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 Mars James M. DOHM＊ and Hideaki MIYAMOTO＊ ［Received 31 March, 2015; Accepted 5 December, 2015］ Abstract This paper overviews water sculpted Martian 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 Global Surveyor, 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, geomorphology, 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 Valles Marineris（ 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 Amazonian–Noachian 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 Neukum（ 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 Cerberus 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 Tharsis（ 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, Olympus Mons（ O.M.）, which is have been identified below the putative shorelines the largest volcano in the solar system, the Tharsis when compared to terrain 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）. Kasei Valles（ 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 South 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 Nili Fossae 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 Jezero impact basin（ Fig. 4d） during et al., 2005; Di Achille and Hynek, 2010）. the Late Noachian–Early Hesperian（ 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.