Global and Planetary Change 70 (2010) 5–13
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Global and Planetary Change
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / g l o p l a c h a
Palaeoflood-generating mechanisms on Earth, Mars, and Titan
Devon M. Burr
a
b
Earth and Planetary Sciences Department and Planetary Geosciences Institute, University of Tennessee Knoxville, 1412 Circle Dr., TN 37996-1410, USA
Carl Sagan Center, SETI Institute, 515 N. Whisman Rd., Mountain View, CA 94043, USA
a r t i c l e
i n f o
Article history:
Accepted 6 November 2009
Available online 15 November 2009
Keywords:
palaeofloods
fluid dynamics
terrestrial comparison
Mars
Titan satellite
volatiles
a b s t r a c t
Channelized, surface, liquid flow occurs on Earth, Mars, and Titan. In this paper, such fluvial flow is
constrained to involve volatiles that can undergo phase transitions under surface conditions and can be
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
flow vary for each body. On Earth, the mechanism that generated the largest flooding is widespread
glaciation (e.g. the catastrophic release of water from glacial lakes), which requires an atmospheric cycle of a
volatile that can assume the solid phase. Volcanism is also a prevalent cause for terrestrial megaflooding,
with other mechanisms producing smaller, though more frequent, floods. On Mars, the mechanism for flood
generation has changed over the history of the planet. Surface storage of floodwater early in the history of
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
The scientific study of palaeoflooding has risen and fallen largely in
concert with prevailing scientific paradigms (Baker, 1998). The
paradigm in the seventeenth and eighteenth centuries was catastrophism, the idea that the geological record and landscape change is the
product of sudden, short, violent events. In palaeoflood science this
paradigm was expressed as diluvialism — the view that the Biblical
flood was historically accurate and could explain various geological
formations and fossil remains. Catastrophism gave way to uniformitarianism, first introduced in 1830, which stipulated that the Earth's
formations are the product of slow, gradual change. Under this
paradigm, geological formations previously explained by catastrophic
(Biblical) flooding were often transmuted into the effects of sustained
river flow or glaciation. The Channeled Scabland in Washington, USA,
was originally interpreted at glacial in origin, either directly (through
flow of glacial ice) or indirectly (through diversion of rivers).
Consequently, the proposal by J Harlan Bretz of massive flooding as
an explanation for the Channeled Scabland was originally treated as
an absurd or outrageous hypothesis (cf. Davis, 1926; Baker, 1978).
Decades of study demonstrated the verity of this hypothesis,
establishing palaeoflood hydrology and geomorphology as a subdiscipline of geology (e.g. Kochel and Baker, 1982).
E-mail address: [email protected].
0921-8181/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.gloplacha.2009.11.003
During this time, the study of palaeofloods was also extended to
Mars. The Mariner 9 and Viking orbiter missions first observed Mars'
gigantic flood channels, identified on the basis of their morphological
similarity to the Channeled Scabland (Mars Channel Working Group,
1983). Future analysis and spacecraft missions to Mars supported
continued study of these and other flood channels (Baker, 1982; Carr,
1996), providing quantitative estimates of discharge and other flow
parameters, although with large error bars (Wilson et al., 2004). As a
result, aqueous catastrophic flooding was established as an extraterrestrial phenomenon.
The Cassini–Huygens mission to the Saturnian system (Matson et al.,
2002) opened up a new frontier in planetary geology. Mission data
revealed the surface of Titan, Saturn's largest moon, as having strikingly
Earth-like landforms – including aeolian dunes (Lorenz et al., 2006) and
lakes (Stofan et al., 2007) – although composed of exotic materials that
are uncommon on Earth. In particular, the volatile cycle on Titan
involves hydrocarbons, namely methane and the nitrogen dissolved in
it. Fluviatile features have been interpreted as river channels (Elachi
et al., 2005; Porco et al., 2005; Tomasko et al., 2005), which may have
discharges of the order of 104 m3 s− 1 (Jaumann et al., 2007). Though
much smaller than the megaflood discharges on Mars or Earth, such
discharges would be large for terrestrial rivers.
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
conditions. This constraint derives from using terrestrial aqueous
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D.M. Burr / Global and Planetary Change 70 (2010) 5–13
flooding as the template. On Earth, flooding results from the rapid
release of liquid water from atmospheric, surficial, and underground
reservoirs (O'Connor and Costa, 2004). The conditions for such release
are created by atmospheric cycling of this volatile and its ability to
undergo phase transitions among solid, liquid, and gas phases at
surface temperatures and pressures. Similar conditions have occurred
on Mars, where water currently exists as a gas in the atmosphere and
as a solid on and near the surface. Ancient valley networks indicate
that water has existed as a liquid on the surface in the past (Pieri,
1976, 1980; Carr, 1996; Irwin and Howard, 2002; Howard et al.,
2005). The interpretation of gullies on Mars as aqueous flow features
(Malin and Edgett, 2000; McEwen et al., 2007) also implies that liquid
water may exist on the surface today, if only temporarily and/or in
isolation. At Titan temperature (∼ 94 K), water occurs only as a solid,
except in limited examples of cryovolcanism (Lopes et al., 2007). By
analogy with flooding on Earth and Mars, flooding on Titan would
involve liquid methane–nitrogen. As on Earth, nitrogen comprises
most (94%) of Titan's atmosphere; methane is the next largest fraction
(5%) (Niemann et al., 2005). Methane undergoes phase changes
between liquid and gaseous phases at Titan surface pressure and
temperature, and gaseous nitrogen dissolves in the liquid methane. In
addition to the gaseous and liquid phases, methane–nitrogen may
even occur or have occurred in solid form on the surface of Titan,
although this evidence is currently limited (Robshaw et al., 2008).
Floods of geological materials that undergo only solid–liquid transitions (e.g. rocks melting to produce flood lavas) are outside the scope
of this work.
Some of the materials and conditions on these three bodies are
listed in Table 1. These different materials and conditions cause each
body to have its own mechanisms for generating floods. This paper
compares and contrasts flooding on these three bodies by reviewing
the flood morphologies and the flood generation mechanisms by
discharge for each body.
2. Palaeoflooding on Earth
2.1. Glaciation
The largest palaeofloods on Earth, with discharges of the order of
106–107 m3 s− 1, are associated with glaciation (Baker 1996, O'Connor
and Costa 2004). The primary mechanism that produced these
‘megafloods’ (cf. Baker, 2009) was the sudden release of water from
glacial lakes. Proglacial lakes result from damming by the glacier of
pre-existing river valleys. These glacial lakes and their outbursts
occurred frequently during the Quaternary glaciations as a result of the
disruption of pre-existing fluvial drainage (Starkel, 1995). Icemarginal lakes result from impoundment of the glacial meltwater
against the glacier margin, commonly by moraine debris. Ice-marginal
Table 1
Key parameters of relevance for flood generation on Earth, Mars, and Titan.
Parameter
Earth
Mars
Titan
Surface gravity (m/s2)
Surface temperature
(K)
Surface pressure
(bar)
Fluvial liquid, density
(kg/m3), viscosity
(Pa s)
Fluvial sediment,
density (kg/m3)
Atmospheric
composition
Surface area (km2)
9.81
287
3.71
210
1.35
94
1.01
0.007
1.44
Water, 1000,
1 × 10− 3 Pa s
Water, 1000,
1 × 10− 3 Pa s
CH4/N2, 450,
2 × 10− 4 Pa s
Quartz, 2650
Basalt, 2900
∼ 95% CO2, 2.7% N2
Water ice, 992
Organics, 1500
∼95%N2, ∼5%CH4
145 × 106 km2
83 × 106 km2
∼ 79% N2, 20% O2
6
2
510 × 10 km
Titan fluid density and viscosity values from Lorenz et al. (2003); organic density from
Khare et al. (1994), but lower values are possible.
(or moraine basin) lakes usually store less water than proglacial lakes,
and so generally produce relatively smaller discharges (O'Connor and
Beebee, 2009). The most extensive known examples of glacial lake
outburst flooding are located in the northern mid-latitudes, where
continental ice sheets disrupted pre-existing fluvial flow (e.g.,
Grosswald, 1998). The most voluminous examples (in terms of
discharge) have been found in areas of significant relief where deep
valleys supported tall ice dams that impounded large lakes. The largest
ice-dam failure floods on Earth include: the Kuray and other floods in
the Altai Mountains, Siberia, during the Late Pleistocene (Baker et al.,
1993; Rudoy, 1998; Carling et al., 2002; Herget, 2005); the Missoula
floods that carved the Channeled Scabland in Washington, USA (Baker,
2009, and references therein); and early Holocene floods down the
Tsangpo River gorge in southeastern Tibet (Montgomery et al., 2004).
The largest ice-marginal or moraine dam failure floods include the
Lake Agassiz overflow floods in North America (Teller et al. 2002;
Kehew et al., 2009, and references therein). It has been argued that
freshwater outbursts to the Atlantic from Lake Agassiz may have been
an important driver of the environmental changes that led to the
Younger Dryas event and other periods of cooling in the Northern
Hemisphere during the last deglaciation (Teller et al. 2002).
2.2. Volcanism
In addition to directly producing palaeofloods on Earth, glaciation
contributes to flood production indirectly in conjunction with nearsurface volcanism or enhanced geothermal heat flow (Björnsson, 2009).
Some of the largest floods associated with volcanism result from direct
melting of overlying snow and ice by eruption or geothermal heat flow.
Persistent sub-glacial hydrothermal systems may result from the
interaction of the resultant glacial meltwater with shallow magmatic
intrusions. Continued increase of the sub-glacial lake volume eventually
breaks the hydraulic seal of the overlying glacier, and the meltwater
is catastrophically released. This process takes place today in Iceland,
as in the 1996 sub-glacial eruption and resultant jökulhlaup from
the Grímsvötn cauldron beneath the Vatnajökull glacier (Fig. 1)
(Guðmundsson et al., 1997) and the 1918 jökulhlaup from beneath
the Kötlujökull glacier (Tómasson, 1996). Early Holocene jökulhlaups
with discharges of the order of 105 m3 s− 1 drained northward from the
Kverkfjöll glacier (Carrivick, 2009 and references therein), carving the
Jökulsá á Fjöllum flood channel (Waitt, 2002).
Volcanism also causes flooding by other means than direct melting.
Caldera-lake breaching can produce discharges of similar magnitude
to ice-dam breaches, as in the case of a late Holocene flood from the
Aniakchak volcano, Alaska (Waythomas et al., 1996). More commonly,
breaching of water-filled calderas produces floods one to two orders of
magnitude lower. The near instantaneous failure of a large ignimbrite
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
Tarawera caldera, New Zealand (Manville et al., 2007) and the
Holocene flood from Mount Mazama, USA (Conaway, 1999, in
O'Connor and Beebee, 2009). Large floods resulted from Pleistocene
lava dams on the Colorado River, USA (Fenton et al., 2006), and
pyroclastic flows, lahar deposits, and landslides triggered by eruptions
have likewise caused flooding.
2.3. Tectonic basin overflow
Volcanic calderas are one of three types of geological basins that
produce flooding, the other two being moraine basins (reviewed in the
section on glacial flooding above) and tectonic basins. Floods from
tectonic basins can be volumetrically the largest of the three basinbreach type floods because they can impound the most water (see
O'Connor and Costa, 2004 and O'Connor and Beebee, 2009, from which
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
the basin becoming filled with water due to a sustained positive water
balance. An example of this process took place during the Quaternary
in the basin-and-range province, western USA, where natural tectonic
basin filling coincided with glacial advance (Baker, 1983; Reheis et al.,
2002). Lake Bonneville (Malde, 1968; O'Connor, 1993) and other
Pliocene–Pleistocene lakes in the western USA (see Currey, 1990;
Reheis et al., 2002) provide examples of tectonic basin-breach floods
fed by combined rainfall and groundwater flow.
More rarely, floods may be produced from tectonic basins by a
sudden input of water; this has been postulated for the lower
Colorado River, which may have connected to the upper Colorado
drainage about 5 Ma ago due to a series of upstream basin breaches
involving large floods (House et al., 2005). Catastrophic breaching of
these basins may have carved Hells Canyon and the western Snake
River gorge, leading to integration with the Columbia River (Othberg,
1994; Wood and Clemens, 2002).
The geological record on Earth also records immense marine
floods, both into and out of tectonic basins, and these episodes have
generated much debate. Flooding of the Mediterranean Basin via a
breach at the present-day Straits of Gibraltar in the Late Miocene
(Hsü, 1983) represents an example of marine flooding of a semiisolated basin. A smaller flood may have occurred when the rising sea
level led to the flooding of the Black Sea from the Mediterranean via
the Bosporus Strait (Ryan and Pitman, 1998, Ryan 2007).
A common geological setting for these tectonic basin-breach floods
is a semi-arid environment during changing hydrological conditions
(O'Connor and Beebee, 2009). The negative water balance in semiarid climates minimizes basin integration, allowing large volumes of
water to accumulate in isolated basins over geological time. The
changing hydrological conditions provide the input to eventually
trigger the flooding. Freshwater floods are associated with pluvial
periods during glacial advance, and marine floods probably resulted
from sea level rise during deglaciation.
(O'Connor and Costa, 2004). Landslide dams can form in a wide
variety of settings, and are most commonly triggered by lubrication
due to water (precipitation or snowmelt) or by earthquakes. As with
glacial ice or moraine dams, the potential discharge increases with
increasing dam height (O'Connor and Beebee, 2009).
Ice jam floods account for three of Earth's largest known floods
(O'Connor and Costa, 2004), all of which occurred on the Lena River,
Russia (Rodier and Roche, 1984). Ice jam flooding occurs when broken
river ice accumulates at constrictions or bends in the river, causing
ponding upstream of the constriction and sudden release when the ice
jam breaks. Large, northward-flooding Arctic rivers are thus most
susceptible to ice jam flooding because the southern reaches are more
likely to thaw and flow before the northern reaches.
2.5. Precipitation
Precipitation-fed floods, along with ice jam floods, fall within the
range of Earth's smallest floods. These ‘meteorological floods’
(O'Connor and Costa, 2004) result from extreme precipitation events
such as those associated with monsoon rainfall. Thus, the largest such
floods occur in tropical climates dominated by monsoons or tropical
storms. Flood size is also correlated with river basin size, because the
largest river basins receive the largest total amount of rain. The
Amazon River in South American, which drains the world's largest
basin with an area of 7 million km2, has produced three of the four
largest recorded precipitation-fed floods, and the Yangtze River of
China has produced the fourth.
In summary, the largest floods on Earth result (indirectly) from
glaciation and related processes (such as ice margin retreat past
critical topographic thresholds), with smaller discharges contributed
by volcanism in regions with large glaciers or small ice sheets, and
much smaller floods from ice jams on polar rivers. Thus, for these
mechanisms, the planetary body needs a volatile that is close to its
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
conditions caused two of the 27 largest documented terrestrial floods
The causes of flooding on Mars are less well known than for
flooding on Earth because in situ data are not yet available from
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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 chronostratigraphic 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 mesooutflow 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, or about 9% of Mars (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
water volume could have been stored in the present-day source basin
to carve Ma'adim Vallis with reasonable water:sediment ratios (see
Irwin and Grant, 2009, and references therein).
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
topography, overflow from Margaritifer would first spill over at lower
points to the east rather than entering the head of Mawrth Valles.
Possible explanations for this geological conundrum include tilting
due to growth of the nearby Tharsis volcanic edifice and/or the influx
of ULM floodwaters as discussed above (Irwin and Grant, 2009).
3.2. Hesperian flooding from chaos and chasmata
Mars' largest flood channels are enormous in size, with widths up to
several tens of kilometres, lengths of hundreds of kilometres, and depths
of over a thousand metres (Fig. 2A) (Smith et al., 1998). These channels
debouch into Chryse Planitia, located northeast of the Valles Marineris
canyon system, and are referred to collectively as the circum-Chryse
channels. Smaller Hesperian-aged channels are also found at the eastern
end of Valles Marineris, south of Chryse Planitia. All these channels date
to the Hesperian epoch (3.7–3.0 Ga), an era of changing surface
conditions from a warm, wet climate to a cold, dry environment.
Fig. 2. Hesperian-aged circum-Chryse channels. (A) Topography showing the westernmost circum-Chryse channel and associated features. The black box shows the location
of Fig. 2B. (Image credit: MOLA/Google Mars/ASU.). (B) THEMIS day infrared image
mosaic showing an example of chaos terrain located in Eos Chasma (image centre).
(Image credit: THEMIS/ASU).
These Hesperian-aged channels may be grouped according to their
size and morphology at origin (Coleman and Baker, 2009). The largest,
circum-Chryse channels generally originate at chasmata. Chasmata
are deep, steep-sided depressions, which commonly contain chaos,
areas of kilometre-sized or larger blocks surrounded by flatter, lower
elevation terrain (Fig. 2B) (see http://planetarynames.wr.usgs.gov/
jsp/append5.jsp for definitions of descriptor terms). Chasmata and
their inset chaos are attributed to surface collapse as a result of
subsurface volatile (water) outburst (e.g., Carr, 1996, and references
therein; Rodriguez et al., 2006). The cause of the outbursting probably
involved pressurisation of groundwater by a downward growing
cryosphere (Clifford and Parker, 2001). Cryosphere disruption could
have been caused by magmatic intrusions (Leask et al., 2006),
cryosphere melting (McKenzie and Nimmo, 1999), and/or by
dissociation of CO2 ice or clathrate (Komatsu et al., 2000). In the
few instances of chaos on channel floors, outbursting may have been
triggered by flood erosion that reduced the overburden pressure
(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
Four of Mars' flood channels are dated to the Amazonian epoch (3.0 Ga
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
(Burr et al., 2009). These channels originate at fossae (e.g, Fig. 3), defined
as long, narrow depressions (see http://planetarynames.wr.usgs.gov/jsp/
append5.jsp). The fossae have varied morphologies, suggesting multiple
possible mechanisms for floodwater release. Some of the fossae have
graben morphology, which is interpreted to reflect the presence of
subsurface dykes (Wilson and Head, 2002; Head et al., 2003a; Ghatan
et al., 2005; Leask et al., 2006). In these cases, the floodwater release
mechanism is inferred to have been dyke-induced cracking of a confining
cryosphere and release of pressurised groundwater. The westward
migration through time of both volcanic and aqueous flooding from the
Cerberus Fossae is consistent with a volcanic triggering mechanism (see
Burr et al., 2009, and references therein).
Alternatively or additionally, fossae may be the result of amagmatic extensional tectonics. Based on inferred hydrological properties
of the Martian crust (Hanna and Phillips, 2005), modelling shows that
extensional stress relief can produce sufficient excess pore pressures
to drive the catastrophic release of groundwater (Hanna and Phillips,
2006). This mechanism may have acted in concert with other factors,
such as increased elevational head in the form of perched aquifers
(cf. Tanaka and Chapman, 1990) or aquifer pressurisation due to
cryosphere growth (Clifford and Parker, 2001), but does not require
them.
Ice-rich ground may have been melted (Zimbelman et al., 1992),
perhaps following dyke intrusion (cf. Head et al., 2003a), contributing
water and sediment to the flood discharge. However, melting by dyke
intrusion alone is not sufficient to release floodwater at the volumetric
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
that head at fossae (see discussion and references in Burr et al., 2009).
Short-term ponding may have occurred in the fossae before breaching
and/or overflowing the fossae rim (Ghatan et al., 2005; Keszthelyi
et al., 2007) thereby contributing to high instantaneous discharges.
Fig. 3. Topography of Mangala Valles. Channel originates at the notch in the northern side
of the fissure (black box) and stretches northward (arrows: note streamlined form in
image centre), before diverging into two channels. Image credit: MOLA/Google Mars/ASU.
Thus, the variety of fossae morphologies at the outflow channels'
origins may be interpreted as resulting from dyke intrusion possibly
combined with melting of ice-rich rock and as amagmatic tectonism.
Given this variety of morphologies, it is likely that multiple floodwater
mechanisms gave rise to the most recent big floods on Mars.
In summary, Noachian floods on Mars were generated by overflow
from impact crater or intercrater topographic basins, whose source is
commonly held to be precipitation or shallow groundwater. These
floods were the lowest in discharge and apparently the fewest in
number, although further improvements in data may modify that
perception (e.g., Mangold et al., 2008). Flooding during the Hesperian
was the most voluminous and it is likely that this flooding involved
both surface and deep subsurface water sources. This confluence of
two different flood-water sources may explain the maximal discharges during this time. Amazonian-aged flooding from fossae was
fed by deep subsurface sources. Changes in megaflooding mechanisms
with time on Mars are summarised in Fig. 4.
To compare the two planets, Mars' Noachian basin overflow
flooding, as well as putative chasmata-lake overflows, is similar to
basin-breach flooding on Earth. As on Earth, this mechanism requires
a climate that generates precipitation. Hypothesized mechanisms that
invoke a pressurised aquifer are dissimilar to any terrestrial mechanisms. However, triggering due to volcano-ice interactions is similar in
concept to terrestrial floods produced by volcanic events, although
different in context (i.e., subsurface versus surface).
4. Palaeoflooding on Titan?
Palaeoflooding on Titan occurred or occurs with very different
materials and conditions than on Earth and Mars. Palaeoflooding on
Earth and Mars entailed water flow and transportation of silicate
material. However, models of Titan relegate any silicate material to its
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
Fig. 4. Schematic plot showing relative frequency of flooding and inferred magnitude
based on channel size through Mars' history.
of Titan's crust (Griffith et al., 2003; Lorenz and Lunine, 2005). Thus,
given sufficient erosion on Titan (Lorenz and Lunine, 1996), fluvial
sediment is likely to be composed of water ice grains. An alternative or
additional source of fluvial sediment is organic material derived from
the atmosphere through photochemical reactions of hydrocarbons
(Waite et al., 2007, and references therein). At Titan's surface
temperature (∼94 K) and pressure (∼1.44 bar), liquid flow would
entail hydrocarbons — most likely a mixture of methane and nitrogen
(Lorenz et al., 2003; Lorenz and Lunine, 2005). Despite these
differences between materials on Titan, Earth, and Mars, however,
theoretical modelling using the values in Table 1 shows that sediment
transport parameters are similar on the three bodies (Burr et al., 2006).
Processes of fluvial erosion may also to be similar (Collins, 2005).
Images from multiple instruments both on the Cassini spacecraft
(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
elongate, sinuous features inferred to be fluvial channels (Fig. 5),
although more likely they are fluvial valleys (Perron et al., 2006; Burr,
2007). Some of these features are a few hundred metres wide, and an
empirical relationship between discharge and width scaled for Titan
gravity suggests that these features, if true channels, may have
conveyed flows of the order of 103 m3 s− 1 of material (Jaumann et al.,
2007, 2008). Such flows are similar to large river systems on Earth.
Of interest here is whether these fluvial channels on Titan show
evidence for palaeoflooding. Some of these features have an
anastomosing appearance in plan view, similar to that of terrestrial
desert flood channels produced by monsoonal rains (Lorenz et al.,
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.
A glacial flood 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.,
2005; Lopes et al., 2007). By analogy with volcanism on Earth,
cryovolcanism on Titan may provide the geological context for debrisdam floods. Calderas large enough to generate floods have not yet been
inferred from Cassini–Huygens published data, but may be also possible.
The impact crater population on Titan is sparse (Lorenz et al., 2007a),
but examples of possible impact craters show elongate sinuous features
on their flanks. These features may be either lava channels or fluvial
channels (Lopes et al., 2007). If they are fluvial channels, they may
indicate floods from craters analogous in mechanism to basin-breach
floods that occur from overflow of Martian impact crater basins. Titan
topography is muted relative to Earth or Mars (Lorenz et al., 2007b), so
topographic basin-breach flooding may be less likely.
5. Discussion
5.1. Comparison of mechanisms
A comparison of flood-generating mechanisms on Earth, Mars, and
Titan is summarised in Table 2. The largest floods on Earth and on Mars
are produced in an atmospheric volatile cycle in which the volatile can
easily assume the solid phase. On Earth, this solid phase is largely a
surface phenomenon, whereas on Mars it is largely a subsurface
phenomenon. On Titan, the temperature and pressure conditions render
such catastrophic palaeofloods unlikely because the volatile is unlikely to
assume the solid phase, although surface features have recently been
interpreted to have resulted from glaciation. However, the least
important mechanism for generating floods on Earth and Noachianage Mars, namely precipitation, is likely to be the most important floodgenerating 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
Fig. 5. Image mosaic from the Descent Imager/Spectral Radiometer on the Huygens probe,
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
of Arizona).
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
argued that Pleistocene freshwater megafloods from the glacial lakes of
the North American Laurentide Ice Sheet disrupted Atlantic meridional
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
Volcanism/ geothermal heat flow
Significant direct cause and most
important indirect cause
Important indirect cause
(post-glaciation); ice jams
Monsoon important in tropics,
snowmelt in continental inlands
Possible, but glacial morphology not
correlated with flood morphology
Hesperian chaos terrain floods, Amazonian
fissure floods (alternative = tectonism)
Noachian basin overflow floods, Hesperian
chasmata floods
Noachian basin overflow floods
Unlikely, given materials and surface
conditions on Titan
Possible, as evidence for cryovolcanism
observed
Dams of cryovolcanic material possible;
moraine dams unlikely
‘Methane monsoon’: the most significant
cause on Titan?
Dam failure
Precipitation
(e.g., Broecker et al., 1989; Broecker, 2002). Sudden influxes of icebergs
during the last glacial interval (∼45–18 ka) discharged from the
Laurentide Ice Sheet have also been inferred from layers of ice-rafted
debris in the North Atlantic Ocean (Heinrich, 1988; Hemming, 2004).
Such “Heinrich Events” have been linked to widespread cooling
episodes (Berger, 1990; Broecker, 2006).
5.2.2. Mars
The most likely effect of megaflooding on Mars is its putative
northern ocean (Lucchitta et al., 1986; Parker et al., 1989, 1993). The
geomorphic evidence for this extensive ocean remains weak (Malin
and Edgett, 1999). This weakness may be due to: 1) less obvious
depositional features on Mars than on Earth because of the easier
transport and therefore broader dispersion of sediment (Komar,
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.
However, the similarity of ages of the vast northern plains deposits
and the late Hesperian outflow channels (reviewed by Coleman and
Baker, 2009) and the spatial contiguity between the deposits and the
outflow channels both support the idea of a Mars northern ocean
filled by flooding (Carr and Head, 2003). Such an ocean would have
induced climate change by adding water vapour (a greenhouse gas) to
the atmosphere. The release of the floodwaters, probably triggered by
enhanced geothermal heat flux or volcanism, may also have been
accompanied by additional radiatively-active gases, which together
may have produced a transient greenhouse (Baker et al., 1991). This
process is envisioned to be cyclical, an effect of the steady downward
propagation of the cryosphere competing with periodic cryosphere
disruption and massive water release by volcanism (Baker et al.,
1991). The various inferred ground ice and glacial landforms on Mars
have been interpreted as support for this hypothesis. Mars has also
experienced recent ground and glacial ice deposition (Head et al.
2003b; Neukum et al., 2004), but this ice deposition is commonly
attributed to orbital forcing, not to Amazonian-aged flooding.
5.2.3. Titan
Contrary to previous predictions (Lunine et al., 1983), Cassini–
Huygens data for Titan do not show evidence of a substantial ocean or
seas and this finding represents a fundamental difference from Earth
and Mars. In their absence, the effects of Titan's relatively smaller floods
may have been minimal. Flooding from monsoonal rainfall apparently
causes surface erosion (Lorenz et al., 2008) and would dry out the
atmosphere, which would likely be replenished slowly due to the lack of
an ocean and the low solar flux result (Lorenz et al., 2005; Lunine and
Atreya, 2008). Once sufficiently replenished, saturation would again
trigger monsoonal flooding. Thus, flooding may be related to global
climate change on Titan as part of a short-term methane cycle (Lunine
and Atreya, 2008), but with a higher and more cyclic frequency than on
Earth and Mars.
6. Summary
Earth, Mars, and Titan all show evidence of liquid flooding, but with
important differences in the discharge and triggering mechanisms. On
Earth, the largest flooding is associated with glaciation, volcanism, or a
combination of the two. However, all well-known flood sites are recent
in age, because of extensive erosion in the presence of an active volatile
cycle. On Mars, the largest palaeoflooding is associated with volcanism
instead of glaciation, but again, a combination of the two mechanisms
may have produced the largest floods (during the Hesperian). The
cessation of the apparent hydrological cycle on Mars has allowed for the
preservation of these much older palaeoflood channels. The preliminary
data from Titan suggest monsoonal rainfall as the primary floodgenerating mechanism, which may be presently operating as in
equatorial regions on Earth. As continued data are returned from
Titan, this speculative hypothesis will be tested.
Acknowledgements
This manuscript benefited from helpful reviews by Vern Manville
and Paul Carling. This work was supported by NASA through the Mars
Data Analysis Program and the Cassini Data Analysis Program.
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