29 Extraterrestrial arid surface processes Gordon L. Wells and James R. Zimbelman

Introduction surface of Venus and attempt to forecast the course of future planetary missions of interest The development of arid zone geomorphology to arid zone scientists. has progressed over the past century from Spacecraft exploration of Mars began with ground-level observations of a limited number of three Mariner flyby missions during the 1960s. locations through the field traverse mapping of The initial impression was of a surface much sizeable arid areas to a broad-scale overview like that of the Moon. This judgement changed of global desert regions assisted by aerial photo- following the Mariner 9 mission in 1971-72, graphy and satellite imagery. Extraterrestrial which relayed more detailed images showing investigations of planetary surfaces and land- a surface with many landforms common to forms have proceeded in exactly the opposite Earth. Beginning in the summer of 1976, views manner. Telescopic observations first provided from the surface were transmitted by two global overviews of distant planets. The early landing craft. On 20 July 1976, Viking 1 landed spacecraft missions of the 1960s relayed hemis- on the vast volcanic plain of Chryse Planitia pheric and regional views, while later missions (22.5°N, 47.8°W). Several weeks later, Viking returned more detailed images comparable to 2 touched down upon Utopia Planitia (48.0°N, satellite images of the Earth. To date, detailed 225.6°W), another northern hemisphere plain. views seen from the surfaces of other terrestrial During the time of Viking lander data collec- planets are restricted to two locations on Mars tion, two Viking orbiters imaged the surface and four landing sites on Venus. As a result, the from an altitude of about 1500 km. approach and scope of our inquiry into arid Though Mars is a small planet, its land- processes are necessarily different when study- surface area is nearly equal to that of the Earth. ing extraterrestrial surfaces. In this review, we Another close similarity is daylength. Because attempt to build a picture of the arid surface of the Martian day is slightly longer than Earth's, Mars starting with a discussion of possible events at the Viking landing sites were microscale and mesoscale processes and later measured in Sols, or Martian solar days, examining the creation of individual arid following each landing. Mars also has distinct landforms and terrains. We conclude with an seasons, but due to the eccentricity of its overview of recent discoveries regarding the orbit, the seasons are of unequal length. The

Arid Zone Geomorphology: Process, Form and Change in Drylands, 2nd edition. Edited by David S. G. Thomas. © 1997 John Wiley & Sons Ltd. 660 Arid zone geomorphology

Table 29.1 Physical characteristics of Earth, Mars and Venus

Earth Mars Venus

Planetary mass (kg) 5.98 x 1024 6.44 x 10" 4.87 x 1024 Planetary radius (km) 6369 3394 6052 Land surface area (km2) 1.47 x 106 1.45 x06 4.61 x 106 Acceleration due to 9.81 3.72 8.85 gravity (m s"1) Composition of N2 (0.78) CO, (0.95) CO, (0.97) atmosphere 02 (0.21) N2 (0.027) N2 (0.03) H20 (<0.03) A (0.016) S02 + S2 (<0.001) A (0.01) Mean surface pressure (mb) 1016 7.5 -90 000 Surface temperature (°C) -53-57 -128--28 374-465 Rotation period (hours 23 h 56 m 24 h 37 m 5832 h (retrograde) and minutes) Revolution period (yr) 1.000 1.881 0.615 Orbit eccentricity 0.017 0.093 0.007 Range: 0.01-0.05 Range: 0.009-0.14 Obliquity* (degrees) 23.5 25.2 -179 Range: 21.8-24.4 Range: -12-38 (0.2-51.4)* Source: Hartmann (1972); Leovy (1979); Carr (1981). *Obliquity history values calculated by Bills (1990). convention for reference to Martian seasons is impacts reduced the quantity of newly frag- the aerocentric longitude (Ls), where in the mented materials. On Earth, Mars and Venus, northern hemisphere the vernal equinox occurs the availability of volatiles allowed the chemical at Ls = 0°, summer solstice at Ls = 90°, autum- breakdown of impact breccia (regolith), and on nal equinox at Ls = 180° and winter solstice at Earth the plate tectonic regime produced a Ls = 270°. chemically differentiated crust suited to chemi- The basic physical characteristics of Earth, cal alteration by a variety of processes. The Mars and Venus (Table 29.1) permit a wide chemical weathering processes occurring on range of surface environments. One of the Mars are in several ways unlike those on Earth, major goals in the development of geomorpho- because the Martian lithosphere does not logical theory is the creation of explanations for undergo plate motion, subduction and crustal landscape processes based upon general princi- recycling, and its regolith is composed primarily ples and not merely the results of descriptive of mafic to ultramafic basalts. speculation. The surfaces of Mars and Venus offer laboratories for testing the assumptions of terrestrial theories under conditions where such PRESENCE OF WATER variables as gravity, atmospheric pressure and surface temperature are different. Attempts to estimate the total amount of outgassed water during Martian history have arrived at figures representing an equivalent Rock disintegration processes on global surface layer of liquid water ranging in Mars depth from 100 to 500 m (Carr 1981; 1986; Squyres 1984; Donahue 1995). The entire The primordial mechanism for rock disintegra- inventory of observable water in the modern tion on planetary surfaces was brecciation atmosphere and ice caps represents a surface caused by impacting meteorites. Following the layer less than 10 m in depth (Rossbacher and first several hundred million years of solar Judson 1981). Analyses performed on soil system history, a decline in the rate of meteorite samples collected at the Viking landing sites Extraterrestrial arid surface processes 661 confirm that at least some of the water has the surface as a seasonal extension of the polar become chemically bound with surficial sedi- cap. If these condensates occupy pore spaces ments. The materials at the landing sites show among the fine surficial sediments and con- high concentrations of iron (as anticipated from dense additional trace amounts of surface water the rusty red surface coloration of the Martian vapour, frost-wedging may liberate fine parti- landscape), sulphur, calcium and magnesium cles that would then be available for aeolian (Baird et al. 1976). The relative abundances transport. suggest source rocks of mafic composition, probably basalts. The Viking results led Baird et al. (1976) to conclude that the fine-grained SALT WEATHERING materials on Mars are predominantly iron-rich clays with additional components of magnesium An unexpected result of the Viking chemical sulphate (10%), carbonate (5%) and iron analyses of Martian surface samples was the oxides (5%). The production of smectite clays discovery of a high concentration of sulphate on Mars may follow several pathways (Carr (MgS04 and NaS04), carbonate (CaC03) and 1981), including hydro thermal alteration in the chloride (NaCl) salts in the Martian soil. The volcanic regions (Griffith and Arvidson 1994), amount of sulphur was found to be enriched in groundwater alteration in subsurface aquifers samples taken from thin layers of duricrust atop (Burns and Fisher 1990; Burns 1994) and, aeolian drift deposits and in pebble-sized soil perhaps, ultraviolet-catalysed reactions with a aggregates, when their composition was com- monolayer of water coating fine-grained basaltic pared to that of unconsolidated fine materials. sediments (Huguenin 1976; Stephens et al. The excess sulphur content strongly indicates 1994). that salt weathering is an active process on Mars (Clark et al. 1976; Clark and Van Hart 1981), and that thin layers of iron-rich clay particles FROST WEATHERING AND are cemented by sulphate and carbonate min- DISAGGREGATION PROCESSES erals (Toulmin et al. 1977). Diffusion of salts to the surface by water vapour passing through The frost point of water on Mars is -7S°C, and the substrate could lead to the formation of all regions poleward of 40° latitude receive at widespread duricrust layers over periods of 5 6 least some seasonal H20 frost accumulation 10 -10 years (Jakosky and Christensen 1986) usually mixed with C02 frost in a clathrate The most dramatic 'geomorphological event' to structure. At both Viking landing sites, water occur during observations at the Viking 1 land- vapour was saturated in the atmosphere, and ing site was the slope failure and slumping of lander measurements appear to confirm a duricrust material covering a drift deposit near seasonal exchanged between surface materials the base of a large boulder (Figure 29.1). The and the atmosphere (Moore et al. 1987; Moore drift deposits in the vicinity of the Viking 1 and Jakosky 1989). Laboratory experiments by lander give the appearance of being stripped by Huguenin (1982), which simulated the Martian deflation. If this interpretation is correct, frost cycle, produced chemical weathering of duricrust formation has proceeded with suffi- silicates (olivine) as a result of the incorporation cient rapidity to encase aeolian drift materials as of hydrogen ions from the H20 frost. In the deposits are in the process of being deflated addition to chemical weathering, frost may and removed from the area. In areas of stable or serve mechanically to disaggregate fine-grained accumulating aeolian deposits, salt weathering surface materials in the high-latitude regions. may lead to the production of considerably The precipitation of frost occurs when atmos- thicker duricrusts. pheric dust particles, which serve as nucleation Salt weathering may attack competent Mar- sites for equatorial water vapour, arrive over tian rocks along several possible routes (Malin the winter hemisphere high latitudes. The 1974) even in the absence of liquid water. As dust-H20 ice particles may also condense C02 occurs in terrestrial environments discussed in ice at its frost point of -125°C. In regions near Chapter 3, hydration by water vapour of salts the poles, the dust-H20-C02 ice particles coat trapped in surface rock pores may physically 662 Arid zone geomorphology

Figure 29.1 Two views of the Viking I landing site taken on (A) VL-1 Sol 25 and (B) Sol 239 record a slump of duricrust material upon a drift deposit. [(A) VL1 #11A143; (B) #11C162] destroy rocks, as may the diurnal thermal site, which indicate a surface age of more than expansion and contraction of salt crystals 3 billion years (Arvidson et al. 1979). At the trapped inside pore spaces of rocks having a simulated rate of aeolian abrasion, all rocky different expansion coefficient. landforms on the scale of 100 m would be abraded to near base-level in less than 50 mil- lion years. AEOLIAN ABRASION At least three factors may explain the discre- pancy between the rate of aeolian abrasion Apparently wind-faceted and pitted rocks are modelled by wind-tunnel experiments and plentiful near the Viking landing sites (Figure the degree of surface modification actually 29.2), and wind-tunnel experiments (Greeley et observed. Broad regions of Mars presently serve al. 1984) simulating Martian conditions have as sediment sinks for aeolian materials, while confirmed high rates of aeolian abrasion. In other areas show signs of the removal of for- fact, the simulated rates are enormous, if basalt merly extensive aeolian mantles. Perhaps in or quartz grains are used as impactors. Abrasion some regions (e.g. around the South Pole) a rates in the range 0.0003-0.002 cmyr"1 have blanket of aeolian deposits protected the ancient been calculated by Greeley et al. (1984) and surface from abrasion (Plaut et al. 1988). A result from the high initial velocities of Martian second possibility follows from the character of particles in saltation and their high terminal saltation on Mars. Both landing sites display velocities under conditions of low atmospheric rocky surfaces. In the Martian atmosphere, drag. Paradoxically, the Martian surface near typical saltation path lengths span several the landing sites is covered with large and metres and require sandy surfaces over such probably very ancient rocks (Mutch and Jones distances for a saltation cloud to develop fully. 1978; Sharp and Malin 1984), while orbital The distribution of rocks could effectively imagery reveals concentrations of small- to dampen the saltation flux and reduce the rate of medium-sized craters near the Viking 1 landing abrasion (Williams and Greeley 1986). Finally, Extraterrestrial arid surface processes 663

Figure 29.2 Two views under different illumination conditions show wind-faceted and pitted rocks at the Viking 1 landing site. Arrows point to small impacts penetrating duricrust by clasts dislodged during the firing of the lander descent rocket. [(A) VL1 #11B144; (B) #11A078] most saltating particles on Mars probably do processes result in transport events having not consist of intact mineral grains. Energetic different magnitudes, frequencies and durations collisions during saltation would lead to short to those of comparable events on Earth lifetimes for basalt fragments and other (Greeley and Iversen 1985). 'kamikaze' grains similar to terrestrial quartz sands (Sagan et al. 1977). Clay aggregates dislodged from duricrusts provide the most DUST DEVILS reasonable candidates for saltation (Greeley 1979), and aggregates formed as a sublimation During summer months, in the lower boundary residue of dust-nucleated ice may supply salt- layer of the clear, cold, thin Martian atmos- able materials in the polar regions (Saunders phere, the large vertical temperature gradient and Blewett 1987; Thomas and Weitz 1989; above the relatively warm surface may support Herkenhoff and Murray 1990a). Impact abra- intense free convection. Under these conditions sion by these low-density particles would be during periods of near calm, thermals will rise much less effective than with intact mineral into the deep Martian convective layer. Gentle grains. surface winds crossing topographic irregularities may create sufficient basal shear to induce vortices lifting dust into the rising thermal. The Transport by suspension, saltation resulting dust devils have been observed on and creep on Mars high-resolution Viking orbiter images of Arcadia Planitia and Phlegre Montes between 30°N and nsAs onun Earth (see Chapter 16), the primary 45°N (Thomas and Gierasch 1985). Of the 99 modes of aeolian transport on Mars are suspen- dust vortices detected, most dust funnels sion, saltation and creep. However, the achieved heights of 1.0-2.5 km. Estimated dust boundary conditions governing Martian aeolian mass suspended aloft in the typical Martian 664 Arid zone geomorphology dust devil is 3000 kg (Thomas and Gierasch take place from late spring until after perihelion 1985). passage, Ls = 250°, near the time of the Although never directly imaged, circumstan- southern hemisphere summer solstice (Peter- tial evidence points to the existence of much freund 1985),when the latitudinal temperature larger Martian aeolian vortices. Dark linear gradient close to the retreating frost outlier filaments approximately 0.5 km wide with reaches its most extreme value, as does surface lengths of 2-75 km lie along approximately heating near the subsolar point. The frequent parallel tracks on the plains of the southern occurrence of point-source dust plumes rising hemisphere between 30°S and 60°S (Grant and from areas recently covered by the seasonal Schultz 1987). The filaments display short gaps frost cap leads to speculation about the role of and make their appearance seasonally from frost-wedging in the production of fines for midsummer until early autumn with slight transport by suspension after water ice has changes in orientation detected from year to sublimed (Figure 29.3). During the modern year. These dark filaments may result from climatic epoch on Mars, the combination of surface dust removal and concentration of fine particles released by frost-wedging and sand-sized sediments by tornadic vortices strong katabatic surface winds sweeping along sweeping across the southern hemisphere mid- the subliming frost cap has created a significant latitudes. Confirmation of such storms occur- dust source in the southern hemisphere ring in a southern hemisphere 'tornado alley' (Christensen 1988). Based upon Viking Infrared awaits their detection by future Mars-orbiting Thermal Mapper (IRTM) observations, Viking spacecraft. camera images and terrestrial telescopic sight- ings, Peterfreund (1985) located 156 dust storm events, 79% of which originated in the BAROCLINIC AND KATABATIC DUST southern hemisphere. STORMS

Global weather systems may produce local dust MARTIAN GLOBAL DUST STORMS storms whenever the poleward temperature gradient is sufficiently large to generate intense When the Mariner 9 spacecraft approached zonal circulation across the mid-latitudes in Mars in November 1971, the entire surface was the form of baroclinic waves. Numerical obscured by a global dust storm. Such enor- simulations of Martian general circulation mous dust storms follow an evolutionary indicate that surface wind velocities in excess of sequence governed by the thermal properties of the saltation threshold should accompany the the Martian atmosphere and global circulation. passage of strong baroclinic waves (Mass and Of the large dust storms, which have been Sagan 1976; Pollock et al. 1981; Barnes et al. observed to grow to global proportions, all 1993; Greeley et al. 1993), and examples of originated south of the equator near the time of dust-transporting extratropical cyclones have southern hemisphere perihelion, Ls = 250° ± 60° been observed in a few instances (Hunt and (Briggs et al. 1979; Kahn et al. 1992). James 1979; Kahn et al. 1992). On the sur- Once entrained in the Martian atmosphere, face, meteorological instruments aboard the dust absorbs incoming radiation, which is then Viking landers recorded peak gust velocities of reradiated as sensible heat. Differential heating 20-26 ms ' during the passage of dust storms of dusty and clear air parcels promotes convec- (Hess et al. 1977; Moore et al. 1987). tive turbulence leading to high-velocity surface Most observations of local dust storms docu- winds along the dust storm margin which, in ment a second type of circulation that operates turn, serve to raise additional dust. On a larger on a regional scale. These dust storms are scale, regional dust storms form an atmospheric produced by katabatic outflow from receding thermal blanket that interacts with clear-air frost outliers of the polar caps and diurnal regions of lower heat capacity to produce winds descending areas of high relief (Lee et al. strong, thermally driven tidal winds (Leovy and 1982; Magalhaes and Gierasch 1982; Kahn et Zurek 1979). These winds will be particularly al. 1992). The great majority of dust storms effective in raising dust from areas along the Extraterrestrial arid surface processes 665

50 km

Figure 29.3 Point-source dust plumes (arrows) rise from an area

recently covered by the seasonal H20 frost cap in the Aonia Terra region (68°S, 69°W) of the Martian southern hemisphere. fVO #248BSS, #248B56] sunset terminator, where a region of dust deposited dust (less than 10"3gcm"2) can warmed by radiative heating adjoins a rapidly result in reflectance values comparable to cooling region of clear air. A regional dust optically thick dust layers, even when deposited storm driven by convective turbulence and upon a dark surface (Wells et al. 1984). Viking thermal tides may propagate to achieve global lander cameras at both landing sites recorded dimensions. the deposition of a thin (less than 1 mm thick), During the late spring and summer, global red dust layer following the two global dust circulation on Mars is dominated by a cross- storms of 1977 (Guinness et al. 1982). The equatorial Hadley cell (Haberle et al. 1993). surface brightening was most noticeable in the The rising limb of the cell is located near the areas surrounding the Viking 2 landing site on subsolar point in the summer hemisphere, and Utopia Planitia (Figure 29.4). As viewed from the descending limb over the winter hemisphere the Viking orbiters, the northern hemisphere mid-latitudes (Toon et al. 1980; Pollock et al. surfaces of Syrtis Major and Acidalia Planitia 1981; Magalhaes 1987). The presence of a brightened measurably immediately after the dusty atmosphere in the southern hemisphere 1977 global storms, then gradually darkened near the time of northern winter solstice greatly again over the following months (Christensen amplifies the Hadley circulation (Haberle et al. 1988). By contrast, Hellas Planitia, a southern 1993). Once pervaded with dust, the global hemisphere basin and site of frequent dust atmosphere becomes stabilised because only the outbreaks, appears to be a sediment source upper portion is effectively heated (Haberle currently undergoing net erosion by aeolian 1986a; 1986b). Convective turbulence is processes (Moore and Edgett 1993). smothered near the surface by an almost isothermal environmental lapse rate. Over a period of several weeks, dust settles out of the BRIGHT AND DARK WIND-STREAKS atmosphere. The northern polar region, with its condensing frost cap, acts as a primary sedi- Extending from many topographic highpoints, ment sink where the dust from global storms crater rims in particular, are light or dark serves in the condensation process to nucleate albedo streaks in the equatorial and mid- H20 and C02 ice (Toon et al. 1980; Barnes latitude portion of Mars between 40°N and 1990). 40°S (Figure 29.5). Both types of streak are Global dust storms constitute the primary intimately connected to the occurrence of global geomorphological process occurring on Mars dust storms. Bright streaks are interpreted to during the present climatic epoch. Laboratory be deposits of fine material which have measurements show that small amounts of been concentrated preferentially in protected 666 Arid zone geomorphology

Figure 29.4 The brightening of the surface around the Viking 2 landing site following dust deposition by the 1977 global storms is apparent in two surface views made under similar illumination conditions on (A) VL-2 Sol 30 prior to the dust storms and (B) after the dustfall. [(A) VL2 #22A2S2; (B) #22G085]

wind-shadow zones during dustfalls from global to reform in the period immediately after the storms (Veverka et al. 1981; Greeley et al. dustfall (Thomas and Veverka 1979). 1992b). Light-coloured streaks are typically Local differences in bright and dark wind- wide, exhibit a feather-like structure downwind streak directions and the creation of bright and reach lengths of 5-20 km. The dark streaks depositional streaks rather than dark erosional result from deflation in the lee of topographic streaks have been attributed to differing obstructions by paired horizontal vortices shed meteorological conditions in a hypothesis from obstacles that accelerate local airflow developed by Veverka et al. (1981). When (Greeley et al. 1974). The dark erosional radiative heating of dust promotes static stabil- streaks that expose the underlying surface are ity during the dustfall stge of a global storm, narrow, sharply defined and attain lengths of airflow in the boundary layer becomes more 5-30 km (Greeley and Iversen 1985). Whereas laminar, leading to concentrated dust deposi- the bright streaks are relatively stable features tion in long wind-shadow zones in the lee of exhibiting only minor changes over the course obstacles. Sediments in bright wind-streaks are of several Martian years, the dark streaks largely deposited at this time. After the atmosphere vanish after deposition of a thin layer of surface clears, the effect of flow-blocking by dust from a single global dust storm and begin terrain diminishes, and vertical motions again Extraterrestrial arid surface processes 667

Figure 29.5 Both bright and dark albedo streaks extend downwind from crater rims in the Syrtis Major region (4aN, 297°W). [VO #496A45] predominate in the boundary layer, allowing of the absolute depth of aeolian deposits in an vortices to scour the recent dustfall layer to area is limited by the spatial resolution of space- create dark wind-streaks. craft cameras. However, some aspects of the The global distribution of bright wind-streak physical properties of Martian surface materials directions serves to map the general circulation can be inferred from data transmitted by the of Mars at the time of streak formation. Since IRTM sensors on the Viking orbiters. most bright streaks are formed during global The equatorial and mid-latitude regions dust storms, the trans-equatorial Hadley circu- display areas of relatively low thermal inertia lation of the southern hemisphere late spring interspersed with areas of relatively high and summer is traced by these surface 'weatiier thermal inertia (Figure 29.6A). The distri- vanes' (Thomas and Veverka 1979; Lee et al. bution of particle sizes inferred from thermal 1982; Greeley et al. 1992b; 1993). inertia values divides Mars into longitudinal sectors (Figure 29.6B). Global circulation patterns influenced by large-scale topography MARTIAN AEOLIAN MANTLE may determine the regional concentrations of fine- and coarse-grained surface materials. It The transport of aeolian sediment across large appears significant that the areas of lower ther- regions on Mars leads to areas of net sediment mal inertia are closely correlated with the areas accumulation and removal (Thomas 1982; of higher albedo (Kieffer et al. 1977). Christensen 1986b; Kahn et al. 1992). Regions The absolute thickness of surface materials of relatively high albedo (0.25-0.4) are inter- distributed across the Martian surface can be preted to contain more dust than regions of broadly constrained by remote sensing measure- lower albedo (0.1-0.2) (McCord and Westphal ments. In high thermal inertia regions, the fine- 1971; Pleskot and Miner 1981). The subdued particle thickness may be as much as tens of surface morphology within regions of higher centimetres. Earth-based radar measurements of reflectance is consistent with the presence of low thermal inertia areas show a large amount a uniformly distributed mantling material of scattering by surface roughness (Harmon et al. (Zimbelman and Kieffer 1979). Our knowledge 1982). This result suggests an upper limit to the 668 Arid zone geomorphology

Figure 29.6 Martian surface thermal inertia in units of 10~3 calcm'3 s~'12 K~' derived from temperatures measured by the Viking IRTM. (A) Data separated by thermal inertia units (0 = lowest; 18 = highest). (B) Data separated according to average particle size for an idealised uniform particle surface (Kieffer et al. 1973) depth of fine materials of 1-2 m (Christensen Center have clarified differences in particle 1986a; 1986b). Moreover, in low thermal inertia saltation that take place on Earth, Mars and areas, fine materials are not sufficiently thick to Venus (Figure 29.7). On Mars, the most easily mask the thermal effects of more competent entrained sediment grains have a diameter of materials, such as large blocks or boulders, which 0.115 mm (Iverson and White 1982). Thermal are observed in varying abundances over most of inertia measurements of aeolian sediments held the Martian surface (Christensen 1986c). These within crater dunefields indicate the presence of observations indicate that potentially mobile fine considerably coarser grains with diameters of sediments are dispersed around the entire planet. 0.4-0.6 mm (Edgett and Christensen 1991). The lower atmospheric pressure results in threshold friction velocities roughly one order of TRANSPORT BY SALTATION AND CREEP magnitude higher than terrestrial velocities for lifting particles of similar size. On Mars, a Experiments performed using the Mars Surface threshold friction speed of 2.4 ms"1 is required Wind Tunnel (MARSWIT) and Venus Wind to entrain a 0.1 mm particle with the density of Tunnel (VWT) at the NASA Ames Research basalt (2.5 gem'3). With the arrival of the first Extraterrestrial arid surface processes 669

given the boundary conditions for saltation to be initiated and the anticipated nature of salta- tion clouds. The very long path lengths of saltating particles cause fewer impacts per unit area under wind conditions just above the saltation threshold. On Mars, surface creep will be inhibited by the relatively small number of neighbouring impacts setting stationary parti- cles rolling across the surface.

Depositional bedforms on Mars

The appearance of Martian dunes, dunefields and ergs suggests a strong connection to bed- form sand transport processes in terrestrial deserts. Yet there exist intriguing differences between the two planets in the global distri- bution of sand dunes and relative abundance of particular dune types.

0075 BARCHANOID AND TRANSVERSE DUNES

0.01 _L J ' 'I'll 001 0.02 0.05 0.1 0.2 0.5 1.0 Nearly all dunes on Mars are either barchanoid or transverse bedforms (Figure 29.8). Individ- PARTICLE DIAMETER (mm) ual barchans appear in planform to be almost Figure 29.7 Threshold friction velocities for entraining identical to their terrestrial counterparts and aeolian sediments on Mars, Earth and Venus as occur as simple bedforms and as compound determined by wind-tunnel experiments (modified after megadunes, where smaller, secondary dunes Iversen and White 1982) rest atop the major bedform (Breed et al. 1979; Tsoar et al. 1979). In dunefields, barchans have collided to form barchanoid ridge dunes, and global dust storm of 1977, wind velocities areas with an abundant sediment supply contain measured 1.6 m above the surface at the Viking transverse ridge dunes. With the exception of 1 landing site reached 17.7 ms ' with instan- isolated dunefields within crater basins taneous gusts of more than 25 ms"1 (Ryan and (Lancaster and Greeley 1987; Edgett and Henry 1979). These velocities are close to the Christensen 1991), most regions of Martian required threshold for saltation, when extrapo- dunes are situated in the high latitudes between lated to the surface. In a typical terrestrial sand 70°N and 85°N (Ward et al. 1985). Whether desert, significant saltation may occur during this asymmetry reflects characteristics of Mar- about 10% of the days in a year. Saltation events tian global circulation, latitudinal differences in occur much less frequently on Mars, and pro- available sources of sediment or a combination bably happen during only 0.01-0.1% of the days of these factors remains an open question. in a Martian year. Despite its relative rarity, A more fundamental question is the com- saltation on Mars should be much more vigorous position of Martian dunes. The petrological than on Earth because initial particle velocities evolution of Mars has limited the amount of are much higher, and atmospheric drag is mini- siliceous rock for breakdown into quartz grains, mal, leading to high terminal velocities. and highly energetic particle collisions over the Surface creep, or the rolling of grains in course of geological time would have long ago traction across the surface, may play a substan- smoothed the surface and reduced the saltating tially smaller part in aeolian transport on Mars, population of intact mineral grains to dust-sized 670 Arid zone geomorphology

#lf

rf •:v

Mlfct 10 km Pf° I .#'* *££• Figure 29.8 Martian barchan and transverse dunes. (A) Barchans ofthe North Polar erg (1TN, 97°W). (B) Transverse bedforms in the ancient cratered terrain of the southern hemisphere (41°S, 340°W). [(A) VO #S19B27; (B) #S7SB60] fragments. Perhaps the saltating sediments of diminishes, the dunes may become etched and Mars need not be long-lived to construct bed- grooved by wind erosion. forms. On Earth, aggregates of clay occasionally are transported to create small dunes with slip faces, even though the entire dune structure SCARCITY OF LINEAR DUNES becomes cemented after minimal rainfall (Bowler 1973). Dunes composed of silt-clay Unlike terrestrial sand deserts, Mars lacks large aggregates are suggested to occur on Mars concentrations of linear dunes. Painstaking (Greeley 1979). As a possible terrestrial ana- investigation has uncovered only a few examples logue, the gypsum dunes of White Sands, New of linear dunes and other bedforms resulting Mexico, are barchanoid and transverse bed- from bi- and multidirectional sediment-moving forms (McKee 1966) constructed by strongly winds. The least ambiguous linear bedforms lie cohering grains. Breed et al. (1979) noted close within a 70-km-diameter crater in the southern morphological similarities between Martian hemisphere (S9°S, 343°W), where 4 5 km barchanoid ridges in the northern polar region dune ridges extend from a field of barchanoid and those at White Sands in terms of scale dunes (Edgett and Blumberg 1994). In this width, length and spacing. They also detected case, local topographic obstructions appear to groove-like features on Martian dunes compar- influence the direction of winds crossing the able to the wind-eroded surfaces of partially crater floor. Limited numbers of Martian star cemented gypsum dunes at White Sands. dunes in the Mare Tyrrhenum region are Many dunes in the northern polar region are similarly situated in an area subject to local covered by frost in the winter. Frost-wedging, topographic disruptions of regional airflow chemical weathering and deflation of their sur- (Edgett and Blumberg 1994). faces and interdunes may dislodge aggregates Several reasons may explain the scarcity of from duricrust layers, liberating particles for linear bedforms on Mars. In their discussion of saltation. So long as Martian saltation events are the nortnern polar region, two investigations in equilibrium with aggregate production, the (Tsoar et al. 1979; Lee and Thomas 1995) dunes will remain intact. If aggregate production point to barchans with elongated wings as Extraterrestrial arid surface processes 671 examples of seif dunes. Tsoar et al. (1979) maximum extent between 77-83°N and suggest that the dunes are in an early stage of 110-220°W (Tsoar et al. 1979). With an area bedform evolution. This idea fails to accord of approximately 680 000 km2, the North Polar with the high degree of bedform organisation erg is larger than any individual terrestrial sand displayed by regional megadune patterns, which sea containing mobile dunes and holds an requires a minimum of several thousand years estimated 1158 km3 of dune sediment (Lancas- to develop on Earth and probably much longer ter and Greeley 1990). on Mars. In an alternative hypothesis, Breed et Two principle circulation patterns are respon- al. (1979) regard the dunes of the northern sible for shaping the dunes of the North Polar polar region and other Martian dunefields to be erg. Low-pressure fronts tracking along the geomorphologically mature formations, where northernmost latitude of zonal circulation most dunes now lie in topographically restricted produce dune-forming westerly and south- areas, such as crater basins. Saltable sediments westerly winds below 80°N. Easterly and no longer pass across great distances between northeasterly sand-moving winds have created sedimentary basins. An analogy can be made to the dunes above 80°N between 120°W and the mass-transport flux of aeolian sediments 240°W in the region nearest to the perennial in the Saharan sand basins (Mainguet and polar ice cap (Tsoar et al. 1979). The airflow Chemin 1985). In the Sahara, linear dunes are regime is the consequence of katabatic winds in the primary mode of bedform transport from the cyclonic circumpolar vortex which follow an the upwind source regions of a basin and across outflow pattern traced by frost streaks into interbasin sills into topographically confined dunefields surrounding the perennial ice cap sediment sinks. (Howard 1980). The concentration of dark A likely explanation for the scarcity of linear polar dunes into a narrow latitudinal band may dunes is that Martian saltation events take be aided by strong surface winds induced along place under a limited range of meteorological the sharp regional temperature gradient and conditions. The climatological setting for these albedo boundary in a manner similar to the low-frequency events may include only the creation of a terrestrial sea breeze (Thomas and most energetic phases of Hadley circulation Gierasch 1995). The location of dunefields over the equatorial regions, zonal circulation downwind of the polar cap suggests that polar crossing the mid-latitudes, katabatic outflow layered deposits are the most likely source of from the polar caps and downslope winds from dune sediment (Thomas 1982; Thomas and high terrain. Each of these systems creates Weitz 1989; Lancaster and Greeley 1990). relatively unidirectional airstream trajectories In contrast to Tsoar et al. (1979), Breed et al. that would lead to the production of bar- (1979) argued that the North Polar erg is chanoid and transverse bedforms. An ancient, citing the absence of sand-passing atmospheric General Circulation Model of longitudinal dunes, the high degree of organisa- Mars, which incorporates several boundary tion in dune patterns and signs of wind erosion conditions for surface shear stress reflecting upon some dune surfaces. Another feature that different saltation threshholds, generated signifies an older age for the erg is the occur- theoretical values for the resultant drift poten- rence of several very large barchanoid ridges in tial divided by the total drift potential (RDP/ the upwind areas of dunefields. These 'framing DP ratios) for global regions (Lee and Thomas dunes' are now displaced from neighboring 1995). The calculated RDP/DP ratios com- dune ridges located downwind by a distance monly exceed 0.9 and point to unidirectional several times the mean wavelength separating sediment movement under a unimodal wind other ridges in the dunefields. Such a circum- regime in most regions of Mars. stance can arise only after many reconstitution cycles (the period required for a bedform to move its own length). In an earlier stage of ERGS bedform development, more rapidly migrating upwind dunes collided with a barchanoid ridge The North Polar sand sea (Figure 29.9) forms causing its volume and height to increase a dark circumpolar collar that reaches its and rate of advance to slow. The adjacent 672 Arid zone geomorphology

Figure 29.9 The dark dune collar of the North Polar erg surrounds the perennial H20 ice cap of the North Pole. [Mariner 9 #4297-46] downwind ridges of the dunefield then began to friable deposits in several regions confirms that move away from the larger barchanoid ridge. aeolian erosion proceeds more rapidly where The existence of windward framing dunes surface units are composed of fine-grained, separated at some distance from other ridges loosely consolidated materials (Figure 29.10). together with the likelihood that reconstitution cycles for Martian megadunes are considerably longer than the c. 1000-year periods for equiva- AERODYNAMICALLY SCULPTURED lent bedforms on Earth belie the creation of LANDFORMS the North Polar erg on the c. 104-106-year timescale of the present climatic period. Yardangs are most plentiful on the equatorial plains overlain by layered deposits. Mature terrestrial yardangs have a width-to-length ratio Aeolian erosion on Mars of about 1:4 (Ward and Greeley 1984). By contrast, in many cases Martian yardangs In the vicinity of the Viking landing sites, pitted appear to be extremely elongated, a factor that rocks with wind-faceted forms have been cre- Ward (1979) ascribes to more intensive abra- ated by aeolian abrasion (Figure 29.2). Though sion within the corridors separating Martian abrasion has modified many smaller rocks at the yardangs than that which occurs on Earth. The landing sites, the presence of boulder fields of paucity of detailed field research and laboratory rather pristine appearance and probable age of experiments dealing with the evolution of ter- more than a billion years demonstrates that, in restrial yardangs limits an attempt to understand regions where lag materials protect the surface, Martian yardang elongation, but the spacing of wind erosion occurs very slowly. On a broader joints and fracture patterns within the layered scale, the stripping of aeolian mantles and other deposits and granulometric characteristics of Extraterrestrial arid surface processes 673

*, V 0

:

#%' '/;,, Figure 29.10 The aeolian stripping of a mantle of layered deposits in the Amazonis Planitia region (llaS, 177°W) exhumes an ancient cratered surface. fVO #438S01, #438S03]

the sediments may also contribute to the character with ignimbrite formation, but is in development of elongated yardangs. keeping with the anticipated weathering pro- Though some yardangs occur in the mid- cesses of a loess-like aeolian mantle. latitude and polar regions of Mars, the majority are located on the equatorial plains. These low- latitude yardang concentrations resemble terres- DEFLATION BASINS trial wind-sculptured landforms developed upon ignimbrite sheets in the central Andes (Figure The most Earth-like type of wind-eroded basin 29.11). The proximity of the Amazonis Planitia on Mars takes the form of a cusp-shaped layered deposits to the Olympus Mons and depression in which the apex of the cusp points Tharsis Montes volcanic massifs has led some upwind along the axis of deflation (Figure investigators to conclude that the layered 29.16). The blunted, downwind portion of deposits were emplaced by pyroclastic flows cuspate basins can be more than 100 m deep, (Scott and Tanaka 1982; Ward et al. 1985). while the apex grades to the level of the sur- Other features suggest they are layers of an rounding surface. On Earth, shallower basins aeolian mande deposited in a sedimentary basin but with similar shapes in planform have been to the west of Tharsis Montes. For instance, created upon sandy surfaces along the margins several of the layers are capped by a resistant unit of ancient lake-beds in the western United approximately 5-10 m thick. Terrestrial ignim- States, where loosely consolidated sediments brites form welded horizons only near the base of favour lateral deflation in the direction of the flow surges and have easily eroded surfaces. An formative wind (Goudie and Wells 1995). indurated horizon within the uppermost portion The most unusual Martian deflation basins of a layer in the Martian deposits appears out of are distinctiy crescentic pits developed upon the 674 Arid zone geomorphology

Figure 29.11 Martianyardangs (A) ofthe Amazonis Planitia region (l°N, 171°W) closely resemble terrestrial yardangs (B) formed upon the Cerro Galan ignimbriie of northwestern Argentina (2S.S°S, 66.8°W). [(A) VO #7728A64; (B) Landsat TM 50787-13503] equatorial layered deposits. These small basins with the greatest implication for possible slope have classically aerodynamic shapes similar to processes is the abundance of H20 ice in the an inverted mould cast from a barchan dune. regolith near the surface. Studies of Martian All occur at the windward base of incipient impact crater morphology have generally taken yardangs, where the most intense abrasion the presence of craters surrounded by unusually occurs upon yardangs formed on Earth (Ward large ejecta-flow deposits in the latitudinal bands and Greeley 1984). No basins of similar mor- along 40°N and 45°S to be evidence of H20 phology have been reported from investigations near the surface (Mouginis-Mark 1979; 1987; of terrestrial yardang localities. Squyres 1979; Barlow 1994). Areas of fretted terrain in the northern hemisphere may present the best case for volatile-controlled surface Slope processes on Mars modification. These regions are composed of isolated mesas with steep valley wall profiles Gravity serves to modify slopes on all planetary separated by level surfaces and are found bodies. On most planetary surfaces, this involves between 30°N and 50°N and in isolated locations the slumping of debris, including the rolling of at equivalent latitudes of the southern hemis- individual rocks detached from upper slopes by phere (Carr 1986). Under current Martian local meteorite impacts or seismic tremors. conditions, H20 ground ice is unstable between Martian mass movements extend from the 40°N and 40°S and will sublime to the atmos- gradual slope retreat and talus creation of dry phere from near-surface regolith (Farmer and mass-wasting to the sudden and catastrophic Doms 1979). The occurrence of fretted terrain emplacement of giant debris avalanches. only in the areas above equatorial latitudes has prompted several investigators to suggest that ground ice processes may mobilise slope mater- EXPOSURE OF SUBSURFACE VOLATILES ials in the mid-latitude regions (Carr 1981; 1986; Rossbacher and Judson 1981; Lucchitta 1984). Of all the unresolved issues of consequence to The most obvious means for destabilising the formation of landforms on Mars, the one slopes in the fretted terrain is the exposure of Extraterrestrial arid surface processes 675 subsurface volatiles contained behind valley Planitia, a widespread aeolian mantle blankets walls either suddenly by tectonic activity or the much of the region (Tanaka and Leonard shock of a meteorite impact or more gradually 1994). through the undercutting of scarps by deflation. Once exposed to the surface, the volatiles will sublime and destabilise the resulting slope, MASS WASTING which may then collapse to form a debris flow or slowly disintegrate to build a debris apron. Within the equatorial canyons, numerous talus chutes form a spur-and-gully morphology entrenched into canyon walls (Lucchirta TERRAIN SOFTENING 1978a). The shapes and spacings of the talus chutes change from irregular crenations dissect- Squyres and Carr (1986) have identified a ing the resistant caprock of canyon walls to global-scale degradation of surface relief at parallel chutes uniformly incised into mesas of latitudes poleward of 30°. The terrain softening layered units located within the canyons (Figure is interpreted to represent a pervasive expres- 29.12). The distinctive change in erosional sion of surface debris movement in response to styles within the same region may reflect downslope creep caused by interstitial ice. The differing mechanical properties of materials three landforms most often associated with composing wall rocks and layered units. Mass- terrain softening are convex lobate debris wasting processes causing lateral slope retreat aprons (Squyres 1978), lineated valley fill along escarpments may explain the fretted consisting of multiple ridges parallel to valley terrain landscapes in the northern hemisphere walls (Squyres 1979) and concentric crater-fill mid-latitudes and similar areas of the southern indicated by circular ridges and grooves within hemisphere highlands (Squyres 1979). craters (Squyres 1979; Squyres and Carr 1986). In each case, the volatile content is inferred from the ridge-and-groove topography taken to DEBRIS AVALANCHES indicate the flow of surface material in a given direction (Squyres 1989). At the larger scale, gravity driven processes on The distinctive morphology of each type of Mars have resulted in the creation of colossal terrain feature associated with icy regolith has landslide debris avalanches triggered by been illustrated primarily with Viking orbiter catastrophic slope failure. The most con- images of moderate resolution. The available spicuous avalanche deposits occur in the deep, images of greater detail supply clues for an broad-floored Valles Marineris canyon system alternative to the volatile-related origin of some between S0°W and 90°W (Figure 29.13A). terrain-softening features (Zimbelman 1987). In this region several dozen debris avalanche The northern mid-latitude regions of Mars deposits ranging in size between 40 and show evidence of manding by materials that 7000 km2 have been discovered (Lucchirta uniformly buried surface landforms and were 1978b; 1979; 1987; McEwen 1989). subsequently partially or completely exhumed The landslides in Valles Marineris were (Williams and Zimbelman 1988; Moore 1990). triggered along canyon walls with heights of High spatial resolution images of deposits both 2-7 km and slopes of about 30°. The upper- within and around various craters exhibit layer- most sections of the canyon walls are interpreted ing (Zimbelman et al. 1988). These character- to be resistant sills of lava, while the underlying istics support an aeolian alternative for the materials may be composed of regolith (Luc- deposition and removal of the ubiquitous chitta 1979). The proximal areas of many mantling materials. The distinctive morphology debris avalanches display giant backward- of certain terrain-softening features may be rotated slide blocks of the resistant units explained as surface expressions of multi-lay- underlying the plateau surface. Hummocky ered aeolian deposits subjected to differential topography often appears down flow from the weathering and erosion. At equivalent latitudes slide blocks and probably represents coherent in the southern hemisphere basin of Hellas megablocks transported far from the collapse 676 Arid zone geomorphology

Figure 29.12 Mass wasting of two different lithologies in the Candor Chasma region (6°S, 7TW) produces irregular crenations (A) along canyon walls and parallel chutes (B) incised into layered deposits with the canyon. [Viking Orbiter mosaic] amphitheatre. The distal reaches of most Mar- lens rising high in the canyon wall (Lucchitta tian landslides are characterised by longitudinal 1979; 1987). The predominance of longitudinal ridges and grooves radiating outward tens of ridge and groove terrain, the extremely low kilometres from the avalanche source (Figure apparent coefficient of friction implied by long 29.13B). In several instances flow levees have avalanche travel distances and the estimated formed along the lateral margins of an aval- high velocities of emplacement led Lucchitta to anche, but are usually absent along the distal the conclusion that Martian avalanches must margin. have been lubricated by water. Recent work on One possible mechanism for the emplace- terrestrial volcanic debris avalanches challenges ment of debris avalanches in Valles Marineris this interpretation. has been suggested to be high-velocity mud- In a study of the major volcanic debris aval- flows resulting from the sudden outbreak of anches in the central Andes, Francis and Wells liquid water from an aquifer held behind an ice (1988) discovered a suite of components Extraterrestrial arid surface processes 677

Figure 29.13 The deposits of giant landslides are found on the floor of Valles Marineris. (A) An oblique view across Ganges Chasma (9°S, 45°W) shows landslides with coherent slide blocks alongside collapse amphitheatres and debris streams radiating far from the avalanche source areas. (B) A detailed view illustrates the longitudinal ridges and grooves in the distal portion of a debris avalanche deposit in Coprates Chasma (llaS, 67°W). [(A) Viking Orbiter mosaic; (B) VO #080A01]

(rotated slide blocks, hummocky topography, Francis 1993), yet lacks any evidence for water- longitudinal ridges and grooves, and marginal mobilised emplacement in the form of debris- levees) reflecting elements of Martian landslides. flow-mudflow transitions, runoff channels or ice While Lucchitta (1979; 1987) presumes that the escape structures. The Andean landslide deposits occurrence of longitudinal ridge and groove with ridge and groove terrain appear to have been terrain is rare for terrestrial avalanches, the formed as dry debris avalanches deposited by deposits emplaced from the Lastarria, Llullail- enormous sturzstroms (Hsu 1975; Melosh 1979; laco, Aucanquilcha, San Pedro and Irruputunca Francis and Wells 1988; Campbell 1989). volcanoes on the Andean Altiplano are domi- Calculations of the yield strength of materials nated by such radiating linear features (Naranjo involved in Martian landslides are similar to and Francis 1987; Francis and Wells 1988; values obtained for dry debris avalanches on de Silva and Francis 1991). The well-studied Earth (McEwan 1989). From these lines of Socompa volcano debris avalanche deposit in evidence, it may be concluded that Martian northeastern Chile possesses each of the Martian landslides flowed across the surface in a similar landslide components (Francis and Self 1987; manner without the aid of a lubricating fluid. 678 Arid zone geomorphology

Climatic change on Mars variations of about 8% (Kutzbach 1987). Martian changes of high-latitude insolation The most ancient terrains on Mars show ample approach 50% during a million-year cycle evidence of water flowing across the surface (Murray et al. 1973), and obliquity forced during the first several hundred million years insolation changes may have been even greater of the planet's geological history (Fieri 1980; over the past 10 million years (Bills 1990). Baker 1982). More controversial interpretations of the Martian landscape have offered clues for the existence of ancient oceans (Baker et al. EVIDENCE FOR CHANGING 1991), lacustrine deposits (Parker et al. 1989; CIRCULATION PATTERNS Scott et al. 1992; Williams and Zimbelman 1994) and glacial features (Lucchitta 1982; The polar regions contain layered deposits Kargel and Strom 1992), though such conten- almost unblemished by impact craters, which tions remain highly speculative. During the suggests a surface age of only a few million present climatic epoch and probably for the past years. Each layer extends uniformly over a vast 3 billion or more years, liquid water has not area and is approximately 30 m thick. High- existed on the Martian surface in equilibrium and low-albedo units alternate to form a with the atmosphere. Despite the absence of sequence of layered deposits exposed along liquid water and an Earth-like hydrological spiral troughs radiating outward from the poles cycle, some regions of Martian landforms (Figure 29.15). At the highest latitudes of the indicate a response to climatic fluctuations northern hemisphere, a perennial H20 ice cap occurring on a timescale between 10 000 and rests upon the polar layered deposits (Kieffer 1 million years. et al. 1976), whereas a perennial C02 frost cap covers the South Pole (Kieffer 1979). The albedo of the north polar ice cap is only MARTIAN MILANKOVITCH CYCLES about 0.4, an indication of substantial aeolian sediment mixed with the ice (Kieffer et al. On Earth, slight changes in the angle of the spin 1977). axis to the orbital plane (obliquity), the axial tilt A model proposed to account for deposition direction (precession) which controls the sea- of polar layered deposits is regulated by the sonal timing of the closest approach to the Sun, climatic change created by Milankovitch cycles and the shape of the orbit around the Sun (Toon et al. 1980). At times of low obliquity (eccentricity) produce cyclical differences in the (12-20°), atmospheric C02 condenses in the intensity of incoming radiation (Table 29.2). polar regions and leads to a decrease in atmos- Mars also displays cyclical variations in its pheric pressure to about 1 mb, thus halting the orbital geometry (Figure 29.14). The changes movement of windblown sediments. During are much greater for Mars because its obliquity periods of low obliquity, largely sediment-free cycle encompasses a much greater range of H20 ice and C02 ice are deposited at the poles. values. The most important characteristic of the When Martian obliquity reaches high values Martian Milankovitch cycles is their combined (30-38°), C02 ice sublimes from the poles and effect upon the amplitude of insolation change. regolith to raise atmospheric pressure to per- Terrestrial ice ages and deglaciations appear haps 30 mb (Toon et al. 1980; Francois et al. to be determined by high-latitude insolation 1990). The increased temperature gradient near

Table 29.2 Milankovitch cycles for Mars and Earth (years)

Mars Earth

Obliquity 1.2 x 10s and 1.3 x 106 9.5 x 10" Eccentricity 9.5 x 10" and 2.0 x 106 4.1 x 10" Precession 7.2 x 10" and 1.7 x 10s 2.3 x 10" and 1.9 x 10" Extraterrestrial arid surface processes 679

Eccentricity 0.16 T 0.12 iu 0.08 m ^##% 0.04 hid^lWj;V ,\ ..,•'.. -1.0 -2.0 -3.0 -4.0 -SO

Obliquity 40 -,

30 o « 20 4#N#M

10 —r— —t— —r— I I -1.0 -2.0 -3.0 -4.0 -5.0

Change of Insolation (NH Summer Solstice)

, Figure 29.14 Cyclical changes of .48- . i 1 i j - . j l 11 i • • . I , : t , I | i. J i Martian orbital eccentricity and obliquity « "\- 6i^M;#Hm'W###'4%#^ cause periodic fluctuations in the amount z °~\, ",' •>• , u . '• i ; '. ,-',,"• '•' J "• ,' '" ' ,' '..-'' •' ' of incident radiation. The shaded portion -48- of the northern hemisphere insolation —I— T T I curve represents the range of variation in -1.0 -2.0 -3.0 -4.0 the Earth's insolation during the past 5 MILLIONS OF YEARS BEFORE PRESENT million years

Figure 29.15 Alternating bright and dark units compose the terrain of layered deposits in the north polar region (82"N, 269°W). [VO #006SB67-69] 680 Arid zone geomorphology the polar caps and higher atmospheric pressure man and Wells 1987). During periods of low promote frequent global dust storms and salta- obliquity, no aeolian sedimentation occurs, tion events during summer in both hemis- and an indurated horizon may form in the pheres. During periods of high obliquity, upper portion of the surface layer. At a time of condensates of dusty ice accumulate on the high obliquity (or intermediate obliquity, high winter pole. The colour and albedo character- eccentricity and perihelion during northern istics of polar layered deposits are consistent hemisphere summer), cross-equatorial airflow with a mixture of dusty ice and saltated returning to the northern hemisphere etched sediment (Herkenhoff and Murray 1990b). the set of yardangs aligned north-south Following sublimation, a dark layer of aeolian (Figure 29.17A). During a phase of sediment will insolate the underlying ice to intermediate obliquity and low to intermediate produce alternating bright and dark bands eccentricity, convergent circulation along the deposited under high- and low-obliquity equator produces easterly winds that transport conditions. A 120 000-year cycle controls layer sediments from the Tharsis Montes region and deposition, according to this hypothesis (Toon beyond for deposition in Amazonis Planitia etal. 1980). (Figure 29.17B). Possible southern sources for Layered deposits without volatiles are found the aeolian mantle burying the north-south at equatorial latitudes. In the region of Ama- yardang set are excluded by the apparent lack zonis Planitia (7°N, 141°W), the deposits are of sediments within that region during the 3.5 km in thickness with individual layers of preceding interval when yardangs were etched about 100 m. Two sets of yardangs with direc- by southerly winds. Following the decline of tions nearly perpendicular to each other have available sediments from eastern source formed upon the layered deposits (Figure regions, the uppermost layer was excavated by 29.16). The older, northward-trending yard- easterly winds to expose the older yardang set angs are partially buried by the uppermost and create a new set of east-west yardangs layer. Deflation by easterly winds created the (Figure 29.17C). Though the Amazonis younger yardangs and exhumed the older set. Planitia layered deposits comprise one of the A model to explain the sequence of layer youngest surfaces on Mars, it is unlikely that deposition and yardang formation by winds of the episodic layer deposition and yardang different directions invokes climatic change formation represent the consequences of a governed by the Milankovitch cycles (Zimbel- single obliquity cycle.

V- 2 / ' :•: J

% &• Figure 29.16 Two sets of yardangs

V 1 w were formed by winds of different fT 1* - \ * V V A- directions in the Amazonis Planitia region (7°N, 141°W). The older yardangs (Y,) were buried by a deposit 100 m in thickness and later exposed by easterly winds that formed the younger yardangs (YJ and a cuspate deflation \i >Y2 basin (CB). fVO #471S18] Extraterrestrial arid surface processes 681

- -^^r~^ Figure 29.17 A model to account for episodic layer deposition and yardang formation in the Amazonis Planitia region to the south of Olympus Mons (OM). (A) The older yardang set is eroded by circulation linked to a period of high obliquity (or intermediate obliquity, high eccentricity and northern hemisphere perihelion). (B) During a period of intermediate B obliquity, convergent circulation transport sediments from the east of Tharsis Monies (TM) with additional material carried by katabatic winds (K) from the volcanic massif. (C) The younger yardangs form after the eastern source of sediments diminishes

Arid surface processes on Venus The abundance of carbon dioxide in the atmosphere, coupled with trace amounts of With a surface temperature hotter than the sulphur dioxide, sulphuric acid and water melting point of lead, Venus offers a near- vapour, produces an extreme greenhouse effect, ultimate case for the action of arid surface warming the surface by an additional 227°C processes. While seemingly exotic in many (Cattermole 1994). High surface temperatures aspects, the forces shaping the Venusian land- are held constant because the obliquity of the scape can produce results which are surprisingly Venusian rotation axis creates no seasonal terrestrial in their final forms, making Venus changes of insolation between hemispheres more a twin of our own planet than many (Sagan 1976), and heat retention by the dense would expect based upon general physical atmosphere allows only a 1-3°C difference principles (Table 29.1). Information gained between day and night-time temperatures from the Magellan radar mapping mission of (Zolotov and Volkov 1992). Differences in the early 1990s established that, like Earth, surface temperature and pressure stem solely most of the surface of Venus is geologically from changes in altitude and vary from 465°C young with an age of less than 500 million years and 96 bars at the lowest elevations to 374°C (Head 1994; Strom et al. 1994). Unlike Earth, and 41 bars at peak elevations in the highlands however, with its oceanic basins, mobile conti- (Greeley 1994). The high level of atmospheric nental plates and continually recycled crust, carbon dioxide is probably buffered by the Venus experienced a global volcanic flooding mineral reaction between calcite and quartz event which covered the entire planet with producing wallastonite and carbon dioxide, a mafic silicate basalts approximately 300 million reaction favoured by the range of surface tem- years ago (Kargel 1994). Physiographically, peratures and pressures on Venus (Urey 1952; 65% of the Venusian surface consists of rolling McGill et al. 1983). Chemically reactive gases plains, 27% lies in basins below the mean (CO, C02, COS, S02, H20, H2S, HC1, HF) surface datum (the 6051 km planetary radius), in the lower Venusian atmosphere raise the and 8% forms highlands with altitudes greater possibility of surface weathering by gas-solid than 2 km (Sharpton and Head 1986). Most of interactions (Florensky et al. 1976; Nozette Venus is covered by exposed bedrock with only and Lewis 1982; Zolotov and Volkov 1992). about one-quarter of the surface overlain by Although mixing ratios of corrosive gases in the unconsolidated sediments (Greeley 1994). lower atmosphere are poorly known, chemical 682 Arid zone geomorphology analyses performed using X-ray florescence at perhaps, minor additional contributions from three landing sites (Venera 13, 14; Vega 2) volcanic eruptions of basaltic cinders and ashes suggest an enrichment of sulphur in surface (Basilevsky et al. 1992). In contrast to Earth materials as a result of chemical weathering and Mars, the sandblasting of surface rocks processes (Basilevsky et al. 1992). Another sign does not play a significant role in particle for- of weathering is the near-infrared brightness of mation. Aeolian abrasion of coherent rocks is soil and surface rocks at the Venera 13 landing orders of magnitude less efficient on Venus site, which may be attributable to the formation than on Earth and Mars because low surface of ferric oxides in an oxidising environment wind velocities fail to transmit sufficient kinetic (Pieters et al. 1986). energy to airborne mineral grains (McGill et al. Altitude-dependent chemical weathering 1983). As documented in a series of panoramic processes may explain an unusual feature views relayed by the Venera 13 lander showing detected by the Magellan radar imaging system. the sequential removal of fine materials from At elevations approximately 2.8 km above the the lander support ring, aeolian transport of mean planetary radius, a sharp boundary occurs sediments by saltation and suspension occurs between lower regions having low radar reflec- on Venus (Basilevsky et al. 1992). Wind-tunnel tion coefficients and highland areas with high experiments under Venusian surface conditions reflection coefficients (Arvidson et al. 1991; confirm that saltation involving a 0.06 mm 1992; Tyler et al. 1991). Contacts between grain will take place at a threshold friction radar-bright and radar-dark surfaces can be speed of 0.17 ms"1 (Iversen and White 1982; traced for hundreds of kilometres with little White 1986), which may be extrapolated to a change in elevation, leading to the conclusion wind velocity of approximately 1ms"1 measured that a global chemical phase change controlled at a level 1 m above the surface (Figure 29.7). by atmospheric temperature and pressure occurs Saltation path lengths are short and trace low- at a specific altitude above the surface. Such a angle trajectories on Venus, and should create phase change would alter the dielectric constant ripple-scale bedforms having higher amplitudes of exposed rock surfaces by producing a different and shorter wavelengths than terrestrial sand kind of weathering crust containing magnetite, ripples (Greeley and Iversen 1985). An unusual haematite, ilmenite or other minerals that remain characteristic of basaltic materials placed in unstable at lower elevations (Arvidson et al. saltation under the high temperatures and 1991; Zolotov and Volkov 1992). At still higher pressures of the Venusian surface is the growth elevations, surfaces again become radar-dark in of thin weathering veneers created by debris areas where highland summit rocks may be comminuted from basaltic grains onto impacted coated with pyrite-bearing minerals stable only at rock surfaces (Greeley et al. 1987). the highest elevations on Venus (Arvidson et al. In a manner similar to comparable features 1992). An intriguing consequence of these on Mars, wind-streaks on Venus are closely chemical equilibrium changes is the possibility associated with impact craters, volcanic domes that metal halides liberated in vapour state in the and other topographic barriers that disrupt and hot (c. 460°C) lowlands of Venus are trans- accelerate airflow (Figure 29.18). Radar-bright ported by atmospheric circulation to higher, streaks typically extend 10-20 km in the lee of cooler (c. 380°C) elevations, where they precipi- obstructions and represent zones swept free of tate to create a uniform surface layer (Bracken et aeolian sediments (Greeley et al. 1992a); thus, al. 1994). Crystal growth in rock pore spaces by the radar-bright streaks of Venus correspond to condensing metal halides may mechanically the visibly dark crater streaks seen on Mars destroy highland rocks and may explain the (Figure 29.5). Other wind-streaks are radar- increased radar roughness characteristics noted dark and represent the concentration of a sur- for highland terrains. face layer of unconsolidated aeolian sediments Despite ample evidence for active chemical at least 10-70 cm deep (Greeley et al. 1992a). weathering processes on Venus, most fine The global pattern of wind-streak directions granular material on the surface appears to can be used to construct a map of prevailing originate from ejecta blankets deposited during surface winds related to the global Hadley impact crater formation (Garvin 1990) with, circulation (Schubert 1983; Greeley et al. Extraterrestrial arid surface processes 683

1994). On Venus, the general circulation is symmetrical about the equator and consists of rising limbs of Hadley cells along the equator, poleward winds aloft, down flow over the polar regions and equatorward surface winds. A global map of wind-streak directions beautifully reflects the strongly meridional character of Venusian surface winds (Greeley et al. 1994). Evidence for aeolian deposition on Venus includes landscape features created by both dust- and sand-sized particles. Dark horseshoe- shaped haloes associated with young impact craters are thought to be the consequence of pre-impact shock waves which swept dust from the surface to altitudes greater than 10 km, where the dust clouds interacted with easterly winds of the atmospheric super-rotation (Arvid- son et al. 1992; Greeley et al. 1992a). The parabolic haloes of radar-dark sediments extend hundreds of kilometres to the west of several of the largest and youngest craters in a pattern which suggests aeolian dispersal by easterly winds. Given the low rate of large impact crater formation during the past billion years, the presence of parabolic haloes adjacent to approximately 10% of all large craters leads to the conclusion that dust deposits may persist as distinct units for tens of millions of years on the Venusian surface (Arvidson et al. 1992). Imaging radar systems do not facilitate detec- tion of aeolian bedforms because dry, granular sediments are poor radar reflectors. Despite this inherent difficulty in the use of Magellan radar imagery, two small dunefields have been located on Venus. The Algaonice dunefield (25°N, 340°E) of transverse ridge bedforms covers 1290 km2 and lies near the radar-bright ejecta deposits of Algaonice Crater (Greeley et al. 1992a). The Fortuna-Meshkenet dunefield (67.7°N, 90.5°E) is also situated in a heavily cratered region and contains 17 120 km2 of transverse dunes. Bedform lengths of the trans- verse dunes range from 0.5 to 10 km with average widths of 200 m and crest-to-crest spacings of approximately 500 m (Greeley et al. 1992a). The apparent scarcity of dunes on Venus may be related to the limited supply of Figure 29.18 Radar-bright wind-streaks extend in saltable sediments available from crater ejecta fan-shaped patterns in the lee of small volcanic cones in blankets. Alternatively, there may remain many the Guinevere Planuia region of Venus (22.3"N, undiscovered dunes on Venus which are simply 332.1°E). [Magellan F-MIDR #20N334] too small to be detected by the Magellan imag- ing radar system. 684 Arid zone geomorphology

Another kind of aeolian deposit may be more missions. Several missions are planned for the ubiquitous than any other discussed in this remainder of the century. section. It is possible that the bedded slab-like The Mars Global Surveyor (MGS) spacecraft rocks seen in surface views of the Venera 10, 13 is scheduled to provide the initial recovery and 14 landing sites are indurated aeolian following the loss of the Mars Observer space- sediments (Florensky et al. 1977; McGill et al craft in 1993. Three of the MGS instruments 1983). The low bulk density (c. 2 gem"3) of will be of particular importance to investigations rock samples examined at the sites is consistent of arid surface processes on Mars. The Mars with the expected properties of an extraterres- Observer Camera (MOC) will provide global trial eolianite (McGill et al 1983). synoptic images of clouds and large regions of Although wind erosion is orders of magnitude the surface at low spatial resolution (7.5 km per less effective on Venus than on Earth and Mars, pixel) and many very high resolution (1.4 m per yardang-like features are present in the region of pixel) views of selected sites around the planet Aphrodite Terra (9°N, 60.5°E) and have been (Malin et al 1992). The Thermal Emission ascribed to the aeolian stripping of a friable Spectrometer (TES) will gather emission ejecta deposit derived from Meade Crater spectra of the surface and atmosphere in the 6 (Greeley et al. 1992a). The parallel ridges and to 50 jim range at a spatial resolution of 3 km grooves attain lengths of 25 km with widths of (Christensen et al. 1992). The Mars Observer 0.5 km and are aligned in the same direction as Laser Altimeter (MOLA) will obtain aldmetry wind-streaks in the region. measurements with 1.5 m vertical resolution Mass-wasting processes are apparent in over 160 m swaths, which will generate surface surface views of the Venusian landscape and elevations and gradients over a global grid of from radar imagery of highland regions. At 0.2° x 0.2° cells (Zuber et al 1992). The Mars the Venera 9 landing site, the panoramic Global Surveyor is scheduled to be placed into scene displayed detached rocks and slab-like a mapping orbit around Mars in 1998. Images boulders descending a 20° slope (Florensky et and measurements by the MOC, TES and al 1977). Steep slopes in mountainous regions MOLA will permit new numerical tests to be serve as sources for slump blocks and large applied to many of the hypotheses discussed in landslides observed on radar images (Arvidson this chapter regarding surfaces processes, et al 1991; Malin 1992). Debris avalanche landform development, the existence of volatiles deposits resembling those emplaced by giant and the climatic history of Mars. terrestrial and Martian landslides occur in the Another NASA spacecraft should arrive on region of Maxwell Monies (Vorder-Bruegge Mars before the MGS mission. Mars Pathfinder 1994). is scheduled to place an instrumented lander Careful inspection of the landforms dis- on the Martian surface in July of 1997. The covered to date on Venus has yet to reveal any lander's colour imaging system will have the evidence for climatic change. Since Venus does capability to generate stereoscopic views and to not experience significant cyclical variations in image wind socks at three discrete heights above its orbital geometry, a possible mechanism for the Martian surface, providing the first extra- global climatic change is not readily apparent. It terrestrial measurements of boundary layer would seem that Venus has suffered a monot- structure. A small solar-charged, battery- onously torrid environment for at least the last powered, 'rover' vehicle will carry an alpha several hundred million years. proton X-ray spectrometer to be placed in direct contact with rocks identified using the lander's colour imaging system (Golombek and Future planetary research Spear 1994). Mars Pathfinder is designed primarily as an engineering test-bed, but After reading this review of the current state of important scientific results should be obtained knowledge concerning arid surface processes on during its mission. 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