An intense terminal epoch of widespread fluvial activity on early
Mars:
1. Valley network incision and associated deposits
Alan D. Howard
Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia, USA
Jeffrey M. Moore
NASA Ames Research Center, Moffett Field, California, USA
Rossman P. Irwin III
Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, D. C., USA
Received 15 April 2005; revised 26 July 2005; accepted 8 August 2005; published 4 November 2005.
[1] We present evidence that a final epoch of widespread fluvial erosion and deposition in
the cratered highlands during the latest Noachian or early to mid-Hesperian was
characterized by integration of flow within drainage networks as long as 4000 km and
trunk valley incision of 50 to 350 m into earlier Noachian depositional basins. Locally
deltaic sediments were deposited where incised valley systems debouched into basins.
Large alluvial fans of sediment deposited from erosion of alcoves in steep crater
walls probably formed contemporaneously. The depth of incision below Noachian
surfaces correlates strongly with the gradient and the total valley length, suggesting
consistent regional hydrology. Estimated discharges from channel dimensions indicate
flow rates equivalent to mean annual floods in terrestrial drainage basins of equivalent
size. Such high flow rates imply either runoff directly from precipitation or rapid
melting of accumulated snow. Development of duricrusts on the Noachian landscape may
have contributed to focusing of late-stage erosion within major trunk drainages. This late-
stage epoch of intense fluvial activity appears to be fundamentally different than the
fluvial environment prevailing during most of the Noachian Period, which was
characterized by widespread fluvial erosion of highlands and crater rims, deeply infilling
crater floors, and intercrater basins through ephemeral fluvial activity, and development of
local rather than regionally integrated drainage networks.
Citation: Howard, A. D., J. M. Moore, and R. P. Irwin III (2005), An intense terminal epoch of widespread fluvial activity on early
Mars: 1. Valley network incision and associated deposits, J. Geophys. Res., 110, E12S14, doi:10.1029/2005JE002459.
1. Introduction
[2] The highlands of Mars record extensive fluvial
erosion that occurred during the Noachian Period, extend-
ing from the earliest observable features until the Hespe-
rian at about 3.5 Ga B.P. [Hartmann, 2005]. Considerable
debate has occurred since the first global observations of
Mars by Mariner 9 (1971?2) about the source of runoff
and the total erosion required to produce the widespread
valley networks. Global topographic mapping by the Mars
Orbiter Laser Altimeter (MOLA) (1997?2001) has
revealed that fluvial erosion may have stripped several
hundred meters of surface materials on the observable
landscape and infilled crater floors [Hynek and Phillips,
2001; Craddock and Howard, 2002]. This extensive fluvial
redistribution of sediment has led to an emerging consensus
that a hydrologic cycle consisting of precipitation and runoff
was required during the Noachian [Grant, 2000; Hynek and
Phillips, 2001, 2003;Craddock andHoward, 2002; Irwin and
Howard, 2002; Forsberg-Taylor et al., 2004].
[3] Following the Noachian, the climate changed due to
a reduction of atmospheric temperature and pressure,
which resulted from loss of atmospheric components to
space and through weathering reactions [Fanale et al.,
1992; Carr, 1999]. This reduction in mass diminished the
capability of the atmosphere to support a precipitation-
based hydrologic cycle, and some of the early water
inventory was incorporated into a cryosphere and polar
deposits [Clifford, 1993; Carr, 1996; Clifford and Parker,
2001]. During the Hesperian and Amazonian time periods
fluvial activity was primarily limited to formation of
outflow channels, primarily thought to result from local
melting of the cryosphere by volcanic processes or
impacts [Carr, 1979, 1996; Clifford, 1993; Tanaka,
1999; Clifford and Parker, 2001]. However, valley net-
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, E12S14, doi:10.1029/2005JE002459, 2005
Copyright 2005 by the American Geophysical Union.
0148-0227/05/2005JE002459$09.00
E12S14 1 of 20
works of Hesperian and Amazonian age have been found,
located primarily on the slopes of volcanoes or within the
Tharsis volcanic province [Gulick and Baker, 1989; Scott,
1995]. These have usually been attributed to hydrothermal
pumping of groundwater or to local precipitation from
condensation of volcanic venting [Gulick and Baker,
1990; Gulick, 1998, 2001; Dohm and Tanaka, 1999],
although recent high-resolution Thermal Emission Imag-
ing System (THEMIS) images have revealed integrated
drainage networks elsewhere in the Tharsis region of
possibly late Hesperian age that are attributed to precip-
itation [Mangold et al., 2004]. Fluvial erosion on the
equatorial highlands since the Hesperian has apparently
been restricted to small gullies on steep slopes [Malin and
Edgett, 2000a], although outflow channel activity has
continued through the Amazonian [Mouginis-Mark,
1990; Burr et al., 2002].
[4] The apparent decline in widespread fluvial activity
across much of Mars associated with rapid loss of surface
atmospheric volatiles has resulted in a paradigm of grad-
ually declining fluvial activity through the Noachian into
the Hesperian Periods, with only localized erosion during
the Hesperian [Tanaka, 1986; Carr, 1996]. Alternatively,
Grant and Schultz [1990] suggest that erosion and depo-
sition was episodic during the Noachian. It has also been
suggested that this decline in activity was associated with
a change in location and formative process of fluvial
erosion. On the basis of Viking images, Baker and
Partridge [1986] distinguished between older ??degraded??
valley networks on the Martian highlands, eroded by
precipitation runoff, and younger ??pristine?? reaches
restricted to downstream portions of the networks, which
were eroded by groundwater sapping. This evolution from
runoff to sapping has also been proposed by Harrison and
Grimm [2005].
[5] In this paper we present observations from recent
high-resolution images from the Mars Orbiter Narrow
Angle Camera (MOC NA) and the THEMIS visual
(VIS) and infrared wavelength (IR) cameras that indicate
that a distinctive style of fluvial erosion and deposition
occurred at a time period near the Noachian-Hesperian
boundary. We term this ??late-stage?? fluvial activity in this
paper. In accord with the observations of Baker and
Partridge [1986] we find deep, late-stage incision of
high-order (downstream) segments of valley networks in
the equatorial regions of Mars. In addition, local occur-
rences of large alluvial fans and fluvial deltas record a
period of deposition by sustained fluvial flows that appear
to have had both an abrupt onset and cessation. Erosion
earlier in the Noachian, although extensive, apparently was
not conducive to deep channel incision or to formation of
large alluvial fans and deltas. We suggest that these
observations are consonant with a period of intense fluvial
activity in the highlands of Mars lasting at least a
millennium. We begin with a short review of the erosional
process and landforms of the Noachian to contrast with
those of the Noachian-Hesperian transition. We limit our
discussion to the equatorial regions (latitude range ?30
)
because strong mantling and post-Noachian ice-related
processes at more polar latitudes complicate landform
analysis [e.g., Soderblom et al., 1973; Head et al.,
2003]. A companion paper [Irwin et al., 2005b] focuses
on lacustrine and deltaic features of equivalent age and the
relationship of late-stage fluvial activity to the evolution of
the highlands-lowlands boundary.
2. Noachian Fluvial Processes
[6] MOC, THEMIS, and MOLA data support sugges-
tions that extensive fluvial erosion and deposition occurred
throughout the highlands during the Noachian [e.g.,
Craddock and Maxwell, 1993; Craddock et al., 1997;
Grant, 2000; Hynek and Phillips, 2001, 2003; Craddock
and Howard, 2002; Grant and Parker, 2002; Irwin and
Howard, 2002]. Sediment yields were high, infilling
>10 km diameter craters with several hundred meters of
sediment and creating alluvial ramps at the foot of
highland relief, thus implying active chemical or physical
weathering [Craddock and Howard, 2002; Forsberg-
Taylor et al., 2004]. Erosion of massifs and inner and
outer crater wall slopes reached close to divides, and
drainage densities locally approached terrestrial values
[e.g., Irwin and Howard, 2002; Hynek and Phillips,
2003; Stepinski and Collier, 2004]. Integrated drainage
networks hundreds of kilometers long were formed where
regional slopes existed. Erosion of the highlands may
have averaged several hundred meters, although rates
should have varied with regional slope and local climate.
Estimates derived from depth of incision or basin infilling
are probably conservative because frequent impact crater-
ing reset the topography and disrupted drainage networks
[Irwin and Howard, 2002]. Both during and subsequent
to their formation these fluvial features were highly
degraded by impacts, eolian transport, mass wasting and
in many areas by extensive airfall mantling by dust,
large-impact ejecta, or volcanic ash [e.g., Grant and
Schultz, 1990; Moore, 1990; Malin and Edgett, 2000b;
Arvidson et al., 2003]. The statistics of degraded crater
infilling and crater counts in the highlands are consistent
with degradation primarily by fluvial processes at a rate
that was proportional to the rate of new impacts, imply-
ing a gradual decline in fluvial activity accompanying the
gradual decline in impact rate [Forsberg-Taylor et al.,
2004].
[7] The environment that supported such intensive
erosion during the Noachian is controversial, but the
prevailing interpretation is that widespread, although
episodic, precipitation (as snow or rain) occurred with
associated (melt) runoff and groundwater seepage. The
abundance of alluvial plains at the base of crater walls
and the small number of breached crater rims suggests
an arid climate, with ephemeral lakes in the larger
craters. Overall, however, long-term erosion rates during
the mid to late Noachian were slow, being comparable
to terrestrial hyperarid deserts (cold or warm) [Craddock
and Maxwell, 1993; Craddock et al., 1997; Hynek and
Phillips, 2001; Golombek and Bridges, 2000]. This
suggests either infrequent precipitation events, perhaps
correlated with brief optimal periods associated with the
pronounced quasi-cyclic climatic variation ofMars [Laskar et
al., 2004], or possibly associatedwith short optima associated
with impacts producing craters in the 100 km+diameter range
[Carr, 1989; Segura et al., 2002; Colaprete et al., 2004]. We
will discuss further the contrast between Noachian and the
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late-stage erosional environment based upon observations
presented below.
3. Late-Stage Fluvial Incision
[8] Relatively undegraded fluvial valleys are found
widely in the equatorial highlands. Sparse, arroyo-like net-
works incise 50?350 m into what appear to be earlier
Noachian fluvial basin deposits. We present observations
from four channel systems in the Martian highlands: the
southern Isidis basin margin, Evros Valles, Parana Valles,
and a region in the Margaritifer Sinus highlands (Figure 1).
3.1. Isidis Basin Margin
[9] Integrated valley systems up to 800 km in length drain
northward into the Isidis basin from its high-relief southern
rim [Crumpler and Tanaka, 2003] (Figures 2 and 3). We
examine below the geomorphic context of these valleys and
quantify the patterns of valley incision.
3.1.1. Geomorphic Relationships
[10] In their downstream reaches these valleys are typi-
cally 2?4 km wide, with flat floors and valley walls
approaching 30
 in steepness (Figures 4 and 5). Except
where they pass through deeper canyons where they
traverse degraded rim remnants of the Isidis Basin (the
Libya Montes), the valleys are typically sharply incised
50?350 m below relatively smooth intercrater plains
surfaces. Both Craddock and Howard [2002] and Crumpler
and Tanaka [2003] interpreted these plains to be Noachian
sedimentary deposits shed from surrounding uplands.
Figure 3a shows an elevation-cued interpretive map of
the downstream portions of one of these valley systems
(the ??eastern valley?? of Crumpler and Tanaka [2003])
showing the dissected basin deposits and contributing
eroded highlands. Figure 3b presents an interpretive
physiographic map of the region shown in Figure 3a.
This map partially overlaps the area mapped by Crumpler
and Tanaka [2003], and we identify in braces the units on
their map that most closely correspond to our mapping.
Uplands (brown) are high areas that are moderately
fluvially dissected {Nm}. Massifs (red) are dissected moun-
tains that comprise the Libya Montes {Nm}. Dissected
basin floors (cyan) lie at the foot of uplands or massifs
and appear to be generally planar surfaces that have been
strongly fluvially dissected {NHf}. Smooth basin floors
(blue) appear smooth at 100?200 m/pixel image resolution,
although they may be rough at meter scale {Hi}. This unit
occupies the lowest relative positions in the intercrater
regions and may locally be entrenched by through-flowing
drainage. Smooth crater floors (gray) comprise the interior
of strongly degraded Noachian craters {Hi}. Isidis basin
deposits (magenta) are mapped as an undifferentiated unit
{Hk and Ht}. Slightly degraded craters (light green) have
been only slightly modified by fluvial processes, and
postfluvial craters (light green) were emplaced after the
cessation of fluvial activity. The major valley networks
(yellow lines) were dated as Lower to Upper Hesperian
age on the basis of crater counts by Crumpler and Tanaka
[2003] {Hd}. We include the incised channels within our
late-stage features.
[11] In summary, by our interpretation the massifs and
uplands of the southern highlands were extensively eroded
during the Noachian, with sediment accumulating in inter-
crater basins and on crater floors. Some fraction of the
eroded sediment was delivered to the Isidis basin floor.
During the late-stage erosion the fluvial networks became
integrated and valley incision dominated over deposition in
the intercrater basins.
Figure 1. Shaded relief global map of Mars, extending from 0
 to 360
E. Locations of the main study
sites indicated as follows: a, Isidis rim (Figure 2); b, Evros Valles region (Figure 11); c, Parana Valles
region (Figure 15); d, Pediment map region (Figure 16). Note that circles for locations c and d are not
scaled to their corresponding figures.
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[12] Locally in the dissected and smooth basin floor units,
the upper walls of craters and incised valleys exhibit layered
sediments (e.g., Figure 6; also Figure 9 of Crumpler and
Tanaka [2003]), supportive of a depositional origin for the
basin deposits.
[13] The incised channels seldom extend headward onto
crater walls or upland slopes, but they can extend far
downstream, rejuvenating the Noachian drainage paths.
Although Baker and Partridge [1986] described such
valleys as ??pristine,?? MOC NA images show that they
were modified by cratering, mass wasting, and eolian
deposition, but to a lesser degree than the antecedent
network (Figure 5). The incised valleys may gradually
shallow and narrow until they merge imperceptibly into
the undissected basin floors or upland surfaces. Locally,
however, the valleys terminate abruptly in steep, amphithe-
ater headwalls (Figure 7). Note that the incised valley
shown in Figure 7 heads in an upland basin just outside
the rim of a 75-km-diameter crater whose floor is about
1500 m below the amphitheater head of the incised valley.
[14] The upper walls of incised valleys and the intervalley
surfaces of smooth and dissected basin floors often display
an irregular surface texture with decameter-scale knobs and
furrows, and, locally, a scabby texture (Figures 5, 8, and 13).
Figure 2. Shaded relief map of the southern rim of the Isidis basin. Map is centered on 90
E and 0
N,
covering approximately 80
 to 100
E and 10
S to 10
N. Axis labels are in kilometers relative to center
of figure. Channel profiles were constructed for the drainage networks shown in white and red (e.g.,
Figure 9), with the red channels being used to quantify the amount of incision as a function of gradient
(see Figure 10). Location of Figure 3 shown in black outline.
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Where present, this textured surface often appears light-
colored in MOC NA images. These same surfaces often
appear bright in nighttime THEMIS IR images (Figures 4b,
7b, 12), indicating a high thermal inertia.
[15] Exposed channels within valleys are rare on Mars
due to postformation eolian infilling and mass wasting.
However, a THEMIS VIS image reveals an inner channel
on the east branch of the ??eastern valley?? (Figure 8).
3.1.2. Valley Profiles and Valley Incision
[16] We investigated the longitudinal profiles and the
spatial pattern of incision of the channels on the Isidis basin
margin using MOLA data. The centerline of the major
valley systems draining toward the northern lowlands
(Figure 2) was digitized from the Viking Orbiter Mars
Digital Image Mosaic (MDIM) 2.1 global image mosaics at
about 3 km intervals, providing longitude and latitude pairs
for each point. For each such point all MOLA PEDR (shot)
observations within a specified search radius (3 km here)
were utilized to determine the point with the lowest eleva-
tion, which is generally the valley floor. In addition, the
maximum and average elevations within the search radius
were recorded, as well as the 25th, 50th, and 75th percentile
of elevations sorted from lowest to highest. If no MOLA
observations fall within the search box, that digitized point
Figure 3. Landforms of the central southern Isidis Rim,
centered on the ??eastern valley?? of Crumpler and Tanaka
[2003]. (a) Elevation-cued image from the MDIM 2.1
Viking image mosaic. Image dimensions 600 
 600 km,
centered on 90
E and 0
N. See Figure 2 for context. Box 1
shows location of Figure 4, box 2 is location of Figure 7,
and box 3 is location of Figure 5. (b) Interpretive landform
map on shaded-relief image from MOLA topography. Key:
yellow, major valleys; magenta, Isidis basin deposits
(undifferentiated); blue, smooth basin floor; cyan, dissected
basin floors; brown, uplands; red, massifs; gray, smooth
crater floors; dark green, postfluvial craters; light green,
slightly degraded craters.
Figure 4. Incised channels and paleolake. (a) THEMIS
daytime IR images overlain on MDIM 2.1 mosaic. Located
at box 1 in Figure 2a. Image width 97.3 km. (b) THEMIS IR
nighttime images of the same location. The valleys in the
north end of the figure are interpreted to be incised into
earlier alluvial basin deposits deposited at the foot of
highlands bordering the basin (Figure 3b). Incised valley
floors have low TI (dark in right image) due to eolian
infilling, but upper rims and the dissected basin deposits are
relatively bright, suggesting coarse sediment or indurated
deposits. A double crater in the lower right has both
entrance and exit breaches, suggesting that it was a
paleolake. The valley system continues about 450 km
upstream from the paleolake (Figure 9b). Box 1 shows
location of Figure 6, and box 2 shows location of Figure 8.
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is dropped from the database. Because the valleys are
generally sharply incised below a comparatively planar
surface, the 75th percentile elevation is assumed to closely
correspond to the level of the surface into which the valley
has incised. Figure 9a shows a profile of the lowest and
75th percentile elevations for the westernmost red channel
in Figure 2. As can be seen the search procedure generally
provides smooth valley and upland estimated profiles. This
contrasts with construction of valley profiles from gridded
data which results in more irregular profiles that are biased
toward higher elevations because the gridding uses averages
of MOLA observations. The use of the 75th percentile
elevations diminishes the influence of local highs such as
nearby crater rims in defining the upland surface.
[17] Valley profiles as estimated by the MOLA shot
data are generally smooth and downstream-sloping. The
major artifacts from the present procedure occur where
postincisional cratering occurred on or near the channel
(e.g., Figure 9a), and occasional apparent local deforma-
tion by wrinkle ridges. This indicates that the Martian
valley systems were well-integrated, although they are
commonly stepped in the Isidis region, with locally steep
Figure 5. Incised valleys and basin deposits (MOC NA
image R1002826). Image width 3.03 km. Located at box 3
in Figure 3a. Note the steep valley walls, the eolian valley
infilling, and the rough, scabby texture of the intervalley
surface.
Figure 6. Layered deposits exposed in the upper wall of a
crater (MOC NA image E0902318). Located at box 1 in
Figure 4. Image width 3.11 km.
Figure 7. Incised valley. (a) THEMIS daytime IR images
overlain on MDIM 2.1 mosaic. Located at box 2 in
Figure 3a. Image width 79.7 km. (b) THEMIS IR nighttime
images of the same location. Note that the amphitheater-like
valley head extends nearly to the outer rim of the 70 km
diameter crater whose floor is about 1500 m below the
valley head. The valley margins and the dissected basin
deposits in the upper right corner of the image are bright in
the nighttime thermal IR, indicating high thermal inertia.
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reaches where the valleys pass through massif or upland
ridges. An exception is the occurrence of a 180-m
depression along the course of the valley profiled in
Figure 9b at the location where the valley passes through
the crater basin shown in Figure 4. This crater basin has
both entrance and exit breaches, suggesting that the basin
was at least occasionally occupied by an overflowing
lake.
[18] From examination of a number of valley profiles it
became apparent that the depth of incision below the upland
surface was greater where the valley gradient was steepest.
We quantified this by measuring valley gradient, apparent
depth of incision, and channel length measured from the
upstream limit of the incised valley network along 41
reaches in the Isidis basin rim (red lines in Figure 2). In
selecting reaches we utilized only valley segments at least
ten kilometers in length along which the valley profile had
nearly constant gradient and depth of incision. We avoided
sites where the valley incised through narrow ridges or
where it was affected by postfluvial impacts. The relation-
ship between apparent depth of incision, DZ, valley
gradient, S, and cumulative downstream length, L, was
determined by linear regression of the logarithms of the
variables, yielding power function equations. The simplest
relationship is between incision depth and valley gradient
(Figure 10):
DZ ? 1452 S0:48; R2 ? 0:60; ?1?
where R2 is the fractional data variance explained by the
regression and DZ is in meters. When downstream length is
included,
DZ ? 993 L0:15 S0:55; R2 ? 0:68; ?2?
Figure 8. Inner channel within incised valley. Located at
box 2 in Figure 4. A portion of THEMIS VIS image
V03442003. Image width about 8.96 km.
Figure 9. Representative profiles of valley systems in the
Isidis region constructed from MOLA PEDR data. Valley
bottoms are the lowest elevations recorded in the vicinity of
the digitized location, and the upland surface is the 75th
percentile (measured from the lowest local elevation)
elevation within a 6 km search window. Profile a is along
the westernmost red channel shown in Figure 2, and profile
b is along the south-trending red valley in the center of
Figure 2. Figure 4 shows about 200 km of this valley system
between about downstream distance 400?600 km. Note the
180 m depression within the presumed paleolake basin
shown in Figure 4.
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where L is in kilometers. The analysis thus confirms a
positive relationship between incision depth, gradient, and
cumulative downstream length.
3.2. Evros Valles
[19] The Evros Valles system south of the Schiaparelli
basin drains westward from 20
E to 5
E at about 12
S. It
lies at elevations from 0 to +1500 m. The main valley and
major tributaries are incised 100?300 m into a broad basin
surface graded to the center of the basin through which the
valley flows (Figure 11). Faintly visible tributaries flow
across the basin surface toward the main valley (Figure 11).
These valleys are shallower, with gentler and less well-
defined sidewalls than the incised master drainage, and they
often join the incised valleys through profile convexities
(knickpoints).
[20] As with the Isidis rim valleys, the upper valley walls
appear bright in nighttime THEMIS IR, implying a high
thermal inertia (Figure 12). These upper valley walls also
have a rough, massive texture that appears bright in MOC
NA images (Figure 13). The upper valley walls and fresh
crater walls on the Evros basin locally display crude
layering, suggesting the valleys have incised into sedimen-
tary deposits (Figure 14).
[21] As with the Isidis basin valleys, the depth of
incision along Evros Vallis correlates with profile gradient
(Figure 10), although for a given gradient the depth of
incision is greater than is typical for the Isidis basin
valleys.
3.3. Margaritifer Sinus Region
[22] The Parana Valles system (Figure 15) also exhibits
a branching valley system deeply entrenched below the
level of a broadly sloping upland surface that forms the
eastern rim of a 330-km-diameter basin (the Parana
Basin of Grant and Parker [2002]) between the Parana
Valles system and its presumed continuation on the
northwestward side of the basin as Loire Valles [Grant,
1987, 2000; Grant and Parker, 2002]. The origin of the
upland surface is uncertain because it lacks highlands
surrounding the basin which could have created a depo-
sitional platform. Strong Noachian denudation of this
upland is suggested by the two highly degraded craters
on the northeast side of Figure 15, the easternmost one of
which has an outer rim that is highly eroded. Simulations
by Craddock and Howard [2002] show that such upland
??ghost?? craters with flat floors can result from long-
continued fluvial erosion of craters that form on local
Figure 10. Relationship between the average depth of
incision and valley gradient for segments of valleys 10 km
or more in length. Locations of valleys in the Isidis Rim
region included in the database are shown in Figure 2.
Figure 11. Evros Vallis overview. THEMIS daytime IR on MDIM 2.1 mosaic. Image centered at
14.51
E, 12.98
S. Image width 219 km. Box 1 shows location of Figure 12, box 2 shows location of
Figure 13, and boxes 3 and 4 are Figures 14b and 14c.
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highlands. The crater walls retreat, infilling the basin
floor while external rim erosion creates deeply incised
outer rims. The basin interior remains undissected until
the outer wall has breached, which appears to have
occurred on the westernmost of the two degraded craters
just prior to the cessation of fluvial erosion. Thus the
highly degraded craters suggest deep erosion of the
upland surface, but the broad intervalley flats between
the incised valleys are most consistent with localized late-
stage valley incision into a relatively planar upland.
[23] As with the Isidis rim and Evros Valles system, the
depth of valley incision correlates with downstream valley
gradient (Figure 10), with incision depths consistent with
averages for Isidis rim valleys.
[24] Another valley system in the Margaritifer Sinus
region exhibits a suite of landforms that suggest a compli-
cated erosional history (Figure 16). A broad ridge trending
northwestward exhibits a radial drainage pattern which is
segmented into concentric zones of similar degree of
dissection bordered by downslope-facing scarps. These
surfaces, described below, are most clearly expressed on
the slope extending to the northeast from the ridge line. The
highest zone is the narrow summit plateau mapped in red in
Figure 16c. This ridge appears to be a remnant of a broad,
sparsely dissected upland to the southeast of the mapped
area which, during the Noachian, was being incised and
backwasted by a series of northwestward-draining valleys,
including the main drainages in Figure 16. This narrow
plateau is bordered by a highly dissected sloping surface
(blue in Figure 16c) which terminates downslope in a low
scarp. Below this scarp lies a less dissected surface (green in
Figure 16c) also bordered by a low scarp, below which is a
broad, undissected valley floor (yellow) along the main
drainageway.
[25] Interpretation: The spatial and vertical sequence of
these surfaces, and the increasing degree of dissection
with height suggest an evolutionary scenario involving
alternating entrenchment and stability of the main drain-
ageways. Erosion along the main drainageways initiated
dissection of the highland surface (red). A period of
relative stability of the main channels encouraged devel-
opment of a low-relief pediment or alluvial fan surface
leading from the edge of the retreating highland scarp to
Figure 12. Nighttime THEMIS IR mosaic of Evros Valles.
Located at box 1 in Figure 11. Image width 43.4 km.
Figure 13. Detail of MOC NA image M1102194 of Evros
Valles. Image width 2.18 km. Located at box 2 in Figure 11.
Crater on valley floor is possibly exhumed, which would
indicate sedimentary burial of the basin floor during the
Noachian.
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the main drainageway, probably in the general location of
the present valley axis along the main valley (yellow).
This was followed by an episode of entrenchment along
the valley axis, initiating erosional dissection of the
pediment/fan. Main valley incision again paused, allowing
development of an erosional scarp moving headward into
the earlier (blue) pediment surface coupled with formation
of a new pediment/fan surface (green) graded to the
valley axis. Finally, another pulse of erosion followed
by stability caused incision below the green pediment/fan
surface and development of a new surface (yellow) along
the valley axis. Similar sets of pediments/fans of increas-
ing age and dissection with distance from the master
drainage occur in the Southwestern United States [Bull,
1991], particularly along the Rio Grande valley [Ruhe,
1967]. Episodes of valley erosion followed by stability or
aggradation of drainageways can be caused either by
headward migration of knickpoints or climatic oscillations
affecting the balance of erosional sediment influx versus
discharges through the fluvial system.
4. Alluvial Fans
[26] Large alluvial fans on the interior of a few 50+ km
craters have been discovered in recent high-resolution
images of the equatorial highlands [Moore and Howard,
2005]. These fans are 10 to 40 km in length and extend
across the crater floors from intricately dissected alcoves
eroded into the crater walls (e.g., Figure 17). These fans
have gradients and morphometric relationships to their
source basins that are similar to terrestrial counterparts.
Probable remnants of distributary channels are often
visible on the fan surfaces, radiating from the fan apex
at the base of the source basins (Figure 17). Age dating
of these fans from crater counts [Moore and Howard,
2005] places their origin to approximately the Noachian-
Hesperian boundary, so that we consider below the
possibility that they formed coevally with the incised
valley systems. On terrestrial fans rainfall directly on
the fans typically results in development of shallow valley
networks on inactive portions of the fan. The Martian
fans exhibit no on-fan drainage networks. The fans also
usually lack fan-head trenches common on terrestrial fan
systems and commonly associated with climatic transi-
tions from Quaternary pluvial climates to more arid
interglacials.
5. Deltas
[27] Recent high-resolution images have also revealed
probable deltas within crater basins. The most definitive
case occurs in the provisionally named Eberswalde crater
at 326
E and 24
S [Malin and Edgett, 2003; Moore et
al., 2003; Bhattacharya et al., 2005], where meandering
Figure 14. Exposed layering in the Evros Valles drainage
basin in MOC NA images. Figures 14a and 14b show
layering exposed in the walls of craters on the basin floor,
and Figure 14c shows layering in the sidewall of a tributary
to Evros Valles. Figure 14b is located at location 3 in
Figure 11, and Figure 14c is at location 4. Figure 14a is
downstream of Figure 11 at 9.4
E and 12.2
S. The plateau
surfaces above the valley are labeled ??T??.
Figure 15. A portion of the Parana Valles. THEMIS IR
mosaic on MDIM 2.1 mosaic. Image centered at 349.29
E,
22.72
S. Image width 78 km.
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distributary channels on the delta surface have been
etched into positive relief by wind erosion. Other prob-
able deltas have subsequently been identified in THEMIS
or MOC images [Irwin et al., 2004b, 2005c; Fassett and
Head, 2005] and others were postulated on the basis of
lower-resolution Viking Orbiter images [Cabrol and Grin,
1999; Ori et al., 2000]. Figure 18 shows a probable delta
in a crater northeast of the Hellas basin that radiates
about nine kilometers onto the crater floor from the
termination of a canyon. The contributing valley eroded
through the crater rim, fed by a network of valleys
draining a perched basin bordered by the rims of three
nearby craters. Figure 19 shows a THEMIS VIS mosaic
of the delta, which displays several fingers protruding
beyond the main delta front that may have been small
birdfoot distributaries. A few possible linear features that
may be remnants of distributary channels occur on the
delta surface, but they are poorly preserved. This delta,
like the one in Eberswalde, has been differentially eroded
resulting in inverted relief of channel deposits. The delta
top slopes at about four degrees toward the center of the
crater, and the marginal scarp (probable foreset beds)
rises about 60 m from the crater floor. On the basis of
topographic profiles from MOLA orbits crossing the
delta, the volume of sediments is approximately
10.2 km3. The canyon connecting the delta to the upland
basin is about 240 m deep, and channels on the upper
basin range from 10 to 100 m in depth. A crude estimate
of the volume of the valley network feeding the delta
based upon width, depth, and total length of the valleys
visible in Figure 18 is 7.5 km3. Thus it appears that the
delta is primarily or wholly constructed of sediment
eroded from the visible valley network, and that it formed
coevally with the network. The gradient of the perched
basin surface immediately above the canyon through the
crater rim equals that of the delta top surface, which is
also supportive of a coeval fluvial origin. A similar
conclusion was reached for the Eberswalde crater delta
and its upland valley network [Moore et al., 2003].
[28] The craters that surround the upland basin, including
the one containing the probable delta appear to be strongly
degraded, with obliteration of ejecta and infilling of crater
floors. The majority of this degradation must have occurred
prior to the incision of the visible valley network and
formation of the deltaic deposit.
6. Discussion: Implications for Martian Erosional
History and Environments
[29] The suites of landform features that we discuss
above appear to present a consistent pattern among widely
distributed regions of the Martian equatorial cratered
highlands. In the following discussion we present our
interpretation of the sequence of events forming these
Figure 16. Pediments and fans in Margaritifer Sinus region. (a) THEMIS daytime IR image I01686001.
(b) Image with elevation cueing. (c) Interpretation based upon landforms northeast of the central ridge
(red) and speculatively extended elsewhere. Age sequence from oldest to youngest is red-blue-green-
yellow (final drainage level). Image located at 357.02
E, 25.86
S. Image width 31 km.
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features, the possible environmental controls producing
them, and remaining issues and uncertainties.
6.1. Erosional and Depositional History
[30] The trunk channels of valley networks generally are
sharply incised 50 to 350 m below relatively planar upland
surfaces (Figures 3, 4, 7, 9, 11, and 15). Valley sidewalls are
steep and generally show an abrupt scarp at the edge of the
upland surfaces (Figures 5, 8, 13, and 20). The steep valley
edges and deep incision prompted Baker and Partridge
[1986] to label these as ??pristine?? valleys, although recent
high-resolution images reveals that they have undergone
significant postfluvial modification by eolian infilling, mass
wasting, and impact cratering. We prefer the more neutral
term ??incised valleys??. It is likely that most of the valley
networks mapped from Viking images [e.g., Pieri, 1980;
Carr and Clow, 1981; Carr and Chuang, 1997] represent
incised valleys of the type discussed here.
[31] Most commonly the valleys dissect smooth-surfaced
(at 100+ m scales) intercrater basins whose outer margins
slope upward toward crater rims or eroded uplands (e.g.,
Figures 3 and 4). In some localities the upper walls of
incised valleys and impact craters excavated into the basin
surfaces exhibit crude layering (e.g., Figures 6 and 14). This
suggests that the incised valleys dissect basins formed from
sediment shed from adjacent uplands. The layering is less
well defined than that of the intracrater deposits and
regional blankets described by Malin and Edgett [2000b].
The fine-scale layering and ease of erosion of the latter
deposits suggests fine-grained deposits of lacustrine or
airfall origin, whereas the less distinct and less regular
layering of the valley walls is consistent with coarser fluvial
sedimentation. The pattern of basins dissected by the incised
valleys suggests a temporal change from fluvial aggradation
or planation of basin surfaces to a later epoch of incision.
[32] In some locations the pattern of incision appears
to suggest a more varied history. The region shown in
Figure 16 appears to have experienced alternating periods
of formation of broad pediment or fan surfaces, presumably
associated with stable or aggrading main channels, and
incisional epochs, leading to fluvial dissection of the fans
and pediments. Overall, however, these valley systems have
undergone long-term incision and the areal extent of
depositional pediments and valley bottoms appears to have
been increasingly restricted toward the valley axis through
time, whereas the earlier, broader pediment surfaces became
increasingly dissected by fluvial erosion.
Figure 17. Fan complex North of the Hellas Basin. This is
fan K of Moore and Howard [2005]. THEMIS daytime IR
mosaic centered at 72.78
E, 21.81S. Image width 80.6 km.
Figure 18. Delta and contributing basin NE of Hellas.
THEMIS daytime IR. Arrows point to delta front. Mosaic
centered at 83.03
E, 28.31
S. Image width 51.5 km.
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[33] Where incised channel systems debouched into
enclosed basins, such as at Eberswalde crater [Malin and
Edgett, 2003; Moore et al., 2003] and the basin shown in
Figure 18, deltas were occasionally deposited [also see
Irwin et al., 2005b]. The equivalence of delta volumes to
that of the feeding incised channel systems suggests that
they formed coevally with the incised channels. The for-
mation of deltas suggests sufficient duration of flows to
create lakes.
[34] A number of large alluvial fans with runoff and
sediment sources from steep, intricately dissected alcoves
in crater walls have been identified on Mars [Moore and
Howard, 2005] (Figure 17). Because their source basins are
restricted to inner crater walls we cannot definitively link
their origin to the same time period and same hydraulic
regime as the incised channels and deltas. However, age
dating based upon crater counting suggests an age in the
same general time period of Martian history. The fans, like
the deltas, imply appreciable, probably repeated flows
extending over a time interval of many hundreds or
thousands of years [Moore et al., 2003; Moore and Howard,
2005]. Both the deltas and the fans share the important
characteristic that the formative flows ceased abruptly,
because the deltas were not dissected as a result of waning
flows and lower lake levels [Irwin et al., 2005b], and the
fans likewise did not experience fan-head trenching or late-
stage restriction of deposition to near the fan apex. Further-
more, the fans and deltas appear to lack examples of
equivalent features formed earlier in Martian history. Al-
though degraded Noachian craters have inward-sloping
walls suggestive of alluvial deposition of sediment derived
from erosion of crater rims [Craddock and Howard, 2002;
Forsberg-Taylor et al., 2004; Moore and Howard, 2005],
the gradients of these surfaces are less than the large alluvial
fans and they appear to have formed as bajadas from
relatively uniform dissection of crater walls rather than
localized incision of alcoves as in the case of the younger
alluvial fans. Although numerous crater basins have inflow-
ing valleys through crater wall breaches, obvious deltas are
limited to where late-stage incised valleys occur upstream
from the delta, and the delta volumes approximate the
volume of the incised valleys. The similar age and deposi-
tional history suggest that both fans and deltas formed
during the same time period on Mars. Notably, we have
not been able to identify older, degraded alluvial fans or
deltas of similar morphology to those discussed here within
highland basins or craters. This suggests that the Noachian
Figure 19. THEMIS VIS mosaic of delta shown in
Figure 18. Image width 17.4 km.
Figure 20. Incised valley breaching a crater rim. The
probable cause of the incision is headward migration of
knickpoint after breaching of the rim of the crater at the left
of the image. THEMIS daytime IR image I01684010,
centered at 29.5
E, 0.0
N. Image width 31 km. Deeply
incised channels with sharp rims are labeled P, for
??pristine,?? and shallow valleys with indistinct rims are
labeled D, for ??degraded.?? See text for further discussion.
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erosional environment differed markedly from that forming
the late-stage fluvial features.
6.1.1. Hydrologic Characteristics of Late-Stage Fluvial
Activity
[35] The relative freshness of the late-stage fluvial fea-
tures discussed here has permitted estimation of the mag-
nitude of associated flows. Moore et al. [2003] utilized the
width and meander wavelength of distributaries on the
Eberswalde crater delta to estimate a formative discharge
of approximately 700 m3/s based upon scaling of terres-
trial hydraulic geometry relationships to Martian gravity.
Jerolmack et al. [2004] made a similar discharge estimate
based upon a sedimentary model of fan/delta deposition.
This discharge is approximately equivalent to the mean
annual flood in terrestrial drainage networks of similar
contributing area.
[36] Using similar hydraulic geometry scaling Irwin et al.
[2005a] have utilized the width of inner channels in incised
valleys to estimate discharges. They also estimated dis-
charges of the same order of magnitude as mean annual
floods in terrestrial drainage networks of similar contribut-
ing drainage area. The inner channel shown in the Isidis rim
drainage in Figure 8 was not included in the Irwin et al.
[2005a] database. The floor of the inner channel averages
183.6 m in width. Irwin et al. [2005a] utilize the scaled
empirical relationship
Q ? 1:4W 1:22 ?3?
to estimate the channel-forming discharge, where discharge,
Q, is in m3/s and channel floor width, W, is in m. This
provides an estimated discharge of 809 m3/s. Irwin et al.
[2005a] also use the estimated discharges and contributing
drainage areas to develop a formative discharge ? drainage
area relationship for incised channels on the Martian
highlands:
Q ? 57A0:33; ?4?
where drainage area, A, is in km2. The inner channel in
Figure 8 lies about 370 km downstream from its headwater
source. Using a scaling relationship relating drainage area to
mainstream length developed for terrestrial drainage net-
works (??Hack?s Law?? [Hack, 1957; Rodriguez-Iturbe and
Rinaldo, 1997]):
L ? 1:3A0:6; ?5?
the indirectly estimated discharge from equations (4) and (5)
is 1276 m3/s. The more direct estimate from channel floor
width is probably more accurate, but the estimate based
upon drainage area suggests that the Isidis channel
discharge is consistent with other highlands inner channels.
The consistent relationship of discharge with contributing
area, combined with the large magnitude of the discharges,
strongly suggests flows through the valley networks were
fed by precipitation, either directly from rainfall-runoff or
indirectly from snowmelt, as suggested earlier by Craddock
and Howard [2002].
[37] Whether the interior channels, such as that shown
in Figure 8, have alluvial or bedrock banks is uncertain.
If they are alluvial, the channel-forming flows would be
equivalent to flows with recurrence interval of one to two
years in terrestrial channels of equivalent size and con-
tributing area. On the other hand, if the channel banks are
bedrock, even larger channel-forming flows might have
been necessary.
[38] These discharge estimates suggest appreciable dis-
charges through the late-stage valley systems, but they leave
uncertain the frequency and duration of such flows. If flows
shared the recurrence interval characteristics of terrestrial
drainage networks fed by precipitation, then the Eberswalde
delta would have required thousands to tens of thousands of
years to form [Moore et al., 2003; Bhattacharya et al.,
2005]. By contrast, Jerolmack et al. [2004] suggest the
overall flow period might have been only tens of years if
flows were continuous rather than intermittent, and Moore
et al. [2003] also acknowledge that the formation interval
could be short if precipitation were caused by climatic
change induced by a large impactor [Segura et al., 2002].
6.1.2. Potential Causes of Valley Incision
[39] A pervasive pattern of late-stage valley incision
occurs throughout the Martian cratered highlands within
the trunk valleys. This incision contrasts with a dominance
of local erosion of uplands and deposition within nearby
basins earlier in the Noachian, with, apparently, less well-
developed through-flowing drainage. A number of geologic
and hydrologic controls could potentially cause valley
incision. We evaluate possible causal conditions within
the context of the observations and analyses presented
above.
[40] River systems draining long regional slopes will
gradually lower due to long-term erosion of the entire
landscape. More or less spatially uniform downcutting of
master drainage channels is documented in many terrestrial
river systems. Such long-term erosion will not result in
incised valleys unless either (1) the valley system is
young and cutting into a previously uneroded surface or
(2) environmental changes or tectonic deformation accel-
erate the erosional potential of the master streams relative
to their smaller tributaries. The incised Martian valley
systems appear to be superimposed upon a landscape
created by extensive prior erosion (e.g., the Isidis rim
region shown in Figures 2 to 8) so that an uneroded
constructional surface is not apparent. Situations of this
second type are discussed below.
[41] Over longer timescales most terrestrial fluvial sys-
tems undergo gradual reduction of relief within the entire
drainage basin, which results in a diminishment in the
quantity and coarseness of the sediment supply. Such land-
scapes, however, are characterized by minimal relief in the
higher-order valleys and broad alluvial plains. Valley pro-
files are concave. Most Martian valley networks retain high
relief, and high order valleys are incised rather than allu-
viated. Valley profiles are commonly irregular, with alter-
nating steep and gentle reaches (Figure 9).
[42] Channels may incise due to headward migration of
knickpoints. Knickpoints (locally steep reaches) commonly
migrate headward through terrestrial valley systems. This
downstream control of channel incision can occur due to,
for example, local steepening of valleys by tectonic
faulting or warping. Local incision of master valleys can
also cause headward migration of knickpoints within
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tributaries. Irwin et al. [2004a], for example, conclude
that rapid incision of Ma?adim Vallis due to paleolake
overflow caused tributaries to be oversteepened at their
junction with the stem valley (hanging valleys). This
apparently caused headward migration of knickpoints
through the tributary valley systems. Another potential
cause of knickpoint formation and headward migration
occurs when fluvial channels breach crater rims, creating
a locally steep reach at the breach site that erodes head-
ward. Irwin and Howard [2002] cite possible examples in
Martian valley networks, and Figure 20 shows a likely
example of a breached crater with an incised reach
upstream, probably caused by headward migration of
knickpoints. Simulation modeling of long-term erosion
of cratered landscapes provides examples of headward
incision driven by crater rim breaching (Figure 21). In this
simulation rim breaching occurs as a joint result of
erosional attack of the inner crater wall and infilling of
the basin by sediment eroded from adjacent uplands.
Although crater rim breaching, possible local faulting,
and flood incision of master drainage might account for
local cases of valley incision, they are unlikely to have
caused the pervasive late-stage incision within the Martian
cratered highlands. In particular, the drainage network
shown in Figure 16 flows into a flat floored 360 km
diameter basin, so that the base level for the valley system
has either been stable or aggrading.
[43] Baker and Partridge [1986] and Harrison and
Grimm [2005] suggest that incised (??pristine??) valleys
formed by headward migration of knickpoints driven by
sapping erosion as groundwater emerged in the lower
reaches of valley networks. According to this scenario,
the groundwater outflow represents either the last stages
of draining of aquifers that were recharged by precipita-
tion during the Noachian or a hydrologic cycle driven by
basal melting of snow and ice accumulated through
precipitation [Carr and Head, 2003]. A number of
observations cast doubt on the efficacy of groundwater
flow to create the incised valley networks. The dimen-
sions of inner channels and delta distributaries suggest
formative discharges through late-stage valleys approxi-
mately equal to terrestrial channel-forming flows for
equivalent contributing areas. Such discharges could only
occur from effluent groundwater if permeability were very
high, such as open-framework gravels. However, mainte-
nance of such flows over a sufficient time to incise valley
networks 50?350 m would require appreciable, areally
distributed recharge through precipitation. Incised valleys
occur at elevated locations with drainage basins (Figure 7)
which are unlikely to be discharge locations for regional
groundwater flow. In the situation shown in Figure 7 it
would be most likely for regional groundwater flow to
emerge at the floor of the nearby crater, some 1500 m
below the level of the incised valley. Finally, depth of
incision within valley networks shows a consistent relation
to contributing area (parameterized by downstream dis-
tance) and valley gradient (equation (2)), which would be
expected for basin-wide incision but unlikely for head-
water migration of knickpoints.
[44] Valley incision can also occur because of changes
in the hydrologic regime and sediment supply. In bedrock
channel systems the rate of erosion depends upon flow
Figure 21. Simulated formation of incised channels due to
entrance breaching of an impact crater. Details of the
simulation model process formulation are presented by
Howard [1994, 1997] and Forsberg-Taylor et al. [2004].
The simulated domain is about 100 km across. (a) A
simulated fresh impact crater has formed in the right-center
of the image within a relatively low elevation region. Note
that random impacts and fluvial erosion have been
occurring prior to this impact. (b) At a later stage the rim
of the impact crater has been breached in several locations,
resulting in formation of deeply incised valleys through
headward migration of knickpoints. Breaching occurs both
due to lowering of the crater rim by erosion occurring on the
inner wall and due to continued infilling of the basin
surrounding the crater by sediment deposited from erosion
of surrounding uplands. In this simulation it is assumed that
no runoff ponding occurs, so that breaching is not a
consequence of lake overflow. Groundwater sapping is also
not included as an erosional process. Note that additional
impact craters have formed during the intervening time. The
fluvial erosion is scaled to terrestrial semiarid environments,
with about 200,000 simulated years occurring between
Figures 21a and 21b.
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magnitude (see below), so that changes in flow regime
could result in altered incision rates. In alluvial channel
systems gradient depends functionally on the discharge
regime, and the size and quantity of sediment supplied
from the basin. In sand-bed channels, gradient increases
with the sediment throughflow and decreases with in-
creasing discharge. In gravel channels the balance is
primarily between the size of supplied sediment and the
discharge [e.g., Howard, 1980]. Thus incision might
occur because of increased effective discharges or reduc-
tion in the size or quantity of supplied sediment.
[45] Valley incision due to a change in hydrologic regime
or sediment supply is supported by the consistent rela-
tionship between the depth of incision, valley gradient,
and position downstream within the drainage network
(equation (2)). In fact, this relationship is consistent with
models of bedrock channel incision in which the rate of
downcutting, @z/@t, functionally depends upon some mea-
sure of the flow intensity, J:
@z
@t
? K J Jc? ?; ?6?
where K is substrate erodibility and Jc is a critical flow
intensity required to initiate incision. For simulation of
long-term channel incision the flow intensity is commonly
parameterized as a power function of contributing drainage
area, A, and local gradient, S, through the use of equations of
hydraulic geometry [e.g., Howard, 1994, 1997; Forsberg-
Taylor et al., 2004]:
@z
@t
? K C Am Sn  Jc? ?; ?7?
where C is a constant. A functional dependence of erosion
rates of this type has been observed in a number of terrestrial
drainage basins, ranging from badlands to high-relief
mountains [e.g., Howard and Kerby, 1983; Stock and
Montgomery, 1999; Tucker and Whipple, 2002]. In parti-
cular, if the appropriate measure of flow intensity is assumed
to be the mean shear stress exerted on the bed and discharge
is assumed to be a power function (exponent e) of drainage
area:
Q / Ae; ?8?
then the exponents take the values [Howard, 1994]:
m  0:3e; ?9?
and
n ? 0:7: ?10?
The observed exponent relating Martian incised valleys in
equation (2) (0.55) is close to the gradient exponent, n, for a
shear stress dependency. If equations (4), (5), and (9) are
combined, the predicted exponent for shear stress depen-
dency of incision rate on channel length equals 0.17, close to
the 0.15 observed in equation (2).
[46] Many of the low-gradient valley segments utilized in
the incision database are probably primarily alluvial chan-
nels. The rate of bed erosion in alluvial channels depends
upon the spatial divergence of sediment flux (which can be
expressed in terms of a spatial variation in shear stress)
rather than the local value of shear stress, which is hypoth-
esized to correlate with bedrock channel incision rates. The
observation that Martian valley incision rates correlate with
surrogate measures of shear stress (gradient and down-
stream length) suggests that erosion of the alluvial channel
reaches is limited by the rate of incision into bedrock
exposures within or immediately below the alluvial reaches.
[47] Incised valleys can also occur due to erosion through
layered bedrock. An incised channel can result were a
channel cuts through a resistant rock unit into a weaker
underlying unit. This is a potential explanation for valleys
such as Nirgal Valles, which cuts into Hesperian basalt
flows, possibly having deepened and widened upon expo-
sure of underlying weaker rocks. Exposure of weak layers
in the highland bedrock stratigraphy, however, is unlikely to
be a universal explanation for late-stage valley incision. The
presence of rough-textured, apparently indurated material
exposed in the upper walls of many incised valleys (e.g.,
Figures 5, 8, 13, and 14) raises the possibility that a resistant
layer, presumably a duricrust, has hardened the surface
layers of much of the cratered highlands. In terrestrial arid
and semi-arid environments duricrusts sometimes develop
sufficient strength and thickness to cause inverted topogra-
phy during subsequent erosion [Dixon, 1994]. Weak dur-
icrusts have been found at the Viking, Pathfinder, and MER
lander sites, but they would not be of sufficient strength to
affect erosion. Unfortunately, no landed mission has exam-
ined intact Noachian-age surfaces, so that direct confirma-
tion is not possible at this time. The high thermal inertia of
many upper incised canyon walls (e.g., Figures 4b, 7b, and
12) could be consistent with a resistant duricrust. More
speculatively, the restriction of formation of large alluvial
fans to localized sites on crater walls [Moore and Howard,
2005] (e.g., Figure 17) could result from erosion of alcoves
only in locations where a duricrust has been breached. If
such ??case hardening?? of the Noachian surface did occur, it
would impose a critical shear stress (e.g., equation (7)) upon
fluvial erosion and sediment transport that could restrict
subsequent erosion only to downstream locations where
sufficient water accumulates. It could also reduce sediment
supply from headwaters, also tending to increase incision.
[48] In summary, the most likely explanations for the
widespread late-stage incision of valley networks are a
change in the hydrologic regime or sediment supply within
the drainage basins, or possibly development and erosional
breaching of a widespread duricrust layer. Possible envi-
ronmental controls for such changes are discussed next.
6.1.3. Environmental Conditions During Late-Stage
Fluvial Activity
[49] The observations cited above present a consistent
picture of the late-stage fluvial activity in the equatorial
Martian cratered highlands. During the late Noachian the
highlands had been extensively eroded by fluvial processes,
eroding inner and outer crater rims as well as Isidis rim
massifs and steeper regional slopes (e.g., Figures 15 and
16), deeply filling crater interiors [Craddock and Howard,
2002; Forsberg-Taylor et al., 2004] and creating broad
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alluvial plains within intercrater basins. At about the Noa-
chian-Hesperian transition (or possibly as late as mid-
Hesperian) the major drainages incised into the earlier
Noachian landscape, generally creating valleys 50?350 m
deep in the intercrater basins. On the basis of the dimen-
sions of delta distributaries and inner channels within
incised valleys, fluvial flows during this time period were
of a magnitude similar to channel-forming floods within
terrestrial drainage networks for equivalent drainage areas.
Flows at least episodically continued sufficiently long to
flood craters along the valley axes up to several tens of
kilometers in diameter, eroding both entrance and exit
breaches through the crater walls (e.g., Figure 4). Where
incised valleys entered craters, deltaic deposits were some-
times deposited. Probably during the same time period the
walls of some deep craters were locally eroded into intri-
cately dissected alcoves with the sediment being deposited
as large alluvial fans on the crater floor (e.g., Figure 17).
Equivalent types of fans and deltas seem not to have formed
earlier in the Noachian. This period of fluvial activity
appears to have ended abruptly, because fans and deltas
were not entrenched by late-stage flows, and the last active
inner channels within incised valleys record large flows.
[50] A number of environmental scenarios might have
produced the observed fluvial features. In earlier discussion
we have discounted scenarios involving a slow decay of the
hydrological cycle resulting in less frequent or less intense
flows. A change from runoff to sapping erosion likewise
appears insufficient to provide the large discharges inferred
from dimensions of fluvial channels formed during the late-
stage incision. As discussed in section 6.1.3, channel
incision could have resulted from a reduction of size or
quantity of supplied sediment or from an increased magni-
tude of fluvial discharges. An increase in critical thresholds
for erosion or sediment transport (e.g., equation (7)) could
also focus erosion within larger channels. We suggest the
following scenarios as possible candidates for the observed
late-stage landform suite.
6.1.3.1. Increase in Global Temperature and in
Precipitation Intensity
[51] Such a scenario might reduce the rate of physical
weathering, and thus the amount and size of supplied
sediment. Increase in precipitation intensity, although in-
creasing both channel flows and sediment entrainment from
slopes and low order channels, generally results in channel
incision rather than aggradation. For example, Howard and
Kerby [1983] noted that badland channels incise during the
summer time period of intense thunderstorms and aggrade
during the winter when rain intensity is lower and frost-
induced mass wasting is greater. Such conditions might be
triggered, for example, by greenhouse gasses introduced by
a period of intense volcanism [Phillips et al., 2001], by
outflow floods spreading water across the northern lowlands
[Gulick and Baker, 1989; Baker et al., 1991; Scott, 1995;
Kreslavsky and Head, 2002; Fairen et al., 2003], by
extreme excursions of obliquity [Laskar et al., 2004], or
by environmental effects of large impacts [e.g., Carr, 1989;
Segura et al., 2002].
6.1.3.2. Change From Rainfall-Runoff to Snowmelt
Runoff
[52] Snowmelt-dominated fluvial systems tend to have
hydrographs characterized by long flood peaks but rela-
tively small sediment loads. Within the Colorado River
system, for example, the bulk of sediment supplied to the
river is delivered by local summer thunderstorms, but the
largest discharges and most of the work of sediment
transport and erosion are accomplished by early summer
snowmelt, primarily deriving from the higher mountain
ranges [Howard and Dolan, 1981]. In order to cause
discharges sufficient to produce the observed dimensions
of channels, melting of a snowpack would have had to be
relatively rapid, for example, due to seasonal rather than
obliquity climatic cycles, or due to some sudden, large-
scale environmental perturbation. Fassett and Head [2004]
suggest that snowmelt may have formed valley systems on
Martian volcanoes. We do not think that the scenario of
basal melting of a thick (hundreds of meters) snow/ice
pack [Carr and Head, 2003] is a sufficient mechanism,
both because of the high flow rates required and the
occurrence of deltas and fans in basins, which are clearly
subareal rather than sub-ice landforms.
6.1.3.3. Formation and Subsequent Erosion of a
Regional Duricrust
[53] In this scenario a duricrust would develop on the
Noachian landscape, perhaps gradually or during an epi-
sodic climate favoring its development, such as, perhaps,
low intensity but frequent precipitation as snow or rain. A
subsequent climate with higher intensity rainfall or rapid
snowmelt would cause incision of major valleys within
which flow rates would be sufficient to erode through the
duricrust. A duricrust would also inhibit delivery of sedi-
ment to the channel system, leading to incision of alluvial
streambeds.
[54] We have little evidence to choose among these
scenarios at present, and combinations of the scenarios, or
others we have not imagined, might have caused the late-
stage features. In any case, conditions favorable to valley
incision and formation of fans and deltas ceased rather
abruptly sometime in the Hesperian on the equatorial
cratered highlands.
6.2. Unresolved Issues
[55] We have presented observations and analyses that
suggest a late-stage epoch of intense fluvial activity within
the cratered highlands characterized by deep valley network
incision and, locally, deposition of alluvial fans and deltas.
We discuss here a number of issues that remain unresolved
or poorly constrained.
[56] The timing of the late-stage fluvial activity relative to
the established Martian chronology is uncertain within wide
limits. The incised valleys, deltas, and fans occupy a small
fraction of the total area of the highlands. What crater
counts have been done [e.g., Baker and Partridge, 1986;
Crumpler and Tanaka, 2003; Moore and Howard, 2005]
yield large error limits but suggest ages from late Noachian
to mid Hesperian. As a result, our evidence for synchronic-
ity of valley network incision throughout the highlands and
for equivalent ages for late-stage valley incision and for
alluvial fan deposition remains circumstantial.
[57] The simplest scenario for late-stage fluvial activity
would involve a single episode occurring throughout the
highlands. The actual situation may have been more
complex. For example, the multiple pediment surfaces in
Figure 16 suggest episodes of alternating incision and
E12S14 HOWARD ET AL.: TERMINAL EPOCH OF MARS FLUVIAL EROSION, 1
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deposition, and stepped fan or delta deposits [Irwin et al.,
2005b] may likewise represent oscillating environmental
conditions. Episodic erosional and depositional events in
the highlands were suggested by Grant and Schultz
[1990].
[58] The late-stage valley networks imply integrated flow
through channel systems often exceeding several hundred
kilometers in length (e.g., Figures 2 and 9) up to a
maximum of more than 4,000 km [Irwin et al., 2005b].
Yet valley profiles remain irregular and in some cases
broadly convex (e.g., Figure 9). This irregularity and the
lack of deeply incised canyons suggest that the mid to late
Noachian epoch preceding the late-stage erosion was capa-
ble of extensive local redistribution of sediment (resulting,
for example, in deeply filled basins and highly eroded
craters (e.g., Figures 15 and 16), but that flows were
ephemeral. The persistence of an unfilled crater 450 km
downstream within a late-stage Isidis rim valley network
(Figures 4 and 9) also suggests negligible long-distance
sediment transport during the mid to late Noachian. The
contrast between the hydrology of the late-stage valley
networks and that during the Noachian are discussed more
fully by Irwin et al. [2005b].
[59] Although a number of late-stage valley systems
entering enclosed basins exhibit deltas or fan-deltas (e.g.,
Figure 17 and examples by Malin and Edgett [2003], Moore
et al. [2003], and Irwin et al. [2005b]), many others do not,
even though the basins may have both entrance and exit
breaches (e.g., Figure 4). Because many of the valley
systems entering these basins are large and experienced
extensive late-stage entrenchment, sediment either was
routed through the basins, or effective processes of sediment
redistribution or removal must have occurred. Possible
scenarios are discussed by Irwin et al. [2005b].
[60] The Isidis rim valley networks do not continue onto
the floor of the Isidis basin. Incised valleys flowing toward
the basin disappear at about 3000 m, although a few
shallow sinuous channels extend lower. Crumpler and
Tanaka [2003] suggest that the valleys transition to ??termi-
nal valley deposits?? interpreted to be alluvial fans that
extend outward to the basin floor at about 3800 m.
However, obvious fluvial features such as the numerous
distributaries seen on highlands fans and deltas [Malin and
Edgett, 2003; Moore et al., 2003; Moore and Howard,
2005] are lacking, although a few isolated, shallow sinuous
channels extend across the terminal plains. Rather, this
elevation zone has an unusual surface of low benches,
ridges, depressions and splotchy textures that, as indicated
by of their brightness in THEMIS nighttime IR images,
have high thermal inertia. Only the slope below the termi-
nation of the westernmost valley in Figure 2 has a fan-like
topographic form. The Isidis rim valleys clearly delivered
appreciable sediment to the basin during late-stage erosion,
however. A possible explanation for the paucity of fluvial
depositional features is that fluvial flows interacted with
standing water or ice deposits, or were later modified by ice
or water within the basin.
[61] The validity and implications of the distinction
made by Baker and Partridge [1986] between pristine
and degraded channels remains uncertain. Pristine valleys
are defined to be more deeply incised and to have steep,
regular valley walls (e.g., ??P?? sites in Figure 20), whereas
degraded valleys are shallower with more irregular walls
(??D?? in Figure 20). Similarly, the main channel and major
tributaries of the Evros Valles system in Figure 11 are
pristine, but the basin surface supports numerous barely
distinguishable degraded valleys. As discussed earlier,
??pristine?? and ??degraded?? are somewhat inappropriate
terms since all such valleys are heavily modified by
later eolian, cratering, and mass wasting processes (e.g.,
Figures 5 and 13). A more important concern is whether
the Baker and Partridge [1986] interpretation of a differ-
ence in formational age and process is valid. One possi-
bility is that pristine versus degraded morphology merely
reflects differential effects of postfluvial modification in
valleys of differing initial depth and width, such that
shallower valleys are more easily modified. A second
possibility is that degraded valleys formed early in the
late-stage fluvial incision of Noachian basins (the dissected
basin floors of Figure 3b and the ??fluvially modified
terrain?? of Crumpler and Tanaka [2003]), with later
erosion (the incised valleys in Figure 3b) being restricted
to major channels (yellow lines in Figure 3b). As dis-
cussed previously, we do not agree with the process
distinction made by Baker and Partridge [1986] between
runoff forming older degraded channels and subsequent
groundwater sapping forming the pristine channels.
[62] The temporal relationship of the late-stage fluvial
activity to other events occurring during its possible age
range (late Noachian to mid Hesperian) remains uncertain,
including outflow channel development, volcanic activity,
mantling and glaciation in mid and high latitudes. If the age
range can be narrowed, possible environments and causes of
enhanced fluvial activity could be better evaluated.
7. Conclusions
[63] In this and a companion paper [Irwin et al., 2005b]
we present evidence that the final epoch of widespread
fluvial erosion and deposition in the cratered highlands
was characterized by integration of flow within drainage
networks as long as 4000 km and trunk valley incision of
50 to 350 m into earlier Noachian depositional basins.
Deltaic deposits formed locally where these incised valley
systems debouched into enclosed basins. Large alluvial
fans of sediment deposited from erosion of alcoves in
steep crater walls probably formed contemporaneously
with the late-stage erosion. The depth of incision below
already established Noachian surfaces correlates strongly
with the gradient and the total valley length, suggesting
consistent regional hydrology. Discharge within the chan-
nel systems estimated by channel dimensions and scaling
to terrestrial empirical relationships indicate flow rates
equivalent to channel-forming floods in terrestrial drainage
basins of equivalent size. Such high flow rates imply either
runoff directly from precipitation or rapid melting of
accumulated snow.
[64] This late-stage epoch of intense fluvial activity
appears to be fundamentally different than the fluvial
environment characteristic of most of the previous Noachian
period. The late-stage epoch was characterized by incision
of trunk valleys into basins that earlier had been sites of
aggradation. We find no earlier Noachian equivalents of the
large alluvial fans and deltas associated with the late-stage
E12S14 HOWARD ET AL.: TERMINAL EPOCH OF MARS FLUVIAL EROSION, 1
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fluvial activity. The earlier Noachian Period, however, was
characterized by widespread fluvial erosion of highlands
and crater rims, deeply infilling crater floors and intercrater
basins. We speculate that the previous Noachian was
characterized by a climate of ephemeral fluvial activity
and local, rather than regional integration of drainage
networks.
[65] The change in conditions that caused the enhanced
late-stage fluvial erosion and deposition remains uncertain.
One possibility is enhanced runoff volumes, either due to
high rates of precipitation or to rapid melting of accumu-
lated snowpacks. Another possibility is the development of
a thick chemically indurated duricrust layer over much of
the highlands, which would enhance runoff, reduce sedi-
ment yields, and focus erosion within larger channels.
[66] A number of issues remain uncertain concerning the
late-stage fluvial activity, including the timing of the fluvial
activity relative to Martian chronology (preliminary esti-
mates place the late-stage activity within the range of late
Noachian to mid-Hesperian) and the fate of sediment
delivered from the highlands to the northern lowlands,
including the possibility of interaction with oceans or ice
covers. The relationship of the late-stage highland denuda-
tion to other activities such as outflow channel flooding,
regional mantling episodes, formation of ice-related high-
latitude features, and volcanic resurfacing remain uncertain.
We do, however, find evidence that late-stage fluvial inci-
sion and deposition continued through the time period of
fretted terrain development in the Aeolis Mensae region
[Irwin et al., 2004b; Irwin et al., 2005b].
[67] We are continuing to investigate the late-stage fluvial
activity through a number of means. The profiles and
incision amounts of valley networks throughout the high-
lands are being digitized to expand the quantitative evalua-
tion of incision amount and to look for regional differences.
Hydrologic routing of runoff across the Martian highlands is
being developed, including depression storage and evapora-
tion. Spatial variations in modeled flow rates can be com-
pared with differences in amounts of late-stage valley
incision. The effects of duricrusts on patterns of erosion
are being investigated using the DELIM simulation model
[Howard, 1994, 1997; Forsberg-Taylor et al., 2004]. We
continue to search for diagnostic erosional and depositional
features in high-resolution images and to narrow the geo-
logic age of the late-stage activity through crater counting
and regional geologic relationships.
[68] Acknowledgments. This research was supported by grants from
NASA?s Mars Data Analysis Program and the Planetary Geology and
Geophysics Program. Helpful reviews were provided by Victor Baker
and James Head III.
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A. D. Howard, Department of Environmental Sciences, University of
Virginia, P.O. Box 400123, Charlottesville, VA 22904-4123, USA.
(ah6p@virginia.edu)
R. P. Irwin III, Center for Earth and Planetary Studies, National Air and
Space Museum, Smithsonian Institution, 6th Street and Independence
Avenue SW, Washington, DC 20013-7012, USA. (irwinr@si.edu)
J. M. Moore, NASA Ames Research Center, MS 245-3, Moffett Field,
CA 94035, USA. ( jeff.moore@nasa.gov)
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