Geological history of Mars
Much of a planet's history can be deciphered by looking at its surface and asking what came first and what came next. For example, a lava flow that spreads out and fills a large impact crater is clearly younger than the crater, and a small crater on top of the same lava flow is younger than both the lava and the larger crater. This principle, called the law of superposition, and other principles of stratigraphy, first formulated by Nicholas Steno in the 17th century, allowed geologists of the 19th century to divide the history of the Earth into the familiar eras of Paleozoic, Mesozoic, and Cenozoic. The same methodology was later applied to the Moon and then to Mars.
Another stratigraphic principle used on planets where impact craters are well preserved is that of crater number density. The number of craters greater than a given size per unit surface area (usually million km2) provides a relative age for that surface. Heavily cratered surfaces are old, and sparsely cratered surfaces are young. Old surfaces have a lot of big craters, and young surfaces have mostly small craters or none at all.
These stratigraphic concepts form the basis for the Martian geologic timescale.
Relative vs. absolute ages 
By using stratigraphic principles, we can usually delineate rock units only in terms of their relative age to each other. For example, knowing that Mesozoic rock strata making up the Cretaceous System lie on top of (and are therefore younger than) rocks of the Jurassic System tells us nothing about how long ago the Cretaceous or Jurassic Periods were. Other methods, such as radiometric dating, are needed to determine absolute ages in geologic time. On Earth, we have this information and know that the Cretaceous Period began around 146 million years ago (Mya) and ended 65 Mya with the extinction of the dinosaurs. Absolute ages are also known for selected rock units of the Moon based on samples returned to Earth.
Assigning absolute ages to rock units on Mars is much more problematic. Numerous attempts have been made over the years to determine an absolute Martian chronology (timeline) by comparing estimated impact cratering rates for Mars to those on the Moon. If we know with precision the rate of impact crater formation on Mars by crater size per unit area over geologic time (the production rate or flux), then crater densities also provide a way to determine absolute ages. Unfortunately, practical difficulties in crater counting and uncertainties in estimating the flux still create huge uncertainties in the ages derived from these methods. Martian meteorites have provided datable samples that are consistent with ages calculated thus far, but the locations on Mars from where the meteorites came (provenance) are unknown, limiting their value as chronostratigraphic tools. Absolute ages determined by crater density should therefore be taken with some skepticism.
Crater density timescale 
Studies of impact crater densities on the Martian surface have delineated three broad periods in the planet's geologic history. The periods were named after places on Mars that have large-scale surface features, such as large craters or widespead lava flows, that date back to these time periods. The absolute ages given here are only approximate. From oldest to youngest, the time periods are:
- Pre-Noachian Represents the interval from the accretion and differentiation of the planet about 4.5 billion years ago (Gya) to the formation of the Hellas impact basin, between 4.1 and 3.8 Gya. Most of the geologic record of this interval has been erased by subsequent erosion and high impact rates. The crustal dichotomy is thought to have formed during this time, along with the Argyre and Isidis basins.
- Noachian Period (named after Noachis Terra): Formation of the oldest extant surfaces of Mars between 4.1 and about 3.7 billion years ago (Gya). Noachian-aged surfaces are scarred by many large impact craters. The Tharsis bulge is thought to have formed during the Noachian, along with extensive erosion by liquid water producing river valley networks. Large lakes or oceans may have been present.
- Hesperian Period (named after Hesperia Planum): 3.7 to approximately 3.0 Gya. Marked by the formation of extensive lava plains. The formation of Olympus Mons probably began during this period. Catastrophic releases of water carved extensive outflow channels around Chryse Planitia and elswhere. Ephemeral lakes or seas formed in the northern lowlands.
- Amazonian Period (named after Amazonis Planitia): 3.0 Gya to present. Amazonian regions have few meteorite impact craters but are otherwise quite varied. Lava flows, glacial/periglacial activity, and minor releases of liquid water continued during this period.
The date of the Hesperian/Amazonian boundary is particularly uncertain and could range anywhere from 3.0 to 1.5 Gya. Basically, the Hesperian is thought of as a transitional period between the end of heavy bombardment and the cold, dry Mars seen today.
Mineral alteration timescale 
In 2006, researchers using data from the OMEGA Visible and Infrared Mineralogical Mapping Spectrometer on board the Mars Express orbiter proposed an alternative Martian timescale based on the predominant type of mineral alteration that occurred on Mars due to different styles of chemical weathering in the planet’s past. They proposed dividing the history of the Mars into three eras: the Phyllocian, Theiikian and Siderikan.
- Phyllocian (named after phyllosilicate or clay minerals that characterize the era) lasted from the formation of the planet until around the Early Noachian (about 4.0 Gya). OMEGA identified outcropping of phyllosilicates at numerous locations on Mars, all in rocks that were exclusively Pre-Noachian or Noachian in age (most notably in rock exposures in Nili Fossae and Mawrth Vallis). Phyllosillicates require a water-rich, alkaline environment to form. The Phyllocian era correlates with the age of valley network formation on Mars, suggesting an early climate that was conducive to the presence of abundant surface water. It is thought that deposits from this era are the best candidates in which to search for evidence of past life on the planet.
- Theiikian (named after sulphurous in Greek, for the sulphate minerals that were formed) lasted until about 3.5 Gya. It was an era of extensive volcanism, which released large amounts of sulphur dioxide (SO2) into the atmosphere. The SO2 combined with water to create a sulphuric acid-rich environment that allowed the formation of hydrated sulphates (notably kieserite and gypsum).
- Siderikan (named for iron in Greek, for the iron oxides that formed) lasted from 3.5 Gya until the present. With the decline of volcanism and available water, the most notable surface weathering process has been the slow oxidation of the iron-rich rocks by atmospheric peroxides producing the red iron oxides that give the planet its familiar colour.
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