TY - CHAP
T1 - Layered sediments on Mars deposited by impacts instead of by liquid water
AU - Burt, Donald M.
N1 - Funding Information:
I herewith freely acknowledge numerous major contributions to the above observations and ideas by my former coauthors, colleagues, and mentors, including the late R.S. Dietz, M.F. Sheridan, K.H. Wohletz, and especially L.P. Knauth. They provided inspiration and invaluable instruction and insights that were extensively used in arriving at unconventional interpretations. I am also grateful to the late N.G. Barlow and other organizers of and participants in the Planetary Cratering Consortium meetings, which have provided me with a critical sounding board for some of these ideas. In addition, I am grateful to the imaging enthusiasts at the website unmannedspaceflight.com, who have kept me updated on Mars rover imagery from the National Aeronautics and Space Administration (NASA; not reproduced here owing to space constraints, but widely available in NASA press releases, at various websites, and in Mars Exploration Rover and Mars Science Laboratory publications), and of course to NASA itself, for making the rover imagery and information so freely available. Finally, I acknowledge useful reviews by R. Sulpizio and P. Claeys.
Publisher Copyright:
© 2022 The Geological Society of America.
PY - 2022/5/3
Y1 - 2022/5/3
N2 - Layered deposits on Mars imaged by the three rovers are generally inferred to have been deposited by liquid water (or wind or volcanism), consistent with interpretations based on orbital imaging. This interpretation implies early Mars was warm and wet, despite long-standing problems with modeling this case. As an alternative hypothesis, rapid sediment deposition during Late Noachian impact bombardment followed by local hydration and alteration of sediment by surficial acid condensates and (at least in Gale Crater) by chemically neutral groundwater can explain all the observed sediment features, such as ubiquitous low-angle cross-bedding, primitive basaltic compositions, persistent acidic salts, abundant amorphous materials, immature clays, high friability with low bulk densities, planar scoured unconformities, and rounded cobbles from rock tumbling. In other words, the ground-observed mineralogy, geochemistry, and sedimentology do not require and even are inconsistent with deposition from liquid water. Unlike the Moon, early Mars is believed to have had an atmosphere and water, perhaps mostly frozen. If so, impacts should have formed turbulent ground-hugging impactoclastic density currents capable of traveling hundreds of kilometers, and even globally. As terrestrial analogs, smaller-scale density currents are widespread around explosive volcanoes and nuclear test sites, whereas terrestrial impact analogs are lacking. Steam condensation on particles causes accretionary lapilli to form, grow to a maximum size, and fall out on layered deposits, and similar spherules have been observed by two of three rovers. Explaining these spherules as normal sedimentary concretions at Meridiani Planum required ignoring some of the observations. Ancient sediments on Mars that superficially resemble terrestrial aqueous deposits could therefore actually have resulted from impact cratering, the dominant geologic process in the early solar system.
AB - Layered deposits on Mars imaged by the three rovers are generally inferred to have been deposited by liquid water (or wind or volcanism), consistent with interpretations based on orbital imaging. This interpretation implies early Mars was warm and wet, despite long-standing problems with modeling this case. As an alternative hypothesis, rapid sediment deposition during Late Noachian impact bombardment followed by local hydration and alteration of sediment by surficial acid condensates and (at least in Gale Crater) by chemically neutral groundwater can explain all the observed sediment features, such as ubiquitous low-angle cross-bedding, primitive basaltic compositions, persistent acidic salts, abundant amorphous materials, immature clays, high friability with low bulk densities, planar scoured unconformities, and rounded cobbles from rock tumbling. In other words, the ground-observed mineralogy, geochemistry, and sedimentology do not require and even are inconsistent with deposition from liquid water. Unlike the Moon, early Mars is believed to have had an atmosphere and water, perhaps mostly frozen. If so, impacts should have formed turbulent ground-hugging impactoclastic density currents capable of traveling hundreds of kilometers, and even globally. As terrestrial analogs, smaller-scale density currents are widespread around explosive volcanoes and nuclear test sites, whereas terrestrial impact analogs are lacking. Steam condensation on particles causes accretionary lapilli to form, grow to a maximum size, and fall out on layered deposits, and similar spherules have been observed by two of three rovers. Explaining these spherules as normal sedimentary concretions at Meridiani Planum required ignoring some of the observations. Ancient sediments on Mars that superficially resemble terrestrial aqueous deposits could therefore actually have resulted from impact cratering, the dominant geologic process in the early solar system.
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U2 - 10.1130/2021.2553(27)
DO - 10.1130/2021.2553(27)
M3 - Chapter
AN - SCOPUS:85143853335
T3 - Special Paper of the Geological Society of America
SP - 347
EP - 354
BT - Special Paper of the Geological Society of America
A2 - Foulger, G.R.
A2 - Hamilton, L.C.
A2 - Jurdy, D.M.
A2 - Stein, C.A.
A2 - Howard, K.A.
A2 - Stein, S.
PB - Geological Society of America
ER -