Palaeoflood-Generating Mechanisms on Earth, Mars, and Titan

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Global and Planetary Change 70 (2010) 5–13

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Global and Planetary Change

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Palaeoﬂood-generating mechanisms on Earth, Mars, and Titan

Devon M. Burr a Earth and Planetary Sciences Department and Planetary Geosciences Institute, University of Tennessee Knoxville, 1412 Circle Dr., TN 37996-1410, USA b Carl Sagan Center, SETI Institute, 515 N. Whisman Rd., Mountain View, CA 94043, USA article info abstract

Article history: Channelized, surface, liquid flow occurs on Earth, Mars, and Titan. In this paper, such fluvial flow is Accepted 6 November 2009 constrained to involve volatiles that can undergo phase transitions under surface conditions and can be Available online 15 November 2009 stable as liquids over geologically significant periods of time. Evidence for channelized, surface, liquid flood flow has been observed on Earth and Mars and hypothesized for Titan. The mechanisms for generating flood Keywords: flow vary for each body. On Earth, the mechanism that generated the largest flooding is widespread palaeofloods fl glaciation (e.g. the catastrophic release of water from glacial lakes), which requires an atmospheric cycle of a uid dynamics fl terrestrial comparison volatile that can assume the solid phase. Volcanism is also a prevalent cause for terrestrial mega ooding, fl fl Mars with other mechanisms producing smaller, though more frequent, oods. On Mars, the mechanism for ood Titan satellite generation has changed over the history of the planet. Surface storage of floodwater early in the history of volatiles that planet gave way to subsurface storage as the climate cooled. As on Earth, flooding on Mars is an effect of the ability of the operative volatile to assume the solid phase, although on Mars, this solid phase occurred in the subsurface. According to this paradigm, conditions on Titan preclude extensive megaflooding because the operative volatile, which is methane, cannot as easily assume the solid phase under current conditions. Mechanisms that produce smaller but more frequent floods on Earth, namely extreme precipitation events, are likely to be the most important flood generators on Titan in the recent past. © 2009 Elsevier B.V. All rights reserved.

1. Introduction During this time, the study of palaeofloods was also extended to Mars. The Mariner 9 and Viking orbiter missions first observed Mars' The scientific study of palaeoflooding has risen and fallen largely in gigantic flood channels, identified on the basis of their morphological concert with prevailing scientific paradigms (Baker, 1998). The similarity to the Channeled Scabland (Mars Channel Working Group, paradigm in the seventeenth and eighteenth centuries was catastro- 1983). Future analysis and spacecraft missions to Mars supported phism, the idea that the geological record and landscape change is the continued study of these and other flood channels (Baker, 1982; Carr, product of sudden, short, violent events. In palaeoflood science this 1996), providing quantitative estimates of discharge and other flow paradigm was expressed as diluvialism — the view that the Biblical parameters, although with large error bars (Wilson et al., 2004). As a flood was historically accurate and could explain various geological result, aqueous catastrophic flooding was established as an extrater- formations and fossil remains. Catastrophism gave way to uniformi- restrial phenomenon. tarianism, first introduced in 1830, which stipulated that the Earth's The Cassini–Huygens mission to the Saturnian system (Matson et al., formations are the product of slow, gradual change. Under this 2002) opened up a new frontier in planetary geology. Mission data paradigm, geological formations previously explained by catastrophic revealed the surface of Titan, Saturn's largest moon, as having strikingly (Biblical) flooding were often transmuted into the effects of sustained Earth-like landforms – including aeolian dunes (Lorenz et al., 2006)and river flow or glaciation. The Channeled Scabland in Washington, USA, lakes (Stofan et al., 2007) – although composed of exotic materials that was originally interpreted at glacial in origin, either directly (through are uncommon on Earth. In particular, the volatile cycle on Titan flow of glacial ice) or indirectly (through diversion of rivers). involves hydrocarbons, namely methane and the nitrogen dissolved in Consequently, the proposal by J Harlan Bretz of massive flooding as it. Fluviatile features have been interpreted as river channels (Elachi an explanation for the Channeled Scabland was originally treated as et al., 2005; Porco et al., 2005; Tomasko et al., 2005), which may have an absurd or outrageous hypothesis (cf. Davis, 1926; Baker, 1978). discharges of the order of 104 m3 s−1 (Jaumann et al., 2007). Though Decades of study demonstrated the verity of this hypothesis, much smaller than the megaflood discharges on Mars or Earth, such establishing palaeoflood hydrology and geomorphology as a sub- discharges would be large for terrestrial rivers. discipline of geology (e.g. Kochel and Baker, 1982). This paper compares and contrasts evidence for palaeofloods on these three planetary bodies. In this work, the operative liquid is constrained to be a volatile that can undergo phase change at surface E-mail address: [email protected]. conditions. This constraint derives from using terrestrial aqueous

flooding as the template. On Earth, flooding results from the rapid (or moraine basin) lakes usually store less water than proglacial lakes, release of liquid water from atmospheric, surficial, and underground and so generally produce relatively smaller discharges (O'Connor and reservoirs (O'Connor and Costa, 2004). The conditions for such release Beebee, 2009). The most extensive known examples of glacial lake are created by atmospheric cycling of this volatile and its ability to outburst flooding are located in the northern mid-latitudes, where undergo phase transitions among solid, liquid, and gas phases at continental ice sheets disrupted pre-existing fluvial flow (e.g., surface temperatures and pressures. Similar conditions have occurred Grosswald, 1998). The most voluminous examples (in terms of on Mars, where water currently exists as a gas in the atmosphere and discharge) have been found in areas of significant relief where deep as a solid on and near the surface. Ancient valley networks indicate valleys supported tall ice dams that impounded large lakes. The largest that water has existed as a liquid on the surface in the past (Pieri, ice-dam failure floods on Earth include: the Kuray and other floods in 1976, 1980; Carr, 1996; Irwin and Howard, 2002; Howard et al., the Altai Mountains, Siberia, during the Late Pleistocene (Baker et al., 2005). The interpretation of gullies on Mars as aqueous flow features 1993; Rudoy, 1998; Carling et al., 2002; Herget, 2005); the Missoula (Malin and Edgett, 2000; McEwen et al., 2007) also implies that liquid floods that carved the Channeled Scabland in Washington, USA (Baker, water may exist on the surface today, if only temporarily and/or in 2009, and references therein); and early Holocene floods down the isolation. At Titan temperature (∼94 K), water occurs only as a solid, Tsangpo River gorge in southeastern Tibet (Montgomery et al., 2004). except in limited examples of cryovolcanism (Lopes et al., 2007). By The largest ice-marginal or moraine dam failure floods include the analogy with flooding on Earth and Mars, flooding on Titan would Lake Agassiz overflow floods in North America (Teller et al. 2002; involve liquid methane–nitrogen. As on Earth, nitrogen comprises Kehew et al., 2009, and references therein). It has been argued that most (94%) of Titan's atmosphere; methane is the next largest fraction freshwater outbursts to the Atlantic from Lake Agassiz may have been (5%) (Niemann et al., 2005). Methane undergoes phase changes an important driver of the environmental changes that led to the between liquid and gaseous phases at Titan surface pressure and Younger Dryas event and other periods of cooling in the Northern temperature, and gaseous nitrogen dissolves in the liquid methane. In Hemisphere during the last deglaciation (Teller et al. 2002). addition to the gaseous and liquid phases, methane–nitrogen may even occur or have occurred in solid form on the surface of Titan, 2.2. Volcanism although this evidence is currently limited (Robshaw et al., 2008). Floods of geological materials that undergo only solid–liquid transi- In addition to directly producing palaeofloods on Earth, glaciation tions (e.g. rocks melting to produce flood lavas) are outside the scope contributes to flood production indirectly in conjunction with near- of this work. surface volcanism or enhanced geothermal heat flow (Björnsson, 2009). Some of the materials and conditions on these three bodies are Some of the largest floods associated with volcanism result from direct listed in Table 1. These different materials and conditions cause each melting of overlying snow and ice by eruption or geothermal heat flow. body to have its own mechanisms for generating floods. This paper Persistent sub-glacial hydrothermal systems may result from the compares and contrasts flooding on these three bodies by reviewing interaction of the resultant glacial meltwater with shallow magmatic the flood morphologies and the flood generation mechanisms by intrusions. Continued increase of the sub-glacial lake volume eventually discharge for each body. breaks the hydraulic seal of the overlying glacier, and the meltwater is catastrophically released. This process takes place today in Iceland, 2. Palaeoflooding on Earth as in the 1996 sub-glacial eruption and resultant jökulhlaup from the Grímsvötn cauldron beneath the Vatnajökull glacier (Fig. 1) 2.1. Glaciation (Guðmundsson et al., 1997) and the 1918 jökulhlaup from beneath the Kötlujökull glacier (Tómasson, 1996). Early Holocene jökulhlaups The largest palaeofloods on Earth, with discharges of the order of with discharges of the order of 105 m3 s−1 drained northward from the 106–107 m3 s−1, are associated with glaciation (Baker 1996, O'Connor Kverkfjöll glacier (Carrivick, 2009 and references therein), carving the and Costa 2004). The primary mechanism that produced these Jökulsá á Fjöllum flood channel (Waitt, 2002). ‘megafloods’ (cf. Baker, 2009) was the sudden release of water from Volcanism also causes flooding by other means than direct melting. glacial lakes. Proglacial lakes result from damming by the glacier of Caldera-lake breaching can produce discharges of similar magnitude pre-existing river valleys. These glacial lakes and their outbursts to ice-dam breaches, as in the case of a late Holocene flood from the occurred frequently during the Quaternary glaciations as a result of the Aniakchak volcano, Alaska (Waythomas et al., 1996). More commonly, disruption of pre-existing fluvial drainage (Starkel, 1995). Ice- breaching of water-filled calderas produces floods one to two orders of marginal lakes result from impoundment of the glacial meltwater magnitude lower. The near instantaneous failure of a large ignimbrite against the glacier margin, commonly by moraine debris. Ice-marginal dam at Lake Taupo, New Zealand, for example, produced a caldera flood of the order of 105 m3 s− 1 (Manville et al., 1999). Other examples of large floods from breached volcaniclastic dams include floods from Table 1 Tarawera caldera, New Zealand (Manville et al., 2007) and the Key parameters of relevance for flood generation on Earth, Mars, and Titan. Holocene flood from Mount Mazama, USA (Conaway, 1999, in Parameter Earth Mars Titan O'Connor and Beebee, 2009). Large floods resulted from Pleistocene Surface gravity (m/s2) 9.81 3.71 1.35 lava dams on the Colorado River, USA (Fenton et al., 2006), and Surface temperature 287 210 94 pyroclastic flows, lahar deposits, and landslides triggered by eruptions (K) have likewise caused flooding. Surface pressure 1.01 0.007 1.44 (bar) 2.3. Tectonic basin overflow Fluvial liquid, density Water, 1000, Water, 1000, CH4/N2, 450, (kg/m3), viscosity 1×10− 3 Pa s 1×10− 3 Pa s 2×10− 4 Pa s (Pa s) Volcanic calderas are one of three types of geological basins that Fluvial sediment, Quartz, 2650 Basalt, 2900 Water ice, 992 produce flooding, the other two being moraine basins (reviewed in the density (kg/m3) Organics, 1500 section on glacial flooding above) and tectonic basins. Floods from Atmospheric ∼79% N2, 20% O2 ∼95% CO2, 2.7% N2 ∼95%N2, ∼5%CH4 composition tectonic basins can be volumetrically the largest of the three basin- Surface area (km2) 510×106 km2 145×106 km2 83×106 km2 breach type floods because they can impound the most water (see Titan fluid density and viscosity values from Lorenz et al. (2003); organic density from O'Connor and Costa, 2004 and O'Connor and Beebee, 2009, from which Khare et al. (1994), but lower values are possible. the following discussion is taken). D.M. Burr / Global and Planetary Change 70 (2010) 5–13 7

Fig. 1. Oblique aerial photo of the November 1996 jökulhlaup at Skeiðarársandur, Iceland. Flow from Vatnajökull glacier is toward the viewer; the two-lane bridge in the centre of the photograph provides scale. (Photo by Magnus Tumi Guðmundsson and Finnur Paulson).

Most commonly, floods from closed tectonic basins are a result of (O'Connor and Costa, 2004). Landslide dams can form in a wide the basin becoming filled with water due to a sustained positive water variety of settings, and are most commonly triggered by lubrication balance. An example of this process took place during the Quaternary due to water (precipitation or snowmelt) or by earthquakes. As with in the basin-and-range province, western USA, where natural tectonic glacial ice or moraine dams, the potential discharge increases with basin filling coincided with glacial advance (Baker, 1983; Reheis et al., increasing dam height (O'Connor and Beebee, 2009). 2002). Lake Bonneville (Malde, 1968; O'Connor, 1993) and other Ice jam floods account for three of Earth's largest known floods Pliocene–Pleistocene lakes in the western USA (see Currey, 1990; (O'Connor and Costa, 2004), all of which occurred on the Lena River, Reheis et al., 2002) provide examples of tectonic basin-breach floods Russia (Rodier and Roche, 1984). Ice jam flooding occurs when broken fed by combined rainfall and groundwater flow. river ice accumulates at constrictions or bends in the river, causing More rarely, floods may be produced from tectonic basins by a ponding upstream of the constriction and sudden release when the ice sudden input of water; this has been postulated for the lower jam breaks. Large, northward-flooding Arctic rivers are thus most Colorado River, which may have connected to the upper Colorado susceptible to ice jam flooding because the southern reaches are more drainage about 5 Ma ago due to a series of upstream basin breaches likely to thaw and flow before the northern reaches. involving large floods (House et al., 2005). Catastrophic breaching of these basins may have carved Hells Canyon and the western Snake 2.5. Precipitation River gorge, leading to integration with the Columbia River (Othberg, 1994; Wood and Clemens, 2002). Precipitation-fed floods, along with ice jam floods, fall within the The geological record on Earth also records immense marine range of Earth's smallest floods. These ‘meteorological floods’ floods, both into and out of tectonic basins, and these episodes have (O'Connor and Costa, 2004) result from extreme precipitation events generated much debate. Flooding of the Mediterranean Basin via a such as those associated with monsoon rainfall. Thus, the largest such breach at the present-day Straits of Gibraltar in the Late Miocene floods occur in tropical climates dominated by monsoons or tropical (Hsü, 1983) represents an example of marine flooding of a semi- storms. Flood size is also correlated with river basin size, because the isolated basin. A smaller flood may have occurred when the rising sea largest river basins receive the largest total amount of rain. The level led to the flooding of the Black Sea from the Mediterranean via Amazon River in South American, which drains the world's largest the Bosporus Strait (Ryan and Pitman, 1998, Ryan 2007). basin with an area of 7 million km2, has produced three of the four A common geological setting for these tectonic basin-breach floods largest recorded precipitation-fed floods, and the Yangtze River of is a semi-arid environment during changing hydrological conditions China has produced the fourth. (O'Connor and Beebee, 2009). The negative water balance in semi- In summary, the largest floods on Earth result (indirectly) from arid climates minimizes basin integration, allowing large volumes of glaciation and related processes (such as ice margin retreat past water to accumulate in isolated basins over geological time. The critical topographic thresholds), with smaller discharges contributed changing hydrological conditions provide the input to eventually by volcanism in regions with large glaciers or small ice sheets, and trigger the flooding. Freshwater floods are associated with pluvial much smaller floods from ice jams on polar rivers. Thus, for these periods during glacial advance, and marine floods probably resulted mechanisms, the planetary body needs a volatile that is close to its from sea level rise during deglaciation. freezing point. Tectonism in semi-arid climates can also produce significant floods through basin breaching. 2.4. Other natural dam failures: landslides and ice jams 3. Palaeoflooding on Mars Two other types of dam breaches cause catastrophic flooding. Landslides that were not directly related either to glacial or volcanic The causes of flooding on Mars are less well known than for conditions caused two of the 27 largest documented terrestrial floods flooding on Earth because in situ data are not yet available from 8 D.M. Burr / Global and Planetary Change 70 (2010) 5–13

Martian flood channels and because Martian flooding has not been observed. Thus, unlike for Earth, flooding on Mars cannot be analyzed solely on the basis of flood-producing mechanisms. However, Martian channels have distinctive morphologies that may be grouped into three categories which roughly correspond with Mars' three chron- ostratigraphic epochs. On the basis of these morphologies, flood mechanisms can be discussed.

3.1. Noachian ﬂooding from large basins

Mars' oldest channels date to the late Noachian epoch (N3.7 Ga; dates of Martian epochs are based on Tanaka, 1986, and Hartmann and Neukum, 2001), a time of inferred warmer and wetter climate than at present (Carr, 1996). These channels originate at large basins formed either by impact cratering or by intercrater plains topography (Irwin and Grant, 2009). Several such basin overflow channels have been inferred for Mars (see Irwin and Grant, 2009, Table 1), of which the three most distinct are discussed here. Mars' longest basin overflow channel is the segmented meso- outflow Uzboi–Ladon–Margaritifer (ULM) channel system (Saunders, 1979; Baker, 1982). The ULM channel originates full width at the northern edge of the large Argyre impact crater basin, from where it extends over several hundred kilometres northward to the Margaritifer topographic basin. Argyre itself was fed through several other large valley systems (Parker et al., 1989, 1993), so that the total contributing area to the ULM system was about 107 km2,orabout9%ofMars(Phillips et al., 2001). Subsequent cratering of proximal ULM has obscured the immediate outlet area, and thus the water release mechanism. However, deeply incised reaches, nearly constant channel widths, hanging valleys, small streamlined forms, and a poorly developed tributary network all suggest formation by flooding. Discharge estimates vary from 150,000 m3 s−1 to 450,000 m3 s−1 (see Irwin and Grant, 2009,and references therein). Ma'adim Vallis, like the ULM system, has a wide, deep main channel and originates full width at the northern side of an enclosed basin. This source basin is a product of intercrater plains topography, with multiple sub-basins, and covers an area of approximately 106 km2 (Irwin et al., 2002, 2004). Flooding in Ma'adim Vallis is suggested by the nearly constant width of the channel, the adjacent hanging valleys, and the longitudinal ridges on the channel floor. The calculated discharge is consistent with a dam breach of ∼100 m depth, which is less than half of the total observed breach depth. An adequate Fig. 2. Hesperian-aged circum-Chryse channels. (A) Topography showing the western- most circum-Chryse channel and associated features. The black box shows the location water volume could have been stored in the present-day source basin of Fig. 2B. (Image credit: MOLA/Google Mars/ASU.). (B) THEMIS day infrared image to carve Ma'adim Vallis with reasonable water:sediment ratios (see mosaic showing an example of chaos terrain located in Eos Chasma (image centre). Irwin and Grant, 2009, and references therein). (Image credit: THEMIS/ASU). Mawrth Valles is the smallest of these late Noachian flood channels. It appears to have originated at the Margaritifer basin, an intercrater basin that would have received discharge from the ULM channel system (Irwin and Grant, 2009). However, with current These Hesperian-aged channels may be grouped according to their topography, overflow from Margaritifer would first spill over at lower size and morphology at origin (Coleman and Baker, 2009). The largest, points to the east rather than entering the head of Mawrth Valles. circum-Chryse channels generally originate at chasmata. Chasmata Possible explanations for this geological conundrum include tilting are deep, steep-sided depressions, which commonly contain chaos, due to growth of the nearby Tharsis volcanic edifice and/or the influx areas of kilometre-sized or larger blocks surrounded by flatter, lower of ULM floodwaters as discussed above (Irwin and Grant, 2009). elevation terrain (Fig. 2B) (see http://planetarynames.wr.usgs.gov/ jsp/append5.jsp for definitions of descriptor terms). Chasmata and 3.2. Hesperian flooding from chaos and chasmata their inset chaos are attributed to surface collapse as a result of subsurface volatile (water) outburst (e.g., Carr, 1996, and references Mars' largest flood channels are enormous in size, with widths up to therein; Rodriguez et al., 2006). The cause of the outbursting probably several tens of kilometres, lengths of hundreds of kilometres, and depths involved pressurisation of groundwater by a downward growing of over a thousand metres (Fig. 2A) (Smith et al., 1998). These channels cryosphere (Clifford and Parker, 2001). Cryosphere disruption could debouch into Chryse Planitia, located northeast of the Valles Marineris have been caused by magmatic intrusions (Leask et al., 2006), canyon system, and are referred to collectively as the circum-Chryse cryosphere melting (McKenzie and Nimmo, 1999), and/or by channels. Smaller Hesperian-aged channels are also found at the eastern dissociation of CO2 ice or clathrate (Komatsu et al., 2000). In the end of Valles Marineris, south of Chryse Planitia. All these channels date few instances of chaos on channel floors, outbursting may have been to the Hesperian epoch (3.7–3.0 Ga), an era of changing surface triggered by flood erosion that reduced the overburden pressure conditions from a warm, wet climate to a cold, dry environment. (Coleman, 2005). D.M. Burr / Global and Planetary Change 70 (2010) 5–13 9

Following outbreak of water from a pressurised aquifer, lakes apparently formed in these chasmata and then overflowed, as indicated by topographic relationships among inferred spillways, scabland, or other flood-formed features (see Coleman and Baker, 2009, and references therein). The interpretation of layered deposits within the chasmata as lacustrine sediments (Lucchitta et al., 1992) was an early indication of this possibility. The overflow flooding may have been triggered by failure of debris-barriers or ice-barriers between chasmata, resulting in sudden influxes of water from other chasmata-lakes (Coleman and Baker, 2009). However, other suggested mechanisms for flooding, such as catastrophic release from extensive caverns (Rodriguez et al., 2005), do not require surface storage of water. Although smaller, the Hesperian-aged channels east of Valles Marineris may have had a similar flood-generating mechanism. These channels lack chasmata at their origins and so are unlikely to have had lakes or other surface water sources. However, various surface features suggest lateral subsurface flow routes from a distal chasmata to channel origins, perhaps augmented by ground ice melting (Carr, 1996; Cabrol et al., 1997; Rodriguez et al., 2003). The relative elevations of these channels in comparison to the massive Tharsis volcanic edifice would have allowed magmatic recycling of groundwater (cf. Gulick, 1998) to have fed these inferred palaeolakes (Coleman et al., 2007). Thus, the source for Mars' Hesperian-aged flood channels was a mixture of ponded surface water and pressurised subsurface water. Contributions from both these sources may have given rise to these largest of Martian floods.

3.3. Amazonian flooding from extensional fossae Fig. 3. Topography of Mangala Valles. Channel originates at the notch in the northern side fi Four of Mars' flood channels are dated to the Amazonian epoch (3.0 Ga of the ssure (black box) and stretches northward (arrows: note streamlined form in image centre), before diverging into two channels. Image credit: MOLA/Google Mars/ASU. to present), with model ages ranging from earliest Amazonian to the last few Ma. Width and depth dimensions are roughly an order of magnitude smaller than those of Mars' Hesperian-aged, circum-Chryse channels Thus, the variety of fossae morphologies at the outflow channels' (Burr et al., 2009). These channels originate at fossae (e.g, Fig. 3), defined origins may be interpreted as resulting from dyke intrusion possibly as long, narrow depressions (see http://planetarynames.wr.usgs.gov/jsp/ combined with melting of ice-rich rock and as amagmatic tectonism. append5.jsp).Thefossaehavevariedmorphologies, suggesting multiple Given this variety of morphologies, it is likely that multiple floodwater possible mechanisms for floodwater release. Some of the fossae have mechanisms gave rise to the most recent big floods on Mars. graben morphology, which is interpreted to reflect the presence of In summary, Noachian floods on Mars were generated by overflow subsurface dykes (Wilson and Head, 2002; Head et al., 2003a; Ghatan from impact crater or intercrater topographic basins, whose source is et al., 2005; Leask et al., 2006). In these cases, the floodwater release commonly held to be precipitation or shallow groundwater. These mechanism is inferred to have been dyke-induced cracking of a confining floods were the lowest in discharge and apparently the fewest in cryosphere and release of pressurised groundwater. The westward number, although further improvements in data may modify that migration through time of both volcanic and aqueous flooding from the perception (e.g., Mangold et al., 2008). Flooding during the Hesperian Cerberus Fossae is consistent with a volcanic triggering mechanism (see was the most voluminous and it is likely that this flooding involved Burr et al., 2009, and references therein). both surface and deep subsurface water sources. This confluence of Alternatively or additionally, fossae may be the result of amag- two different flood-water sources may explain the maximal dis- matic extensional tectonics. Based on inferred hydrological properties charges during this time. Amazonian-aged flooding from fossae was of the Martian crust (Hanna and Phillips, 2005), modelling shows that fed by deep subsurface sources. Changes in megaflooding mechanisms extensional stress relief can produce sufficient excess pore pressures with time on Mars are summarised in Fig. 4. to drive the catastrophic release of groundwater (Hanna and Phillips, To compare the two planets, Mars' Noachian basin overflow 2006). This mechanism may have acted in concert with other factors, flooding, as well as putative chasmata-lake overflows, is similar to such as increased elevational head in the form of perched aquifers basin-breach flooding on Earth. As on Earth, this mechanism requires (cf. Tanaka and Chapman, 1990) or aquifer pressurisation due to a climate that generates precipitation. Hypothesized mechanisms that cryosphere growth (Clifford and Parker, 2001), but does not require invoke a pressurised aquifer are dissimilar to any terrestrial mechan- them. isms. However, triggering due to volcano-ice interactions is similar in Ice-rich ground may have been melted (Zimbelman et al., 1992), concept to terrestrial floods produced by volcanic events, although perhaps following dyke intrusion (cf. Head et al., 2003a), contributing different in context (i.e., subsurface versus surface). water and sediment to the flood discharge. However, melting by dyke intrusion alone is not sufficient to release floodwater at the volumetric 4. Palaeoflooding on Titan? rate modelled from surface channel size and slope (cf. McKenzie and Nimmo, 1999), which is of the order of 105–106 m3 s− 1 for channels Palaeoflooding on Titan occurred or occurs with very different that head at fossae (see discussion and references in Burr et al., 2009). materials and conditions than on Earth and Mars. Palaeoflooding on Short-term ponding may have occurred in the fossae before breaching Earth and Mars entailed water flow and transportation of silicate and/or overflowing the fossae rim (Ghatan et al., 2005; Keszthelyi material. However, models of Titan relegate any silicate material to its et al., 2007) thereby contributing to high instantaneous discharges. core (Tobie et al., 2006), and water is in the solid phase forming much 10 D.M. Burr / Global and Planetary Change 70 (2010) 5–13

2008). On the basis of theory and cloud observations, Titan is hypothesized to have a ‘methane monsoon’ which would deluge the surface every few hundred years (Lorenz and Mitton, 2002; Lorenz et al., 2005). Under normal (inter-monsoonal) conditions, rain would fall about six times more slowly on Titan than on Earth and would probably evaporate before reaching the ground (Lorenz, 1993). A greater portion of those drops that do reach the ground would be more likely to infiltrate, given their slow descent, which would also mitigate against monsoon-style runoff. Both remote telescopic and in situ observations suggest that at the time of data acquisition rain may have been falling slowly and continuously (Tokano et al., 2006; Ádámkovics et al., 2007). Thus, if the anastomosing features are indeed flood channels, they may reflect this episodic monsoon. Initial estimates (Jaumann et al., 2007, 2008) show that any flood discharges generated from monsoonal conditions were orders of magnitude smaller than megafloods on Earth and Mars. Fig. 4. Schematic plot showing relative frequency of flooding and inferred magnitude based on channel size through Mars' history. Aglacialflood mechanism is unlikely on Titan, given the triple points of hydrocarbons (see Lorenz and Lunine, 2005). However, images and spectra indicate the occurrence of cryovolcanism on Titan (Sotin et al., of Titan's crust (Griffith et al., 2003; Lorenz and Lunine, 2005). Thus, 2005; Lopes et al., 2007). By analogy with volcanism on Earth, given sufficient erosion on Titan (Lorenz and Lunine, 1996), fluvial cryovolcanism on Titan may provide the geological context for debris- sediment is likely to be composed of water ice grains. An alternative or dam floods. Calderas large enough to generate floods have not yet been additional source of fluvial sediment is organic material derived from inferred from Cassini–Huygens published data, but may be also possible. the atmosphere through photochemical reactions of hydrocarbons The impact crater population on Titan is sparse (Lorenz et al., 2007a), (Waite et al., 2007, and references therein). At Titan's surface but examples of possible impact craters show elongate sinuous features temperature (∼94 K) and pressure (∼1.44 bar), liquid flow would on their flanks. These features may be either lava channels or fluvial entail hydrocarbons — most likely a mixture of methane and nitrogen channels (Lopes et al., 2007). If they are fluvial channels, they may (Lorenz et al., 2003; Lorenz and Lunine, 2005). Despite these indicate floods from craters analogous in mechanism to basin-breach differences between materials on Titan, Earth, and Mars, however, floods that occur from overflow of Martian impact crater basins. Titan theoretical modelling using the values in Table 1 shows that sediment topography is muted relative to Earth or Mars (Lorenz et al., 2007b), so transport parameters are similar on the three bodies (Burr et al., 2006). topographic basin-breach flooding may be less likely. Processes of fluvial erosion may also to be similar (Collins, 2005). Images from multiple instruments both on the Cassini spacecraft 5. Discussion (Porco et al., 2005; Elachi et al., 2005; Tomasko et al., 2005; Barnes et al., 2007) and on the Huygens probe (Tomasko et al., 2005) show 5.1. Comparison of mechanisms elongate, sinuous features inferred to be fluvial channels (Fig. 5), although more likely they are fluvial valleys (Perron et al., 2006; Burr, A comparison of flood-generating mechanisms on Earth, Mars, and 2007). Some of these features are a few hundred metres wide, and an Titan is summarised in Table 2. The largest floods on Earth and on Mars empirical relationship between discharge and width scaled for Titan are produced in an atmospheric volatile cycle in which the volatile can gravity suggests that these features, if true channels, may have easily assume the solid phase. On Earth, this solid phase is largely a conveyed flows of the order of 103 m3 s− 1 of material (Jaumann et al., surface phenomenon, whereas on Mars it is largely a subsurface 2007, 2008). Such flows are similar to large river systems on Earth. phenomenon. On Titan, the temperature and pressure conditions render Of interest here is whether these fluvial channels on Titan show such catastrophic palaeofloods unlikely because the volatile is unlikely to evidence for palaeoflooding. Some of these features have an assume the solid phase, although surface features have recently been anastomosing appearance in plan view, similar to that of terrestrial interpreted to have resulted from glaciation. However, the least desert flood channels produced by monsoonal rains (Lorenz et al., important mechanism for generating floods on Earth and Noachian- age Mars, namely precipitation, is likely to be the most important flood- generating mechanism on Titan. Whereas on Earth and Mars, this mechanism generated floods indirectly through basin overflow, on Titan, heavy precipitation appears likely to have caused smaller floods directly.

5.2. Comparison of effects

5.2.1. Earth In addition to voluminous erosion and reorientation of continental drainage networks, terrestrial megaflooding has caused global climate change through disruption of ocean currents. Massive (of the order of 106–107 m3 s−1) ocean currents transport heat poleward as a result of temperature and salinity contrasts. In particular, the northward flowing Gulf Stream evaporates and becomes more saline with distance poleward, causing the water to sink in the North Atlantic and flow southward as deep ocean currents. On a global scale, such thermohaline circulation regulates meridional heat transport. However, it has been Fig. 5. Image mosaic from the Descent Imager/Spectral Radiometer on the Huygens probe, argued that Pleistocene freshwater megafloods from the glacial lakes of showing inferred fluvial channels and a dry lake bed shoreline. The image is ∼12 km wide. The arc in the lower right is an artificial marking. (Image Credit: ESA/NASA/JPL/University the North American Laurentide Ice Sheet disrupted Atlantic meridional of Arizona). overturning circulation, changing the climate of Earth for a millennium D.M. Burr / Global and Planetary Change 70 (2010) 5–13 11

Table 2 Summary of comparison among ﬂood-generating mechanisms on Earth, Mars, and Titan.

Cause Earth Mars Titan

Glaciation and related processes Most significant indirect cause Possible, but glacial morphology not Unlikely, given materials and surface correlated with flood morphology conditions on Titan Volcanism/ geothermal heat flow Significant direct cause and most Hesperian chaos terrain floods, Amazonian Possible, as evidence for cryovolcanism important indirect cause fissure floods (alternative = tectonism) observed Dam failure Important indirect cause Noachian basin overflow floods, Hesperian Dams of cryovolcanic material possible; (post-glaciation); ice jams chasmata floods moraine dams unlikely Precipitation Monsoon important in tropics, Noachian basin overflow floods ‘Methane monsoon’: the most significant snowmelt in continental inlands cause on Titan?

(e.g., Broecker et al., 1989; Broecker, 2002). Sudden influxes of icebergs Earth, the largest flooding is associated with glaciation, volcanism, or a during the last glacial interval (∼45–18 ka) discharged from the combination of the two. However, all well-known flood sites are recent Laurentide Ice Sheet have also been inferred from layers of ice-rafted in age, because of extensive erosion in the presence of an active volatile debris in the North Atlantic Ocean (Heinrich, 1988; Hemming, 2004). cycle. On Mars, the largest palaeoflooding is associated with volcanism Such “Heinrich Events” have been linked to widespread cooling instead of glaciation, but again, a combination of the two mechanisms episodes (Berger, 1990; Broecker, 2006). may have produced the largest floods (during the Hesperian). The cessation of the apparent hydrological cycle on Mars has allowed for the 5.2.2. Mars preservation of these much older palaeoflood channels. The preliminary The most likely effect of megaflooding on Mars is its putative data from Titan suggest monsoonal rainfall as the primary flood- northern ocean (Lucchitta et al., 1986; Parker et al., 1989, 1993). The generating mechanism, which may be presently operating as in geomorphic evidence for this extensive ocean remains weak (Malin equatorial regions on Earth. As continued data are returned from and Edgett, 1999). This weakness may be due to: 1) less obvious Titan, this speculative hypothesis will be tested. depositional features on Mars than on Earth because of the easier transport and therefore broader dispersion of sediment (Komar, Acknowledgements 1980; Burr et al., 2006) and 2) less obvious tidal erosional features on Mars than on Earth because of the lack of a single large moon. This manuscript benefited from helpful reviews by Vern Manville However, the similarity of ages of the vast northern plains deposits and Paul Carling. 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