**1. Introduction**

### **1.1 Mars planet: environment and geological history**

As it is known, Mars is the fourth and the last of the inner and rocky planets of our Solar system. Beyond the "Red Planet", it extends the belt of asteroids just before the giant Jupiter and the other gaseous external planets of the system. It has a mass: 6.417 x 1023 Kg, a density of 3,940 g/cm3 , equatorial diameter of 6,792 Km and a mean temperature of −63°C, a distance from Sol of 228,000,000 km.

Mars orbits the Sun at an average distance of 230 million km and its revolution period is about 687 days; while his solar day is a little longer than ours: 24 hours, 37 minutes and 23 seconds. The Martian axial inclination is 25.19°, which is similar to that of Earth. Due to the discrete eccentricity of its orbit of 0.093, its distance from Earth to opposition can range between about 100 and about 56 million kilometers; only Mercury has a higher eccentricity in the Solar System. However, Mars used to follow a much more circular orbit: about 1.35 million years ago its eccentricity was equivalent to 0.002, which is much lower than the current Earth's. Mars has an eccentricity cycle of 96,000 Earth years compared to Earth's 100,000. Over the past 35,000 years, Martian orbit has become increasingly eccentric due to the gravitational influences of other planets, and the closest point between Earth and Mars will continue to decline over the future time.

The planet is enveloped by a thin atmosphere dominated by the gas carbon dioxide as the 95.3% of the whole. The other chemical elements with their respective rates are: nitrogen (2.7%), Argon (1.6%) and oxygen–carbon monoxide-water steam (0.4%). The weakness of the Martian atmosphere and the lack of a magnetic field do not allow any effective defence against ultraviolet radiations and solar winds.

The environmental and geological history of early Mars is mostly written in its rocks, in their composition and morphological/structural signatures. The first few billion years of Mars' geologic history records surface environments considerably different than the surface today, prompting a succession of coordinated surface and orbiter missions over the past two decades aimed ultimately at determining if Mars ever had an early biosphere. Water is the sine qua non for all life as we understand it and, therefore, past missions have sought environments where water was abundant and possibly long-lived [1, 2].

Mars was formed 4.6 billion years ago, with a history similar to the other three Terrestrial-type planets, i.e. as a result of the condensation of the solar nebula, most probably silicates. Due to the upper distance from the Sun from Earth, during the initial phase of formation in Mars' orbit there was a higher concentration of elements with low boiling points, such as chlorine, phosphorus and sulfur, probably driven away from the inner orbits by the strong solar wind of the young Sun [3].

During the first Martian Era, known as Noachian period, 4.1–3.7 Gys ago, its environment, that had formed, was moderately similar to the one on present Earth. Liquid water was widespread in a neutral environment, volcanic activity and heat flow more vigorous, and atmospheric pressure and temperature were higher than today. Morphological evidences are represented by river delta and river meanders, drainage networks and lakes; such morphologies are accompanied by the occurrence of consistent sedimentary, layered deposits. These conditions may have favoured the spread of life on the surface of Mars [4]. In this period the planet was subject to intense late bombardment, to which Earth was also a victim. In fact, about 60% of the surface has markers dating back to that era, particularly impact craters. The largest of these is located in the northern hemisphere and has a diameter of about 10,000 km, almost half the circumference of the planet. The formation of this crater is probably due to the impact with a big asteroid, which left a deep depression (the Boreal Basin), covering about 40% of the planet, brutally changing the history and environment of planet [5]. Eloquents morphological rest of such big impacts are extensive water flow formations in the Tharsis region; a region subject, towards the end of the era to a very active volcanism and flooded, by a large amount of water.

Slowly, in just over a billion and a half years, Mars went from a warm and humid phase characteristic of the Noachian to that of a cold and on-surface arid planet observable today. This transitional phase occurred during the Hesperian; a period characterized by continuation of intense volcanic activity (like those of Olympus Mons), deposition of evaporitic sedimentary sequences, and catastrophic floods that dug immense canals along the surface [6]. The continuous eruptions brought large amounts of sulphur dioxide and hydrogen sulfide to the surface, changing the large expanses of liquid water into small basins of high acidity water due to the sulphuric acid that formed. Although the disappearance of rivers and lakes is generally considered attributable towards the end of this era, recent dating on Gale Crater outcroppings open up the existence of water lake about billion years ago, during the Amazonian era [7].

One era, this last, from about 3 billion years ago to today, that is characterized by a poor period of meteor bombardment and by a continuation of cold and arid

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**Figure 1.**

*Pictures show the landing site of NASA Curiosity rover, at GaleCrater, located near the Martian Equator; a site where intertidal and lacustrine clayey deposit have been found, as well as organic molecules and microstructures resembling microbialtes and terrestrial algae have been found. Below: a geological sketching of* 

*the ancient lake deposits (from NASA reports, modified).*

*Life on Mars: Clues, Evidence or Proof? DOI: http://dx.doi.org/10.5772/intechopen.95531*

#### **Figure 1.**

*Pictures show the landing site of NASA Curiosity rover, at GaleCrater, located near the Martian Equator; a site where intertidal and lacustrine clayey deposit have been found, as well as organic molecules and microstructures resembling microbialtes and terrestrial algae have been found. Below: a geological sketching of the ancient lake deposits (from NASA reports, modified).*

climatic conditions, until today. Conditions producing surface aridity and deepening of water tables, whose existence has been proven by recent space missions.

In fact, the Martian poles are covered with water ice and permafrost layer extends to latitudes of about 60°, and large amounts of water are believed to be trapped under the thick Martian cryosphere [8]. The formation of Valles Marineris and its spill channels and sink holes, shows that during the early stages of Mars' history there was a large amount of liquid water. The presence of large amount of ground water and ice in the south pole of Mars was confirmed by the European Mars Express probe in January 2004 and by MARSIS radar near the Chryse Planitia region. Radar analyses conducted from 2012 to 2015 by Mars Express revealed presence of liquid salt water under the southern ice cap. In 2015, based on the MRO's monitoring, NASA announced evidence that liquid salt water flows on the surface of Mars in the form of small streams [9].

Recent data of spatial missions and in particular of NASA rovers instrumentation proved on Gale crater the existence of lacustrine and intertidal deltas deposits (**Figure 1**), presence of all the life elements, occurrence of complex organic molecules, sedimentary structures similar to terrestrial microbialites and the existence of an environment favorable to microbial life [1, 10, 11].

Stromatolite-like structures were also found by NASA rovers Opportunity, Spirit and Curiosity on the laminated Martian outcroppings of Meridiani Planum [12–18]. These findings have given strength to the hypothesis of stromatolites presence [19], that could be probably quite widespread from the Noachian to the Hesperian geological era of Martian life.
