JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, E09008, doi:10.1029/2009JE003548, 2010
Updated global map of Martian valley networks and implications
for climate and hydrologic processes
Brian M. Hynek,1,2 Michael Beach,2 and Monica R. T. Hoke1,3
Received 18 November 2009; revised 23 February 2010; accepted 30 April 2010; published 22 September 2010.
[1] Martian valley networks have long been viewed as some of the best evidence of
prolonged surface water on Mars. Analysis from Viking data showed that valleys were
primarily contained on the ancient crust, drainage networks were immature, and large
undissected regions occurred in between individual systems. These observations led
many to propose that they formed primarily by groundwater processes, and limited or
no climate change from the present state was required. Using more recent and higher‐
resolution data sets, including visible, infrared, and topographic data, we have manually
remapped valleys on a global scale. More than eight times as many valleys have been
identified on the surface, and calculated drainage densities are, on average, higher by a
factor of 2. Further, regions previously thought to be undissected have significant incision
by fluvial processes. Most of these systems seem to have formed around the Noachian‐
Hesperian boundary (∼3.8–3.6 Ga). Minor valley formation continued through the
Hesperian and into the Early Amazonian epoch, approximately 2.8 Ga ago. The new data
reveal characteristics of sustained precipitation and surface runoff including inner braided
channels, terraces, multiple periods of formation, complex network morphology, and
correlation with other fluviosedimentary features and chloride salts on Mars. Groundwater
processes or a transient steam atmosphere generated by impacts played, at most, a
minor role. These new results imply that Mars had a long‐lived period or periods of
clement conditions toward the end of the Noachian epoch that supported a hydrologic
cycle and potentially a biosphere.
Citation: Hynek, B. M., M. Beach, and M. R. T. Hoke (2010), Updated global map of Martian valley networks and implications
for climate and hydrologic processes, J. Geophys. Res., 115, E09008, doi:10.1029/2009JE003548.
1. Introduction
[2] The ancient terrains of Mars are covered with signatures of past water. Branching channel systems on Mars,
known as “valley networks,” have long been viewed as some
of the best evidence that water flowed across the surface
[Milton, 1973; Carr, 1996]. These features are predominately
found on the heavily cratered, southern highland terrains.
They occur in nearly all geological settings including on
crater walls, volcanoes, the sides of massifs, intercrater
plains, and along the crustal dichotomy boundary. Given the
ancient nature of these landscapes and the lack of many
valleys on younger surfaces, most valleys are thought to have
formed in the Noachian epoch (>∼3.7 Ga; ages herein are
from Hartmann and Neukum [2001]). This coincides with
evidence for a warm and wet climate that includes abundant
1
Laboratory for Atmospheric and Space Physics, University of
Colorado at Boulder, Boulder, Colorado, USA.
2
Department of Geological Sciences, University of Colorado at
Boulder, Boulder, Colorado, USA.
3
Department of Astrophysical and Planetary Sciences, University of
Colorado at Boulder, Boulder, Colorado, USA.
Copyright 2010 by the American Geophysical Union.
0148‐0227/10/2009JE003548
water‐altered minerals [Bibring et al., 2006; Murchie et al.,
2009] that occurred during active Tharsis volcanism
[Anderson et al., 2001; Phillips et al., 2001], a dying or dead
magnetic dynamo [Acuña et al., 1999; Nimmo and Tanaka,
2005], a declining rate of bombardment by bolides [Ivanov,
2001; Hartmann and Neukum, 2001], and a thinning atmosphere [Jakosky and Phillips, 2001]. In addition, erosion rates
during the Noachian were approximately 1000 times higher
than at present [Craddock and Maxwell, 1993; Hynek and
Phillips, 2001; Craddock and Howard, 2002; Golombek
et al., 2006].
[3] Valley networks on Mars were noted as evidence for
ancient precipitation and surface runoff [Masursky, 1973;
Milton, 1973; Sharp and Malin, 1975]; however, further
scrutiny of the data revealed an abundance of immature
drainage systems consisting of widely spaced, U‐shaped
valleys that sometimes had alcove‐like heads [e.g., Pieri,
1980; Carr and Clow, 1981; Carr, 1995, Carr1996]. Moreover, large undissected regions were observed between networks [Carr, 1995]. Groundwater flow was thought to be
responsible for these characteristics, and this process was
hypothesized to dominate Martian valley formation [Squyres
and Kasting, 1994; Goldspiel and Squyres, 1991, 2000; Carr,
1995, 1996].
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[4] Data from Mars acquired during the last decade have
shed new light on valley network formation. It is now
believed that the majority of valleys were formed by precipitation and surface runoff [e.g., Craddock and Howard,
2002; Hynek and Phillips, 2003; Howard et al., 2005;
Irwin et al., 2005] instead of groundwater processes. These
authors note the extensive erosion that is seen globally as
well as complex valley networks with their heads at topographic divides. High drainage densities are also reported
and are comparable to the low end of terrestrial values
[Hynek and Phillips, 2003]. Putative deltas have been
observed in recent images [e.g., Malin and Edgett, 2003;
Moore et al., 2003; Di Achille and Hynek, 2010a, 2010b].
These observations, among others, imply that climatic
conditions were drastically different on early Mars from
those at present, since the modern ∼6 mbar atmospheric
pressure and subfreezing temperatures prevent liquid water
being stable at the surface. Thus, studying the characteristics
and timing of the valley networks provides important clues
to the putative “warm and wet” early climate of Mars.
[5] Some observations remain consistent with groundwater processes, and this mechanism likely acted on Mars,
particularly toward the end of valley formation. Numerous
valley networks have both U‐ and V‐shaped valley cross
sections [Williams and Phillips, 2001], and it is possible
that this represents a reactivation of networks by sapping
processes toward the end of their formation [Baker and
Partridge, 1986; Williams and Phillips, 2001]. Carr and
Malin [2000] noted that at high resolution (few meters/
pixel), the Noachian highlands were largely undissected and
suggested that this observation implies precipitation and
that surface runoff may not have been important across
much of Mars. However, others have noted that the antiquity
of the valley networks is consistent with the removal of their
smaller tributaries through geologic time through resurfacing
processes including eolian deposition, dust and volcanic ash
mantling, impact crater ejecta, and impact crater gardening
[Irwin and Howard, 2002; Craddock and Howard, 2002;
Hynek and Phillips, 2003; Irwin et al., 2008].
[6] To date, two global maps of Mars valley networks that
relied primarily on Viking images for mapping have been
produced [Carr, 1995; Scott et al., 1995]. The Viking data
have tens to often hundreds of meters of spatial resolution
and nonuniform quality, and thus, the ability to discern local
features varies greatly depending on lighting geometries.
Many more recent local to regional studies on the basis of
post‐Viking data have elucidated the characteristics of
Martian valley networks and made inferences to the
amount, duration, timing, and capacity of water required for
their formation [e.g., Gulick, 2001; Williams and Phillips,
2001; Hynek and Phillips, 2001, 2003; Craddock and
Howard, 2002; Irwin and Howard, 2002; Howard et al.,
2005; Irwin et al., 2005; Harrison and Grimm, 2005; Irwin
et al., 2008; Fassett and Head, 2008a, 2008b; Hoke and
Hynek, 2009]. Yet, to understand the climate history
of Mars as inferred from valley networks, a global
approach is necessary. Here, we present an updated global
map of Martian valley networks using post‐Viking data,
including the observations of, and inferences regarding, the
conditions of formation. The new data are consistent with
widespread precipitation and surface runoff on ancient
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Mars that required substantially different climatic conditions than at present.
2. Data and Methods
[7] After a 20 year lull, renewed interest in our neighboring
planet has come to fruition. NASA has sent orbiting or landed
spacecraft to Mars in the years 1996, 1998, 1999, 2001, 2003,
2005, and 2007, and the European Space Agency added to the
queue with a mission in 2003. Many of these highly successful missions are still conducting operations at Mars. The
1996 launch of the Mars Global Surveyor orbiter spacecraft
was the first successful mission to Mars in two decades by any
nation. The mission operated for 10 Earth years, tripled its
design lifetime, and provided a new perspective of the red
planet. High‐resolution topographic data were collected with
the Mars Orbiter Laser Altimeter (MOLA). The MOLA
experiment [Zuber et al., 1992; Smith et al., 2001] consisted
of a 1.024‐mm wavelength laser that bounced electromagnetic pulses off the surface at a frequency of 10 Hz and collected the reflected light with a small telescope. By the end of
the experiment, more than half a billion pulses estimating
elevation were collected, resulting in a global topographic
grid consisting of ∼30 cm/pixel vertical resolution and
∼460 m/pixel horizontal resolution at the equator [Smith
et al., 2001]. Also onboard was the Mars Orbiter Camera
(MOC), capable of acquiring meter‐resolution images over a
few percent of the planet [Malin and Edgett, 2001].
[8] In 2001, Mars Odyssey went into orbit around Mars and
included the Thermal Emission Imaging System (THEMIS)
experiment [Christensen et al., 2004]. The spectrometer
collects visible (∼19 m/pixel resolution) and infrared (IR;
100 m/pixel) data. To date, more than 90% of the planet (P. R.
Christensen, personal communication, 2009) has been
mapped at 100 m/pixel in both the daytime and the nighttime
IR, and a ∼230 m/pixel mosaicked product has been released
to the public [Christensen et al., 2004]. This product, combined with the MOLA data, was the base for our mapping.
Where THEMIS IR data did not exist, we used the Viking
Mars Digital Image Mosaic 2.1 or more recent products for
mapping. Daytime THEMIS IR data are acquired in midafternoon (local time) and include nine spectral bands ranging
from 6.78 to 14.88 mm. The 12.57 mm band is quite sensitive
to surface brightness temperature with shaded surfaces being
cooler and sunlit slopes being warmer in the data. Topography provides much of the temperature contrast in the data, and
therefore, images provide a pseudo hillshade of the landscape.
This product, combined with actual topography from MOLA,
permits near‐complete identification and characterization of
valley networks on a global basis at this scale. Often, an order
of magnitude increase in the number of valleys is seen compared with the Viking‐based results of Carr [1995] and Carr
and Chuang [1997], and drainage densities are comparably
higher (Figure 1). Figure 1c shows a difference map of our
new results and those of Carr [1995] and Carr and Chuang
[1997]. Almost everywhere on Mars where valleys were
originally identified, additional valleys were noted, and
drainage density was higher.
[9] Analysis of data sets at even higher‐resolution data
than THEMIS and MOLA does not give similar results:
beyond the 230 m/pixel resolution of global mosaicked
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Figure 1. Global comparison of valleys identified by Carr [1995] from (a) Viking data and (b) our
updated map using THEMIS data on top of a MOLA shaded relief map and MOLA topography from
high (red) to low (blue). (c) Drainage density calculated from Figure 1a, subtracted from Figure 1a. In the
new map (Figure 1b), denser concentrations of valleys are seen almost everywhere, and some areas with
few identified valleys from Viking data show significant dissection (e.g., south of Hellas Basin; 55°S,
70°E). Line colors on Figure 1b, represent inferred valley ages determined by the youngest terrain unit
they incise (blue, Amazonian; purple, Hesperian; red, Noachian) (unit ages are from Scott and Tanaka
[1986] and Greeley and Guest [1987]). AP, Alba Patera; AT, Arabia Terra; HB, Hellas Basin; NT,
Noachis Terra. Figure 6 is a higher‐resolution example comparing the two maps.
THEMIS daytime IR, combined with MOLA data, few
additional valleys are seen. Coregistered THEMIS visible
data (a factor of 12 better in spatial resolution) yield only a
slight increase (less than a factor of 2) in the number of
identified valleys and a modest increase in total valley length
(typically <20%) [Hoke and Hynek, 2009]. Moving to the few
meters/pixel MOC data, very few additional valleys are seen
(typically a few percent compared with the THEMIS visible).
This result is in contrast to terrestrial data analyses that show
an increasing number and length of valleys with finer and
finer resolution, down to the meter scale. This absence of
additional valleys at the meter scale is likely due to the res-
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Figure 2. Comparisons of Luo and Stepinski’s [2009] (orange) automated detection of valleys versus
this study (blue), with manual mapping in THEMIS daytime IR and MOLA data. More valleys were
detected in this study, particularly the smaller tributaries, and this is likely a factor of the coarser data used
in the automated detection method. (a) Apollinaris Patera (roughly centered at 175°E, 9°S). (b) Valleys
around 48°E and 9°N.
urfacing effects after most of the valleys formed some 3.5 Ga
ago or more, as discussed by Irwin and Howard [2002],
Craddock and Howard [2002], and Hoke and Hynek [2009].
From these observations, we conclude that our new global
map on the basis of THEMIS daytime IR and MOLA data
represents most of the valleys on Mars. Certainly, there are
local exceptions such as those noted by Mangold et al. [2004,
2008]. Although more detailed local to regional analyses may
supersede our mapping, this work likely represents the global
standard until higher‐resolution global imagery is obtained.
[10] The valleys were manually mapped using similar
determining characteristics to those of Carr [1995] for direct
comparison (i.e., sublinear, erosional channels that form
branching networks, slightly increasing in size downstream
and dividing into smaller branches upslope). Valleys are
typically hundreds of meters to 20 km wide and up to a few
hundreds of meters deep. We mapped valleys as vector‐
based lines within the ArcGIS software, which were identified in THEMIS daytime IR (231 m/pixel), MOC wide
angle (231 m/pixel), and Viking images (tens to hundreds of
meters/pixel; generally ∼250 m/pixel), plus topographic data
from MOLA (∼500 m/pixel). At low latitudes, we used an
equidistant cylindrical projection, and at mid to high latitudes, we used sinusoidal and polar stereographic projections,
respectively, to represent and analyze the data. Topographic
troughs that had a visual indication of valleys formed by
fluvial processes were traced, and adjoining ones were
connected into networks. A confidence factor was included
for each valley segment that denoted the likelihood of the
trough being formed by fluvial processes, on a scale of 1 to
3. Number and total length of valley segments plus other
network properties (e.g., stream order) were computed
within the geographic information system (GIS). We then
used the GIS software for the morphometric, topological,
comparative, and statistical analyses discussed in the next
paragraphs.
[11] Our approach of manual mapping of valleys is a
tedious and somewhat subjective process that can be influenced by albedo variations and image quality. This is partially abated by using the THEMIS daytime IR mosaic and
MOLA data as our primary bases for mapping, which provide uniform resolution and lighting conditions. While a
number of automated routines to globally map valleys have
been attempted with MOLA gridded data and other products
[e.g., Molloy and Stepinski et al., 2007; Luo and Stepinski,
2009], these have even greater limitations. Particularly,
some of these use the D8 algorithm to determine downhill
direction in the MOLA data and thus map out valleys [e.g.,
Stepinski and Collier, 2004]. However, the resolution of the
MOLA data is too coarse to resolve many of the smaller
features. The size of the smallest valley the algorithm can
detect is limited by the resolution and completeness of the
gridded MOLA data at a given latitude. At the equator, one
MOLA pixel is ∼500 m on a side. The smallest valley that
can be reliably resolved is ∼3 km wide (6 pixels). However,
this assumes that there is complete coverage. Because of the
orbit of the Mars Global Surveyor, approximately 60% of
the pixels in the MOLA gridded data at the equator are
interpolated, which often eliminate the signature of a few‐
kilometer‐wide valley. Indeed, Molloy and Stepinski [2007]
found that their MOLA‐based algorithm identified on average 69% of the valleys they could manually identify in
THEMIS images. Missing ∼1/3 of the valleys, particularly all
the smaller ones, results in an incomplete picture of fluvial
processes on Mars. Further, MOLA data have higher spatial
resolution and more complete coverage toward the poles, and
this would bias the geographic detection of valleys.
[12] Recently, Luo and Stepinski [2009] created the most
complete global map of Martian valley networks using an
automated routine and MOLA gridded data. Valleys were
mapped by delineating troughs detected in the MOLA data
and then manual editing of this map to remove spurious
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Figure 3. Number of valleys versus east longitude.
valley detections (faults, troughs unrelated to fluvial activity, etc). Overall, this method produced a data set that is
similar to our completed map and shows similar trends in
valleys detected and drainage density. However, at a fine
scale, the maps differ significantly. Figure 2 shows two
examples of the differences. In these cases, both techniques
identified many of the same prominent valleys, but we
identified many more valleys overall because the smaller
valleys we identified in the THEMIS + MOLA data sets are
not always detectable in just the MOLA data used by Luo and
Stepinski [2009]. This resulted in much greater total valley
lengths with values of 1.68 and 2.65 times those of Luo and
Stepinski [2009], resulting in correspondingly higher drainage densities. A second issue with Luo and Stepinski’s [2009]
method of mapping valley networks occurs in complex
regions where valleys are tough to identify by an automated
routine. For example, the Alba Patera volcano has been significantly faulted subsequent to formation. It is also covered
with many fluvial valleys and discrimination between troughs
from extensional faulting and those incised by water can be
difficult. Using our valley detection criteria in Alba Patera,
we mapped out 1245 valleys totaling 22,070 km in length.
Luo and Stepinski [2009] chose to disregard this difficult
region of troughs from both valleys and faults and reported no
valleys. A similar case occurs in northern Arabia Terra fretted
terrain that has been sculpted by ice‐related processes [Carr,
1996]. Here, we mapped nearly an order of magnitude of
greater valley length than Luo and Stepinski [2009] did.
Certainly, both approaches (automated and manual detection)
have pros and cons. Manual issues include more subjectivity
in valley identification and potentially missing valleys in
regions without a strong signature of incision on the surface.
Meanwhile, automated routines will map every trough on the
planet and a user must manually determine which ones are
real detections. Because our manual mapping method uses a
base map with a higher spatial resolution by a factor of 2 and
multiple data sets (not just topography), we think that this
approach results in a more complete picture of incision
across the surface of Mars. The largest difference is the many
additional fine‐scale features we identify that are not evident
in MOLA data alone (Figure 2).
3. Results
3.1. Geographic Location
[13] Consistent with previous results [Carr, 1995; Scott et
al., 1995; Carr and Chuang, 1997; Luo and Stepinski,
2009], valleys were mainly found on the cratered Noachian‐
aged (>3.7 Ga) highlands at low to mid latitudes (Figure 1). In
fact, the majority of Noachian‐aged surfaces (time stratigraphy from Scott and Tanaka [1986] and Greeley and Guest
[1987]) contain some evidence of fluvial dissection, which
is consistent with the high erosion rates hypothesized for
this epoch [e.g., Hynek and Phillips, 2001; Craddock and
Howard, 2002; Golombek et al., 2006]. Figures 3 and 4
show every valley centroid divided by age versus longitude
and latitude. Here, valley centroid represents the midpoint of a
valley segment between junctions or a junction and a source;
thus, each mature network may have hundreds of valley segments. In terms of longitude, Noachian valleys are relatively
well distributed, with peaks around the prime meridian and
140°E and with deficiencies in the western hemisphere because
of Tharsis and around 100°E because of the Hellas Basin.
Hesperian valleys are well distributed, whereas the majority of
Amazonian valleys are found on Alba Patera, centered around
240°E. In terms of latitude, Noachian valleys peak at low
southern latitudes and decrease toward both poles. Hesperian valleys are distributed throughout the southern latitudes. Amazonian‐aged valleys have a bimodal distribution
with peaks around 40°S and 50°N, corresponding to Hellas
and Alba Patera plus Arabia fretted terrains, respectively.
Very few valleys are found outside the ±60° latitude.
[14] Most ancient surfaces at latitudes less than 60° typically contain significant dissection by valleys (Figures l and 4),
although a few regions still appear relatively undissected.
The two largest of these regions are northwest Arabia Terra
and west and southeast of Hellas Basin in Noachis Terra
(Figure 1). We speculate that, in these regions, additional
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Figure 4. Number of valleys versus latitude.
resurfacing has obscured the signature of valleys, and this is
supported by the heavily degraded surface and infilled craters.
Within NW Arabia Terra, it was argued that extensive
denudation removed substantial amounts of crust and likely
valley networks [Hynek and Phillips, 2001]. Abundant
inverted ridges thought to be exhumed and differentially
eroded paleovalleys occupy this area, supporting a history
of resurfacing [Williams, 2007]. Within Noachis Terra,
significant resurfacing is also evident. Large impact craters
are highly degraded, noted by their extensively gullied rims
and infilled floors, and the regional processes that acted on
the crust likely obliterated signatures of valley incision. In
addition, Hesperian terrain thought to be regions resurfaced
by volcanics, basin infilling, or eolian deflation (Hr unit from
Scott and Tanaka [1986]; Greeley and Guest [1987]) has
overprinted the Noachian crust locally.
[15] Beyond ancient terrains, valleys also occur on the
slopes of low‐latitude to midlatitude youthful volcanoes and
within large impact basins, especially Hellas. We conducted
a cursory assessment at polar latitudes for completeness and
the highest latitude valley that matched our mapping criteria
maxed out at 73°S. We hypothesize that polar locales had
limited precipitation and runoff to carve valleys and that
valleys that did form have been modified beyond recognition
by past and current ice‐related processes.
3.2. Distribution by Age
[16] Table 1 shows that in terms of age, approximately
91% of newly mapped valley segments lie entirely within
Noachian terrains (>3.7 Ga ago), 6% cross into or are
entirely contained within Hesperian‐aged surfaces (3.7–
3.0 Ga), and 3% occur on Amazonian terrain (<3.0 Ga;
Figure 2 and Table 1). Considering the ages of each valley
segment, as opposed to networks, provides a maximum age
since connected valleys might cross a boundary onto younger
terrain. Regardless, the ages reported here are similar to those
reported from Viking‐based mapping [Carr, 1995] and still
argue for relatively rapid climate change at the end of the
Noachian. This is supported by the abrupt drop in drainage
densities on Martian volcanoes and other terrains spanning
this time boundary (discussed later). A number of the larger
valley systems have been age‐dated with crater density
measurements [Fassett and Head, 2008a; Hoke and Hynek,
2009]. These authors suggested that most of these valleys
formed toward the end of the Noachian or the start of the
Hesperian epochs. Hoke and Hynek [2009] crater age–dated
10 of the largest valley systems on Mars. Unlike most efforts that assign an age to the valleys on the basis of the age
of the terrain on which they lie, which gives an upper age
limit, this work focused on the actual age of valley formation, which gives a lower age limit. These workers found
that most of the valleys formed within the latest Noachian
and earliest Hesperian epochs, roughly 3.8 to 3.6 Ga ago.
These results are consistent with a similar study by Fassett
and Head [2008a] on a larger data set. No ancient large
networks of older or younger age were seen, indicating that
perhaps most of the global valley networks formed during this
constrained period. In some cases, systems were reactivated
after a period of up to hundreds of millions of years, and this
may indicate that the climate was not continuously warm and
wet, but instead was more episodic and regional in scope
[Hoke and Hynek, 2009].
Table 1. Comparison of Global Martian Valleys and Network
Characteristics Identified from Viking Imagery [Carr, 1995] and
Recently Acquired Topographic and THEMIS Daytime IR Data
(This Study)
Number of valleys
Total length (km)
Highest stream order
Highest network drainage density
(length/area) (km−1)
Age breakdown (Ga)
>∼3.7
∼3.7–3.0
<∼3.0
Average drainage density (km−1)
>∼3.7
∼3.7–3.0
<∼3.0
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Carr [1995]
This Study
10,748
342,384
4
∼0.02
82,217
781,393
7
0.14
90%
5%
5%
91%
6%
3%
0.00557
0.00162
0.000678
0.0115
0.00261
0.00146
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HYNEK ET AL.: UPDATED GLOBAL MAP OF MARTIAN VALLEYS
Figure 5. Plot of valley network stream order [Strahler,
1958] using data from Carr [1995] and this study. Many
more complex systems are evident in the new work, indicative of prolonged hydrologic activity on Mars.
[17] Valley network density being highest on Noachian
terrains supports the notion that most valleys formed during
this period. A lack of significant incision of valleys on the
oldest terrains (Noachian Basement and Noachian Hilly
Plateau from Scott and Tanaka [1986] and Greeley and Guest
[1987]) is intriguing and suggests that valleys did not start
forming until almost the end of the Noachian epoch or that
preservation on ancient terrains is very low. Tian et al. [2009]
modeled loss from the early Martian atmosphere under the
hypothesized strong EUV flux from the sun at that time. They
concluded that a CO2‐dominated atmosphere could not have
been maintained until the Late Noachian owing to the very
efficient thermal escape of carbon under these conditions. These
results indicate that Mars may have started cold and could
only support valley formation after the Middle Noachian epoch.
[18] Valleys are now identified in terrains thought to be
undissected from previous work (Figure 1). Particularly,
many valleys are now recognized south of the Hellas Basin,
which exist on Hesperian‐aged terrain, as well as some volcanoes (Figure 1). South of Hellas marks a dense set of valley
networks that can be up to almost 1000 km long and are
continuous down more than 6 km elevation. Many of these
originate on the slopes of the Hesperian‐aged Amphitrites
Patera volcano. The remainder of Hesperian‐aged valleys is
peppered throughout the cratered highlands, typically on Hr,
Hesperian Ridged terrain from Scott and Tanaka [1986] and
Greeley and Guest [1987]. The Amazonian‐aged valleys
occur predominantly on the flanks of volcanoes, including
the Early Amazonian‐aged Tyrrhena, Hadriaca, and Alba
Paterae. Alba Patera contains the densest set of young
valleys, and drainage density is within the range of Noachian
values (i.e., 0.0196 km−1 on unit Aam from Scott and Tanaka
[1986]). The other major concentration of Amazonian
valleys is on the eastern slopes of Hellas Basin.
3.3. Morphometry and Network Topology
[19] Morphometry of valley networks on Earth and Mars
has been used to discern their formation mechanisms and the
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amount and duration of water responsible. Many properties
have been used, but the system of stream order by Strahler
[1958] is often invoked. In this model, the headward‐most
valleys are assigned a stream order of 1. When two first‐
order valleys come together, the resultant downslope valley
is given an order of 2 and so on. If two different‐order
valleys join, the result is the highest order in the upstream
segments. Higher‐order networks show more complexity
and integration, and it is thought that larger amounts of
water and time are necessary to form high‐order systems.
Viking‐based global maps of Martian valley networks often
revealed a single‐trunk segment with sparse stubby tributaries
[Carr, 1995], resulting in low stream order of most networks.
Carr [1995] and Carr and Chuang [1997] did not identify a
single network greater than fourth order in the Strahler system
(Figure 5). Low stream order combined with other morphometric properties of some valleys (e.g., alcove‐like heads,
U‐shaped valleys) led these workers to conclude that significant precipitation was not necessary to form the valleys,
and perhaps most are a result of groundwater processes. In
our updated map, the many additional valleys we find are
manifest in complex, high‐order networks, implying that
earlier studies of Carr [1995] and Carr and Chuang [1997]
were limited by poor resolution and image quality. We have
found 1 seventh‐order, 9 sixth‐order, and 24 fifth‐order networks (Figure 5). For comparison, larger rivers in the United
States such as the Ohio and Platte rivers have a Strahler order
of 8 determined using equally high or higher spatial resolution
than available for Mars. However, terrestrial comparisons
are limited because of the higher‐resolution data and their
youthfulness that result in much higher stream orders.
3.4. Drainage Density
[20] Individual valley network drainage density was
determined by summing the total lengths of valleys in a
network and dividing by the drainage area using the convex
hull method as described by Hoke and Hynek [2009].
Although this method assumes that valleys reach the drainage
divides, and thus may overestimate drainage density, it is
appropriate here because significant subsequent modification by many processes (volcanism, tectonism, impacts,
etc.) has often obliterated the original boundaries of a
drainage basin. Global drainage density was determined by
mathematically determining the total length of valleys
within a set radius (235 km in this case) of each output raster
cell. For drainage density on volcanoes (section 3.5), we
used the area of the entire volcanic construct and summed
length of valleys identified on it. As expected, many more
valleys were seen in THEMIS daytime IR data compared with
those detected in Viking images. We used ages from Scott and
Tanaka [1986] and Greeley and Guest [1987] to determine
that Noachian units as mapped on the global geologic maps
have an overall drainage density of 0.0115 km−1, whereas the
drainage densities on Hesperian and Amazonian terrains
(excluding the polar terrains) are 0.00261 and 0.00146 km−1,
respectively (Table 1). These are all approximately a factor of
2 higher than those of Carr [1995] determined in the same
manner. In total, the updated global map shows more than
eight times as many valleys identified, totaling a summed
length of approximately two times greater than that of Carr
[1995] (Table 1). In local cases, drainage density of individual valley networks can be as high as 0.20 km−1, and a greater
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cipitation and surface runoff were likely required because of
the dense drainage, high stream order, and many small
tributaries that reach right up to the drainage divides.
[22] Carr and Chuang [1997] used Landsat data degraded
to Viking resolution to map out precipitation‐formed valleys
in various geographic locales and climates on Earth for
comparison to Martian data. Their terrestrial drainage density
values ranged from 0.079 to 0.21 km−1 at the 1:1,000,000
scale, which is roughly equivalent to our mapping scale of
∼1:750,000. We included multiple data sets in our mapping,
so the results are not easily directly comparable. Our average
Noachian units drainage density falls below their low end of
values, but this is because we included all Noachian units,
some of which have been greatly modified in more recent
times (section 3.1). Local values of drainage density, determined with a method similar to that of Carr and Chuang
[1997], show much higher drainage densities that span the
range of their terrestrial systems.
Figure 6. Comparison of valleys mapped by Carr [1995]
from (a) Viking data to those identifiable in (b) THEMIS
daytime IR and MOLA data near 3°S, 5°E; both are on a
THEMIS daytime IR base map.
complexity of drainage is evident. Many smaller tributaries
that were not evident with Viking data, as well as structure
within the valleys including braided channels and terraces
indicative of sustained flow, are seen.
[21] Figure 6 shows an example of valleys mapped by
Carr [1995] compared with this study. In the original
mapping, a few, often unconnected, valleys without many
tributaries could be discerned (Figure 6a). Figure 6b shows
that, with data of higher image quality and topographic
information, valleys can be resolved into a dense, mature
drainage network. In this example, Carr mapped 16 valley
segments totaling 1492 km in length; giving a corresponding
drainage density of 0.034 km−1. In contrast, our updated
mapping shows 462 valley segments totaling 6031 km in
length, giving a corresponding drainage density of 0.14 km−1.
In this case, the higher‐resolution mapping reveals an
increase in stream length and drainage density by a factor
of 4. The characteristics of the originally mapped valleys
are consistent with the formation by groundwater processes.
However, the updated map indicates that sustained pre-
3.5. Valleys on Volcanoes
[23] Valleys occur on many, but not all, of the major
Martian volcanoes. Baker et al. [1991] and Baker [2001]
suggested that many valleys, on Alba Patera especially,
may have formed by precipitation when global climate
became temporarily warm and wet as a result of the massive
flooding and ponding of water that occurred during outflow
channel formation. A rise in temperature and vapor pressure
would have allowed regions with higher elevations adjacent
to the transient ocean to be incised with precipitation‐fed
valleys. Although somewhat sporadic, the majority of outflow channels likely formed in the Late Hesperian to Early/
Middle Amazonian epochs [Rotto and Tanaka, 1995].
[24] We mapped the valleys and crater age–dated every
channel‐covered volcano on the planet to understand
valley formation (timing and mechanism) on these terrains.
We then inferred the origins of valley networks (namely,
precipitation‐fed runoff versus hydrothermal) from the
valleys’ morphometry and surrounding geological context.
We also measured drainage densities and investigated the
maturity of the valley systems. Subsequently, volcano surfaces
were age‐dated through measurements of crater density, which
were then related to the Martian isochrons of Hartmann and
Neukum [2001]. Craters greater than 5 km were primarily
adopted from previous studies [i.e., Barlow, 1988] that used
Viking data. We used higher‐resolution THEMIS data to
enhance this count and added smaller‐crater‐diameter bins,
thereby increasing the accuracy of surface ages compared
with those of many previous studies. The age of each volcanic
construct with valleys was inferred from cumulative crater
plots. With the surface ages of 18 major volcanoes determined, along with identification of precipitation‐fed systems,
it became possible to analyze valley network formation due to
precipitation during the course of Martian history.
[25] Figure 7 shows drainage density through time on
Martian volcanoes. The results indicate a sharp decrease in
drainage density over time, similar to valley network formation across the rest of the planet. The relatively high
drainage densities, valley morphology, and complicated
network patterns on older volcano surfaces imply surface
runoff from precipitation (e.g., Figures 7 and 8). The majority
of our drainage densities on Noachian era volcanoes are
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Figure 7. Normalized drainage density on all dissected volcanoes versus their surface age determined by
crater counting. Black line represents our interpretation of drainage density over time.
similar to values calculated for densely dissected Noachian
highlands.
[26] Since 2.9 Ga ago, drainage densities on more recent
volcanoes approaches zero (Figure 1), and the complexity of
the few valley networks observed decreases significantly
(few tributaries, low bifurcation ratio, etc.). Furthermore,
most valleys that formed after this time tend to appear related
to nonimpact pit crater chains associated with volcanic processes. The northwestern flank of Arsia Mons serves as an
example (Figure 8). These traits are characteristic of valleys
Figure 8. Comparison of valley networks inferred to have been formed by (a) hydrothermal processes
and (b) surface runoff/precipitation. Figure 8a is taken from Arsia Mons, age of 0.6 Ga, with a drainage
density of 0.0024 km−1. Figure 8b is taken from Hecates Tholus, age of 3.6 Ga, with a drainage density of
0.051 km−1. Colors represent MOLA topography (blue, low; red, high).
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may support the theory of youthful valley formation related
to outflow channel formation (Figure 7). The one volcano
(Alba Patera) whose surface formed during high outflow
activity does in fact show a spike in what we infer to be
precipitation‐fed drainage systems relative to the overlying exponential decay of drainage density through time
(Figure 7). However, this conclusion is limited by only
one volcano dated to the period of significant outflow
channel activity. As Figure 7 shows, there is a lack of volcanic
surfaces between 2.9 and 1 Ga ago. The dissected volcanic
surfaces that formed after 3.0 Ga ago are all found in the
Tharsis region. The final spurts of valley‐forming activity
were between 1.0 and 0.6 Ga ago, and most of these valleys
appear to have characteristics consistent with formation by
hydrothermal processes.
Figure 9. Distribution of ages and elevations of all identified valleys on Mars. (a) Distribution of valleys vs. elevation. (b) Land area vs. elevation, separated by age.
formed by groundwater processes (hydrothermal activity) as
opposed to surface runoff from precipitation, consistent with
Viking‐based results of Gulick and Baker [1990]. The change
of valley morphology over time shows the abrupt decline of
the putative warm and wet Noachian climate in which global
precipitation was frequent enough to erode and modify volcanoes into the cooler dry climate found today.
[27] The Martian volcano with the highest drainage density is Ceranius Tholus, with a density of 0.14 km−1 and an
age of 3.7 Ga. Some volcanoes had no detectable valleys at
the THEMIS IR resolution. It is important to note that the
THEMIS visible data (19 m/pixel) show valleys on some
volcanoes that appear undissected in the IR daytime global
mosaic used in this work. However, the higher‐resolution
images suggest that these valleys are likely hydrothermal in
nature and do not reflect precipitation.
[28] In summary, valley formation on Martian volcanoes
shows similar trends to the rest of the planet. Our results
3.6. Comparison to Other Data Sets
and Atmospheric Models
3.6.1. MOLA Topography
[29] Figures 1 and 9 show the relative distribution of Mars
valley networks across the three defined major geological
epochs, and Figure 9b shows land area versus elevation
separated by age. As previously understood, most of the
valleys occur on ancient terrains, which are typically high in
elevation [Carr, 1995; Carr and Chuang, 1997]. These
valleys have a rough Gaussian distribution centered around
1500 m (Figure 9), and the binned elevation of Noachian
terrains is similar in nature (Figure 9b). Thus, valleys formed
across most ancient terrains, and elevation was not a major
factor in the location of incision. Hesperian‐aged valleys,
comprising 10% of the total, developed roughly evenly across
all elevations of Hesperian terrain (Figures 1 and 9) but with
concentrations around the Hellas Basin and on volcanoes of
this age. The Amazonian‐aged valleys developed almost
exclusively on high volcanoes that formed during this period.
These valleys formed after climatic conditions that favored
precipitation, and stable surface water on a global scale
ended. The youngest valleys probably originated from
hydrothermal circulation on volcanoes or from catastrophic
floods or bolide impacts that could force climate change
locally (Baker et al. [1991] and Segura et al. [2002],
respectively). We conclude that the majority of valleys
formed on Mars were evenly distributed across terrains and
do not show any strong preference for elevation.
3.6.2. Derived Elemental/Mineralogical Data Sets
[30] The updated global valley network map was compared with other data sets including global databases of
fluvial/sedimentary features and mineralogical and chemical
data. K/Th ratios are often used to assess the extent of
aqueous alteration because their host minerals behave
differently in aqueous solutions. Data from the Gamma
Ray Spectrometer on Mars Odyssey have recently resulted
in elemental maps of the upper meter of the Martian surface
[Boynton et al., 2008]. These data have a low spatial resolution of ∼300 km/pixel, so small alteration signatures from
individual valley floors would be lost in the noise. Still,
regions that experienced significant precipitation may have
crust that records a period or periods of pervasive aqueous
alteration. Comparison of our map with the K/Th map
[Taylor et al., 2006] shows no strong correlations
existing between valley density and the K/Th ratio. It is also
the case that the isolated locales and regions exhibiting
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Figure 10. Global drainage density calculated by mathematically determining the total length of valleys
within a set radius (235 km in this case) of each output raster cell in relation to putative paleolake deposits
(green circles, [Fassett and Head, 2008b]), fan deltas (dark red circles, Di Achille and Hynek [2010a,
2010b]), and chloride deposits (pink circles, [Osterloo et al., 2010]). There is a strong correlation
between high drainage density and reported fluviosedimentary deposits.
water‐altered minerals detected from the IR spectrometers
Observatoire la Minéralogie, l’Eau, les Glaces, et l’Activité
and Compact Reconnaissance Imaging Spectrometer for
Mars (Bibring et al. [2006] and Murchie et al. [2009],
respectively) show no significant correlation with regions of
dense fluvial incision. Perhaps eolian and impact modification processes occurring since valley formation have obscured
the signature of water‐altered minerals, or that such materials
never made up a large portion of the incised regions.
[31] Figure 10 shows newly mapped global valley drainage density. Overlaid on this map are global locations of
putative paleolakes, fan delta deposits, and occurrences of
chlorides as detected by THEMIS (databases from Fassett and
Head [2008b], Di Achille and Hynek [2010a, 2010b], and
M. M. Osterloo (personal communication, 2010), respectively). Unlike most of the elemental and mineralogical data,
there is a strong correlation between the density of valleys
and the documented fluviosedimentary deposits (Figure 10).
It should be noted that, at the mouths of most valley networks, no fluviosedimentary deposits are seen. We hypothesize that downstream portions of most valley networks have
been resurfaced since formation. Most valleys terminate in
large basins that have likely experienced significant infilling
since valley activity, even if clear landforms are not present
[Howard, 2007]. Clearly, the volume of material excavated
from the valleys was deposited somewhere, but the antiquity
of the valleys on a planet that experienced substantial subsequent geological modification after their formation has
obliterated depositional features in most cases. However,
locations where paleolakes and fan deltas have been noted on
the surface of Mars are often in regions of high drainage
density. Fassett and Head [2008b] mapped out valley‐fed
open‐lake basins across Mars. These paleolake basins spatially correlate with regions of high drainage density, in
particular, in the Terra Cimmeria, Margaritifer Terra, and
Terra Sirenum regions (Figure 10). The main region lacking
significant correlation is north of Syrtis Major, where
numerous paleolakes are noted in a region of low drainage
density. Fassett and Head [2008b] attribute these particular
paleolakes to be groundwater‐fed systems, and their occurrence is consistent with models of groundwater upwelling
[Andrews‐Hanna et al., 2007]. Similarly, Di Achille and
Hynek [2010a] mapped and compiled 54 fan delta deposits
on Mars, which formed at the mouths of valleys. These fan
deltas also correlate with valleys, albeit in regions of generally moderate drainage density (Figure 10). Finally, Osterloo (personal communication, 2010) used THEMIS data to
characterize locales thought to be rich in chloride salts. The
detections of chloride deposits are also in highly dissected
regions of the planet (Figure 10). These relationships show
correlation between known fluviosedimentary deposits and
aqueously formed salts. It is likely that clement conditions
capable of supporting an active and long‐lived hydrological
cycle were required and that there is a causal relationship
between incision of valleys and these sedimentary features
and chemical data.
3.6.3. Climate Models
[32] Recently, Soto et al. [2010] used the National Center
for Atmospheric Research’s Community Atmosphere Model
to simulate a putative warm and wet climate on Mars. Specifically, the workers flooded Mars up to −4.5 and −3 km
elevation relative to the Martian datum and observed daily
and annual precipitation patterns. They found that, without
standing bodies of water, it was nearly impossible to get
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significant rainfall anywhere on Mars. Most terrestrial valley
formation is greatly affected by the distance to the nearest
water body. For example, the interior of Pangaea was thought
to be very dry [Crowley and North, 1991], and similar constraints would have likely operated on ancient Mars. Using
modern topography, Soto et al. [2010] found that rainfall on
Mars would have predominately occurred in low to mid
southern latitudes, and their high annual precipitation
regions in most models coincide with regions of dense
valley dissection. A Noachian ocean on Mars is supported
by recent geological evidence reported by Di Achille and
Hynek [2010b] and the distribution of ancient valleys
reported here are generally upslope and proximal to the
inferred shoreline.
4. Valley Formation Mechanisms and the Early
Climate of Mars
[33] The widespread and complex dissection of ancient
terrains and correspondingly high drainage densities imply a
long‐lived hydrological cycle that spanned the latest Noachian and earliest Hesperian epochs. Some valleys are more
than 1000 km long and have well‐integrated drainage patterns. We infer that precipitation during this period was
sufficient over most terrains, excluding polar regions, to
allow surface runoff and incision of the crust. The question
of how a dense atmosphere is produced and maintained to
allow warm enough temperatures for valley formation to
occur remains an enigma. Lower solar luminosity with a
high EUV flux, Mars’s distance from the sun, and a dynamo
that shut off before 4.0 Ga [Nimmo and Tanaka, 2005] conflict with a warm and wet early Mars [e.g., Kasting 1991;
Segura et al., 2008; Tian et al., 2009].
[34] Two main methods of supplying an atmosphere have
been proposed; transient steam atmospheres produced by
large (few tens to hundreds of kilometers) impacts [Segura
et al., 2002, 2008] and volcanic outgassing [e.g., Phillips
et al., 2001; Jakosky and Phillips, 2001]. The impact
model is inconsistent with the observations, particularly the
inferred age of the majority of valleys. Most valleys were
likely incised in the latest Noachian and earliest Hesperian
epochs [Fassett and Head, 2008a; Hoke and Hynek, 2009],
when few large impact basins were being formed [e.g.,
Nimmo and Tanaka, 2005]. This timing disparity between
valley and large impact basin formation is significant; a few
hundreds of millions of years passed between large basin and
valley formation. Formation of all the modest‐sized (tens of
kilometers) impact craters contributed ∼2 to 46 m of erosion
on a global scale [Segura et al., 2008], although the authors
contend that these numbers are minimal. In addition, the
oldest terrains (pre‐Late Noachian) show few signs of dissection, although this is the period when the greatest flux and
size of impactors were hitting Mars, and abundant valleys are
predicted. Perhaps preservation of valleys on these degraded
terrains is an issue. Still, the extent of dissection and integration of networks implies long‐lived surface water, and it is
unclear how short‐lived transient impact‐generated atmospheres could produce the geological observations. In regional
modeling of the Parana Basin, Barnhart et al. [2009] showed
that episodic precipitation of more than 105 to 106 years was
required to form the features seen in that region, and they
discounted impact‐generated events as a major contributor.
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Other studies, such as that of Craddock and Howard [2002],
used crater degradation and other lines of geological evidence
to argue also that impacts could not be a significant source of
water for sculpting of the observed Martian landscape. These
authors note that degraded craters are generally at least 500 m
to as much as 3000 m shallower than fresh craters at equivalent
diameters and that the tens of meters of total erosion predicted by all smaller impact craters forming during this period
[Segura et al., 2008] do not match these values. Evidence
of paleolakes [e.g., Fassett and Head, 2008b; Di Achille
et al., 2009], numerous fan deltas [Di Achille and Hynek,
2010a], and a past ocean [Di Achille and Hynek, 2010b] are
also at odds with short‐lived transient steam atmospheres.
[35] Thus, it appears likely that Mars had a substantial and
long‐lived atmosphere around 3.8 to 3.6 Ga ago. Volcanic
outgassing could have been the source of volatiles. Mars is
covered with signs of magmatism/volcanism, and the heat
flux on early Mars was likely a factor of 2 to 3 higher than at
present [e.g., Hauck and Phillips, 2002]. Phillips et al. [2001]
showed that the majority of the Tharsis complex formed in the
Noachian epoch. They noted that this activity likely released
the integrated equivalent of a 1.5 bar CO2 atmosphere and a
120 m thick global layer of water. Additional greenhouse
gases are associated with volcanism that could have contributed to warm wet conditions for hundreds of millions of
years required for valley formation. Moreover, more than half
of the volcanoes with valleys formed in the Noachian epoch,
providing additional volatiles (Figure 7). As Tian et al. [2009]
noted, the high solar EUV flux before the late Noachian likely
prohibited the build up of a thick atmosphere. Soto et al.
[2010] showed that without large bodies of water on early
Mars, little precipitation would occur. In addition, their
annual precipitation models in the case of a Mars with seas
or oceans result in precipitation concentrated in the regions
of dense dissection in Figure 10. We hypothesize that the
majority of valley formation occurred during the few‐hundred‐
million‐year period of clement conditions and that the source
of volatiles was widespread volcanic outgassing, particularly
from the Tharsis region with only minor additional contribution from impact processes.
5. Conclusions
[36] An updated global map of valleys on Mars has been
completed and released to the scientific community on the
USGS Astrogeology’s PIGWAD Web site, and the valley
network map will also be included as a layer in the new
global geologic map of Mars [Tanaka et al., 2009]. The
230 m resolution global THEMIS daytime IR data, combined with MOLA topography and other imagery, allow an
unprecedented look at these features formed by surface water.
Many more valleys are seen globally than in earlier studies,
and drainage density measurements are, on average, a factor
of 2 higher, indicating that more water was required than
previously thought. The majority of Noachian terrains are
dissected, and all units of this age have an average drainage
density of 0.0115 km−1, but locally, drainage density can be
as high as 0.2 km−1. Complex drainage networks are visible
with stream orders of up to 7. Some valleys are seen with
interior anatomizing channels and terracing, and certain valley networks show signs of multiple formation periods [Hoke
and Hynek, 2009]. These new results are consistent with a
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warm and wet early climate that incised the crust. Hesperian
and Early Amazonian valleys also show signs of formation by
precipitation and surface runoff, although at much decreased
levels through time. The youngest valleys on the planet are
confined to volcanic constructs and most likely had a hydrothermal origin. Areas on Mars that have higher valley density
correlate with fluviosedimentary deposits that have been noted
by a number of workers. Our results and those from others
show that valleys formed on Mars during the late Noachian and
earliest Hesperian epochs, perhaps spanning a couple hundred
million years, across most terrains of these ages. Widespread
and long‐lived surface water could have supported microbial
life if it ever arose on Mars. It is unlikely that transient impact‐
generated steam atmospheres could have produced the magnitude of dissection during this period, and instead, most
Martian valleys and associated deposits were likely formed by
long‐lived surface runoff from precipitation.
[37] Acknowledgments. We appreciate comments and suggestions
provided in conversations with Gaetano Di Achille. We thank Danielle
Russell who helped complete GIS analysis of the valleys. This work
was supported under NASA Mars Data Analysis Program award
NNX06AE08G.
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M. Beach, Department of Geological Sciences, University of Colorado at
Boulder, Boulder, CO 80309‐0392, USA.
M. R. T. Hoke and B. M. Hynek, Laboratory for Atmospheric and Space
Physics, University of Colorado at Boulder, 392 UCB, Boulder, CO
80309‐0392, USA. ([email protected])
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