46th Lunar and Planetary Science Conference (2015)
K. W. Lewis1, W. E. Dietrich2, L. A. Edgar3, J. P. Grotzinger4, S. Gupta5, L. C. Kah6, N.
Mangold , D. M. Rubin8, K. M. Stack-Morgan9, R. M. E. Williams10, and the MSL science team. 1Johns Hopkins
University, Dept. of Earth and Planetary Sciences, Baltimore, MD 21218 ([email protected]), 2UC Berkeley, Berkeley, CA, 3USGS, Flagstaff, AZ, 4California Institute of Technology, Pasadena, CA, 5Imperial College, London, UK,
University of Tennessee, Knoxville, TN, 7Université de Nantes, Nantes, France, 8UC Santa Cruz, Santa Cruz, CA,
Jet Propulsion Laboratory, Pasadena, CA, 10Planetary Science Institute, Tucson, AZ.
Introduction: Over the first 800 sols of its mission, the Curiosity rover traversed several kilometers
across the floor of Gale crater from its landing site on
Aeolis Palus toward its destination at Aeolis Mons
(Mount Sharp). This initial phase of the mission culminated with the recent arrival at the base of Mount
Sharp, represented by an outcrop informally known as
the Pahrump Hills. Over this route, the rover explored
several distinct geologic units representing the transition zone between sediments of the Peace Vallis fan
and Mount Sharp. From its landing site, the rover has
climbed nearly 50 meters in elevation to date, with the
Pahrump Hills outcrop nearly 70 meters above the
lowest point in the traverse, at Yellowknife Bay. The
natural topography along the route has provided an
opportunity to explore the nature of this stratigraphic
transition in three dimensions. Understanding the rela-
Figure 1: Projected extent of strata within the lower
formation of Mount Sharp above Curiosity's current location at Pahrump, which dip at roughly 4
degrees to the northwest. Curiosity is currently
located near minimum of the cross-section shown.
Regional topography from MOLA.
tionships between these sedimentary systems is critical
to determining the mechanisms and timing of deposition and erosion within Gale crater. In particular, the
lower strata of Mount Sharp are most promising for
determining whether Gale crater experienced longterm lacustrine phases early in its history. Here we
describe a combination of orbital and rover-based
mapping, integrating stereo images and topography
from the HiRISE camera with those from Curiosity’s
Navcam and Mastcam instruments. Outcrop and bed
geometries are used to identify the location of this geological transition, as well as to constrain physical stratigraphic relationships and depositional mechanisms.
Bedding Geometry: From orbit, the lower strata of
Mount Sharp have been observed to dip consistently
away from the center of the mound [1]. In the vicinity
of the planned Curiosity ascent, strata have a consistent
dip of ~4 degrees to the northwest where they can be
measured, [1,2]. Extrapolation suggests the layers of
the Lower Unit of Mount Sharp might have extended
hundreds of meters above the current topography of
the areas traversed by the rover, but did not completely
fill the crater. This trend is not yet observed from the
ground, suggesting a change in bedding attitude between Pahrump and the lowermost strata measurable
from orbit, at the Hematite Ridge of [3].
From the surface, Curiosity has encountered a diverse array of bedded sedimentary rocks ranging from
fluvial conglomerates to lacustrine mudstones [4-6].
Since the Cooperstown waypoint (Fig. 2), the rover has
imaged recurring outcrops of consistently southdipping crossbedded rocks. These occur primarily in a
geomorphic terrain type mapped as the Striated Unit
from orbit. Figure 2 shows a number of instances of
south-dipping beds exposed along the traverse, typically inclined at 5-15 degrees from horizontal. This pattern indicates a sediment transport direction from the
north, possibly originating from the crater rim. However, the elevation of the striated unit (of order 1 meter
vertical thickness) increases to the south at a slope of
~1 degree, implying an uphill flow direction (aeolian)
or an aggradation-dominated environment (deltaic).
Interpretations: Currently, Curiosity is poised at
the boundary between the sedimentary units of Aeolis
Palus and those of Mount Sharp. Given the measured
differences in bedding geometry between these re-
46th Lunar and Planetary Science Conference (2015)
gions, the nature of the transition will provide clear
information regarding the formation of and evolution
of Mount Sharp. Plausible endmember scenarios include an onlap relationship of crater floor units onto
the base of Mount Sharp (as inferred from orbital mapping [2,7]), or a smooth transition with more complex
interfingering, as suggested by some ground-based
observations [8]. In either scenario, the units observed
Yellowknife Bay
Mean dip direction
MSL Traverse
Hidden Valley
Dingo Gap
Figure 2: Average bedding orientations observed at
several major waypoints along the rover traverse.
Since Cooperstown, layers have exhibited consistent
southward dips even as terrain increases in elevation to the south.
by Curiosity to date are not correlative with the inclined layers found higher on Mt. Sharp, and shown in
Figure 1. We explore the current observations in support of these multiple hypotheses arising from the diverse geologic units in the transitional region at the
base of Mount Sharp. New geologic units exposed
near this boundary include repetitive thin-bedded rocks
exposed at Hidden Valley (Fig. 3). These beds are
relatively flat-lying compared to other units observed
in the area, and may represent a distal fluvial or lacustrine environment. Further work is needed to determine the significance of the cm-scale repetitive layering observed at Hidden Valley, and any potential relation to annual or other periodic climate variations.
Detailed analysis of the section now being explored at
Pahrump Hills, inferred to be the lowermost exposed
portion of the orbitally-defined Mount Sharp Lower
Unit, will further constrain the nature of this basal
References: [1] Kite, E.S. et al., (2013) Geology
41 (5), 543-546 [2] Le Deit, L. et al., (2013) JGR
Planets 118 (12), 2439-2473. [3] Fraeman, A. A. et al.,
(2013) Geology 41 (10) 1103-1106. [4] Grotzinger,
J.P. et al., Science 343 (6169). [5] Williams, R.M.E. et
al, Science 340 (6136) 1068-1072. [6] Edgar, L.A. et al
(2014) LPS XLV, Abstract #1648. [7] Anderson, R. B.
and Bell, J. F. (2010) Mars 5 (76-128). [8] Stack, K.
M. et al., (2015) LPS XLVI.
Figure 3: Mastcam view of repetitive, flat-lying layering observed near the base of Mount Sharp at Hidden
Valley on Sol 710. This and other transitional units exposed in the area provide information regarding the
relationship between Mount Sharp and surrounding units on the floor of Gale crater.