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  • Writer's pictureDale DeBakcsy

Sarah Stewart Johnson, Mars, and the Search for Life as We Don’t Know It.

The Red Planet has not always been kind to those who have given their lives to its study. Before the rise of rover-based observation, Mars had a particular genius for revealing itself only in tantalizing snips and glimpses seized upon by our planet’s most fertile minds to form the basis of compelling theories of life beyond Earth, and a greater genius still for tearing those theories apart with each new set of data provided by our evolving observational technology. From Giovanni Schiaparelli’s canals to Carl Sagan’s macroorganisms to David McKay’s analysis of meteorite ALH84001, the history of Martian biological studies is littered with exhilarating moments cruelly cut short, all leading to the question, “What sort of person, seeing all those careers broken against the rocks of Martian obstinacy, would choose to go into the field today?”

A dauntless person, certainly, a person who sees the failure of previous methods not as a sign to quit, but as a challenge to rethink old definitions and cast wider the net of human curiosity. Somebody who sees the history of Martian exploration not as a succession of easily deluded dreamers, but as one of almost painfully brilliant humans wringing every last drop of meaning from the technology available to them before passing the torch to a new generation in the hope that, some day, all of that concentrated genius-to-come might compel the universe to reveal truths about the nature of life unimaginable in their own time.

Such an individual is Sarah Stewart Johnson, the planetary scientist and geochemist whose 2020 book The Sirens of Mars was a celebration of the last century and a half of planetary observers whose evolving theories continued to push the boundary of the possible in interplanetary observation technology, and of the spirit of the modern rover projects and those who devote themselves to their creation, direction, and maintenance. Born in Kentucky, Johnson spent her youth in warm proximity to the scientific enthusiasms of her father, who had thrilled to the exploits of Mariner 4 as it flew by Mars in 1965, and who regularly pulled the family vehicle off to the side of the road to investigate interesting geological strata with his two children, Sarah and her sister Emily, in close tow.

As very young children, Sarah and Emily had watched Carl Sagan’s Cosmos series when it debuted in 1980, and had enjoyed hours of backyard astronomy with their father, who put the star maps in Astronomy magazine to good use following the passions that his paying job at the state health department didn’t give him scope to fulfil. Johnson enjoyed the feeling of connection to the universe and to the astronomers of the past that those timeless moments in the backyard provided, and further enjoyed the rigorous world of mathematical thinking so much that, one time during a long stay with sick relatives, her mother, to give her something to do, ordered a correspondence mathematics course that allowed her to delve into a self-guided trip through the worlds of trigonometry, logarithms, and polynomials long before her contemporaries did so at school.

Her youth spent almost entirely within the physical confines of Kentucky, Johnson’s mind wandered the universe, and in particular she paid close attention to the new generation of Mars rovers that were arriving at the Red Planet as she was preparing to start on her own adventure, attending college at Washington University in St. Louis. In an article about the Pathfinder project, which was launched in 1996 and which landed on Mars in 1997, deploying the outrageously successful Sojourner rover soon thereafter, Johnson read about a scientist named Ray Arvidson who happened to be a WUSL professor who also worked on Pathfinder. Upon arriving on campus, she made straight for his classes on the subtle and powerful art of determining different chemical and physical aspects of planetary objects from a distance, and in the summer of her freshman year she worked in Arvidson’s research lab developing an “aerobot” to survey the surface of planets at much lower altitudes than orbiting spacecraft could.

The summer of my freshman year, I beat Final Fantasy VII on the Playstation. So, yeah, pretty similar levels of accomplishment, really.

During her sophomore year, she joined an expedition to Hawaii that had a profound impact on her life when she kicked over a stone at the desolate top of a volcano, some fourteen thousand feet above sea level, and saw growing there, in that spot beneath where the rock had lain, a small fern. That fern represented all of life’s tenacity, its ability to overcome every conceivable restriction and, somehow, find a way to be. Johnson describes that moment, gazing down at the resolute fern in the middle of omnipresent barrenness, as the instant when something in her clicked over a notch, and her path as a planetary scientist was decided.

After a time as a Rhodes Scholar at Oxford University in the early 2000s, Johnson ended up at MIT, working under one of the legends of Mars exploration, Maria Zuber, whose work on the Mars Global Surveyor mission in the late 1990s had resulted in topographical maps of exquisite detail which, in tandem with the contemporaneous success of the Surveyor rover, revived public interest in NASA’s Martian program. In 2004, Johnson had an invitation to visit the famous Jet Propulsion Laboratories (JPL) in Pasadena, which served as the command center for the Spirit and Opportunity rovers that had recently made Marsfall. Here, she came into contact with people who had adjusted their sleep schedule to better approximate the length of Martian days, and who were on the front lines as new chemical and optic vistas were being uncovered, and decisions about new explorations were being made.

It was a heady atmosphere and, understandably, Johnson decided to stay there for more than a few days. WIth Zuber’s blessing, she stayed on at JPL, analyzing the chemical processes that had occurred on Mars as its playa lakes receded over time. Months passed away in a state of complete commitment to the cause of understanding the chemistry of a distant and lost world. Johnson did ultimately return to MIT, and received her PhD in 2008 for her study of the evolution of the Martian surface during the late Noachian period (approximately 3.7 billion years ago).

There are three ways to understand what has happened to Mars over time - one is to visit there with rovers and collect data directly, a project Johnson was involved with during her time at JPL, the second is to go to places on Earth that have Mars-like conditions and study how life has existed there, and what tell-tale signs it has left behind, as a way of informing the design of new apparatus for future Mars missions, as Johnson has done several times on expeditions to some of our planet’s most extreme environments, and the third is perhaps the most daunting, and which is among the goals of her current lab’s research - the search for Life As We Don’t Know It, otherwise known as Agnostic Biosignatures.

This type of research is based on the likelihood that, the further we get from our own planet, the more likely the life we find will be based on other biochemical pathways, other energetic systems, than the ones we have taken for granted as the basis of life on Earth. Agnostic biosignature science, then, is dedicated to studying ways that we can search for signs of life that don’t employ the compounds and chemical reactions which we associate with terrestrial biology. How could life be other than it is, and how could we find it at a distance?

It’s a daunting puzzle which generalizes life as a matter of energy transfer, the concentration of sophisticated chemicals in patterns not replicated by mineral processes, and the development of complex chemical structures, divorced from any particular preferred biochemistry, and then develops potential ways to detect the remains of those organisms that one time, perhaps billions of years ago, were alive in a way we could scarcely imagine twenty years ago, let alone when Schiaparelli discovered his Martian “canals” a century and a half ago.

Past failures to detect life as we know it on the red planet disappointed many, but inspired many as well to think deeper and look harder, to move on beyond photosynthesis and workaday cellular respiration and imagine new ways of propagating complexity over time using the constituent components of vastly different worlds. Johnson is today a part of a vast system of research, with exploring rovers, guided by chemical intuitions derived from extreme forms of ancient life on Earth, regularly uncovering startling new chemical possibilities on an equally ancient but deeply foreign world, which in turn expand the horizons for new planetary biologists to propose new equipment for new rovers and spacecraft to drive another turn of the great wheel of exobiological studies, per aspera ad planetas.


Sirens of Mars (2020) is several things within its 186 pages - a fascinating look at the people who have observed Mars for the last century and a half, and the equipment and theories they have developed in the process, as well as a collection of autobiographical moments scattered throughout the text. The book doesn’t overly much highlight Johnson’s current work, however, a good summary of which can be found on NASA’s website here, while the full spectrum of her current projects can be found here. She can be found on Twitter at @biosigs, and while you’re there, why not follow Maria Zuber as well at @maria_zuber !


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