Research Articles

using the instrument suite aboard the In Situ Radiometric and Exposure Age Curiosity rover exploring Gale crater. Geologic Setting Dating of the Martian Surface Gale is a ~150 km diameter impact crater that formed near the Late Noa- chian - Early Hesperian boundary (5, ,1 ,2 ,2 ,1 K. A. Farley * C. Malespin P. Mahaffy J. P. Grotzinger P. M. 6), or around 3.7-3.5 Ga according to 3 4 5 5 2 Vasconcelos, R. E. Milliken, M. Malin, K. S. Edgett, A. A. Pavlov, impact crater models (3, 7). In addition J. A. Hurowitz,6 J. A. Grant,7 H. B. Miller,1 R. Arvidson,8 L. Beegle,9 to the ~5 km high central mountain of 9 2 10 2 stratified rock informally known as Mt. F. Calef, P. G. Conrad, W. E. Dietrich, J. Eigenbrode, R. Sharp, the crater is partially filled with 11 12 13 13 14 Gellert, S. Gupta, V. Hamilton, D. M. Hassler, K.W. Lewis, S. sedimentary rocks derived from the M. McLennan,6 D. Ming,15 R. Navarro-González,16 S. P. crater rim. Crater density distributions 17 18 1 19 imply a somewhat younger age (Early Schwenzer, A. Steele, E. M. Stolper, D. Y. Sumner, D. to Late Hesperian, or ~3.5-2.9 Ga) for 20 9 9 Vaniman, A. Vasavada, K. Williford, R. F. Wimmer- at least some of these deposits (5, 6). Schweingruber,21 and the MSL Science Team† While heading for its destination at Mt. Sharp, Curiosity traversed a gently 1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, sloping plain where local outcrops of USA. 2NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA. 3School of Earth Sciences, 4 pebble conglomerate were encountered, University of Queensland, Brisbane, Queensland, QLD 4072, Australia. Department of Geological recording the presence of an ancient Sciences, Brown University, Providence, RI 02912, USA. 5Malin Space Science Systems, San Diego, CA 6 7 stream bed (8). Most of this surface is 92121, USA. Department of Geosciences, Stony Brook University, Stony Brook, NY 11794, USA. Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, 6th at heavily cratered and covered with rock Independence SW, Washington, DC 20560, USA. 8Department of Earth and Planetary Sciences, and soil (Fig. 1). However, a substantial Washington University in St. Louis, St. Louis, MO, 63130, USA. 9Jet Propulsion Laboratory, California expanse of bare bedrock was encoun- Institute of Technology, Pasadena, CA 91109, USA. 10Earth and Planetary Science Department, University tered in an ~5 m deep topographic of California, Berkeley, CA 94720, USA. 11Department of Physics, University of Guelph, Guelph, ON N1G trough (Yellowknife Bay) representing 2W1, Canada. 12Department of Earth Science and Engineering, Imperial College London, London SW7 13 14 an erosional window through a se- 2AZ, UK. Southwest Research Institute, Boulder, CO 80302, USA. Department of Geosciences, quence of stratified rocks known as the Princeton University, Princeton, NJ 08544, USA. 15NASA Johnson Space Center, Houston, TX 77058, USA. 16 17 Yellowknife Bay formation (Fig. S1 Universidad Nacional Autonoma de Mexico, Coyoacan, Mexico City 4510, Mexico. Department of Physical Sciences, CEPSAR, Walton Hall, Milton Keynes MK7 6AA, UK. 18Carnegie Institution, Geophysical and (9)). These rocks consist mostly of Laboratory, Washington, DC 20015, USA. 19Department of Geology, University of California, Davis, CA distal alluvial fan and lacustrine facies 95616, USA. 20Planetary Science Institute, Tucson, AZ 85719, USA. 21University of Kiel, Kiel D-24098, of basaltic bulk composition and were Germany. derived from the crater rim (9, 10). *Corresponding author. E-mail: [email protected] Compared to adjacent geological units, the Yellowknife Bay trough is notable †MSL Science Team authors and affiliations are listed in the supplementary materials. for its lack of visible craters and its apparently greater degree of stripping We determined radiogenic and cosmogenic noble gases in a mudstone on the floor of impact ejecta and wind-blown soil. of Gale crater. A K-Ar age of 4.21 ± 0.35 Ga represents a mixture of detrital and This appearance suggests active ero- authigenic components, and confirms the expected antiquity of rocks comprising sion, distinctly different from the adja- 3 21 36 the crater rim. Cosmic-ray-produced He, Ne, and Ar yield concordant surface cent map units (9). However, the exposure ages of 78 ± 30 Ma. Surface exposure occurred mainly in the present depositional age of the Yellowknife geomorphic setting rather than during primary erosion and transport. Our Bay formation, its stratigraphic rela- observations are consistent with mudstone deposition shortly after the Gale impact, tionship to the adjacent alluvial fan and or possibly in a later event of rapid erosion and deposition. The mudstone remained to the strata of Mt. Sharp, and the caus- buried until recent exposure by wind-driven scarp retreat. Sedimentary rocks es and timing of its exposure are all exposed by this mechanism may thus offer the best potential for organic biomarker presently uncertain. preservation against destruction by cosmic radiation. As shown in Fig. 1, the Sheepbed mudstone and stratigraphically overly- ing Gillespie Lake sandstone of the Multiple orbiter and rover missions have documented a rich geologic Yellowknife Bay formation (9) form a rock couplet of apparently varia- record on the surface of Mars, likely spanning almost the entire history ble rock hardness, and their contact has eroded to form a decimeter-scale of the planet (e.g., 1). However, interpretation of this record is impeded topographic step. The Sheepbed-Gillespie Lake contact is sharp and by our limited understanding of the connection between the materials marked by scouring of the underlying mudstone, producing a small scarp observed and their absolute age. Impact crater densities are the primary and in some locations an overhang. Calved-off blocks of the sandstone means for establishing a chronology for geologic features on Mars and occur at the base and a few meters outboard of the scarp, but not beyond other solar system bodies (2–4), but crater-based dating methods suffer (see figure 3 of (9)). This suggests that residual fragments of the degrad- from multiple sources of uncertainty that obscure both absolute and rela- ing scarp are removed efficiently enough to leave only a very thin lag tive ages. In contrast, on Earth, isotopic dating methods provide a pow- deposit at locations where the Gillespie Lake member is now completely erful quantitative framework for defining geologic history and for absent. identifying the rates and causes of geologic phenomena. Here we report The distinctive erosional profile of the Sheepbed-Gillespie Lake con- results of an attempt to apply in-situ isotopic dating methods to Mars,

/ http://www.sciencemag.org/content/early/recent / 9 December 2013 / Page 1 / 10.1126/science.1247166 tact can be traced around the full width of Yellowknife Bay (9) with the energy neutrons into the Martian atmosphere (e.g., 19). Sheepbed unit making up the floor of the encircled trough. This trough is The concentration of a single cosmogenic isotope yields a non- elongated in the NE-SW direction, creating an amphitheater-shaped unique exposure history for the rock. The customary end-member inter- planimetric form open to the NE. Beyond the Gillespie Lake contact, the pretive models are to assume either no erosion, in which case a surface more resistant units higher in the section form similar scarps stepping exposure age is obtained, or steady erosion from great depth, in which upward a total of ~5 m to the uppermost unit of the Yellowknife Bay case a mean erosion rate is obtained (e.g., 17). Figure 3 shows that this formation (Fig. S1). Within the Sheepbed unit, mm- to cm-scale topo- non-uniqueness can be eliminated by combining spallogenic and neutron graphic protrusions form fields of small ridges that have been stream- capture isotope measurements. In particular, a rock of Sheepbed compo- lined in the NE-SW direction (Fig. 2). Along with the morphology of the sition initially at a depth of greater than a few meters that was instanta- Yellowknife Bay trough, this observations indicates a dominant role for neously exposed at the surface would have 36Ar/3He of ~1.5 and SW-directed wind erosion. These observations implicate eolian process- 36Ar/21Ne of ~13. In contrast, if steady erosion causes progressive es in eroding the mudstone, in efficiently wearing down any blocks that downward migration of the surface toward the sample, then the sample are delivered to its surface from the encircling scarps or from distal im- integrates the entire depth profile including the large subsurface 36Ar pacts, and in expanding the exposure of the Yellowknife Bay formation. peak. Such a rock would have 36Ar/3He~4 and 36Ar/21Ne~30. In both Scientific investigations at Yellowknife Bay focused on the Sheep- erosion scenarios the 3He/21Ne ratio is ~8; this isotope pair provides a bed mudstone. This fine-grained (<50 μm) rock was drilled twice for cross-check but no additional information on exposure history. mineralogical and evolved gas analyses (11, 12). In addition, multiple chemical analyses were obtained on outcropping mudstone as well as the Results and Discussion drill tailings. The two drilled samples, referred to as John Klein and Geochronology measurements were performed on the Cumberland Cumberland, were located ~3 m apart, and yielded very similar results. drilled powder (21). After drilling, the sample was sieved and an aliquot The mudstone is composed of detrital and authigenic components con- with estimated mass of 135 ± 18 mg (22) was introduced into a quartz sisting of both crystalline and x-ray amorphous phases (11). The detrital cup in the Sample Analysis at Mars (SAM) instrument. The cup was fraction includes minerals typical of volcanic rocks: plagioclase, pyrox- moved into position in an oven, sealed against Mars atmosphere, and enes, and minor olivine, sanidine and ilmenite. Crystalline authigenic evacuated. The sample was then heated to a maximum temperature of phases include smectitic clay (possibly saponite) and magnetite, thought ~890°C for 25 min, and the evolved gases were purified and admitted to have formed as a result of chemical alteration of detrital olivine, and into the quadrupole mass spectrometer for analysis. All reported meas- minor akaganeite, pyrrhotite, bassanite, and hematite. Quartz and halite urements used the high sensitivity semi-static analysis mode. were reported, but very near the CheMin detection limit. Chlorate or perchlorate was also tentatively identified (12). A model for the amor- K-Ar Age phous component, comprising about one third of the sample, indicates it The Ar concentration and the APXS-determined K2O concentration is composed mainly of Si, Fe, Ca, S, and Cl (11). It likely includes poor- yield a K-Ar age of 4.21 ± 0.35 Ga (1σ) for the Cumberland sample ly crystalline or finely crystalline materials and may include detrital (Table 1). This age calculation assumes all measured 40Ar is radiogenic volcanic and impact-derived glass. The presence of smectite, magnetite, (23). Although a fairly uniform level of excess Ar is thought to be pre- and akaganeite suggests that the mudstone has not experienced burial sent in some young Martian meteorites (24), that same level of excess heating above ~200°C, (11, 13). It is possible that the sample experi- would contribute <0.03 Ga to the age of this high K2O, high Ar concen- enced no burial heating at all. 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 Geochronology Overview: K/Ar and Cosmogenic Isotopes despite a fairly low extraction temperature. This surprisingly high yield Noble gas isotopes can quantitatively constrain the age and erosion (25) may result from rapid diffusive loss from the inherently fine grain 40 40 history of the Sheepbed mudstone. Ar from K decay will record a size of the mudstone (probably enhanced by the powdering process of potassium-weighted average of the formation or cooling ages of the the drill (26)). Alternatively, the volatile constituents in the mudstone 36 21 3 multiple components of the mudstone. Ar, Ne, and He are produced may promote loss via flux-induced melting. Since fine grain size and by irradiation of the uppermost ~2-3 m of the Martian surface by galactic flux-melting will affect the noble gas isotopes similarly, effective extrac- cosmic rays (GCRs) (14). These cosmogenic isotopes are commonly tion of 36Ar, 21Ne and 3He might also be expected. The high age also used to assess exposure histories of meteorites (e.g., 15, 16) and erosion implies that a very large fraction of the radiogenic Ar was retained in the rates and styles of terrestrial rocks (e.g., 17). Because the Martian at- mudstone over geologic time. mosphere is very thin and the magnetic field very weak (18), the Martian The interpretation of the K-Ar age depends critically on where po- surface is only weakly shielded from GCRs. Thus cosmogenic isotope tassium is located in the rock. If a fraction FD of the potassium is in the production rates are much higher than on Earth (14, 19). The chemistry detrital phases and the remainder in authigenic phases, then the bulk age of the Sheepbed mudstone (10) is such that the most abundant cosmo- (TB) is a potassium-weighted average of the age of the detritus (TD) and 36 genic isotope is likely to be Ar produced from the capture of cosmo- the age of authigenesis (TA): TB=(1-FD)TA+FDTD. If all of the potassium 3 genic thermal neutrons by Cl, followed by He produced by spallation is carried in the detrital components (mainly sanidine, plagioclase and 21 mainly of O, Si, and Mg, followed by Ne from spallation of Mg, Si, possibly basalt glass), FD=1. In this case the K-Ar system records a mix- and Al (20). The mudstone’s exposure history is thus recorded by multi- ture of the ages of components present in the crater rim, principally bed- ple isotopic systems. rock ranging from minimally to completely reset by the Gale-forming The depth-dependence of the production functions of the two spalla- impact. Crater rim lithologies as well as the crater itself are thought to tion isotopes are very similar: a small maximum at ~15 cm below the range from Noachian to early-Hesperian in age, or about 4.1 to 3.5 Ga surface reflecting development of the nuclear cascade in the uppermost (5, 6, 27–29), in agreement with the K-Ar age of 4.21 ± 0.35 Ga we layers of rock, followed by an exponential decay with an e-folding depth measured. Because formation of K-bearing authigenic phases can only of ~1 m as the energetic particles attenuate (Fig. 3). In contrast, the pro- lower the mudstone age below that of the detrital component, we con- 36 duction rate of Ar from neutron capture has a larger subsurface maxi- clude with 95% confidence (30) that the potassium-weighted mean age mum at greater depth (~60 cm) arising from the combined effects of the of the materials in the Gale crater wall exceeds 3.6 Ga. Alternatively, if production of neutrons within the descending cascade and the loss of low 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 Noachian-Hesperian boundary (37). Because wet conditions favor rapid records the formation age of those phases. In this case 4.21 Ga would erosion, in either scenario the cosmogenic isotope observations would represent a minimum age of mudstone deposition. support deposition of the Sheepbed mudstone shortly after Gale for- At present we have no definitive way to determine the potassium mation. Alternatively, deposition may have occurred in association with distribution in the mudstone, but we can make a reasonable estimate. K- a later event in which material was eroded and transported rapidly bearing phases determined by CheMin include sanidine (1.8%), andesine enough to prevent detectable cosmogenic production. A late erosional feldspar (21%) and an unknown fraction of basaltic or impact glass (11). event has also been suggested for Amazonian-age fan formation in some CheMin cannot determine the K2O content of any of these phases, so we other Martian craters (38–40). assume representative values of 13%, 0.3%, and 1%, respectively (31). Topography, stratigraphy and surface exposure dating suggest that By assuming the mudstone contains 10% glass, we conclude that the the Yellowknife Bay trough is currently a focus of erosion, and that ero- detrital component accounts for 80% of the measured K2O. The remain- sion is occurring by scarp retreat. The Sheepbed mudstone lies below a der could then be in the authigenic clay (18% of the mudstone) with sequence of relatively more resistant units that define a series of topo- 0.6% K2O, in agreement with K2O concentrations in saponite in a Mar- graphic steps (A-A’ in Fig. S1). The pace of wind erosion for the entire tian meteorite (32). If we assume TD<4.1 Ga based on crater-count ages escarpment will be set by the retreat rate of these more resistant units. of the Gale impact and host terrains, and use the 95% confidence lower Deflation of the softer units, such as the mudstone, however, undercuts limit on the K-Ar age of 3.6 Ga, we obtain a current best estimate that and contributes to collapse of the resistant layers (Fig. 1), such that both the mudstone was deposited prior to 1.6 Ga. Assuming either a younger direct wind abrasion of resistant units and removal of the softer units age for the detrital component or a smaller fraction of it in the mudstone contribute to slope retreat. Resistant units and corresponding local steps would cause this estimate to rise. will thicken or thin as erosion sweeps into units with variable thickness- es and resistance properties. This predominately lateral erosion has Cosmogenic 36Ar, 21Ne, and 3He caused scarp migration to the southwestward and possibly other direc- 36Ar, 21Ne, and 3He were all detected at levels that cannot be at- tions, and produced several meters of surface lowering. The general tributed to sources other than cosmic ray irradiation (33). In the mud- flattening of the topographic profile (Fig. S1) as it extends into the Yel- stone, 36Ar/3He and 36Ar/21Ne are 1.7 ± 0.5 and 12 ± 5, respectively (34). lowknife Bay trough may indicate that a more resistant unit beneath the These ratios are within error of predictions of the no-erosion scenario Sheepbed mudstone has slowed continued removal of this unit. (1.5 and 13) and very different from the prediction of the steady erosion In such a model, the scarp retreat rate can be estimated from the sur- scenario (4 and 30). Thus we cast our results in terms of surface expo- face exposure ages. To prevent cosmogenic nuclide accumulation re- sure age, which for 3He, 21Ne, and 36Ar are 72 ± 15, 84 ± 28 and 79 ± 24 quires at least 2-3 m of overburden. Southwestward from the drill site Ma, respectively. These ages could be the sum of multiple shorter expo- (and downwind, according to Fig. 3) this amount of overburden is first sure intervals separated by burial events, e.g., by exposure during initial encountered about 60 m away (Fig. S1). Thus, in a model in which the deposition and then again by recent re-exposure, or alternatively by buri- mudstone is rapidly exposed from >2-3 m depth to the surface, the scarp al and exhumation associated with migrating eolian deposits. However, retreat rate over the last ~80 Ma averages ~0.75 m/Ma. At this rate, the these events would have to involve rapid burial and removal of at least a full extent of the Yellowknife Bay exposure could have been produced few meters of cover to maintain the good match of measured nuclide over several hundred million years of steady lateral erosion. Alternative- concentration ratios to those of production at the surface. ly, wind erosion and associated scarp retreat may be highly episodic, The agreement among these results argues for complete extraction of perhaps in response to climatic variations tied to Mars’ obliquity cycle all three noble gases in the SAM oven, and against substantial loss of (41, 42). Either way, surface exposure ages may vary substantially noble gases over the ~78 Myr period of cosmic ray exposure. For exam- around the Yellowknife Bay outcrop and other similar exposures, in ple, diffusive loss of He from amorphous phases, plagioclase and phyllo- principle offering a test of the scarp retreat hypothesis. 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- Implications for Organic Preservation ly, dissolution and reprecipitation of a water-soluble Cl-rich phase by Cosmic rays penetrating the uppermost several meters of rock create occasional wetting of the mudstone might release accumulated 36Ar. If a cascade of atomic and subatomic particles, electrons, and photons that this occurred to a significant extent, then the agreement between the 36Ar ionize molecules and atoms in their path (e.g., 43, 44). Complex organic exposure age and the 21Ne and 3He exposure ages would have to be for- molecules are particularly susceptible to degradation by such particles, tuitous. While not impossible, we see no evidence for it in these data. and because these particles also produce the cosmogenic nuclides, we A significant portion of the 3He and 21Ne must be carried by detrital have a way to assess the likely magnitude of this degradation in the minerals including plagioclase and pyroxene because these are important Cumberland sample. Degradation of organic molecules in Martian sur- host phases for the major target elements (36). In contrast, the enrich- face rocks subjected to cosmic ray irradiation has been modeled (44). ment of Cl compared to a typical Martian basalt suggests much of this When scaled to the long-term cosmic ray dose implied by the cosmogen- element was added to the mudstone from solution (9, 10). This distinc- ic nuclides in the Cumberland sample, these rates predict reductions of tion implies that, while 3He and 21Ne might record cosmic ray irradiation between 2 and 3 orders of magnitude in the concentration of organic that occurred during primary exposure and transport in addition to that molecules in the 100-400 AMU range. acquired during modern exposure at Yellowknife Bay, the same is not The scarp retreat hypothesis offers a possible strategy for minimiz- true of 36Ar. The logical interpretation of the good agreement among all ing the degree of organic degradation in samples obtained during the three exposure ages is that all three reflect exposure only in the modern further course of exploration by Curiosity and possible future missions. setting. Because exposure ages and thus cosmic ray doses should decrease to- The apparent absence of a detrital cosmogenic signal favors deposi- ward a bounding scarp (at least in the downwind direction), such a loca- tion of the mudstone in an environment in which erosion and transport tion may offer the best potential for organic preservation. Given our were fairly rapid and/or in which the Martian surface was shielded from estimated scarp retreat rate, locations within a few tens of cm of a few- cosmic rays by a reasonably dense atmosphere. The transition from meter-scale scarp may have been exposed to cosmic rays for less than a comparatively wet conditions and a thick atmosphere to cold and dry few Ma. Such a short exposure compares favorably to what can reasona- conditions with a thin atmosphere is thought to have occurred around the bly 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. 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. References and Notes Schoppers, A. Sengstacken, J. J. Simmonds, N. Spanovich, M. de la Torre 1. M. H. Carr, J. W. Head III, Geologic History of Mars. Earth Planet. Sci. Lett. Juarez, A. R. Vasavada, C. R. Webster, A. Yen, P. D. Archer, F. Cucinotta, J. 294, 185–203 (2010). doi:10.1016/j.epsl.2009.06.042 H. Jones, D. Ming, R. V. Morris, P. Niles, E. Rampe, T. Nolan, M. Fisk, L. 2. E. M. Shoemaker, R. J. Hackman, R. E. Eggleton, “Interplanetary correlation Radziemski, B. Barraclough, S. Bender, D. Berman, E. N. Dobrea, R. Tokar, of geologic time,” in Advances in the Astronautical Sciences (Plenum, New D. Vaniman, L. Leshin, T. Cleghorn, W. Huntress, G. Manhes, J. Hudgins, T. York, 1962), vol. 8, pp. 70–89. Olson, N. Stewart, P. Sarrazin, J. Grant, E. Vicenzi, S. A. Wilson, M. Bullock, 3. W. K. Hartmann, G. Neukum, Cratering chronology and the evolution of Mars. B. Ehresmann, V. Hamilton, D. Hassler, J. Peterson, S. Rafkin, C. Zeitlin, F. Space Sci. Rev. 96, 165–194 (2001). doi:10.1023/A:1011945222010 Fedosov, D. Golovin, N. Karpushkina, A. Kozyrev, M. Litvak, A. Malakhov, 4. W. K. Hartmann, Martian cratering 8: Isochron refinement and the chronology I. Mitrofanov, M. Mokrousov, S. Nikiforov, V. Prokhorov, A. Sanin, V. of Mars. Icarus 174, 294–320 (2005). doi:10.1016/j.icarus.2004.11.023 Tretyakov, A. Varenikov, A. Vostrukhin, R. Kuzmin, B. Clark, M. Wolff, S. 5. L. Le Deit, E. Hauber, F. Fueten, M. Pondrelli, A. P. Rossi, R. Jaumann, McLennan, O. Botta, D. Drake, K. Bean, M. Lemmon, E. M. Lee, R. Sequence of infilling events in Gale crater, Mars: Results from morphology, Sucharski, M. A. P. Hernandez, J. J. Blanco Avalos, M. Ramos, M.-H. Kim, stratigraphy, and mineralogy. J. Geophys. Res. Planets n/a (2013). C. Malespin, I. Plante, J.-P. Muller, R. N. Gonzalez, R. Ewing, W. Boynton, doi:10.1002/2012JE004322 R. Downs, M. Fitzgibbon, K. Harshman, S. Morrison, O. Kortmann, A. 6. B. J. Thomson, N. T. Bridges, R. Milliken, A. Baldridge, S. J. Hook, J. K. Williams, G. Lugmair, M. A. Wilson, B. Jakosky, T. B. Zunic, J. Frydenvang, Crowley, G. M. Marion, C. R. de Souza Filho, A. J. Brown, C. M. Weitz, K. Kinch, S. L. S. Stipp, N. Boyd, J. L. Campbell, R. Gellert, G. Perrett, I. Constraints on the origin and evolution of the layered mound in Gale Crater, Pradler, S. VanBommel, S. Jacob, T. Owen, S. Rowland, H. Savijarvi, E. Mars using Mars Reconnaissance Orbiter data. Icarus 214, 413–432 (2011). Boehm, S. Bottcher, S. Burmeister, J. Guo, J. Kohler, C. M. Garcia, R. M. doi:10.1016/j.icarus.2011.05.002 Mellin, R. W. Schweingruber, T. McConnochie, M. Benna, H. Franz, H. 7. B. A. Ivanov, Mars/Moon cratering rate ratio estimates. Space Sci. Rev. 96, 87– Bower, A. Brunner, H. Blau, T. Boucher, M. Carmosino, S. Atreya, H. Elliott, 104 (2001). doi:10.1023/A:1011941121102 D. Halleaux, N. Renno, M. Wong, R. Pepin, B. Elliott, J. Spray, L. 8. R. M. E. Williams, J. P. Grotzinger, W. E. Dietrich, S. Gupta, D. Y. Sumner, R. Thompson, S. Gordon, J. Williams, P. Vasconcelos, J. Bentz, K. Nealson, R. C. Wiens, N. Mangold, M. C. Malin, K. S. Edgett, S. Maurice, O. Forni, O. Popa, J. Moersch, C. Tate, M. Day, G. Kocurek, B. Hallet, R. Sletten, R. Gasnault, A. Ollila, H. E. Newsom, G. Dromart, M. C. Palucis, R. A. Yingst, Francis, E. McCullough, E. Cloutis, I. L. ten Kate, R. Arvidson, A. Fraeman, R. B. Anderson, K. E. Herkenhoff, S. Le Mouélic, W. Goetz, M. B. Madsen, D. Scholes, S. Slavney, T. Stein, J. Ward, J. Berger, J. E. Moores, MSL A. Koefoed, J. K. Jensen, J. C. Bridges, S. P. Schwenzer, K. W. Lewis, K. M. Science Team, Martian fluvial conglomerates at Gale crater. Science 340, Stack, D. Rubin, L. C. Kah, J. F. Bell, 3rd, J. D. Farmer, R. Sullivan, T. Van 1068–1072 (2013). doi:10.1126/science.1237317 Medline Beek, D. L. Blaney, O. Pariser, R. G. Deen, O. Kemppinen, N. Bridges, J. R. 9. J. P. Grotzinger et al., A habitable fluvio-lacustrine environment at Johnson, M. Minitti, D. Cremers, L. Edgar, A. Godber, M. Wadhwa, D. Yellowknife Bay, Gale Crater, Mars. Science, published online 9 December Wellington, I. McEwan, C. Newman, M. Richardson, A. Charpentier, L. 2013 (10.1126/science.1242777). Peret, P. King, J. Blank, G. Weigle, M. Schmidt, S. Li, R. Milliken, K. 10. S. M. McLennan et al., Elemental geochemistry of sedimentary rocks in Robertson, V. Sun, M. Baker, C. Edwards, B. Ehlmann, K. Farley, J. Griffes, Yellowknife Bay, Gale Crater, Mars. Science, published online 9 December H. Miller, M. Newcombe, C. Pilorget, M. Rice, K. Siebach, E. Stolper, C. 2013 (10.1126/science.1244734). Brunet, V. Hipkin, R. Leveille, G. Marchand, P. Sobron Sanchez, L. Favot, G. 11. D. Vaniman et al., Mineralogy of a mudstone on Mars. Science, published Cody, A. Steele, L. Fluckiger, D. Lees, A. Nefian, M. Martin, M. Gailhanou, online 9 December 2013 (10.1126/science.1243480). F. Westall, G. Israel, C. Agard, J. Baroukh, C. Donny, A. Gaboriaud, P. 12. D. W. Ming et al., Organic, volatile, and isotopic compositions of a Guillemot, V. Lafaille, E. Lorigny, A. Paillet, R. Perez, M. Saccoccio, C. sedimentary rock in Yellowknife Bay, Gale Crater, Mars. Science, published Yana, C. A. Aparicio, J. Caride Rodriguez, I. Carrasco Blazquez, F. Gomez online 9 December 2013 (10.1126/science.1245267). Gomez, J. G. Elvira, S. Hettrich, A. Lepinette Malvitte, M. Marin Jimenez, J. 13. L. P. Keller, K. L. Thomas, R. N. Clayton, T. K. Mayeda, J. M. DeHart, D. S. M. Frias, J. M. Soler, F. J. M. Torres, A. Molina Jurado, L. M. Sotomayor, G. McKay, Aqueous alteration of the Bali CV3 chondrite: evidence from Munoz Caro, S. Navarro Lopez, V. P. Gonzalez, J. P. Garcia, J. A. Rodriguez mineralogy, mineral chemistry, and oxygen isotopic compositions. Geochim. Manfredi, J. J. R. Planello, S. Alejandra Sans Fuentes, E. Sebastian Martinez, Cosmochim. Acta 58, 5589–5598 (1994). doi:10.1016/0016-7037(94)90252-6 J. Torres Redondo, R. U. O’Callaghan, M.-P. Zorzano Mier, S. Chipera, J.-L. Medline Lacour, P. Mauchien, J.-B. Sirven, H. Manning, A. Fairen, A. Hayes, J. 14. D. Lal, Cosmogenic and nucleogenic isotopic changes in Mars: their rates and Joseph, S. Squyres, P. Thomas, A. Dupont, A. Lundberg, N. Melikechi, A. implications to the evolutionary history of Martian surface. Geochim. Mezzacappa, J. DeMarines, D. Grinspoon, G. Reitz, B. Prats, E. Atlaskin, M. Cosmochim. Acta 57, 4627–4637 (1993). doi:10.1016/0016-7037(93)90188-3 Genzer, A.-M. Harri, H. Haukka, H. Kahanpaa, J. Kauhanen, M. Paton, J. Medline Polkko, W. Schmidt, T. Siili, C. Fabre, J. Wray, M. B. Wilhelm, F. 15. O. Eugster, Cosmic-ray exposure ages of meteorites and lunar rocks and their Poitrasson, K. Patel, S. Gorevan, S. Indyk, G. Paulsen, D. Bish, J. Schieber, significance. Chemie Der Erde-Geochemistry 63, 3–30 (2003). B. Gondet, Y. Langevin, C. Geffroy, D. Baratoux, G. Berger, A. Cros, C. doi:10.1078/0009-2819-00021 Uston, J. Lasue, Q.-M. Lee, P.-Y. Meslin, E. Pallier, Y. Parot, P. Pinet, S. 16. K. Marti, T. Graf, Cosmic-ray exposure history of ordinary chondrites. Annu. Schroder, M. Toplis, E. Lewin, W. Brunner, E. Heydari, C. Achilles, D. Rev. Earth Planet. Sci. 20, 221–243 (1992). Oehler, B. Sutter, M. Cabane, D. Coscia, C. Szopa, F. Robert, V. Sautter, M. doi:10.1146/annurev.ea.20.050192.001253 Nachon, A. Buch, F. Stalport, P. Coll, P. Francois, F. Raulin, S. Teinturier, J. 17. T. Cerling, H. Craig, Geomorphology and in-situ cosmogenic isotopes. Annu. Cameron, S. Clegg, A. Cousin, D. DeLapp, R. Dingler, R. S. Jackson, S. Rev. Earth Planet. Sci. 22, 273–317 (1994). Johnstone, N. Lanza, C. Little, T. Nelson, R. B. Williams, A. Jones, L. doi:10.1146/annurev.ea.22.050194.001421 Kirkland, A. Treiman, B. Baker, B. Cantor, M. Caplinger, S. Davis, B. 18. M. H. Acuña, J. E. P. Connerney, P. Wasilewski, R. P. Lin, K. A. Anderson, Duston, D. Fay, C. Hardgrove, D. Harker, P. Herrera, E. Jensen, M. R. C. W. Carlson, J. McFadden, D. W. Curtis, D. Mitchell, H. Reme, C. Mazelle, Kennedy, G. Krezoski, D. Krysak, L. Lipkaman, E. McCartney, S. McNair, B. J. A. Sauvaud, C. d’Uston, A. Cros, J. L. Medale, S. J. Bauer, P. Cloutier, M. Nixon, L. Posiolova, M. Ravine, A. Salamon, L. Saper, K. Stoiber, K. Mayhew, D. Winterhalter, N. F. Ness, Magnetic field and plasma observations Supulver, J. Van Beek, R. Zimdar, K. L. French, K. Iagnemma, K. Miller, R. at Mars: Initial results of the Mars global surveyor mission. Science 279, Summons, F. Goesmann, S. Hviid, M. Johnson, M. Lefavor, E. Lyness, E. 1676–1680 (1998). doi:10.1126/science.279.5357.1676 Medline Breves, M. D. Dyar, C. Fassett, D. F. Blake, T. Bristow, D. DesMarais, L. 19. M. N. Rao, D. D. Bogard, L. E. Nyquist, D. S. McKay, J. Masarik, Neutron Edwards, R. Haberle, T. Hoehler, J. Hollingsworth, M. Kahre, L. Keely, C. capture isotopes in the martian regolith and implications for Martian McKay, L. Bleacher, W. Brinckerhoff, D. Choi, P. Conrad, J. P. Dworkin, J. atmospheric noble gases. Icarus 156, 352–372 (2002). Eigenbrode, M. Floyd, C. Freissinet, J. Garvin, D. Glavin, D. Harpold, P. doi:10.1006/icar.2001.6809 Mahaffy, D. K. Martin, A. McAdam, A. Pavlov, E. Raaen, M. D. Smith, J. 20. Cosmogenic production rate calculations are given in the supplementary Stern, F. Tan, M. Trainer, M. Meyer, A. Posner, M. Voytek, R. C. Anderson, materials. A. Aubrey, L. W. Beegle, A. Behar, D. Brinza, F. Calef, L. Christensen, J. A. 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. 46. O. Bachmann, M. A. Dungan, P. W. Lipman, The Fish Canyon magma body, 23. Atmospheric and excess argon are assumed negligible, as described in the San Juan volcanic field, Colorado: Rejuvenation and eruption of an upper- supplementary materials. crustal batholith. J. Petrol. 43, 1469–1503 (2002). 24. D. Bogard, J. Park, D. Garrison, 39Ar-40Ar “ages” and origin of excess 40Ar doi:10.1093/petrology/43.8.1469 in Martian shergottites. Meteorit. Planet. Sci. 44, 905–923 (2009). 47. Calculations provided in supplementary materials. doi:10.1111/j.1945-5100.2009.tb00777.x 48. P. R. Mahaffy, C. R. Webster, M. Cabane, P. G. Conrad, P. Coll, S. K. Atreya, 25. Full release of Ar for Ar geochronology often requires much higher R. Arvey, M. Barciniak, M. Benna, L. Bleacher, W. B. Brinckerhoff, J. L. temperatures. See supplementary materials for a discussion of this issue. Eigenbrode, D. Carignan, M. Cascia, R. A. Chalmers, J. P. Dworkin, T. 26. R. C. Anderson, L. Jandura, A. B. Okon, D. Sunshine, C. Roumeliotis, L. W. Errigo, P. Everson, H. Franz, R. Farley, S. Feng, G. Frazier, C. Freissinet, D. Beegle, J. Hurowitz, B. Kennedy, D. Limonadi, S. McCloskey, M. Robinson, P. Glavin, D. N. Harpold, D. Hawk, V. Holmes, C. S. Johnson, A. Jones, P. C. Seybold, K. Brown, Collecting samples in Gale Crater, Mars; an overview Jordan, J. Kellogg, J. Lewis, E. Lyness, C. A. Malespin, D. K. Martin, J. of the Mars Science Laboratory sample acquisition, sample processing and Maurer, A. C. McAdam, D. McLennan, T. J. Nolan, M. Noriega, A. A. handling system. Space Sci. Rev. 170, 57–75 (2012). doi:10.1007/s11214-012- Pavlov, B. Prats, E. Raaen, O. Sheinman, D. Sheppard, J. Smith, J. C. Stern, 9898-9 F. Tan, M. Trainer, D. W. Ming, R. V. Morris, J. Jones, C. Gundersen, A. 27. D. H. Scott, M. G. Chapman, Geologic and topographic maps of the Elysium Steele, J. Wray, O. Botta, L. A. Leshin, T. Owen, S. Battel, B. M. Jakosky, H. paleolake basin, Mars. U.S. Geol. Surv. Misc. Invest. Map, I-2397 (1995). Manning, S. Squyres, R. Navarro-González, C. P. McKay, F. Raulin, R. 28. R. Greeley, J. E. Guest, Geologic map of the eastern equatorial region of Sternberg, A. Buch, P. Sorensen, R. Kline-Schoder, D. Coscia, C. Szopa, S. Mars. U.S. Geol. Surv. Misc. Invest. Map I-1802-B (1987). Teinturier, C. Baffes, J. Feldman, G. Flesch, S. Forouhar, R. Garcia, D. 29. J. J. Wray, Gale crater: the Mars Science Laboratory/Curiosity rover landing Keymeulen, S. Woodward, B. P. Block, K. Arnett, R. Miller, C. Edmonson, S. site. Int. J. Astrobiol. 12, 25–38 (2013). doi:10.1017/S1473550412000328 Gorevan, E. Mumm, The Sample Analysis at Mars investigation and 30. Using the cumlative normal distribution. instrument suite. Space Sci. Rev. 170, 401–478 (2012). doi:10.1007/s11214- 31. Potassium concentrations were estimated from terrestrial examples of 012-9879-z volcanic sanidine and andesine [e.g., (45, 46)]. The relatively high K2O 49. National Geochronological Database, United States Geological Survey Open- content assumed for the basalt glass is consistent with ongoing analyses of File Report 2003-23, http://geopubs.wr.usgs.gov/open-file/of03-236/ (2013). basalts in Gale crater. The K2O concentration and glass abundance can be 50. P. R. Renne, G. Balco, K. R. Ludwig, R. Mundil, K. Min, Response to the traded off to yield the same overall mass balance. comment by W.H. Schwarz et al. on “Joint determination of 40K decay 32. J. C. Bridges, S. P. Schwenzer, The nakhlite hydrothermal brine on Mars. constants and 40Ar*/40K for the Fish Canyon sanidine standard, and improved Earth Planet. Sci. Lett. 359-360, 117–123 (2012). accuracy for 40Ar/39Ar geochronology” by P.R. Renne et al. (2010). Geochim. doi:10.1016/j.epsl.2012.09.044 Cosmochim. Acta 75, 5097–5100 (2011). doi:10.1016/j.gca.2011.06.021 33. Possible alternative sources and why they can be beglected are described in 51. P. R. Mahaffy, C. R. Webster, S. K. Atreya, H. Franz, M. Wong, P. G. the supplementary materials. Conrad, D. Harpold, J. J. Jones, L. A. Leshin, H. Manning, T. Owen, R. O. 34. Mass errors cancel in these ratios. Pepin, S. Squyres, M. Trainer, O. Kemppinen, N. Bridges, J. R. Johnson, M. 35. Laboratory data bearing on the question of diffusive noble gas loss are given Minitti, D. Cremers, J. F. Bell, L. Edgar, J. Farmer, A. Godber, M. Wadhwa, in the supplementary materials. D. Wellington, I. McEwan, C. Newman, M. Richardson, A. Charpentier, L. 36. Likely host minerals for each of the noble gases are given in the supplemntary Peret, P. King, J. Blank, G. Weigle, M. Schmidt, S. Li, R. Milliken, K. materials. Robertson, V. Sun, M. Baker, C. Edwards, B. Ehlmann, K. Farley, J. Griffes, 37. J.-P. Bibring, Y. Langevin, J. F. Mustard, F. Poulet, R. Arvidson, A. Gendrin, J. Grotzinger, H. Miller, M. Newcombe, C. Pilorget, M. Rice, K. Siebach, K. B. Gondet, N. Mangold, P. Pinet, F. Forget, M. Berthé, J. P. Bibring, A. Stack, E. Stolper, C. Brunet, V. Hipkin, R. Leveille, G. Marchand, P. S. Gendrin, C. Gomez, B. Gondet, D. Jouglet, F. Poulet, A. Soufflot, M. Sanchez, L. Favot, G. Cody, A. Steele, L. Fluckiger, D. Lees, A. Nefian, M. Vincendon, M. Combes, P. Drossart, T. Encrenaz, T. Fouchet, R. Merchiorri, Martin, M. Gailhanou, F. Westall, G. Israel, C. Agard, J. Baroukh, C. Donny, G. Belluci, F. Altieri, V. Formisano, F. Capaccioni, P. Cerroni, A. Coradini, A. Gaboriaud, P. Guillemot, V. Lafaille, E. Lorigny, A. Paillet, R. Perez, M. S. Fonti, O. Korablev, V. Kottsov, N. Ignatiev, V. Moroz, D. Titov, L. Saccoccio, C. Yana, C. Armiens-Aparicio, J. C. Rodriguez, I. C. Blazquez, F. Zasova, D. Loiseau, N. Mangold, P. Pinet, S. Douté, B. Schmitt, C. Sotin, E. G. Gomez, J. Gomez-Elvira, S. Hettrich, A. L. Malvitte, M. M. Jimenez, J. Hauber, H. Hoffmann, R. Jaumann, U. Keller, R. Arvidson, J. F. Mustard, T. Martinez-Frias, J. Martin-Soler, F. J. Martin-Torres, A. M. Jurado, L. Mora- Duxbury, F. Forget, G. Neukum, Global mineralogical and aqueous mars Sotomayor, G. M. Caro, S. N. Lopez, V. Peinado-Gonzalez, J. Pla-Garcia, J. history derived from OMEGA/Mars Express data. Science 312, 400–404 A. R. Manfredi, J. J. Romeral-Planello, S. A. S. Fuentes, E. S. Martinez, J. T. (2006). doi:10.1126/science.1122659 Medline Redondo, R. Urqui-O’Callaghan, M.-P. Z. Mier, S. Chipera, J.-L. Lacour, P. 38. J. A. Grant, S. A. Wilson, Late alluvial fan formation in southern Margaritifer Mauchien, J.-B. Sirven, A. Fairen, A. Hayes, J. Joseph, R. Sullivan, P. Terra, Mars. Geophys. Res. Lett. 38, L08201 (2011). Thomas, A. Dupont, A. Lundberg, N. Melikechi, A. Mezzacappa, J. doi:10.1029/2011GL046844 DeMarines, D. Grinspoon, G. Reitz, B. Prats, E. Atlaskin, M. Genzer, A.-M. 39. J. A. Grant, S. A. Wilson, A possible synoptic source of water for alluvial fan Harri, H. Haukka, H. Kahanpaa, J. Kauhanen, O. Kemppinen, M. Paton, J. formation in southern Margaritifer Terra, Mars. Planet. Space Sci. 72, 44–52 Polkko, W. Schmidt, T. Siili, C. Fabre, J. Wray, M. B. Wilhelm, F. (2012). doi:10.1016/j.pss.2012.05.020 Poitrasson, K. Patel, S. Gorevan, S. Indyk, G. Paulsen, S. Gupta, D. Bish, J. 40. N. Mangold, E. S. Kite, M. G. Kleinhans, H. Newsom, V. Ansan, E. Hauber, Schieber, B. Gondet, Y. Langevin, C. Geffroy, D. Baratoux, G. Berger, A. E. Kraal, C. Quantin, K. Tanaka, The origin and timing of fluvial activity at Cros, C. d’Uston, O. Forni, O. Gasnault, J. Lasue, Q.-M. Lee, S. Maurice, P.- Eberswalde crater, Mars. Icarus 220, 530–551 (2012). Y. Meslin, E. Pallier, Y. Parot, P. Pinet, S. Schroder, M. Toplis, E. Lewin, W. doi:10.1016/j.icarus.2012.05.026 Brunner, E. Heydari, C. Achilles, D. Oehler, B. Sutter, M. Cabane, D. Coscia, 41. J. Laskar, A. C. M. Correia, M. Gastineau, F. Joutel, B. Levrard, P. Robutel, G. Israel, C. Szopa, G. Dromart, F. Robert, V. Sautter, S. Le Mouelic, N. Long term evolution and chaotic diffusion of the insolation quantities of Mars. Mangold, M. Nachon, A. Buch, F. Stalport, P. Coll, P. Francois, F. Raulin, S. Icarus 170, 343–364 (2004). doi:10.1016/j.icarus.2004.04.005 Teinturier, J. Cameron, S. Clegg, A. Cousin, D. DeLapp, R. Dingler, R. S. 42. W. R. Ward, Large-scale variations in the obliquity of Mars. Science 181, Jackson, S. Johnstone, N. Lanza, C. Little, T. Nelson, R. C. Wiens, R. B. 260–262 (1973). doi:10.1126/science.181.4096.260 Medline Williams, A. Jones, L. Kirkland, A. Treiman, B. Baker, B. Cantor, M. 43. L. R. Dartnell, L. Desorgher, J. M. Ward, A. J. Coates, Modelling the surface Caplinger, S. Davis, B. Duston, K. Edgett, D. Fay, C. Hardgrove, D. Harker, and subsurface Martian radiation environment: Implications for astrobiology. P. Herrera, E. Jensen, M. R. Kennedy, G. Krezoski, D. Krysak, L. Lipkaman, Geophys. Res. Lett. 34, L02207 (2007). doi:10.1029/2006GL027494 M. Malin, E. McCartney, S. McNair, B. Nixon, L. Posiolova, M. Ravine, A. 44. A. A. Pavlov, G. Vasilyev, V. M. Ostryakov, A. K. Pavlov, P. Mahaffy, Salamon, L. Saper, K. Stoiber, K. Supulver, J. Van Beek, T. Van Beek, R. Degradation of the organic molecules in the shallow subsurface of Mars due Zimdar, K. L. French, K. Iagnemma, K. Miller, R. Summons, F. Goesmann, to irradiation by cosmic rays. Geophys. Res. Lett. 39, L13202 (2012). W. Goetz, S. Hviid, M. Johnson, M. Lefavor, E. Lyness, E. Breves, M. D. doi:10.1029/2012GL052166 Dyar, C. Fassett, D. F. Blake, T. Bristow, D. DesMarais, L. Edwards, R. 45. W. A. Deer, R. A. Howie, J. Zussman, Rock-Forming Minerals, Volume 4A: Haberle, T. Hoehler, J. Hollingsworth, M. Kahre, L. Keely, C. McKay, M. B. Framework Silicates - Feldspars. (Geological Society of London, London, Wilhelm, L. Bleacher, W. Brinckerhoff, D. Choi, J. P. Dworkin, J. 2001). 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. 63. T. Cerling, Dating geomorphic surfaces using cosmogenic 3He. Quat. Res. 33, Meyer, A. Posner, M. Voytek, R. C. Anderson, A. Aubrey, L. W. Beegle, A. 148–156 (1990). doi:10.1016/0033-5894(90)90015-D Behar, D. Blaney, D. Brinza, F. Calef, L. Christensen, J. A. Crisp, L. 64. A. Jambon, J. S. Shelby, Helium diffusion and solubility in obsidians and DeFlores, B. Ehlmann, J. Feldman, S. Feldman, G. Flesch, J. Hurowitz, I. Jun, basaltic glass in the range 200–300°C. Earth Planet. Sci. Lett. 51, 206–214 D. Keymeulen, J. Maki, M. Mischna, J. M. Morookian, T. Parker, B. Pavri, (1980). doi:10.1016/0012-821X(80)90268-X M. Schoppers, A. Sengstacken, J. J. Simmonds, N. Spanovich, M. T. Juarez, 65. R. Wieler, in Noble Gases in Geochemistry and Cosmochemistry, D. Porcelli, A. R. Vasavada, A. Yen, P. D. Archer, F. Cucinotta, D. Ming, R. V. Morris, C. J. Ballentine, R. Wieler, Eds. (2002), vol. 47, pp. 125–170. P. Niles, E. Rampe, T. Nolan, M. Fisk, L. Radziemski, B. Barraclough, S. 66. S. R. Taylor, S. M. McLennan, Planetary Crusts: Their Composition, Origin, Bender, D. Berman, E. N. Dobrea, R. Tokar, D. Vaniman, R. M. E. Williams, and Evolution (Cambridge University Press, Cambridge, 2009), pp. 378. A. Yingst, K. Lewis, T. Cleghorn, W. Huntress, G. Manhes, J. Hudgins, T. 67. G. J. Taylor, The bulk composition of Mars. Chemie Der Erde-Geochemistry, Olson, N. Stewart, P. Sarrazin, J. Grant, E. Vicenzi, S. A. Wilson, M. Bullock, published online 5 October 2013 (10.1016/j.chemer.2013.09.006). B. Ehresmann, V. Hamilton, D. Hassler, J. Peterson, S. Rafkin, C. Zeitlin, F. Acknowledgments: The authors are indebted to the Mars Science Laboratory Fedosov, D. Golovin, N. Karpushkina, A. Kozyrev, M. Litvak, A. Malakhov, Project engineering and management teams for their exceptionally skilled and I. Mitrofanov, M. Mokrousov, S. Nikiforov, V. Prokhorov, A. Sanin, V. diligent efforts in making the mission as effective as possible and enhancing Tretyakov, A. Varenikov, A. Vostrukhin, R. Kuzmin, B. Clark, M. Wolff, S. science operations. We are also grateful to all those MSL team members who McLennan, O. Botta, D. Drake, K. Bean, M. Lemmon, S. P. Schwenzer, R. B. participated in tactical and strategic operations. Without the support of both Anderson, K. Herkenhoff, E. M. Lee, R. Sucharski, M. A. P. Hernandez, J. J. the engineering and science teams, the data presented here could not have B. Avalos, M. Ramos, M.-H. Kim, C. Malespin, I. Plante, J.-P. Muller, R. been collected. Three anonymous reviewers provided many helpful Navarro-Gonzalez, R. Ewing, W. Boynton, R. Downs, M. Fitzgibbon, K. suggestions. Some of this research was carried out at the Jet Propulsion Harshman, S. Morrison, W. Dietrich, O. Kortmann, M. Palucis, D. Y. Laboratory, California Institute of Technology, under a contract with the Sumner, A. Williams, G. Lugmair, M. A. Wilson, D. Rubin, B. Jakosky, T. National Aeronautics and Space Administration. Data presented in this paper Balic-Zunic, J. Frydenvang, J. K. Jensen, K. Kinch, A. Koefoed, M. B. are archived in the Planetary Data System (pds.nasa.gov). 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. Supplementary Materials Boehm, S. Bottcher, S. Burmeister, J. Guo, J. Kohler, C. M. Garcia, R. www.sciencemag.org/cgi/content/1247166/DC1 Mueller-Mellin, R. Wimmer-Schweingruber, J. C. Bridges, T. McConnochie, Materials and Methods M. Benna, H. Bower, A. Brunner, H. Blau, T. Boucher, M. Carmosino, H. Figs. S1 to S3 Elliott, D. Halleaux, N. Renno, B. Elliott, J. Spray, L. Thompson, S. Gordon, Tables S1 to S4 H. Newsom, A. Ollila, J. Williams, P. Vasconcelos, J. Bentz, K. Nealson, R. References (48–67) 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. 14 October 2013; accepted 25 November 2013 Arvidson, A. Fraeman, D. Scholes, S. Slavney, T. Stein, J. Ward, J. Berger, J. Published online 09 December 2013 E. Moores; MSL Science Team, Abundance and isotopic composition of 10.1126/science.1247166 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

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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. 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 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.

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