GEO 330 Lecture Notes - Lecture 27: Gusev (Martian Crater), Alkali Basalt, Pedestal Crater
14 May 2018
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Common on Husband Hill and Cumberland Ridge
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Only found as “float” -- no outcrops observed
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Lower Fe and Cr and higher Al, Ti, and P compared to Adirondack and Clovis class rocks
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Dominant primary minerals are plag, pyx, olv, with minor ilmenite and apatite
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MI images show poorly sorted angular clasts in a fine matrix
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Resemble terrestrial ash-flow tuffs
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Rocks only minimally altered
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Gusev Crater -- Wishstone Class Rocks
Only two examples of Peace- class rocks have been found
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Fine-grained, cemented, and clastic
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Form finely-layered outcrops
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If layering is not present, they often have a spongy texture
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Silicate portion only mildly weathered, but salts are abundant as cements
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High abundances of Mg and Ca-bearing sulfates
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Gusev Crater -- Peace Class
Only found in part of Columbia Hills called Cumberland Ridge
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Forms rugged-looking, stratified outcrops
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Rocks are variable in composition and texture, but have high Ti, P, S, Cl, and Br and low Cr
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Variability in chemistry can be explained by mixing Peace and Wishstone class rocks at varying
ratios
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Interpreted as fine-grained ejecta from an impact into a mixture of Peace and Wishstone rocks
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Rocks are isochemically altered, mostly as a result of oxidation
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Alteration was likely at low water/rock ratios, perhaps in an impact-induced hydrothermal system
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Gusev Crater -- Watchtower Class Rocks
Alkaline volcanic rocks
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Found because of unique Mini-TES spectrum
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Backstay is an olivine basalt similar to Adirondack class, but with higher Ti, Al, K, and lower Fe
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Less altered than any other rock in Columbia Hills
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Irvine similar to Backstay, but enriched in Fe, Mn, and Ni and depleted in Al, Na, K, and P
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Several other classes of rocks have been identified based on minor variations in chemistry. Most
have only one or two examples
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Gusev Crater -- Backstay and Irvine Class Rocks
Home Plate is a plateau ~90m in diameter and 2-3 m high
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Contains coarsely and finely layered units that include cross-stratified sedimentary layers
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Composition is a moderately altered alkali basalt that is enriched in volatile elements
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Most likely formation scenario involves interaction of an alkali basalt magma with a
subsurface water or brine layer
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Sedimentary textures suggest layers have a pyroclastic origin
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Other rocks and soils in the region are extremely silica-rich, implying leaching by an acidic fluid
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Gusev Crater -- Home Plate and Surrounding Region
All Columbia Hills rocks except for Backstay-class are significantly more altered than plains
Adirondack class basalts
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Some Clovis and Watchtower class rocks are so altered that only minor amounts of the primary
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Gusev Crater -- Summary
Landing Sites -- Surface Properties and Processes
Thursday, May 3, 2018
1:45 PM
GEO 330 Page 1
Some Clovis and Watchtower class rocks are so altered that only minor amounts of the primary
minerals are still present -- these rocks are very soft
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Columbia Hills probably Noachian aged, post-dating formation of Gusev Crater, but pre-dating
plains formation
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No Columbia Hills rocks (with possible exception of Peace class) appear to be lacustrine
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Columbia Hills probably formed as a result of uplift related to an impact event
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Mission ended on May 25, 2011
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Presence of coarsely crystalline hematite on Meridiani plains implied action of water
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South of landing site, the boundary is sharp. The layered unit cuts across pre-existing craters
and valleys
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In other regions, the boundary is poorly defined, with complex patterns of ridges, mesas,
and hills that have an “etched” appearance
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Landing site was the upper surface of a 600 m thick section of layered rocks that lie
uncomformably on a cratered Noachian surface
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Pedestal craters, outward-facing cliffs, mounds of sediment within craters, and etched
remnants of the layered sequence suggest formation was once much more extensive
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Layered sequence extends for at least 800 km in both the E-W and N-S directions
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Where layered sequence is eroded to form etched terrain, hematite is not detected
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From orbit, hematite only present on the top surface
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Deposits do not lie in a clearly defined basin as might be expected if they were deposited in a large
body of water
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Surface has a two-toned appearance -- some regions are a mottled mixture of light and dark,
others are mostly dark with the exception of bright rings, which correspond to rims of
craters
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Bright areas correspond to regions where a bright underlying unit pokes up through the dark
surface unit
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Near landing site, the sequence is minimally eroded, providing a relatively smooth level surface
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Age of layered unit is difficult to determine due to effects of infilling and erosion on crater counts
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Implies small craters have been preferentially destroyed
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Based on number of large craters, it was suggested that upper surface is late Noachian to
early Hesperian in age
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Number of small craters is much smaller than would be expected based on number of large
craters
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Meridiani Planum -- Geologic Context
Opportunity landed inside Eagle crater, and immediately imaged the first bedrock ever seen on
Mars
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Soils inside Eagle and on the plains are dark, mainly composed of fine-grained (50-150µm) basaltic
sands and spherules 4-5mm across that are the main source of hematite
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Fe-sulfate jarosite, indicates an aqueous environment with a pH <5
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Outcrop is a sandstone composed of silicic clastic debris (plagioclase and amorphous silica) and an
evaporitic component (Ca, Mg, and Fe-bearing sulfates)
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After spending almost three months in Eagle crater, Opportunity moved across the plains, briefly
stopping at Fram crater before exploring Endurance crater in detail
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Spherules are most likely concretions which formed when fluids dissolved Fe-rich material in
the rocks and reprecipitated hematite or a precursor mineral
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About 1m of outcrop needs to have eroded away to produce the density of spherules
seen on the surface
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Hematite forms a lag on the surface
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Examination of outcrop in Endurance Crater indicates that hematite spherules are eroding out of
sandstone
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Meridiani Planum -- Rocks and Soils
GEO 330 Page 2
Document Summary
O(cid:374)ly fou(cid:374)d as (cid:862)float(cid:863) -- no outcrops observed. Lower fe and cr and higher al, ti, and p compared to adirondack and clovis class rocks. Dominant primary minerals are plag, pyx, olv, with minor ilmenite and apatite. Mi images show poorly sorted angular clasts in a fine matrix. Only two examples of peace- class rocks have been found. If layering is not present, they often have a spongy texture. Silicate portion only mildly weathered, but salts are abundant as cements. Only found in part of columbia hills called cumberland ridge. Rocks are variable in composition and texture, but have high ti, p, s, cl, and br and low cr. Variability in chemistry can be explained by mixing peace and wishstone class rocks at varying ratios. Interpreted as fine-grained ejecta from an impact into a mixture of peace and wishstone rocks. Rocks are isochemically altered, mostly as a result of oxidation.
