The Amazonian is a geologic system and time period on the planet Mars characterized by low rates of meteorite and asteroidimpacts and by cold, hyperarid conditions broadly similar to those on Mars today. The transition from the preceding Hesperian period is somewhat poorly defined. The Amazonian is thought to have begun around 3 billion years ago, although error bars on this date are extremely large. The period is sometimes subdivided into the Early, Middle, and Late Amazonian. The Amazonian continues to the present day.
Description and name origin
The Amazonian System and Period is named after Amazonis Planitia, which has a sparse crater density over a wide area. Such densities are representative of many Amazonian-aged surfaces. The type area of the Amazonian System is in the Amazonis quadrangle around. ImageSize = width:800 height:50 PlotArea = left:15 right:15 bottom:20 top:5 AlignBars = early Period = from:-4500 till:0 TimeAxis = orientation:horizontal ScaleMajor = unit:year increment:500 start:-4500 ScaleMinor = unit:year increment:100 start:-4500 Colors= id:prenoachicol value:rgb id:noachicol value:rgb id:hespericol value:rgb id:amazonicol value:rgb PlotData= align:center textcolor:black fontsize:8 mark: width:25 shift: text:Amazonian from:-3000 till:0 color:amazonicol text:Hesperian from:-3700 till:-3000 color:hespericol text:Noachian from:-4100 till:-3700 color:noachicol text:Pre-Noachian from:start till:-4100 color:prenoachicol
Because it is the youngest of the Martian periods, the chronology of the Amazonian is comparatively well understood through traditional geological laws of superposition coupled to the relative dating technique of crater counting. The scarcity of craters characteristic of the Amazonian also means that unlike the older periods, fine scale surface features are preserved. This enables detailed, process-orientated study of many Amazonian-age surface features of Mars as the necessary details of form of the surface are still visible. Furthermore, the relative youth of this period means that over the past few 100 million years it remains possible to reconstruct the statistics of the orbital mechanics of the Sun, Mars, and Jupiter without the patterns being overwhelmed by chaotic effects, and from this to reconstruct the variation of solar insolation – the amount of heat from the sun – reaching Mars through time. Climatic variations have been shown to occur in cycles not dissimilar in magnitude and duration to terrestrial Milankovich cycles. Together, these features – good preservation, and an understanding of the imposed solar flux – mean that much research on the Amazonian of Mars has focussed on understanding its climate, and the surface processes that respond to the climate. This has included:
System and Period are not interchangeable terms in formal stratigraphic nomenclature, although they are frequently confused in popular literature. A system is an idealized stratigraphic column based on the physical rock record of a type area correlated with rocks sections from many different locations planetwide. A system is bound above and below by strata with distinctly different characteristics that indicate dramatic changes in the dominant fauna or environmental conditions. At any location, rock sections in a given system are apt to contain gaps analogous to missing pages from a book. In some places, rocks from the system are absent entirely due to nondeposition or later erosion. For example, rocks of the Cretaceous System are absent throughout much of the eastern central interior of the United States. However, the time interval of the Cretaceous still occurred there. Thus, a geologic period represents the time interval over which the strata of a system were deposited, including any unknown amounts of time present in gaps. Periods are measured in years, determined by radioactive dating. On Mars, radiometric ages are not available except from Martian meteorites whose provenance and stratigraphic context are unknown. Instead, absolute ages on Mars are determined by impact crater density, which is heavily dependent upon models of crater formation over time. Accordingly, the beginning and end dates for Martian periods are uncertain, especially for the Hesperian/Amazonian boundary, which may be in error by a factor of 2 or 3.
