43rd Lunar and Planetary Science Conference (2012) 2778.pdf

EVIDENCE FOR MELT-FED MEANDERING RIVERS IN THE GALE-AEOLIS-ZEPHYRIA REGION, MARS. Edwin S. Kite1,2, 1Department of Earth and Planetary Science and Center for Integrative Planetary Science, U.C. Berkeley, 2Division of Geological and Planetary Sciences, Caltech ([email protected])

Introduction: The largest concentration of inverted Images showing multiple SMRFs include channels (ICs) on Mars is in the Gale-Aeolis-Zephyria B19_016981_1746_XI_05S205W and B17_016137 region [1]. These ICs and their associated terminal fans _1745_XI_05S205W. may constrain ancient Mars climate, and the nominal Mars Science Laboratory traverse includes inspection of an IC within Gale’s mound. Paleodischarge 101 – 103 m/s has been inferred from channel width and me- ander wavelength [2]. However, the water source for these channels – rain, snow melt, ice melt, or springs – remains unclear. Here I describe watershed-scale SHALLOW topographic relations that suggest the presence of large MOAT near-surface ice bodies at the time the rivers were flowing, and evidence that Gale-Aeolis-Zephyria riv- ers were fed by seasonal melt. Channel inversion and watershed inversion: Mars ICs are diverse, and this abstract focuses on a subset (n > 10) of the Gale-Aeolis-Zephyria ICs, in which trellis-like networks of rough-topped channels RIDGE drain through breaks in ridges into fans (Figure 1). FAN The channel inversion process in Gale-Aeolis- Zephyria appears to be differential aeolian erosion, with channel materials left highstanding either because Figure 1. CTX image of a Shallow Moat-Ridge-Fan. The channel of grainsize differences or channel-bed cementation rises >80m downstream on passing through the ridge. N is toward the top of the image. Fan is ~5km across. A HiRISE DTM of this [2]. In addition to IC inversion, CTX DTMs [3], feature (with 1m post spacing) may be downloaded at HiRISE DTMs and MOLA PEDR show that these http://climatefutures.com/stereo/ #AZPlanum. watersheds are inverted – the paleo-headwaters are 2 situated behind ridges within terrain that is O(10 )m Midlatitude glacial analogs: Each element of the lower elevation than the paleo-lowlands. Drainage SMRFs has analogs in Mars’ midlatitudes. <30ºN, direction is inferred from point-bar truncation direc- debris-covered glaciers are removed by sublimation to tion, terminal fan apex direction, and (least reliably) leave “ghost moats.” [5] These glaciers were cold- meander asymmetry, because present-day slope cannot based and not erosive, but the moats are now lower be used. This watershed-scale inversion is not the re- than surrounding terrain because the glaciers were sult of regional tectonics – present-day slope does line embayed by lava and other deposits, defining a mar- up well with paleoflow over most of Mars [4]. The ginal ridge or rise [5]. topographic changes have a length-scale and amplitude Supraglacial channels on Mars [6] and their termi- [3] that is unlikely for differential compaction of po- nal fans (if present) become inverted when the main rous sediments. Removal of ice is a better explanation. glacier begins to sublime. Examples include:- Lyot Shallow moat - Ridge – Fans: Figure 1 shows a Crater (fans and inverted channels, e.g. typical shallow moat-ridge-fan assemblage (SMRF). ESP_019372_2300); Acheron Fossae (inverted chan- 2 Shallow moat. The O(10 )m deep moats have irregular nel, ESP_018178_2165); and Reuyll Vallis. What margins, and are typically the source for several chan- happens when the glacier sublimes entirely? A crater nels. Channels within moats are boader and less well- near Tempe Fossae (G01_018545_2187_XN_ defined. 3 channel networks emanate from a nearly- 38N072W; I thank Caleb Fassett for drawing my at- closed depression trough within the central valley of tention to this crater) appears to show the largely in- part of the Medusae Fossae Formation verted channel network associated with a past glacier. (P16_007105_1774_XN_02S209W). Ridge or rise. At the equator, snowmelt is more likely than in The moats are bounded by a rise, typically blocky at midlatitudes. Many more melt channels would have the kilometer scale. Fans. On passing through a gap in formed, but near-surface massive ice is unstable near the ridge, the channel rises ~100m and forms a dis- the equator and has sublimed away entirely, leaving a tributuary fan or (less commonly) a meandering IC or channel skeleton. (less commonly) both. Interpretation: The simplest explanation of these 43rd Lunar and Planetary Science Conference (2012) 2778.pdf

observations is that the annual-averaged temperature Weaknesses and omissions: The relationship of Tav < 273K to allow for thick water ice bodies, and that the river deposits to the material between them (flood- melting to produce runoff for the rivers was confined plains, or preexisting aeolian material?) is not under- to a seasonally-active layer as in Earth’s Antarctic Dry stood. The stratigraphic correlation between the ICs in Valleys [7]. the Medusae Fossae Formation and those in Gale We interpret the shallow moats as pits formerly oc- Crater is poorly constrained [11-12]. cupied by cold-based ice or by snow-sediment mix- Environmental interpretation and implications tures, which formed the water source for the rivers. for Gale: Basal melting is unlikely to account for me- We interpret the networks in the headwaters region as andering IC discharges [2]. For the lithosperic heat ice-associated channels (such as eskers or supraglacial flow Ql < 50 mW/m2 [13], a 104 km2 watershed would channels), the ridges/rises as candidate moraines or produce <1.5 m3/s by basal melting. Insolation pro- protalus ramparts, and the fans and well-defined me- vides much more energy: an ice melt rate of 1 mm/hr andering channels as proglacial. Based on channel requires at least 84 W/m2 (direct melting) plus 316 continuity visible in HiRISE images and DTMs, we W/m2 (radiative losses), which is much less than the interpret most the exhumed SMRFs we see today as 670 W/m2 (plus greenhouse effect) maximum near the corresponding to the same broad interval of fluvial Early Mars equator [14-16]. At LPSC, I will present a activity, rather than a palimpsest. watershed-scale energy balance for a closed region The pattern of watershed-scale subsidence supports that is the source of 3 SMRFs. the past presence of near-surface ice that directed or Mars’ high-amplitude orbital variability appears to confined channel flow. If the rivers flowed over deep- regulate sedimentary rock layering [17]. We speculate ly-buried past ice, then it would be suprising that fans that the meandering ICs record peak runoff during rare so often have apices at the edge of the shallow moats. orbital conditions of moderate-to-high obliquity, high Global-groundwater flow is all but ruled out as a water source, because thick permafrost is impermea- eccentricity, and equinoctial perihelion, which are op- ble. The only known spring discharge flowing through timal for seasonal melting [16]. Orbital variations in thick permafrost, on Axel Heiburg Island, flow only insolation cannot support orbital cycle-averaged tem- <20 km and to <640 m depth [8]. If the river waters peratures Torb << 273K and orbitally-optimal Tav > flowed as groundwater then they were still locally de- 273K, unless there were major changes in atmospheric rived. composition on orbital frequencies. Rain is also disfavored because a planet on which This preliminary work indicates that Tav < 273K the annual mean equatorial temperature <273K is un- when the Gale-Aeolis-Zephria channels formed, in- likely to support much atmospheric liquid water aero- consistent with a global-groundwater water source for sol (especially given the offset between surface and these channels. near-surface atmospheric temperatures in thin atmos- Acknowledgements: I thank Bill Dietrich, Devon pheres; [9]). Earth glaciers supplied by rain are near Burr, Caleb Fassett, Noah Finnegan, Leif Karlstrom, oceans, which draw energy from warmer latitudes. Maarten Kleinhans, Alexandra LeFort, Michael Man- These ICs are near Mars’ equator, which is already the ga, and Tim Michaels for valuable discussions. warmest latitude for most orbital conditions, and there is no direct geologic evidence for Mars oceans at this References: [1] Burr, D.M. et al. (2009), Icarus 200, 52. [2] time. Burr D.M. et al. (2010) JGR, 115 E0711. [3] Lefort, A. et al. (2011), Not a single episode of warming: If the rivers LPSC 42, 1608. [4] Phillips, R.J. (2001) Science, 291, 2587 [5] Hau- formed during transient warm conditions, and the heat ber E., et al. (2008) JGR 113, E02007. [6] Fassett, C. I., et al. (2010), pulse began quickly enough, then buried ice need not Icarus 208, 86. [7] Doran, P.T., et al., eds. (2010), Life in Antarctic melt even if Tav > 273K at the surface. deserts and other cold dry environments, C.U.P. [8] Anderson, D. The main evidence against this comes from a well- (2002) JGR 107, 5015; see also correction to this paper, Anderson, imaged part [10] of the headwaters of the longest me- D. (2005), JGR 110, E04007. [9] Pierrehumbert, R.T. (2010) Princi- andering river on Mars. Here (ESP_021728_1740), ples of Planetary Climate, C.U.P. [10] Howard, A.D. (2009), PNAS two generations of sinuous, inverted channels follow 106, 17245 [11] Kerber, L., and J.W. Head (2010), Icarus 206, 669. the same general trend. The required sequence of [12], Zimbelman, J.R. (2011), LPSC 42, 1840. [13] Solomon, S.C. events is:- incision of the first-generation channels (2005) Science 307, 1214. [14] Clow G.D. (1987) Icarus 72, 95- (broad, highly sinuous, with meander cutoffs); incom- 127. [15] Hecht, M. H. (2002), Icarus 156, 373. [16] Kite, E.S., et al. plete infilling of the entire landscape; incision of the (2011) JGR 116, E07002. [17] Lewis, K.W., and Aharonson, O. second-generation channels (narrow and less sinuous); (2008), Science 322, 1532 . and deflation to the current level. This is unlikely for a single transient episode of warm conditions.