Latent outflow activity for western Tharsis, Mars:
Significant flood record exposed
J. M. Dohm,1 R. C. Anderson,2 V. R. Baker,1,3 J. C. Ferris,1 L. P. Rudd,1
T. M. Hare,4 J. W. Rice Jr.,3 R. R. Casavant,1 R. G. Strom,3
J. R. Zimbelman,5 and D. H. Scott6
Abstract. Observations permitted by the newly acquired Mars Observer Laser Altimeter
data have revealed a system of gigantic valleys northwest of the huge Martian shield
volcano, Arsia Mons, in the western hemisphere of Mars (northwestern slope valleys
(NSVs)). These features, which generally correspond spatially to gravity lows, are obscured
by veneers of materials including volcanic lava flows, air fall deposits, and eolian materials.
Geologic investigations of the Tharsis region suggest that the system of gigantic valleys
predates the construction of Arsia Mons and its extensive associated lava flows of mainly
late Hesperian and Amazonian age and coincides stratigraphically with the early
development of the outflow channels that debouch into Chryse Planitia. Similar to the
previously identified outflow channels, which issued tremendous volumes of water into
topographic lows such as Chryse Planitia, the NSVs potentially represent flooding of
immense magnitude and, as such, a source of water for a northern plains ocean.
1. Introduction
The images of Mars acquired by the Mariner 9 and Viking
orbiters during the 1970s revealed geologic terrains that dis-
play assemblages of geomorphic features characteristic of
flood-carved surfaces on Earth such as the Channeled Scab-
land of the western United States [Bretz, 1969; Baker and
Milton, 1974; Baker, 1978, 1982; Mars Channel Working Group,
1983]. Assemblages of relict landforms, which include anasto-
mosing channel patterns, streamlined islands, longitudinal
grooves, scour depressions, depositional bars, inner channels,
and channel sources, strongly indicate ancient catastrophic
flood events on Mars. Although smaller outflow channels are
recorded elsewhere on Mars including Mangala [Scott and
Tanaka, 1986; Chapman and Tanaka, 1993; Zimbelman et al.,
1994; Craddock and Greeley, 1994], Ma?adim [Greeley and
Guest, 1987; Scott et al., 1993; Scott and Chapman, 1995], Dao
[Greeley and Guest, 1987; Crown et al., 1992; Crown and Gree-
ley, 1993; Scott et al., 1995; Price, 1998], Harmahkis [Greeley
and Guest, 1987; Crown et al., 1992; Crown and Greeley, 1993;
Scott et al., 1995; Price, 1998], Reull [Greeley and Guest, 1987;
Crown et al., 1992; Crown and Greeley, 1993; Scott et al., 1995;
Price, 1998], and Marte [Plescia, 1990; Scott et al., 1995] Valles,
the large outflow channels, which debouch into Chryse Planitia
(circum-Chryse outflow channel systems; Figure 1) and gener-
ally source near broken surfaces marked by the Valles Marin-
eris canyon system and by fractures, hills, and mesas forming
chaotic terrain, have received the greatest attention [e.g., Mil-
ton, 1974; Baker and Milton, 1974; Scott and Tanaka, 1986;
Chapman et al., 1991; DeHon, 1992; DeHon and Pani, 1992;
Scott, 1993; Rotto and Tanaka, 1995; Scott et al., 1995; Rice and
DeHon, 1996; Chapman and Tanaka, 1996; Nelson and Greeley,
1999]. Geologic mapping indicates that the outflow systems
may have been carved by several flood episodes over pro-
tracted time intervals. For example, Kasei Valles is one of the
largest previously identified systems of outflow channels on
Mars and a major contributor of water to Chryse Planitia. It
formed by multiple flood events during the Hesperian and
early Amazonian [e.g., Scott, 1993]. Another example is Man-
gala Valles, where at least two episodes of flooding have been
recorded [Chapman and Tanaka, 1993; Zimbelman et al., 1994;
Craddock and Greeley, 1994].
One commonly held explanation for the origin of the out-
flow channels involves episodic magmatic-driven processes
[e.g., Baker et al., 1991; Dohm et al., 2000a], perhaps aided by
gas hydrate dissociation [Max and Clifford, 2000; Kargel et al.,
2000; Komatsu et al., 2000], that explosively expel groundwater
and other materials (e.g., water-rich debris) from an ice-
enriched crust [e.g., Baker and Milton, 1974; Masursky et al.,
1977; Carr, 1979; Nummedal and Prior, 1981; Lucchitta, 1982;
MacKinnon and Tanaka, 1989; Baker et al., 1991]. The expelled
materials are catastrophically discharged down gradient to-
ward the northern plains, sculpting the Martian surface and
entraining boulders, rock, and sediment during passage. The
sediment-charged water eventually enters the northern plains,
contributing to the formation of large bodies of water and
sediment, including various hypothesized oceans, seas, lakes,
and ice sheets [e.g., McGill, 1985; Jo?ns, 1986; Lucchitta et al.,
1986; Parker et al., 1987, 1993; Baker et al., 1991; Scott et al.,
1991a, 1991b, 1995; Chapman, 1994; Scott and Chapman, 1995;
Kargel et al., 1995; Head et al., 1999]. The ?MEGAOUTFLO?
hypothesis [Baker et al., 2000] first proposed by Baker et al.
[1991] genetically links such activity to relatively short lived
climatic perturbations from a common cold and dry Mars (e.g.,
short-term hydrological cycles), which include enhanced ero-
1Department of Hydrology and Water Resources, University of Ar-
izona, Tucson, Arizona.
2Jet Propulsion Laboratory, Pasadena, California.
3Lunar and Planetary Laboratory, University of Arizona, Tucson,
Arizona.
4U.S. Geological Survey, Flagstaff, Arizona.
5Smithsonian Institution, Washington, D. C.
6Deceased August 9, 2000.
Copyright 2001 by the American Geophysical Union.
Paper number 2000JE001352.
0148-0227/01/2000JE001352$09.00
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. E6, PAGES 12,301?12,314, JUNE 25, 2001
12,301
sion by precipitation, landslides/mass wasting (both land and
submarine), glacial activity, and the presence of large standing
bodies of water such as the Amazonian-aged Oceanus Borea-
lis.
Here, we discuss a system of immense valleys (northwestern
slope valleys (NSVs) [Dohm et al., 2000b]; Figure 1) as defined
using Mars Observer Laser Altimeter (MOLA) data; these
valleys have been identified as topographic troughs [DeHon,
1996]. The system of valleys has largely gone unnoticed since it
was first imaged by the Mariner and Viking orbiters because it
is extensively veneered by late Hesperian and younger mate-
rials, including lava flows and ash deposits of the Tharsis Mon-
tes shield volcanoes and middle Amazonian and younger pu-
tative ash flow deposits of the Medusae Fossae Formation
[Malin, 1979; Scott and Tanaka, 1986; Greeley and Guest, 1987].
Though several modes of formation cannot be ruled out for the
NSVs (see section 4), geologic and paleohydrological investi-
gations and topographic and geophysical analyses using
MOLA and gravity data indicate that the NSVs likely repre-
sent structurally controlled valleys that routed catastrophic
floods. Such potential latent outflow activity of western Tharsis
may have contributed tremendous volumes of water to the
northern plains, an explanation consistent with the putative
existence of large bodies of water in the northern plains and
the ?MEGAOUTFLO? hypothesis of Baker et al. [1991, 2000].
2. Geologic and Physiographic Setting
A conspicuous system of gigantic, northwest trending valleys
as much as 200 km wide, referred to as the NSVs, are partly
defined by enormous elongated promontories and are located
along the margin of the northern lowland plains and the south-
ern highlands, west of the huge northeast trending chain of
Tharsis Montes shield volcanoes (Figure 1). The NSVs occur
within a large topographic depression (Plate 1) interpreted to
be the result of long-term crustal response to loading by Thar-
sis [Phillips et al., 2000] and generally correspond to gravity
lows. The northern and southern provinces represent a physi-
ographic and geologic dichotomy of the Martian crust. Several
hypotheses have been advanced for the origin of the dichot-
omy, including mantle convection associated with core forma-
tion [Wise et al., 1979], a colossal impact that formed a basin in
the north polar region [Wilhelms and Squyres, 1984], extensive
southward erosional retreat of the heavily cratered highland
plateau [Scott, 1978; Hiller, 1979], and plate tectonics [Sleep,
1994]. Regardless of origin, the boundary clearly separates
relatively young, uncratered materials of the lowlands from the
highly cratered, ancient highland rock assemblages. The low-
land materials are interpreted to have been emplaced by both
subaerial and subaqueous processes, which include eolian, vol-
canic, fluvial, mass-wasting, lacustrine, and marine processes
Figure 1. Mars Observer Laser Altimeter (MOLA) shaded relief map of the Tharsis and surrounding
regions (courtesy of the MOLA Science Team), which shows the newly identified northwestern slope valleys
(NSVs), scarps (s) that may indicate late-stage fluvial activity, and centers of tectonic activity [Anderson et al.,
1998; Dohm et al., 1998] interpreted to represent magmatic-driven doming events (ASWD, Arsia-southwest
dome; CVD, central Valles dome; SD, Syria dome).
DOHM ET AL.: LATENT OUTFLOW ACTIVITY FOR WESTERN THARSIS, MARS12,302
12,303DOHM ET AL.: LATENT OUTFLOW ACTIVITY FOR WESTERN THARSIS, MARS
[e.g., Scott and Tanaka, 1986; Tanaka, 1986; Greeley and Guest,
1987; Parker et al., 1993; Scott et al., 1995]. The ancient high-
lands consist of a me?lange of rocks that are interpreted to
mainly include lava flows, impact breccias, and eolian, fluvial,
and colluvial deposits [e.g., Scott and Tanaka, 1986; Tanaka,
1986; Greeley and Guest, 1987; Dohm and Tanaka, 1999], al-
though glacial deposits have also been proposed [e.g., Kargel et
al., 1995]. Amazonis Planitia, located to the north of and down
gradient from the NSVs, is the site of a hypothesized ocean
[e.g., Parker et al., 1993] and (or) paleolake [e.g., Scott et al.,
1995]. Heavily cratered plains materials crop out up gradient
and to the south of the NSVs [e.g., Scott and Tanaka, 1986]. A
more abrupt change in slope occurs to the east and northeast
of the NSVs as a result of the prominent shield volcanoes and
associated lava flows of Tharsis Montes [Scott and Tanaka,
1986; Scott and Zimbelman, 1995; Scott et al., 1998] and Olym-
pus Mons [Scott and Tanaka, 1986; Morris and Tanaka, 1994],
respectively (Figure 1 and Plate 1).
Arsia Mons is the southernmost shield volcano of the north-
east trending volcanic chain of Tharsis Montes, located be-
tween the NSVs and Syria Planum (Figure 1). Arsia exhibits a
summit caldera, reaches a height of greater than 17 km, and
displays a basal diameter greater than 300 km. Some of the
youngest flows of Arsia overflowed the northeast rim of the
caldera while other young flows were extruded from fissures
and collapse pits on the northeast and southwest sides of the
volcano along the trend of a regional fault zone [Carr et al.,
1977; Scott and Tanaka, 1986; Scott and Zimbelman, 1995]. The
lower northwestern flank of Arsia Mons is covered by fan-
shaped deposits composed of three distinct facies [Scott and
Zimbelman, 1995] similar to smaller occurrences on the north-
west flanks of Pavonis [Scott et al., 1998] and Ascraeus [Scott
and Tanaka, 1986] Montes; the facies may indicate mass move-
ment such as landsliding and debris avalanching, whereas oth-
ers are characteristic of facies suggesting ash flow or glacial-
periglacial processes. Lava flows emanating from Arsia Mons
extend to the northwest at least as far as the newly identified
NSVs, as evidenced by MOLA data that show lava flows trace-
able from within the NSVs to near the western base of Arsia
Mons [Zimbelman et al., 2000].
Syria Planum, including Noctis Fossae, located to the east of
Arsia Mons, forms the western margin of a complex system of
canyons, Valles Marineris (Figure 1). Syria is a site of long-
lived magmatic-tectonic activity including domal uplift and as-
sociated radial and concentric faulting and volcanism [Tanaka
and Davis, 1988; Anderson et al., 1997, 1998, 1999; Anderson
and Dohm, 2000; Dohm and Tanaka, 1999]. Valles Marineris
appears to have developed, in large part, along fault systems
associated with the early development of the Tharsis rise [e.g.,
Plescia and Saunders, 1982]. Additionally, rifting, magma with-
drawal, and tension fracturing have been proposed as possible
processes involved in the initiation and development of the
canyons. Lucchitta et al. [1992] noted that the depth of the
Figure 2. Three-dimensional topographic projection of the NSVs and surrounding region using ARCVIEW
(MOLA data courtesy of MOLA Science Team). The white arrow represents potential Noachian/early
Hesperian catastrophic flooding from source region (Arsia-SW/Syria dome complex). Also shown are centers
of tectonic activity [after Anderson et al., 1998; Dohm et al., 1998] interpreted to represent magmatic-driven
magmatic doming events (UD, Ulysses dome; ASD, Ascaeus-south dome; ASWD, Arsia-southwest dome;
CVD, central Valles dome; SD, Syria dome).
Plate 1. (opposite) (a) MOLA shaded relief map showing
features of interest, including NSVs, Tharsis rise, circum-
Chryse outflow channel systems, and centers of tectonic activ-
ity [Anderson et al., 1998; Dohm et al., 1998] interpreted to
represent magmatic-driven doming events (UD, Ulysses dome;
ASD, Ascaeus-south dome; ASWD, Arsia-southwest dome;
CVD, central Valles dome; SD, Syria dome), (b) part of the
geologic map of the western equatorial region of Mars (rep-
resentative map units are shown [Scott and Tanaka, 1986]), and
(c) generalized geologic cross section (a-a9, transect of A and
B; PBS, potential location of basement structures; TMl, Thar-
sis Montes lavas; MF, Medusae Fossae materials).
DOHM ET AL.: LATENT OUTFLOW ACTIVITY FOR WESTERN THARSIS, MARS12,304
large troughs may have been caused by (1) collapse of near-
surface materials due to withdrawal of underlying material or
opening of tension fractures at depth, (2) development of
keystone grabens at the crest of a bulge, or (3) failure and
subsequent drifting of plates. Lucchitta [1987] also recognized
that many of the valley faults associated with Valles Marineris
may have been associated with volcanic activity. Extending
from the northern and eastern margins of Valles Marineris
into Chryse Planitia are well-documented circum-Chryse out-
flow channel systems that cut heavily and moderately cratered
surfaces of the southern cratered highlands (Figure 1 and
Plate 1).
3. Stratigraphy
The stratigraphic positions of rock units in the region of
interest (the NSVs, Tharsis, Valles Marineris, and circum-
Chryse outflow channel systems; Figure 1) have been estab-
lished regionally and planet-wide by previous mapping inves-
tigations including impact crater counts [e.g., Scott and
Tanaka, 1986; Tanaka, 1986; Scott and Chapman, 1991; Morris
et al., 1991; Chapman et al., 1991; Witbeck et al., 1991; DeHon,
1992; Scott, 1993; Chapman and Tanaka, 1993; Craddock and
Greeley, 1994; Zimbelman et al., 1994; Morris and Tanaka, 1994;
Scott and Zimbelman, 1995; Rotto and Tanaka, 1995; Scott et
al., 1995; Chapman and Tanaka, 1996; Rice and DeHon, 1996;
Scott et al., 1998; Schultz, 1998; Dohm and Tanaka, 1999; Nel-
son and Greeley, 1999]. The stratigraphic history of the region
of interest is summarized in sections 3.1?3.4 and portrayed in
cross section (Plate 1c).
3.1. Noachian System
The Noachian System consists of ancient crustal rocks
formed during the period of late heavy bombardment that
produced a high density of impact craters including enormous
basins. Impact cratering, along with water and wind erosion,
tectonic deformation, and magmatic-related resurfacing, have
modified or destroyed many of the primary morphologic fea-
tures of Noachian units, including a substantial part of the
ancient crater population. Thus Noachian surfaces are usually
characterized by impact, erosional, tectonic, and magmatic-
related features that postdate the material. Noachian rocks are
interpreted to consist mainly of impact breccias, volcanic ma-
terials (shields and flows), and eolian and fluvial sediments that
mantle and subdue parts of the heavily cratered terrain.
Densely cratered terrain is located to the south and southwest
of the system of large valleys. Heavily and moderately cratered
terrain also occurs to the north-northeast of Valles Marineris,
on the opposite side of the magmatic complex of Tharsis Mon-
tes, several thousands of kilometers from the NSVs. Several of
the major Noachian rock units, which form the Martian high-
lands, are observed in the region of interest (Plate 1), including
hilly (unit Nplh), cratered (unit Npl1), ridged (unit Nplr), and
subdued cratered (unit Npl2) materials [e.g., Scott and Tanaka,
1986].
The hilly material at the base of the plateau sequence forms
heavily cratered terrain of high-standing plateaus, irregular
topographic highs, prominent ridges, and highly degraded, an-
cient crater rims mainly to the south of the NSVs and to the
northeast of Valles Marineris in the Xanthe Terra region.
Locally, tectonic and impact processes [Watters, 1993; Schultz
and Tanaka, 1994] may have contributed to the prominent
relief of the hilly unit. The cratered material, which is the most
extensive unit in the western equatorial region [Scott and
Tanaka, 1986; Tanaka, 1986], forms most of the basal rocks to
the south of the NSVs and partly embays the hilly material.
The cratered material records a high density of superposed and
partly buried and degraded impact craters of all sizes formed
during the middle Noachian. Because of continued bombard-
ment and possible early, widespread volcanism [Saunders,
1979; Greeley and Spudis, 1981] the cratered unit probably
consists mostly of impact breccias and volcanic materials. The
ridged material outcrops to the south of the NSVs [e.g., Scott
and Tanaka, 1986] and is generally marked by rough, promi-
nent, sublinear to irregular ridges. Generally, the Noachian
ridges are less continuous and less evenly spaced and have
more relief than Hesperian ridges, but they follow similar re-
gional trends, forming an arcuate pattern around the southern
part of the Tharsis rise [e.g., Scott and Tanaka, 1986; Schultz
and Tanaka, 1994].
Toward the end of the late Noachian many intercrater and
intracrater areas of older materials were largely resurfaced by
a thin mantle mapped as the subdued cratered material (unit
Npl2) of the plateau sequence of mainly volcanic, eolian, and
fluvial origin [e.g., Scott and Tanaka, 1986]. The material em-
bays most craters .10 km across and mainly crops out far to
the south of the NSVs and to the northeast of Valles Marineris
in the Xanthe Terra region. It is marked in a few places by
small ridges, channels, and possible flow fronts. Also during
this time, centers of tectonic activity, many of which are inter-
preted to be the sites of magmatic-driven domal uplifts and
associated volcanic eruptions, are documented near Arsia
Mons (predating the Tharsis Montes volcanoes), Syria Planum,
and the central part of Valles Marineris [Anderson et al., 1997,
1998, 1999; Anderson and Dohm, 2000; Dohm et al., 1998;
Dohm and Tanaka, 1999] (Figures 1 and 2 and Plates 1 and 2).
Deep-seated magmatic bodies have been previously inferred
for the western equatorial region where fault swarms have
been deflected around their central cores such as at the central
part of Valles Marineris [Scott and Dohm, 1990]. The uplift of
the Thaumasia plateau may have also occurred during this
time [Dohm and Tanaka, 1999] (Figures 1 and 2 and Plates 1
and 2).
Comprehensive geologic investigations of the northeast part
of the Thaumasia region indicate that magmatic-tectonic ac-
tivity occurred at central Valles Marineris as early as the late
Noachian and diminished substantially during the early Hes-
perian, prior to the emplacement of the younger ridged mate-
rials (unit Hr) [Dohm et al., 1998; Dohm and Tanaka, 1999];
such activity may have accompanied substantial volcanism.
Similar to the central part of Valles Marineris, Syria Planum is
also a site of long-lived magmatic-tectonic activity, which in-
cludes domal uplift and associated radial and concentric fault-
ing during the late Noachian?early Hesperian, but at a much
larger scale than is observed at the central part of Valles
Marineris [Tanaka and Davis, 1988; Anderson et al., 1998, 1999;
Anderson and Dohm, 2000; Dohm and Tanaka, 1999].
Magmatic-related activity such as doming underlying Arsia
Mons and located at Syria Planum and central Valles Marin-
eris during the late Noachian?early Hesperian may be genet-
ically associated with the early development of the circum-
Chryse outflow channel systems [e.g., Dohm et al., 1998;
McKenzie and Nimmo, 1999]. Importantly, such activity, which
includes volatile and magma interactions, especially underlying
Arsia and at Syria, may have also contributed to the formation
of the NSVs that occur to the south of Amazonis Planitia
12,305DOHM ET AL.: LATENT OUTFLOW ACTIVITY FOR WESTERN THARSIS, MARS
(Figures 1 and 2 and Plates 1 and 2). A proposed Noachian
drainage basin/aquifer system may have provided a sufficient
source of water necessary to carve the circum-Chryse outflow
channel systems and the NSVs [Dohm et al., 2000c].
3.2. Hesperian System
The Hesperian System records extensive volcanism includ-
ing the emplacement of wrinkle-ridged plains materials (unit
Hr) [e.g., Scott and Tanaka, 1986]. Wrinkle ridge materials
dominate the Lunae Planum region located to the north of
Valles Marineris (Plate 1b) and outcrop, in places, south of the
NSVs. In addition, widespread resurfacing of early Hesperian
and older materials, which includes the deposition of colluvial
and fluvial deposits and the dissection of rock materials by
outflow activity north and northeast of Valles Marineris, is
observed during the early Hesperian [e.g., Rotto and Tanaka,
1995; Nelson and Greeley, 1999]. Incipient Alba Patera [Scott
and Tanaka, 1986] and other centers of tectonic activity lo-
cated in the Syria, Pavonis (pre-Tharsis Montes), and Ulysses
Fossae regions [Anderson et al., 1997, 1998, 1999; Anderson and
Dohm, 2000; Dohm and Tanaka, 1999], which are interpreted to
be magmatic-related domal uplifts, are also observed in the geo-
logic record during this time; such activity is indicative of growth
of the magmatic complex of Tharsis (Plate 1 and Figure 2).
Also during the early Hesperian period, outflow channel
development is recorded at Mangala Valles located to the
southwest of the NSVs [Craddock and Greeley, 1994; Zimbel-
man et al., 1994] and at the circum-Chryse outflow channel
systems located to the north-northeast of Valles Marineris
[e.g., Rotto and Tanaka, 1995; Nelson and Greeley, 1999], on the
opposite side of the magmatic complex of Tharsis Montes
several thousands of kilometers from the NSVs, although sig-
nificant outflow channel formation is also recorded during the
late Hesperian and into the early Amazonian such as at Kasei
[Chapman et al., 1991; Scott, 1993; Chapman and Tanaka,
1996] and Mangala [Chapman and Tanaka, 1993; Craddock
and Greeley, 1994; Zimbelman et al., 1994] Valles.
Significant magmatic activity is also recorded during the late
Hesperian and early Amazonian including the development of
Olympus Mons and the Tharsis Montes shield volcanoes [Scott
and Tanaka, 1986; Morris et al., 1991; Morris and Tanaka, 1994;
Scott and Zimbelman, 1995; Scott et al., 1998] and the emplace-
ment of voluminous sheet lavas centered at the large shield
volcanoes and at Syria Planum [Scott and Tanaka, 1986; Dohm
and Tanaka, 1999], which include members 1 and 2 of the
Olympus Mons Formation, members 1?4 of the Tharsis Mon-
tes Formation, and members 1 and 2 of the Syria Planum
Formation. Lava flows centered at Arsia Mons, for example,
extend at least 1300 km to the northwest [Zimbelman et al.,
2000], partly embaying the gigantic northwest trending prom-
ontories and partly infilling the NSVs (Figure 1 and Plate 1).
3.3. Amazonian System
Continued development of Tharsis Montes and Olympus
Mons is recorded during the Amazonian period including the
emplacement of associated lavas and aureole deposits [Scott
and Tanaka, 1986; Morris et al., 1991; Morris and Tanaka, 1994;
Scott and Zimbelman, 1995; Scott et al., 1998]. In addition,
late-stage outflow channel development is observed at Man-
gala Valles [Chapman and Tanaka, 1993; Craddock and Gree-
ley, 1994; Zimbelman et al., 1994], Marte Valles, which is in-
terpreted to represent a spillway between the Elysium and
Amazonis basins [Scott et al., 1995], and the circum-Chryse
outflow channel systems [Scott and Tanaka, 1986; Chapman et
al., 1991; Scott, 1993; Rotto and Tanaka, 1995; Chapman and
Tanaka, 1996; Nelson and Greeley, 1999]. Medusae Formation
materials, which partly blanket the NSVs, form a broad but
discontinuous band of wind-etched materials that trends east-
west along the highland-lowland boundary [e.g., Scott and
Tanaka, 1986; Greeley and Guest, 1987; Scott and Chapman,
1991]. On the basis of its morphologic characteristics the Me-
dusae Fossae Formation has been interpreted to consist of ash
flow tuffs [Malin, 1979; Scott and Tanaka, 1982, 1986; Zimbel-
man et al., 1997], ancient polar deposits [Schultz and Lutz,
1988], pyroclastic and eolian materials [Greeley and Guest,
1987], or a host of additional (but less likely) origins [Zimbel-
man et al., 1997]. Significant to the stratigraphic history of the
NSVs region, which has been clouded by late-stage volcanic
and other activity, Sakimoto et al. [1999] have completed Mars
Orbiting Camera (MOC) and MOLA investigations of previ-
ously mapped outcrops of Medusae Fossae Formation; they
indicate that the outcrops may be comprised of eolian or vol-
canic deposits that were emplaced on top of existing topogra-
phy (e.g., older competent materials) and subsequently eroded.
Similarly, we interpret the NSVs to be carved Noachian and
possibly early Hesperian materials draped by late Hesperian
and younger materials (Figure 1 and Plate 1). In addition,
relatively small erosional terraces (Figure 1), located at the
southwest and western parts of the NSVs, may indicate more
recent, local fluvial activity, which is consistent with the recent
identification of several small fluvial channels (Figure 3) that
intermingle with the Medusae Fossae Formation materials in
the region of the NSVs [Zimbelman et al., 2000]. Such fluvial
activity may be associated with late-stage development of Man-
gala Valles [Chapman and Tanaka, 1993; Craddock and Gree-
ley, 1994; Zimbelman et al., 1994].
3.4. Stratigraphic Summary of the NSVs
Noachian materials were faulted and carved by the late
Noachian?early Hesperian, forming a system of gigantic north-
west trending promontories and valleys that are associated
stratigraphically with pre-Tharsis Montes magmatic-driven
doming events of the Arsia, Syria Planum, and central Valles
regions. The geometric shapes and geomorphic character of
the gigantic, northwest trending promontories, which include
ghost craters, in comparison to other much smaller outcrops of
Medusae Fossae located to the west, indicate that the features
mostly comprise promontory-forming, older materials. Late
Hesperian and younger materials, which may include lavas, air
fall deposits from Tharsis Montes, possible ash flow deposits,
and eolian materials, appear to blanket and embay the gigantic
promontories and partly infill the valleys, subduing the relict
landforms, especially when viewed from Mariner and Viking
images. Terraces located to the south of the system of valleys
and, in places, along the valley floors may have resulted from
local, late-stage fluvial activity. In addition, eolian, glacial, and
mass-wasting processes may have modified the existing valley
morphology.
4. Discussion
In addition to the previously identified outflow channels,
observations permitted by MOLA data have highlighted the
NSVs located along the highland-lowland dichotomy to the
northwest of Arsia Mons and located south of Amazonis Plani-
tia, the site of a postulated ocean [e.g., Parker et al., 1993] and
DOHM ET AL.: LATENT OUTFLOW ACTIVITY FOR WESTERN THARSIS, MARS12,306
(or) a paleolake [Scott et al., 1995], in the western hemisphere
of Mars (Figures 1 and 2 and Plates 1 and 2). Veneers of lava
flows and volcanic air fall deposits and possible ash flow de-
posits have partly obscured the NSVs. The veneers suppress
evidence (e.g., relict features), making it difficult to rule out
modes of valley origin. In addition to volcanic modification,
local eolian, fluvial, and mass movement processes further
obscure clues that would otherwise aid in better understanding
the origin of the valleys.
In sections 4.1?4.5, we present several hypotheses that at-
tempt to explain the origin of the NSVs. Following careful
consideration of the stratigraphic, geomorphic, paleotectonic,
topographic (using MOLA data), and geophysical data, we
present these hypotheses, which include catastrophic flooding,
tectonism, mass wasting, glaciation, and wind erosion. It is
important to keep in mind that combinations of any of the
activities listed in sections 4.1?4.5, occurring concurrently or at
different times, may have contributed to valley formation.
4.1. Hypothesis 1: Catastrophic Flooding
The premise that the NSVs located near the western slope of
Tharsis resulted from one or more catastrophic floods assumes
that magmatic-driven processes triggered sudden releases of
large volumes of water into Amazonis Planitia (forming a wa-
tershed to the northwest of the Arsia-Syria dome complex)
(Figures 1 and 2 and Plates 1 and 2), similar to the driving
mechanism often envisioned for the formation of the smaller
circum-Chryse outflow channel systems that debouch into
Chryse Planitia. Spatial and temporal associations of the NSVs
with magmatic-driven, late Noachian to early Hesperian dom-
ing events located near Arsia (pre-Tharsis Montes) and Syria
Planum may corroborate such activity. During the same time,
tremendous volumes of water were issued into Chryse Planitia
from source areas near Valles Marineris, a region of recorded
magmatic and tectonic activity. Similar to circum-Chryse out-
flow channel systems, the implications of uncovering such a
significant flood record of the ancient Martian past are of great
significance in that such activity supports northern ocean(s)
and (or) large paleolakes in the northern plains.
While observations of the NSVs do not individually confirm
hypothesis 1, collectively, they corroborate tremendous flood-
ing. Some of the more significant observations include (1) the
tremendous size of the system of valleys and promontories; (2)
the unique shape of the gigantic promontories with respect to
smaller topographic highs located to the west of the NSVs,
including teardrop-shaped ends and tremendously long, rela-
tively straight margins (the shapes indicate that the giant out-
crops mostly comprise competent materials similar to the
flood-carved mesas north and east of Valles Marineris (Figure
1)); (3) terraces that, in places, mark the margins of the gigan-
Plate 2. Schematic diagram showing late Noachian?early Hesperian activity. Centers of tectonic activity:
Tempe, Uranius, Ceraunius, Arsia-SW, Syria, central Valles, Claritas, and Warrego [Anderson et al., 1998;
Dohm et al., 1998] are interpreted to be magmatic-related doming events and, in many cases, associated
volcanic events. The formation of NSVs may be partly related to the development of the Arsia-SW?Syria
dome complex. Also shown is modification along the highland-lowland boundary and Thaumasia plateau
(uplift shown by narrow arrows), Coprates rise, Thaumasia highlands, early circum-Chryse outflow channel
systems, early Valles Marineris canyon system, Uzboi, fault and fracture system (straight lines), and wrinkle
ridge development (wavy lines).
12,307DOHM ET AL.: LATENT OUTFLOW ACTIVITY FOR WESTERN THARSIS, MARS
tic promontories; (4) a distinctively smooth surface located
directly north of the NSVs in Amazonis Planitia, along the
western margin of Olympus and associated lava flows (in con-
trast to the knobby terrain to the west), that closely resembles
subaqueous sedimentation in abyssal plains and sedimentary
basins [Aharonson et al., 1998] or flood-scoured desert terrains
on Earth; (5) the system of valleys that generally correspond
spatially to gravity lows and occur within a large topographic
depression (Plate 1), similar to the eastern Chryse Planitia
outflow channels [Phillips et al., 2000] (the gravity lows may
represent low-density materials that partly infill the valleys and
large topographic depression); (6) the late Noachian?early
Hesperian age of the NSVs that corresponds stratigraphically
with the early outflow development of the circum-Chryse out-
flow channel systems and with magmatic-driven, pre-Tharsis
doming events, which occurred up gradient and to the south-
east of the NSVs near Arsia Mons and at Syria Planum (Fig-
ures 1 and 2 and Plates 1 and 2); and (7) the trends of the
gigantic outcrops, especially the southeast parts in conjunction
with linears such as scarps/terraces located to the northeast of
the promontories (Figures 1 and 2), which point toward the
potential source region, Arsia Mons (pre-Tharsis Montes) and
Syria Planum.
Unlike the development of the circum-Chryse outflow chan-
nel systems, which may have occurred from the late Noachian
to early Amazonian, stratigraphic evidence suggests that po-
tential northwestern slope flooding was limited to the late
Noachian?early Hesperian, possibly cut off from a massive
water supply by a drainage divide formed by a fully developed
Syria-Arsia dome complex and (or) late-stage growth of the
Tharsis Montes shield volcanoes, although minor, local fluvial
activity occurred later in the southern and western parts of the
NSVs. As with the occurrence of mass wasting (see hypothesis
3), it is important to realize that catastrophic flood events are
not necessarily limited to streamflow. There is little doubt that
the NSVs signify multiple flood events of various magnitudes
prior to the late Hesperian. This raises the possibility that
phases of hyperconcentrated-flow and debris flow activity may
have been associated with the flooding in this area. Such vari-
ations in flow behavior would create a unique assemblage of
Figure 3. Smooth channel floor (black arrows; at 30 m/pixel) exposed among outcrops and boulders of
Medusae Fossae Formation (MFF) materials. The smooth floor does not constrain whether the flow preceded
or postdated MFF, but the lack of any streamlining on knobs within the channel may argue for an earlier stage
of development. The channel gently slopes (0.228; toward upper left) along the bottom of a broad topographic
valley revealed by MOLA data [Zimbelman et al., 2000]. Unusual flow-like features (white arrows) crop out
from beneath the MFF deposits on both sides of the channel and are visible through the MFF cover (right
center); these flows have been interpreted as possible peperites [Gregg and Schultz, 1997], implying that the
flows may have been emplaced within early MFF materials made wet by flow along the channel. The area
shown is 3.08?4.08N, 140.08?141.28W [U.S. Geological Survey, 1995].
DOHM ET AL.: LATENT OUTFLOW ACTIVITY FOR WESTERN THARSIS, MARS12,308
deposits [Webb et al., 1989] that may have at least partly filled
these canyons between flood events.
4.2. Hypothesis 2: Tectonism
Tectonic structural control may have played a significant
role in channeling catastrophic flood waters from the Arsia-
SW?Syria dome complex region (Plate 2) to the Amazonis
Planitia region, resulting in significant enhancement of the
original valley geometry of the NSVs. For that reason, it is
important not to understate the importance of this hypothesis
in relation to catastrophic flooding. Large-scale tectonic fab-
rics, for example, exist within and adjacent to the NSVs. These
include (1) ridged material, which crops out to the south of the
NSVs and is composed of sublinear to irregular ridges that
exhibit trends common to the NSVs, (2) faults that cut Noa-
chian surfaces to the southwest of the special site of interest,
and (3) large-scale tectonic fabrics adjacent to and within the
NSVs, such as Gordii Dorsum [e.g., Scott and Tanaka, 1986].
Although tectonic features may have routed the erosional
agent, tectonic activity by itself probably did not result in the
gigantic size of the NSVs.
4.3. Hypothesis 3: Mass Wasting
The formation of outflow channels emptying into the Chryse
Planitia on Mars has been attributed to debris flow activity
[Tanaka, 1999]. Mass movements are common in the Valles
Marineris canyon system and may be responsible for the de-
velopment of the aureole deposits on the flanks of Olympus
Mons [Hodges and Moore, 1994]. Therefore the possibility that
mass wasting may have created the NSVs must be thoroughly
examined.
Debris flows on Earth are capable of moving huge quantities
of material great distances (more than 800 km in the subaque-
ous Storegga slide) [Bugge et al., 1988]. Most debris flows move
in existing topographic depressions, however, and may occur as
a brief phase accompanied by subsequent streamflow or hy-
perconcentrated flow [Melis et al., 1997]. Typically, mass move-
ments will create a scarp or headwall in their source region
where the slide material is removed and a lobate zone of
deposition where the material stops moving. Erosion is great-
est at the source area where debris flow activity has been
observed; in some cases, such activity may remove all of the
existing unconsolidated material in the source area, leaving
exposed bedrock [Johnson and Rodine, 1984]. Landslide depos-
its such as debris flow levees, slump blocks, or a pile of rubble
produced by a rock topple are the most typical landforms
produced during these mass movements.
No direct evidence of deposits created by mass movement
can be found in or around the NSVs. In addition, no scarps or
headwalls are visible near Arsia Mons where either mass
movements or floodwaters would have originated. Higher-
resolution images of the floors of the NSVs and Amazonis
Planitia should be examined for deposits typical of mass move-
ments, especially debris flow deposits which would be the most
likely subareal mass movements to travel great distances from
their source. Subaqueous landslides have the longest recorded
runouts of any mass movements on Earth [Bugge et al., 1988].
If the area north of the NSVs was filled with water at any time
in the Martian geologic past, subaqueous landslides could have
moved through the area, moving large amounts of material out
of the channels and into Amazonis Planitia. Such subaqueous
slides on Earth have moved huge, intact blocks of material
great distances from the slide?s source region [Bugge et al.,
1988]. Similar activity on Mars could deposit large blocks at
great distances out into the smooth plains north of the pro-
posed channels.
It is not likely that mass movement activity alone created the
NSVs. Depositional or erosional features typically associated
with mass movements are not visible in the region of special
interest. In addition, mass movements usually concentrate
their erosive activity in their source region and thus would not
be likely to create channels a significant distance down slope
from their source region. Mass movements are expected as
part of the streamflow that may have created these channels.
Postflood slumping could have occurred on the sides of the
valleys, and some debris flow activity may have occurred in the
valleys themselves as a phase in an intermittent streamflow
regime.
4.4. Hypothesis 4: Glaciation
Ice streams have been proposed to have carved the circum-
Chryse outflow channel systems [e.g., Lucchitta and Anderson,
1980]. In order for glaciers to have formed the NSVs the
climate had to differ significantly from the present. The
?MEGAOUTFLO? hypothesis first demonstrated by Baker et
al. [1991] genetically links magmatic-triggered activity with rel-
atively short lived climatic perturbations from a common cold
and dry Mars (e.g., short-term hydrological cycles), which in-
clude enhanced erosion by precipitation, landslides/mass wast-
ing (both land and submarine), glacial activity, and the pres-
ence of large standing bodies of water such as the Amazonian-
aged Oceanus Borealis. A synthesis of the stratigraphic,
erosional, and paleotectonic records indicates that potential
pulses of magmatic activity may have triggered climatic per-
turbations such as those during the late Noachian?early Hes-
perian [Dohm et al., 2000a] when the NSVs were carved. In
addition, in order for glaciers to develop and move down the
northwest flank of the pre-Tharsis Montes, magmatic-driven
Arsia uplift, a plentiful supply of water must have been
present. This water could be provided through an Earth-like
hydrologic cycle, which may have existed episodically on Mars
[Baker et al., 1991, 2000], or subsurface water that froze upon
contact with the surface, creating an ice stream [Lucchitta et al.,
1981; Lucchitta, 1982].
Glacier formation by accumulation of snow through a hy-
drologic cycle assumes that precipitation (most likely oro-
graphic) would have resulted from prevailing winds rising over
the Arsia uplift. Such precipitation would have to continue
over a sufficient period of time for the necessary thickness of
ice to build up for glacial flow to initiate. Interpretations of
striations on the southwestern flank of Arsia Mons as glacial
moraines [Hodges and Moore, 1994] and fan-shaped deposits
on the northwest flanks of the Tharsis Montes volcanoes,
which consist of facies that are interpreted to be the result of
glacial and volcanic/ice interactions [Scott and Zimbelman,
1995; Scott et al., 1998], indicate that variations in climate from
the present one have occurred in the recent geologic past.
These features, however, are dwarfed by the system of gigantic
valleys that formed much earlier in Martian time, during the
late Noachian?early Hesperian.
The major drawback to the glacial origin hypothesis is the
fact that no characteristic features resulting from glacial ero-
sion or deposition are seen in the region encompassing the
NSVs. Depositional features such as eskers, drumlins, or mo-
raines are absent as are erosional features such as cirques at
the heads of the valleys or are?tes along the ridgelines between
12,309DOHM ET AL.: LATENT OUTFLOW ACTIVITY FOR WESTERN THARSIS, MARS
the valleys. Particularly, there are no moraines or eskers down
gradient from the NSVs. An explanation for this is that all
evidence of glaciation, except for the gigantic promontories,
has been buried and (or) obscured by materials (including
volcanic, eolian, colluvial, possible marine, and possible
lacustrine) or destroyed by wind and water erosion.
4.5. Hypothesis 5: Wind Erosion
During the absence of an active hydrologic cycle for much of
the history of the planet [Baker et al., 2000], wind erosion
played an important role in the modification of the surface of
Mars [Greeley et al., 1999]. Thus it is important to consider the
possibility that the NSVs are the result of wind erosion. On
Earth, winds are efficient at sculpting existing surfaces but not
sufficient to carve bedrock canyons of the magnitude observed
for the NSVs.
Wind erosion would be most effective in eroding unconsol-
idated surficial deposits, so the competence of the material in
the study area is a major factor to take into account in the
assessment of the efficacy of wind erosion. The unique shape of
the gigantic promontories with respect to smaller topographic
highs mapped as Medusae Fossae Formation materials located
to the west of the region of interest, however, indicate that the
gigantic landforms comprise competent materials similar to
the flood-carved mesas north and east of Valles Marineris
(Figure 1). Evidence of recent wind activity exists in and
around the special site of interest, especially yardangs and wind
streaks that mark the Medusae Fossae Formation surfaces
[e.g., Scott and Chapman, 1991]; the wind streaks provide ev-
idence of winds originating from several different directions in
recent times. Wind erosion is an unlikely sole candidate for the
formation of the NSVs for the following reasons: (1) Wind
erosion on Earth lacks the power needed to be the sole erosive
force for carving gigantic bedrock valleys, (2) wind-formed
features of a much smaller scale than the gigantic promonto-
ries indicate a variety of wind directions for the special region
of interest whereas a single wind direction would have to be
maintained for a long period of time for efficient wind erosion
to occur, and (3) a well-established, strong wind pattern in this
area capable of large-scale erosion would have produced other
features of similar size in the area at the same time. No such
wind-related features of gigantic proportions can be found
elsewhere on Mars.
5. Paleohydrology
After careful consideration of the processes mentioned in
section 4, structurally controlled flooding appears to best ex-
plain the formation of the NSVs, although the other processes
probably further modified the NSVs. Here we attempt to de-
termine the ancient hydrologic conditions on the basis of our
knowledge of the geometry of the valleys, as they exist today.
This task, however, is daunting because of subsequent volcanic,
eolian, fluvial, and gravity-driven modification of the NSVs.
Extensive masking of the walls of the NSVs by the Medusae
Fossae Formation means that the earlier fluvial erosion of
whole channel cross sections must be inferred in order to
perform the calculations that follow.
The paleohydrology of the NSVs was estimated from an
analysis of the geometry of the valleys (e.g., several MOLA-
based valley profiles were generated and analyzed). There are
limitations imposed on these calculations. The ancient flow
depths, for example, can only be approximated because volca-
nics as well as other materials of unknown thickness obscure
the original valley geometries. Thus we sought to determine
the sensitivity of the discharge value by altering the constituent
variables of the modified Manning equation [Sellin, 1969;
Komar, 1979]:
u 5 @~ gmhs!/Cf#1/ 2,
where u is the average flow velocity, gm is the Martian gravi-
tational acceleration, h is the hydraulic radius, s is the sine of
the channel bed slope, and Cf is a dimensionless drag coeffi-
cient approximated by
Cf 5 ge~n2/h1/3! ,
where ge is the gravitational acceleration of Earth and n is the
Manning roughness coefficient. Using the method of Komar
[1979], discharge calculations for the NSVs were made
(Table 1).
The greatest source of error in the Manning equation is the
value of the Manning roughness coefficient. After careful con-
sideration of hydrologic and geologic factors a value of 0.025
was selected for the Manning roughness coefficient. It is inter-
esting to note that this value, determined independently,
agrees with the value used by Robinson and Tanaka [1990] in
the calculation of flood discharge for Kasei Valles, an outflow
channel system that is generally agreed to be the largest cur-
rently known on Mars.
A series of calculations were made to determine what effect
certain hydrologic variables would have on the discharge val-
ues. For example, we performed numerous alterations to the
valley geometries, including (1) lowering the valley floors by
intervals of 100 m until reaching a depth of 1 km, in order to
approximate valley profiles prior to volcanic infilling of the
NSV, (2) decreasing the valley widths by 20 and 40%, to ac-
count for the possible removal of canyon wall material by
wave-cut action, erosion, and (or) mass movements, (3) mod-
ifying the valley slopes, in order to accurately gage the sensi-
tivity of the calculations to the shallowing of the regional slope
due to infilling volcanic materials, and (4) changing the water
Table 1. A Comparative Analysis Between Some of the Previously Defined Martian Outflow Valley Systems and the Newly
Defined Northwestern Slope Valleysa
Reference Channel D, m W, km S V, m/s Q, m3/s
Komar [1979] Mangala 100 14 0.003 15 2 3 107
Baker [1982] Maja 100 80 0.02 38 3 3 108
Komatsu and Baker [1997] Ares 500?1000 25 0.02?0.0001 25?150 5 3 108
Robinson and Tanaka [1990] Kasei 400?1300 80 0.009 30?75 1?2 3 109
Dohm et al. [2000b] NSVs 1200?2400 100?700 0.004?0.005 30?40 ;109?1010
a D, depth; W, width; S, slope; V, velocity; Q, discharge.
DOHM ET AL.: LATENT OUTFLOW ACTIVITY FOR WESTERN THARSIS, MARS12,310
depth from a bare minimum amount needed to cover the
channel floor (roughly 10% maximum capacity), up to 100% of
the NSV?s maximum capacity. Although the valley geometries
were modified in almost every conceivable way possible, be-
cause of the immense size of this valley system the discharge
values remained consistently large.
More specifically, in the calculation of discharge values the
valley profiles were corrected to reflect the infilling of volcanic
and other materials since the late Hesperian (e.g., Plate 1c).
Progressively away from Arsia Mons, the valley floors were
lowered 500 m, 250 m, and 0 m, respectively. The modified
geometry of the channel system and a Manning roughness
coefficient value of 0.025 yielded a maximum discharge of 2 3
1010 m3/s. The discharge value for an assumed flow depth is
summarized in Figure 4.
The most interesting result of these calculations is that no
matter what modifications were made to the observed valley
geometries, whether it be the narrowing of the valleys, the
lowering of the valley floors, altering the regional slope, or
assuming that the NSVs were never filled by more than 10% of
their maximum capacity, the resultant discharge values remain
large. All other calculated discharge values for the circum-
Chryse outflow channel systems of the northeastern watershed
of the Tharsis region combined fail to equal many of the
maximum discharge estimates for the NSVs.
The NSVs potentially represent catastrophic flooding from
an ancient (late Noachian?early Hesperian) watershed located
to the northwest of the Arsia-Syria dome complex, perhaps
related to the early development of the circum-Chryse outflow
channel systems, as well as a potential source of water for a
northern plains ocean. Two shorelines have been proposed for
the northern plains: an inner, younger one that is close to an
equipotential line and an older one that deviates from an
equipotential line [Parker et al., 1993; Head et al., 1999]. The
inner, younger ocean is estimated to have had a volume of
;1.4 3 107 km3, and the larger, older one is estimated to have
had a volume of ;9.6 3 107 km3 [Head et al., 1999]. The
ancient topography, however, most likely varied from the
present one. Thus the estimated calculated volume of the pu-
tative larger ocean may be considerably different. It is inter-
esting to note that for the maximum discharge rate of 2 3 1010
m3/s, assuming the flow rate was sustained, the fill time would
have been ;8.1 days and ;8 weeks for the smaller and larger
oceans, respectively. If the Chryse outflows were occurring at
the same time at rates of ;109 m3/s to 1010 m3/s, then the fill
rates would be considerably shorter, from ;7.7 to 5.4 days for
the smaller ocean. The discharge rates, however, were most
likely not sustained at these peak values, and a greater time
period would be required to fill these hypothesized oceans. In
addition, other hydrogeologic activities associated with cata-
strophic flooding and related short-lived (;104?105 years) ep-
isodes of quasi-stable climatic conditions [Baker et al., 1991,
2000] may have also contributed water to the hypothesized
oceans by mechanisms such as spring-fed activity along areas of
the highland-lowland boundary. Another potential contributor
to the northern plains ocean is a large quantity of ground ice
(e.g., stagnant ice sheets) already in place in the northern
plains during this time. This large quantity of ice would be the
likely consequence of an earlier warm and wet phase of Mars,
possibly induced by catastrophic flooding [Baker et al., 1991,
2000]. As new floodwaters washed over the northern plains,
the additional heat would melt the upper layers of ice, and the
gradients created would allow the meltwater to cycle into the
hydrologic system.
6. Implications
Although several processes were most likely involved with
the present-day valley geometries of the NSVs, catastrophic
flooding prior to late Hesperian and younger volcanism (in-
cluding other depositional and erosional modification) contrib-
uted significantly to the formation of the NSVs. The implica-
tions for flood-carved NSVs are enormous; the NSVs
potentially represent (1) previously undocumented Martian
catastrophic floods, (2) a watershed to the northwest, perhaps
related to the early development of the circum-Chryse systems
of outflow channels, and (3) a potential source of water for a
northern plains ocean. The discharge rates have been esti-
Figure 4. Plot of discharge value for an assumed flow depth.
12,311DOHM ET AL.: LATENT OUTFLOW ACTIVITY FOR WESTERN THARSIS, MARS
mated between ;109 and 1010 m3/s. Even if maintained for
only a short time, these rates would have significantly contrib-
uted to the formation of a northern plains ocean.
Acknowledgments. Review by R. DeHon is gratefully acknowl-
edged. This work was supported by the NASA MARS Data Analysis
Program, grant NAG5-9790.
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(Received August 10, 2000; revised January 15, 2001;
accepted January 25, 2001.)
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