 181, 260–262 (1973). doi:10.1126/science.181.4096.260 Medline 43. L. R. Dartnell, L. Desorgher, J. M. Ward, A. J. Coates, Modelling the surface and subsurface Martian radiation environment: Implications for astrobiology. Geophys. Res. Lett. 34, L02207 (2007). doi:10.1029/2006GL027494 44. A. A. Pavlov, G. Vasilyev, V. M. Ostryakov, A. K. Pavlov, P. Mahaffy, Degradation of the organic molecules in the shallow subsurface of Mars due to irradiation by cosmic rays. Geophys. Res. Lett. 39, L13202 (2012). doi:10.1029/2012GL052166 45. W. A. Deer, R. A. Howie, J. Zussman, Rock-Forming Minerals, Volume 4A: Framework Silicates - Feldspars. (Geological Society of London, London, 2001). 46. O. Bachmann, M. A. Dungan, P. W. Lipman, The Fish Canyon magma body, San Juan volcanic field, Colorado: Rejuvenation and eruption of an uppercrustal batholith. J. Petrol. 43, 1469–1503 (2002). doi:10.1093/petrology/43.8.1469 47. Calculations provided in supplementary materials. 48. P. R. Mahaffy, C. R. Webster, M. Cabane, P. G. Conrad, P. Coll, S. K. Atreya, R. Arvey, M. Barciniak, M. Benna, L. Bleacher, W. B. Brinckerhoff, J. L. Eigenbrode, D. Carignan, M. Cascia, R. A. Chalmers, J. P. Dworkin, T. Errigo, P. Everson, H. Franz, R. Farley, S. Feng, G. Frazier, C. Freissinet, D. P. Glavin, D. N. Harpold, D. Hawk, V. Holmes, C. S. Johnson, A. Jones, P. Jordan, J. Kellogg, J. Lewis, E. Lyness, C. A. Malespin, D. K. Martin, J. Maurer, A. C. McAdam, D. McLennan, T. J. Nolan, M. Noriega, A. A. Pavlov, B. Prats, E. Raaen, O. Sheinman, D. Sheppard, J. Smith, J. C. Stern, F. Tan, M. Trainer, D. W. Ming, R. V. Morris, J. Jones, C. Gundersen, A. Steele, J. Wray, O. Botta, L. A. Leshin, T. Owen, S. Battel, B. M. Jakosky, H. Manning, S. Squyres, R. Navarro-González, C. P. McKay, F. Raulin, R. Sternberg, A. Buch, P. Sorensen, R. Kline-Schoder, D. Coscia, C. Szopa, S. Teinturier, C. Baffes, J. Feldman, G. Flesch, S. Forouhar, R. Garcia, D. Keymeulen, S. Woodward, B. P. Block, K. Arnett, R. Miller, C. Edmonson, S. Gorevan, E. Mumm, The Sample Analysis at Mars investigation and instrument suite. Space Sci. Rev. 170, 401–478 (2012). doi:10.1007/s11214012-9879-z 49. National Geochronological Database, United States Geological Survey OpenFile Report 2003-23, http://geopubs.wr.usgs.gov/open-file/of03-236/ (2013). 50. P. R. Renne, G. Balco, K. R. Ludwig, R. Mundil, K. Min, Response to the comment by W.H. Schwarz et al. on “Joint determination of 40K decay constants and 40Ar*/40K for the Fish Canyon sanidine standard, and improved accuracy for 40Ar/39Ar geochronology” by P.R. Renne et al. (2010). Geochim. Cosmochim. Acta 75, 5097–5100 (2011). doi:10.1016/j.gca.2011.06.021 51. P. R. Mahaffy, C. R. Webster, S. K. Atreya, H. Franz, M. Wong, P. G. Conrad, D. Harpold, J. J. Jones, L. A. Leshin, H. Manning, T. Owen, R. O. Pepin, S. Squyres, M. Trainer, O. Kemppinen, N. Bridges, J. R. Johnson, M. Minitti, D. Cremers, J. F. Bell, L. Edgar, J. Farmer, A. Godber, M. Wadhwa, D. Wellington, I. McEwan, C. Newman, M. Richardson, A. Charpentier, L. Peret, P. King, J. Blank, G. Weigle, M. Schmidt, S. Li, R. Milliken, K. Robertson, V. Sun, M. Baker, C. Edwards, B. Ehlmann, K. Farley, J. Griffes, J. Grotzinger, H. Miller, M. Newcombe, C. Pilorget, M. Rice, K. Siebach, K. Stack, E. Stolper, C. Brunet, V. Hipkin, R. Leveille, G. Marchand, P. S. Sanchez, L. Favot, G. Cody, A. Steele, L. Fluckiger, D. Lees, A. Nefian, M. Martin, M. Gailhanou, F. Westall, G. Israel, C. Agard, J. Baroukh, C. Donny, A. Gaboriaud, P. Guillemot, V. Lafaille, E. Lorigny, A. Paillet, R. Perez, M. Saccoccio, C. Yana, C. Armiens-Aparicio, J. C. Rodriguez, I. C. Blazquez, F. G. Gomez, J. Gomez-Elvira, S. Hettrich, A. L. Malvitte, M. M. Jimenez, J. Martinez-Frias, J. Martin-Soler, F. J. Martin-Torres, A. M. Jurado, L. MoraSotomayor, G. M. Caro, S. N. Lopez, V. Peinado-Gonzalez, J. Pla-Garcia, J. A. R. Manfredi, J. J. Romeral-Planello, S. A. S. Fuentes, E. S. Martinez, J. T. Redondo, R. Urqui-O’Callaghan, M.-P. Z. Mier, S. Chipera, J.-L. Lacour, P. Mauchien, J.-B. Sirven, A. Fairen, A. Hayes, J. Joseph, R. Sullivan, P. Thomas, A. Dupont, A. Lundberg, N. Melikechi, A. Mezzacappa, J. DeMarines, D. Grinspoon, G. Reitz, B. Prats, E. Atlaskin, M. Genzer, A.-M. Harri, H. Haukka, H. Kahanpaa, J. Kauhanen, O. Kemppinen, M. Paton, J. Polkko, W. Schmidt, T. Siili, C. Fabre, J. Wray, M. B. Wilhelm, F. Poitrasson, K. Patel, S. Gorevan, S. Indyk, G. Paulsen, S. Gupta, D. Bish, J. Schieber, B. Gondet, Y. Langevin, C. Geffroy, D. Baratoux, G. Berger, A. Cros, C. d’Uston, O. Forni, O. Gasnault, J. Lasue, Q.-M. Lee, S. Maurice, P.Y. Meslin, E. Pallier, Y. Parot, P. Pinet, S. Schroder, M. Toplis, E. Lewin, W. Brunner, E. Heydari, C. Achilles, D. Oehler, B. Sutter, M. Cabane, D. Coscia, G. Israel, C. Szopa, G. Dromart, F. Robert, V. Sautter, S. Le Mouelic, N. Mangold, M. Nachon, A. Buch, F. Stalport, P. Coll, P. Francois, F. Raulin, S. Teinturier, J. Cameron, S. Clegg, A. Cousin, D. DeLapp, R. Dingler, R. S. Jackson, S. Johnstone, N. Lanza, C. Little, T. Nelson, R. C. Wiens, R. B. Williams, A. Jones, L. Kirkland, A. Treiman, B. Baker, B. Cantor, M. Caplinger, S. Davis, B. Duston, K. Edgett, D. Fay, C. Hardgrove, D. Harker, P. Herrera, E. Jensen, M. R. Kennedy, G. Krezoski, D. Krysak, L. Lipkaman, M. Malin, E. McCartney, S. McNair, B. Nixon, L. Posiolova, M. Ravine, A. Salamon, L. Saper, K. Stoiber, K. Supulver, J. Van Beek, T. Van Beek, R. Zimdar, K. L. French, K. Iagnemma, K. Miller, R. Summons, F. Goesmann, W. Goetz, S. Hviid, M. Johnson, M. Lefavor, E. Lyness, E. Breves, M. D. Dyar, C. Fassett, D. F. Blake, T. Bristow, D. DesMarais, L. Edwards, R. Haberle, T. Hoehler, J. Hollingsworth, M. Kahre, L. Keely, C. McKay, M. B. Wilhelm, L. Bleacher, W. Brinckerhoff, D. Choi, J. P. Dworkin, J. Eigenbrode, M. Floyd, C. Freissinet, J. Garvin, D. Glavin, A. Jones, D. K. / http://www.sciencemag.org/content/early/recent / 13 December 2013 / Page 5 / 10.1126/science.1247166
    • Martin, A. McAdam, A. Pavlov, E. Raaen, M. D. Smith, J. Stern, F. Tan, M. Meyer, A. Posner, M. Voytek, R. C. Anderson, A. Aubrey, L. W. Beegle, A. Behar, D. Blaney, D. Brinza, F. Calef, L. Christensen, J. A. Crisp, L. DeFlores, B. Ehlmann, J. Feldman, S. Feldman, G. Flesch, J. Hurowitz, I. Jun, D. Keymeulen, J. Maki, M. Mischna, J. M. Morookian, T. Parker, B. Pavri, M. Schoppers, A. Sengstacken, J. J. Simmonds, N. Spanovich, M. T. Juarez, A. R. Vasavada, A. Yen, P. D. Archer, F. Cucinotta, D. Ming, R. V. Morris, P. Niles, E. Rampe, T. Nolan, M. Fisk, L. Radziemski, B. Barraclough, S. Bender, D. Berman, E. N. Dobrea, R. Tokar, D. Vaniman, R. M. E. Williams, A. Yingst, K. Lewis, T. Cleghorn, W. Huntress, G. Manhes, J. Hudgins, T. Olson, N. Stewart, P. Sarrazin, J. Grant, E. Vicenzi, S. A. Wilson, M. Bullock, B. Ehresmann, V. Hamilton, D. Hassler, J. Peterson, S. Rafkin, C. Zeitlin, F. Fedosov, D. Golovin, N. Karpushkina, A. Kozyrev, M. Litvak, A. Malakhov, I. Mitrofanov, M. Mokrousov, S. Nikiforov, V. Prokhorov, A. Sanin, V. Tretyakov, A. Varenikov, A. Vostrukhin, R. Kuzmin, B. Clark, M. Wolff, S. McLennan, O. Botta, D. Drake, K. Bean, M. Lemmon, S. P. Schwenzer, R. B. Anderson, K. Herkenhoff, E. M. Lee, R. Sucharski, M. A. P. Hernandez, J. J. B. Avalos, M. Ramos, M.-H. Kim, C. Malespin, I. Plante, J.-P. Muller, R. Navarro-Gonzalez, R. Ewing, W. Boynton, R. Downs, M. Fitzgibbon, K. Harshman, S. Morrison, W. Dietrich, O. Kortmann, M. Palucis, D. Y. Sumner, A. Williams, G. Lugmair, M. A. Wilson, D. Rubin, B. Jakosky, T. Balic-Zunic, J. Frydenvang, J. K. Jensen, K. Kinch, A. Koefoed, M. B. Madsen, S. L. S. Stipp, N. Boyd, J. L. Campbell, R. Gellert, G. Perrett, I. Pradler, S. VanBommel, S. Jacob, S. Rowland, E. Atlaskin, H. Savijarvi, E. Boehm, S. Bottcher, S. Burmeister, J. Guo, J. Kohler, C. M. Garcia, R. Mueller-Mellin, R. Wimmer-Schweingruber, J. C. Bridges, T. McConnochie, M. Benna, H. Bower, A. Brunner, H. Blau, T. Boucher, M. Carmosino, H. Elliott, D. Halleaux, N. Renno, B. Elliott, J. Spray, L. Thompson, S. Gordon, H. Newsom, A. Ollila, J. Williams, P. Vasconcelos, J. Bentz, K. Nealson, R. Popa, L. C. Kah, J. Moersch, C. Tate, M. Day, G. Kocurek, B. Hallet, R. Sletten, R. Francis, E. McCullough, E. Cloutis, I. L. ten Kate, R. Kuzmin, R. Arvidson, A. Fraeman, D. Scholes, S. Slavney, T. Stein, J. Ward, J. Berger, J. E. Moores; MSL Science Team, Abundance and isotopic composition of gases in the martian atmosphere from the Curiosity rover. Science 341, 263– 266 (2013). doi:10.1126/science.1237966 Medline 52. V. A. Krasnopolsky, G. R. Gladstone, Helium on Mars and Venus: EUVE observations and modeling. Icarus 176, 395–407 (2005). doi:10.1016/j.icarus.2005.02.005 53. K. A. Farley, J. Libarkin, S. Mukhopadhyay, W. Amidon, Cosmogenic and nucleogenic He-3 in apatite, titanite, and zircon. Earth Planet. Sci. Lett. 248, 451–461 (2006). doi:10.1016/j.epsl.2006.06.008 54. K. A. Farley, Cenozoic variations in the flux of interplanetary dust recorded by 3He in a deep sea sediment. Nature 376, 153–156 (1995). doi:10.1038/376153a0 55. D. D. Bogard, K-Ar dating of rocks on Mars: Requirements from Martian meteorite analyses and isochron modeling. Meteorit. Planet. Sci. 44, 3–14 (2009). doi:10.1111/j.1945-5100.2009.tb00713.x 56. W. S. Cassata, P. R. Renne, Systematic variations of argon diffusion in feldspars and implications for thermochronometry. Geochim. Cosmochim. Acta 112, 251–287 (2013). doi:10.1016/j.gca.2013.02.030 57. L. Gourbet, D. L. Shuster, G. Balco, W. S. Cassata, P. R. Renne, D. Rood, Neon diffusion kinetics in olivine, pyroxene and feldspar:Retentivity of cosmogenic and nucleogenic neon. Geochim. Cosmochim. Acta 86, 21–36 (2012). doi:10.1016/j.gca.2012.03.002 58. D. L. Shuster, K. A. Farley, Diffusion kinetics of proton-induced 21Ne, 3He, and 4He in quartz. Geochim. Cosmochim. Acta 69, 2349–2359 (2005). doi:10.1016/j.gca.2004.11.002 59. P. H. Blard, N. Puchol, K. A. Farley, Constraints on the loss of matrix-sited helium during vacuum crushing of mafic phenocrysts. Geochim. Cosmochim. Acta 72, 3788–3803 (2008). doi:10.1016/j.gca.2008.05.044 60. T. D. Swindle, D. A. Kring, M. K. Burkland, D. H. Hill, W. V. Boynton, Noble gases, bulk chemistry, and petrography of olivine-rich achondrites Eagles Nest and Lewis Cliff 88763: Comparison to brachinites. Meteorit. Planet. Sci. 33, 31–48 (1998). doi:10.1111/j.1945-5100.1998.tb01605.x 61. D. D. Bogard, L. E. Nyquist, B. M. Bansal, H. Wiesmann, C. Y. Shih, 76535 Old Lunar Rock. Earth Planet. Sci. Lett. 26, 69–80 (1975). doi:10.1016/0012821X(75)90178-8 62. J. N. Goswami, N. Sinha, S. V. S. Murty, R. K. Mohapatra, C. J. Clement, Nuclear tracks and light noble gases in Allan Hills 84001: Preatmospheric size, fail characteristics, cosmic-ray exposure duration and formation age. Meteorit. Planet. Sci. 32, 91–96 (1997). doi:10.1111/j.19455100.1997.tb01244.x 63. T. Cerling, Dating geomorphic surfaces using cosmogenic 3He. Quat. Res. 33, 148–156 (1990). doi:10.1016/0033-5894(90)90015-D 64. A. Jambon, J. S. Shelby, Helium diffusion and solubility in obsidians and basaltic glass in the range 200–300°C. Earth Planet. Sci. Lett. 51, 206–214 (1980). doi:10.1016/0012-821X(80)90268-X 65. R. Wieler, in Noble Gases in Geochemistry and Cosmochemistry, D. Porcelli, C. J. Ballentine, R. Wieler, Eds. (2002), vol. 47, pp. 125–170. 66. S. R. Taylor, S. M. McLennan, Planetary Crusts: Their Composition, Origin, and Evolution (Cambridge University Press, Cambridge, 2009), pp. 378. 67. G. J. Taylor, The bulk composition of Mars. Chemie Der Erde-Geochemistry, published online 5 October 2013 (10.1016/j.chemer.2013.09.006). Acknowledgments: The authors are indebted to the Mars Science Laboratory Project engineering and management teams for their exceptionally skilled and diligent efforts in making the mission as effective as possible and enhancing science operations. We are also grateful to all those MSL team members who participated in tactical and strategic operations. Without the support of both the engineering and science teams, the data presented here could not have been collected. Three anonymous reviewers provided many helpful suggestions. Some of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Data presented in this paper are archived in the Planetary Data System (pds.nasa.gov). Supplementary Materials www.sciencemag.org/cgi/content/1247166/DC1 Materials and Methods Figs. S1 to S3 Tables S1 to S4 References (48–67) 14 October 2013; accepted 25 November 2013 Published online 09 December 2013 10.1126/science.1247166 / http://www.sciencemag.org/content/early/recent / 13 December 2013 / Page 6 / 10.1126/science.1247166
    • Table 1. Geochronology data. Sheepbed Mudstone Cumberland Sample Sample Mass K-Ar System K2O (wt %) 40 Ar (nmol/g) K-Ar Age (Ga) 0.135 ± 0.018 g ±1σ 0.50 0.08 11.95 1.71 4.21 0.35 Cosmogenic Isotopes Isotope 3 He 21 Ne 36 Ar pmol/g 33.7 4.49 55.6 ± 6.9 1.52 16.8 PR (pmol g−1 Ma−1)A Surface 2m Average 0.466 0.254 0.054 0.034 0.714 1.029 Exposure AgeB (Ma) ± 72 15 84 28 78 24 Notes: elemental composition from APXS measurement of Cumberland drill tailings. AModel isotope production rate (47). BSurface exposure age assuming no erosion. / http://www.sciencemag.org/content/early/recent / 13 December 2013 / Page 7 / 10.1126/science.1247166
    • Fig. 1. Mastcam view looking 79° west of North. With increasing distance the Yellowknife Bay formation includes the Sheepbed mudstone, Gillespie Lake sandstone, and Point Lake outcrop (36 m away). In the distance, the mostly rockand sand-covered Bradbury rise is visible. The large outcrop on the near horizon (marked “x”) is 240 m distant and stands 13 m above the Gillespie Lake-Sheepbed contact. This is a portion of a 40 frame M-100 mosaic taken on Sol 188 between 13:27 and 13:48 LMST (2013-02-14 23:41:35 and 2013-02-15 00:03:23 PST). The images in the full mosaic, acquired as sequence mcam01009, have picture IDs between 0188MR0010090000202415I01 and 0188MR0010090390202454I01. / http://www.sciencemag.org/content/early/recent / 13 December 2013 / Page 8 / 10.1126/science.1247166
    • Fig. 2. Mars Hand Lens Imager (MAHLI) image of brushed, gray bedrock outcrop of Sheepbed mudstone near the Cumberland drill hole. Protrusion of nodules (9) results from eolian scouring of rock surface, creating wind-tails. Preference for steep faces of wind-tails on NE side suggests long-term averaged paleowind direction from NE to SW. This is a portion of MAHLI image 0291MH0001970010103390C00, acquired on Sol 291. Illumination from the top/upper left. Fig. 3. Depth dependence of cosmogenic isotope production rates 3 modeled for a rock of Cumberland mudstone chemistry on Mars. He 21 36 and Ne are spallation isotopes, while Ar is produced by capture of 3 21 cosmogenic neutrons. Note the multiplicative factors applied to He and Ne. 3 A mudstone bulk density of 2.6 g/cm was assumed to convert overburden mass to linear depth. / http://www.sciencemag.org/content/early/recent / 13 December 2013 / Page 9 / 10.1126/science.1247166