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Home , Amazonian (Mars), Link (Mars), Noachian, Valley network (Mars)

46th Lunar and Planetary Science Conference (2015) 2166.pdf

VALLEY NETWORKS AND THE NATURE OF THE LATE . B.M. Hynek 1Laboratory for Atmospheric and Space Physics, University of Colorado-Boulder, 3665 Discover Drive, Boulder, CO 80303, 2Dept. of Geological Sciences, University of Colorado-Boulder [email protected]

Introduction: Valley networks remain the best ev- valley networks formed and individual events were too idence that Mars had a long-lived, integrated hydro- short in duration to match expected formation time- sphere covering much of the planet, which seems to scales [1]. Recently, two Mars climate modeling ef- imply a warm/wet climate for 10s to 100s of millions forts show promise at making Mars warm during the of in the Late Noachian. Most of these connect- faint Noachian period: SO2 warming of the planet ed and often dendritic valley systems are consistent during periods of punctuated [8] and micro- with a fluvial formation via and surface physical models of the early Mars climate [9]. These runoff [e.g., 1-2]. Cross-cutting relations with ancient models provide intriguing ways to allow an active hy- cratered highlands [1] and network crater- dates [3- drosphere during the Noachian; however, the connec- 4] indicated that a majority (~90%) of these systems tion to formation and temporally- seem to have formed in the Late Noachian up through resolved details of the models require further work. the Noachian‐ boundary (∼3.7 Ga) (Fig. 1). Additionally, the valley networks formed shortly There is evidence that valley incision was active in after the Late Heavy Bombardment. Could that deliv- different regions of Mars throughout this time period ery of volatiles and associated climate disruption been and also that many valleys had several periods of ac- required for valley network formation? tive formation [3]. Smaller, immature valley networks Timing: Dating the age of valley networks is chal- occur on younger terrains of Mars (Hesperian- lenging. Cross-cutting relations with the terrains they ) with a majority found on volcanic edifices incised and crater-age dates only provide minimum or and within Hellas basin [1] (Fig. 1). These smaller termination ages, respectively. Determining when younger systems are more apt to preserve terminal valleys were actively being incised is further exacer- deposits in the forms of fan deltas or alluvial fans [5-6] bated by many systems showing evidence of multiple The largest valley networks are similar in length periods of formation through time [3]. It is crucial to and drainage area to Earth’s largest systems and characterize the inception of valley network formation individual trunk-segment valleys can be 20 km wide and the spatial and temporal components through time, and >1 km deep. Few of the larger valley systems not just the shut-off of incision. Were valley networks have sedimentary deposits at their termini; however, active in the Middle and Early Noachian? Or was many of the lower reaches of the large ancient valley there really a climate optimum in the Late Noachian? networks have been obscured by younger resurfacing. Formation timescales: Hoke et al. [7] showed that Analytical methods were used to assess the formation grain size-frequency and water depth greatly influence timescales of eight larger networks. Results indicate valley formation timescale models. These are uncon- that it took 105-107 years to transport enough strained variables for Mars, since we have never ob- out of the valley network to match the observed vol- served Mars’ larger valley networks from the ground umes [7]. Though, considering the large uncertainty in with landed assets. How can we better model the these calculations and the combination of parameters length of time required for valley network formation? that produced minimum timescales, it is possible that Distribution: While valley networks cover most of the formation timescales approach durations similar to the ancient highland terrains, several regions have a their span in ages of ~108 years. paucity of valleys (e.g., ) or elsewhere Outstanding Questions: While strong evidence there are expansive undissected reaches between sys- exists that the valley networks required a cli- tems. Were valleys formed on these terrains and then mate capable of producing sustained precipitation and later obliterated? Or did they never form in these re- for formation, many questions still ex- gions? Fine scale tributaries are also lacking across ist: much of Mars. Is this just a result of the antiquity of Climatic Triggers: Morphometric arguments, mod- valleys on a geologically active planet? els, and sediment transport calculations indicate that Links to aqueous alteration: The orbiting spec- many of Mars’ larger valley networks required an ac- trometers and rovers have shown significant wa- tive hydrosphere with long-lived precipitation (at least ter:rock alteration of the Martian into several 10+ m.y. periods). Impact-induced warm/wet phyllosilicates, sulfates, silica, and even carbonates conditions has been proposed as a climate trigger; [e.g., 10]. Several deltas also exhibit alteration miner- however, the larger impacts needed to significantly als [e.g., 11]. Yet it remains unclear whether or not induce warm/wet conditions occurred long before most the widespread fluvial episodes that formed the valley 46th Lunar and Planetary Science Conference (2015) 2166.pdf

networks contributed to the chemical alteration of the Martian crust (i.e., it could have been altered at an earlier time and just transported during valley for- mation). Links to volcanism: The majority of the mass was emplaced in the Noachian and altered the long-wavelength topography of the planet on which the valleys were incised [12]. Additionally, Hesperi- an-aged volcanism resurfaced ~1/3 of the planet and was complete shortly after the majority of valley net- works formed [13]. The temporal coincidence be- tween valley formation and extensive volcanic outgas- sing may provide a causal , especially considering the possible warming effect of Mars’ climate through volcanism [8]. Yet this link has not been demonstrat- ed. Lack of terminal deposits: Martian valley networks the size of the Mississippi River seldom have terminal deposits (e.g., deltas). What happened to all the mate- rial excavated from and transported through the net- works? It is easy to dismiss it as subsequent ero- sion/modification/obliteration, but is that the case? Is an ocean required?: Modeling by [14] showed that without a large standing body of water in the northern plains of Mars, a long-lived warm/wet cli- mate was impossible. Further, their regions of en- hanced precipitation when including a northern ocean in the models correlated with the regions of dense val- ley dissection observed of the highland crust. It is well known that the interior of the Pangean supercontinent on Earth was extremely arid due to the great distance from a body of water, as are the deep interior of conti- nents today (e.g., Antarctica). Did Mars need an ocean to produce the valley networks? References: [1] Hynek, B.M. et al. (2010) JGR, 115, E09008. [2] Craddock, R.A., & A.D. Howard (2002) JGR, 107(E11), 21-1. [3] Hoke, M.R.T., & B.M. Hynek (2009), JGR, 114, E08002. [4] Fassett, C.I., & J.W. Head (2008), Icarus, 195, 61–89. [5] Di Achille, G., & B.M. Hynek, (2010) Nat. Geosci., 3, doi:10.1038/NGEO891. [6] Moore, J.M., & A.D. Howard, A. D. (2005) JGR, 110(E4). [7] Hoke, M.R.T. et al. (2011) EPSL, 312, 1–12. [8] Halevy, I., & J.W. Head III, (2014). Nat. Geosci. 7, 865–868. [9] Urata, R.A. and O.B. Toon (2013) Icarus, 226, 229– 250. [10] Ehlmann, B.L., et al., (2011), Nature, 479(7371), 53-60. [11] Ehlmann, B.L., et al., (2008), Nat. Geosci. 1(6), 355-358. [12] Phillips, R.J., et al., Figure 1. Top to Bottom: (1) Valley networks on Noachian terrains (2001) Science, 291, 2587–2591. [13] Head, J.W., & (red). (2) Hesperian (blue) and Noachian valleys. (3) Noachian- L. Wilson, (2011) LPSC XLII Abs. 1214. [14] Soto, Hesperian-Amazonian () valleys [1]. (4) Valleys (blue) with A., et al., (2010) LPSC, XLI, Abs. 2397. [15] Fassett, in cold-warm colors. black = fan deltas [5], dark red C.I., & J.W. Head III, (2008) Icarus, 198(1), 37-56. = open-lake basins [15], pink = chloride salts [16] [16] Osterloo, M.K. et al., (2010) JGR, 115, E10012.