Procaedngs of the 20tb Lunar and Phnetmy Science Conference, pp. 525-530 
Lunar and Planemy Institute, Houston, 1990 
Outliers of Dust Along the Southern Margin 
of the Tharsis Region, Mars 
Center for Bartb and PJanerary Studies, National Air and Space Museum, 
Sndtrbsonian I- Washington, DC 20560 
This work documents the distinctive characteristics of two locations near the southeastern margin of 
the Tharsis low thermal inertia region on Mars that support the inferred south-to-north migration of dust 
deposits under present climatic conditions (Christensen, 1986,1988). Low thermal inertia and high 
albedo indicate the presence of he-grained surface materials on the highly firaauted terrain of northern 
Claritas Fossae (-16O, 1 lo0), with a narrow zone of k e  material connecting this location to the southern 
margin of the Tharsis low thermal inertia region. A similar deposit occurs in Sinai Planum (-16O, SO0) 
on an isolated exposwe of hctured terrain. In comparison to their surroundings, both locations have 
higher visual albedo, lower thermal inertia, and higher topopphic relief coupled with lower reflectivity 
and higher roughness at radar wavelengths. The Sinai Planum location has less contrast with its 
surroundings at a l l  wavelengths than the Claritas Fossae location. The remote-sensing data are interpreted 
to indicate erosional stripping of dust deposits that progressed in a northerly direction, with dust 
preferentially preserved on rough sutfaces due to reduced dciency of aeolian erosion. 
INTRODUCTION region occurred dunng the V i  mission, with downslope 
winds carrying materials away from the Tharsis shield 
wing Of the thermal Of identified volcanoes (Lee et al, 1982). However, changes in the overall 
three continent-scale regions of low thermal inertia, inter- distribution of the large regional dust are likely driven preted to consist of very fine (silt-sired) particles at the surhce by weS in wte and global ckdtion (-, ( ~ i e f f i  et al., 1977; Zimbelman and Kieffet; 1979; Palluconi 1986, 988). 
and Kiejfi, 1981). Recent synthesis of global properties at It seems unlikely that the migration of the dust deposits can 
visual' and radar d* led the be observed directly, due to the long timescales (ranging h m  
that the low thermal inertia regions consist of dust deposits, tens of to of to be mted 
on the order of meters in thickness, that are redistributed 
around the planet as changing insolation conditions alter the 
global wind regime (Clmkttensen, 1986). Variations in the 
pattern of atmospheric clearing following global dust storms 
indicate a net south-to-north transport of dust under present 
climatic conditions (CBrkterzsen, 1988). However, the lack of 
large, identillable dust deposits in the southern hemisphere at 
present makes it diBicult to relate the dust transportation to 
the evolution of the regional dust deposits. In this paper the 
distinctive characteristics of two locations near the southeast- 
ern margin of the Tharsis low thermal inertia region are 
documented and are shown to support the inferred south-to- 
north migration of dust deposits under present climatic 
conditions. 
BACKGROUND 
The llmsis and Elysium volcanic provinces and cratered 
highlands in the Arabia region all have distinctive remote- 
sensing characteristics. Comparison of the physical properties 
in these three regions provide consistent results of low thermal 
inertia, high visual albedo, and considerable roughness at radar 
wavelengths, interpreted to result kom the penetration of the 
radar signal through a comparatively thin dust mantle with 
subsequent scattering by roughness elements on the underly- 
ing volcanic and craterea terrains (Christensen, 1986). 
Imaging observations of wind streaks and variable features 
indicate that si@cant sediment transport within the Tharsis 
- - 
Planum %> 
m m  
-301 . e *  . I 
150 120 Longitude 90 60 
Fig. 1. Location map for the Claritas Fossae (A) and Sinai 
P w u m  (B) features described in the text. The large shield 
volcanoes of Tharsis (dark pattern) and Valles Marineris 
(stippled pattern) are shown for reference, as well as the names 
of sawal prominent features. The lined pattern indicates the 
extent of the 'Iharsis low thermal inertia region within the map 
area The boundary of the 'Ihacsii region comesponds to the 
- 10 K contour (equivalent to a thermal inertia of about 4 X loe3 
cal ~ m - ~  s e ~ - " ~  F1) of the predawn residual temperature maps 
of Kkffer et d. ( 1977) and Zimbelmrm and Kfeffer (1979). 
. , ' 8 5 
526 Proceedings of tbe 20tb Lunar ;Ld planeta'& Science Conference' t * ?  ' .  
with climatic changes on Mars (e.g., Ward, 1974). This 
conclusion is consistent with thermal measurements made 
throughout the Viking mission that failed to detect evidence 
of a change in the position of the margins of the three low 
thermal inertia regions (Zimbelman and Kieffer, 1979), 
implying that the present margins of the dust deposits are 
stable on a scale of tens of kilometers. Even if the migration 
* . " .  Z Longitude 
aiFig. 2. Late afternoon temperature residuals (the difference 
between observed temperature and that predicted for a Mars 
average thermal inertia of 6.5 x cal c m 2  set-'I2 K-I and 
albedo of 0.25) for Claritas Fossae (modified from Fig. 4B of 
K i e f f  et al, 1976). Dashed pattern indicates the extent of 
ancient, fractured terrain (from Scott and T a w  1986). 
Temperature residuals of -28 K, -8 K, and 0 K are equivalent 
to thermal inertias of 2.5, 4.5, and 6.5 X cal ~ m - ~  s e ~ ' ~ ' ~  
K-I, respectively. The low temperature residuals (low thermal 
inertia) on Claritas Fossae are concentrated north of -20? lat., 
and they are comparable to the tc3nperaNe residuals within 
the nearby Tharsis region. ,.  , , .-,. ,.
of the dust deposits cannot be observed directly, there may 
be remnants of a former dust mantle that could indicate the 
general direction in which the dust deposits have moved under 
recent climatic conditions. The approach adopted here is to 
look for evidence of a previous dust mantle that can be related 
to the present dust deposits. 
The largest of the three regional dust deposits surrounds the 
Tharsis Montes extending over 130' of longitude and 60' of 
latitude and encompassing an area of 22 million km2, only 
slightly smaller than the area of the North American continent 
(Zimbelman and Kieffer, 1979). The southern margin of the 
Tharsis region skirts the northern edge of the Valles Marineris 
canyon system and follows an irregular course across Sinai, 
Syria, and Daedalia Planae (Fig. 1). The eastern margin includes 
a pronounced embayment into the low thermal inertia material 
along Kasei Vallis, the site of considerable aeolian activity and 
abundant sand-sized material confined within the channel 
(Chisttenen and Kieffer, 1979). The south-to-north transport 
of dust observed under present climatic conditions 
(Christenen, 1988) implies that evidence for previous dust 
deposits should exist along the southern margin of the deposit. 
Claritas Fossae is a complex assemblage of grabens and 
hctured terrain associated with a projection of low thermal 
inertia material into the higher thermal inertia materials of the 
southern hemisphere (A Fig. 1). The distinctive thermal 
properties of Claritas Fossae were first noted early in the Viking 
mission (Kiefer et d, 1976; see Fig. 2), but this area received 
less attention than did the unusual thermal characteristics on 
and around Arsia Mons. During an examination of the 
published physical properties of Claritas Fossae, a second 
location of hctured terrain in Sinai Planum (B in Fig. 1) was 
observed to have many characteristics similar to Claritas Fossae 
but generally with less contrast in comparison to its 
surroundings (Zimbelman, 1989). Both locations have 
bedrock interpreted to be highly deformed, hctured materials 
of Noachian age, the oldest of the martian stratigraphic systems 
(Scott and Tanuka, 1986). Many additional outcrops of this 
old, hctured terrain are present in the cratered highlands 
south of the Valles Marineris (Scott and T d a ,  1986), but 
only the two locations described above have remote-sensing 
characteristics comparable to those of the regional dust 
deposits. The variations in the remotely observed properties 
of the ancient, hctured materials provide strong clues to the 
direction in which a migrating dust mantle could have moved 
' 'CI",*Y*.T .l 
across Mars. 
The physical properties of both the Claritas Fossae and Sinai 
Planum locations of ancient, fractured terrain are Summarized 
below. All of the data have been published previously, but here 
the results for Claritas Fossae and Sinai Planum are examined 
within the context of the proposed global transport of dust 
on Mars. 
Thermal In& 
The northern portion of Claritas Fossae (Fig. 2) has 
temperature residuals indicative of a thermal inertia much 
lower (I < 3) than its immediate surroundings (I > 5). A zone 
Zimbelman: Dust outliers south of tbe Tharsis region, Mars 
surroundings, particularly at red wavelengths (Fig. 3). It is not 
a coincidence that the phimetric shape of the high albedo 
region on northern Claritas Fossae corresponds to the general 
shape of the low thermal inertia material at this same location 
(compare Figs. 2 and 3). Global mapping has revealed a 
planetwide correlation between low thermal inertia and high 
visual albedo (Kieffer et al, 1977; Arvidson et al, 1982), a 
relationship that holds down to a scale of a few kilometers, 
the highest resolution of the Viking hfrared Thermal Mapper 
data ( Z i a l m a n  and L&n, 1987). Results for northern 
Claritas Fossae are very consistent with this well-documented 
global trend. South of -20" lat. Claritas Fossae lacks a 
wominent albedo contrast with its surroundings, consistent 
vith the reduced contrast of this area at thermal wavelengths. 
The Sinai Planum location has a slightly higher albedo on 
he southern portion of the fractured terrain (Fig. 4), 
coincident with the area that shows the subtle thermal 
contrast. As was the case at thermal wavelengths, the high 
albedo portion of the Sinai Planum location has considerably 
less contrast with its surroundings at visual wavelengths than 
does northern Claritas Fossae. The reduced contrast does not 
keep this location fiom being visible in the color mosaic 
(McEwen, 1987). 
Fig. 3. Northern portion of Claritas Fossae. The area of low 
thermal inertia corkqonds to the high albedo material visible 
~~~~h~ and s&e ~~~~h~~ 
here (compare with Fig. 2). The lowest thermal inertias occur 
on the highly tkmured terrain (-16O lat., 110? long.) below Both the northern Claritas Fossae and the Sinai Planum dust 
the center of the scene shown here. Portion of Vi deposits are located on terrain possessing considerable 
frame 334S12 (centered on -14O lat., 110? long.), red filter, 
730 m/pixel, shading corrected reailinear version 
of low thermal inertia material connects the Tharsis region 
with the low thermal inertia material on the northern portion 
of Claritas Fossae, centered approximately on -16" G, 110" I 
long. The thermal propertieses6f northern Claritas Fossae are 
comparable to the properties of the large regions of low 
thermal inertia, interpreted to be a surface mantled by silt-size 
material (Kieffer et d, 1977). It is signillcant that the k t u r e d  I 
terrain of Claritas Fossae south of -20" lat. has thermal 1 
properties very similar to the moderate thermal inertias typical 
of the southern midlatitudes of Mars, in great contrast to the 
distinctive thermal properties of northern Claritas Fossae. 
Fractured terrain in Sinai Planum (centered approximately 
on -16" lat., 80" long.) also has temperature residuals 
indicative of a thermal inertia lower than its surroundings, 
although the thermal contrast is considerably reduced 
compared to that of northern Claritas Fossae. Global mapping 
of predawn temperature residuals (Fig. 9b of Kieffer et al, 
1977) shows the Sinai Planum location to be between 0 and 
2 K (I = 6.5), M e  the surrounding plains hpe temperature 
residuals between 4 and 6 K (I = 7.5). Such a small thermal 
Contrast might seem insi@cant if it did not also correlate ~ig. 4. Fractured terrain in Sinai Planum. The high albedo 
with distinct properties at both visual and radar wavelengths. material on the southern portion of the fractured terrain has 
physical properties similar to the northern portion of Claritas 
VisualAlbedo Fossae (see Fig. 3), but the Sinai Planum location generally has 
less contrast with its surroundings than does the Claritas Fossae 
Subtle albedo variations in the vicinity of Valles Marineris are location Portion of V i  Miter  frame 334.~46 (centered on 
portrayed dramatically in a recent color mosaic (McE~en, -15" lat., 80" long.), red filter, 730 m/pixel shading corrected 
1987). Northern Claritas Fossae has a higher albedo than its rectihearvemion 
528 Proceedings of tbe 20tb Lunar and Planetary Science Conference 
. % - I 8  , . - . - , . . . 
9, 115 110 105 
Longitude (deg) 
Fig. 5. Topographic profiles across northern Claritas Fossae 
obtained from Earth-based radar measurements (modifled from 
Fig. 7 of Roth et al, 1980). The dust deposit is associated with 
the prominent ridge posesing relief on the order of one 
kilometer, in contrast to the more subdued plains on either 
side. Dashed line shows the portion of a profile hidden by an 
intervening profile in this pfilaaicular projection. The base of 
each profile is plotted at the latitude of the corresponding 
groundtrack (&om north to south these latitudes are -14.50?, 
-15.91?, and -17.46O). Profiles are shown at lOOX vertical 
exaggeration. 
" "  
topographic relief. Ed-based radar measurements indicate 
that northern Claritas Fossae consists of a broad ridge that 
stands more than one kilometer above the uniform plains on 
either side of the ridge (Fig. 5). The topographic ridge 
corresponds closely with the location of the dust deposit 
inferred fkom the visual and thermal measurements. S W l y ,  
the fixctured terrain in Sinai Planum also possesses relief 
- 
Longitude (deg) 
Pig. 6. Topographic profiles across the Sinai Planum fractured 
terrain obtained from Earth-based radar measurements 
(modified from Fig. 8 of Rotb et d, 1980). As is the case with 
Claritas Fossae (see Fig. 5), the dust deposit is associated with 
relief on the order of one kilometer, in contrast to the more 
subdued plains on either side. Dashed line shows the portion 
of a profile hidden by an intervening profile in this parti& 
projection. The base of each profile is plotted at the latitude 
of the corresponding groundtrack (from north to south these 
latitudes are -14.50?, - 15.80?, -15.91?, and - 17.08'). Profiles 
are shown at lOOX vertical exaggeration. 
greater than one kilometer, again in contrast to the relatively 
&t plains surroun* the topographic high (Fig. 6). The area 
of greatest relief within the fractured terrain of Sinai Planum 
occurs at a latitude of -16O, precisely where the dust deposit 
is inferred h m  the other remote sensing data 
Radar measurements also provide information on the 
scattering characteristics of the martian surface on a scale 
comparable to or larger than the radar wavelength (12 cm for 
the data described below). Northern Claritas Fossae and the 
hctured terrain in Sinai Planum both stand out h m  their 
surroundings in several parameters obtained from the 
characteristics of the reflected radar signal (Fig. 7). Both 
locations have low dectbity and high roughness and RMS 
slopes, relative to the surrounding terrain, but the contrast is 
120 100 80 
Longitude 
Fig. 7. Surface properties obtained from Earth-based radar 
measurements (modified from Fig. 7 of Downs et d, 1975). 
The data are for a latitude of -16O, where A and B indicate 
the locations of hctured terrain in northern Claritas Fossae and 
Sinai Planum, respectively. Note that the C-factor (center 
diagram) is smaller for rough surkes than for smooth &faces. 
Both locations have decreased reflectiviw and increased 
roughness and RMS slope, relative to their &rroundings, but 
the magnitude of the contrast is much greater at northern 
Claritas Fossae than at Sinai Planum. 
Zimbelman: Dust outliers south of the Tharsis region, Mars 529 
much more pronounced at Claritas Fossae than at Sinai 
Planum. The RMS slope values are particularly instructive; 
Claritas Fossae has a maximum RMS slope greater than 10" 
at long. 110?, while Sinai Planum has a fairly uniform RMS 
slope of 3O, and both locations are surrounded by plains with 
slopes of 1" or less (Fig. 7). These distinctive radar properties 
are unique within the longitude range of 0" to 120?, at a 
latitude of - 16", except for Valles Marineris and Arsia Mom. 
Enhanced roughness and/or decreased reflectivity in the 
Tharsis region correlate well with lava flows that are observed 
to have a relatively thin aeolian mantle when examined in 
Viking images with a resolution of 15 m/pixel or better 
(Schaber, 1980). Although images of this resolution do not 
exist for the areas of Claritas Fossae and Sinai Planum under 
discussion here, it seems reasonable to conclude that the radar 
data are sensing a rough, irregular surface (on a scale of 1 to 
loom) that do not contribute to the remote-sensing 
characteristics at visual and thermal wavelengths. 
DISCUSSION 
AU of the remote-sensing data for northern Claritas Fossae 
and the fractured terrain in Sinai Planum are consistent with 
a surface that is rough and irregular at the meter scale but 
that is mantled by fine dust that dominates the surface 
properties at the centimeter (and smaller) scale. This is 
precisely the surface condition inferred by f2wktensen ( 1986) 
to characterize the large, low thermal inertia dust deposits of 
the northern midlatitudes. Of equal importance is the fact that 
several other nearby locations of ancient, fractured materials 
do not have anomalous remote-sensing properties, but rather 
these features have properties quite similar to their surround- 
ings. The fractured terrains having characteristics of dust 
deposits are at a latitude comparable to the current southern 
margin of the nearby Tharsis dust deposit (see Fig. l), while 
the fractured terrain locations lacking evidence of a dust 
covering are south of -20" lat., strengthening the likelihood 
of a causal relationship between the large and small dust 
deposits. 
The requirement that the surface properties be dominated 
by dust at thermal wavelengths implies an average dust 
thickness of at least several centimeters, while the penetration 
of the dust by the radar signals, to the underlying rough 
surface, implies an average dust thickness that is probably not 
much greater than a few meters (Christmen, 1986). A similar 
analysis procedure indicates that northern Claritas Fossae has 
a dust deposit comparable to the large regional dust deposits, 
while the fractured terrain at Sinai Planum probably has a dust 
cover on the order of centimeters thick, still sdcient to affect 
visual albedo and the diurnal variation of surface temperatures 
but thin enough to cause reduced contrast between the dust 
deposit and the surrounding plains. 
How did this nonuniform distribution of dust on isolated 
areas of rough terrain come about? It is possible that the 
dominance of roughness on the fractured terrain may be so 
strong that subsequent aeolian erosion may be unimportant. 
If surface roughness is the primary agent responsible for 
producing dust deposits, then one should expect to find dust 
trapped on all of the numerous outcrops of fractured terrain 
mapped south of -20" lat. between 70" and 110' long. (Scott 
and Tanu&u, 1986). The lack of anomalous remote sensing 
properties for Nectaris Fossae (fractured terrain between - 18" 
and -25" lat. along long. 58") indicates that the lack of 
preserved dust deposits on fractured terrains is not solely due 
to latitude but also is related to increased distance from the 
present margins of the Tharsis dust deposit. These factors make 
it highly unlikely that roughness alone can explain the isolated 
dust deposits. 
Is there evidence that the isolated dust deposits are the 
result of aeolian activity? The available image resolution is not 
d c i e n t  to provide diagnostic evidence for the process of 
deposit formation, but temporal variations provide an 
important clue. Images firom both Mariner 9 and Viking show 
considerable variation in the occurrence and orientation of 
bright streaks in Syria Planum just north of the dust deposit 
on northern Claritas Fossae ( Z h m  and V i a ,  1979). 
These same images show virtually no change in the high albedo 
material on northern Claritas Fossae. Also, streak orientations 
indicate winds capable of moving surface materials are blowing 
toward the north on the western side of Claritas Fossae, while 
winds blow to the south on the eastern side of Claritas Fossae 
( Z B o m  and V i a ,  1979). Thus, there is evidence of 
considerable aeolian activity in the vicinity of Claritas Fossae 
and the nonuniform wind direction may contribute to the 
relative stability of surface materials on Claritas Fossae itself. 
It seems reasonable to infer an aeolian origin for the dust 
deposit on northern Claritas Fossae and, by analogy, for the 
thinner dust deposit in Sinai Planum. 
Why are the dust deposits located on rough terrain? Field 
studies, laboratory simulations, and theoretical considerations 
indicate that increased roughness at the surface increases the 
ditEculty of moving material by the wind by decreasing the 
wind shear at the particulate surface (BagmI4 1941, pp. 53- 
55). It is important to realize that surface roughness can vary 
both on small (less than one meter) and large (greater than 
many meters) scales where small-scale roughness can protect 
erodible elements, such as dust, while large-scale roughness 
tends to enhance wind shear and the possibility of erosion (Lee 
et d, 1982). It is the small-scale roughness that affects the 
wind shear at the surface and creates an aerodynamic regime 
in which mobile particles (dust or sand) are protected from 
the lifting forces of the wind. There is no reason to expect 
dust to be preferentially deposited on the rough terrains on 
Mars, but once on the surface it is more difEcult to remove 
the dust from rough terrains without reducing the local 
roughness. 
Are there isolated dust deposits near the southern margins 
of the other regional dust deposits on Mars? There do not 
appear to be isolated deposits elsewhere along the southern 
margin of the Tharsis region nor along the southern margins 
of the Elysium or Arabia regions. This result may be partly due 
to the lack of isolated regions with properties as distinctive 
as those observed at northern Claritas Fossae, but it may also 
be related to the lack of exposures of ancient, fractured terrain 
near the southern margins of the regional dust deposits except 
at the locations discussed here. Isolated deposits smaller than 
530 Proceedings of the 20th Lunar and Planetary Science Conference 
about 25 km probably are present, but these deposits likely will 
be detectable only with the highest resolution data 
The interpretations described here do not provide conclu- 
sive verification of the migration of regional dust deposits on 
Mars, but the spatial relationship and the local contrast of the 
remote-sensing properties at northern Claritas Fossae and Sinai 
Planum provide some compelling circumstantial evidence for 
the regional migration. Erosional removal of a previously 
widely distributed dust deposit seems the simplest explanation 
for the occurrence of isolated dust deposits on rough terrain 
near the margin of a regional dust deposit, while other 
exposures of similar rough terrain do not display evidence of 
the dust deposit. Alternative mechanisms can be proposed for 
isolated dust accumulations, but it seems unlikely that a 
nonaeolian process would be as consistent with the spatial 
distribution and the observed remote-sensing characteristics. 
Since Sinai Planum is both further removed kom the present 
margin of the Tharsis dust deposit (see Fig. 1) and has less 
dust preserved than at northern Claritas Fossae, the inferred 
removal of the hypothesized dust mantle should have 
progressed from south to north, as inferred from the 
hypothesis of Chnktensen ( 1986). 
,554 t ' s t  ' CONCLUSIONS 
I '  
Remote-sensing characteristics at visual, thermal, and radar 
wavelengths are u$ed to infer the presence of isolated dust 
deposits on areas of hctured terrain in northern Claritas 
~o-&ae and Sinai Planum. The dust deposit at Sinai Planum is 
probably less thick than the deposit at northern Claritas Fossae, 
which is comparable to the dust present within the nearby 
Tharsis low thermal inertia region. Numerous exposures of 
hctured terrain are present in this area south of -20' lat. but 
these locations lack distinctive contrast with their surroundings 
in the remote-sensing data Aeolian removal of a previously 
widely distributed dust deposit is interpreted to be the most 
likely explanation for the isolated dust deposits that have been 
preserved because they exist on rough terrain that decreases 
the local efficiency of aeolian erosion The different distances 
of the Claritas Fossae and Sinai Planum dust deposits h m  the 
southern margin of the Tharsis low thermal inertia region, in 
combination with a thinner dust deposit at Sinai Planum, 
indicate that the removal of a widely distributed dust deposit 
progressed from the south to the north, in agreement with the 
global movement of dust proposed by Chnktensen ( 1986). 
Acknowlerlgments. The comments of S. Lee, S. W i ,  and an 
anonymous reviewer were very helpful during the revision of the 
manuscript. This work was supported by NASA grant NAGW-1390. 
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