The fluvial history of Mars

Abstract

River channels and valleys have been observed on several planetary bodies in addition to the Earth. Long sinuous valleys on Venus, our Moon and Jupiter's moon Io are clearly formed by lava, and branching valleys on Saturn's moon Titan may be forming today by rivers of methane. But by far the most dissected body in our Solar System apart from the Earth is Mars. Branching valleys that in plan resemble terrestrial river valleys are common throughout the most ancient landscapes preserved on the planet. Accompanying the valleys are the remains of other indicators of erosion and deposition, such as deltas, alluvial fans and lake beds. There is little reason to doubt that water was the erosive agent and that early in Mars' history, climatic conditions were very different from the present cold conditions and such that, at least episodically, water could flow across the surface. In addition to the branching valley networks, there are large flood features, termed outflow channels. These are similar to, but dwarf, the largest terrestrial flood channels. The consensus is that these channels were also cut by water although there are other possibilities. The outflow channels mostly postdate the valley networks, although most are still very ancient. They appear to have formed at a time when surface conditions were similar to those that prevail today. There is evidence that glacial activity has modified some of the water-worn valleys, particularly in the 30-50° latitude belts, and ice may also be implicated in the formation of geologically recent, seemingly water-worn gullies on steep slopes. Mars also has had a long volcanic history, and long, sinuous lava channels similar to those on the Moon and Venus are common on and around the large volcanoes. These will not, however, be discussed further; the emphasis here is on the effects of running water on the evolution of the surface.

Full text

For Review Only
The fluvial history of Mars
Journal:
Philosophical Transactions A
Manuscript ID:
RSTA-2011-0500
Article Type:
Research
Date Submitted by the Author:
19-Oct-2011
Complete List of Authors:
Carr, Michael; USGS
Subject:
Geology < EARTH SCIENCES
Keywords:
Mars, valleys, outflow channels, running water
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short title: Mars
The fluvial history of Mars
By M
ICHAEL
H.
C
ARR
*
U.S. Geological Survey, 346 Middlefield Rd., Menlo Park CA 9402, USA
River channels and valleys have been observed on several planetary bodies in addition to the
Earth. Long sinuous valleys on Venus, our Moon, and Jupiter’s moon Io are clearly formed
by lava, and branching valleys on Saturn’s moon Titan may be forming today by rivers of
methane. But by far the most dissected body in our Solar System apart from the Earth is
Mars. Branching valleys which in plan resemble terrestrial river valleys are common
throughout the most ancient landscapes preserved on the planet. Accompanying the valleys
are the remains of other indicators of erosion and deposition, such as deltas, alluvial fans, and
lake beds. There is little reason to doubt that water was the erosive agent and that early in
Mars’ history climatic conditions were very different from the present cold conditions and
such that, at least episodically, water could flow across the surface. In addition to the
branching valley networks there are large flood features, termed outflow channels. These are
similar to but dwarf the largest terrestrial flood channels. The consensus is that these
channels were also cut by water although there are other possibilities. The outflow channels
mostly postdate the valley networks, although most are still very ancient. They appear to have
formed at a time when surface conditions were similar to those that prevail today. There is
evidence that glacial activity has modified some of the water-worn valleys particularly in the
30-50° latitude belts and ice may also be implicated in the formation of geologically recent,
seemingly water-worn gullies on steep slopes. Mars also has had a long volcanic history, and
long, sinuous lava channels similar to those on the Moon and Venus are common on and
around the large volcanoes. These will not, however, be discussed further; the emphasis here
is on the effects of running water on the evolution of the surface.
Keywords: Mars; valleys; outflow channels; running water
• Author for correspondence (carr@usgs.gov)
One contribution of 10 to a Theme Issue ‘River History’.
___________________________________________________________________________
1. Introduction
Because there are no rivers flowing on Mars today, our perception of its fluvial
history is based almost entirely on remote sensing, and particularly on the morphology of the
surface as observed in satellite imagery. Fortunately we have excellent coverage. Almost the
entire planet has been imaged at 100 m/pixel resolution and substantial fractions at
resolutions down to 1 m/pixel. In addition, we have global altimetry at a few hundred
m/pixel and compositional data from infrared spectroscopy with spatial resolutions of a few
tens of meters. These data are supplemented by views from the ground at several locations,
particularly at the two sites explored by the rovers Spirit and Opportunity.
To provide some context for discussion of the fluvial features, this paper begins with a
summary of present conditions on Mars and an outline of the planet’s geologic history insofar
as we know it. There follows a detailed discussions of the valley networks, the large flood
features and other water-worn or water-deposited forms. Included will be brief discussions of
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evidence for former oceans and speculation on the nature of the hydrologic system that
resulted in the water-worn features that we see. Table 1 provides a few useful facts to aid the
discussion.
1. Present climatic conditions
Discovery of Mars’ branching valley networks during the Mariner-9 mission in 1972
was a complete surprise because, by that time, we already knew that Mars had very thin CO
2
atmosphere and that the surface was much too cold to permit streams of liquid water at the
surface. At 1.5 times the Earth’s distance from the Sun (Table 1) , and with almost no
greenhouse effect, mean surface temperatures are well below freezing. The mean daily
temperature at the equator is close to -60 °C (213 K) and drops to -125 °C (148 K) at the
winter pole. During the day, however, temperatures at the surface may fluctuate widely. At
noon they can reach as high as 30°C (303 K) depending on season, location, and surface
properties, although this is somewhat deceptive in that temperatures are close to the daily
mean of 60° below freezing just a centimeters or two below the surface. The mean surface
pressure is close to the triple point pressure for water of 6.1 mbar. Thus at higher elevations,
where the pressure is below the triple point, ice if heated will sublime rather than melt.
Roughly 30% of the CO
2
in the atmosphere freezes out in the winter hemisphere to form
a seasonal polar cap. Growth and recession of the two seasonal caps causes movement of
CO
2
back and forth between the two hemispheres. As the seasonal caps recede in summer
they expose semi-permanent water-ice caps around 3 km thick at each pole. Water ice is also
pervasive just below the surface at latitudes greater than 60° and may be present locally
below the surface at lower latitudes. Despite the widespread presence of water ice, however,
liquid water can exist near the surface only temporarily and in very small amounts under
present conditions. Widespread presence of seemingly water-worn channels suggests
therefore that climatic conditions were very different at times in the past.
The tilt of Mars’ rotational axis (25.2°) is similar to the Earth’s (23.5°), so Mars, like the
Earth, has seasons. Mars’ orbit has, however, a significant eccentricity. This causes the
seasons to have different lengths. At present, southern summer (158 days) is shorter and
hotter than northern summer (183 days) but the length and intensity of the seasons change
slowly with time as the various orbital and rotational parameters change. One astronomical
parameter that has a significant effect on the water cycle is the tilt of the rotational axis, or
obliquity. The Earth’s tilt undergoes little change but Mars’ obliquity changes significantly.
During the last 10 million years it has been as low as 15° and as high as 45°. Laskar et al.
(2004) estimate that there is a 63% probability that the obliquity reached 60° in the last 1
Gyr. At high obliquities when the summer pole faces the Sun, water ice sublimes from the
poles and precipitates out at lower latitudes. Warming of ice on sun-facing slopes during
high obliquities could then provide meltwater to cut small channels.
2. Geological overview
Two thirds of the martian surface is heavily cratered like the surface of the highlands on
our Moon (figure 1). The surface has clearly survived from the period of heavy bombardment
that all the planetary bodies in the inner Solar system experienced prior to around 3.7 Gyr
ago. The geological history of Mars has been divided into three periods: the Noachian,
Hesperian and Amazonian (for a detailed summary of the geologic history see Carr (2006)).
The Noachian refers to the period of heavy bombardment. The rest of the planet’s history is
divided somewhat arbitrarily into the Hesperian and Amazonian, largely on the basis of
numbers of superimposed impact craters. The absolute age of the boundary between the
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Hesperian and Amazonian is not known but is estimated to be between 2.9 and 3.3 Gyr ago
(Hartmann & Neukum 2001). The heavily cratered Noachian terrain is mostly in the southern
hemisphere and is at an elevation roughly 6 km higher than the more sparsely cratered
lowlands in the north. The contrast between the two hemispheres has been called the global
dichotomy and attributed to a giant impact very early in the planet’s history. The valley
networks are mostly incised into the heavily cratered Noachian uplands. Other than the
dichotomy, the largest surviving impact basin on the planet is Hellas, which is 4000 km
across, has a rim that reaches 2 km above the datum and a depth that extends to 8 km below
the datum. The Hellas basin dominates drainage in the eastern part of the southern
hemisphere. Elsewhere drainage is mostly controlled by the north south regional slope
created by the global dichotomy.
Much of the geologic activity subsequent to the end of heavy bombardment was related
to the volcanic province of Tharsis, a broad elevated area 5000 km across and10 km high
centered on the equator at 265° E (figure 1). The volcanic pile that forms the Tharsis bulge
had probably largely accumulated by the end of heavy bombardment as the drainage pattern
of the Noachian valleys is consistent with the present topography around the bulge. The
province continued, however, to be a focus of volcanic activity for the rest of the planet’s
history. Several
volcanoes formed within the province and most of intervening plains are characterized by
numerous lava flows one upon another. The volcanoes are enormous by terrestrial standards.
The present edifice of Olympus Mons is 550 km across, 21 km high, and surrounded by a
cliff up to 9 km high. Alba Patera, the largest volcano on the planet in areal extent, is 2000
km across but only a few kilometers high. The large size of the volcanoes has been attributed
to the lack of plate tectonics, the long life of the magma sources, and the buoyancy of the
magmas. Although the present surfaces of the volcanoes have few impact craters, which
indicates that the surface is young , the volcanoes probably have been active throughout the
history of the planet, with large eruptions spaced widely in time. Elysium, another much
smaller volcanic province, is the source of several large, seemingly fluvial, channels.
The presence of the Tharsis volcanic pile has deformed the crust and created a vast array
of radial fractures around the pile. These may have acted as conduits for groundwater and
enabled release of groundwater to the surface. Rifting along the radial fractures has also
resulted in the formation of a vast canyon system, the Valles Marineris. The canyons extend
from the summit of the Tharsis bulge eastward for 4000 km where they merge with some
large channels. In the central section, where several canyons merge, the depression that they
form is 600 km across and several kilometers deep. The alignment of many of the canyon
walls radial to Tharsis indicates that faulting played a major role in the formation of the
canyons, although branching side canyons and landslides show that other processes were
involved. Within the canyons are thick, layered deposits containing hydrated sulfates. The
canyons may once have contained lakes that drained catastrophically eastwards to form the
large channels with which they merge. The deposits within canyons are mostly Hesperian in
age, significantly younger than most of the valley networks.
At each pole is a 3 km thick stack of finely layered, ice-rich deposits that extend out
roughly to the 80° latitude circle. The deposits are young. Crater counts suggest 10
5
to 10
7
years, with those in the south being somewhat older than those in the north. The layering is
caused by different proportions of dust incorporated into the ice as the caps accumulated, the
depositional rates being modulated by variations in the planet’s orbital and rotational
motions. The young ages are indicative of the aforementioned episodic removal of the caps
during periods of high obliquity. Dust storms are common at present during summer months,
some occasionally achieving global proportions. Drifts and dunes that are seen in almost
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every high resolution image of the surface attest to the efficacy of the wind in moving
fragmental material around the surface.
3. Valley networks
The Noachian uplands are highly eroded. Craters, the dominant landform in the uplands,
occur in widely ranging states of preservation. Some craters are almost perfectly preserved;
others, even some hundreds of kilometers across are so filled and eroded as to be barely
discernible. Clearly the Noachian terrain has undergone extensive erosion, with the younger
craters being barely eroded and the oldest visible craters having been almost completely
eliminated. In contrast, impact craters on post-Noachian surfaces are almost all perfectly
preserved. It appears that there were continuously high or episodically high rates of erosion
during the Noachian, that rates fell precipitously at the end of the period, around 3.7 Gyr ago,
then remained low for the rest of the planet’s history. Average erosion rates during the
Noachian are estimated to be around 10
-6
-10
-5
m/yr, the low end of continental denudation
rates on Earth (Golombek et al. 2006). These rates should, however, be viewed with some
caution since we do not know how old the oldest features that we can see are. According to
the late heavy bombardment model, no topographic features have survived from prior to
around 3.9 Gyr ago; in other models some of the large basins such as Hellas may be as old as
4.4 Gyr. Average post-Noachian erosion rates, were 2-5 orders of magnitude lower than
those in the Noachian although there were episodic, local, erosional events, such as the large
floods, that accomplished a significant amount of erosion.
Branching valley networks, that in plan resemble terrestrial river systems, are common
throughout the Noachian uplands. In view of their branching morphology, the presence of
subsidiary features such as deltas, the general availability of water, at least as ice, and the
lack of plausible alternatives, there is little reason to doubt that the valleys were cut by water.
Most of the valley networks are incised into the degraded Noachian landscape (Carr 1995;
Howard et al. 2005; Hynek et al. 2010), and one issue is the extent to which fluvial processes
such as those that formed the branching valleys contributed to the general degradation.
Although all the heavily cratered terrain is degraded, not all of it is dissected by valley
networks. Broad areas in the southern hemisphere to the east and southwest of the Hellas
impact basin are largely undissected, as is another large area, northwest Arabia, in the
northern hemisphere (figure 1). Elsewhere, however, the old terrain is mostly dissected, at
least at latitudes below 50°. Crater counts and transection relations indicate that most of the
valleys formed in the late Noachian or early Hesperian. Dissection of terrains younger than
lower Hesperian by branching networks is rare but has occurred locally. Some Hesperian
lava plains adjacent to Echus Chasma are, for example, densely dissected by branching
valleys (Mangold et al. 2004) and several late Hesperian to Amazonian volcanoes (Hecates
Tholus, Ceraunius Tholus, Alba Patera) have numerous valleys on their flanks.
Most of the martian valleys portrayed in figure 1 are narrow and inset into an otherwise
poorly dissected terrain. They more resemble terrestrial river channels than terrestrial river
valleys. The valleys are mostly 1-4 km wide and have U-shaped to rectangular cross-sections
downstream. Upstream they either become shallower and more V-shaped or they retain their
downstream cross-section until they terminate in a broad alcove. The more prominent valleys
are typically incised into the surface to a depth of 50-300 m (Williams & Phillips 2001) and
the depth of incision may remain almost constant for large distances. Typically the more
prominent valleys are outlined by steep walls with an abrupt scarp at the edge of the upland
surface. Most of the valleys are less than 200 km long and drain into local depressions, but a
few reach lengths of over 2000 km. Drainage densities in the Noachian terrain widely range
widely from undissected to densely dissected local areas with values in the 0.1 to 1 km
-1
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range (Hynek et al. 2010; Irwin et al. 2008). The average drainage density for the Noachian
terrain as a whole is 0.01 km
-1
, 2 to 3 orders of magnitude less than typical terrestrial values.
Details of the valley floors are usually masked by later deposits, particularly dunes.
Irwin et al. (2005) and Jaumann et al. (2005) have, however, identified a number of valleys
with inner channels from which they were able to estimate stream discharges, which ranged
from 300-5000 m
3
/sec. The rates are comparable to terrestrial rates for similar drainage
areas. From the contributing areas they derived runoff production rates of 0.1-6 cm/day.
Jaumann et al. (2005) estimated that it would take 1800 years to cut a valley with an inner
channel in the Libya Montes that they studied, but thought it more likely that the channel was
cut over a much more extended period of time during episodic fluvial events. This
conclusion is consistent with modelling studies of landscape development under the influence
of episodic fluvial and impact events which suggest that the incised valleys formed over
hundreds of thousands to millions of years by multiple, modest-sized fluvial events separated
widely in time (Howard 2007).
Most of the narrow incised valleys portrayed in Figure 1 appear to have formed in the
late Noachian and early Hesperian under environmental conditions that were fundamentally
different from the earlier conditions under which crater rims were eroded and local lows
filled (Howard et al. 2005). The younger valleys are incised into the alluvium that fills the
local depressions and where deltas occur (see below) they are comparable in volume to the
incised valleys, there has been minimal contribution from the rest of the drainage basin. The
period of late incision appear to have started abruptly, ended abruptly and contributed little to
the general degradation of the Noachian landscape.
Low drainage densities, abrupt terminations of many tributaries, and the difficulties in
envisaging how early Mars could be warm enough for precipitation and surface run-off
suggested to many early researchers that the valley networks could have formed by
groundwater sapping alone (e.g. Pieri 1976; Carr & Clow 1981; Goldspiel & Squyres
2000). Better imaging now shows that the early measurements of extremely low drainage
densities were due in part to the poor resolution of the then available images. We now know
that area filling drainage is common (figure 2), although average drainage densities are still
lower than typical terrestrial values (Hynek et al. 2010). Precipitation and surface runoff
have clearly been contributors to formation of the valley networks. Nevertheless,
groundwater sapping has also likely played a role, particularly in formation of some
prominent younger valleys such as Nirgal Vallis (figure 3) and Nanedi Vallis, which have
very open networks and tributaries with abrupt, alcove like terminations. In the early
Hesperian there may have been a transition from precipitation dominated to groundwater
controlled valley formation (Harrison & Grimm 2005).
Although dissection of plains younger than lower Hesperian is rare, several volcanoes
with surfaces that are much younger are densely dissected. It is not clear whether these
volcano flank valleys are similar in origin to the typical Noachian valleys. They could have
formed by mechanisms unique to volcanoes such erosion of pyroclastics by nuées ardentes,
hydrothermal circulation of groundwater, melting of ice, or they could be lava channels. They
will not be discussed further.
4. Drainage basins
Altimetry shows that, independent of the impact craters, the Noachian terrain is poorly
graded, having numerous closed depressions. During fluvial episodes these depressions
presumably contained lakes. Several global scale basins can be extracted from the altimetry.
These are not true drainage basins in the sense that drainage paths are continuous from the
divides to the basin exits, as would normally be the case for large basins on Earth. They do,
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however, show how global drainage would converge and divide if all local lows were filled.
In the southern hemisphere most of the regional drainage is toward either the Argyre or the
Hellas basins. In the northern hemisphere drainage is into the northern lowlands that form
part of the hemispheric dichotomy. Despite considerable relief along the dichotomy boundary
and around Hellas, large drainage basins analogous to the Mississippi and Amazon basins did
not develop. Seemingly, the cumulative effects of erosion, alluviation and stream capture
were insufficient to result in integration of drainage over large areas and growth of large
basins. The elimination of relief by erosion and infilling was not rapid enough keep pace with
creation of new relief by impacts and volcanism, so large, integrated drainage basins did not
form.
The lack of regional drainage is well demonstrated by Hellas. Despite over 10 km of
relief between the floor of Hellas and its rim crest and despite the area having possibly
experienced 100 to 400 million years of erosion during the Noachian, there are no significant
valleys draining into Hellas from the north and west. Even if Hellas formerly contained a
lake to the -3.1 km level (Moore & Wilhelms 2001), there are still 5 km of relief from the rim
crest down to the proposed lake level to enable drainage. If the observed degradation of
craters superimposed on the rim was due to fluvial erosion, then most of the drainage must
have been local with the water accumulating in local depressions to be lost by infiltration or
evaporation. Such a scenario is also consistent with the apparent failure to transport large
amounts of sediment from the Hellas drainage basin into the central depression. Comparing
the present topography of Hellas with the models of the original topography suggests that the
basin probably contains no more than 2 x 10
4
km
3
of fill. The Hellas drainage basin covers
1.5 x 10
7
km
2
so, despite the highly eroded craters on the rim, which imply hundreds of
meters of erosion, no more than about 1m has been eroded from the basin rim and transported
into the basin. The sediment eroded from the highs must simply have accumulated locally.
Quantitative measures of stream profiles and basin shapes support the supposition that
most martian drainage basins are less well developed than their terrestrial counterparts
(Aharonson et al. 2002; Stepinski & O’Hara 2003). In the case of terrestrial basins, over
long time scales, relief is slowly reduced over the entire basin. Higher order streams
commonly have little relief and form alluviated plains. In most terrestrial basins, the local
slope S is ~ A
-θ
, where A is the area upstream of the given point and θ is called the
concavity exponent. The larger the upstream area that drains through a point in a basin, the
lower the slope at that point; the higher the value of the exponent, the more concave is the
basin. Terrestrial basins typically have values that range from 0.3 to 0.7, with the smallest
exponents in basins of sporadic runoff or where groundwater sapping dominates or both.
Concavity exponents of most martian basins are in the 0.2-0.3 range, indicating poor
concavity. Another indicator of basin shape is how the circularity of the basin shape varies
with elevation. The higher the elevation slice through a typical terrestrial basin the more
circular the basin outline. This tendency to higher circularities at higher elevations is
significantly less pronounced with martian basins (Stepinski 2003). The low circularity is
most evident in the longest streams, those that extend from the highs on the Hellas rim,
northwestward toward Chryse basin. The poor development of basins is also reflected in the
basin divides. Because terrestrial basins tend to have significant concavity, the steepest
slopes tend to be around the periphery of a basin. In contrast, because of their low
concavities, martian basin divides are difficult to discern in images and most can only be
determined from the altimetry (Grant & Parker 2002).
5. Lakes and Deltas
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Lakes were probably common throughout the poorly graded Noachian terrain while it
was undergoing fluival erosion (Cabrol & Grin 1999, 2010). Most valleys terminate in
closed depressions such as craters or low areas between craters, where at least transient,
closed lakes almost certainly formed. Many of these areas are underlain by seemingly fine
grained, horizontally layered, easily erodible sediments (Malin and Edgett 2000). Chlorine-
rich deposits (Osterloo 2008) and sulfates (Bibring et al. 2006) in local depressions within
the Noachian uplands may be the result of evaporation from such lakes. Lakes were common
in impact craters as indicated by numerous crater rims breached by ingoing and /or outgoing
valleys. A prominent example is the crater Gusev in which the Spirit rover landed. It is
breached by a large channel, Ma’adim Vallis, which itself appears to have formed by
drainage of a large lake in the uplands to the south of Gusev (Irwin et al. 2002). Many old
impact craters have flat floors underlain by finely layered sediments. A striking characteristic
of many of these sediments is their remarkably regular, rhythmic layering. The layering
could result from a variety of causes such as successive impacts and volcanic events, or
changes in the erosional regime as a result of climate change. While all these processes may
have contributed, the extreme regularity of some of the layering argues against impacts and
volcanism as primary causes. The rhythmic depositional patterns suggest an astronomical
forcing. Erosion and sedimentation may have been modulated by climate changes that
resulted from variations in the orbital and rotational motions of the planet.
Almost 300 depressions have been identified within the highlands that have both inlet
and outlet valleys indicating the former presence of an open basin lake i.e. one that
overflowed (Fasset & Head 2008). In most of these lakes, the lake volume is proportional to
the watershed area which suggests that the lakes were fed mostly by precipitation and runoff
rather than by groundwater upwelling. Modelling by Matsubara et al. (2010) suggests that
the open basin lakes had rates of evaporative loss over surface runoff comparable to lakes in
the Great Basin in the western U.S. If climatic conditions were similar to the Great Basin, the
typical lake would have had to persist for several hundred years to maintain the observed lake
levels. However, most basins are closed. From the fact that most local basins did not
overflow, coupled with estimates of peak discharges from channel dimensions and computer
simulations of fluvial erosion of cratered landscapes, Howard (2007) and Barnhart et al.
(2009) concluded that the valley network system did not result from a few deluge-type
events, such as might be caused by large impacts, but rather from modest, episodic fluvial
events spread over extended periods of time.
Most valleys terminate at grade in depressions with little or no indication of deposits at
their mouths which suggests that in most cases the materials eroded to form the valleys were
either too fine grained to form a delta or were distributed across the depressions to form
subaerial alluvial fans. Where lakes were present the lake levels may have fluctuated and so
no delta formed. Despite their general absence, a few tens of deltas have been recognized
(Irwin et al. 2005; Achille & Hynek 2010). Most are in local depressions such as craters but
some that occur over a narrow elevation range along the dichotomy boundary have been
interpreted as having formed where valleys debouch into a northern ocean (Achille & Hynek
2010). The deltas typically are fan shaped, outlined by an outward facing scarp and have
distributary channels on their upper surfaces (figure 4). The channels are either incised into
the upper surface or left as positive features after preferential erosion of the interchannel
areas. Two particularly striking deltas are in the craters Eberswalde (Malin & Edgett 2003)
and Jazero (figure 5). If peak flows, estimated at 10
2
to 10
3
m
3
/s from input channel
dimensions, were continuously sustained, both these deltas could have formed in a few tens
of years. More likely they formed by intermittent flow over much longer periods (Jerolmack
2004; Moore et al. 2003; Fasset & Head 2005). Lack of deep incision of the deltas as the
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lake levels declined suggests that the fluvial activity that resulted in delta formation declined
abruptly.
In summary, Mars experienced a period of fluvial incision around 3.7 Gyr ago that left
much, but not all of the Noachian highlands dissected by branching valley networks. Prior to
that time high erosion rates had produced a highly degraded landscape. The branching
valleys added little to that degradation. Little evidence survives of the processes that caused
the early degradation. Although water erosion was likely involved, integration of drainage
over large areas did not occur either in the early phase or in the subsequent incision phase.
During the late incision, lakes were common throughout the Noachian, some with deltas
where streams entered. The valley and delta dimensions, coupled with lake volumes and
drainage basin areas suggest that incision involved peak stream discharges comparable to
terrestrial rivers. The time over which most of the valleys formed is poorly constrained.
Failure to fill most closed basins suggests that the valleys were not formed by a few deluge
events but by intermittent modest-sized events, perhaps spread over an extended period of
time.
6. Outflow channels
Large flood channels or outflow channels, are the characteristic fluvial feature of the late
Hesperian era. They are very different from the branching valley networks just described.
They have no tributaries. They start full size from a single source, commonly in rubble filled
depressions or at a graben. They are mostly very large, but vary greatly in size. The largest,
Kasei Vallis, is over 400 km across at its mouth and in places over 2.5 km deep (figure 6).
Others are only a few kilometers across at their source. They have low sinuosity, smoothly
curving walls and most contain teardrop-shaped islands around which flow has diverged and
converged. On the channel floors are a range of bedforms including longitudinal striae,
cataracts, plucked zones and inner channels. In toto, they are remarkably similar to large
terrestrial floods features, such as the Channeled Scablands of eastern Washington state in the
U.S. (Baker & Milton 1974; Baker & Nummedal 1978). The size of the largest channels
implies huge discharges. Estimates for Kasei Vallis range as high as 10
8
- 10
9
m
3
s
-1
(Robinson & Tanaka 1990; Komatsu & Baker 1997) , as compared with 10
7
m
3
s
-1
for the
Channeled Scablands (Baker 1982) and a peak of 3 x 10
4
m
3
s
-1
for the Mississippi. While the
resemblance of some of the channels to terrestrial flood features is impressive, a fluvial origin
has been questioned. Leverington (2004), for example, argues that the channels were cut
during large eruptions of fluid lava. But the consensus is that the channels were cut by water,
based on the strong resemblance with terrestrial flood features, on the availability of water as
indicated by other indications of hydrologic activity such as the valley networks, and on
geophysical modeling of channel formation, as discussed below. We will assume in the
following discussion that the outflow channels were cut by water though recognizing that
there are other possibilities
Most outflow channels formed in the late Hesperian (Tanaka et al. 2005), or around 3-
3.5 Gyr ago (Hartmann and Neukum 2001), and most occur around the Chryse basin, a low
area in the northern hemisphere centered on 330° E, between the Noachian uplands of Arabia
to the east and the Tharsis rise to the west. The basin is open to the low-lying plains to the
north. The channels start in the higher ground around the basin, extend into the basin then
northward into the low-lying, northern plains. Many of the channels emerge full-size from
closed, rubble-filled depressions (Figure 7). Maja Vallis, for example, emerges from a deep
depression with an outlet 4 km above the floor of the depression. If formed by water then a
lake must have been left in the depression after the flood, a conclusion that is supported by
the presence stacks of layered sulfates on the depression’s floor. Other source depressions
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also contain sulfate deposits. Some of the Chryse channels merge to the southeast with the
floors of the large canyons that form the eastern end of Valles Marineris. A likely possibility
is that the canyons once contained lakes that were released catastrophically to the east to cut
the channels. Again, thick sulfate deposits and other layered sediments in the canyons
support the former presence of lakes. Elsewhere, channels start at graben. A particularly
prominent example, Mangala Vallis, several hundred km long and in places over 100 km
across starts at a notch cut in a graben wall (Figure 8). Another channel, Athabasca Vallis in
Elysium, that starts at a graben, has an unusually young age of less than 10
7
years, as
indicated by the number of superimposed impact craters. Several outflow channels start
adjacent to volcanoes in both of the two largest volcanic provinces Tharsis and Elysium.
Thus outflow channels vary greatly in size, age and geologic context. What they have in
common is that they start almost full-size with no indication of convergent surface drainage
to provide their discharge
The full-size start and lack of tributaries indicate that the outflow channels formed not by
precipitation and surface runoff but by rapid release of a stored volume of water. In the case
of the channels that emerge from the eastern end of the Valles Marineris the stored volume
was probably intra-canyon lakes. But many of channels, particularly those around the Chryse
basin, start in rubble filled hollows with floors at elevations well below the elevation of the
channel floors and with volumes too small to explain the size of the channels. Carr (1979)
proposed that the outflow channels that start in rubble-filled hollows were caused by
eruptions of groundwater trapped under high pressure below a kilometers thick cryosphere.
Thick aquifers, high hydrostatic pressures and high permeabilites are needed to account for
the high discharges. A kilometers thick cryosphere is needed to contain the pressurized
groundwater until containment s breached such as by a large impact or a tectonic event.
After the flood, the surface collapses to form a rubble filled hollow. The necessity of a thick
cryosphere implies that in contrast to formation of the valley networks, a cold climate like
today’s is needed to form the outflow channels. This model has since been elaborated upon
in considerable detail by Andrews-Hanna & Phillips (2007) and Hanna & Phillips (2005)
who suggest that tectonic forces could have contributed the pressurizing the aquifer and that
superlithostatic pore pressures could cause opening of fractures within the aquifer, thereby
increasing its permeability and enabling high discharges. Hydrofracturing of the cryosphere
may also be one cause of release of water from the confined groundwater. Their modelling
results are sensitive to the aquifer properties but in a typical result for Ares Vallis, a peak
discharge of 8 x 10
6
m
3
sec
-1
is reached after 7 hours after which the discharge rate drops
quickly. After 23 days 1000 km
3
of water has been released. After the flood the path
through cryosphere would refreeze and the aquifer, if of large areal extent, could be
recharged by diffusion of groundwater from distil sources, and the flood repeated. If this
modelling is realistic, then a large channel like Ares Vallis, which has a volume of roughly
10
5
km
3
, may have formed by multiple, short lived floods, each with a far smaller volume of
water than the total volume needed eroded the channel. Another possibility is that flow
around the channel to the surface is not restricted by the intrinsic permeability of the
fractured aquifer but instead the aquifer partly disintegrates and some is carried to the surface
by the massive discharge thereby leading to collapse to form the rubble-filled hollow after the
event. In this way, larger floods would be possible and fewer would be needed to form an
Ares-like channel.
The start of the outflow channels at graben is simply explained by the pressurized aquifer
model: the faulting disrupted both the aquifer and the cryosphere seal, thereby providing
pathways for flow through the aquifer and to the surface. Some graben, such as the Cerberus
Fossae, appear to have been the source of eruptions of both groundwater and lava which
suggests that in some cases floods may not be simply triggered by tectonic activity but also
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by injection of dikes (Head et al. 2003). Such an origin seems particularly likely for several
channels that originate in graben around the periphery of the large volcano Elysium Mons.
Eruption of groundwater along the large faults that formed the equatorial canyons may also
be the cause of the lakes that are postulated to have been formerly present within the canyons.
The source of the water in the aquifers is uncertain. Much of it may have been inherited
from the earlier era, represented by the valley networks, when warm surface conditions
permitted precipitation and infiltration. As surface conditions, changed, water involved in the
earlier hydrologic cycle may have become trapped beneath the growing cryosphere.
Alternatively, or in addition, water could have been added from primary magma sources
beneath the volcanic regions of Tharsis and Elysium. Other possibilities are that aquifers
were recharged by basal melting of ice deposits at the poles (Clifford 1987) or melting of ice
deposits that formed in Tharsis during periods of high obliquity (Harrison & Grimm 2004).
7. Oceans
We saw above that abundant evidence of surface runoff during the Noachian suggests
that large bodies of water may have been present at that time, at least episodically, both as
sources and sinks. A number of linear features around the northern plains (Clifford & Parker
2001) and in Hellas (Moore &Wilhelms 2001) have been suggested as possible shorelines, as
have the presence of deltas at similar elevations (Achille & Hynek 2010). The ages of many
of the proposed shorelines are uncertain but some in the uplands could be Noachian.
However, the linear features, being discontinuous, at varying elevations, and being open to
other interpretations are not compelling evidence of former oceans. Nor are oceans necessary
for precipitation. The valley networks may have formed by the melting of ice driven from
the poles during periods of high obliquity, or they could result from precipitation of water
injected into the atmosphere during large impacts (Segura et al. 2002). In addition, we saw
above that large drainage basins with large streams to carry surface runoff into the major
basins did not develop. If large bodies of water were present in low areas such as Hellas they
were probably fed by groundwater. The evidence for oceans is thus equivocal. While the
simplest explanation of the high erosion rates and abundance of valley networks in the
Noachian is that conditions were Earth-like and that evaporation from ocean-like bodies of
water was in quasi-equilibrium with precipitation, runoff and infiltration, other scenarios are
possible.
The evidence for post-Noachian oceans is perhaps somewhat more convincing. Large
outflow channels drain into the northern plains. If the channels were each eroded by a single
or a few large events, then large volumes of water must once have been present in the
northern plains Linear features on Hesperian surfaces around the northern plains are better
preserved than the Noachian ones because of the low Hesperian erosion rates. Ghost craters
in low parts of the northern plains suggest the presence of effluent deposited during the
floods (Head et al. 2002). Many landforms in the lower areas of the northern plains have been
compared to terrestrial features that form by meltwater under a static ice sheet (Kargel et al.
1995) that could have been a frozen remnants of an ocean. There are thus several lines of
evidence suggestive of the former presence of large bodies of water. On the other hand, if
Andrews-Hanna &Phillips (2007) are correct and individual flood volumes are much smaller
than the channel volumes then large bodies of water need not have been present. So we are
left with similar ambiguities as with the Noachian.
8. Gullies
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Gullies are by far the most common fluvial feature that has formed in the last few Gyr of
martian history. They are common on steep slopes in the 30-60° latitude belts with a
preference for poleward facing slopes (figure 9). They typically consist of an upper theater-
shaped alcove that tapers downslope to converge on a channel that extends further downslope
to terminate in a debris fan (Malin & Edgett 2000). The channels are mostly several meters
wide and hundreds of meters long. Their origin is controversial. Although initially attributed
to groundwater seeps, this origin now seems unlikely given the probable thick cryosphere
during the second half of Mars’ history and the common presence of gullies at locations
where groundwater is unlikely, as on slopes around mesas and central peaks and at crater rim
crests. Dry mass-wasting may contribute to their formation but this also seems to be an
unlikely cause since many of the gullies cut through bedrock ledges. Erosion by wind or ice
appears ruled out by their morphology, and erosion by liquid or gaseous CO
2
appears ruled
out by stability relations. All the morphologic attributes are consistent with water erosion,
and the broad consensus is that that is their cause.
In the southern highlands at mid-latitudes, where most of the gullies occur, average daily
summer temperatures are in the 220-230 K range and surface pressures are below the triple
point of water. While groundwater seeps might temporarily exist under such conditions,
particularly in the presence of salts, accumulation of sufficient liquid to erode gullies is
unlikely. One possibility is that the gullies result from the temporary presence of water
produced by the melting of snow and ice deposited at mid latitudes during periods of high
obliquity (Christensen 2003). Such an origin is supported by modelling studies (Costard et
al. 2002; Williams et al. 2009) and by observations of gullies emerging from beneath what
appear to be ice deposits on steep slopes. The age of the gullies cannot be accurately
determined but they probably have been forming episodically, when obliquities were high
enough, throughout the last 3 billion year and possibly longer. They appear fresh because of
the extremely low erosion rates, but are unlikely to have been forming continuously since
there is little evidence that they have caused significant backwasting of slopes and filling of
the craters despite the long times over which they probably have been forming. Thus, fluvial
activity during the last 3 billion years of Mars' history has been minor, and restricted mainly
to rare groundwater eruptions, very rare valley network formation of unknown causes, and
the gullying of steep slopes, probably from melting of ice during high obliquities.
9. Global implications
The relatively high erosion rates and presence of valley networks imply that during the
Noachian Mars was at least episodically warm and wet. This conclusion is supported by the
surface mineralogy. Widespead presence phyllosilicates in Noachian terrains (Bibring et al.
2006, Murchie et al. 2009), indicates weathering under warm, moist conditions. In contrast,
in the Hesperian, erosion rates are extremely low and valley networks are rare. The
characteristic erosional feature of the Hesperian is the outflow channel, many of which
probably formed formed by eruptions of groundwater from below a thick cryosphere.
Phyllosilicates are not found in the late Hesperian terrains but evaporitic minerals,
particularly sulfates are common. All the evidence points to a period of transition in the late
Noachian-early Hesperian time period (figure 10). Temperate conditions prevailed when
valley networks were forming in the late Noachian, but a thick cryosphere had developed by
the late Hesperian when most of the outflow channels formed.
Andrews-Hanna & Lewis (2011) used global scale hydrologic models to explore what
might have happened during this transitional period. They suggest that during the Noachian,
water was abundant, warm conditions prevailed and precipitation kept the near-surface close
to saturation. However, loss of water as a result of large impacts and solar wind interactions
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resulted in a lowering of the groundwater table, precipitation became more rare but
groundwater upwellings driven by topographic variations occurred locally. Their modelling
demonstrates that the preferred locations for upwellings are those places where sulfates are
found, such as Meridiani Planum where the rover Opportunity landed. The upwellings
presumably created local lakes, which on evaporation left behind the sulfate deposits that we
observe. Further cooling and additional water losses led to more lowering of the global
groundwater table and ultimately trapping of groundwater beneath a thick cryosphere.
A major issue is how Noachian Mars became warm and wet. The geologic evidence for
an early warm, wet Mars is compelling yet how such conditions were achieved is very
unclear. According to stellar evolution models, 3.8 Gyr ago the luminosity of the Sun was 75
percent of its present value (Gough 1981). This would imply a mean surface temperature on
Mars of 196K if there were no greenhouse warming (Haberle 1998). To achieve the
warming necessary to bring the mean surface temperature up to 273K would require that the
atmosphere intercept 85 percent of the radiation from the surface as compared with 56
percent for the Earth (Haberle 1998). A very powerful greenhouse is needed.
Until the 1970’s the early atmospheres of Earth and Mars were thought to be reducing.
If so, then several greenhouse gases (CH
4
, NH
3
, H
2
S, H
2
O) could have provided the
necessary warming. It is now thought, however, that early core formation and massive loss
of hydrogen left both the mantle and atmosphere oxidized (Stevenson et al., 1983; Kuhn &
Atreya 1979; Pepin 1984), and the atmosphere dominated by CO
2
and H
2
O. With the low
early solar luminosity and as a result of cloud formation, it may not be possible to warm the
martian surface higher than about 230 K, with just a CO
2
/H
2
O atmosphere, no matter how
thick it is (Kasting 1991). In addition, with Mars’ low gravity, it is difficult to retain a thick
atmosphere against blow-off by large impacts (Melosh & Vickery 1989) and a warm
CO
2
/H
2
O atmosphere tends to self-destruct by forming carbonates. Carbonates are detected
at the martian surface, although not in the amounts expected from a massive CO
2
/H
2
O
atmosphere.
There is thus considerable uncertainty. One possibility is that episodic, massive volcanic
events introduced large amounts of CO
2
, SO
2
and possibly other greenhouse gases into the
atmosphere (Baker 2001). This resulted in temporary warmings, melting of water ice and
initiated hydrologic cycles. The warm conditions ended when the strong greenhouse gases
were flushed out. Another possibility is that temporary warm conditions with accompanying
rainfall were episodically caused by large impacts (Segura et al. 2002). But both these
hypotheses have many unresolved issues and we must conclude that how conditions on early
Mars became such as to allow widespread fluvial erosion remains a mystery.
One final mystery concerns life. Conditions on early Mars appear to have been Earth-
like. We know that life started early on Earth, although exactly when is still controversial
(Mojzis & Mark 2000; Lepland et al. 2005). Even if some form of life did start on early
Mars, its unambiguous detection will be difficult, as exemplified by the controversy over the
martian meteorite ALH84001 (McKay et al. 1996; Anders 1996). Nevertheless, as in situ
analyses become more sophisticated and return of samples carefully selected for their
habitability and preservation potential becomes more feasible, the answer to whether some
form of life ever started on Mars may not be that far in the future.
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Table 1 Some Useful Mars Facts
Orbit semi-major axis 1.524AU
Inclination of rotational axis (Obliquity) 25.2°
Mean solar day 24 h. 40 m.
Year 687 Earth days
Mean equatorial radius 3396 km
Surface elevation range -8200 m to 21230 m
Surface area 1.44 x 10
8
km
2
Ratio surface area to Earth’s land area 0.976
Surface acceleration 3.71 m sec
-1
Mean atmospheric pressure at surface 6.1 mbar (560 Pa)
Atmosphere composition 95.3% CO
2
, 2.7% N
2
, 1.6% Ar
____________________________________________________________
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FIGURES.
Figure 1. A. Global topography and some of the place names mentioned in the
text. The topography is dominated by the north-south dichotomy with high elevations
in the south and low elevations in the north, and by the presence of the Tharsis high in
the west. B. The distribution of the larger valley networks. Most of the valleys are
in heavily cratered areas, but not all heavily cratered areas are dissected. Most of the
valleys are likely water-worn, but some in the volcanic regions of Tharsis and
Elysium may be have been cut by lava.
Figure 2. Warrego Vallis at 42° S, 267° E, one of the most densely dissected
areas of the planet. The drainage density strongly supports precipitation and surface
runoff.
Figure 3. Nirgal Vallis at 28° S, 320° E. This is a younger valley than Warrego
Vallis in the previous image. Its more open system of tributaries and their alcove-like
terminations, as seen in the inset, suggest that groundwater sapping played a more
prominent role in its formation than in the case of Warrego Vllis.
Figure 4. This delta at 27° S, 326° E lies within a large impact crater and is at the
mounth of a valley, just off the bottom of the picture, that breached the crater rim.
Differential erosion has left remnants of distributary channels as positive features on
the delta surface.
Figure 5. Delta in the 40 km Jezero crater at19° N, 77° E. The dimensions of the
channels indicate maximum discharges of a few to several hundred m
3
/sec. The crater
has an overflow channel. Fasset & Head (2005) estimate that if the calculated
discharge was continuous the crater would take a few tens of years to fill and
overflow. They argue, however, that the delta was more probably built with
intermittent flow over thousands of years.
Figure 6. Kasei Vallis (25° N, 300° E), the largest outflow channel on the planet,
emerges from a shallow N-S canyon to the west of the scene shown here. To the east
the channel extends deep into the northern plains.
Figure 7. Mangala Vallis (18° S, 210° E). The channel starts at a 7 km wide gap
in a graben wall, widens northward and can be traced for well over 1000 km into the
northern plains.
Figure 8. Ravi Vallis (1° S, 316° E) , like several other channels in the south
Chryse region, emerges from a rubble filled depression. The floor of the depression is
1.5 km below the outlet. The channel is thought to have formed by an eruption of
groundwater. After the event, the ground collapsed into a depression excavated by
the flood to form the rubble filled hollow that we see.
Figure 9. Gullies on the south facing slope of Nirgal Vallis (Figure 3). The
gullies, around 10 m across, are common on pole-facing slopes in the southern
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uplands. The parallel linear ridges at the bottom of the image are dunes on the valley
floor.
Figure 10. Comparison of geologic time scales on Earth and Mars. The timing
of water-related activity on Mars indicates major changes from the late Noachian
through the Hesperian as Mars changed from an era of high erosion rates, widespread
fluvial activity and widespread aqueous weathering, to one characterized by minimal
fluvial activity, massive floods and accumulation of sulfate-rich deposits. Around 3
Gyr ago the main flood-sulfate era was over and Mars entered its present phase of
extremely low erosion rates and rare fluvial activity other than gully formation
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Carr Fig 1 colour
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Carr fig 1 grayscale
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Carr fig 2
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Carr fig 3
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Carr fig 4
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Carr fig 5
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