planetary landscape systems: a limitless frontier

Planetary landscape systems 1341

Copyright © 2008 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 33, 1341–1353 (2008)DOI: 10.1002/esp

Earth Surface Processes and LandformsEarth Surf. Process. Landforms 33, 1341–1353 (2008)Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/esp.1713

Planetary landscape systems: a limitless frontierVictor R. Baker*Department of Hydrology and Water Resources, and Department of Planetary Sciences, University of Arizona, Tucson, AZ 85721, USA

AbstractIf it is to be a complete science of landforms and landscapes, geomorphology is not appropri-ately limited geographically to the terrestrial portions of Earth’s surface. Various systems oflandforms and their generative processes are best understood in a full planetary context.Moreover, by extending its purview to include the nature of landscapes on Earth-likeplanets, geomorphological inquiry is not appropriately limited in its philosophical presump-tions to the reductionist views that have so successfully guided much of physics. Holisticthinking, exemplified by some aspects of evolutionary biology, and a systems framework mayprove to be particularly fruitful for understanding future extraterrestrial discoveries and thegeneral nature of landforms and landscapes. Copyright © 2008 John Wiley & Sons, Ltd.

Keywords: Geomorphology; systems; planets; landforms

Received 15 April 2007;Revised 8 October 2007;Accepted 3 November 2007

*Correspondence to: Victor R.Baker, Department of Hydrologyand Water Resources, andDepartment of PlanetarySciences, University of Arizona,Tucson, AZ 85721, USA.E-mail: [email protected]

Introduction

Landscapes, landforms and their origins comprise the obvious subject matter of geomorphology. Nevertheless, recentdiscoveries and the etymology of this science’s name lead to a question. Should geomorphology restrict itself to a wayof thinking (‘Logos’) about planetary form or surface (‘morphos’) for Earth (‘geo’ or ‘Gaia’) alone, or should itspurview extend to the surfaces of all Earth-like planets? Obviously, both the science of geomorphology and its namederive from a historical tradition of studying the land surface of Earth. Yet, discoveries from recent solar systemexplorations invite, if not compel, the extraterrestrial extension of the subject (Sharp, 1980: Baker, 1984, 1985a,1993). Of the ‘top ten’ most cited geomorphology papers of the last quarter of the 20th century, two were directlyconcerned with topics in extraterrestrial geomorphology (Dorn, 2002).

Despite spectacular discoveries in planetary geomorphology over the past 40 years, conventional geomorphologyhas been surprisingly slow to incorporate insights gleaned from this rapidly accelerating research area (Baker, 1993).Evidence for this can be found in the standard textbooks for the field (e.g., Ahnert, 1998; Bloom, 1998; Easterbrook,1999; Ritter et al., 2002), which make scant reference to any extraterrestrial landform or process, although Summerfield(1991) is a notable exception. Geographical geomorphology has been particularly resistant to planetary studies,despite many pleas on their behalf (e.g., Baker, 1981; Ford, 1984; Pike, 1974, 1987). The formal position in geogra-phy seems to be that of Rhodes and Thorn (1996), who restrict geomorphology proper to study of Earth’s terrestrialsurface landforms and processes. This view excludes not just the surfaces of other planets, but also the rather alien-looking landscapes of the submarine 70 per cent of Earth’s surface.

Stated another way, the question can be asked as to whether geomorphology should become a complete science oflandforms and landscapes. Perhaps some other examples will aid in addressing this question. Because of the etymol-ogy of its name, does one think of geometry as limited to ‘Earth measurement?’ Obviously, geometry evolved longago to a much broader and more complete science than that from which its name derives, and geomorphology mayalso so evolve. In another example, suppose certain biologists were to restrict their studies solely to life on Earth, oreven to life on just a portion of Earth. Suppose now that life is discovered on some other planet. What would we thinkof the attitude of those hypothetical biologists who might insist that biology is not a science of life in general, and thatit should be restricted to the study of life on one restricted portion of a planetary sample of one? Is it particularlyenlightened scientific inquiry to purposefully ignore what could be learned about life in general from the comparisonsmade possible by new discoveries? In contrast to biology, which has not yet been presented with extraterrestrialdiscoveries, new landforms and landscapes are increasingly being discovered beyond the terrestrial land surface. Howthese discoveries become incorporated into its scientific programme will determine whether geomorphology becomesa broad and complete science of landforms and landscapes.

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Historical Background

It is interesting to consider the historical irony that it was through the study of extraterrestrial landforms thatgeomorphology arguably first became a scientific enterprise. Galileo’s discovery of numerous circular depressions onthe surface of the Moon stimulated what were likely the first geomorphological experiments. These were performed in1665 by Robert Hooke, who also delivered Britain’s first known geomorphology lectures (Davies, 1969). In the late19th century Hooke’s seminal experimental study of lunar crater formation was extended by Grove Karl Gilbert,whose scientific methodology provided a major inspiration for the modern paradigm of process geomorphology(Baker and Pyne, 1978). Of course, one might dismiss Hooke’s and Gilbert’s experiments as ‘selenomorphology.’ Thiswould follow the logic of identifying a science with the specific location of its objects of study. However, suchrestriction also limits the generalizations made possible by the science’s methods. Geomophology’s methods largelyderive from the use of analogies (Gilbert, 1886; Schumm, 1991; Baker, 1996), which can be broadly construed toinclude all kinds of models (Baker, 1985b). Gilbert’s classic experiments functioned as analogies of formative pro-cesses in order to resolve the competing hypotheses of volcanism versus impact origins of lunar craters. Like Hookebefore him, Gilbert (1893) propelled balls of clay and metal into various target materials. He also considered terres-trial analogues to the various crater origins. Gilbert developed compelling evidence for an impact origin by his carefulstudy of similarities in morphology among Earth landforms, experimental data, and the lunar craters that he observedthrough the relatively advanced Naval Observatory telescope in Washington, DC.

During the 1960s and 1970s contemporary process studies became the dominant theme for academic geomorphology.In the early part of the 20th century, great syntheses had been developed to explain whole landscapes, many of whichwere first explored during the previous century (Baker and Twidale, 1991). Unfortunately, these theories proveddifficult to verify with the then-available quantitative tools appropriate to the study of very large temporal and spatialscales. Partly in reaction to the perceived failure of this earlier thinking, the geomorphology of the later 20th Centuryprogressively became more focused on physical-based mathematical treatments of small-scale processes. As observedby Thornes and Brunsden (1977, p. 116), ‘The current paradigm is one in which process studies prevail effectedprincipally and increasingly through mathematical and stochastic models.’ The ‘new geomorphology’ even developedinto a kind of stipulative revolution, insisting that the mathematical modelling and empirical measurement of pro-cesses constitute a more effective scientific approach than does the explanatory description of whole landscapes.

The process geomorphological agenda had the partly unintended consequence that the temporal and spatial scales ofinquiry were diminished. This was simply because small-scale processes operate on the spatial and temporal scalesthat are most amenable to quantitative measurement, employing such devices as current meters, soil augers andpainted pebbles. Such studies naturally led to an emphasis on process associations with individual landforms, therebyfocusing study to individual features of characteristic form that could be associated with well-constrained processes.Examples include studies of river meanders, barchan dunes, moraines, shingle beaches, ice-wedge polygons anddolines. Such features, properly understood, might then presume to naturally assemble respectively into the fluvial,aeolian, glacial, coastal, periglacial, and karstic landscapes that comprise Earth’s land surface. However, this laterinference assumes a kind of Cartesian metaphysical reductionism: that, having effectively analysed the fine scale ofthings, one can then simply sum up the fine-scaled components to generate what occurs at much larger scales of timeand space. Presumption is not science, and it is now obvious that one cannot simply extrapolate the process measure-ments from fine scales of time and space, extending them to problems of large-scale landscape evolution (Church andMark, 1980). Landscapes, not unlike living organisms, display complexities of organization at large scales that are notsimply constituent of their finer scaled parts.

Another analogy from biology comes to mind in regard to assumed reductionism of process geomorphology. Thegreat evolutionary biologist, Ernst Mayr was once asked if there could possibly be anything more to complex livingorganisms other than their molecular and physical components. Mayr quickly answered in the affirmative, stating thatevery known organism also has its own unique history. This is one biological version of holism, the view of wholesystems being greater than the sums of their parts. Nevertheless, the reductionism/holism question seems to bephilosophical rather than scientific, unless and until some molecular biologists mix the right ingredients in the rightway in a test tube, with the result that some living thing crawls out.

It should be noted that uniqueness of history does not mean uniqueness of form. Despite their unique histories, bothorganisms and landforms may be generally singular in form without being unique in form (Schumm, 1991). Variationsoccur within species or landform type that derive from randomness, and this quality, combined with problems ofsampling, results in difficulties for extrapolation and prediction (Schumm, 1991), but it is still useful scientifically tosearch for the commonalities of landforms (and organisms) that are appropriate to various types.

The age of discovery for new planetary landscapes began with spacecraft explorations of the solar system in the1960s and 1970s. Unlike the small-scale process studies that then dominated conventional geomorphology, the new



planetary exploration began at regional scales and subsequently focused to finer scales. It thus proceeded, bothhistorically and methodologically, in exactly the opposite manner to terrestrial studies. Its major tools were remotesensing imagery from spacecraft and theoretical modelling of large-scale processes. At the very large scales, its focuswas on the relating of landscapes to regional factors of tectonics and global environmental change.

Although a renewed interest in ‘megageomorphology’ was becoming apparent in the 1980s (Baker and Head, 1985;Baker, 1986), it has only more recently become prominent in conventional geomorphology, stimulated by advancesin geochronology, remote sensing and computer modelling (Summerfield, 2005). Thus, technological advances arecritical both to extraterrestrial geomorphology and to the new megageomorphological concerns. Using a terminologysuggested by Laudan (1982), the changes to the science that will be brought about by these trends are adventitious, asopposed to stipulative. Stipulative agenda, or ‘bandwagons’ (Jennings, 1973), have too long preoccupied the otherwiseproductive efforts of too many geomorphologists. Stipulative change in science (Laudan, 1982) specifies what isproper in aims, methods, and problem orientations. Its advocates commonly employ philosophical arguments tobolster particular views, while ignoring the fact that philosophy is filled with profound and convincing counterargumentsto nearly every possible position. Adventitious change, in contrast, arises when new technologies yield discoveries thatstimulate fundamental advances in understanding. Thus, it is not a matter of stipulating that geomorphologists shouldembrace the adventitious path; it is merely a matter of observing whether the spirit of their scientific inquiry is suchthat they will indeed do so.

Systems Concepts

Systems analysis has been employed in geomorphology since at least the 1950s and 1960s (Strahler, 1952; Chorley,1962). This organizing framework has proven to be useful for dealing with the complexities of landform associations,and especially for illustrating process interactions. Systems are usually defined as structured sets of objects or charac-teristics plus the relationships or processes that allow these to interact as a complex whole. Thus, the landscape can beidealized as a series of elements linked by flows of mass and energy. The relevant processes are then organized asinputs, outputs, transfers and transformations that involve cascades, equilibrium states, feedbacks and thresholds (e.g.,Schumm, 1977). Such organization might be limited to an exercise in classification, without appreciably advancingscience. More productively, however, it comprises an early step toward the formulation and testing of predictivemodels. Given the rapidly accelerating advances in computational technology, the use of predictive mathematicalmodels is rapidly advancing to become the new methodological paradigm for geomorphology (e.g., Wilcock andIverson, 2003). It is interesting that much of the larger-scale modelling work currently in vogue has been madepossible because of the detailed mechanistic understanding afforded by earlier process studies. Moreover, systemsanalysis also leads to the introduction of new mathematical formulations. Over the past few decades these haveincluded catastrophe theory, chaos theory and complexity theory. The latter currently subsumes all study of complexsystems, which are presumed to be emergent, as opposed to reductive, in that the whole systems cannot be deducedfrom the properties of their components.

One way to conceive of a planetary landscape system is in a functional set of relationships, in much the same waythat Jenny (1941) conceived of soil properties as functions of complex interacting factors, e.g. (from Baker, 1984):

λ = ƒ (g, h, v, p, a, c, s)

where λ is the planetary landscape defined as a function (ƒ) of multiple factors, including: gravity (g); internal heatflow (h); volatiles (v), including atmospheric gases, such as H2O, CO2, CH4; physical properties (p) of rock, ice, etc.;atmospheric and hydrospheric properties (a); celestial mechanics (c), such as tidal forcing, orbital parameters, impactingbodies; and surface temperature (s). Table I provides examples of how some of these factors vary for selected Earth-likeobjects in the solar system.

A currently popular application of systems concepts is that of whole planet systems, including Earth-system science.For Earth and other rocky, Earth-like planets (excluding gas planets such as Jupiter, Saturn, Uranus and Neptune),there are major subsystems, including those for the atmosphere, biosphere, hydrosphere (oceans), upper lithosphere(crust), mantle, core and extraplanetary space (impacts from meteors and comets). Table II illustrates some of thecurrent understanding as to how these subsystem components influence various Earth-like planetary objects. Thesesubsystems exert their influence on three broad classes of planetary surface processes (Table III): cosmogenic (impact)processes, endogenic (tectonic and volcanic) processes and exogenic (fluvial, lacustrine, coastal, hillslope/mass move-ment, glacial, periglacial, aeolian, weathering) processes. The following section will provide very brief overview ofthese process systems as they relate to planetary landscapes.

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Table I. Parameters that affect geomorphological processes on some planetary surfaces

Planet surface Surface gravity Surface Averageor moon (m s−1) pressure (Pa) temperature (°C) Volatiles present (% of each)

Mercury 3·95 <10−5 ~400 H, He, O (traces)Venus 8·88 107 (92 bar) 460 CO2 (97), H2SO4 (trace)Earth 9·78 105 (1 bar) 20 N2 (78), O2 (21), CO2 (0·03), H2O (1–5), CH4 (trace)Mars 3·73 700 (7 mb) −46 N2 (2·7), CO2 (96), H2O (0·03), formaldehyde (trace),

CH4 (trace)Io 1·796 Trace −140 SO2 (90)Titan 1·44 1·5 × 105 −180 N2 (90+), CH4 (few %), Ar (few %)Triton 0·782 1 −235 N2 (99·9), CH4 (0·01)

Table II. Importance of planetary subsystem components for surface processes on some selected solar system bodies

Component Mercury Venus Earth Mars Titan

Impact Dominant Low Low Low LowAtmosphere Low Moderate High Moderate (today) High (past) HighBiosphere None None (?) High None (today?) (Past?) ?Surface ocean None None (today) (Past?) High None (today) Moderate (past) NoneUpper lithosphere Moderate High (plume tectonics) High (plate tectonics) Moderate ModerateMantle Low Moderate Moderate Moderate LowCore Low Low Moderate Low (Today) None(Magnetic field) Active None Active Remnant None

Table III. Active (A), relict (R) and exotic (E) geomorphological processes on some selected planetary surfaces

System Process Mercury Venus Earth Mars Io Icy satellites Titan

Cosmogenic Impact cratering A, R A, R A, R A, R A, R A, R A, REndogenic Tectonic R R A, R R R A, R R

Volcanic R A, R A, R R A, R E EExogenic Fluvial – E A, R R – – E

Lacustrine – – A, R R – – ECoastal – – A, R R – – EHillslope/mass movement R R A, R A, R R R RGlacial – – A, R R – E EPeriglacial – – A, R A, R – – –Aeolian – A A, R A, R – – A, EWeathering – E A, R R – – E

Planetary Surface Process Systems

Cosmogenic landscape systemsImpact craters are the most common landforms on the rocky objects in the solar system. Only on Earth, Venus and afew outer solar system satellites have active resurfacing processes, both exogenic and endogenic, erased the impactscars of the primordial epoch of heavy bombardment from the earliest history of the solar system. For the inner solarsystem, the heavy bombardment may have included an especially intense phase at around 3·9 Ga, from which ispreserved extensive heavily cratered landscapes on Mercury, the Moon and Mars (Strom et al., 2005).

Due in part to extensive research programmes on cratering induced by nuclear explosions, the mechanics of impactcratering are very well known (Melosh, 1989). Thus, it is possible to elegantly predict the morphology of impact



craters within complex systems that, among other variables, consider differences in scale, projectile velocity, projectilestrength and target properties. Although only about 175 geologically confirmed cosmogenic impact structures havebeen recognized so far for Earth (Earth Impact Database, 2007), there are millions of impact features on the extra-terrestrial rocky surfaces of the solar system. Such an immense population of examples affords tremendous opportunityto study the full range of factors that apply to genesis of these landforms.

Endogenic landscape systemsVolcanic landforms. A rich variety of basaltic volcanic landforms occurs on the Moon, as well as on the terrestrialplanets: Mars, Mercury and Venus, and Earth. Silicic volcanism also occurs on Earth and Mars. The most extensivevolcanic landscapes are great plains that were emplaced by effusive lava flows on the terrestrial planets. Variousvolcanic constructs also occur, including the immense Martian shield volcano Olympus Mons, which measures 700 kmacross and rises to a height of 25 km. Although most extraterrestrial volcanoes are relict (Table III), there are spectacu-lar active volcanic plumes emanating from Io, a satellite of Jupiter. These propel debris 300 km above the satellite’ssurface and up to 600 km from the eruptive vents. Various sulphur compounds play the critical role in this process,and heat is supplied by the dissipation of tidal energy within the gravitational system of Jupiter and its moons. Activeplumes also occur on satellites of Neptune (Triton) and Saturn (Enceladus), and these, like those on Io, seem tooperate by processes more similar to those of geysers than to volcanoes. Triton erupts nitrogen, dust, or methanecompounds, whereas Enceladus erupts mostly water vapour.

Cryovolcanism has occurred on many icy satellites of the outer solar system (Kargel, 1995). This process isgenerally attributed to very low melting point mixtures of water and ammonia, which permit viscous lava-like fluidflow at temperatures so low that pure water ice behaves as a very brittle rock (Kargel et al., 1991). Extensive evidenceor cryovolcanism, including viscous flows, volcanic constructs, and calderas, have recently been discovered onSaturn’s moon Titan (Lopes et al., 2007).

Tectonic Landforms. The most common tectonic features of the rocky planets and satellites are various fractures,faults and grabens. A very early phase of compressional deformation on Mercury, perhaps associated with the forma-tion of its massive core, led to the development of huge, planetary-scale thrust faults (Strom and Sprague, 2003). Marshas immense fracture and graben systems, many of which are associated with the Tharsis volcanic region (Anderson etal., 2001). Only Earth shows the broad assemblage of mid-oceanic ridges, subduction zones and other features thatcomprise plate tectonics. Although Venus possesses a radius, density and other geophysical similarities to Earth, itlacks evidence for plate tectonics, thereby raising the question as to what makes plate tectonics unique to Earth as aplanet.

Plate tectonics proved to be of critical importance for understanding the megageomorphology of Earth. It is instruc-tive for any generalization of Earth science, therefore, that it was not so much study of Earth’s land surface, butinstead discoveries from submarine landscapes, including mid-oceanic ridges, sea-mount chains and oceanic fracturesystems, that ultimately led to the modern understanding of plate tectonics (Menard, 1986).

Exogenic landscape systemsAeolian. The relatively dense atmospheres of Venus, Earth, Mars and Titan result in the generation of extensive suitesof landforms related to the action of wind on a planetary surface (Greeley and Iversen, 1985). Mars has the greatestvariety of aeolian landforms, including various crescent-shaped and transverse dunes, wind ripples, yardangs, pittedand fluted rocks, and various dust streaks. There are also remarkable tracks produced by Martian dust devils. Morepuzzling are the huge areas of longitudinal dunes on Titan (Lorenz et al., 2006). Given the varying properties ofatmospheres on these bodies (Table I), it is clear that aeolian systems can be compared across a broad range ofcontrolling factors.

Hillslopes and mass movement. Slopes occur on all the rock planets. On airless bodies, only gravity and impactprocesses generate slope processes. However, the atmospheres of Mars, Venus and Titan invite comparisons to otherprocesses that operate on Earth. One very interesting problem is that of very large (millions of cubic metres) rock anddebris slides and avalanches. Such masses on Earth show evidence of surprisingly high mobility over relatively flatterrains. Various causes have been proposed for this, including the cushioning effects of air or water to reduce the effectivepressure of the slide mass that tends to resist broad lateral spreading. By studying similar-appearing mass movementson the Moon and Mars, which may have formed under different causal conditions, it is possible to contemplate a kindof natural experiment to distinguish various causal mechanisms, including those applicable to Earth.

A recent discovery is that of numerous small gullies developed on hillslopes associated with crater rims and channelor valley walls (Malin and Edgett, 2000; Berman et al., 2005). Morphological similarities of these hillslope gullies to

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terrestrial high-latitude, periglacial gullies suggests an origin by aqueous debris flows, involving the melting of near-surface ground ice. The flow processes, not necessarily aqueous, are active today (Malin et al., 2006; McEwen et al.,2007). The gullies are uncratered, and their associated debris-flow fan deposits are superimposed on both aeolianbedforms (dunes or wind ripples) and polygonally patterned ground, all of which cover extensive areas that are alsouncratered (Malin and Edgett, 2000). The patterned ground is itself a very strong indicator of near-surface, ice-relatedprocesses in the active (seasonally thawed) layer above the Martian permafrost zone (Siebert and Kargel, 2001).

At the regional scale, gullies occur in high-latitude bands on Mars. They are associated with a variety of otherlandforms that indicate direct emplacement and local degradation of mantles of ice and dust, possibly even dirty snow,all derived from the atmosphere (Head et al., 2003; Schorghofer, 2007). The evidence consists of small-scale polygo-nal or patterned terrains, similar to the ice-wedge phenomena of Earth’s high-latitude permafrost regions; the mobili-zation of rocky debris on slopes, similar to the rock glaciers of Earth’s periglacial regions (Mahaney et al., 2007); anda regional smoothing of small-scale topography by deposits a few to several metres thick that are internally layeredand locally eroded. The emplacement of ice-rich deposits at low to mid-latitudes seems to be consistent with geologi-cally recent episodes of higher tilt (obliquity) in the axis of Mars’ planetary spin (Head et al., 2003). This would resultin warming of the polar caps, thereby increasing the sublimation of water ice and migration of water vapour to thethen cooler lower latitudes.

Fluvial. Besides Earth, water-related fluvial action seems to have extensively occurred only on Mars, where themost extensive fluvial processes were active in the remote past. The two main varieties of fluvial landforms on Marsare valley networks and outflow channels (Baker, 1982, 2006). Many of valley networks occur in the very ancientcratered highlands of Mars. While the largest Martian channels were generally well characterized by the older imagingsystems of the 1970s, it was not until a new generation of orbital imaging capabilities that major advances occurredfor understanding the nature of valley networks. These valleys dissect the Martian highlands much more extensivelythan was apparent from the earlier images. Many valleys contain relict channels comparable in their dimensions to theactive river channels associated with terrestrial valleys (Irwin et al., 2005). There are even some striking examples ofunderfitness (Figure 1). Large alluvial fans occur in ancient highland craters at middle-to-low southern latitudes. Theyare remarkably similar to low-relief terrestrial alluvial fans formed dominantly by fluvial, as opposed to debris flow,processes (Moore and Howard, 2005). They probably formed during an episode of surface runoff processes duringepisodes with Earth-like precipitation that occurred very early in Martian history, about 3·9 billion years ago (Craddockand Howard, 2002).

The Martian outflow channels involved immense outpourings of floodwater and debris from subsurface sources.Channels are elongated troughs that display clear evidence for large-scale fluid flow across their floors and on parts oftheir walls or banks. Immense channels, with widths of tens of kilometres and lengths of up to a few thousandkilometres, display a suite of morphological attributes that are most consistent with genesis by cataclysmic flows ofwater and sediment (Baker, 1982, 2001). On Earth such flows produced the distinctive landforms of the ChanneledScabland (Baker, 1978). An important recent discovery is that Martian flood channel activity, involving outbursts ofwater and associated lava flows, occurred in the Cerberus Plains region on the order of 10 million years ago (Bermanand Hartmann, 2002; Burr et al., 2002). The huge discharges associated with these floods and the concurrent volcanismshould have introduced considerable water into active hydrological circulation on Mars. The megafloods that formedthe Martian outflow channels had maximum discharges comparable to those of Earth’s ocean currents and itsthermohaline circulation (Baker, 2002). On both Earth and Mars, abrupt and episodic operation of these megascaleprocesses are probably major factors in global climatic change.

Coastal and Lacustrine. On Earth, bodies of standing water include (1) lakes, which are limited areas of inundationsurrounded by extensive land areas; (2) seas, in which saline waters cover extensive parts of the planetary surface; and(3) the ocean, which is the vast, interconnected body of water that covers about 70 per cent of Earth’s surface. ForMars, unlike Earth, there is no direct evidence for an ocean that covered the majority of the planet’s surface through-out its long geological history. Nevertheless, there is morphological evidence for large bodies of water that episodi-cally covered the northern plains of Mars. This evidence, apparent since the late 1980s, includes the morphologicalcharacteristics of sedimentary deposits, and, more dramatically, a pattern of surrounding shorelines (Clifford andParker, 2001). Although the shoreline interpretations have been systematically criticized (Carr, 1996), the generalconcept of past inundations on the Northern Plains, constituting an ‘Oceanus Borealis,’ at least for geologically shortepisodes, has been found to be generally consistent with considerable evidence. The distinctive waterlain sedimentarydeposits that cover parts of the Northern Plains, known as the Vastitas Borealis Formation, affords the most convinc-ing case, including (1) margins that roughly mark the surface to which a body of water would approximate, (2) adistinctive population of impact craters indicating associated ice and sediments, (3) a phenomenally flat and smoothsurface expression, similar to that of abyssal plains in Earth’s ocean basins, and (4) spatial and temporal associationwith the megaflood events that shaped the largest outflow channels (Baker et al., 1991; Fairen et al., 2003).



Figure 1. Incised meanders and inset channel of Nanedi Vallis, Mars (Latitude 5·2° N, Longitude 311·8° E). This image from theHigh Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter shows a scene 6 km wide.

Though the debate over the Martian ‘ocean’ has received much attention, even more compelling evidence supportsthe existence of numerous lakes and seas, which were temporarily extant on the surface of Mars at various times in theplanet’s history (Cabrol and Grin, 2002). The more ancient lakes occupied highland craters during the heavy bombard-ment epoch, spilling over to feed valley systems that may contain deltas, terraces and other fluvial landforms (Achilleet al., 2006). Abundant crater palaaeolakes seem to have developed just after the heavy bombardment, and especiallylarge lakes occupied the floors of the largest impact basins, Hellas and Argyre.

Glacial and periglacial landscapes. Glaciated landscapes are some of the most important landform features to bedocumented with the newer high-resolution data. Earlier arguments for extensive glaciation on Mars (Baker et al.,1991; Kargel et al., 1995) were severely criticized, in part because glaciation has significant hydrological andclimatological implications (Carr, 1996). The growth and persistence of large glaciers requires a dynamic hydrologicalsystem that moves large quantities of water from surface-water reservoirs, such as lakes and seas, through theatmosphere to sites of precipitation. Resistance to the idea of ancient glaciers on Mars was especially curious, given

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that there was a general scientific consensus that Mars displays a great variety of periglacial landforms, most of whichrequire the activity of ground ice. The periglacial landforms include debris flows, polygonally patterned ground,thermokarst, frost mounds, pingos and rock glaciers. On Earth most of these landforms develop under climate condi-tions that are both warmer and wetter than the conditions for cold-based glacial landforms that we now know toabound on Mars (Baker, 2001, 2006).

The new evidence of glaciation is distinguished by its abundance, the complex detail of its assemblages, and thecommonly very young geological ages (Kargel, 2004). The glacial landforms of Mars include erosional grooves,streamlined/sculpted hills, drumlins, horns, cirques and tunnel valleys; depositional eskers, moraines and kames; andice-marginal outwash plains, kettles and glaciolacustrine plains. These landforms occur in spatial associations, proximal-to-distal in regard to past ice margins, exactly paralleling terrestrial glacial geomorphological settings. Long recognizedareas of past glaciation on Mars include lobate debris aprons near uplands surrounding Argyre and Hellas and alongthe highlands/lowlands boundary north of Arabia Terra (Kargel, 2004). These lobate debris aprons show clear morpho-logical evidence of glacier-like viscous flow (Figure 2). Extensive accumulations of glacial ice persist in the polarareas up to the present day (Plaut et al., 2007). Huge glaciers marked the western flanks of the Tharsis volcanoes.Geologically recent ice-rich rock glaciers (or debris-covered glaciers) occur at the base of the Olympus Mons scarp,where they are superimposed on older, much larger relict debris-covered piedmont glacial lobes (Head et al., 2005).

Exotic Landforms and Analogical Reasoning

Most of the landforms described above will have a considerable degree of familiarity to those with a broad knowledgeof terrestrial geomorphology. However, by extending study to the unfamiliar, some of which may seem to have astrangely familiar component, one encounters some of the most fascinating aspects of extraterrestrial geomorpholology.For example, on the phenomenally hot surface of Venus, lava may behave geomorphically like running water (Komatsuet al., 1993; Baker et al., 1997). On the phenomenally cold surfaces of satellites in the outer solar system, ice behavesas rock, and water–ammonia mixtures may behave like lava. On Saturn’s largest moon, Titan, methane may mimic thegeomorphological activity of water.

Titan is about half the diameter of Earth, and its nitrogen atmosphere has comparable total pressure to that of Earth(Table I). Unlike Earth, however, Titan is immensely cold. The temperature regime is such that the next most abundantgas in its atmosphere, methane, can condense and rain on to the surface, where it will produce liquid flows andponding. This strange, cold mimicry of Earth’s hydrological cycle is made even more striking because of the discov-ery of dendritic drainage networks (Tomasko et al., 2005) and even lakes (Stofan et al., 2007) on the moon’s surface.Figure 3 shows the dendritic networks that were imaged by the Huygens descent probe that ultimately landed onTitan’s surface.

Gilbert (1886, p. 287) formally described analogical reasoning in the earth sciences. To have some chance ofsuccess, such reasoning requires that we have a phenomenon A (e.g., channels on Titan) whose cause we seek (i.e., theformative processes for these channels). To do this we seek another phenomenon B (similar channels to A), which (1)has key features in common with A (similar in morphological details to A), and (2) whose causes we truly know.I have placed emphasis on the last two points because they must be valid for any success in the logical propositionthat gives force to Gilbert’s argument, that is, if A and B are indeed similar, the known causes of B (channels knownto be analogous to the Titan channels) will allow us to infer the causes of A (the channels on Titan). While Gilbert’sanalogy by no means proves the causal association, it does initiate fruitful lines for further reasoning, via abductive(or, retroductive) inference (Baker, 1996).

Terrestrial analogues are commonly used in planetary geomorphology because of Gilbert’s point (2), that is, interrestrial cases there is a relatively good chance that we indeed know the causes. (This is also a rationale for thepresent as the key to the past – the actualistic form of uniformitarianism). Fluvial geomorphology is concerned withthe forms and processes generated by rivers, presumed for Earth to be restricted to the flowing of water. In the contextof planetary surfaces, however, rivers must be interpreted more broadly to encompass flow by other relatively low-viscosity fluids besides water. We now know that fluvial-like landforms occur on at least five of the solar systembodies. ‘Riverine’ features on Venus and Earth’s moon seem to have been produced by lava (Baker and Komatsu,1999) and those on Titan by methane. Water-related forms are abundant on both Earth and Mars, although they arerelict for the latter (Table III).

The Venus channels are especially remarkable, with one having a length of 6800 km, making it longer than anymodern terrestrial river (Baker et al., 1992). Though a minority view, an aqueous origin is still under consideration forthe Venus channels (Jones and Pickering, 2003). Much of the aqueous argument for Venus channels relies uponsimilarities of morphology to terrestrial fluvial features, recognition of which was fully apparent at the time of their



discovery (Baker et al., 1992). Here, another important aspect of analogical reasoning comes into play. If we provi-sionally accept an analogy as true, then we must infer what would be expected in a planetary system that would beconsistent with that presumption. Obviously, we then need to see if the expected associations are indeed present. Foran aqueous origin of Venus channels to be reasonable we would expect many other water-related landforms to bepresent on Venus. However, Venus is dominated by volcanic landforms; there are no associated water-related landforms,other than the channels, all of which exist in volcanic contexts.

Figure 2. Lobate debris apron (relict, debris-covered glacial lobe) in Deuteronilus Mensae, Mars (Latitude 39·8° N, Longitude23·1° E). This HiRISE image shows a scene 6 km wide. Glacial flow features (Kargel, 2004) indicate ice movement converging at thetop of the scene (north) and extending toward the lower left, and terminating in a lobate flow front. Profiles for similar lobatedebris aprons are consistent with solid state deformation of ice up to a few hundred metres thick (Mangold and Allemand, 2001).

1350 V. R. Baker


Figure 3. Dendritic network on Titan revealed on a mosaic of three images acquired by the Huygens Descent Imager. The widthof the imaged scene is approximately 6·5 km. The scene has been stretched to enhance the contrast between the relatively darkchannels and the brighter upland terrains into which they are incised.

The history of another fluvial controversy is instructive in this regard. In the early history of lunar exploration it wasthought that sinuous rilles (channels) on the Moon’s volcanic plains might be fluvial in origin. This was proposed at atime when the Moon was thought to have formed wet (like Earth) and even to have extensive sedimentary rocks(Gilvarry, 1968). The fluvial hypothesis seemed so consistent with other then-presumed facts about the Moon, that itwas fully accepted by a famous Nobel laureate scientist (Urey, 1967). Subsequently, however, the Apollo missionsreturned lunar samples that showed that the Moon was extremely dry and never had a hydrosphere, even for itsearliest history. As with the Venus example, the volcanic associations provide the most consistent and coherent picturein regard to the origin of these lunar channels.

Analogical reasoning cannot provide complete explanations for the origin of planetary landforms. Instead, analogiesinitiate a line of inquiry, which, if properly pursued, places the investigator on a reliable path toward achieving thoseexplanations. Inquiry can then be advanced, bringing to bear all forms of systems analysis, including the quantitativemodelling of various system components.



Discussion and Conclusions

The future for discovery of extraterrestrial landforms and landscapes is essentially unlimited. As with the geomorphologyof the late 19th century, planetary geomorphology affords a frontier that is filled with wonder, excitement and evenenchantment (Baker and Twidale, 1991). Abundant opportunities exist to compare landform and landscape systems forvarying parameters in our solar system (e.g., Tables I and II). Essentially all terrestrial geomorphological processes arerepresented on extraterrestrial rocky surfaces (Table III). Moreover, as of this writing, about 100 planetary objectshave been discovered in systems that surround stars other than our own sun. The billions of solar systems to beexplored in the universe will surely contain both the familiar and the exotic to stimulate thinking for the geomorphologistsof future generations.

The study of extraterrestrial landscapes is not just a means for bringing excitement to geomorphology. It is integralto the logic of systems thinking that geomorphological systems of landforms and landscapes cannot be restrictedsolely to the subset of those features that just happen to reside on the terrestrial land surface. To the degree thatextraterrestrial landscapes are consistent with the components of the landscape system defined for analysis, then theycomprise essential parts of the relevant system for geomorphological inquiry. In short, their exclusion is not merelyunproductive; from a systems viewpoint, it is illogical.

Geomorphology will continue to contribute to the understanding of extraterrestrial planetary landscapes, but thiscontribution may be less in the realm of theoretical models derived from first principles and more in the realm ofdiscovery of unique realities that point to new evolutionary histories. A final observation from a biologist makes thiscase for planetary geomorphology. Based on his survey of discoveries made about the icy surfaces of moons in theouter solar system, Stephen Jay Gould proposed a ‘principle of planetary individuality’, which he described as follows(Gould, 1991, pp. 506–508):

‘. . . the surfaces of planets and moons cannot be predicted from a few general rules. To understand planetarysurfaces, we must learn the particular history of each body as an individual object . . . their major features are setby unique events – mostly catastrophic – that shape their surfaces . . . Planets are like individual organisms, notwater molecules; they have irreducible personalities built by history . . .’

AcknowledgementsMy planetary research was supported in the past by the U.S. National Aeronautics and Space Administration. Devon Burr, PaulCarling and Stephen Rice provided comments that improved the manuscript.

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