Research Articles

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Multiple orbiter and rover missions have documented a rich geologic record on the surface of Mars, likely spanning almost the entire history of the planet (e.g., 1). However, interpretation of this record is impeded by our limited understanding of the connection between the materials observed and their absolute age. Impact crater densities are the primary means for establishing a chronology for geologic features on Mars and other solar system bodies (2–4), but crater-based dating methods suffer from multiple sources of uncertainty that obscure both absolute and rela-tive ages. In contrast, on Earth, isotopic dating methods provide a pow-erful quantitative framework for defining geologic history and for identifying the rates and causes of geologic phenomena. Here we report results of an attempt to apply in-situ isotopic dating methods to Mars,

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 Noa-chian - Early Hesperian boundary (5, 6), or around 3.7-3.5 Ga according to impact crater models (3, 7). In addition to the ~5 km high central mountain of stratified rock informally known as Mt. Sharp, the crater is partially filled with sedimentary rocks derived from the crater rim. Crater density distributions imply a somewhat younger age (Early to Late Hesperian, or ~3.5-2.9 Ga) for at least some of these deposits (5, 6). While heading for its destination at Mt. Sharp, Curiosity traversed a gently sloping plain where local outcrops of pebble conglomerate were encountered, recording the presence of an ancient stream bed (8). Most of this surface is heavily cratered and covered with rock and soil (Fig. 1). However, a substantial expanse of bare bedrock was encoun-tered in an ~5 m deep topographic trough (Yellowknife Bay) representing an erosional window through a se-quence of stratified rocks known as the Yellowknife Bay formation (Fig. S1 and (9)). These rocks consist mostly of distal alluvial fan and lacustrine facies of basaltic bulk composition and were derived from the crater rim (9, 10). Compared to adjacent geological units, the Yellowknife Bay trough is notable for its lack of visible craters and its apparently greater degree of stripping of impact ejecta and wind-blown soil. This appearance suggests active ero-sion, distinctly different from the adja-cent map units (9). However, the depositional age of the Yellowknife Bay formation, its stratigraphic rela-tionship to the adjacent alluvial fan and to the strata of Mt. Sharp, and the caus-es and timing of its exposure are all presently uncertain.

As shown in Fig. 1, the Sheepbed mudstone and stratigraphically overly-ing Gillespie Lake sandstone of the

Yellowknife Bay formation (9) form a rock couplet of apparently varia-ble rock hardness, and their contact has eroded to form a decimeter-scale topographic step. The Sheepbed-Gillespie Lake contact is sharp and marked by scouring of the underlying mudstone, producing a small scarp and in some locations an overhang. Calved-off blocks of the sandstone occur at the base and a few meters outboard of the scarp, but not beyond (see figure 3 of (9)). This suggests that residual fragments of the degrad-ing scarp are removed efficiently enough to leave only a very thin lag deposit at locations where the Gillespie Lake member is now completely absent.

The distinctive erosional profile of the Sheepbed-Gillespie Lake con-

In Situ Radiometric and Exposure Age Dating of the Martian Surface K. A. Farley,1* C. Malespin,2 P. Mahaffy,2 J. P. Grotzinger,1 P. M. Vasconcelos,3 R. E. Milliken,4 M. Malin,5 K. S. Edgett,5 A. A. Pavlov,2 J. A. Hurowitz,6 J. A. Grant,7 H. B. Miller,1 R. Arvidson,8 L. Beegle,9 F. Calef,9 P. G. Conrad,2 W. E. Dietrich,10 J. Eigenbrode,2 R. Gellert,11 S. Gupta,12 V. Hamilton,13 D. M. Hassler,13 K.W. Lewis,14 S. M. McLennan,6 D. Ming,15 R. Navarro-González,16 S. P. Schwenzer,17 A. Steele,18 E. M. Stolper,1 D. Y. Sumner,19 D. Vaniman,20 A. Vasavada,9 K. Williford,9 R. F. Wimmer-Schweingruber,21 and the MSL Science Team† 1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA. 2NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA. 3School of Earth Sciences, University of Queensland, Brisbane, Queensland, QLD 4072, Australia. 4Department of Geological Sciences, Brown University, Providence, RI 02912, USA. 5Malin Space Science Systems, San Diego, CA 92121, USA. 6Department of Geosciences, Stony Brook University, Stony Brook, NY 11794, USA. 7Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, 6th at Independence SW, Washington, DC 20560, USA. 8Department of Earth and Planetary Sciences, Washington 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 of California, Berkeley, CA 94720, USA. 11Department of Physics, University of Guelph, Guelph, ON N1G 2W1, Canada. 12Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, UK. 13Southwest Research Institute, Boulder, CO 80302, USA. 14Department of Geosciences, Princeton University, Princeton, NJ 08544, USA. 15NASA Johnson Space Center, Houston, TX 77058, USA. 16Universidad Nacional Autonoma de Mexico, Coyoacan, Mexico City 4510, Mexico. 17Department of Physical Sciences, CEPSAR, Walton Hall, Milton Keynes MK7 6AA, UK. 18Carnegie Institution, Geophysical Laboratory, Washington, DC 20015, USA. 19Department of Geology, University of California, Davis, CA 95616, USA. 20Planetary Science Institute, Tucson, AZ 85719, USA. 21University of Kiel, Kiel D-24098, Germany.

*Corresponding author. E-mail: farley@gps.caltech.edu

†MSL Science Team authors and affiliations are listed in the supplementary materials.

We determined radiogenic and cosmogenic noble gases in a mudstone on the floor of Gale crater. A K-Ar age of 4.21 ± 0.35 Ga represents a mixture of detrital and authigenic components, and confirms the expected antiquity of rocks comprising the crater rim. Cosmic-ray-produced 3He, 21Ne, and 36Ar yield concordant surface exposure ages of 78 ± 30 Ma. Surface exposure occurred mainly in the present geomorphic setting rather than during primary erosion and transport. Our observations are consistent with mudstone deposition shortly after the Gale impact, or possibly in a later event of rapid erosion and deposition. The mudstone remained buried until recent exposure by wind-driven scarp retreat. Sedimentary rocks exposed by this mechanism may thus offer the best potential for organic biomarker preservation against destruction by cosmic radiation.

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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 topo-graphic protrusions form fields of small ridges that have been stream-lined 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 process-es in eroding the mudstone, in efficiently wearing down any blocks that are delivered to its surface from the encircling scarps or from distal im-pacts, and in expanding the exposure of the Yellowknife Bay formation.

Scientific investigations at Yellowknife Bay focused on the Sheep-bed 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 con-sisting of both crystalline and x-ray amorphous phases (11). The detrital fraction includes minerals typical of volcanic rocks: plagioclase, pyrox-enes, 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 amor-phous component, comprising about one third of the sample, indicates it is composed mainly of Si, Fe, Ca, S, and Cl (11). It likely includes poor-ly 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 experi-enced 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 at-mosphere 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 cosmo-genic isotope is likely to be 36Ar produced from the capture of cosmo-genic 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 multi-ple isotopic systems.

The depth-dependence of the production functions of the two spalla-tion 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 pro-duction rate of 36Ar from neutron capture has a larger subsurface maxi-mum 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 non-

unique exposure history for the rock. The customary end-member inter-pretive 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 compo-sition initially at a depth of greater than a few meters that was instanta-neously exposed at the surface would have 36Ar/3He of ~1.5 and 36Ar/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 meas-urements 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 pre-sent 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 concen-tration 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 extrac-tion 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 po-tassium 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 mix-ture of the ages of components present in the crater rim, principally bed-rock 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 con-clude 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-

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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. K-bearing 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 remain-der could then be in the authigenic clay (18% of the mudstone) with 0.6% K2O, in agreement with K2O concentrations in saponite in a Mar-tian 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 36Ar, 21Ne, and 3He were all detected at levels that cannot be at-

tributed to sources other than cosmic ray irradiation (33). In the mud-stone, 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 expo-sure 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 expo-sure intervals separated by burial events, e.g., by exposure during initial deposition and then again by recent re-exposure, or alternatively by buri-al 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 exam-ple, diffusive loss of He from amorphous phases, plagioclase and phyllo-silicates might occur at Mars ambient temperature (35), but the 3He/21Ne ratio of ~7.5 ± 2.6 matches the expected production ratio of ~8. Similar-ly, 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 for-tuitous. 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 enrich-ment of Cl compared to a typical Martian basalt suggests much of this element was added to the mudstone from solution (9, 10). This distinc-tion 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 deposi-tion 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 for-mation. 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 ero-sion is occurring by scarp retreat. The Sheepbed mudstone lies below a sequence of relatively more resistant units that define a series of topo-graphic 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 thickness-es and resistance properties. This predominately lateral erosion has caused scarp migration to the southwestward and possibly other direc-tions, and produced several meters of surface lowering. The general flattening of the topographic profile (Fig. S1) as it extends into the Yel-lowknife 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 sur-face exposure ages. To prevent cosmogenic nuclide accumulation re-quires 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. Alternative-ly, 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 sur-face rocks subjected to cosmic ray irradiation has been modeled (44). When scaled to the long-term cosmic ray dose implied by the cosmogen-ic 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 minimiz-ing 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 to-ward a bounding scarp (at least in the downwind direction), such a loca-tion may offer the best potential for organic preservation. Given our estimated scarp retreat rate, locations within a few tens of cm of a few-meter-scale scarp may have been exposed to cosmic rays for less than a few Ma. Such a short exposure compares favorably to what can reasona-bly be expected from alternative sampling strategies relying on recent

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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 5 / 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/s11214-012-9898-9

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 upper-crustal 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/s11214-012-9879-z

49. National Geochronological Database, United States Geological Survey Open-File 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. Mora-Sotomayor, 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 6 / 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:10.1016/0012-821X(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.1945-5100.1997.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

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Table 1. Geochronology data.

Sheepbed Mudstone Cumberland Sample

Sample Mass 0.135 ± 0.018 g K-Ar System

±1σ

K2O (wt %) 0.50 0.08

40Ar (nmol/g) 11.95 1.71

K-Ar Age (Ga) 4.21 0.35

Cosmogenic Isotopes

PR (pmol g−1 Ma−1)A Exposure AgeB

Isotope pmol/g ± Surface 2m Average (Ma) ±

3He 33.7 6.9 0.466 0.254

72 15

21Ne 4.49 1.52 0.054 0.034

84 28

36Ar 55.6 16.8 0.714 1.029

78 24 Notes: elemental composition from APXS measurement of Cumberland drill tailings. AModel isotope production rate (47). BSurface exposure age assuming no erosion.

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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 rock- and 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.

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Fig. 3. Depth dependence of cosmogenic isotope production rates modeled for a rock of Cumberland mudstone chemistry on Mars. 3He and 21Ne are spallation isotopes, while 36Ar is produced by capture of cosmogenic neutrons. Note the multiplicative factors applied to 3He and 21Ne. A mudstone bulk density of 2.6 g/cm3 was assumed to convert overburden mass to linear depth.

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.