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Geomorphology 37Ž. 2001 303–328 www.elsevier.nlrlocatergeomorph

Extraterrestrial coastal geomorphology

Timothy J. Parker a,), Donald R. Currey b a Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak GroÕe DriÕe, Pasadena, CA 91109, USA b Department of Geography, UniÕersity of Utah, Salt City, UT 84112, USA Received 1 April 1995; received in revised form 1 May 1996; accepted 14 January 1999

Abstract

Earth is the only in the where large amounts of liquid have been stable at the surface throughout geologic time. This unique trait has resulted in the production of characteristic landforms and massive accumulations of aqueous sediments, as well as enabled the evolution of advanced and diverse forms of . But while Earth is the only planet with large bodies of water on its surface today, and may have once had or as well. More exotic fluids may be stable in the outer solar system. Prior to the Voyager flybys of the outer during the 1970s and 1980s, the moon of Neptune, Triton, was thought to be much larger than the Voyager cameras revealed it to be, and predictions that liquid lakes or oceans might be found were made. The moon of Saturn, Titan, however, was found to have a massive atmosphere, so the possibility remains that it may have, or may once have had, lakes or oceans of liquid hydrocarbons. The recent, high-resolution synthetic aperture radar imaging of Venus has failed to reveal any evidence of any putative clement period, but the results for Mars are much more intriguing. Herein, we briefly review work on this subject by a number of investigators, and discuss problems of identifying and recognizing martian landforms as lacustrine or marine. In addition, we present additional examples of possible martian coastal landforms. The former presence of lakes or oceans on Mars has profound implications with regard to the climate history of that planet. q 2001 Published by Elsevier Science B.V.

Keywords: Earth; Mars; Solar system

1. Introduction sometimes quite strange surfaces on other solid bod- ies in our solar system from Mercury to the moon of Beginning with the first deep space missions of Neptune, Triton. Pluto remains the only major planet the early 1960s, our geomorphic horizons have been not visited by spacecraft to date. We have seen that expanded beyond Earth to include hundreds of mil- impact cratering has been one of the most important lions of square kilometers of previously unknown, processes operating throughout the history of the solar system. Thirty years ago, however, debate raged as to whether lunar craters were of impact or vol- ) Ž. Corresponding author. canic origin e.g., , 1971 . Evidence for other, E-mail addresses: [email protected] more familiar processes such as volcanism, was Ž.T.J. Parker , [email protected] Ž D.R. Currey . . quickly recognized on planetary surfaces, though the

0169-555Xr01r$ - see front matter q 2001 Published by Elsevier Science B.V. PII: S0169-555XŽ. 00 00089-1 304 T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 tremendous size of many volcanic features on Venus, to the accumulation of massive marine and lacustrine Mars, and the moon of , Io, has been surpris- sedimentary deposits and characteristic recent and ing. Structural features such as fault scarps and modern coastal landforms. But has Earth always grabens had been recognized on the moon soon after been the only planet with standing bodies of water? telescopes were able to resolve them, so they were What about other, exotic fluids that might be stable quickly recognized on other planetary surfaces as in liquid form elsewhere in the solar system? Could well, once spacecraft images became available. these mimic the behavior of water on Earth and Differences in the sizes of familiar features on collect into lakes, seas, or even oceans? other planets can usually be attributed to the astro- In this paper, we take a look at recent discussions nomical, geophysical, or climatological conditions about and geomorphic evidence for extraterrestrial specific to a particular planetary body. For example, lacustrine or marine environments in the current the moon of Jupiter, Io, appears to experience global literature and comment on the potential for further volcanic resurfacing because its interior is kept hot discoveries. We are interested in lakesroceans in the through tidal friction caused by gravitational tugging AtraditionalB senseŽ. to the extent that there is one , between Jupiter and the other Galilean satellites and so we will exclude lava lakes from this discus- Ž.Gailitis, 1982 , and yet it is comparable in size to sion. our own moon, on which regional-scale volcanism appears to have ceased over 3 billion years agoŽ e.g., . Wilhelms, 1987 . The largest shield volcanoes in the 2. Narrowing the field solar system have developed on Mars because that planet lacks terrestrial-style plate tectonism, such We shall limit the scope of this study to solid- that the lithosphere cannot move relative to mantle surface bodies that have or may have had atmo- hot spots, and because its thin atmosphere and cold, spheres in which some abundant liquid might have dry surface environment preclude terrestrial rates of been stable long enough to collect into topographic erosion. At the same time, Mars is scarred by gigan- depressions and leave characteristic geomorphic sig- tic flood channels that require up to three orders of natures. Further limiting this study to objects whose magnitude greater discharges than the largest known surfaces have been imaged leaves Venus, Earth, similar floods on Earth, and by abundant valley Mars, the moon of Saturn, Titan, the moon of Nep- networks that suggest long-term groundwater sap- tune, Triton, and Pluto. ping and surface runoffŽ e.g., Mars Channel Working Group, 1983. . These hark back to an earlier, wetter climate, and also attest to the extremely slow de- struction of landforms on Mars over much of geo- 3. The outer solar system logic time when compared to terrestrial rates of erosion. On the icy satellites of the outer planets and on For many terrestrial surface processes, analogous Pluto, water ice is an important lithic component. processes have been identified, or at least inferred, Indeed, many landforms seen during the Voyager on other planetary surfaces. Earth is unique in the flybys of the satellites of outer planets have been solar system, however, in one important aspect, in interpreted as produced by eruption of cryogenic that two thirds of its entire surface is covered by solutions containing water as a volcanic melt, to water. Because of the atmospheric pressure and dis- which the term AcryovolcanismB has been applied tance from the sun, water can exist in all three Ž.e.g., Helfenstein et al., 1992 . These cryogenic solu- physical states somewhere on the surface or in the tions might temporarily collect in a depression or atmosphere of Earth; at least, enough water to insure caldera on one of these satellites, as appears to have that not all can be contained in the subsurface. The happened on Triton, but these are lava lakes in the presence of so much surface water in the oceans strictest sense. Extant lakes composed of liquids throughout geologic time, and in continental basins stable in the outer solar system have yet to be seen for geologically significant periods of time, has led directly, though there may be good reason to suspect T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 305 a presence on Titan. In the absence of extant lakes, it bons, perhaps even a global , might be found is difficult to predict what paleolakeshores might on the surface today, or that evidence for paleolakes look like on one of these satellites, because we have might be foundŽ e.g., Caldwell, 1978; Hunten, 1992; no experience with interactions between large bodies Flasar, 1983; Lunine et al., 1983; Lunine, 1993. . of liquids and icy terrain that would be stable at Lunine’sŽ. 1993 summary of this work points to the these cold temperatures; constructing physical fascinating probability that one, or parts of all these models, therefore, would also be difficult. Neverthe- models will be verified during the course of the less, predictions of lakes or paleolakes have been Mission to Saturn. Recent earth-based obser- made for Titan and Triton. Pluto is thought to be vations at radar wavelengthsŽ. Muhleman et al., 1990 very similar in size and composition to Triton, and and with the Hubble Space Telescope in the near its distance from the sun is comparable, so we will infraredŽ. et al., 1996 , however, have revealed accept the current hypotheses that predict a similar large regions with topographic or albedo variations, surface environment, at least until spacecraft data thus ruling out the hypothesis of a global ocean. become available. Tidal models of the evolution of the orbit of Titan would also seem to rule out the global ocean, but 3.1. Titan disconnected seas or lakes remain a possibilityŽ Der- mott and , 1995; Sears, 1995; Sohl et al., At 5150 km in diameter, Titan is the second 1995.Ž . Currently enroute Lebretson and Matson, largest moon in the solar systemŽ the moon of Jupiter, 1992. , Cassini includes the Saturn Orbiter, which Ganymede, is 5262 km in diameter. . The existence will make numerous flybys of Titan employing syn- of an atmosphere on Titan had been known since thetic aperture radarŽ. SAR , and the atmo- was able to detect gaseous in its spheric probe equipped with a spectral radiometer, spectrum in 1944Ž. Kuiper, 1944 . Prior to the Voy- which will be deployed into the atmosphere of Titan. ager flybys, the density of this atmosphere was generally thought to be only on the order of a few 3.2. Triton times as high as the atmosphere of Mars, or a few tens of millibarsŽ. e.g., Kaufmann, 1978 , though In the case of Triton, speculations that lakes of HuntenŽ. 1978 correctly predicted that nitrogen might liquid nitrogen might be found were made prior to be a primary constituent of a massive atmosphere. the Voyager flyby in 1989Ž e.g., Cruikshank et al., One of the hopes of the Voyager science team was to 1984, 1988. . These were based on the assumptions image the surface of Titan during the flybys, though made, before Voyager, that Triton was much larger at least a partial, and possibly a global, cloud cover than it turned out to be, perhaps even nearly as large had been anticipated based on the very small varia- as MarsŽ. Kaufmann, 1978 , and that a greenhouse tions in the brightness of Titan viewed from Earth as effect kept surface temperatures warm enough to it rotates. Titan turned out to have an impenetrable, allow liquid nitrogen to be stable. These early esti- global photochemical haze of organic compounds, mates that Triton might be the largest satellite in the including ethane and , so its surface could solar system were made based on its brightness and not be imaged at optical wavelengths from orbit. an assumed range of AreasonableB albedos. Triton More important gaseous nitrogen, which was not turned out, however, to be one of the more reflective detected spectroscopically from Earth, was found to moons in the solar system such that, at only 2706 km be a major component of the Titan atmosphere, with in diameter, it is less than half of the diameter of the result that the surface pressures are actually on Mars and is actually smaller than our own moon the order of one and a half to three times as high as Ž.3476 km . For these reasons, instead of a substantial those on EarthŽ. e.g., Lunine, 1993 . atmosphere and greenhouse effect, the atmosphere of Several investigators have proposed, based on this Triton is actually on the order of a few microbars discovery and models of the evolution of the surface Ž.e.g., Brown et al., 1991 . As a result, nitrogen is and atmosphere of Titan through the history of the stable only as an ice or at the surface, though a solar system, that large bodies of liquid hydrocar- number of active nitrogen geysers were seen by 306 T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328

VoyagerŽ. e.g., Lee et al., 1991 . This evidence that from orbiting spacecraft, provided the pixel scale Triton is geologically active today, such that volatiles Ž.and the landform is large enough to enable identifi- are being injected into the atmosphere, suggests the cation. Stratigraphic evidence will probably require possibility of a denser ancient atmosphere with an confirmation by surface landers, though it might be associated greenhouse effect. recognizable in erosional scarps and hollows from orbit or in multi-spectral orbiter images that provide 3.3. Pluto unequivocal compositional information. The similarity of Pluto to Triton was inferred 4.1. Venus based on the information obtained about size and surface temperatures of Triton during the Voyager Venus presents us with the opposite extreme in flyby. The dimensions of Pluto had been fairly accu- surface conditions from the outer planetary satellites. rately known for several years longer than those for Whereas aqueous melts appear to be volcanic fluids Triton, however. The discovery of the satellite of in the outer solar system, silicate melts on Venus Pluto, Charon, in the late 1970s, and recent observa- appear to have carved long, meandering channels, tions of mutual occultations, has enabled accurate including the longest known channel in the solar determinations of the and dimensions of the system, Baltis VallisŽ formerly Hildr Fossa; e.g., two bodies. Pluto is approximately 2300 km in diam- Baker et al., 1992. . Venus has a massive, dominantly eter, whereas Charon is about 1200 km in diameter, CO atmosphere, producing a substantial green- making it the largest moon in the solar system in 2 house. Surface pressures are as high as 90 bars or proportion to its primary. The occultation studies more and temperatures are on the order of 750 K, have even allowed crude global albedo maps to be hotter even than daytime equatorial temperatures on constructed of Pluto and CharonŽ e.g., Buie et al., Mercury. Metals like lead and zinc would be molten 1992. . Like that of Triton, the atmosphere of Pluto is at the surface of Venus, as would sulfur and many quite tenuous, and the principal volatiles probably sulfur compoundsŽ. Baker et al., 1992 . partly condense seasonally onto the surface as solids. The Magellan Orbiter mission used synthetic ŽPluto is currently closer to the sun in its 248.5-year aperture radarŽ. SAR to map more than 98% of the orbit than Neptune.. At only 41–42 K, the surface of surface of Venus at resolutions between 120 and 350 Pluto is far too cold for liquid N , and pure He and 2 mrpixelŽ such that more of the surface of Venus is H are not likely to be present in significant quanti- 2 covered at this scale compared to the surface Earth. . ties to form lakes. It would seem unlikely, therefore, These data reveal a surface dominated by volcanism that we can expect to find bodies of liquids on Pluto and tectonism, with evidence of limited impact cra- once spacecraft data become available sometime next tering, eolian sediment transport, and chemical centuryŽ. Staehle et al., 1993 . Like Triton, however, weathering. Though there had been hopes that an- ancient greenhouse atmospheres remain a possibility cient surfaces might retain a record of a past clement for Pluto, so evidence of ancient lakes may eventu- epoch on Venus, when the atmospheric pressure may ally be discovered. have been lowerŽ in the present surface pressure environment, water boils around 570 K. and liquid 4. The inner solar system water may have been available to carve channels and coastal landforms, none have been found. The num- Though only the atmosphere of Earth is the right ber and apparently uniform global distribution of temperature and density today to permit liquid water, impact craters seems to suggest a surface age on the the ancient atmospheres of Venus and Mars may order of several hundred millions of years or less have allowed liquid water as well. But these planets Ž.Phillips et al., 1992; Schaber et al., 1992 . lack surface water today, so verification of the for- Planetary evolution models have placed a water- mer existence of lakes or oceans requires geomor- rich Venus at billions of years agoŽ e.g., Matsui and phic or stratigraphic evidence. Searches for geomor- Abe, 1986; Kasting, 1989; Grinspoon, 1993. . High phic evidence can be made with images acquired deuteriumrhydrogen ratios in the upper atmosphere T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 307 S . From Atlas of Mars: 1:25 M Topographic Series Map I-2179, U.S. Geological 8 N to 57.5 8 Ž. Fig. 1. Shaded relief and albedo mercator projection of Mars from 57.5 Survey 1991 . 308 T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 of Venus imply either early abundant surface water and many valley networks appear Že.g., Donahue and Hodges, 1992; Hartle et al., to head at the surface with no distinct chaotic terrain 1996. , on the order of lakes or oceans worth, or or theater-headed source regionŽ Parker and Gorsline, recent replenishment from the mantle or cometary 1991a,b; Parker, 1994. , requiring spillover of surface impactsŽ. Grinspoon, 1993 . If Venus ever did pos- lakes or atmospheric precipitation to initiate channel sess an Earth-like climate, we will probably have to flow. search for the evidence in the stratigraphic record. The question of the fate of the water discharged Even coarse stratigraphic resolution will probably by the channels is dependent on how much water require complex extended missions by a surface was discharged during a single flood eventŽ in the rover, or perhaps an intermediate-stage, multi-spec- case of the outflow channels. andror what the pre- tral aerial reconnaissance imaging mission. Still, the vailing climatic and substrate conditions were at the implication that Venus may have lost its oceans time. In essence, whether fluvial sedimentation into because of a runaway greenhouse may have a pro- martian basins occurred in a subaerial or lacustrine found impact on our understanding of how the cli- setting would have depended on the volume of trans- mate of Earth has evolved and might evolve in the ported material, the sedimentrwater ratio, the rate future. The development of search strategies for and duration of fluvial discharge, the number of indicators of former aqueous processes on Venus simultaneously active channels draining into a given should at least be considered for follow-on surface basin, and the stability of liquid water and ice at the science missions to that planet. at the latitude of the basin at the time. For the purpose of searching for evidence of 4.2. Mars paleolakes, therefore, it is reasonable to focus on In terms of environmental properties, the present- basins that have inward-draining channel systems day Mars is the most Earth-like planet in the solar that are large or have large catchment areas relative system. Because the atmosphere of Mars cannot to the volume or area of the basin. Better still, basins support liquid water today, several mechanisms for with both inward- and outward-draining systems can producing observed channels were suggested that do be sought, as the outward-draining channel would not necessarily require liquid water. Proposed fluids indicate overflow from a former lake within the ranged from windŽ. Cutts and Blasius, 1981 to lava basin. Several large basins, and hundreds of small to Ž.Carr, 1974 . Today, however, most investigators intermediate-size craters, have channels terminating conclude that liquid water, in an earlier wetter envi- within them. These include most of the largest im- ronment, was responsible for the outflow channels pact basins on the planet, such as Argyre, Chryse, and valley networks. Hellas, and Isidis. Chryse and Isidis are Aembay- The presence of the channels evokes three impor- mentsB in Mars’ vast northern lowland plains, which tant questions. What was the source of the water that comprise approximately one third of the surface of carved the channels? What was the fate of the water? the planetŽ. Fig. 1 . What were the climatic conditions that enabled chan- nel formation? The largest and most widely known channels are those along the western and southern 5. Martian paleolakes and oceans Ž. rim of Figs. 1 and 7 . These chan- 5.1. Origins of martian lacustrine basins nels are typically on the order of several tens of kilometers wide and up to thousands of kilometers Because the diameter of MarsŽ. about 6800 km is longŽ. Mars Channel Working Group, 1983 . All the just over half that of Earth, it does not exhibit global circum-Chryse channels head in large collapse de- tectonism on a scale comparable to Earth and Venus. pressions, or chaotic terrain, implying a subterranean But because it is still a large body compared to sourceŽ. e.g., Carr, 1979 . Often-cited examples of Mercury and the moon, it has had an atmosphere and valley networks also head in incised, theater termina- climate over solar system history. This is why Mars tions, suggesting groundwater sappingŽ Mars Chan- has been able to retain surfaces produced both nel Working Group, 1983. . However, several other through volcanic and climatic processes that are T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 309 largely intermediate in age between volcanic sur- to its sheer abundance of surface water. Only a small faces on the moon and Mercury and both types of percentage of the 1.37=109 km3 of water in the surfaces on Venus and Earth. For the purposes of oceans can be stored in continental and polar reser- this discussion, this has important implications about voirs at any given time; the nonmarine reservoirs the origins and evolution of topographic depressions contain ;2.3=1083 km of waterŽ Hutchinson, that potentially may have contained lakes. 1957. . Tectonism is probably the most important process On the other hand, the volatile reservoirs on on Earth for producing closed depressions on the Mars, the megaregolithŽ the impact-brecciated upper continents, and is clearly responsible for mainte- few kilometers of the crust. , the polar caps, and nance of the ocean basins through geologic time. In potential near-surface massive ice depositsŽ e.g., addition, tectonism is probably the most important Costard and Kargel, 1995. that may be protected process for forming depressions in the highland ter- from sublimating by thin insulating mantles, are of rains and lowland plains of Venus. On Mars, how- sufficient volume to contain its entire remaining ever, tectonism appears limited to relatively small water inventory. If we assume that the pore volume amounts of regional extension, compression, and of ;108 km3 for the megaregolith estimated by vertical motion largely because of crustal loading of CliffordŽ. 1981 is reasonable and that very little of the two major volcanic provinces— and Ely- the original water has been lost to space, and we siumŽ Banerdt et al., 1982; Plescia, 1991; Tanaka et allocate his 600 m global average layer to the north- al., 1991; Watters, 1993. . The global crustal di- ern plains, this yields an average ocean depth of over chotomy separating the heavily cratered southern 1.5 km. highlands from the sparsely cratered northern low- The actual value of the total martian water inven- land plains is an ancient feature, resulting from tory is still poorly known, but is likely to beŽ have tectonism or the formation of a single, or several been. between one and two orders of magnitude overlapping giant impact basins in the northern plains smaller than the inventory on Earth. Recently, Don- Že.g., McGill and Squyres, 1991; McGill and Dim- ahueŽ. 1995 has suggested that deuteriumrhydrogen itriou, 1990; Breuer et al., 1993. . ratios from SNC meteorites, thought to have origi- Impact craters and large impact basinsŽ including nated on Mars, imply a modern water inventory all or parts of the northern plains. are clearly more equivalent to a global layer several meters deep. important sites for potential lake basins on Mars, Extrapolating to the early HesperianŽ see Fig. 2 for though they were likely more important on Earth and Martian time scale. , Donahue believes this inventory Venus as well during the period of heavy meteorite would have amounted to several hundred meters bombardment prior to 3.5 Ga. Comparisons of the spread globally. How much water could have been relative importance of other formative processes on present at the martian surface during and Mars with those on Earth are less obvious, and some time depends on the rate at which juvenile may be quite speculative, because our understanding andror recycled water was produced through vol- of the early martian environment is still rather lim- canic outgassing and heavy bombardment relative to ited. Nevertheless, a preliminary assessment of the the rate at which it would be lost to the megare- origins of lacustrineŽ. and oceanic basins on Mars, golith, the cryosphere, and to space. Both processes based on those of terrestrial lakes by Hutchinson were likely important prior to 3.5 Ga, but volcanism Ž.1957 and Currey’s Ž 1994a,b . discussion of hemi- would have been the only significant contributor arid lake basins, is listed in Table 1. after that time. The permeability of the megaregolith is also poorly knownŽ. MacKinnon and Tanaka, 1989 , 5.2. The martian water budget, and the aÕailability but it likely decreased as the meteorite impact flux of surface water through time declined over geologic time, allowing chemical weathering and cementation of surface rocks to oc- It is perhaps fair to state, even considering our cur in the presence of liquid waterŽ. or ice and inexperience with other, truly Earth-like planets, that giving an opportunity to form permafrost seams. the geomorphic uniqueness of Earth can be attributed Percolation would have been further retarded in a 310 T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328

Table 1 Proposed origins of martian lacustrine and oceanic basins Structuralrgeomorphic Proposed type localityŽ. if any Reference Ž one listed for each . Expected closure cause of containment depths:Ž. 1 shallow; Ž.2 moderate; Ž. 3 deep Cosmogenic basins Large impact basins Hellas Moore and Edgett, 1993 3 Argyre Parker, 1994 2–3 Chryse Rotto and Tanaka, 1991 3 Isidis Scott et al., 1991a 2Ž. ? UtopiaŽ. ? Scott et al., 1991a 3 Chapman, 1994 Impact craters 158S, 1848W Goldspiel and Squyres, 1991 2Ž. ? Parana–Loire Valles 238S, 138W Goldspiel and Squyres, 1991 1–2 numerous highland craters and Williams, 1994 1–2 Uzboi VallisŽ. Forsythe and Zimbelman, 1995 1 Ž. ? Parker, 1985 External Ž.crater-dammed valleys

Tectonic basins Regional downwarp Northern PlainsŽ. ? Lucchitta et al., 1986 2–3 Graben Mareotis FossaeŽ. ? Parker et al., 1989, 1993 1 Faultrwrinkle-ridge dammed valleys N flank, Alba Patera 1

FluÕial Natural levee-dammedŽ. ? Alluvial fan-dammedŽ. ? Delta-dammedŽ. ? Abandoned channels Uzboi VallisŽ. flow from Parker, 1985 2 Ž. ? Flood-scoured pools Kasei VallesŽ. ? ? 1–2 Ž. ?

Eolian Deflationrblowouts Argyre 1–2Ž. ? Hellas 2–3Ž. ? Utopia PlanitiaŽ. ? 1 Glacier-impounded Utopia Kargel et al., 1995 2 Hellas Kargel and Strom, 1992 3 Argyre Kargel and Strom, 1992 2 Northern Plains Baker et al., 1991 3

Thermokarst Permafrost thaw Utopia Costard and Kargel, 1995 1 Kettle Argyre Kargel and Strom, 1992 1

cold martian paleoclimate because outgassed water find on Mars, at least until unequivocal surface would simply snow out onto the surface and remain compositions can be identified. Free liquid water is there until removal through basal melting or sublima- confined by gravity to an equipotential upper surface tion. that intersects topography at an essentially constant elevation over large regions. This makes it possible 5.3. Recognition of coastal landforms on Mars to distinguish shorelines from fault scarps where Coastal morphology is probably the most diagnos- both feature types occur, such as in the Great Basin tic evidence of former lakes that we can expect to of the southwestern United States. Fault scarps cross T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 311

Fig. 2. Chronologies for Earth, moon, and MarsŽ. adapted from Tanaka, 1986 .

undulations in topography with little or no lateral land, resulting in erosion and sediment transport and deflection, whereas even minor topographic undula- deposition that is focused within a few meters of the tions profoundly affect the lateral position of a shore- water level. Longshore transport moves sediment line. Shoreline elevation varies somewhat locally, eroded from headlands into embayments, producing because of tidesŽ. in very large basins , wind stresses beaches that transit from erosional to depositional, Ž.seiches , and a number of other, though less impor- with wave-cut cliffs grading along shore into con- tant, factors. Martian tides would have been much structional barriers and spitsŽ. Fig. 3 . Spits and smaller than terrestrial tides because of the lack of a coastal barriers remain as large, often arcuate or large natural satellite and its greater distance from cuspate sets of beach ridges after recession of the the sun. Winds blowing across open water generate lake. Predictable longshore trends of geomorphic ripples and waves that transfer their to the development distinguish wave-cut and wave-built 312 T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328

Fig. 3. Stereo aerial photograph pair from the northern Lake region of Utah. Prominent peak at center is a headland massif with wave-cut cliffs and a prominent wave-built platform, or apron, that was produced during the Bonneville highstand of the lake. The Bonneville shoreline separates highly dissected pre-lake surfaces from a much less dissected, horizontally banded surface that is below the elevation of maximum inundation. Portions of aerial photos VBIV 6-127,128, March 1966.

terraces from structure-controlled benches in se- How would surface waves on Mars differ from quences of layered rocks. those on Earth, and would they tend to increase or Shore platforms and sedimentary bedforms asso- decrease the chances of recognizing coastal land- ciated with dissipative shorelinesŽ e.g., et al., forms from orbiter images of Mars? Beginning with 1979. can be extensive, so many terrestrial paleo- scaling for martian gravity, we have identified some lakeshores can be seen in moderate- to high-resolu- interesting differences between water waves on the tion orbiting spacecraft image data. Steeply shelving two planetsŽ. Figs. 4–6 . To keep the comparison as shore zones dominated by wave reflection are much simple and straightforward as possible, we restricted narrower in plan view, so that many steep this exercise to linear or waves in deep water mountain-front paleolakeshores may be undetectable Ž.e.g., Komar, 1976 and assumed a denser early in orbiter images. Narrow shore platforms require martian atmosphere and above-freezing tempera- high spatial resolutions to be recognizable, typically tures, at least seasonally, to allow a free air–water moderate- to high-resolution aerial photographs. surface and enable wind-driven waves. Coastal ero- T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 313

Fig. 4. Comparison of terrestrialŽ. black line and Martian Ž. gray line wave phase velocities Ž. celerity for Airy waves in deep water.

sion in an ice-covered sea or ocean is also possible, lar relationship between martian and terrestrial wave- but only if that cover is thin enough that it can be lengths, defined by: mobilized by means of seasonal breakup. 2 The phase velocity of an Airy wave in deep water gT L` s Ž.2 is expressed by: 2p such that martian waves must travel about one third gT as fast or have about one third the wavelength as C` s Ž.1 2p terrestrial waves with the same periodŽ. Fig. 5 . An important factor to consider with regard to the in which C` is the phase velocity in deep water, g is erosive ability of martian waves is the diameter of the gravitational accelerationŽ mrs2 . , and T is the particle orbits beneath the waves and the depth at periodŽ. s , from one wave crest to the next. The ratio which this motion essentially ceases Ž.Awave baseB . of martian to terrestrial wave phase velocity is about This, in turn, will determine the slope and width of one-thirdŽ. Fig. 4 . This is accommodated by a simi- the shorezone. In deep water the particle orbits be-

Fig. 5. Comparison of terrestrialŽ. black line and Martian Ž. gray line wavelengths for Airy waves in deep water. 314 T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 neath waves are essentially circular, so the horizontal few meters or tens of meters of presentŽ eustatically and vertical components of the orbit are equal. The and isostatically evolved. sea level and range in orbit diameter is then determined by: widthŽ. normal to the coast from less than 1 km to at least 60 km. Martian analogs of the strandflats would 4p 2 Z s 0 almost certainly be detectable as broad, abandoned d H exp2 Ž. 3 ž/gT shore platforms from orbit. Although the originŽ or where d is the particle orbit diameter, H is the wave origins. of strandflats on Earth is still a matter of debateŽ. e.g., Trenhaile, 1987 , the most likely forma- height, and Z0 is the depth below the still water level to the center of the particle orbit. The results of tive processes—Ž. 1 coastal zone frost weathering substituting martian and terrestrial gravity into the coupled with debris removal by wave action;Ž. 2 equation for the specific case of 1-m high waves of rock scour by moving shelf ice, both glacial and 10-s period are shown in Fig. 6. On Mars, d de- non-glacialŽ the latter exemplified by Ward Hunt Ice creases much more rapidly with depth than it does Shelf on Ellesmere Island and ice islands in the on Earth. This suggests that terrestrial waves are Arctic Ocean.Ž. ; 3 rock plucking and removal by r much more effective at moving sediment at depth shore-fast sea ice; and orŽ. 4 rock scour by drifting than AequivalentB martian waves. At the same time, sea ice or circulating pack ice—are not inconsistent it also indicates that martian waves entering shallow with plausible martian paleoclimatic conditions. water Afeel bottomB later than terrestrial waves and that martian wave energy is focused within a nar- 5.4. Martian paleolake studies rower depth range than terrestrial wave energy. On Mars, this should lead to the production of dissipa- Martian paleolake studies have taken two ap- tive shorezones that are broader and have gentler proaches. One group of investigators has focused on slopes than those on Earth. This might permit the the heavily cratered, relatively ancient southern high- development of coastal landforms on Mars that are lands as the most likely place to preserve evidence of large enough to be visible from orbit, while reducing a former wet climate. The valley networks, while the AEarth-likeB climate requirements somewhat. they may not always indicate atmospheric precipita- An additional or complementary mechanism for tion, nevertheless require significant discharges for potentially producing broad paleoshore platforms on sustained periods of time. The other group of investi- Mars may be found in the strandflats of Earth. On gators has focused on the basin-forming northern Earth, and particularly along numerous segments of lowlands as a likely site for paleolakes or even a the North Atlantic coast, strandflats occur within a paleo-ocean. The primary sources of influx into this

Fig. 6. Comparison of terrestrialŽ. black line and Martian Ž. gray line particle orbit diameters beneath a 1-m high Airy wave of period Ts10 s. T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 315 basin are the circum-Chryse outflow channels and In the southern highlands, several basins on the several smaller outflow channels and valley net- order of several tens to a few hundred kilometers works west and of the Elysium volcanic com- across and parts of have been found plexŽ. Fig. 7 . The Chryse channels alone are large to contain remnants of former alluvial or lacustrine enoughŽ Carr, 1986; Rotto and Tanaka, 1991; DeHon sedimentary deposits. Many of these basins were and Pani, 1992; DeHon, 1993. to have produced simply in the paths of short-term catastrophic floods, several large lakes, even a sea or ocean, within the and thus may not have contained lakes for very long northern plains in a time period as short as a few Ž.DeHon, 1992 . The thick Valles Marineris layered months or years. deposits occupy deep graben and collapse depres-

Fig. 7. Polar projection of northern plains of Mars relative to major bounding provinces. Moderate gray indicates southern cratered uplands and volcanic terrains. Pale gray stipple indicates northern plains. White indicates residual north polar ice cap. The dichotomy between the northern plains and southern highlands is indicated as either AgradationalB Ž.Ž.Rossbacher, 1985 or AfrettedB Sharp, 1973 , except where regional volcanic surfaces appear to obscure it. Major outflow channels are indicated by light gray stipple. Mappable distributions of two major northern plains boundaries, which may be ancient shorelines, to which the terms Agradational boundaryB and Ainterior plains boundaryB were applied in Parker et al.Ž. 1993 , are indicated by thick, dark gray line Ž coincident with lowlandrupland boundary for the most part.Ž. and thick, black line, respectively. Map adapted from Parker et al. 1989 . 316 T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 sions in the martian crust, and appear to have been notable examples of highland basins have been the fed by groundwater rather than overland flowŽ Nedell subjects of proposals for future lander missions to et al., 1987; Komatsu et al., 1993. . The deposits are Mars that might search for evidence of fossil organic on the order of kilometers thick and appear finely materialsŽ. summarized in Landheim et al., 1993 . laminatedŽ. in high resolution Viking Orbiter images , The first of these, the 135 km crater GusevŽ 158S and so probably indicate the presence of lakes for lat., 1848 lon., Fig. 8. , lies at the mouth of the large, long periods of time. These layered deposits and two tributary-fed channel Ma’adim Vallis and contains

Fig. 8. Terminal deposit at mouth of Ma’adim Vallis, in the 135-km-diameter , GusevŽ. 158S latitude, 1848 longitude . This feature, expressed as partially eroded, flat-topped mesas along the south-central margin of the impact basin, may be the remnants of an alluvial fan or delta at the mouth of the channel. Portions of Mars Digital Image Mosaics MI15S182, MI15S187. T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 317 the remnants of deltaic or alluvial fan deposits and can be seen in other large, shallow basins in the possible lake sedimentsŽ. Schneeberger, 1989 . The Phaethontis region of Mars at 358S latitude, 1778 other site is an unnamed, 200 km diameter depres- longitudeŽ. and nearby at 308S lati- sion along the Parana–Loire valley system in Mar- tude, 1708 longitudeŽ. , Fig. 10 . On garitifer SinusŽ. 238S lat., 138 lon., Fig. 9 . This basin Earth, eroded lake deposits are among the more contains a peculiar, hummocky deposit that has been common host terranes for badlands topography, interpreted as the eroded remnants of lake sediments which bears some resemblance to the martian de- by Goldspiel and SquyresŽ. 1991 . Similar deposits posits, though fluvial dissection is more pronounced.

Fig. 9. AEtchedB deposit in an intercrater depression in Margaritifer SinusŽ. 238S lat., 138 lon. . This depression lies at the termini of several valley networksŽ.Ž Parana Valles, eastern part of image mosaic , and is the source for Loire Vallis large valley, northwest part of image mosaic. . Such depressions, with channels that drained into them and others that flowed out from them, are not uncommon in the martian highlands, and are perhaps the most convincing evidence of paleolakes in the ancient cratered terrain of Mars. Viking Orbiter images 615a44-48,65-68. 318 T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328

Fig. 10. Three large, degraded basins, which are interpreted to be old impact structures, contain AetchedB deposits similar to those found in Gusev and Margaritifer Sinus, but are more extensive. Though several valley networks terminate within these depressions, none appear to flow from them. Basin deposits, Atlantis ChaosŽ. 358S lat., 1778 lon. , are located in the northwest part of the mosaic. Basin deposits, Gorgonum ChaosŽ. 388S, 1708 lon. , occupy the basin located in the southwest part of mosaic. Portions of Mars Digital Image Mosaics MI30S167, MI30S172, MI30S177, MI45S167, MI45S172, MI45S177.

The largest highland basins, ArgyreŽ 900 km inte- Vallis, and Palacopas Vallis, which head near pale- rior diameter.Ž and Hellas 1500 km interior diame- opolar deposits at high southern , drained ter. appear to contain massive accumulations of lay- into Argyre basin and , an outflow ered sediments that are now being exposed through channel, appears to have spilled northward from a eolian deflation. Those in Argyre may be on the lake within the basinŽ e.g., Parker, 1989, 1994; Parker order of hundreds of meters or more thickŽ Parker and Gorsline, 1991a,b, 1993. . and Gorsline, 1993. , whereas those in Hellas appear The origin and evolution of the northern plains of to have been as much as a few kilometers thick Mars, once largely ignored in favor of focused stud- Ž.Moore and Edgett, 1993 . Two outflow channels, ies of highland and volcanic landforms that are often and HarmakhisrReull Valles, terminate much more dramatic in appearance, has recently within the eastern rim of Hellas Basin with no outlet. become a subject of high scientific interest. The Three large valley networks, Surius Vallis, Dzigai reasons for this relate largely to the need to under- T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 319 stand the evolution of the water inventory on the channels, which they feel limits any standing water planet, particularly its surface water inventory, to one or a few large, ephemeral lakes. They based through geologic time. This, in turn, has important the locations of these lakes on the identification of implications regarding the question of exobiology on broad, shallow topographic basins on the present Mars, particularly if long periods of geologic time Martian topographic mapsŽ U.S. Geological Survey, with available surface water are indicated. 1989.Ž. . Similarly, Scott et al. 1991a have indicated Over the past several years, a number of investi- evidence for several large lakes across the northern gators have described geomorphic evidence, which plains, some exhibiting connecting spillways, that indicates that large standing bodies of water or ice were fed by a variety of channel sources peripheral sheets once occupied the northern lowland plains of to the plains. Delineation of these lakes was based on MarsŽ including paleoclimatic significance; e.g., Jons,¨ a similar assessment of the topography but also 1985, 1986, 1990; Lucchitta et al., 1986; McGill, included the local identification of shore morphol- 1985, 1986, McGill and Hills, 1992; Parker et al., ogy. 1987a,b, 1989, 1993; Baker et al., 1991; Scott et al., All the above studies are now being evaluated in 1991a,b; Rotto and Tanaka, 1991; Chapman, 1994; light of recent acquisition of high-resolution topogra- Kargel et al., 1995. . The details of the timing, em- phy afforded by the reflight of the Mars Observer placement mechanisms, and sizes of these water Laser AltimeterŽ. Zuber et al., 1992 in late 1996. For bodies differ markedly, however, from one group of example, estimates of basin volume in Parker et al. investigators to another. For example, Jons¨ Ž 1985, Ž.1993 are loosely based on the available topography 1986. envisioned a Amud oceanB covering much of with its very large vertical errors. These estimates, the northern plains, with sediment slurries derived when compared to estimates by others of the water from a variety of peripheral sources, including the discharged by the Chryse outflow channels, suggest fretted terrains and outflow channels. Lucchitta et al. the possibility that the volumes required to fill the Ž.1986 pictured an ice-covered ocean, fed by large basin may be at or beyond the high end of the circum-Chryse ice streams, analogous to those in estimated volumes available from the channels. High Antarctica. Parker et al.Ž. 1989, 1993 indicated two resolution topography is needed to sort out the com- or more highstands of a sea or ocean that, during the mon modifiers of shoreline elevation, such as tecton- early , would have been charged by catas- ism and isostatic reboundŽ with lower amplitudes trophic floods, but may have existed more or less than on Earth, perhaps. and sediment desiccation and permanently during Noachian and Hesperian time compaction; these modifiers likely altered the topog- Ž.Fig. 2 . Baker et al. Ž. 1991 pictured a plains-wide raphy of the northern plains after the putative surface ocean emplaced by the major outflow channels rela- water was lost, so that the original topography can be tively late in martian history during the early and reconstructed. middle Amazonian, and coined the term AOceanus Local elevations can be derived using the cur- BorealisB for this ocean. Similarly, the ice-sheets rently available high-resolution topographic tools. described by Kargel et al.Ž. 1995 and Chapman The precision of topographic measurements taken Ž.1994 are attributed to the early Amazonian. Inter- from stereo image pairs can be maximized by using estingly, the shorelines of Jons’¨ mud ocean, Luc- high-resolution stereo pairs and medium-resolution chitta et al.’s ice-covered ocean, and Parker et al.’s pairs with large parallax angles. Photoclinometry Ž.1993 most recent sea, or Ainterior plainsB, coincide provides the highest topographic measurement preci- almost precisely around the northern lowlands, sion currently available, but contains a number of though the details of the mechanisms by which potential sources of errorsŽ e.g., see discussion by boundary morphologies are thought to have been Jankowski and Squyres, 1991. . Photoclinometric produced differ. profiles should be corroborated, whenever possible, Taking a more conservative approach to the ques- with other measurements. The vertical precision af- tion of standing water in the northern plains, Rotto forded by photogrammetry is less than that provided and TanakaŽ. 1991 relied on volume estimates of by photoclinometry. Photogrammetrically derived el- maximum discharge from the circum-Chryse outflow evations, however, can be correlated regionally 320 T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 within a stereo scene, whereas photoclinometric ele- Until the current mission, vations cannot. The last, and simplest, topography elevations derived using the available high-resolution tool, shadow measurements, is most suited to steep topographic tools could not be accurately tied to the slopes with sharp breaks in slope and low sun eleva- global datumŽ. e.g., Zuber et al., 1992 . Nevertheless, tions. High-resolution images with low sun eleva- these tools are very useful for measuring the relief of tions are relatively uncommon, particularly from late local features that are distributed regionally. For in the Viking mission, but even medium- to low-res- example, massifs within the northern lowlands com- olution images can be quite suitable with a low sun monly exhibit a surrounding apron with a horizontal and modest relief. upper surface that may indicate the remnants of a

Fig. 11. Portion of Viking Orbiter image 070a02, showing massif with relatively steep slopes immediately above a surrounding flat-topped apron. Asymmetric photoclinometric profilesŽ. bottom are indicated on left image. These profiles, based on a Aflat fieldB brightness value, approximate the height and overall shape of the hill and surrounding apron. The apparent difference in height of the apron to either side is due to changes in albedo of the surface from one side to the other. The enlargement of the hillŽ. at right exhibits a faint dark layer Ž arrowed . below the top of the escarpment on its southwestern and northwesternŽ. sunlit sides. On the northwestern side, the layer dips toward the plains, paralleling the hill’s topographic surface, suggesting that the hill has been tilted toward the northeast. The horizontal surface of the apron surrounding the massif is cut at an angle to this layer. North is toward top of scene. T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 321 formerly continuous surface above the modern plains surements of the heights of the flat upper surfaces of Ž.Parker et al., 1987b, 1993 . These massifs appear the aprons can be used to provide local estimates of similar to a wave-eroded headland massif in Lake the depth of the paleolake or paleo-ocean. We pre- Bonneville, depicted in Fig. 3, and may indicate sent examples of measurements of this type for the coastal erosion of inselbergs within the plains. eastern AcidaliarCydonia Mensae region, for which If a coastal interpretation for these features is uniform high-resolution coverage exists from the correct, photoclinometric and photogrammetric mea- Viking Orbiters over a region of several tens of

Fig. 12. Massifs in the northeastern region exhibit a prominent bench at the outer margin of the surrounding apron, particularly in the upsun and downsun directionsŽ. where shading maximizes their detectability . Photoclinometric profiling has consistently revealed this bench to lie between 20 and 40 m below the top of the apron. Because this bench can be identified on massif aprons over many hundreds of kilometers, it may be useful as a AmarkerB bench. Image an asymmetric profile from portion of Viking Orbiter image 072a01. North is toward top of scene. 322 T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 scene. ll number of stereo easurements with other profiles compares the height Ž. Ž Ž. Ž . . Fig. 13. To insure that the measurements derived from the photoclinometric profiles are as accurate as possible, it is advisable to cross check those m pairs at high-resolution . Thedetermined most via reliable an check asymmetric of profile asymmetric photoclinometric left profiles are measurements and using a a symmetric symmetric profile profile. This right pair of . Both profiles returned a height of approximately 130 m. North is toward top of techniques. These checks can include shadow measurements of the massifs when possible , and stereo parallax measurements severely limited due to sma T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 323 thousands of square kilometers, and from northern lie between 20 and 40 m below the upper surface of , where high-resolution stereo images the apron around the associated massif. are available. Preliminary photoclinometric measure- In northern Acheron Fossae, stereo photogramme- ments place the upper surfaces of the massif aprons try was used to measure the height of a horizontal in northern Cydonia Mensae between 150 and 200 m bench in the rim of a large, degraded craterŽ. Fig. 14 . above the surrounding plainsŽ. Figs. 11–13 . Addi- The crater lies on the north-sloping margin of tional benches are also evident at the margins of Acheron Fossae, and is partially buried by northern many of the massif apronsŽ. Fig. 12 . These appear to plains materials on its lower, northwest sideŽ Fig.

Fig. 14. 35-km-diameter degraded crater at northern edge of Acheron Fossae. Northwest part of crater rim is partially buried by plains materials. Much of the rest of the crater appears subdued, except for a sharply defined portion of the east rim. This area is covered in stereo in the following figure. Viking Orbiter image 035b17. 324 T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328

Fig. 15. Stereo pair of east rim of crater in Fig. 14 with stereo parallax measurement points plotted. The prominent bench at east rim appears to have been cut horizontally into the northward tilted rim of the crater. ASoftenedB crater rim is below this bench, sharply defined rim is above. Profiles of the crater floorŽŽ..ŽŽ..ŽŽ.. points labeled p for plains , the crater rim points labeled c , and the bench points labeled b and a plateau of AperchedB chaotic terrain material, possibly a remnant of a former deposit within the craterŽŽ.. points labeled ch for Fig. 16 are indicated. Portions of Viking Orbiter images 112a32Ž. left and 130a21 Ž right . .

15. . The bench lies between 140 and 180 m above A lacustrinermarine origin of these features would the plains surfaceŽ. Fig. 16 . have far-reaching implications with regard to martian

Fig. 16. Stereo parallax measurement points are projected in the y-planeŽ. see Fig. 15 . Vertical precision, based on the 308 parallax and the image resolution of the stereo pair, is approximately 70 m. T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 325 surface volatile history and exobiology studies, par- or surface rovers capable of identifying ancient sedi- ticularly if long periods with surface water are indi- mentary deposits, will be required. cated. Mars is much less geologically active than Venus, so very ancient landforms that point to a warmer andror wetter paleoclimate are preserved to this day. Several investigators have suggested that Mars may 6. Conclusions have had lakes, ice sheets, and even oceans prior to about 2.5 Ga, and perhaps until as recently as several We have examined the current planetary datasets hundred million years ago. for evidence of extraterrestrial coastal landforms. In our review of the evidence for lakes or oceans The search was limited to solid surface planetary on Mars, we conclude the following: Mars lacks bodies that have, or may once have had, atmo- large, stable surface waterrice reservoirs todayŽ and spheres. This includes Venus, Mars, the moon of probably at least since middle Amazonian time. Saturn, Titan, the moon of Neptune, Triton, and mainly for these reasons:Ž. 1 volcanic and impact Pluto. production and recycling of declined In the outer solar system, compounds other than over time until it could no longer keep pace with loss water would be necessary to collect into standing to subsurface reservoirs through percolation, chemi- bodies at the surface, because of the very low tem- cal weathering, permafrostrpolar cap formation, and peratures far from the sun. Liquid nitrogen, which photodissociation and loss to space;Ž. 2 the total was once thought to be stable at the surface of water inventory remaining on the planet is less than Triton, is instead stable only in solid or gaseous the pore volume and water-volume-equivalent of form. Pluto is comparable in size and bulk composi- chemical weathering products in the megaregolith, tion to Triton, and is at a similar distance from the and the ice content of the polar caps;Ž. 3 the lack of sun, so environmental conditions are thought to be plate tectonism means that Mars has no mechanism similar. Titan, however, is now known to have a to ArejuvenateB large topographic depressions, so massive atmosphere. Several investigators are basins are likely to have become broader and shal- proposing that Titan may have lakes or oceans of lower with each lake occupation, eventually convert- liquid hydrocarbons. The Cassini Mission to Saturn, ing the lake basins to playa basins. due to reach the planet early next century, will image portions of Titan with a Synthetic Aperture Radar on board the Saturn Orbiter, and will deploy the Huy- Acknowledgements gens probe into the atmosphere of Titan. This probe includes a descent imager that will operate from The authors thank Jeffrey Kargel for a thorough when the parachute opens to the surface. The poten- and helpful review of this paper. This work was tial verification of abundant stable liquids at the supported initially by a grant from the NASA Mar- surface of Titan, either in the past or at present, is an tian Surface and Atmosphere Through Time program exciting prospect for future planetary geomorphic at the University of Southern California. Final fund- studies. ing was through the NASA Planetary Geology and Considering the planetary neighbors in the inner Geophysics program at the Jet Propulsion Labora- solar system, we have seen that Venus and Mars may tory. have had abundant liquid water on the surfaces at some time in the past. The Magellan Mission has revealed Venus to have experienced an episode of References global resurfacing, such that surfaces older than sev- eral hundred million years have not been preserved. A B Baker, V.R., Strom, R.G., Gulick, V.C., Kargel, J.S., Komatsu, Since a clement period on Venus, if it ever had G., Kale, V.S., 1991. Ancient oceans, ice sheets and the one, is thought to date back billions of years, further hydrological cycle on Mars. Nature, 352, 589–594. exploratory missions, such as aerial reconnaissance Baker, V.R., Komatsu, G., Parker, T.J., Gulick, V.C., Kargel, J.S., 326 T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328

Lewis, J.S., 1992. Channels and valleys on Venus: preliminary Forsythe, R.D., Zimbelman, J.R., 1995. A case for ancient evapor- analysis of Magellan Data. J. Geophys. Res., 97, 13421–13444. ite basins on Mars. J. Geophys. Res.,wx Planets 100, 5553–5563. Banerdt, W.B., Phillips, R.J., Sleep, N.H., Saunders, R.S., 1982. Gailitis, A., 1982. Tidal heating of Io and orbital evolution of the Thick shell tectonics on one-plate planets: applications to jovian satellites. Mon. Not. R. Astron. Soc., 201, 415–420. Mars. J. Geophys. Res., 87, 9723–9733. Goldspiel, J.M., Squyres, S.W., 1991. Ancient aqueous sedimenta- Breuer, D., Spohn, T., Wullner, U., 1993. Mantle differentiation tion on Mars. Icarus, 89, 392–410. and the crustal dichotomy of Mars. Planet. Space Sci., 41, Greeley, R., Williams, S.H., 1994. Dust deposits on Mars—the 269–283. parna analog. Icarus, 110, 165–177. Brown, R.H., Johnson, T.V., Goguen, J.D., Schubert, G., , Green, J., 1971. Copernicus as a lunar caldera. J. Geophys. Res., M.N., 1991. Triton’s global heat-budget. Science, 251, 1465– 76, 5331–5719. 1467. Grinspoon, D.H., 1993. Implications of the high DrH ratio for the Buie, M.W., Tholen, D.J., 1992. Albedo maps of Pluto and sources of water in Venus’ atmosphere. Nature, 363, 428–431. Charon—initial mutual event results. Icarus, 97, 211–227. Hartle, R.E., Donahue, T.M., Grebowsky, J.M., Mayr, H.G., Caldwell, J.S., 1978. Low surface pressure models for Titan’s 1996. and deuterium in the thermosphere of Venus- atmosphere. In: Hunten, D.M., Morrison, D.Ž. Eds. , The Sat- Solar-cycle. J. Geophys. Res.,wx Planets , 101, 4525–4538. urn System. pp. 113–126NASA Conf. Publ., DP-2068. Helfenstein, P., Veverka, J., McCarthy, D., Lee, P., Hillier, J., Carr, M.H., 1974. The role of lava erosion in the formation of 1992. Large quasi-circular features beneath frost on Triton. lunar rilles and martian channels. Icarus, 22, 1–23. Science, 255, 824–826. Carr, M.H., 1979. Formation of martian flood features by release Hunten, D.M., 1978. A Titan atmosphere with a surface tempera- of water from confined aquifers. J. Geophys. Res., 84, 2995– ture of 200 K. In: Hunten, D.M., Morrison, D.Ž. Eds. , The 3007. Saturn System. NASA Conf. Publ. CP-2068. Carr, M.H., 1986. Mars: a water-rich planet? Icarus, 68, 187–216. Hunten, D.M., 1992. The thermal structure of the atmosphere of Chapman, M.G., 1994. Evidence, age, and thickness of a frozen Titan. Proceedings Symposium on Titan. pp. 37–40Eur. Space paleolake in , Mars. Icarus, 109, 393–406. Agency Spec. Publ. SP-338. Clifford, S.M., 1981. A pore volume estimate of the martian Hutchinson, G.E., 1957. A Treatise on Limnology. Wiley, 1015 megaregolith based on a lunar analog. 3rd Int. Colloq. on pp. Mars. LPI Contribution, vol. 441,, pp. 46–48. Jankowski, D.G., Squyres, S.W., 1991. Sources of error in plane- Costard, F.M., Kargel, J.S., 1995. Outwash plains and thermokarst tary photoclinometry. J. Geophys. Res.,wx Planets , 96, 20907– on Mars. Icarus, 114, 93–112. 20922. Cruikshank, D.P., Brown, R.H., , R.N., 1984. Nitrogen on Jons,¨ H.-P., 1985. Late sedimentation and late sediments in the Triton. Icarus, 58, 293–305. northern lowlands on Mars. Lunar. Planet. Sci. Cong., vol. Cruikshank, D.P., Brown, R.H., Tokunaga, A.T., Smith, R.G., XVI, Lunar and Planet. Inst., Houston, TX, pp. 414–415. Piscetelli, J.R., 1988. Volatiles on Triton: the spectral Jons,¨ H.-P., 1986. Arcuate ground undulations, gelifluxion-like evidence, 2.0–2.5 mm. Icarus, 74, 413–423. features and Afront toriB in the northern lowlands on Mars— Currey, D.R., 1994a. Hemiarid lake basins: hydrographic patterns. what do they indicate? Lunar and Planet. Sci. Conf., vol. In: Abrahams, A.D., Parsons, A.J.Ž. Eds. , Geomorphology of XVII, Lunar and Planet. Inst., Houston, TX, pp. 404–405. Desert Environments. Chapman & Hall, London, pp. 405–421. Jons,¨ H.-P., 1990. Das relief des Mars: versuch einer zusammen- Currey, D.R., 1994b. Hemiarid lake basins: geomorphic patterns. fassenden ubersicht. Geol. Rundsch., 79, 131–164. In: Abrahams, A.D., Parsons, A.J.Ž. Eds. , Geomorphology of Kargel, J.S., Strom, R.G., 1992. Ancient glaciation on Mars. Desert Environments. Chapman & Hall, London, pp. 405–421. Geology, 20, 3–7. Cutts, J.A., Blasius, K.R., 1981. Origin of martian outflow chan- Kargel, J.S., Baker, V.R., Beget, J.E., Lockwood, J.F., Pewe, nels: the eolian hypothesis. J. Geophys. Res., 86, 5071–5102. T.L., Shaw, J.S., Strom, R.G., 1995. Evidence of ancient DeHon, R.A., 1992. Martian lake basins and lacustrine plains. continental glaciation in the martian northern plains. J. Geo- Earth, Moon, Planets, 56, 95–122. phys. Res.,wx Planets , 100, 5351–5368. DeHon, R.A., 1993. Duration and rates of discharge—, Kasting, J.F., 1989. How Venus lost its oceans. Oceanus, 32, Mars. J. Geophys. Res.,wx Planets , 98, 9129–9138. 54–57. DeHon, R.A., Pani, E.A., 1992. Flood surge through the Lunae Kaufmann III, W.J., 1978. Exploration of the Solar System. Planum outflow complex, Mars. Proceedings of Lunar and Macmillan, New York, 575 pp. Planet. Sci., vol. XXII, Lunar and Planet. Inst., Houston, TX, Komar, P.D., 1976. Beach Processes and Sedimentation. pp. 63–71. Prentice-Hall, New Jersey, 429 pp. Dermott, S.F., Sagan, C., 1995. Tidal effects of disconnected Komatsu, G., Geissler, P.E., Strom, R.G., Singer, R.B., 1993. hydrocarbon seas on Titan. Nature, 374, 238–240. Stratigraphy and erosional landforms of layered deposits in Donahue, T.M., 1995. Evolution of water reservoirs on Mars from Valles Marineris, Mars. J. Geophys. Res.,wx Planets , 98, drh ratios in the atmosphere. Nature, 374, 432–434. 11105–11121. Donahue, T.M., Hodges, R.R., 1992. Past and present water-budget Kuiper, G.P., 1944. Titan: a satellite with an atmosphere. Astro- of Venus. J. Geophys. Res.,wx Planets , 97, 6083–6091. phys. J., 100, 378–383. Flasar, F.M., 1983. Oceans on Titan? Science, 221, 55–57. Landheim, R., Greeley, R., Des Marais, D., Farmer, J.D., Klein, T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328 327

H., 1993. Mars exobiology landing sites for future exploration. Vallis, Mars? Lunar Planet. Sci., vol. XXII, Lunar and Planet. Lunar and Planet. Sci. Conf., vol. XXIV, Lunar and Planet. Inst., Houston, TX, pp. 1033–1034. Inst., Houston, TX, pp. 845–846. Parker, T.J., Gorsline, D.S., 1993. Extent and timing of fluvial and Lebretson, J.P., Matson, D.L., 1992. An overview of the Cassini lacustrine events in , Mars. Am. Geophys. Mission. Geophys. Space Phys., 15, 1137–1147. Union, Spring Meet., 1 p. Lee, P., Helfenstein, P., Veverka, J., 1991. Search for glazed Parker, T.J., Schneeberger, D.M., Pieri, D.C., Saunders, R.S., surfaces on Triton. J. Geophys. Res.,wx Planets , 96, 9231–9239. 1987a. Geomorphic evidence for ancient seas on Mars. Sym- Lucchitta, B.K., Ferguson, H.M., Summers, C., 1986. Sedimen- posium on Mars: evolution of its Climate and Atmosphere, tary deposits in the northern lowland plains. Mars. J. Geophys. LPI Tech. Rept. 87-01. Lunar and Planet. Inst., Houston, TX, Res., 91, E166–E174 suppl. pp. 96–98. Lunine, J.I., 1993. Does Titan have an ocean? A review of current Parker, T.J., Schneeberger, D.M., Pieri, D.C., Saunders, R.S., understanding of Titan’s surface. Rev. Geophys., 31, 133–149. 1987b. Curvilinear ridges and related features in southwest Lunine, J.I., Stevenson, D.J., Yung, U.L., 1983. Ethane ocean on Cydonia Mensae, Mars. Rept. Planet. Geol. Prog-1986, NASA Titan. Science, 222, 1229–1230. Tech. Memo. 89810. pp. 502–504. MacKinnon, D.J., Tanaka, K.L., 1989. The impacted martian Parker, T.J., Saunders, R.S., Schneeberger, D.M., 1989. Transi- crust: structure, , and some geologic implications. J. tional morphology in the west region of Geophys. Res., 94, 17359–17370. Mars: implications for modification of the lowlandrupland Mars Channel Working Group, 1983. Channels and valleys on boundary. Icarus, 82, 111–145. Mars. Geol. Soc. Am. Bull., 94, 1035–1054. Parker, T.J., Gorsline, D.S., Saunders, R.S., Pieri, D.C., Schnee- Matsui, T., Abe, Y., 1986. Impact-induced atmospheres and oceans berger, D.M., 1993. Coastal Geomorphology of the Martian on Earth and Venus. Nature, 322, 526–528. Northern Plains. J. Geophys. Res., 98,Ž. E6 , 11061–11078. McGill, G.E., 1985. Age and origin of large martian polygons. Phillips, R.J., Raubertas, R.F., Arvidson, R.E., Sarkar, I.C., Her- Lunar and Planet. Sci. Conf., vol. XVI, Lunar and Planet. rick, R.R., Izenberg, N., Grimm, R.E., 1992. Impact craters Inst., Houston, TX, pp. 534–535. and Venus resurfacing history. J. Geophys. Res.,wx Planets , 97, McGill, G.E., 1986. The giant polygons of Utopia, northern 15923–15948. martian plains. Geophys. Res. Lett., 13, 705–708. Plescia, J.B., 1991. Graben and extension in northern Tharsis, McGill, G.E., Dimitriou, A.M., 1990. Origin of the martian global Mars. J. Geophys. Res., 96, 8883–8895. dichotomy by crustal thinning in the late Noachian or Early Rossbacher, L.A., 1985. Ground ice models for the distribution Hesperian. J. Geophys. Res., 95, 12595–12605. and evolution of curvilinear landforms on Mars. In: Wolden- McGill, G.E., Hills, L.S., 1992. Origin of giant martian polygons. berg, M.J.Ž. Ed. , Models in Geomorphology. Allen & Unwin, J. Geophys. Res.,wx Planets , 97, 2633–2647. Winchester, MA, pp. 343–372. McGill, G.E., Squyres, S.W., 1991. Origin of the martian crustal Rotto, S.L., Tanaka, K.L., 1991. Chryse Planitia region, Mars: dichotomy: evaluating hypotheses. Icarus, 93, 386–393. Channeling history, flood volume estimates, and scenarios for Moore, J.M., Edgett, K.S., 1993. , Mars: site of net bodies of water in the northern plains. Workshop on the dust erosion and implications for the nature of basin floor Martian Surface and Atmosphere Through Time. Lunar and deposits. Geophys. Res. Lett., 20, 1599–1602. Planet. Inst., Houston, TX, pp. 111–112. Muhleman, D.O., Grossman, A.W., Butler, B.J., Slade, M.A., Schaber, G.G., Strom, R.G., Moore, H.J., Soderblom, L.A., Kirk, 1990. Radar reflectivity of Titan. Science, 248, 975–980. R.L., Chadwick, D.J., Dawson, D.D., Gaddis, L.R., Boyce, Nedell, S.S., Squyres, S.W., Andersen, D.W., 1987. Origin and J.M. et al., 1992. Geology and distribution of impact craters evolution of the layered deposits in the Valles Marineris, on Venus: what are they telling us? J. Geophys. Res.,wx Planets , Mars. Icarus, 70, 409–441. 97, 13257–13301. Parker, T.J., 1985. Geomorphology and geology of the southwest- Schneeberger, D.M., 1989. Episodic channel activity at Ma’adim ern Margaritifer Sinus—northern Argyre region of Mars. Mas- Vallis, Mars. Lunar and Planet. Sci., vol. XX, Lunar and ter’s thesis, Geology Department, California State University, Planet. Inst., Houston, TX, pp. 964–965. Los Angeles, 165 pp. Scott, D.H., Chapman, M.G., Rice Jr., J.W., Dohm, J.M., 1991a. Parker, T.J., 1989. Channels and valley networks associated with New evidence of lacustrine basins on Mars: Amazonis and Argyre Planitia, Mars. Lunar Planet. Sci., vol. XX, Lunar and Utopia Planitiae. Proceedings of Lunar and Planet. Sci., vol. Planet. Inst., Houston, TX, pp. 826–827. 22, Lunar and Planet. Inst., Houston, TX, pp. 53–62. Parker, T.J., 1994. Martian paleolakes and oceans. PhD disserta- Scott, D.H., Rice Jr., J.W., Dohm, J.M., 1991b. Martian paleo- tion, Geological Sciences, University of Southern California, lakes and waterways: exobiological implications. Origins Life 200 pp. Evol. Biosphere, 21, 189–198. Parker, T.J., Gorsline, D.S., 1991a. Formation of , Sears, W.D., 1995. Tidal dissipation in oceans on Titan. Icarus, Mars, through catastrophic drainage of a large surface lake. 113, 39–56. Lunar and Planet. Sci., vol. XXII, Lunar and Planet. Inst., Sharp, R.P., 1973. Mars: fretted and chaotic terrains. J. Geophys. Houston, TX, pp. 1031–1032. Res., 78, 4073–4083. Parker, T.J., Gorsline, D.S., 1991b. Where is the source for Uzboi Smith, P.H., Lemmon, M.T., Lorenz, R.D., Sromovsky, L.A., 328 T.J. Parker, D.R. CurreyrGeomorphology 37() 2001 303–328

Caldwell, J.J., Allison, M.D., 1996. Titan’s surface, revealed U.S. Geological Survey, 1991. Shade relief and albedo mercator by H.S.T. imaging. Icarus, 119, 336–349. projection of Mars, scale 1:25,000,000. U.S. Geol. Surv. Misc. Sohl, F., Sears, W.D., Lorenz, R.D., 1995. Tidal dissipation on Inv. Series Map I-2179. Titan. Icarus, 115, 278–294. Watters, T.R., 1993. Compressional tectonism on Mars. J. Geo- Staehle, R.L., Abraham, D.S., Carraway, J.B., Esposito, P.J., phys. Res., 98, 17049–17060. Salvo, C.G., Terrile, R.J., , R.A., Weinstein, S.S., Wilhelms, D.E., 1987. Geologic History of the Moon. U.S. Geol. 1993. Exploration of Pluto. Acta Astronaut., 30, 289–310. Surv. Prof. Pap. 1348, 302 pp. Tanaka, K.L., 1986. The stratigraphy of Mars. J. Geophys. Res., Wright, L.D., Chappell, J., Thom, B.G., Bradshaw, M.P., Cowell, 91, 139–158. P., 1979. Morphodynamics of reflective and dissipative beach Tanaka, K.L., Golombek, M.P., Banerdt, W.B., 1991. Reconcilia- and inshore systems: Southeastern Australia. Mar. Geol., 32, tion of stress and structural histories of the Tharsis region of 105–140. Mars. J. Geophys. Res., 96, 5617–5633. Zuber, M.T., Smith, D.E., Solomon, S.C., Muhleman, D.O., Head, Trenhaile, A.S., 1987. The Geomorphology of Rock Coasts. J.W., Garvin, J.B., Abshire, J.B., Bufton, J.L., 1992. The Mars Clarendon Press, Oxford, 384 pp. observer laser altimeter investigation. J. Geophys. Res., 97, U.S. Geological Survey, 1989. Topographic maps of the western 7781–7797. equatorial, eastern equatorial, and polar region of Mars, scale 1:15,000,000. U.S. Geol. Surv. Misc. Inv. Series Map I-2030.