Establishing a geological chronology on Mars today is very difficult, if not complicated. In fact, unlike with the Earth, we cannot establish an absolute chronology, that is, a numerical one based on radioactive dating methods, since for this we would have to have samples of its surface, and except for some meteorites, we have no possibility , at least for now, to date its surface in this way.

Instead, we have established two relative chronologies, one, on the one hand, based on the density of craters on its surface, and a mineralogical one that tries to assign an age to the mineralogy that is observed on the surface of the planet. They only serve to give us a rough idea of the relative ages of the rocks we observe. Which comes after which other, but little more.

The geological time scale based on the density of the number of craters is based solely and exclusively on counting the number of craters that there are per unit area, and depending on this number, assigning an age. As the number of meteorite impacts per unit of time has been decreasing over time, different models have been created that allow us to associate a density of craters with a given period.

Well, this scale is subdivided into three large periods (or four, depending on whether we divide the first into two) that have the name of large relief forms on Mars, so that they are easily associable: Noachian (by Noachis Terra), Hesperian (by Hesperia Planum) and Amazonian (by Amazonis Planitia).

• Pre-Noachian: It begins 4500 million years ago with the formation of the planet and its internal differentiation (Differentiation is the process by which the different constituent elements of a planet are separated as a consequence of their physical and chemical characteristics, creating the different inner layers of the planet, in which the denser materials sink towards the center, while the lighter ones rise to the surface).

Another remarkable fact of this period is that it was surely when the famous dichotomy was formed. This dichotomy consists of the sharp contrast observed between the elevation of the northern hemisphere, which is lower and topographically smooth, and an elevated, more rugged and covered with craters southern hemisphere. The origin of this dichotomy is still highly debated today, and it is possible that it was created by a large impact, by the coalescence of large impact basins, and even by the redistribution of crustal material from the mantle due to the movement of this

Large impact basins such as Argyre, Isdis and Hellas are thought to have formed during this period, and in fact the formation of Hellas would mark the end of this period 4.1 billion years ago.

• Noachian: It would begin 4100 million years ago with the formation of the oldest existing terrain on Mars and that is now completely covered by impact craters, some of them very large. It is thought that the deformation that caused the bulging of the Tharsis volcanic region may have occurred during this period, either when hot material from the mantle rose and actively deformed the overlying crust, or when it is just a mass of cold igneous material that It is supported by the underlying lithosphere.

It is also believed that the greatest erosion processes produced by liquid water on the surface, drainage networks, and the possible existence of lakes or oceans occurred at this time. Valles Marineris, perhaps one of the best-known forms of Mars, began to form during the Noachian, and probably became inactive at the end of the Hesperian.

• Hesperian: It begins 3700 million years ago and is marked by the formation of large lava plains. Possibly, the formation of Olympus Mons began in this period.

There were also large upwellings of water that carved out the drainage channels of Chryse Planitia and many other places. In this period there could have been ephemeral lakes or seas in the depressed areas of the Northern Hemisphere.

• Amazonian: It is the last period. It began 3 billion years ago and continues to the present day. Few impact craters are visible on surfaces of this age, and there are still lava flows, glacial activity, and small springs of water. The beginning of this period is highly disputed, since it could even have its base 1500 million years ago, since the Hesperian period is a moment of transition between the end of the Late Heavy Bombardment and the climatic evolution of Mars. to what it is today, cold and dry.

The other scale that we are going to discuss is the one based on the mineralogy of the alteration of rocks that is observed on the surface of Mars due to the different styles of chemical weathering of the surface rocks. This scale was proposed in 2006 based on data from the OMEGA spectrometer that travels aboard Mars Express. Like the first scale, it also has three distinguishable periods:

• Philocian: It is so called because the phyllosilicates or minerals from the clay group are characteristic of this period. There are many outcrops of phyllosilicates on Mars, but all in Noachian age rocks. It is the time of the formation of the drainage networks that require for their formation, like the phyllosilicates, abundant water on the surface.
• Theeikinse: It is named for the appearance of sulfates on the surface (and from the Greek sulfurized). It was a period of great volcanism, which released large amounts of sulfur dioxide into the atmosphere, and which, combined with the water, created an environment rich in sulfuric acid that allowed the formation of hydrated sulfates such as kieserite and gypsum.

• Siderikiense: It is the time of formation of iron oxides, and which reaches our present day. With the cessation of volcanism and the disappearance of surface water, the most important chemical weathering has been the slow oxidation of iron-rich rocks by peroxides found in the atmosphere, which have produced red-colored iron oxides that they give Mars that characteristic color.