Late alluvial fan formation in southern Margaritifer Terra, Mars
John A. Grant1 and Sharon A. Wilson1
Received 24 January 2011; revised 24 February 2011; accepted 2 March 2011; published 20 April 2011.
[1] Crater statistics indicate alluvial fans, crater floor, and
fill/mantling deposits within impact craters >50 km in
diameter in southern Margaritifer Terra were likely
emplaced during multiple epochs: fans formed during the
Amazonian or near the Amazonian?Hesperian boundary,
crater floor deposits are likely Hesperian in age, and most
fill/mantling deposits are Amazonian. The regional
distribution of fans points to a late period of widespread
water?driven degradation. Two of the final candidate
landing sites for the 2011 Mars Science Laboratory mission
would land on or near some of these fans, which appear
younger than previously considered. Citation: Grant, J. A.,
and S. A. Wilson (2011), Late alluvial fan formation in southern
Margaritifer Terra, Mars, Geophys. Res. Lett., 38, L08201,
doi:10.1029/2011GL046844.
1. Introduction
[2] Alluvial fans within impact craters have been identi-
fied throughout southern Margaritifer Terra, southwestern
Terra Sabaea, and southwestern Tyrrhena Terra regions of
Mars [Moore and Howard, 2005; Kraal et al., 2008]. The
morphology and age of these fans provides critical insight
into the climate when they formed [e.g., Moore and
Howard, 2005]. Previous analyses of intracrater alluvial
fans primarily used visible (VIS, ?19 m pixel scale) and
infrared (IR, ?100 m pixel scale) images from the Thermal
Emission Imaging System (THEMIS) [Christensen et al.,
2004]. Determining the relative ages of these deposits via
crater statistics was impeded by image resolution, incom-
plete coverage of VIS images, and the small area of the
deposits (generally <1000 km2) [Moore and Howard, 2005].
As a result, the age of fan deposits within the global
framework of fluvial activity has remained uncertain, and
was typically bounded by the age of their host craters.
Because craters hosting alluvial fans are often Noachian in
age [e.g., Scott and Tanaka, 1986], most fans were inferred
to be late Noachian and no younger than mid?Hesperian in
age [e.g., Moore and Howard, 2005].
[3] This study utilizes higher resolution data from the
High Resolution Imaging Science Experiment (HiRISE)
[McEwen et al., 2007] and Context Camera (CTX) [Malin
et al., 2007] instruments on theMars Reconnaissance Orbiter
to reevaluate the distribution and age of alluvial fans. The
coverage and resolution (?0.25 m pixel scale for HiRISE
and 6 m pixel scale for CTX) of these data allowed a sys-
tematic evaluation of fans and confident identification of
smaller craters, thereby allowing for more meaningful crater
statistics.
2. Regional Setting and Classification of Deposits
[4] This study investigates alluvial fans and other deposits
in craters >50 km in diameter in southern Margaritifer Terra
(Figure 1), including Eberswalde and Holden craters, two
finalists for the landing site for the Mars Science Laboratory
(MSL). Based on morphology, we classified deposits in 22
craters as 1) fan deposits, 2) floor deposits, or 3) fill/mantling
deposits (Figure 1).
[5] The eight craters hosting fan deposits typically have
well?developed alcoves, incised walls, and occasionally
etched deposits on their floors inferred to be distal alluvial,
playa or shallow lacustrine deposits. In most instances, the
transition from fan deposits to inferred distal alluvial, playa or
lacustrine deposits is continuous so the deposits are assumed
to be contemporary. Many fan surfaces preserve distributary
channels and lobes that typically grade to the crater floor at a
slope of only a few degrees [Moore and Howard, 2005].
Erosion of fan surfaces in Holden crater leaves distributary
channels standing ?15 m in relief above surrounding surfaces
based on analysis of a HiRISE digital terrain model (DTM).
Comparable erosion of fans in some other craters is inferred
frommorphology of distributaries, but the absence of HiRISE
DTMs precludes quantitative constraint.
[6] Six craters have floor deposits that preserve morpho-
logic features indicative of past fluvial and/or lacustrine
activity (Figure 1). These craters lack well?developed rim
alcoves and extensive alluvial fans, but display incised walls.
The floor deposits, although occasionally locally mantled,
are generally light?toned, layered and erode to a scabby
appearance. Some crater floor deposits are bound by what
appear to be trim lines around the lower crater wall near the
termination of the wall valleys. These crater floor deposits,
therefore, may reflect deposition in a playa or shallow
lacustrine setting, though limited occurrence of associated
fans suggests differences in water and/or sediment discharge
relative to the conditions enabling fan formation.
[7] Finally, eight craters generally located in the south-
ern part of the study area are partially filled or mantled.
For example, crater N (Figure 1 inset) is partially filled by
a blocky deposit that apparently flowed into the crater,
creating abrupt outward facing relief along its margin.
The nature of the small mounds and dark polygonal
fractures in crater N is reminiscent of volcanic materials.
By contrast, the other craters in this class, such as crater I
(see Figure 1 for location and auxiliary material), show
hints of incised walls and light?toned outcrops reminiscent
of distal alluvial fan or playa/lacustrine deposits beneath a
mantle of covering materials.1 There are a few additional
1Center for Earth and Planetary Studies, National Air and Space
Museum, Smithsonian Institution, Washington, D. C., USA.
Copyright 2011 by the American Geophysical Union.
0094?8276/11/2011GL046844
1Auxiliary materials are available in the HTML. doi:10.1029/
2011GL046844.
GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L08201, doi:10.1029/2011GL046844, 2011
L08201 1 of 6
craters in the study area with diameters greater than 50 km,
but gaps in image coverage or subsequent modification
precluded their evaluation.
3. Methods
[8] Crater statistics were generated using areas and craters
mapped on CTX data in ArcGIS using CraterTools software
[Kneissl et al., 2011]. All counts excluded gaps in CTX
image data, obvious secondary clusters, putative lava flows,
central peaks, and etched surfaces (generally based on
evaluation of both HiRISE and CTX data; see auxiliary
material). The resolution of CTX data enabled confident
definition of craters down to approximately 20?30 m in
diameter, but craters smaller than ?50 m were not included
in the statistics to avoid potential effects of image resolution.
For craters with fan deposits, craters were counted on fan
surfaces and in some cases, the non?etched portions of
adjacent putative distal alluvial and/or playa or lacustrine
surfaces based on the assumption they are contemporary.
For all other craters, the area included in the analysis was
defined by the extent of floor or fill/mantling deposits on
encompassing crater floors. Craterstats software was used to
compile reverse cumulative histograms using pseudo log
bins to interpret a relative age for each count [Michael and
Neukum, 2010] based on the chronology function of
Hartmann and Neukum [2001] and production function
from Ivanov [2001]. Relative ages were also interpreted
from the incremental plots using the variable diameter bin?
size method of Hartmann [2005].
4. Results
[9] The areas and number of craters used to generate the
crater statistics for fan deposits are generally more than
1000 km2 and well over 100 craters, respectively (auxiliary
Figure 1. Study area in southern Margaritifer Terra indicating place names in text. Craters >50 km in diameter with col-
ored letters were included in the study. Color of crater labels indicates presence of fan deposits (green, Inset 1), crater floor
deposits (yellow, Inset 2) or crater fill/mantling deposits (red, Inset 3). MOLA topography over subset of THEMIS global
daytime IR mosaic. Shaded white and black indicate gaps in CTX and THEMIS coverage, respectively (as of 12/2010).
White boxes are HiRISE footprints (as of 12/2010). Example fan deposit in Crater S (Inset 1). Subset of
CTXB01_00999_1519. Example crater floor deposit in Crater Q (Noma, Inset 2). Subset of CTX P12_005582_1561.
Example mantling deposit in crater N (Inset 3). Subset of CTX P12_005701_1448. North to top of all images.
GRANT AND WILSON: LATE FAN FORMATION IN MARGARITIFER TERRA L08201L08201
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material). Areas and craters for craters with floor and fill/
mantling deposits (Figure 1) averaged ?800 km2 and usu-
ally more than 100, respectively (auxiliary material).
[10] In both types of plots (Figure 2), fan deposits date
from the Amazonian to near the Amazonian?Hesperian
boundary. Crater floor deposits are likely Hesperian, and fill/
mantling deposits are mostly Amazonian except for the
Hesperian or Noachian?aged deposit in crater K. Despite
variations between individual counts, the clustering of results
within each class relative to one another indicates distinct
differences in ages. The cumulative and incremental plots
yield broadly similar results and are internally consistent,
though absolute ages derived from the incremental plot are
systematically younger due to differences in the production
function and position of hypothesized isochrones [Hartmann,
2005; Carr, 2006]. With the understanding that interpreted
ages represent the minimum age of the surface, we empha-
size the older values derived from the cumulative statistics
(auxiliary material). We generally excluded craters smaller
than ?200 m (except in crater N where the largest crater is
?280 m across) in the interpretation of ages due to defi-
ciencies in their numbers caused by erosion as discussed
below (auxiliary material).
5. Discussion
[11] The variability of relative ages for different surfaces
is likely due as much to small areas considered and paucity
of large craters (>1?2 km in diameter) as it is to real dif-
ferences in age. One check on the general validity of the
crater statistics can be made by comparing the predicted
amount of erosion for the alluvial fan deposits to the onset
diameter of crater deficiency in the statistics relative to the
expected production population. The ?15 m erosion
required to account for topographic inversion of putative
distributary channels on the fans in Holden crater plus ?5 m
erosion needed to account for the original depth of the
channels suggests that erosion should also have destroyed
some craters ?100?200 m in diameter, assuming a crater
depth?to?diameter ratio of about 0.1 to 0.2 for primary and
secondary craters [Melosh, 1989]. Examination of the sta-
tistics for the fan and floor deposits confirms a relative
deficiency of craters smaller than ?250 m, similar to the
expected amount. By comparison, the ejecta of a 7.5 km?
diameter crater on the floor of crater R (auxiliary material)
display a significantly higher crater density relative to
adjacent surfaces. Compilation of statistics for the ejecta of
this crater, however, reveals the higher density is due to the
preferential preservation of craters <200 m in diameter rel-
ative to the adjacent fans/playa. The crater ejecta likely
incorporate materials that are more resistant to erosion,
perhaps excavated from beneath the fan deposits. Overall,
most statistics for both the fan and crater floor deposits are
consistent with the expected production population for cra-
ters with diameters ?200 m up to 1?2 km, thereby implying
the absence of other intervals of burial/erosion.
[12] All but one crater containing fill/mantling deposits
yield ages similar to or younger than the fan and crater floor
deposits, however the range of ages is consistent with the
diversity of mantling materials. Additional fan and/or distal
alluvial or possible playa/lacustrine deposits may be par-
tially buried by younger fill/mantling deposits in some
craters (e.g., crater I, see auxiliary material), especially in
the southern part of the study area. By contrast, there is no
evidence that fill/mantling deposits were emplaced and then
removed subsequent to the formation of fan deposits in this
region. As noted, the fill/mantling deposits are more prev-
alent in the south and at least some of the deposits (e.g.,
putative volcanic material in crater N) appear to consist of
materials that would be difficult to completely remove rel-
ative to surrounding materials (e.g., fans).
[13] The crater statistics from fans evaluated by Moore
and Howard [2005] yielded a mid?Hesperian age, with an
uncertainty ranging from near the Noachian?Hesperian
boundary to the mid Amazonian. Considering the position
of the fans relative to other features, Moore and Howard
[2005] concluded the fans in Margaritifer Terra were no
older than late Noachian, but could be younger. Crater
statistics based on CTX data for alluvial fans in Harris crater
in western Terra Tyrrhena are consistent with a Late Noa-
chian age [Williams et al., 2011], but examination of the
statistics suggests the eastern fan in Harris may be appre-
ciably younger.
[14] Although the derived age of individual fan and floor
deposits varies and may overlap to some extent, the differ-
ences in the morphology of the deposits coupled with their
overall grouping in age suggests the two classes of deposits
have distinct origins and ages. Nevertheless, we cannot
preclude the possibility they are contemporaneous. It is also
possible that alluvial fan deposits bury crater floor deposits
in some locations, perhaps even rejuvenating pre?existing
crater wall valleys or fans. Despite such uncertainties, the
statistics appear consistent with expectations based on the
inferred degradation of associated surfaces and their strati-
graphic position: crater floor deposits do not embay alluvial
fan deposits, whereas the generally younger fill/mantling
deposits may bury some crater floor and/or alluvial fan
deposits in the southern portion of the study area. We
therefore conclude that the fan deposits were emplaced
during the Amazonian to around the time of the Amazonian?
Hesperian transition. Crater floor deposits are older, likely
of Hesperian age, and mantling deposits are generally
Amazonian. If the fans formed as a result of precipitation in
the form of rain or snow [Moore and Howard, 2005;
Howard and Moore, 2011], their young age requires a cli-
mate optimum relatively late in Martian history after most
precipitation?driven fluvial activity is thought to have
occurred [Howard et al., 2005; Moore and Howard, 2005;
Bibring et al., 2006; Fassett and Head, 2008].
[15] Comparison with geomorphic events inferred to have
occurred in southern Margaritifer Terra and elsewhere on
Mars provides a regional context. Episodes of water pond-
ing in Holden crater and adjacent Uzboi Vallis [Grant et al.,
2008] in the late Noachian or well into the Hesperian [Irwin
and Grant, 2011] was likely associated with a widespread,
late interval of precipitation?driven runoff [Howard et al.,
2005; Moore and Howard, 2005; Fassett and Head,
2008]. The ages inferred for the crater floor deposits are
broadly contemporary and may record the effects of this
widespread wetter period. The inferred age of the alluvial
fan deposits appear younger, however, and their formation
may require an even later wet period.
[16] Formation of the fan deposits near or after the
Amazonian?Hesperian boundary could relate to local,
regional, or perhaps global?scale events. Local events could
include the impact?induced release of volatiles, generating
GRANT AND WILSON: LATE FAN FORMATION IN MARGARITIFER TERRA L08201L08201
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Figure 2. (left) Cumulative and (right) incremental crater statistics for (a) fan deposits, (b) crater floor deposits, and (c) fill/
mantling deposits. AH and HN in incremental plots marks the hypothesized position of the Amazonian?Hesperian and Hes-
perian?Noachian boundary, respectively. Error bars reflect ?1/N0.5 and N ? N0.5/A for data points in cumulative and incre-
mental plots, respectively, where N is the number of craters and A is the area. See text and auxiliary material for discussion.
GRANT AND WILSON: LATE FAN FORMATION IN MARGARITIFER TERRA L08201L08201
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runoff, valley incision and the formation of fluvial features
on and near the crater and ejecta [e.g., Maxwell et al., 1973].
Such water?driven activity is thought to be the result of
excavation and melting of subsurface ice during and after
the impact [Jones et al., 2011] and may persist up to 104 or
105 years [Newsom et al., 1996; Abramov and Kring, 2005].
[17] The impact that formed crater Hale, located just south
of the study area (35.7S, 323.6E), is a potential source for
local volatiles. The ejecta from Hale is incised by numerous
valleys [Jones et al., 2011] and the crater likely formed
between the early to middle Amazonian [Jones et al., 2011]
and Amazonian?Hesperian boundary [Cabrol et al., 2001].
Although the fan deposits may be contemporary, there are
several aspects of the Hale valleys and the alluvial fan de-
posits that suggest their formation is not related. First, some
of the fan deposits occur 700?800 km from Hale (e.g., in
craters A, B, and R, Figure 1) and additional fan deposits
may be even farther outside the study area. Second, there is
little correlation between Hale and the azimuth of craters
containing alluvial fan deposits as might be expected for a
local source of water vapor under the influence of prevailing
winds. Third, many craters with floor deposits (e.g., craters
G, H, V, and T, Figure 1) are located closer to Hale than
craters with fan deposits, which is the opposite of what
would be expected if Hale was the source of water for the
fans. Finally, many craters closest to Hale contain fill/
mantling deposits (e.g., crater N, Figure 1). Hence, there is
not a convincing connection between the formation of crater
Hale and the emplacement of the alluvial fan deposits. Other
possible impact crater?related sources of water are likely
older [e.g., Irwin and Grant, 2011] and even less likely to be
associated with fan development.
[18] The widespread occurrence of fans in Margaritifer
Terra seems to require synoptic precipitation, perhaps
enhanced over time and locally by orbital variations and
topography. If synoptic precipitation was responsible (as
either snow or rain), there should be morphologic evidence
of contemporaneous/concurrent water?driven erosion else-
where on Mars.
[19] Late intervals of water?driven erosion that may be
contemporary with the fans include incision of valleys on
some Martian volcanoes [Gulick and Baker, 1990; Fassett
and Head; 2008], hypothesized supraglacial and proglacial
valleys [Fassett et al., 2010], and late geomorphic activity in
Electris [Grant and Schultz, 1990] that included valley
incision [Howard and Moore, 2011]. In addition, Moore
and Howard [2005] suggested fans elsewhere on Mars
formed at the same time as those in southern Margaritifer
Terra, thereby implying global?scale processes. Water
delivered to the northern lowlands during late outflow
channel formation [e.g., Rotto and Tanaka, 1995] and later
redistributed as precipitation in the southern highlands [Luo
and Stepinski, 2009] is one possible source of runoff for the
fans. Under this scenario, fan development occurred during
a relatively short?lived late interval. Because two of the
craters with fan deposits are finalists for the MSL landing
site, such a relatively young age implies that MSL could
sample materials emplaced later in Martian history than
previously considered.
[20] Acknowledgments. We thank the people at the University of
Arizona, Ball Aerospace, Malin Space Science Systems, the Jet Propulsion
Laboratory, and Lockheed Martin that built and operate the HiRISE and
CTX cameras and Mars Reconnaissance Orbiter spacecraft. Reviews by
Cathy Quantin and an anonymous reviewer improved the manuscript. This
work was supported by NASA.
[21] The Editor thanks an anonymous reviewer and Cathy Quantin for
their assistance in evaluating this paper.
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J. A. Grant and S. A. Wilson, Center for Earth and Planetary Studies,
National Air and Space Museum, Smithsonian Institution, PO Box
37012, Washington, DC 20560, USA. (grantj@si.edu)
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