**2. The search of life on Mars**

The search for life on Mars, either in the present or in the past history of the "Red Planet", has been the main motivation behind research programs since the 1970s. The first images, highlighting the evidence for past liquid water on Mars, were carried out by orbital images from Mariner 9 [20]. Then, Mars Observer Camera and Mars Reconnaissance Orbiter [21] provided new images of past fluvial networks on its surface, long time ago.

In fact, the claim of this question stems from the early debates that arose from the Vikings' LR experiments until their recent re-analysis [84]. We can also recall as in 2004, the Phoenix space probe showed the existence of an ice-cemented ground in the northern plains of Mars combined with the presence of perchlorates normally used in terrestrial metabolic pathways of a large number of microorganisms [22–24]. So, perchlorates, found in the ground ice of Mars, is a putative biosignature resulting by a possible microbial activity on Mars [25] and could suggest a chemosynthetic activity carried out by bacteria on the planet surface, a short time ago [26].

Our knowledge about this topic has been increased considerably as a result of recent NASA missions, Opportunity, Spirit and Curiosity, located at Meridiani Planum, Gusev and Gale Craters, respectively. The Curiosity rover landed inside the Aeolis Crater, informally known as Gale Crater, on August 2012 with a complex set of scientific instruments (MSL), able to detect chemical and mineralogical soil composition, environmental data, and record panoramic and microscopic images of high accuracy, obtaining subsequently several thousands of images. Hence, the MSL scientists discovered a fluvial-lacustrine sequence and fine grained sedimentary rocks containing clay and hydrated minerals, deposited inside an almost neutral lake. These deposits were, then, subjected to two post-genetic more acidic phases, revealing that they contain the elements necessary for life, e.g., H, O, S, C, N, P,

**101**

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

and also including Fe, Mg and Mn suitable to support a possible Martian biosphere based on chemolithoautotrophy [1]. In addition, life, if it existed, must have left visible traces of its activity and presence in the sediments, i.e., the rocks, that are now photographed by rovers. Furthermore, microbes and microalgae are the first step on the evolutionary scale of life on Earth and stromatolites are the oldest evidence for them, stretching back at least 3.5 billion years. Hence, these are the structures that

The presence of extraterrestrial microorganisms and, in particular, of cyanobac-

teria, well known as the main builders of terrestrial stromatolites, has been suggested by many authors beginning from the famous discovery of Martian meteorite ALH84001 [27]. This biological approach was further confirmed by some intriguing images of the Martian surface sent by rover Opportunity on 2010, showing a set of rocks partially covered by a dark shiny patina, close to the terrestrial "Desert Varnish" probably formed by the same bacteria that built stromatolites on the Earth [28]. The latest finding is the discovery in CL1 carbonaceous Martian chondrite of some microfossils very close to terrestrial cyanobacteria [29]. All these hypotheses were strengthened by some studies about extremophiles as desert cyanobacteria of the genus Chroccocidiopsis living in extremely hot and cold deserts that sheds a new light on the possible history of Martian microbial life [30, 31]. In particular, it has been pointed out the important role of Cyanobacteria in the formation of organosedimentary rocks as one of the most successful and widespread forms of life on Earth [32] because of their great morphological variability and their bio-stabilization capacity on sun-light exposed sedimentary surfaces, but also in environments characterized by extremely low energy light. In this way, the recent discovery of a new photopigment, found within terrestrial stromatolites and named Chlorophyll f, that can absorb light of even lower photon energy until 720 nm [33], suggests that cyanobacteria could alive also in extreme environments as on Mars. Generally, these microorganisms occupy a very broad range of environments including waters of widely different chemistry and composition so that their involvement in sedimentation processes is equally varied [34]. Cyanobacteria, including more than 2000 species [35], as composite microbial associations, dominate microbial mats and are

ubiquitous, leaving successful records in sediments and sedimentary rocks. Morphological study of images reveals evidence of widespread occurrence of micro, meso, and macro structures recalling for some authors early terrestrial forms of life; such as the "blueberries", concretions possibly induced by chemolithoautotrophic bacteria [10, 36, 37]. These strange and complex structures, for which abiological explanation it's hard to find, have strong morphological parallels with terrestrial microbialites/stromatolites [14–16, 18], a conclusion that seems to be supported by morphometric approaches [12, 13]. Other possible biogenic structures have been observed on Mars, recalling those of terrestrial silica deposits in hydrothermal environments [38, 39] or typical structures, known as Microbially Induced Sedimentary Structures (MISS) and generated by microbial mats of intertidal environments [14]. Despite the many observations, mutually supporting a possible microbialite hypothesis, they do not prove the presence of fossil life on Mars, because biologic explanations for their terrestrial counterparts and for their contained microbial structures are often deeply controversial. In fact, such organosedimentary structures are sediments, and despite having unusual features at meso and macroscales, somewhere contain controversial microbial remnants of micrometric dimensions; while complex and larger structures, as are evolved and more evident microfossils (generally larger than 0.1 mm) or macrofossils (centimetric) are generally more obvious and indisputable. Finally, in the Martian atmosphere it has been detected traces of methane and formaldehyde, changing seasonally and

supporting evidences on the potential habitability of Mars [40].

may be present if life ever existed on Mars comparable to Earth [19].

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

and also including Fe, Mg and Mn suitable to support a possible Martian biosphere based on chemolithoautotrophy [1]. In addition, life, if it existed, must have left visible traces of its activity and presence in the sediments, i.e., the rocks, that are now photographed by rovers. Furthermore, microbes and microalgae are the first step on the evolutionary scale of life on Earth and stromatolites are the oldest evidence for them, stretching back at least 3.5 billion years. Hence, these are the structures that may be present if life ever existed on Mars comparable to Earth [19].

The presence of extraterrestrial microorganisms and, in particular, of cyanobacteria, well known as the main builders of terrestrial stromatolites, has been suggested by many authors beginning from the famous discovery of Martian meteorite ALH84001 [27]. This biological approach was further confirmed by some intriguing images of the Martian surface sent by rover Opportunity on 2010, showing a set of rocks partially covered by a dark shiny patina, close to the terrestrial "Desert Varnish" probably formed by the same bacteria that built stromatolites on the Earth [28]. The latest finding is the discovery in CL1 carbonaceous Martian chondrite of some microfossils very close to terrestrial cyanobacteria [29]. All these hypotheses were strengthened by some studies about extremophiles as desert cyanobacteria of the genus Chroccocidiopsis living in extremely hot and cold deserts that sheds a new light on the possible history of Martian microbial life [30, 31]. In particular, it has been pointed out the important role of Cyanobacteria in the formation of organosedimentary rocks as one of the most successful and widespread forms of life on Earth [32] because of their great morphological variability and their bio-stabilization capacity on sun-light exposed sedimentary surfaces, but also in environments characterized by extremely low energy light. In this way, the recent discovery of a new photopigment, found within terrestrial stromatolites and named Chlorophyll f, that can absorb light of even lower photon energy until 720 nm [33], suggests that cyanobacteria could alive also in extreme environments as on Mars. Generally, these microorganisms occupy a very broad range of environments including waters of widely different chemistry and composition so that their involvement in sedimentation processes is equally varied [34]. Cyanobacteria, including more than 2000 species [35], as composite microbial associations, dominate microbial mats and are ubiquitous, leaving successful records in sediments and sedimentary rocks.

Morphological study of images reveals evidence of widespread occurrence of micro, meso, and macro structures recalling for some authors early terrestrial forms of life; such as the "blueberries", concretions possibly induced by chemolithoautotrophic bacteria [10, 36, 37]. These strange and complex structures, for which abiological explanation it's hard to find, have strong morphological parallels with terrestrial microbialites/stromatolites [14–16, 18], a conclusion that seems to be supported by morphometric approaches [12, 13]. Other possible biogenic structures have been observed on Mars, recalling those of terrestrial silica deposits in hydrothermal environments [38, 39] or typical structures, known as Microbially Induced Sedimentary Structures (MISS) and generated by microbial mats of intertidal environments [14]. Despite the many observations, mutually supporting a possible microbialite hypothesis, they do not prove the presence of fossil life on Mars, because biologic explanations for their terrestrial counterparts and for their contained microbial structures are often deeply controversial. In fact, such organosedimentary structures are sediments, and despite having unusual features at meso and macroscales, somewhere contain controversial microbial remnants of micrometric dimensions; while complex and larger structures, as are evolved and more evident microfossils (generally larger than 0.1 mm) or macrofossils (centimetric) are generally more obvious and indisputable. Finally, in the Martian atmosphere it has been detected traces of methane and formaldehyde, changing seasonally and supporting evidences on the potential habitability of Mars [40].
