JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, E07005, doi:10.1029/2003JE002155, 2004 
Aeolian sediment transport pathways and aerodynamics at troughs 
on Mars 
Mary C. Bourke' 
School of Geography and the Environment, University of Oxford, Oxford, United Kingdom 
Joanna E. Bullard 
Department of Geography, Loughborough University, Loughborough, United Kingdom 
Olivier S. Bamouin-Jha 
Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA 
Received 14 July 2003; revised 4 February 2004; accepted 2 April 2004; published 13 July 2004. 
[i]   Interaction between wind regimes and topography can give rise to complex suites 
of aeolian landforms. This paper considers aeolian sediment associated with troughs 
on Mars and identifies a wider range of deposit types than has previously been 
documented. These include wind streaks, falling dunes, "lateral" dunes, barchan dunes, 
linear dunes, transverse ridges, sand ramps, climbing dunes, sand streamers, and sand 
patches. The sediment incorporated into these deposits is supplied by wind streaks and 
ambient Planitia sources as well as originating within the trough itself, notably from 
the frough walls and floor. There is also transmission of sediment between dunes. The 
flow dynamics which account for the disfribution of aeolian sediment have been modeled 
using two-dimensional computational fluid dynamics. The model predicts flow separation 
on the upwind side of the trough followed by reattachment and acceleration at the 
downwind margin. The inferred patterns of sediment transport compare well with the 
distribution of aeolian forms. Model data indicate an increase of wind velocity by ~30% 
at the downwind trough margin. This suggests that the threshold wind speed necessary for 
sand mobilization on Mars will be more frequently met in these inclined 
locations.        INDEX TERMS: 1815 Hydrology: Erosion and sedimentation; 1824 Hydrology: 
Geomorphology (1625); KEYWORDS: Trough, aeolian, dune, MOC, MOLA, computational fluid dynamic 
modeling 
Citation:    Bourke, M. C, J. E. Bullard, and O. S. Bamouin-Jha (2004), Aeolian sediment transport pathways and aerodynamics at 
troughs on Mars, J. Geophys. Res., 109, E07005, doi:10.1029/2003JE002155. 
1.    Introduction associated with troughs and valleys on Mars, despite the 
.-,.???, ,.        ^  ? ^ ? ,,       . potential for significant infilling and burial of fluvial fea- [2]   A sigmticant proportion o? the surtace oi Mars is i,       .,     <-, u ?* i/-^ ri,/ir^\- u      ^u ai. 
'-^     , . ?      ,.       ,.      ?    r .? r . tures. Mars Orbital Camera (MOC) images snow that there IS 
covered m aeolian deposits. Surtace sediment trequently u    A e ^ u A e 4.       u {^ J, ,?   ?    , ,   ,    ?   , J:-   ^ , ? ?   , an abundance o? transverse bed torms on trough iloors 
forms distinct morphological featoes such as mantles, wmd ^^^?.^ andEdgett, 2001] but other within-valley dune forms 
streaks, dunes and a variety ot sand sheet-like deposits , * u       -J   ^i- j A? n c ^u       i *      * 
r?     , ,, ,??,  X,, ,   ,???,   .?,       , have not been identitied. Valleys on Earth are known to act \ureetey andIversen, ?9o5; Ihomas et at., 1999\. Aimough ,   ., J-       <. ?    j       i- u   rr ,      mo^ 
?   ,       ,; ,        ,. ,   ,? ,.        -, \,       ,   , both as sediment sources in dune iields \Lancaster, 19?6; 
wmd-erodible sediment potentially exists over the whole ,^ ,,   ?      , ?,    ,.      ,noT   ur ^e\o^^     A      ? * 
,      ^ ,, . ,,?_?-, ?   ?        ,       , Mabbutt and Wooding, 1983; Wasson, 19?41 and as inter- planet, the mai or concentration 01 dunes IS in vast sand seas , " ? i   " ui    i ? *        *? A-       *     iu 
r^y ^^     \      -1 ? i:?^o,^/^/^,^,    2 ccptors Or    siuks    blockmg or truncating sediment path- 01 the northern hemisphere covering an area ot 680,000 km , ? ^ A-       ^t. i. ^    A ,.      ? 
, ?    \      i^^^n   T?      1 1 ? ways. In previous studies, the approach to determine [Lancaster and Greeley, 1990]. Elsewhere, they occur m ^^^.^^^^ ^^ ^^^ ^^^ ^^^ ^.^^ ^^^ ^^^^ 
local concentrations oiten trapped by topographic teatures       i u i ? i    r^  J ,    ;    inam   T7        ^ A-     I, 
r?     , , , ,^o^    .? 1- 1    i^^m   T. ?      global m scale ?Anderson et at., 1999]. Few studies have [Greeley and Iversen,  1985; Malm et al.,  1998]. It is     ?        A      ^u   t    A^ I    TU 
.        ^^   ^        ,,,,,, , ? , .    ?   ,     locused on the landiorm scale. There are some exceptions, important to understand the extent to which geological , ,, i  *i, * u     -A    4.-c A      *    A       C ^A A- 
. ^ J,      , r,    ? ,     ,?  -, ,,      ,        , ,      notably work that has identiiied crater dune iields as sedi- 
signatures oi past tluvial activity on Mars have been mod- , r j^irirj,,  inm   ZTJ   ^,      J 
?J-   1 rr,    , ,    ,,       ,     ,        ^??   r 1-     i:- mcut sourccs tor wind streaks [Edgett, 2002; Edgett and itied. To date, there has been little locus on aeolian teatures     ,^ ,.    r.^?^r,  T? .   ;   mon *i, ? ^-       c     c Mahn, 2000; Thomas et ai, 1981], the association ot sen- 
like features with the elongation of barchan horns [Lee and 
?-, Thomas, 1995 ; Tsoar et al., 1979] and links between barchan 
Now at Planetary Science Institute, Tucson, Arizona, USA. ., . ?    n      t. j.     vr^    ^        ^    i    '^r\r\-^-i 
' and transverse dunes m Proctor crater [/^e?to? ei a/., 2003]. 
Copynght 2004 by the American Geophysical Union. t^]    Topography affects both wind velocity and direction. 
0l48-0227/04/2003JE002l55$09.00 Sediment transport and erosion may increase as wind veloc- 
E07005 1 of 16 
E07005 BOURKE ET AL.: SEDIMENT TRANSPORT AND AERODYNAMICS ON MARS E07005 
ities increase on the windward side of topographic features 
such as hills and valleys [Greeley et al, 2002] while zones of 
flow separation on the downwind side may be areas of 
preferential deposition [Wiggs et ai, 2002]. Wind flow 
patterns predicted by general circulation models for Mars 
and Earth generally correlate with the location and orienta- 
tion of aeolian deposits [Anderson et al., 1999; Blumberg 
and Greeley, 1996]. Research on the interactions between 
synoptic-scale wind flow and trough topography on Earth 
suggests that the presence of troughs will initiate not only a 
within-trough wind regime, but also may affect the strength 
and orientation of wind on the adjacent surface [Bullard et 
al, 2000; Slerputowski et al, 1995]. This, in turn, may affect 
the location of aeolian deposits [Wiggs et al, 2002]. 
[4] We use the term trough to include all linear negative 
relief topography. The results reported here are valid for 
fluvial valleys, Chaos regions, Chasma etc., and are partly 
applicable to escarpments. However, the specific geological 
history will determine the sediment type and availability, 
which in turn will influence the potential assemblage of 
aeolian forms. Another influence will be the presence of 
niveo-aeolian deposits. In polar deserts on Earth, sand dunes 
may have permafrost cores and/or contain sand layers inter- 
bedded with snow and ice [Calkin and Rutford, 1974; 
McKenna Neumann, 2004; Steidtmann, 1973]. We are 
investigating the potential for niveo-aeolian dune deposits 
on Mars in ongoing research [e.g., Bourke et al, 2004]. 
[5] This paper focuses on the interaction between trough 
topography and aeolian sediment on Mars. Using detailed 
observations from narrow and wide-angle MOC images of 
troughs in Syrtis Major (and elsewhere), the effects of the 
interaction between regional-scale wind patterns and trough 
topography on aeolian depositional forms are described. We 
highlight the range of dunes found within the trough, 
suggest potential sediment sources and consider the role 
of the trough as a temporary sediment sink and/or store. 
Using work that has been undertaken on valley-wind 
interaction on Earth [Wiggs et al, 2002], a preliminary 
2-D model is developed to investigate similar interactions 
under Martian conditions. We suggest that, through its 
impact on aeolian sediment transport pathways, the trough 
provides an important link (source/sink) for exchange of 
sediment between different aeolian forms and different parts 
of the surface. A conceptual model is presented illustrating 
the sediment transport continuum between the trough and 
the aeolian features both within and adjacent to it. 
2.    Study Area 
[6] Syrtis Major is one of the most prominent Early 
Hesperian (~3.6 Gy) volcanic complexes on Mars. The 
gently sloping (~0.13?) lava plains are estimated to be 
between 0.5 and 1 km thick [Hiesinger and Head, 2004]. 
Syrtis Major is a low-albedo (0.15) and high thermal 
inertia (average 312.53 J m~^ K~' s~''^) [Fergason and 
Christensen, 2002] region on Mars. Recent Thermal Emis- 
sion Spectrometer (TES) data indicate the northern region of 
Syrtis Major to be predominantly basaltic [Bandfield et al, 
2000], specifically comprising feldspar and high calcium 
Pyroxene [Mustard and Cooper, 2002]. The principal 
surface types of the region include the following terrain 
components: old cratered, knobby, locally grooved, etched. 
pitted, polygonal terrain in craters, cratered and smooth 
terrain superimposed with dunes [Hiesinger and Head, 
2004]. 
[7] At a regional scale, Syrtis Major is dominated by 
easterly winds. This is inferred from the orientation of 
wind streaks which are aligned between 260? and 280? 
(Figure 1). Amus Vallis, located in northern Syrtis Major, is 
a sinuous, slot-like trough (Figures 1-3). It has a northeast- 
southwest orientation and crosses the east to west aeolian 
sediment transport path, as indicate by wind streak direc- 
tion. Amus Vallis is 200 km long and centered on 14.1?N 
289.5?W sloping gently down to the north (Figure 4b). 
Other significant regional features include impact craters, 
troughs, wrinkle ridges, lava flow fronts and dark and bright 
wind streaks (Figure 1). The origin of the trough is 
unknown; however proximity to the Nili Patera caldera to 
the south suggests that it is likely to be a lava channel. A 
series of (sub) horizontal beds exposed in the trough wall 
may be lava deposits. The presence of an inner channel 
(Figure 3) suggests that Amus Vallis may have been subject 
to low-viscosity fluid erosion, perhaps water released 
through geothermal activity. 
3.    Results 
3.1. Trough Morphometry 
[8] The MOC and MOLA data indicate that the trough 
has an average width of 775 m and a relative depth of 
~90 m. The high-resolution MOC images show the trough 
to be sinuous, with well-developed convex and concave 
bends (Figure 3). The absence of craters on the trough floor 
indicates that it may be significantly younger than the 
surrounding Planitia surface on which a number of degraded 
craters are visible, although, craters may have been buried 
by trough floor aggradation. The MOLA profile indicates a 
sharp break of slope between the trough and Planitia surface 
that is sufficient to initiate flow separation at the upwind 
trough wall (Figure 4d). 
3.2. Aeolian Deposits on the Planitia Surface 
[9] We have based our observations primarily on a high- 
resolution (2.95 m/pixel) MOC image that reveals a 4.6 km 
length of Amus Vallis in Syrtis Major (Figure 3). Two types 
of aeolian deposits are identified on the Planitia surface, 
wind streaks and drift deposits, and are described below. 
3.2.1.   Wind Streaks 
[10] Wind streaks are identified by variations in albedo 
and take the form of dark, bright and mixed tone streaks that 
generally form in association with topographic obstacles 
such as troughs and craters [Thomas et al, 1981]. In 
Figure 1, a series of bright and dark albedo markings extend 
from positive and negative topographic features (craters, 
ridges, and troughs). The east to west trend of these wind 
streaks (270?) correlates with global climate model predic- 
tions of regional wind directions [Greeley et al, 1992]. 
Using the classification of Thomas et al [1981], the streaks 
are identified as type I bright (b) and dark (d) wind streaks 
and are parallel and fan-shaped. Type I (d) streaks occur on 
the outer margins of approximately 30% of the bright 
streaks in Figure 1. High-albedo markings have been 
interpreted as global dust storm fallout deposits in protected 
wind-shadow zones [e.g., Veverka etal, 1981]. Dark albedo 
2 of 16 
E07005 BOURKE ET AL.: SEDIMENT TRANSPORT AND AERODYNAMICS ON MARS E07005 
Figure 1. Amosaic of MOC wide-angle images: (bottom) E16-01895,12.3?N, 290.12?W, 259.9 m/pixel, 
(middle) E16-00806, 14.17?N, 289.2rw, 261.13 m/pixel, and (top) E16-00279, 15.28?N, 288.75?W, 
261.42 m/pixel. Amus Vallis extends from the lower left. The location of MOC narrow-angle images (see 
Figure 2) is also shown. Arrows indicate examples where dark (black arrow) and bright (white arrow) streaks 
extend from trough; j: wind streak crosses trough; k: wind streak extending from crater does not appear to 
cross trough. 
3 of 16 
E07005 BOURKE ET AL.: SEDIMENT TRANSPORT AND AERODYNAMICS ON MARS E07005 
markings can form by the deposition or erosion of sediment 
[Edgett, 2002; Thomas et al, 1981]. Both bright and dark 
albedo markings feed into and extend from Amus Vallis 
(Figures 1 and 2). On the downwind side the bright streaks 
are similar to the coalesced individual bright streaks 
reported by Thomas et al. [1981]. The dark areas extending 
from the trough are assumed to be coalesced dark wind 
streaks. Dark streaks emanating from troughs have not 
previously been reported. We suggest that the classification 
of Thomas et al [1981] be expanded to include these 
dark albedo features (Type II (d)). Most of the bright and 
dark albedo markings are discontinuous across the trough 
(Figure 1) suggesting a change in streak composition. The 
details of the relatively high-albedo markings are seen on 
the MOC image (e.g., Figure 3) and are identified as drift 
deposits. Some low-albedo areas are also depositional (see 
Figure 2 a). 
3.2.2.   Drift Deposits 
[n] Drift deposits are accumulations of wind blown 
particles not organized into bed forms [Greeley et al, 
2002]. The relatively high-albedo area on the downwind 
side of Amus Vallis (Figure 3) appears to be a thin, 
discontinuous, sediment sheet and the dark albedo area 
upwind appears devoid of (relatively high-albedo) sediment 
at this resolution. The intensity of brightness on the down- 
wind trough margin varies (Figure 3), and areas with higher 
albedo are inferred to be concentrations of aeolian sediment. 
We infer two types: sand patches and sand streamers 
(Figure 3). Sand patches are irregularly shaped areas, the 
largest measuring 1.7 km^. Some appear to be trapped 
in topographic lows such as degraded impact craters 
(Figure 3). Sand streamers are narrow, slightly sinuous 
and sometimes discontinuous. They have an average width 
of 12 m and are of variable length (100-400 m). The 
two types of deposit appear to be linked and streamers 
frequently feed into and extend from patches. Lancaster 
[1996] suggests that sand patches are initiated in a zone of 
spatially and temporally fluctuating winds and disperse as 
surface roughness increases and sand supply is reduced. 
[12] We do not know the thickness of sand streamers on 
Mars but suggest that they are thin. On Earth, a 7 km long, 
0.5 km wide streamer has a maximum depth of 2.5 m 
[Schaber and Breed, 1999], and smaller ones are known to 
Figure 2. Subset of MOC narrow-angle images showing 
sections of Amus Vallis (for location, see Figure 1). From 
north to south: (a) E16-00278, 15.3TN, 289.15?W, 
4.67 m/pixel, (b) E22-01045, 14.69?N, 289.4rW, 6.2 m/ 
pixel, (c) El 1-00553, 14.37?N, 289.44?W, 6.21 m/pixel, 
(d) E16-00805, 14.9?N, 289.61?W, 4.66 m/pixel, (e) M03- 
07414, 13.97?N, 289.97?W, 2.94 m/pixel, (f) E23-01150, 
13.5?N, 290.012?W, 4.65 m/pixel, (g) same as Figure 2f, 
(h) E18-01455, 12.98?N, 290.08?W, 6.2 m/pixel, and 
(i) E16-01845, 12.32?N, 290.51?W, 6.19 m/pixel. The 
lateral dune is visible on the east side of Amus Vallis in all 
images. The dune is estimated to extend continuously 
>165 km. Small-scale (500-800 m wide) low-albedo streaks 
extend westward from the trough. Many are associated with 
cross-trough extensions of the lateral dune (dark arrows). 
This illustrates a cross-trough sediment transport pathway 
from the lateral dune to the dark depositional streaks. 
4 of 16 
E07005 BOURKE ET AL.: SEDIMENT TRANSPORT AND AERODYNAMICS ON MARS E07005 
Figure 3. Geomoiphic map of aeolian features and MOC image of Amus Vallis (for location, see 
Figures 1 and 2e). (a) A large lateral dune extends along the eastern trough wall, (b) Transverse aeolian 
ridges are aligned along the trough floor, and (c) hypothesized sand ramps climb up the western trough 
wall, (d) Drift deposits: (i) sand streamers and (ii) sand patches. White arrow indicates western margin of 
an inner channel, the eastern boundary of which is buried by the lateral dune. Note the contrasting albedo 
on the east and west of the trough. MOC image (M03-07414), 13.97?N, 289.97?W, 2.95 m/pixel. 
be significantly thinner. On Earth, sinuous sand streamers 
have been described as areas where through?ow dominates 
over accumulation and are consequently areas of the highest 
rates of transport [Thomas, 1997]. The albedo of these 
streamers and patches is similar to the distal portion of 
some bright wind streaks and similar sediment texture and 
processes may be found in those locations. 
[n] Greeley et al. [2002] documented small-scale aeolian 
deposits at the Mars Pathfinder landing site (MPL). The 
features include small-scale (<5 m) dunes (whalebacks, 
transverse: barchan, barchanoid), normal ripples, megarip- 
ples and surface lag deposits. We do not have the resolution 
to determine the presence of these features at Amus Vallis, 
but it is possible that the patches and streamers we identify 
5 of 16 
E07005 BOURKE ET AL.: SEDIMENT TRANSPORT AND AERODYNAMICS ON MARS E07005 
15.0 
Distance (km) 
0     10    20    30    40    50 
70.0  70.2  70 4  70 6  70.8  71,0 
Longitude 
1400 
c 1300 
.O 1200 
13 1100 
? 1000 
LU 900 
800 
700 
Orbit 19251 
60 40 
Distance (km) 
1200-) 
5 1100- 
LU 
1000 
-50 m         i 
m Orbit 19251 
B 
' 1 1  1 1 r- 
f 
70 65 60 
Distance (l<m) 
55 50 
75 70 65 
Distance (lim) 
Figure 4 
O 
-1200 m 
Orbit 12134 
El
ev
ai 
o
 
 
 
 
 
 
 
 
 
 
 
 
o
 
o
 
 
 
 
 
 
 
 
 
 
 
o
 
1  
 
 
 
 
 
1 
 
 
 
 
 
 
1 
D 
' 1 
so 
6 of 16 
E07005 BOURKE ET AL.: SEDIMENT TRANSPORT AND AERODYNAMICS ON MARS E07005 
at 2.9 m/pixel resolution may alternatively be collections of 
lag surfaces and small-scale dunes and ripples similar to the 
MPL site. 
3.3.   Aeolian Deposits in Trouglis 
[14] The trough floor of Amus Vallis is almost completely 
overlain by aeolian deposits. Four types of aeolian forms are 
identified. Falling dunes occur on the east of the trough at 
the junction with the Planitia surface; one third of the floor 
of the trough is occupied by a large "lateral" dune; 
transverse aeolian ridges occur in the base of the trough 
and sand ramps and dunes climb the downwind trough wall. 
Although not considered in detail in this paper, additional 
dune types noted in other troughs include linear, barchan, 
dome, network and star. 
3.3.1. Falling Dunes 
[15] Falling dunes are identified in Figure 3 as small-scale 
high-albedo areas between the trough edge and the lateral 
dune. Clearer examples are seen in Figure 11. They are 
between 50 and 75 m long, coalesce laterally and are the 
smallest of the aeolian dune features in the Amus image. 
The relatively dark albedo of the surface adjacent to the 
falling dunes suggests that there is a high throughflow of 
sediment across the Planitia and all sediment is being 
deposited in the trough. However, some upwind locations 
along Amus Vallis do have a high albedo (see Figures 2a 
and 2d), which indicates that deposition may occur on the 
Planitia surface upwind of the trough. We suggest that 
falling dunes concentrate significant sediment volumes into 
the trough and are important components in the sediment 
transport pathway. Falling dunes are not only associated 
with troughs as they have been reported in Proctor Crater 
[Fenton et al., 2003] and observed on terrain with topo- 
graphic obstacles (e.g., see MOC image FHA01366 from 
G. chaos). They are found on Earth on the downwind side 
of obstacles such as mountains {Liu et al., 1999] and within 
valleys [Li et al, 1999; Seppala, 1993]. 
3.3.2. Lateral Dune 
[16] Within the trough, is a distinctive, topographically 
anchored dune that we term a lateral dune. The dune lies 
adjacent to the eastern trough wall and its alignment closely 
mirrors that of the trough. Planimetrically, it is very similar 
to valley-marginal dunes in southem Africa [Bullard and 
Nash, 2000] but in the Amus example, the main body of 
sediment is contained within the trough rather than on the 
surrounding surface, as is found in the African examples. 
The falling dunes described above provide a sediment 
pathway linking the lateral dune to a sediment source on 
the Planitia surface to the east. The top of the dune averages 
80 m wide and has a sinuous crest. The steep upper slope of 
the dune, grades into a lower-angle foot slope. The maxi- 
Figure 5. Trough and dunes in Nili Patera Caldera. MOC 
image M17-00435, 8.99?N, 293.29?W, 5.84 m/pixel. 
(a) Lateral dune, (b) Broad climbing dune, (c) Linear 
extension from a transverse dune. Linear dune (d) supplies 
sediment to barchanoid ridge (d') on the downwind surface. 
mum width of the dune is ~285 m. The sharp crest and 
steep avalanche slopes indicate that this dune is being 
actively maintained. The position of the sinuous crest and 
steep avalanche face suggest that local vortices operating 
within the trough have sculpted the western face of the dune 
and suggest a secondary sedimentary transport path within 
the trough toward the north. 
[17] Two other examples of lateral dunes have been found, 
both located in Nili Patera Caldera (Figures 5 and 6). In the 
first example the dune is located at the upwind wall of two 
parallel troughs. Here the lateral dune is larger than that of 
Amus Vallis and averages 195 m wide (Wmax " 250 m; 
Wmin = 170 m) despite a similar trough width (805 m). In the 
Figure 4. MOLA data for Amus Vallis site. The shot spacing is approximately 300 m, and each shot averages topography 
over a ~100 m ellipse, (a) MOLA topographic data calculated at 1/128 degrees in longitude and 1/256 in latitude 
resolution. Locations of MOLA transects 19251 and 12134 are shown, (b) Poleward plot of transect 19251 over 180 km 
length showing the general gentle dip to the north 0.0031 m/m. The transect generally parallels Amus Vallis. (c) Detail of 
portion of transect 19251 at a zone where it intersects with the trough bottom. Data confirm a northward trending trough 
gradient and indicate that that portion of the trough has a gradient of approximately 0.0014 m/m, which is lower than the 
regional gradient, (d) Detail of portion of transect 12134, which traverses Amus Vallis. The cross-trough profile illustrates 
that the windward side is approximately 25 m higher than the downwind trough side. Along the 1.25 km approach to the 
trough the gradient is 0.004 m/m. On the downwind side there is a convexity which may accelerate wind speeds and 
enhance particle transport. 
7 of 16 
E07005 BOURKE ET AL.: SEDIMENT TRANSPORT AND AERODYNAMICS ON MARS E07005 
Figure 6. A3 km section of a trough in Nili Patera caldera 
in Syrtis Major. A variety of dunes are present: (a) lateral, 
(b) linear, (c) barchan, and (d) transverse aeolian ridges 
(ripples/dunes). The 375 m wide trough has been heavily 
modified by aeolian sediment. Sand and barchan dunes fill 
shallow trough (arrow 1) and barchan limb crosses onto and 
merges with lateral dune in trough (arrow 2). MOC image 
(MlO-01512), 9.27?N, 293.27?W, 2.92 m/pixel. 
second example (Figure 6) the lateral dune averages 145 m 
wide (Wmax = 185 m; Wmin = 95 m), and stands approxi- 
mately 15m high in a trough 730 m wide and 100 m deep. In 
both of these examples the dune location and morphology is 
similar to that in Amus Vallis. 
3.3.3.   Transverse Aeolian Ridges 
[is]   The floor of Amus Vallis has a series of bed forms 
oriented perpendicular to the trough. These ridges are small 
features with wavelengths averaging 18 m (max. = 25 m, 
min. = 12 m). Ridge crest length averages approximately 
140 m. These values fall within the distribution measured 
for similar features in narrow troughs (i.e., <600 m wide) on 
Mars [Bourke et al, 2003]. Transverse forms (dunes and 
ridges) are the most common aeolian bed form on Mars 
[Malin andEdgett, 2001]. Their occurrence has been related 
to the relatively low volume of saltation load available and 
the unimodal transport regime [Zee and Thomas, 1995]. 
Transverse ridges are particularly abundant on valley floors 
[Carr and Malin, 2000], are aligned perpendicular to the 
valley direction, and are assumed to form transverse to the 
main wind direction [Malin and Edgett, 2001]. Zimbelman 
and Wilson [2002] measured the wavelength of similar but 
larger-scale transverse ridges in troughs ("ripple bands") on 
Syrtis Major (mean = 39 ? 16 m; crest length = 370 ? 236). 
On Earth the wavelength of dunes may be controlled by 
sediment supply and by grain size [Wasson and Hyde, 
1983]. The smaller wavelength of the Amus ridges com- 
pared with those in Syrtis Major may be related to a more 
abundant supply of sediment and/or finer grained sediment 
available on the trough floor [Wilson, 1972, 1973]. The crest 
length is controlled, for the most part by local trough floor 
topography, which in Amus Vallis is primarily determined 
by the inner channel width. 
3.3.4. Sand Ramps 
[i9] Low albedo deposits have accumulated along sec- 
tions of the west trough wall of Amus Vallis (Figure 3). 
They reach highest elevations in the north of the concave 
trough sections. The accelerated build-up in these wind- 
protected locations suggests a wind direction from the 
south. Their absence at convex reaches indicates a high 
shear zone along these trough sections with eddies/low 
velocity zones forming in the concave sections. The features 
have scalloped upper margins, and, in places, overlie the 
transverse ridges on the trough floor. MOLA profile data 
intersecting one such feature indicates it climbs to at least 
25 m up the trough wall. The low-albedo sediment contrasts 
with the high albedo of the aeolian forms described previ- 
ously. We infer a predominantly local source of sediment for 
these ramps in Amus Vallis but do not exclude additional 
extemal sources. We interpret these deposits to be a series of 
sand ramps, where fine-grained slope debris from the trough 
wall (and possibly floor) is blown back up the trough wall. 
Sand ramps on Earth are commonly composed of mixed 
materials such as aeolian, fluvial and coUuvial sediment 
[Lancaster and Tchakerian, 1996]. Sand ramps have not 
been previously reported on Mars. 
3.3.5. Climbing Dunes 
[20] In the Nili Patera troughs (Figures 5 and 6) dunes 
climb up the downwind trough wall. In Figure 5b the dune 
extends 1.35 km along the trough wall, appears steep, but 
does not climb over the lip of the trough at this resolution. 
Other examples of climbing dunes tend to be extensions of 
dune limbs that cross the trough floor (Figure 5). Climbing 
dunes have not been reported on Mars previously but are 
frequently observed on Earth associated with topographic 
obstacles along sediment transport pathways [Howard, 
1985; Lancaster and Tchakerian, 1996]. They most com- 
monly form on the upwind side of positive relief features 
such as a hill or escarpment when sand-transporting winds 
are obstmcted, causing a loss of momentum and deposition. 
8 of 16 
E07005 BOURKE ET AL.: SEDIMENT TRANSPORT AND AERODYNAMICS ON MARS E07005 
The obstacle has to have a steep upwind slope (SO- 
SO degrees) in order to effect dune forming conditions. If 
the slope is greater than 50 degrees, an echo dune, rather 
than a climbing dune, will form [Cooke et al, 1993]. Where 
the sediment source is local, the size characteristics of 
particles at the base of the dune will be very similar to 
the source. Sediment size decreases with increased elevation 
up the dune as fine particles are winnowed from the source 
material and carried further up the dune slopes [White and 
Tsoar, 1998]. Climbing dunes are not only formed in 
association with positive relief but can also develop within 
valleys on the downwind valley slopes. Here, the valley 
?oor or sediment entering the valley Irom the surrounding 
plateau, supplies sediment to form the dune. The sediment is 
transported across the valley and accumulates on the down- 
wind valley slope gradually building up into a dune form. 
Within-valley climbing dunes on Earth are particularly 
associated with broad valleys but, with the exception of 
Li et al. [1999], few detailed studies of these features have 
been reported. 
3.4.   Simulated Flow Patterns on Mars 
[21] Having described the aeolian features in the Amus 
Vallis region of Syrtis Major, we can use their morphological 
and morphometric characteristics to consider, first, the 
relationship between regional- and local-scale wind flow 
patterns and, second, the interactions between the trough and 
the Planitia surface winds that determine aeolian sediment 
sources and transport pathways. 
[22] As trough topography can have an impact upon 
regional surface winds, the following is a short review of 
recent wind tunnel and field studies of terrestrial conditions 
across a valley. Wiggs et al [2002] have shown that in 
plateau landscapes with slot-channel relief the presence of 
the valley can initiate an acceleration of wind flow up to 
1 valley width upwind due to a decreasing pressure gradient 
drawing air into the valley (Figure 7). Studies have also 
demonstrated a significant reduction in velocity as flow- 
passes over the upwind edge of a valley or backward 
(downwind) facing escarpment [Sierputowski et al, 1995; 
Mggs et al, 2002]. This is dependent upon the angle of 
approach of the regional wind compared with the valley 
orientation. Immediately downwind of the leading edge of 
the valley, any sediment being transported will be deposited. 
This is the mechanism which leads to the development of 
falling dunes and facilitates aeolian sediment accretion 
adjacent to the valley wall. In Figure 7b the point of flow 
reattachment is 0.7 of the valley width. Within the valley 
walls, wind speed is at a minimum in the center of the valley 
and flow accelerates toward the downwind valley edge 
often exceeding upwind velocities. A second zone of flow 
separation may develop on the plateau surface immediately 
downwind of the downwind valley edge [Bowen and 
Lindley, 1977; Mggs et al, 2002]. On the downwind 
plateau surface flow decelerates to recover toward velocities 
found upwind of the valley [Mggs et al, 2002] (Figure 7a). 
3.4.1.   Computational Fluid Dynamic Modeling 
[23] In order to understand the spatial distribution of 
aeolian sediments at Amus Vallis, we have undertaken 
preliminary 2-D computational fluid dynamic (CFD) mod- 
eling of the flow field generated in an isolated trough in a 
flat, plateau terrain under Martian conditions. As the MOLA 
-400     -200 0        200      400 
Distance (x) 
600     800 
Figure 7. Experimental observations from wind tunnel 
[Garvey et al, 2002] and field data [Wiggs et al, 2002]. 
(a) Field data were collected on the Gaub River Namibia 
[Wiggs et al, 2002]. The widthidepth ratio for the Gaub 
River (7.9) is similar to Amus Vallis (8.6). The graph shows 
the fractional speedup ratio [Wiggs et al, 1996] measured at 
two heights (0.4 m and 2.8 m) above ground for a wind 
blowing normal to the valley axis. The valley position is 
shown at the bottom of the graph, (b) Cross-valley 
streamlines generated from wind tunnel data after Garvey 
et al [2002]. Flow is from the left. Flow reattachment is at 
x// = 0.7, xlh = 5.6. 
spacing (300 m) and footprint (150 m) is too large to 
accurately represent Arnus Vallis in cross section, we 
consider a geometrically similar, but idealized situation 
where a wind blows steadily over the top of a canyon 
consisting of two 90 m vertical walls separated by an 800 m 
flat floor, and an 880 m plateau further downsfream. This 
will permit us to gain a general overview of the flow 
stmctures generated by a valley under Mars atmospheric 
conditions. In addition, a valley with vertical walls repre- 
sents a useful extreme where all possible wind eddies that 
may be generated in a trough can be seen. Changes in valley 
slopes are expected to alter eddy size and may in some 
instances even reduce their number 
[24] This preliminary model does not compute flow along 
a plateau upwind. Instead, we simply assume that wind 
entering the computational domain possesses a flat rather 
than a logarithmic velocity profile. A logarithmic profile is 
typically expected for a well-developed wind traveling 
along an upwind plateau. For Mars, the wind speed 
increases slowly ~100 m above the surface [Greeley and 
Iversen, 1985]. However, field and wind tunnel experiments 
[Garvey et al, 2002; Wiggs et al, 2002] indicate that the 
near surface winds accelerate as they pass over the upwind 
edge of a valley. In order to simulate this acceleration, we 
flatten the wind profile to better compute reasonable flow 
interactions within the trough. In addition, a test was run 
9 of 16 
E07005 BOURKE ET AL.: SEDIMENT TRANSPORT AND AERODYNAMICS ON MARS E07005 
Table 1.  Martian Atmospheric Properties and Parameters Used in 
CFD Model 
Reynolds Number 
Martian Wind Properties Value (Re = uhp/p,) 
Atmospheric density, p 0.02 kgW 
Atmospheric viscosity, p 2.238 X 10"' Pa-s 
Upstream wall height, h 90 m 
Wind speed, u 1 m/s 0.8 X 10' 
10 m/s 8.0 X 10' 
20 m/s 16 X lO' 
Transition to turbulence 1.0 X lO'-l.O xlO* 
with a logarithmic wind profile. The change in the upwind 
re-circulation zone was found to be small, increasing it size 
by ~8%. 
[25] The CFD used for this study was developed by 
Ferziger and Pervic [1996] and uses an implicit finite 
volume scheme with a pressure correction method 
(SIMPLE scheme). Table 1 lists typical Mars wind con- 
ditions, as well as atmospheric and flow parameters that are 
relevant to Amus Vallis. Most notable is the Reynolds 
number {R?) that provides a measure of the inertial to 
viscous forces in a flow. When this number exceeds 10^, 
inertial forces within a flow become important and generate 
random small-scale turbulence [White, 1986]. The scale of 
Amus Vallis and the atmospheric conditions on Mars ensure 
that Re for flows within this trough are just within this 
turbulent regime (Table 1). In addition, surface roughness 
on the upstream plateau and within the Vallis probably 
ensures turbulent flow conditions. We thus use a version of 
the SIMPLE scheme that includes a K-e turbulent flow 
model, where K parameterizes the turbulent energy of a 
flow and e estimates the energy dissipation by turbulence in 
shear zones near surfaces and at the edge of obstacles. We 
use values of K, = 0.015 and e = 0.0037 that are computed 
irom the streamwise turbulence measured in the wind tunnel 
experiments of Garvey et al. [2002] using typical equations 
for estimating these parameters [Barnouin-Jha et al, 1999]. 
[26] The boundary conditions used in our calculations are 
shown in Figure 8a. We assume that no slip occurs at the 
trough walls, floor or on the downstream plateau; that the 
upstream flow possesses a vertical flow profile; and that 
the top boundary permits slip parallel to its surface but no 
flow convection through the surface. This last condition in 
effect turns the top boundary into a symmetry plane, which 
we place far irom the area of interest to ensure that no 
unrealistic accelerations occur in the flow due to area 
changes. Barnouin-Jha et al. [1999] have shown that 
placement of this boundary 800 m above the trough floor 
avoids such accelerations. Because we are considering 
steady state flows, the outflow conditions assume zero 
velocity gradients at the boundary and the same total mass 
flux as at the inlet. 
[27] We performed calculations on three separate grids to 
determine which one would provide us with the most 
reasonable flow solution using the least amount of compu- 
tational resources and time. The coarsest grid used had 
approximately 80 by 160 cells. The mesh was refined in the 
regions of interest, namely within the trough and near the 
downstream wall, but were kept coarse elsewhere. 
The intermediate and high-resolution grids were 4 and 
16 times finer than this coarsest grid. The numerical method 
used in this study generates changes in the flow solution 
following each iteration. The computation was therefore run 
on all three grids employed until convergence where addi- 
tional iterations caused no fiirther change in the computed 
flow. The solutions obtained on the coarsest grid were 
significantly different from that seen on the other two finer 
grids. Very small changes, on the order of a few percent, 
were seen between the intermediate and highest resolution 
grids, although significant savings in computational time 
were obtained with the intermediate resolution grid. We 
therefore used the intermediate resolution grid to generate 
all the results presented in this study. 
3.4.2.   Model Results 
[28] We present results for three wind speeds: Im/s, lOm/s 
and 20m/s. The first two encompass the wind speeds 
measured by several of the Mars Landers on a typical 
Martian day \Greeley and Iversen, 1985; Sullivan et al, 
2000]. The last corresponds to typical wind speeds measured 
during Martian dust storms. Although the structures of 
turbulent flows are typically independent of flow speed, 
we present three flow speeds because Re for these flows are 
just greater than 10 , the transition point between laminar 
and turbulent flow, and some Re dependence could be 
expected. 
[29] Similar flow streamlines are produced for all three 
wind speeds (Figure 8). We see a zone of recirculation 
behind the upstream trough wall, recirculation associated 
with comer flow at the base of the downstream wall, 
acceleration of flow and flow separation at the top of the 
downstream trough wall, and formation of a subsequent 
recirculation zone on the downwind surface. All the flow 
patterns are consistent with the wind tunnel experiments 
presented by Garvey et al. [2002] except for the down- 
stream comer flow re-circulation zone. This is because the 
wind tunnel experiments use shallow 20-25? inclined walls 
while our calculations possess steep 90? walls. 
[30] The size of the recirculation zone generated near the 
upstream wall does not significantly vary with increasing 
flow speed (Re) and confirms that these flows are ?illy 
turbulent. Usually recirculation zones only vary for flow in 
the laminar regime, and under such conditions, depend 
primarily on Re. The location of the reattachment point is 
similar to that seen in the wind tunnel experiments [Garvey 
et al., 2002] {xll = 0.7), where / is valley width and x is 
horizontal distance (x = 0 at the leading edge of the valley). 
From our experiment we suggest that the extent of the 
recirculation region and the location of reattachment should 
be defined in terms of the height of the upstream wall as 
increasing the height of the valley would have more effect 
on the location of reattachment, than increasing valley 
width. If we redefine this reattachment point in terms of 
wall height, we then find that the wind tunnel experiments 
[Garvey et al, 2002] occur at 5.6h where h is the upstream 
wall height. In our numerical results, this location is 
approximately at 5.9?, which is essentially identical to that 
observed in the wind tunnel given that reattachments points 
determined from K-e models typically fall within 20% of 
those observed in wind tunnel experiments [Nallasamy, 
1985]. 
[31] This modeled modification of the synoptic wind 
pattern by the trough has a number of implications for 
sediment transport adjacent to troughs on Mars. First, 
10 of 16 
E07005 BOURKE ET AL.: SEDIMENT TRANSPORT AND AERODYNAMICS ON MARS E07005 
a) 
? 
symmetry plane 
(:)-?? [%h 
-* 
outlet 
-^ - 
wall and floor 
m%] -0 800 m 
y ^-MM^y) = 0 90 m u 890 m 
200 m ^ 
X 1 
bii) 
.30K+?) m/s 
cii) 
.32E^I mli 
dii) 
= 2.64E+0! in/s 
Figure 8. Results of CFD model: (a) the computational domain and the boundary conditions: u, 
horizontal velocity; v, vertical velocity. Streamline on left (bi, ci, di) and flow vector on right (bii, cii, dii); 
b is at 1 m/sec, c is at 10 m/sec, and d is at 20 m/sec. The red arrows are equal to the incoming flow 
speed. Reduced wind speeds are indicated in color from red, through yellow and green, and to blue. The 
arrow length indicates relative flow velocities. Modeled flow is from left to right. Flow reattachment is at 
xll = 0.7, xlh = 5.9. 
although we did not include the upwind plateau, the model 
of Wiggs et al. [2002] suggests that there will be a region 
immediately upwind of the trough where little or no aeolian 
deposition takes place due to accelerated flow. Figure 3 
shows a dark zone on the upwind Planitia surface suggest- 
ing no sediment deposition has taken place, and/or erosion 
of the surface dust veneer by accelerated flow. Second, the 
CFD illustrates a zone of flow separation and consequent 
reduction in velocity at the upwind trough wall. We expect 
that, where sediment is available, significant amounts will 
11 of 16 
E07005 BOURKE ET AL.: SEDIMENT TRANSPORT AND AERODYNAMICS ON MARS E07005 
be deposited at the foot of the slope. Within Amus Valhs, 
falHng dunes and the lateral dune are located immediately 
downwind of the trough edge suggesting rapid flow decel- 
eration. Third, there may be erosion or a zone of low/no 
deposition at the point of reattachment (~5.9?). This zone is 
not clearly identifiable in the troughs examined and sug- 
gests wind flow along the valley axis is important. Fourth, 
there may be deposition in the recirculation zone at the foot 
of the downwind wall. Here we have found a series of 
climbing dunes and sand ramps. Sediment may be eroded 
fi-om the sand ramps as flow accelerates at the trough wall. 
This may explain the process and identify the source of 
sediment to the sand streamers and patches downwind of 
the trough. This sediment movement will be more pro- 
nounced on walls less than 90?. Fifth, as the velocity 
increases at the top of the wall we suggest that sediment 
erosion/entrainment may be enhanced and would expect no 
or low deposition in this reach. Finally, beyond the trough 
we expect a zone of deposition. These findings support the 
conceptual model proposed by Wiggs et al. [2002]. 
[32] Although the model compares well with the distri- 
bution of sediment at many sites along Amus Vallis, there 
are sites where, e.g., deposition is found on both the upwind 
and downwind trough margin. This variability is expected 
as there is dependence on local topography and sediment 
supply. 
[33] The model indicates that maximum wind velocities 
are generated at the trough outlet. These velocities are an 
increase of 30%, 32% and 32% for 1, 10 and 20 m/sec 
upwind values. The topographically accelerated velocities 
are 1.3, 13.2 and 26.4 m/sec. Dunes on Mars have not 
actively migrated in the last ~30 years [Edgett and Malin, 
2000; Zimbelman, 2000], but sand is active on dune crests 
[Malin and Edgett, 2001]. Zimbelman [2000] suggested 
that dune inactivity may be a factor of insufficient wind 
velocities. We have shown that topography can accelerate 
wind velocities under Martian conditions. It may be that 
current dune activity is limited to locations of accelerated 
flow and features such as sand streamers and climbing 
dunes associated with inclined topography should be 
targeted in future dune activity investigations. 
3.5.   Wind Direction 
[34] Valley topography on Earth has been found not only 
to have an impact upon wind velocity, but also on wind 
direction [Bullard et al, 2000; Whiteman and Doran, 
1993]. Bullard et al. [2000] found, in experimental studies, 
that at any wind approach angle less than 90?, upstream 
regional winds are deflected to become more parallel to the 
valley orientation and this impact can extend up to 400 m 
upstream of the valley. Wiggs et al. [2002] found that wind 
deflection is more pronounced when the approach angle is 
<45?. When winds approach perpendicular to a valley there 
is no discernible change in direction, but wind velocities are 
affected. Amus Vallis is oriented both broadly perpendicular 
to the regional wind regime and, in places, deviates away 
fi"om it. We would expect some deflection of winds in these 
latter areas. Although bright and dark wind streaks are 
discontinuous across the trough, by examining streak 
orientation on either side we see that their orientation 
(270?) is not offset by the presence of the trough. Elsewhere 
on Mars, Thomas et al. [1981] reported that bright wind 
streaks that cross-trough complexes can be slightly offset 
but also show little or no deflection when they cross 
topography of 50 m or more. 
3.6. Witliin-Trougli Wind Regimes 
[35] At the local scale, valleys on Earth can generate 
complex within-valley wind regimes as a result of processes 
such as thermal forcing, downward momentum transport 
and pressure-driven channeling. This can have an impact on 
aeolian bed form development in the trough with trough 
floor dunes usually forming perpendicular to the main 
trough strike as a result of unidirectional along trough 
winds. Most trough floor dunes and ripple-like features on 
Mars have a similar relative orientation suggesting that 
along-trough wind is the main determinant of the transverse 
ridge form. In Amus Vallis, the transverse aeolian ridges 
have probably formed as a result of along-trough winds. 
The planimetric characteristics of the lateral dune, some of 
the larger transverse ridges and the sand ramps suggest 
that north-flowing winds have recently had a greater influ- 
ence on landform development than south-flowing winds 
(Figure 3). 
3.7. Aeolian Sediment Source 
[36] Four potential sediment sources for aeolian deposits 
in troughs have been identified; wind streaks, ambient 
Planitia sediment, dunes and the trough (walls and floor). 
In addition, intradune nourishing is proposed as an impor- 
tant component in the sediment transport pathway. The 
following section is represented in Figure 9. 
3.7.1. Supply to the Trough 
3.7.1.1. Wind Streaks 
[37] Type I b wind streaks feed sediment into the upwind 
side of Amus Vallis but are discontinued on the downwind 
trough margin. This indicates that Amus Vallis interrupts 
the sediment transport continuum across Syrtis Major. One 
of the controls on streak discontinuity may be proximity to 
the streak source. In a location to the north along Amus 
Vallis, a bright streak extends continuously across the 
trough (Figure 10). The streak is initiated at a crater 
3.5 km from the trough. The continuity of the streak and 
the absence of shadow in the trough suggest that sediment 
from the wind streak has infilled the trough. It is also 
possible that the deposit is dust fallout. As there are no 
MOC data for this location, Figure 11, from Candor 
Chasma, indicates a potential mechanism for sediment 
transport across a trough. Low-albedo sand enters the 
trough as falling dunes (Figure 11a). Linear forms cross 
the trough and climb the downwind wall facilitating a 
sediment transport continuum across the trough. The Amus 
Vallis example illustrates the potential importance of wind 
streaks as a source of within-trough sediment. Others have 
found that in addition to dust, streaks may contain fine sand 
[e.g., Edgett, 2002; Pelkey et al, 2001; Thomas et al, 1981; 
Zimbelman, 1986]. Pelkey et al [2001] were unable to 
determine the grain size of wind streaks south of Amus 
Vallis within the limitations of the available data. This study 
suggests that some wind streaks in Syrtis Major may contain 
a fine sand fraction which, upon reaching the trough is 
stored in a range of aeolian bed forms. Troughs are not 
necessarily endpoints of the transport pathway as shown by 
dark streaks that extend downwind from a trough in Candor 
12 of 16 
E07005 BOURKE ET AL.: SEDIMENT TRANSPORT AND AERODYNAMICS ON MARS E07005 
Figure 9.   Conceptual model of sediment transport pathways associated with trough and valley features 
on Mars. 
Chasma (Figure 11). The barchan dunes indicate that these 
dark albedo streaks are composed of sand. This suggests 
that, despite acting as a sediment store, troughs do not trap 
the sand fraction but permit throughflow through the 
construction of sand ramps, climbing dunes and at topo- 
graphic low points (see also Figures 5 and 6). 
3.7.1.2. Ambient Planitia Sediment 
[38] Some relatively high-albedo areas upwind of Amus 
(Figures 2a and 2d) are not obviously associated with wind 
streaks and may be examples of ambient sediment supply to 
the trough. The sediment appears to be arranged into sand 
streamers and patches similar to those described above. Any 
link to wind streaks is as yet unproven, but they may be a 
mechanism of sediment transport across the surface of Mars 
that is particularly active in locations where velocities are 
topographically accelerated. 
3.7.1.3. Dunes 
[39] In Nili Patera Caldera, barchan dunes are observed 
upwind and downwind of the troughs. In Figure 6 the 
elongated arm of a barchan dune is seen to extend over 
the trough wall and onto the surface of the lateral dune. This 
is clear evidence that a principal source of sediment for the 
lateral dune is from outside the confines of the valley, 
specifically, dunes on the upwind surface. 
3.7.2.   Within-Trougli Sources 
[40] The trough walls contribute sediment to the trough 
by slope failure (Figure 3), and probably aeolian abrasion 
and granular disintegration.  Slope sediment has been 
5t m 
mm 
?^ 
ir'' 
IP' --/A ^?^^ ?, '..<.., \ V __al
Figure 10. Viking image (S52188, 341 m/pixel) showing principal surface features in the region of 
Amus Vallis (image center). Location of enlarged subset is shown by box. Subset indicates the location of 
the hypothesized trough filling (arrow) by sediment from a wind streak. 
13 of 16 
E07005 BOURKE ET AL.: SEDIMENT TRANSPORT AND AERODYNAMICS ON MARS E07005 
Figure 11. Trough in central Candor Chasma, MOC 
image E03-00746, 7.0rs, 72.69?W, 4.30 m/pixel. Image 
shows significant infilling of trough with sand: (a) low- 
albedo falling dunes, (b) linear dune, and (c) low-albedo 
linear and barchan dunes on downwind side of trough. 
remobilized into sand ramps and climbing dunes. It is likely 
that the trough floor is a source of sediment for aeolian 
dunes, particularly the transverse aeolian ridges. However, 
as yet, this can only be inferred as no data currently exist to 
confirm the hypothesis. Others have also interpreted a local 
sediment source for bright transverse forms \Fenton et al, 
2003; Malin et al, 1998]. 
[4i] Within troughs, dunes may supply sediment to other 
dune forms. The MOC image from Candor Chasma 
(Figure 11) illustrates the movement of sediment across a 
700 m wide trough. In the upper part of the figure a series of 
falling dunes (one 314 m long), supplies sediment to linear 
dunes that extend across the trough floor. These then form 
climbing dunes and feed dark streaks on the downwind side 
of the trough. A similar relationship is seen in the Nili 
Patera (Figures 5 and 6). 
3.7.3.   Trough as a Source for Planitia Sediment 
[42] The absence of dark albedo sediment on the western 
Planitia surface in Figure 3 suggests that the low-albedo 
sediment in sand ramps and climbing dunes may not extend 
over the trough wall in all reaches of Amus Vallis (although 
see Figures 2a and 2d). MOC images of troughs in Candor 
Chasma and Nili Patera suggest that the supply of sediment 
from the trough to the adjacent surface downwind is not 
dependent on a continuous climbing dune form over the lip 
of the trough. In Figure 5 a linear extension fi-om a dune on 
the trough floor extends up the trough wall. Although the 
dune does not overtop the trough edge, there is an alignment 
of sediment trailing directly from the climbing dune toward 
a barchan form on the downwind surface (d-d'). A second 
example is seen in Figure 5c where the linear limb of a 
barchan form on the trough floor extends across a breach in 
the trough wall to feed into a small barchan. 
3.7.4.   Summary of Sediment Sources and Patliways 
[43] Several sediment sources for aeolian deposits in 
troughs have been identified. A trough can act as both a 
throughflow area for aeolian sediment as well as a store, 
sometimes being significantly modified by the aeolian 
deposits. Wind streaks and ambient Planitia sediments, 
and in certain locations dunes, are sediment sources and 
means by which sediment is transported across a surface on 
Mars. Troughs (and valleys) will act as topographic barriers 
to sediment flux, temporally storing sediment in a range of 
bed forms. Under appropriate conditions and nourished by 
sources within the trough, sediment is supplied to the 
downwind Planitia surface. These data show that different 
styles of aeolian deposit can be both physically connected 
and linked as sinks and sources along sediment transport 
pathways. Geomorphic signatures of fluvial processes along 
these pathways can be significantly modified. 
4. Estimating Sediment Storage 
[44] Syrtis Major has been identified as a significant depo- 
center for global-scale aeolian sediment fluxes {Anderson 
et al, 1999]. In addition, it is an area that has been subjected 
to major aeolian deflation {Hiesinger and Head, 2004] and so 
is presumably itself a significant source of sediment. There- 
fore understanding sediment fluxes across the region is 
important. We have identified that sediment moving across 
the Planitia is trapped in Amus Vallis. The large linear dune 
is present in all available MOC images for the trough 
(Figures 2a-2i). Using the MOC and MOLA data sets and 
assuming that the lateral dune extends for the full length of 
the trough, we have calculated that the lateral dune in Amus 
Vallis stores approximately 1.4 billion m^ of aeolian sand at 
present. 
5. Conclusion 
[45] Valleys and troughs on Mars may be important 
storage areas for aeolian deposits. This paper has described 
a wider range of aeolian deposits associated with valleys 
and troughs than has previously been documented. These 
include: wind streaks, falling dunes, lateral dunes, barchan 
dunes, linear dunes, transverse ridges, sand ramps, climbing 
dunes sand streamers and sand patches. 
[46] The source of sediment for aeolian deposits in 
troughs and the potential importance of a regional-scale 
input have not been previously considered. The aeolian 
sediment transport system across Syrtis Major (as indicated 
by the example of Amus Vallis) illustrates the complex and 
dynamic nature of linked sediment storages that can exist on 
14 of 16 
E07005 BOURKE ET AL.: SEDIMENT TRANSPORT AND AERODYNAMICS ON MARS E07005 
Mars. Assemblages of deposits are often physically linked 
and form an active sediment transport continuum. 
[47] The range of aeolian deposits recorded at Amus 
Vallis (and in the Nili Patera Caldera and Candor Chasma) 
may be inherently linked to regions of high sediment flux, 
wind streak development and the presence of troughs and 
valleys. 
[48] Computational fluid dynamic modeling of flow 
across a trough indicates a zone of flow separation at the 
foot of the upwind trough wall, reattachment at 5.9h and 
accelerated flow at the downwind valley wall. These model 
results match the pattern and distribution of aeolian forms 
associated with Amus Vallis. 
[49] Acknowledgments. This research was funded by NASA MDAP 
grant NAG5-11090. The authors acknowledge the use of Mars Orbiter 
Camera images processed by Malin Space Science Systems that are 
available at http://www.msss.com/moc_gallery/. We thank Peter Thomas 
and Giles Wiggs for helpful reviews. M.B. wishes to thank Rebecca 
Williams (Malin Space Science Systems), who targeted Amus Vallis for 
more complete MOC coverage. 
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