Advances in the study of life on Mars. A biological point of view

The quest to uncover signs of life on Mars remains a focal point in planetary science. Establishing the existence of past or even current Martian life forms would constitute a profoundly significant discovery, potentially offering insights into the life’s origin on Earth, helping to delineate the evolution of the biosphere, as well to help us answer one of the most basic scientific and philosophical questions “Are we Alone in the Universe?”. During the first Martian Era, known as Noachian period spanning 4.1-3.7 Ga ago, the planet bore some resemblance to Earth, hosting abundant liquid water in a neutral environment, volcanic activity and heat flow vigorous, high atmospheric pressure and temperatures, perhaps not too dissimilar to those of Earth today. This geological epoch is marked by distinct morphological formations, such as river deltas, meanders, drainage networks, and lakes; coupled with notable sedimentary, layered deposits which might have facilitated the proliferation of life on Mars Since the 1970s, the motivation driving research programs has been the exploration of life on Mars, a journey that commenced with the debated Viking experiments (Levin et al, 1977; Glavin et al., 2001; Miller et al., 2002; Levin & Straat, 2016) and furthered by the subsequent re-evaluation of data from the Viking Mars Mission Labeled Release, hinting at the possibility of existing life forms on Mars (Bianciardi et al. 2012). Following the discovery of Martian meteorite ALH84001 and the pivotal publication by McKay D.S. et al. in 1996, a growing body of literature over the past fifteen years has suggested the potential existence of extraterrestrial microorganisms, including cyanobacteria - primary constituents of terrestrial microbialites/stromatolites (Rizzo & Cantasano, 2009; Bianciardi,2014, 2015, Noffke, 2015) as well algae and microalgae (Rizzo, 2020; Latif, 2021; Bianciardi et al., 2021). Recent studies even underscore the possible presence of lichen (Armstrong, 2021) and fungi (Joseph, 2016) on Mars, as corroborated by several researchers -analyzing the images from the NASA Rovers, Opportunity, Spirit, Curiosity and Perseverance, on the Martian outcroppings, together with chemical and mineralogical converging data. Despite accumulating evidence suggesting the likelihood of life on Mars, either in its past or in its present form, the hypothesis remains speculative, with the data frequently being a subject of debate Schuerger & Clark, 2008; (Kah et al., 2018; Steel et al., 2022) . However, recent discoveries, including the identification of liquid water on contemporary Mars by China's Zhurong rover, the detection of complex organic molecules potentially indicative of biological processes by NASA's Curiosity and Perseverance rovers, the discovery of subterranean water bodies by ESA's Mars Express, and periodic fluctuations in methane and oxygen levels, necessitate a renewed evaluation of Mars' potential to sustain life. Looking forward, it is imperative to leverage more definitive methodologies to verify the existence of life on Mars, both in its historical and current states. We advocate for the utilization of optical microscopy at magnifications of 100-400x to examine potential life forms' morphology and mobility, coupled with fluorescence microscopy and/or flow cytometry (Nadeau et al., 2016; Eny et al, 2021) to detect complex biological molecules intrinsic to life, such as polysaccharides, chlorophylls, and nucleic acids. Moreover, the introduction of innovative technologies, like the 4th generation synchrotron-based techniques (Callefo et al, 2019), could afford more conclusive data concerning the presence of potential fossils in Martian rocks.