Geology and Geochemistry of Noachian Bedrock and Alteration Events, Meridiani Planum, Mars: MER Opportunity Observations. Issue 9 (17th September 2021)

• Record Type: Journal Article
• Authors: Mittlefehldt, David W.; Gellert, Ralf; vanBommel, Scott; Arvidson, Raymond E.; Ashley, James W.; Clark, Benton C.; Crumpler, Larry S.; Farrand, William H.; Golombek, Matthew P.; Grant, John A.; Morris, Richard V.; Schröder, Christian
• Is Part Of: Journal of geophysical research. Volume 126:Issue 9(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-09-17
• Subjects: Endeavour crater -- Mars Exploration Rover mission -- Mars geochemistry -- Mars geology -- Noachian crust; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2021JE006915
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: We have used Mars Exploration Rover Opportunity data to investigate the origin and alteration of lithic types along the western rim of Noachian‐aged Endeavour crater on Meridiani Planum. Two geologic units are identified along the rim: the Shoemaker and Matijevic formations. The Shoemaker formation consists of two types of polymict impact breccia: clast‐rich with coarser clasts in upper units; clast‐poor with smaller clasts in lower units. Comparisons with terrestrial craters show that the lower units represent more distal ejecta from at least two earlier impacts, and the upper units are proximal ejecta from Endeavour crater. Both are mixtures of target rocks of basaltic composition with subtle compositional variations caused by differences in post‐impact alteration. The Matijevic formation and lower Shoemaker units represent pre‐Endeavour geology, which we equate with the regional Noachian subdued cratered unit. An alteration style unique to these rocks is formation of smectite and Si‐ and Al‐rich vein‐like structures crosscutting outcrops. Post‐Endeavour alteration is dominated by sulfate formation. Rim‐crossing fracture zones include regions of alteration that produced Mg‐sulfates as a dominant phase, plausibly closely associated in time with the Endeavour impact. Calcium‐sulfate vein formation occurred over extended time, including before the Endeavour impact and after the Endeavour rim had been substantially degraded, likely after deposition of the Burns formation that surrounds and embays the rim. Differences in Mg, Ca and Cl concentrations on rock surfaces and interiors indicate that mobilization of salts by transient water has occurred recently and may be ongoing. Plain Language Summary: Data returned by the Mars Exploration Rover Opportunity was used to investigate rock origins along the western rim of Endeavour crater on Meridiani Planum, Mars. The Shoemaker formation consists of impact‐formed breccia of two types: coarser‐grained upper subunits and finer‐grained lower subunits. The lower units represent ejecta from at least two older, more distant craters, while the upper units are ejecta from Endeavour crater. Subtle compositional differences are caused by differences in post‐impact alteration along the crater rim. The lower Shoemaker units represent part of the pre‐Endeavour rock sequence. An alteration style unique to these rocks is formation of Si‐ and Al‐rich structures crosscutting bedrock. Post‐Endeavour alteration is dominated by sulfate formation. Fracture zones in the rim include regions of alteration that produced Mg‐sulfates as a dominant phase, plausibly closely associated in time with the Endeavour impact. Calcium‐sulfate vein formation occurred over extended time, some before the Endeavour impact and some much later, likely after deposition of the sulfate‐rich sandstones of Meridiani Planum. Differences in composition of rock surfaces and interiors indicate that mobilization of salts by transient water has occurred recently and may be ongoing on Mars. Key Points: Clast‐poor impact breccias on Endeavour crater rim are distal ejecta from at least two earlier impacts Enrichments in Si and Al, and smectite formation are unique to pre‐Endeavour rocks; post‐Endeavour alteration formed Ca‐ and Mg‐sulfates Magnesium, Ca and Cl have been mobilized by transient thin films of water acting on salts very recently, and could be ongoing