This blog is written by Steven Banham, a planetary researcher at Imperial College London who has worked there since 2015. His research focuses on aeolian duneforms on Mars. Contact Steve on Twitter @ICPlanetologist.
Are we alone in the universe? How did life start? Is life unique to Earth? Is there life elsewhere in the Solar System? These are questions that science has been asking since humans started looking toward the stars, realised the mechanics of the heavens, and understood that Earth is one planet among millions.
The search for civilisations
The search for life in the solar system was started largely by accident in the 19th Century by Geovani Schiaparelli. While mapping the surface of Mars using a telescope, Schiaparelli saw straight features that he interpreted to be river channels – ‘canallia’ in Italian. This was mistranslated by others to ‘canals’ (man-made water channels) and the idea that an advanced civilisation existed on Mars was born. This idea was spread throughout the 1930s by Percival Lowell, an American astronomer, and inspired science fiction literature such as H.G. Wells’ ‘War of the Worlds’, and E.R. Burroughs’ ‘A princess of Mars’.
Schiaparelli’s 1886 map of Mars, based on telescope observations
The first satellite flyby of Mars took place in 1965, when NASA’s Mariner 4 probe gave us our first close glimpse of the Martian surface. The satellite recorded no evidence of a magnetic field, no surface water, and a very thin atmosphere. Mars looked to be a dead planet, with no evidence for a habitable environment, let alone an advanced civilisation.
Between 1971 and 1972, Mariner 9 became the first probe to orbit Mars, and returned over 7000 images of the surface. These images revealed that Mars was more active than previously thought, and that there may have been liquid water on the Martian surface in the past. Our first observations of the surface were hidden by planet-wide dust storms, indicating an active atmosphere. When the dust storms cleared, valley networks, volcanoes, dune fields, drainage networks, and layered sedimentary rocks were observed, showing us that Mars was geologically active in the past.
In 1975, Viking 1 landed on the Martian surface with the aim of searching for organisms living in the soil. The Viking lander carried out biological experiments but found no conclusive evidence of living organisms in the samples which were tested. The search for widespread extant life on Mars effectively ended here.
The search for ancient life
But what about extinct life on Mars? As far back as Mariner 9, evidence of valley networks had been observed, indicating flowing water on the surface. This has been backed up by more recent observations of islands, channels and canyons (more evidence of flowing water), and by spectral observations of clays, iron oxides and fracture-filled veins (evidence of minerals becoming altered by water beneath the surface).
Scientists now know that Mars had long-lived liquid water, a thicker atmosphere, and a magnetic field in its ancient past (Ehlmann et al 2016; Javaux et al 2018). It is thought these conditions lasted from the creation of the planet (4.6 billion years ago) until about 3.5 billion years ago. Coincidentally, life on Earth likely began sometime between 4.1 and 3.5 billion years ago, at a time when Mars could have been habitable.
Image of streamlined islands, Oxia Palus (Viking 1 NASA)
So, could life have existed for a brief 600 million year period on Mars? Where would you search across the 144.8 million km² surface of Mars for evidence of extinct life?
Much like Earth, the history of Mars’ evolution is written in stone. The rock record tells us the history of surfaces processes, such as water and wind transporting sediments, and the deposits they create through time. If there was life on Mars, there is a chance that some expression of life – a chemical or physical clue – may be recorded in rocks older than 3.5 billion years old, created by the biological processes of living organisms. Biological processes can occur in many surface environments, such as rivers, lakes, and the sea floor. The clues, or ‘biomarkers’, they create are locked into the sediments deposited in these places to become part of the sedimentary rock record. Some microbes can live in the ground, in the gaps between sands grains. The biomarkers they form, which become locked up within pores and fracture spaces within the rocks, influence the processes that turn sediments into stone. Understanding sedimentary environments is therefore key to searching for life on Mars.
Timeline of Earth and Mars’ history. Green box indicates period where life on Earth started, and life may have existed on Mars
Where to search?
Landing site selection for the most recent surface missions has been driven by an understanding of ancient environments: where could biological processes have been happening? Where may biomarkers have been preserved?
Gale crater was selected as the landing site for the Mars Science Laboratory (MSL) mission in 2012 because clay-bearing rocks were detected in the crater floor. Clays are well known on Earth to have formed in association with water, and are a good material for preserving organic molecules with little chance of alteration. Jezzero crater has been selected as the landing site for the Mars2020 mission because the crater hosts a large ancient river delta. We know that on Earth, delta sediments are a good place to preserve organic molecules. The river feeding the delta could have brought in nutrients to feed a habitable lake within the crater. Finally, Oxia Planum was selected in 2019 as the ExoMars landing site based on the clay signatures detected across much of the landing site, and the large ancient river system feeding into the low lying area.
Curiosity at the ‘Glen Etive’ drill site, Mission Sol 2553 (NASA/JPL/MSSS)
Once at the surface, a more detailed look at the rocks tells scientists where to take samples. For the MSL mission in Gale crater, the science team have been able to reconstruct the ancient depositional environment of the crater – a large lake, fed by rivers originating from the crater rim. This understanding has been used to guide sample site selection throughout the mission. Biomarker may be preserved within lake mudstones, sandstones altered by subsurface waters, rocks rich in clays, and mudstones altered to contain iron oxides. Sampling of these locations has given us evidence of organic molecules that could record the starting point of biological processes, which is an exciting finding.
For the Mars2020 mission at Jezzero crater, a similar approach to sample collection will be taken, where an understanding of the rocks, their environments of deposition, and their potential to preserve biomarkers, will guide sample selection and ensure the best samples are returned to Earth for assessment.
Animation of how the lake in Gale crater changed over time, drying out as Mars dried out (NASA/JPL)
Conclusion
The search for ancient life and extant life on other planets continues. Mars will be the subject of exploration for the next decade, and geoscience will play a pivotal role in understanding what surface conditions were like, and whether the rock-samples to be tested could have originated in a habitable environment. Future missions such as NASA’s 2020 mission to Jezzero, ESA’s ExoMars mission to Oxia Planum and even NASA’s dragonfly mission to Titan will require input from geoscientists to select representative samples which may host evidence of ancient or extant life.