Last month, the EXPLORER featured an article on the geology of Jezero crater on Mars where NASA’s Perseverance rover will land on Feb. 18, 2021. After a successful landing, Perseverance will begin an astrogeology mission to search for signs of past life on the Red Planet and to gain knowledge to prepare for human exploration. In this month’s installment on the Perseverance mission, we will take a look back at our historical search for life on Mars. There have been intriguing findings, but no conclusive proof that life has or does exist there.
Man has always stared at Mars in wonder. Its ruddy hue has inspired the lore of war in Babylonian, Hindu, Greek and Roman culture. Famously, Mars was the Roman god of war, symbolized by an arrow protruding to the upper right of the planet’s circle, as a representation of a Roman shield and spear.
Dreams of Martian Waters
With the advent of high-quality telescopes in the 19th century, astronomers began investigating the geographic features of the planet. The particularly imaginative Giovanni Schiaparelli interpreted Mars vistas as continents crisscrossed by canals. While he may not have imagined water-filled canals attributed to intelligent design, others certainly embellished his visions. A plethora of colorful Martian science fiction ensued.
The myth of water canals on Mars was reinforced by American astronomer Percival Lowell’s observations and publications of the early 20th century. The vagaries of Earth-based observation coupled with atmospheric turbulence would continually cultivate the canal myth until the successful Mars flyby of Mariner 4 in 1965. It captured grainy images showing what appeared to be a cratered planet as lifeless as Earth’s moon. Then, in 1971, Mariner 9 successfully entered Mars’ orbit and sent tantalizing images of canyons possibly carved by water, this time bolstered by “close-range” state-of-the-art photography.
Mars continues to stoke our imagination with the wealth of data acquired from successful orbiting platforms and surface landers. We now know that Mars’ first billion years of geologic history was remarkably like Earth’s, influenced by coursing waters sufficient to fill a great ocean in the northern hemisphere. The great river channels of Mars offered their tribute to that ocean for untold ages before the planet became barren and dry. Some xenobiologists even speculate that life evolved on Mars before it did on Earth, preparing a platform for panspermia as cosmic impacts cast adrift enormous pieces of Mars, which would eventually fall to Earth, possibly seeding life here.
Viking Lander Experiments
Reaching Mars has proven to be troublesome and difficult. More than two-thirds of the 40 missions to Mars to date have been failures. There have been 12 landing attempts on Mars by Russia, NASA and the European Space Agency. Only eight landing missions have been successful. All of these have been NASA missions.
Renowned Cornell astronomer, Carl Sagan, was a prime promoter of NASA’s Mars Viking missions. He helped select the landing sites and plan the missions. The Viking 1 and 2 Landers successfully touched down on Mars in July and September of 1976. Meanwhile, their respective orbiters returned images of Mars that convincingly showed that water once flowed on Mars, sometimes in great floods. To date, the most unique accomplishment of the landers were the three independent biological experiments designed to detect life in the Martian soil. For example, the labeled release experiment, designed to incubate microbes and detect radioactive tagged gasses from metabolism, turned a positive result. The other two experiments did not detect any organic molecules in the soil. The current consensus among scientists is that the positive metabolism results were likely caused by non-biological chemical reactions from oxidizing perchlorates present in Martian soil.
The Viking test results are controversial and are still under assessment. In 2008, perchlorates were detected in Mars’ soil by NASA’s Phoenix lander. On Earth, perchlorates have been demonstrated to destroy organics when heated and produce the same chlorobenzene compounds detected by the gas chromatographs on both Viking landers when they heated Mars soil in the Viking molecular analysis experiment. The question of microbial life on Mars remains unresolved. Nonetheless, in 2012, a team of scientists, including original Viking investigators Gilbert Levin and Patricia Ann Straat, published a peer-reviewed analysis of the Viking labeled release experiments that “may suggest the detection of extant microbial life on Mars.”
Studying Mars Up Close, On Earth
Miniaturized robotic labs on Mars landers cannot match the skill, precision and detailed scrutiny of human eyes and instruments in Earth-based labs. While the Perseverance rover will have state-of-the-art, onboard lab instruments, it will cache samples to be returned to Earth on a future robotic mission. Even before we bring that Mars sample bonanza back to Earth, we already have pieces of Mars to study in Earth labs. Over geologic time, Mars impact debris has been blasted into space toward Earth’s orbit. Of the estimated 60,000 meteorites that have been discovered on Earth, more than 100 of these have been determined by isotopic signature to have originated on Mars.
One such Martian meteorite, discovered in Antarctica, rocked the world in 1996 when a science team, led by NASA astrogeologist David McKay, published that they had found “possible relic biogenic activity in Martian Meteorite ALH84001.” Very few scientists found the evidence to be a convincing argument for fossil life from Mars. If the rock had come from Earth, the bar might not have been set so high.
The analysis focused on carbonate that had formed in basalt from the primordial crust of Mars. The carbonate apparently formed around 4 billion years ago, when Mars had surface water, higher atmospheric pressure and higher temperatures. They found some small tube-like structures, in the size range of Earth nanobacteria at only 100-200 nanometers. They found polycyclic aromatic hydrocarbons that could be associated with life. They found strings of magnetite associated with some of the tube-like structures. This is similar to some modern bacteria. Each of these pieces of evidence taken alone is not convincing, but taken together, the possibility, as reported by McKay and his co-authors, could be plausible. More than 20 years later, the debate continues.
Mystery of Martian Methane
The robotic Curiosity rover Mars Science Laboratory has been studying geology in Gale Crater on Mars since 2012, long outliving its planned two-year mission. Curiosity has detected methane with levels peaking in the warmth of Mars’ summer. Chris Webster at NASA’s Jet Propulsion Laboratory speculates that Mars’ methane comes from the deep surface and migrates to adsorb on near-surface soil where an increase in temperature releases it to the atmosphere. While methane can have an inorganic source, on Earth it is generally associated with biological sources. NASA Goddard’s Jennifer Eigenbrode asks, “Are there organisms that are doing those processes? Is it life?”
Viking chlorobenzene findings are consistent with recent detection of chlorobenzene by instruments on the Curiosity rover. In 2018, a paper by a diverse science team with Eigenbrode as lead author, reported that Curiosity detected thiophenic, aromatic and aliphatic compounds in drill samples using gas chromatography–mass spectrometry in its sample analysis instrument. Curiosity has found “complex organic molecules that have been preserved in the clay for 3.5 billion years.”
Curiosity’s Gale Crater was an ancient Martian lake similar to the target landing site for Perseverance in Jezero crater. In March of 2020, Rhawn Joseph and a diverse team of scientists reported that their study of Curiosity traverse images reveal “specimens resembling terrestrial algae, lichens, microbial mats, stromatolites, ooids, tubular-shaped formations, and mineralized fossils of metazoans and calcium-carbonate encrusted cyanobacteria.” Selected images from their paper accompany this article. Their claims are as intriguing as they are controversial.
To add to the drama, cyanobacteria evolved and made the oldest layered fossil stromatolites on Earth about 3.5 billion years ago. This is roughly the same age as sediments Curiosity is studying in Gale Crater on Mars.
In a 2017 paper, Roger Weins and co-authors interpret that similar round features up to 23-centimeters in diameter in Gale Crater are “concretions that could form due to redox reactions around concentrated iron, maybe from meteorite fragments.”
Questions to Explore
Did Mars’ early, water-rich geologic history in the Noachian period, from 4.1 to 3.7 billion years ago, lead to the evolution of life? If so, does life still exist there – perhaps in the subsurface? We have seen teasing, tantalizing hints of possible fossil and extant life on Mars from Viking, meteorite ALH8001 and from the Curiosity rover. What will the Perseverance rover find to shed light on these still unanswered questions of possible extraterrestrial life on Mars? For the search for life on Mars we should adhere to Carl Sagan’s standard, “extraordinary claims require extraordinary evidence.”