Dating of the great Martian periods

The Martian geological time scale is essentially based on the quantitative comparison of the craterization of the different types of Martian terrain, between them on the one hand, and on the other hand with what we know of other planets (among others, the Moon and Mercury).

The dating of geological events is a still widely debated question, to which no definitive answer has yet been given. In general, we admit that the higher the density of impact craters, the older the surface of a planet. Studies on the density and morphology of craters on the surface of Mars, which vary greatly from one region to another, have therefore made it possible to characterize three distinct eras named after representative regions. Source : Wikipedia.

1 Noachian Period

The Noachian (named after Noachis Terra) corresponds to the oldest terrains since the formation of the planet, going back more than 3.7 billion years according to the scale (see below) of Hartmann & Neukum (but of 3.5 billion years on the standard Hartmann scale), strongly cratered and located mainly in the southern hemisphere. Mars probably had a thick atmosphere at that time, the pressure and greenhouse effect of which certainly made possible the existence of large quantities of liquid water. The end of this period would have been marked by the great late bombardment, dated around 4.1 to 3.8énbsp;billion years ago, as well as by the start of intense volcanic activity, especially in the region of present day bulge of Tharsis.
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2 Hesperian Period

The Hesperian (named after Hesperia Planum) corresponds to the land from 3.7 to 3.2 billion years according to the scale of Hartmann & Neukum (but from 3.5 to 1.8 billion years in the standard Hartmann scale), marked by the volcanic activity of the planet. The global magnetic field would have disappeared, allowing the solar wind to erode the atmosphere, whose temperature and pressure on the ground would have started to drop significantly, so that the liquid water would have ceased to exist permanently on the surface of Mars.
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3 Amazonian Period

The Amazonian (named after Amazonis Planitia) corresponds to land less than 3.2 billion years old according to the Hartmann & Neukum scale (but only 1.8 billion years old in the standard Hartmann scale), very little cratered and located mainly in the northern hemisphere, at an altitude below the Martian reference level. It is a time quite poor in geological events, marked by extreme aridity and dominated by several volcanic episodes, less intense than that, major, of the Hesperian, but occurring until very recent dates, perhaps there is only a few tens of millions of years old in some cases.
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Several time scales ...

The first reference geological time scale is the work of the American astronomer William Hartmann and dates back to the 1970s. Called the standard Hartmann scale, it is based on the density and morphology of craters. Widely used in particular among Anglo-Saxons and in the literature prior to the mid-2000s, it nevertheless tends to be replaced.

The German planetologist Gerhard Neukum had developed in parallel a dating system backing in time the milestones of Hartmann and which found in the data of the European probe Mars Express, sent by ESA (European Space Agency) on June 2, 2003, supporting a much longer chronology generally known as the Hartmann & Neukum scale. It is the repository in which the current studies relating to the evolution of the planet Mars, in particular the ESA, tend to be inscribed.

This second scale is more in line with the stratigraphic system proposed in particular by the team of the French astrophysicist Jean-Pierre Bibring of the IAS (Institute of Space Astrophysics) in Orsay from information collected by the OMEGA instrument of the European Mars Express probe, introducing the term "Phyllosian" to define the first era, also called eon, from areas dominated by phyllosilicates.

Chronostratigraphy (study of the age of rock layers) creates the Phyllosian corresponds to eras before 4.2 billion years, with an additional adjustment to the definition of Martian geological eras. The discontinuity between Phyllosian and Theiikian would materialize a catastrophic transition between these two eras underlined by the concept of LHB (Late Heavy Bombardment), which would have hit the solar system between 4.1 and 3.8 billion of years estimated from lunar samples and studies based on the surface of the planet Mercury. Since Mars is both closer than Earth to the asteroid belt and ten times less massive than our planet, these impacts would have been more frequent and more catastrophic on the red planet.

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