Fossils on Mars: Evidence for microbial life on mars
Is there life on Mars? Will we ever know?
On December 27, 1984, meteorite hunters found a 4.3-pound fragment of a rock in the Allan Hills in Antarctica. Researchers determined that the rock was a meteorite from Mars. That fragment of a meteorite has since been known as ALH84001. Researchers determined that the meteorite was formed on Mars some 4,091,000,000 years ago when liquid water was still found on the surface of the planet. It was, the theory went, knocked from the surface of the red planet by a meteor that struck Mars 17,000,000 years ago.
In 1996, exciting findings were announced to much fanfare — by then-President Bill Clinton, of all people. Scientists had discovered using a scanning electron microscope that ALH84001 contained evidence of microscopic fossils of bacteria. Since the evidence was on a meteorite that came from Mars, and since fossils of bacteria indicated there had once been living bacteria, that meant — surely — that there had once been life on Mars.
One NASA report said: “The NASA-funded team found the first organic molecules thought to be of Martian origin; several mineral features characteristic of biological activity; and possible microscopic fossils of primitive, bacteria-like organisms inside of an ancient Martian rock that fell to Earth as a meteorite.”
Unfortunately, ALH84001 didn’t settle the life-on-Mars question once and for all. In fact, its status as proof-of-life was largely debunked. Non-biological explanations for the tiny rod-like features seen in ALH84001 became more persuasive.
More Martians discovered in Antarctica?
Fast-forward to the March 28, 2019 version of the journal Open Astronomy. There, researchers from HAS Research Centre for Astronomy and Earth Sciences in Budapest published peer-reviewed findings from their research into another Martian meteorite — also found in the Allan Hills of Antarctica, by the Japanese National Institute of Polar Research, in 1977 — dubbed ALH-77005.
ALH-77005 is a 175,000,000-year-old meteorite whose composition looks very similar to that of rocks and gases analyzed by spacecraft that have actually been to Mars. For example, argon that the Curiosity rover found in Mars’ atmosphere has also been found in ALH-77005. What else did researchers find in ALH-77005?
Within the meteorite, they found spherical and thread-like formations and biosignatures indicating the mineralized remains of millennia-old-microbes. Fossils of bacteria. Time and further inquiry will tell whether these findings will remain persuasive indicators of one-time bacterial life on Mars. It’s fair to say that, at worst, it’s still an open question.
The romanticized version of fossil hunting on Earth involves dinosaur bones by the layer. That vision of fossil discovery ignores the reality that most life on Earth has been at the microorganism level.
NASA’s continued exploration of Mars has contributed much to the debate. Now, investigating the possibility of life on Mars isn’t limited to looking at meteorites found in Antarctica. Now, researchers — or their robotic colleagues — can go right to the Martian source and send back results.
Exploring Mars on Mars, not just in Antarctica
In January 2004, the NASA rovers Spirit and Opportunity began traversing Mars. Within weeks, Opportunity sent back evidence that water once flowed on the red planet. Opportunity also took samples from Mars’ Meridiani Planum that suggested the rocks were formed billions of years ago in a long-gone sea. The robot rover also captured images of layered rocks that had not been crushed to dust by billions of years of gravity and other forces like similarly aged deposits on Earth. For researchers curious about the presence of fossils on Mars, the existence of intact layers of billions-of-years-old rocks and material in a long-gone sea bed promised a fruitful hunting ground.
Orbiting probes and telescopes from Earth have also detected methane and formaldehyde in Mars’ atmosphere. Here on the blue planet, methane and formaldehyde in the atmosphere are produced, among other things, by microbe activity and other living things. Could the same be said about Mars?
Hunting for meteorites in Antarctica is one thing! How to hunt for fossils on Mars — some 282,000,000 kilometers away — is quite another.
Fossil-hunting on the red planet
In May 2018, the Journal of Geophysical Research: Planets published an article teasingly called “A Field Guide to Finding Fossils on Mars.” It argued that conditions on Mars were right for the preservation of “fossils and isotopic biosignatures…in the depositional environments and mineralizing media thought to have been present in habitable settings on early Mars.” Leveraging research from Earth’s own fossil record and our understanding of Earthly fossil formation, the authors identified the most promising targets for astrobiological — life-hunting — missions to Mars.
They admitted that there is “no compelling evidence for life on Mars, now or in the geological past.” Having said that, though, “there is now a very strong case that the surface of early Mars was habitable.” But where to look for evidence?
We conclude that mudstones rich in silica and iron‐bearing clays currently offer the best hope of finding fossils on Mars and should be prioritized, but that several other options warrant further research.
The authors identified iron-rich rocks near the sites of seeming ancient lakes as priorities for visits to Mars. The rocks along those former lakes may, they theorized, have preserved signs of prior life for these past billions of years. NASA listened.
The Mars 2020 mission
The romanticized version of fossil hunting on Earth involves dinosaur bones by the layer. That vision of fossil discovery ignores the reality that most life on Earth has been at the microorganism level, which means most fossils on Earth are also tiny, tiny motes smaller than the width of a human hair. The same thing, Earth-bound wannabe-fossil-hunters-on-Mars theorize, may be true about Martian fossils.
NASA’s Mars 2020 mission, intended to launch in July 2020, has a goal of searching for biosignatures of past life on Mars. It won’t investigate whether Mars is or was inhabitable — Spirit and Opportunity did that — but will search for actual signs of existing or fossilized life. NASA puts it this way:
The Mars 2020 mission addresses high-priority science goals for Mars exploration, including key questions about the potential for life on Mars. The mission takes the next step by not only seeking signs of habitable conditions on Mars in the ancient past, but also searching for signs of past microbial life itself.
In May 2019, Swedish researches published a paper in Frontiers in Earth Science about the instruments on NASA’s rovers that could detect tiny fossils. They argued that the Mars 2020 rover could enable fossil research in several ways:
- Taking close-up pictures of the rocks for Earth-bound researchers to study
- Scanning rocks for key isotypes
- Looking for chemical changes in the rocks produced by microbes many millennia or more ago
- Physically collecting rocks to return to Earth later
In December 2019, NASA unveiled its as-yet-unnamed Mars 2020 rover which will be transported to Kennedy Space Center in Florida in February 2020 for final assembly. Once fully assembled, the rover will be four-wheeled and the size of a car and launched in July 2020. It will arrive at Mars in February 2021. Upon arrival, the rover will explore an 820-foot deep crater called Jezero Crater that is thought to have been a lake roughly the size of Lake Tahoe.
Around the edge of the former lake at Jezero Crater is a ring of carbonates, detected by NASA’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument. Here on Earth, carbonates form fossils that are billions of years old, such as seashells and stromatolites that are billions of years old. The presence of a former lake and carbonates at Jezero Crater holds out the promise that there may be fossilized signs of life.
For the moment, the rover’s mission at Jezero Crater will be mere collection. The rover will collect up to 30 soil samples, but leave them there. NASA’s Mars 2020 deputy project manager Matt Wallace described the mission this way:
Once we have a sufficient set, we’ll put them down on the ground, and another mission, which we hope to launch in 2026, will come, land on the surface, collect those samples and put them into a rocket, basically.
The search for evidence of life on Mars — be it in fossil form or otherwise — has taken researchers from Antarctica to the Red Planet over decades. Waiting a few more years to bring back some Martian sediment seems do-able. Perhaps, then, we’ll finally have some answers to our most pressing questions.
A deeper dive — Related reading from the 101:
Strange as it may seem, not every “fossil” died away billions of years ago.
NASA’s Martian explorations have been limited to robotic journeys, so far? What does the private sector have in mind, and when?