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地学雑誌 Journal of Geography（Chigaku Zasshi） 125（1）121—132 2016 doi:10.5026/jgeography.125.121

Geomorphological Indication of Ancient, Recent, and Possibly Present-day Aqueous Activity on

James M. DOHM＊ and Hideaki ＊

［Received 31 March, 2015; Accepted 5 December, 2015］

Abstract This paper overviews water sculpted landscapes, ancient through to possibly present day, which have become more pronounced through each new orbiting, landing, and roving mission. Geomorphological evidence of ancient aqueous activity associated with lakes and putative oceans includes a diversity of features. Features include sedimentary sequences, debris flows, fluvial valleys, alluvial fans, giant polygons, and glacial and periglacial landscapes. Arguably one of the most significant geomorphological indicators of a paleoocean is deltaic landforms identified along a topographic zonal boundary which correlates with reported putative shorelines. Other evidence includes distinct geochemical/mineralogical/elemental signatures of aqueous weathering. In addition, relatively high-resolution imaging cameras onboard the , Mars Odyssey, and Mars Reconnaissance Orbiter have detailed features which indicate recent and possibly present-day aqueous activity such as slope streaks, slope linea, gullies which occur along faults and fractures and source from geologic contacts and tectonic structures, and possible open-system pingos, among other feature types. Ancient, recent, and possibly present-day features point to both surface and subsurface aqueous environments throughout time, and thus making Mars a prime target to address the ever- important question of whether life exists beyond the Earth.

Key words： Mars, , hydrology, oceans, outflow channel, gullies, slope linea

Mitchell and Wilson, 2003）. Perhaps the most I．Introduction surprising finding from the post-Viking mission Mars' missions continue to reveal aqueous- are a variety of features that indicate the Mars modified Martian landscapes. Geomorphological has been hydrologically active up until present evidence includes a diversity of features, which day, including features that indicate hydrological include sedimentary sequences, debris flows, activity in （ McEwen et al., 2011, fluvial valleys, alluvial fans, deltas, and glacial 2013）. and periglacial landscapes（ Baker, 1978; Scott The purpose of this paper is to present evidence et al., 1995; Malin and Edgett, 2000a, b; Kargel that points to aqueous activity during ancient 2004; Soare et al., 2007, 2014a, b; Di Achille and （Early – or older than ～ Hynek, 2010; El Maarry et al., 2013）. In addition, 1.03 Ga or 1.23 Ga based on Hartmann（ 2005） there is significant geomorphological evidence of or Hartmann and （ 2001）, respectively） the interaction among magma, water, and base- and recent（ ～younger than 1.03 Ga or 1.23 Ga） ment structures such as in the case of Athabascas times, and even possibly present day, and thus Valles and Fossae（ Burr et al., 2002; making Mars a prime target to test the hypothesis

＊ University Museum, University of Tokyo, Tokyo, 113-0033, Japan

— —121 of whether life exists beyond Earth. Below we discuss ancient aqueous activity on Mars followed by evidence of recent and possibly present-day activity.

II．Ancient aqueous activity on Mars

One of the most distinct ancient water-sculpted features on Mars is the circum-Chryse outflow channel system, which forms the northeast watershed of （ Dohm et al., 2001a, b）. The circum-Chryse outflow channel system is hypoth- esized to have formed from linked Tharsis-driven magmatic activity and flooding, with associated ponding in the northern plains to form oceans and Fig. 1 Oblique view using a Mars Orbiter Laser Altimeter to drive transient climate change and hydrological （MOLA） map looking generally to the west across cycling（ Baker et al., 1991, 2007; Fairén et al., most of the Tharsis, which covers more than 25％ 2003; Komatsu et al., 2004）（ Figs. 1 and 2）. of the total surface area of Mars（ modified from In addition, geochemical /mineralogical /ele- Komatsu et al., 2004）. Tharsis, generally 5,000 km mental signatures of ancient aqueous activity have across, is located in the western hemisphere of Mars along the highland–lowland boundary that sepa- detected through orbital-, lander-, and been rates the northern plains（ N.P.） from the southern rover-based spectroscopic imagers. Minerals cratered highlands（ S.C.H.）. Tharsis is interpreted from aqueous weathering and hydrothermal to be a superplume（ Dohm et al., 2007）, as it has activity include hematite concretions referred many traits of a terrestrial superplume（ Maruyama, 1994）. Valles Marineris, which would extend at least to as blueberries（ Fig. 3）（ Moore, 2004; Ormö et across the United States from Los Angeles（ LA） to al., 2004b; Havics et al., 2009）, sulfates（ Gendrin New York（ NY）（ white line about 4,000 km）, is a et al., 2005; Quantin et al., 2005; Mangold et al., significant component of Tharsis. The vast canyon 2007a, b; Murchie et al., 2009）, clays（ Fialips et system is hypothesized to be genetically linked to al., 2005; Bibring et al., 2006）, chloride-bearing the evolution of pulsating Tharsis, which includes floods that carved the circum-Chryse outflow chan- aterials Osterloo et al., 2008; Davila et al., m （ nel system and that formed oceans in the northern 2011）, carbonates（ Ehlmann et al., 2008; Morris plains（ N.P.） and drove short-lived climatic pertur- et al., 2010）, serpentine（ Ehlmann et al., 2010）, bations（ Baker et al., 1991; Fairén et al., 2003）. The and pure silica（ Squyres et al., 2008）. In addition, most distinct of the outflows cannel system, is Kasei elevated concentrations of potassium and thorium Valles（ K.V.）. Also highlighted are the gigantic shield volcanoes, （ O.M.）, which is have been identified below the putative shorelines the largest volcano in the solar system, the Tharsis when compared to above the shorelines, Montes shield volcanoes, Arsia（ A.M.）, Pavonis likely indicating ancient aqueous activity related （P.M.）, and Ascraeus（ As.M.） Montes. Also shown to oceans（ Dohm et al., 2009）. are the locations of the Viking Lander 1（ VLS） and Pathfinder（ PLS） landing sites, as well as the area Such evidence indicates weathered zones and of debouchment of the circum-Chryse outflow chan- possible paleosols in stratigraphic sequences（ e.g., nels（ white oval）. Mahaney et al., 2001）, transport of water and rock materials to sedimentary basins（ e.g., Dohm et surface, which corroborates the existence of al., 2009）, and the formation of extensive lakes an cient oceans once occupying the northern plains （e.g., Scott et al., 1995; Cabrol et al., 2001） and of Mars, includes deltas at certain topographic possibly transient oceans（ e.g., Fairén et al., 2003; zonal boundaries, consistent with the margin of Greenwood and Blake, 2006） on ancient Mars. the putative ocean, and thus referred to as circum- The growing geomorphological evidence of paleoocean deltas（ e.g., Di Achille and Hynek, ancient hydrological modification of the Martian 2010）（ Fig. 4）. Arguably the most significant

— —122 Fig. 2 Mars possesses massive outflow channels carved by flowing water（ modified from Dohm et al., 2011）. （ top）, dissecting the northeast part of the Tharsis superplume（ Fig. 1）, is one of the longest continuous outflow channels on Mars（ ～2400 km long and up to 480 km wide）. For comparison, the Amazon River in America, Earth's largest river, is shown at the same scale（ bottom）. The map shows the relief near the mouth of the Amazon River and the cross section is inset with Kasei Valles for comparison. Vertical exaggeration of topographic profiles is ×200.（ Shuttle Radar Topography Mission（ SRTM） Earth topography, Farr and Kobrick, 2000; and Mars topography NASA/Mars Global Sur- veyor（ MGS）/Mars Orbiter Laser Altimeter（ MOLA））. indicator of paleooceans on Mars, deltas are also In particular, carbonates occur east-southeast identified in impact basins where river systems of within deltaic deposits that were debouched sediment-laden water under lower- emplaced from a river system into the western energy conditions（ Cabrol and Grin, 1999; Irwin part of the impact basin（ Fig. 4d） during et al., 2005; Di Achille and Hynek, 2010）. the Late Noachian–Early （ Ehlmann

— —123 Ray Spectrometer-based elemental information, which highlights distinctions between the regions below and above the putative shorelines（ Dohm et al., 2009）, long-wavelength topography con- sist ent with deformation caused by true polar wandering（ Perron et al., 2007）, and Mars Advanced Radar for Subsurface and Ionosphere Sounding （MARSIS） data which indicate that regionally occurring fine-grained sediments such as marine deposits and/or ice rather than lava flows best explain the radar signatures（ Mouginot et al., 2012）.

III．Recent and possibly present-day aqueous activity on Mars Fig. 3 Mars Exploration Rover, Opportunity, imaged nu- merous spherules on Mars, referred to as“ blueber- Contrasting with an ancient warmer and wetter ries”, once located in layered deposits that have Mars, as indicated by the above geomorphological since been exposed through differential erosion. The and geochemical evidence, present-day Mars blueberries, generally less than 1 cm in diameter as shown, are circular concretions of iron oxides（ hema- is characterized by drier and colder conditions. tite）, which are believed to be formed with a large With a mean surface pressure of about 8 kPa and amount of liquid water（ Moore, 2004; Havics et al., a mean temperature of －60℃ below the triple 2009）. The blueberries are similar to those formed point of water, liquid water at the surface of Mars on Earth through the precipitation of iron-enriched fluids, often hydrothermal related such as observed is absent except possibly transiently（ e.g., Haberle on the Colorado Plateau, United States（ Chan et et al., 2001; Sears and Moore, 2004; Fairén et al., 2004, 2007; Ormö et al., 2004b）. Thus, they are al., 2009a, b）. Elemental information obtained strong evidence for the existence of liquid water on through the Gamma Ray Spectrometer（ GRS） ancient Mars. instrument onboard the Mars Odyssey Spacecraft has led to the discovery of large quantities of near- et al., 2008）. Regional to global geological and surface water ice（ Boynton et al., 2002; Feldman hydrological considerations, including spatial et al., 2002）. Consistent with the GRS-based ob- and temporal associations among the western servation, the Phoenix Mission to Mars confirmed margin of Isidis basin and a topographic interval ice-cemented ground below a thin layer of dry consistent with other deltaic structures along the through in situ exploration（ et hemispheric dichotomy（ Di Achille and Hynek, al., 2009）. The origin of the ice-cemented ground 2010） and the proposed Noachian–Early Hesperian is still a matter of debate, but the presence of ocean（ Fairén et al., 2003; Ormö et al., 2004a）, ice close to the surface, makes it possible that it indicate that the Jezero paleolake was likely asso- occurs elsewhere and could contribute to present- ciated with hydrological activity（ including elev ated day transient aqueous activity. In addition, brine hydraulic head） of the more extensive ocean. may have also allowed water to remain liquid Additional evidence for ancient hydrological for sufficient time to produce geomorphological systems that included oceans are the giant poly- features indicative of aqueous activity near and on gons similar to those found on the ocean floors of the surface; possible liquid water, likely brine, has Earth（ McGowan, 2011） and impact craters and been observed at the Phoenix landing site（ Smith associated features reminiscent of impacts into et al., 2009; Whiteway et al., 2009）, and possible Earth oceans such as tsunami-like return flow of formation of night-time transient liquid brines the water displace by the impact event（ Ormö et which evaporate after sunrise has been observed al., 2004a）. In addition, the existence of ancient through Mars Science Laboratory（ MSL） data oceans is supported by Mars Odyssey Gamma （Martín-Torres et al. 2015）.

— —124 Fig. 4 The distribution and morphology of currently known candidate Martian deltaic deposits（ modified from Dohm et al., 2011）.（ a） Context camera image（ CTX） showing a multilobate depositional feature at the mouth of Sabrina Vallis.（ b） CTX view of the multilobate deposit within the crater, showing evidence for vertical and lateral accretion and extensive channel avulsion over the putative delta plain.（ c） Perspective view（ CTX image draped over CTX-derived dig- ital elevation model［ DEM］ with 2× vertical exaggeration） of a possible -type fan delta in （ Di Achille et al., 2007, 2009）.（ d） Multilobate deposit within the Jezero crater showing typical characteristics of terrestrial river-dominated deltas, evidence for vertical and lateral accretion, and channel avulsion over the putative delta plain （CTX）.（ e） Possible Gilbert-type fan delta in Nephentes Mensae region（ the same deposit is shown in the lower panel in a perspective view; CTX image draped over HRSC-derived DEM with 2× vertical exaggeration）.（ f） CTX image of a previously unreported deposit in .（ g） Heavily eroded Gilbert-type fan delta at the mouth of Tyras Vallis; the two transversal scarps have been interpreted to be evidence for two standings of the water level（ Di Achille et al., 2006）.（ h） North polar stereographic projection from Mars Orbiter Laser Altimeter data with locations of candidate deltas of（ a）（– g） shown. F indicates the approximate location of the delta fronts. Delta fronts mark the approximate el- evation of the most stable highstand during which the deposits formed.

— —125 Fig. 5 Slope streaks with distinct anastomosing patterns observed in MOC images（ a） M20-00897,（ b） M16-00596,（ c） M13- 00834, and（ d） M10-00967（ Miyamoto et al., 2004a, b）. Also note that a relatively large flow sources from a small domi- cal structure, indicating a continuous source. A competing hypothesis is dust avalanching for the feature（ Sullivan et al., 2001）. There is more and more evidence that points to water being involved with at least some of the dark slopes（ Kre- slavsky and Head, 2009）. In addition, there are more recently identified feature types that occur on Martian hillslopes that point to seasonal and possibly present-day hydrologic activity such as within impact craters and Valles Marineris （McEwen et al., 2011, 2013）.

Fig. 6 HiRISE stereo（ ESP_0028500_1425_ESP_028922_1425） image showing evidence for multiple resurfacing events by dif- ferent processes（ including possible glacial, alluvial, periglacial, fluvial, among others）（ Dohm et al., 2015）. The is buried by flow materials; the margin of the flow has slumped into the impact crater basin（ arrow）.

— —126 Fig. 7 Gullies and graben-like cavities upslope of candidate open-system pingos（ OSPs; dark gray arrows in（ a））, with arcu- ate ridges in between, interpreted to be moraines（ light gray arrows）; HiRISE image ESP_020720_1410（ Soare et al., 2014b）.（ a） Overview of the site, showing the locations of insets b–d and the downslope position of the putative OSPs rel- ative to the gullies and arcuate ridges.（ b） Top of the alcove of the eastern gully, showing an abrupt start of the channel embedded in the graben-like elongated depression. A possible landslide scar is located at the northern tip of the cavity. （c） Top of the alcove of the western gully, with rill-like features running into the graben-like cavity; the features seem to originate upslope from the non-polygonised terrain. Note the polygonal network within the cavity and in the surrounding terrain; black arrow points to location with low-centered polygons.（ d） Mid-part of the eastern gully, with multiple terrac- es（ i, ii） and multiple self-blocking digitate deposits（ iii, iv）, as indicated by black arrows. Note the distinct lineaments, which we interpret to be fractures and faults, as well as a polygonal network within the cavity and in the surrounding terrain. Image credits: NASA/JPL/University of Arizona.

As is often the case of geomorphological feature geomorphological feature types identified through types on Earth, a single Martian type has multiple relatively high-resolution imaging cameras on- interpretations. For example, while dark slope board the Mars Global Surveyor, Mars Odyssey, streaks have been interpreted to be related to and Mars Reconnaissance Orbiter. In addition dust avalanching（ Sullivan et al., 2001）, at least to slope streaks（ Ferris et al., 2002; Miyamoto et a percentage of the total population of streaks al., 2004a）（ Fig. 5）, gully and associated debris could be indicative of present-day aqueous activity aprons（ Malin et al., 2006）, slope linea in and （Ferris et al., 2002）, supported by viscoplastic outside of Valles Marineris（ McEwen et al., 2011, flow numerical modeling techniques applied to 2013）, features indicative of recent glacial（ Kargel, anastomosing-patterned streaks（ Miyamoto et al., 2004; El Maarrry et al., 2013） and periglacial 2004a）. Thus, a single geomorphological feature activity（ Soare et al., 2014a）（ Fig. 6）, gullies on such as a dark slope streak is a controversial dunes（ Miyamoto et al., 2004b）, and gullies and marker of possible recent and possibly present- candidate open-system pingos（ Soare et al., 2014b） day aqueous activity using orbiting spacecraft （Fig. 7） point to recent and possibly present- data. On the other hand, it is hard to refute such day aqueous activity. possible activity when considering a collection of

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— —131 地形学からみた火星における過去から現在までの水成活動

ジェームズ・ドーム＊ 宮 本 英 昭＊

本論では，最近の火星における各軌道衛星・着 ヤー，マーズ・オデッセイ，マーズ・リコネッサ 陸船・探査車による探査計画によって判明した， ンス・オービターに搭載された高解像度カメラ 過去から現在までの水成侵食地形について概観 も，現成の可能性もある新しい水成活動を示す詳 する。過去に存在した湖や海洋に関連する地形 しい地形を撮しだした。スロープ・ストリーク， 的証拠には多種多様なものがある。例えば，堆 スロープ・リニア，断層・断裂に沿ってあるいは 積岩層，土石流跡，河谷，沖積扇状地，巨大多 地質境界や地質構造に端を発するガリー，開放系 角形土，そして氷河・周氷河地形などである。 ピンゴなどが該当する。これらの過去から現在ま おそらく古海洋の存在を示す最大の証拠は，海岸 でのさまざまな地形は，長期にわたる地表や地下 線に対応する地形的境界に沿ってみられるデル の水文環境を示唆すると同時に，火星が「地球外 タ地形であろう。化学・鉱物・元素上の諸特性も 生命はどこに存在するか？」という永遠の問いに 水の関与する風化作用が働いていたことを明示 答える最大の候補地であることを物語っている。 する。さらに，マーズ・グローバルーサーベー

キーワード： 火星，地形，水文，海洋，アウトフロー・チャネル，ガリー，スロープ・リニア

＊ 東京大学総合研究博物館

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