Crater density used relative dating

The Mariner 9, Viking orbiter, and Mars Global Surveyor spacecraft have provided a wealth of data from which the types and abundances of Mars’s geological units can be surveyed, their relative stratigraphic ages derived, and their absolute ages estimated. These three time-stratigraphic systems correspond to three epochs, major time periods having durations and absolute ages that have been estimated from model crater production curves (see the next subsection, “Cratering Chronology”). Swindle, “Applying Noble-gas Geochronology Techniques In Situ on Planets and Asteroids,”Eleventh Annual Goldschmidt Conference, Abstract #3718 (CD-ROM), 2001. The planet’s various geological units are distinguished and characterized on the basis of their morphologic, topographic, and spectral properties. Data from the Mars Global Surveyor (MGS) spacecraft provide rich insight into martian stratigraphic relationships and the timing of major geological events.

The uncertainty in the martian crater production function implies that late Hesperian through mid-Amazonian ages are the most poorly characterized.

The number of observable martian craters is affected by geological processes of erosion and deposition, with the preservation time of a given crater being dependent on its size and the geological processes that have acted to modify it.

Thus, understanding the stratigraphy of Mars is of high priority, as is the ability to date surface units. The most recent system is the Amazonian, represented by the plains and volcanic materials of Amazonis Planitia. Swindle, “In Situ Noble-gas Based Chronology on Mars,”pp.

The absolute ages of surface units will remain necessarily uncertain until samples from known surface locations have been dated in situ and/or on Earth. Volcanic materials of the Elysium and Tharsis Montes volcanic regions are Amazonian in age.

The SNC meteorites (discussed in Chapters 1 and 3) have contributed useful constraints on surface ages, though their exact provenance on Mars is unknown.

Their radiometric ages indicate that some near-surface rocks are as old as 4.5 Gyr and that martian volcanic activity occurred as recently as ~175 Myr ago. Their relative ages are determined through examination of their crosscutting and superpositional relationships and the number of superposed impact craters. Many specific stratigraphic issues have been addressed and new questions have arisen, notably regarding the presence, timing, and extent of liquid water on the surface. Analyses of this sort have allowed confident derivation of a stratigraphic column for Mars and corresponding chronological ordering of the major geological events in the planet’s history (see Figure 4.1). Moreover, MGS images have revealed thick layered sequences, within Valles Marineris and surrounding troughs and elsewhere across the cratered highlands, which may be volcanic and/or sedimentary in origin. : Some Insights into the Timing of Major Events from Mars Global Surveyor Data,”Abst. The figure is subject to amendment; it shows Tharsis volcanism to have occurred in the Hesperian epoch, but recent Mars Global Surveyor data have indicated that the Tharsis complex of volcanoes was formed in the upper Noachian epoch. Carr, “Geological Processes and Evolution,” ages and crater densities has not been established with confidence. exhumation is found in MGS images, the effect on cratering chronology has not yet been studied in detail.

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