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46th Lunar and Planetary Science Conference (2015)
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GLOBAL INVENTORY OF RECTILINEAR AND POLYGONAL RIDGE NETWORKS ON MARS. L.
Kerber1, J.L. Dickson2, E.B. Grosfils3, J.W. Head2, 1Jet Propulsion Laboratory (4800 Oak Grove Dr. Pasadena, CA
91109, [email protected]), 2Department of Earth, Environmental, and Planetary Sciences, Brown University, Box
1846 Providence, RI 02912, 3Geology Department, Pomona College, 185 E. 6 th St. Claremont, CA 91711.
Introduction: Rectilinear and polygonal ridge
networks have been recognized in several different
areas on Mars. While many types of ridges share morphological similarities, they can be created by a variety
of unrelated geological processes. Here we summarize
and review occurrences of rectilinear ridge networks
on Mars, their morphologies, and their probable formative causes. To these we add two widespread, previously undocumented ridge occurrences located in the
Medusae Fossae Formation (MFF) and Hellas Basin.
Ridge Occurrences:
Tharsis, Elysium, and the Highlands. Long, linear
to slightly curving or en echelon (not networked)
ridges have been found in a variety of locations on
Mars, especially on the large Martian shields (Fig. 1a;
[1-5]). These ridges can be tens of meters across and
hundreds of kilometers long, and can transition from
ridges into fissures along strike (white arrow). They
are hypothesized to be magmatic dikes [1-5].
Inca City. A group of intersecting rectilinear
ridges, informally known as “Inca City” and located
within the south polar layered deposits, was observed
in early Mars Orbiting Camera images and hypothesized to be either lithified duneforms, clastic dikes, or
magmatic dikes (Fig. 1b; [6]).
Gale Crater. Smaller networks of linear ridges
(polygons ~25 m across) have been identified in Gale
Crater, where they have been attributed to preferential
cementation of material due to the circulation of fluids
through fracture networks (Fig. 1c, black arrow indicates ridge, while white arrow indicates fracture pattern [7]).
Across Mars. Transverse aeolian ridges can also
form networks of intersecting bedform ridges (usually
in the bottoms of craters or within other confined topography), but these occur on a scale that is orders of
magnitude smaller than the other ridges described
above (Fig. 1d).
On Crater Floors. A network of rectilinear, crisscrossing ridges was described in several craters on the
dichotomy boundary and near Nili Fossae [8-9]. These
chaotic, intersecting ridges, forming irregular polygons
~1 km across, are hypothesized to be brecciated dikes
emplaced during the process of crater formation [8-9].
The Medusae Fossae Formation. MFF networks
consist of thin, dark, intersecting ridges of resistant
material that becomes exposed as the result of the erosion of fine-grained, lighter-toned material surrounding
them. The ridge thicknesses are fairly uniform, unlike
the ridges attributed to cemented fractures or impactinduced clastic diking, which vary in thickness both
between ridges and along strike.
Figure 1. Examples of different types of rectilinear
and polygonal ridges on Mars.
46th Lunar and Planetary Science Conference (2015)
2148.pdf
The tops of the ridges form a generally flat surface,
except at the edges of networks where the distal ends
of ridges tail off. Two types of interlocking patterns
are common: a generally isotropic pattern of intersecting ridges (Fig. 2a), and a curvilinear, fish-scale-like
arrangement of ridges. The ridges tend to be relatively
dark and erode by shedding blocky boulders, while the
material that surrounds them (the MFF) is light-toned
and erodes into loose, fine-grained material. Like the
Gale Crater ridges, the MFF ridges are associated with
fractured terrains: the two patterns of ridges mirror two
different patterns of MFF fracturing that are common
in ridge-forming areas (Fig. 2b). Unlike the Gale
Crater ridges, however, the MFF ridges tend to be associated with lava flows. Ridges often emerge from
remnant mesas of fractured MFF material where these
mesas sit alongside a lava flow (Fig. 2c), and in some
places a rough lava flow surface will transition directly
into a ridge-covered surface (Fig. 2d). It is possible
that the ridges represent the result of a lava flow entering a network of MFF fractures. If this is the case, it
explains the appearance of the ridges (dark and
blocky), their geometric similarity to nearby fracture
networks, and their nearly uniform heights, which
would correspond to the height of the lava flow at the
time of emplacement.
Hellas Basin. These polygonal ridge networks are
widespread in the northwestern part of central Hellas
Basin. They consist of thick, light-toned, intersecting
ridges. Like the MFF ridges, they do not usually
change thickness along strike, but the large networks
can sometimes contain subsidiary networks composed
of smaller ridges forming smaller polygons (Fig. 3).
The Hellas ridges do not shed blocks, but appear extensively fractured. The Hellas ridges are not associated with lava flows, and are more likely to be filled
with sediment.
Figure 3. Ridges in Hellas Basin. ESP_028457_1425.
References: [1] Shean, D.E., Head, J.W., Marchant,
D.R. (2005) JGR, 110, E05001. [2] Head J.W. et al. (2006)
GSA 34, 285–288. [3] Basilevsky A.T. (2006) GRL 33,
L13201. [4] Pedersen G.B.M. et al. (2010) EPSL 294, 424–
439. [5] Wilson L. Mouginis-Mark P.J. Icarus 165, 242-252,
2003. [6] Malin et al. (1998) Science 279, 1681. [7] Thomson
et al. (2011) Icarus 214, 413-432. [8] Head, J.W., Mustard,
J.F. (2006) Meteorit. Planet. Sci. 41, 1675-1690. [9] Saper,
L., Mustard, J., (2013) GRL 40, 245-249.
Figure 2. Ridges in the MFF.