層状堆積岩層を切って崩壊地―ガリー―土石流錐の一連の地形が形成されており，その下端は多角形土のみられるクレーター底（写真右端）に張り出している．斜面頂部（写真左端）にも多角形土が広がる．これらの景観より，高緯度に広く存在する「氷に富む表層（永久凍土）」からなる谷頭部で（融解？）侵食が発生し，土石流の流下によりガリーを発達させ，斜面基部に堆積するという地形変化が最近も起こっている可能性がうかがえる．崖錐表面は季節的なドライアイス層で覆われて白っぽくみえる．HiRISEのホームページにGulick（2014）による本画像の紹介記事がある．写真の左右幅は約5 km． （写真提供：NASA/JPL/University of Arizona；説明：松岡憲知）
Mars has fascinated us in terms of discovering vital activities and examining planetary surface evolutions. Recent explorations detected subsurface ice and methane, which reveal the importance of investigating ancient environments and their evolution. Impact craters and their ejecta morphologies are natural probes for obtaining knowledge on ancient surface and subsurface environments, because detailed morphologies of impact craters and their ejecta are sensitive to impact conditions and degradation processes. Martian enigmatic ejecta morphologies and their currently-proposed origins are summarized, and some degradation processes effective on Martian craters are introduced. Ejecta of Martian impact craters are different from those of lunar or Mercurian impact craters. They possess clear distal ridges and cliffs, and are diverted by preexisting obstacles, while edges of lunar or Mercurian ejecta are ambiguous. Their unique morphologies suggest that the ejecta are emplaced by radial ground-hugging flows generated at the time of the impact. Processes and agents of the fluidization draw attention, because the atmosphere and/or subsurface volatiles are suggested for its formation. Degradation processes on Mars vary, and include superimposed impacts, viscous relaxation, mass movements, lava infilling, fluvial incision, gully formation, lacustrine erosion and deposition, ice-related processes, and airfall deposition. Due to high ancient erosion rates, degraded crater morphologies also provide useful information on the ancient environment.
Cone morphologies with a variety of origins and sizes have been widely identified on Mars using remote sensing data such as ultra-high resolution visible images. Currently, small cones of less than 100 m in bottom diameter can be identified. These Martian cones are located in young surface regions, suggesting they were produced in an environment that existed in recent geological history. They had volcanic, periglacial, and other origins. This paper first introduces a classification of terrestrial cone morphology: volcanic (spatter cones, scoria/pumice cones, maars, tuff rings, tuff cones, and rootless cones), periglacial (pingos), and others (mud volcanoes). Then, it reviews the characteristics of cone morphology on Mars focusing on morphology, morphometry, and distribution. Previous cone studies show the existence of explosive basaltic eruptions on recent Mars, while young lava flows were pervasive. The prevalence of rootless cones suggests the presence of water/ice during their formation at many places on Mars. These discoveries contribute to clarifying the recent surface environment and thermal state of Mars. To further apply terrestrial knowledge to Martian cones, it is necessary to understand the relationship between the morphology and the formation process of cone morphologies on Earth from a wide perspective.
Recent detailed explorations of Mars have revealed various types of landform that are similar to terrestrial periglacial landforms and provided direct and indirect evidence for ground ice. More than 60% of regolith is occupied by ice at high latitudes. Phoenix lander has discovered pure ice by trenching at the landing site, and the volume fraction is up to 90%. Many researchers have investigated formation mechanisms of excess ice, but uncertainties still remain. In addition to acquisition of detailed Martian surface data, a range of water forms such as premelting water and brine water have drawn attention in the Martian environment with the development of premelting dynamics. Premelting water (also called unfrozen water), which is adsorbed to particle surfaces and confined to capillary regions, remains in a liquid state below the nominal melting temperature. Migration and solidification of premelting water causes various periglacial processes such as ice lens formation, frost heave, and soil movement. Although bulk water cannot exist in the recent Martian environment, premelting water can remain in a liquid state due to interaction with regolith and the existence of salt. Electrolytes in water decrease evaporation rate and broaden the stable temperature range of liquid water. Sizemore et al. (2015) investigated the initiation and growth of Martian ice lens by developing an ice lens model and a climate model. Numerical simulations suggest that ice lens initiation is a ubiquitous phenomenon in the high latitudes, but the magnitude of ice lens growth depends highly on soil properties. In particular, deliquescent salts and water vapor are important sources of water. Further development of numerical models and experiments that simulate Martian environments are required.
Following the development of high-resolution imaging, digital elevation models, thermal and hydrological data, and onsite ground information during the early period of the 21st century, the periglacial geomorphology of the Martian surface advanced rapidly. Images can even resolute meter-scale landforms, enabling identification of most periglacial features and analysis of their global distributions and detailed morphologies. This review focuses on progress in research covering permafrost distribution, patterned ground, possible heave and subsidence features, lobate debris forms, and slope-lineated features during the last decade. Most of the Martian high-latitude surface is underlain by ice-rich ground called the latitude-dependent mantle (LDM), which favors permafrost-related features possibly developed under warm-humid conditions during past high-obliquity periods or partially s,till active under the present cold-dry conditions. Thermal contraction cracking is likely to prevail in the LDM, resulting in high-centered, flat-top polygons, possibly underlain by sublimation-type wedges or sand wedges that prevail at high latitudes. The surface patterns change into subdued or peak-top polygons toward the mid-latitudes, probably reflecting long-term sublimation of the LDM. Some researchers attribute stone circles to sorting due to freeze-thaw, but the features are much larger than candidates on the Earth. Small isolated domes with concentric cracks or craters at the top may include pingos, which also prevail at high- to mid-latitudes. Asymmetrical scalloped depressions may result from sublimation or thawing of the LDM, but there is a debate between pole-ward and equator-ward slope retreats. Lobate debris aprons may originate from creep of ice-rock mixtures or debris-covered glaciers, but the distinction between the two origins is unclear, as in the long-lasting debate on terrestrial candidates. Some thin, smaller debris lobes at high latitudes resemble stone-banked solifluction lobes on the Earth, which may indicate the occurrence of seasonal freeze-thaw cycles in the recent past. Time-series images indicate active slope features, including gullies, slope streaks, and recurrent slope lineaes that develop below cliffs. These active features may originate from outflows of brine that thaws far below the melting point of water ice.
Spiral troughs are known to exist on the polar ice caps of Mars. The formative conditions of the troughs, which should be affected by Martian climate processes, have been sources of much debate. Recently, they have been interpreted to be cyclic steps formed by katabatic winds blowing over the ice. Cyclic steps are spatially periodic bedforms observed on relatively steep slopes, characterized by regular upstream-migrating steps delineated by hydraulic jumps. They are relatives of upstream-migrating antidunes. Boundary waves often form at the interface between ice and fluid flowing adjacent to it. Examples include ripples on the underside of a river ice cover and steps on the bed of a supraglacial meltwater channel. Waves on ice may also be formed by wind, such as megadunes observed on the Antarctic ice sheet. There have been, however, few experimental studies on the formation of either cyclic steps or upstream-migrating antidunes on ice. The first part of this paper introduces features of spiral troughs on Mars' North Polar ice cap and recent hypotheses interpreting spiral troughs as cyclic steps created by katabatic winds. The cyclic step framework can explain trough initiation, migration, and all of the major physical characteristics of spiral troughs. The latter part of this paper introduces an analogue experiment on the formation of cyclic steps on ice by flowing water, with the aim of understanding the process whereby spiral troughs are formed on Mars' ice caps. Trains of steps form when the Froude number is larger than a value around unity. The features of those steps allow them to be identified as ice-bed analogs of cyclic steps in alluvial and bedrock rivers. Moreover, the results of experiments fall into a region where a linear stability analysis predicts interfacial instability.
Spiral troughs are observed on the polar ice caps of Mars. Interplanetary explorations indicate the troughs are perpendicular to katabatic winds blowing on the ice surface with jumps observed at the lee sides of troughs. Based on these observations, Smith et al. suggest they are not troughs but bedforms created by the sublimation of water carried by katabatic winds and ice on the floor. To demonstrate the fundamental processes forming bedforms on ice, linear stability analyses of the formation of boundary waves on the water-ice interface under laminar and turbulent flow conditions are presented. In addition, a formulation for the formation of boundary waves on ice due to katabatic winds is also proposed.
Weathering processes on Martian surface are among the essential issues for both understanding landform evolution and exploring water availability on Martian surface. Studies are reviewed on various weathering processes on Martian surface based on images, data collected by rovers, and laboratory approaches. Recent explorations by the Mars Exploration Rovers reveal that chemical weathering occurs on the surface of basaltic regolith. Dissolution of olivine and oxidation of Fe produces weathering rinds on basaltic surface regolith. Rock interiors also show vugs and veins filled with light-toned efflorescence indicative of chemical weathering. In particular, in high-latitude areas the two Viking landers and the Mars Pathfinder observed honeycomb weathering, angular rock fragments, and polygonal cracks in bedrock. Most of these features are also observed in the Antarctica and other cold deserts on Earth, and are generally attributed to physical weathering such as salt crystallization, thermal weathering, and/or frost weathering. Some studies successfully estimate periods and rates of weathering on the Martian surface, which promote a further understanding of environmental changes and landform evolution.
Gullies and lobate deposits commonly occur on the flanks of craters and dune foresets on Mars. These topographic features are thought to have been formed by debris-flow processes. Debris-flow processes suggest the existence of liquid water on the surface of Mars, which is believed to have an extremely cold and dry environment. Debris flows occur when masses of poorly sorted sediment, agitated and saturated with water, surge down slopes in response to gravitational attraction. Thus, recent activities of debris flows imply the existence of liquid water phase materials on the Martian surface. However, the dry granular flow caused by a slope avalanche can also form gullies and lobate deposits, which resemble debris-flow deposits, so there are still uncertainties over the occurrence and origins of liquid water on the modern Martian surface. This study proposes a method for estimating debris-flow properties, in order to distinguish debris-flow processes on Mars. Preliminary results suggest that a lobate deposit on Mars can be formed by a debris flow that shows flow properties similar to water-saturated debris flows on the Earth. If rheological parameters of the flows can be determined from the surface morphology of lobate deposits, it would be helpful for distinguishing debris-flow processes from dry-granular flows. Although our method is in the course of development and it is still difficult to determine flow types from rheological parameters alone, future field surveys and experimental studies will provide criteria for identifying debris flows.
Ancient Mars is now considered to have had an environment somewhat similar to that of Earth in terms of the existence of large bodies of water, a wide range of surface oxidation states, an active dynamo and associated magnetic field, magmatism and tectonism which includes mountain building and basin formation, and appearances of variety of chemical components potentially building blocks of life. Similar to habitable Earth, ancient Mars included hydrological cycling among the atmosphere, ocean, and landmass (southern cratered highlands), and plate tectonism cannot be ruled out. Endogenic activities have continued until even very recently, and recent water-related geological features indicate prolonged existence of aquifer systems, where habitable environments may exist for a significant period of time. Occasional releases of volatiles from such aquifer systems may ultimately account for the detection of methane by the Curiosity rover in the Gale crater and the inconclusive result (i.e., not unambiguous denial) of metabolism-detection instrument onboard Viking landers. Unequivocal evidence of the existence of subsurface aquifers or extant endogenic activity is, however, still lacking possibly due to the existence of homogeneous regolith materials covering the surface of Mars. Also, even if a habitable environment exists at depth, accessing the environment by a spacecraft (either a lander or a rover) has been considered to be challenging especially because such an environment has been generally thought to exist more than several kilometers below the Martian surface. Recent findings of a recurring slope lineae (RSL) point to traces of possible seasonal liquid water flows along slopes, findings of which will likely change the above prevailing view; some of these features might result from the partial discharges from an aquifer. In other words, RSLs might provide a natural bridge between a subsurface aquifer and the surface accessible by a rover. Thus, subsurface structures near such features become prime targets to be explored through future missions. Once the presence of ground water is confirmed, especially an aquifer, mapping and characterizing the distribution of subsurface water would significantly help address the ever-important question of whether life exists on Mars. Given this view, we have selected possible landing sites for a future landing mission to detect life on Mars. Our selection is based on the possibility of the existence of near-surface water and recent geological and hydrological activities; specifically areas with (1) a higher possibility of releases of volatiles, (2) a relatively high water activity (Aw>0.6), (3) a relatively higher maximum environmental temperature (T>250K), and (4) an existence of gradients of free energy. We propose Melas Chasma in Valles Marineris as a prime candidate because of its long-term water enrichment and energy conditions as evidenced through it: (1) comprising confirmed recurring slope lineae (RSL); (2) being the widest and deepest part of the Valles Marineris and thus a major catchment basin of Mars since its formation; (3) being connected to the outflow channels; (4) possible fog for at least part of a Martian day; (5) containing Interior Layered Deposits (ILDs) which comprise various sulfates deposits, as well as phyllosilicates among the canyon units, both of which are suggestive of abundant past water; (6) comprising a volcanic field in its southeast part; and (7) being cut by deep-seated basement structures that served as conduits for the migration of both groundwater and heat. We also propose Tharsis/Elysium Corridor region as among the best candidates, which shows evidence of long-lived water enrichment and recent geologic activity.