13000 years ago, a lonely rock that had been hurtling through space for 16 million years, entered Earth’s atmosphere and made it to Antarctica. About 35 years ago, in 1984, it was discovered in the Allan Hills ice field by a group of people who were hunting for meteorites. However, it wasn’t until 1994 that the rock’s home planet was found to be Mars. This discovery made it popular among scientists who believed that there was life on Mars sometime in its past. About 12 Martian meteorites had been found till then, but this one was the oldest and special. ALH84001, as it came to be known as, became a sensation on August 7 1996 when a group of NASA scientists declared that they had found possible evidence of Martian fossils in the meteorite. In short, they believed life existed on Mars some billions of years ago. And no, they did not find bones of little green people, but pointers to microbial life.
The meteorite’s journey began about 4.5 billion years ago when it was formed in the crust of Mars. Its home is traced to a crater in the southern hemisphere of Mars which was heavily bombarded with meteorites and asteroids. One such impact ejected the ALH84001 from the ground 4 billion years ago. But that event did not throw the meteorite into space and it fell back to Mars. That was lucky for us, because it is at this point that it’s most intriguing history begins. Between 3.6 and 4 billion years ago, water is believed to have seeped into its cracks. Life as we know it requires water to exist even in the harshest of habitats. Water is the universal solvent and necessary to support chemical reactions essential for metabolic activities. Thus it fulfilled the first condition for existence of life. This was also consistent with the findings at that time that Mars had a thicker, warmer atmosphere in the past allowing the existence of liquid water. That’s right, Mars wasn’t always so dry and cold.
Well what did the scientists find? McKay and the team of NASA scientists had found microscopic carbonate globules along cracks and pores of the meteorite, where water is believed to have entered. These globules were essentially carbon bubbles that trapped potential microfossils. The scientists proved that these globules were indeed formed on Mars and not on Earth. This was important to prove because all the subsequent observations that were presented were inside these globules. So if the globules were formed on Mars, the fossils inside would be from Mars too. The age of the globules was around 3.6 billion years, which also proved their origin. Their formation process was also suggested as possibly biogenic, i.e. produced by living organisms.
The team found elongated bacteria like shapes inside the globules that could be possible fossils of the microbes. Their size was attributed to hypothetical nanobacteria because they were smaller than any known lifeforms on Earth. The team also found chains of mineral grains that are typically deposited by bacteria on Earth as waste products. The final evidence was organic molecules that were claimed to be formed by decay of bacteria. These molecules, called Polycyclic Aromatic Hydrocarbons or PAH, are common on Earth and are formed by a variety of processes. These claims were strongly challenged because each one of these evidences, seen in isolation from the other evidences, were non-conclusive proofs and each one could be explained by a different non-biological phenomena. However, McKay and team argued that although these are individually non-conclusive, all of them looked together along with their geographic proximity on the surface are proof that there was in fact life in the past on Mars.
Can Earth provide some answers? Even today, the claim of microfossils on ALH84001 is neither completely accepted nor completely refuted. But this discovery sparked a debate around primitive life itself, that is still going on. Scientists started looking for cues here on Earth in an effort to understand some of these observations. For example, comparing with the evidence on the meteorite, we see that the shapes observed inside the carbonate globules are similar to those of 3.5 billion year old bacterial fossils on Earth. And so Earth analogs have always been a key to unravel the mysteries of other solar system bodies. And even on Earth, microscopic fossils are difficult to identify definitively. But as the field progresses, more clues to early life on Earth emerge. The search for life and its origins on Earth and Mars continue to shed light into each other’s past.
The continuing discoveries of thriving life in the most unlikely places on Earth open the possibility that it might still exist on Mars in ice deposits beneath the surface, if it ever originated in the past. Antarctica is one of the closest analogs we have for Mars because of its dryness, ice sheets and glaciers. It beams with colonies of microbes even in its harsh cold environment. Microbes have been found under ice living without light and oxygen. Even more interestingly, they are found to remain dormant in ice for millions of years and can be revived when exposed to water. They are found beneath the ocean surface, trapped in rocks, surviving for billions of years by radiolysis. It is a process through which hydrogen is produced by breakdown of water from radiation released by rocks. Each new discovery of such microbes increases the possibility of finding extant life on Mars, and thus, the search continues.
How to search for life on Mars? The search for life has been an important scientific goal of most of the robotic missions to Mars. The approach has changed considerably over the decades; from searching for direct evidence of life to trying to first establish habitability, that is to find if the place is favourable to life at all. This search is guided by the “follow the water” philosophy. Water in the form of ice, liquid or minerals is a key to establishing habitability since it is necessary for life as we know it. Other factors include radiation, temperature, toxicity and availability of minerals or some source of energy to support biological processes. The next step is to search and analyze organic molecules. Organic molecules, containing carbon and hydrogen, are the basis of life as we know it. Though they can be products of non-biological processes, their presence increases the possibility of life and a possible prebiotic history which is as interesting. Finally, biomarkers such as remains of cells, organic matter, biominerals and magnetofossils are direct evidence of life.
The dozens of Mars missions so far have provided crucial evidence to further the search for life, even if their objective is not directly so. Although we still don’t have conclusive evidence of life on Mars, each of the missions have brought together a piece of the puzzle. And these findings also drive the research on Earth, explaining observations such as those found inside ALH84001.
Prior to the 1964 Mariner 4 probe, the possibility of stumbling upon multicellular organisms crawling on the surface was quite real. But as Mariner 4 flew past Mars, taking 2 dozen photographs of its surface, a dry and cratered surface was revealed. It also found that Mars did not have any magnetic field, making it susceptible to cosmic radiation which would mean no life as we know it could survive on the surface of Mars. The most likely place for extant life (still surviving) to exist was below the surface and in the form of microbes, which can brave the harsh environment and still survive. This narrowed down search for life drove the subsequent missions. The 1971 Mariner 9 orbiter provided evidence for a thicker than expected atmosphere, and also pointed to a past when the climate was milder and there was an abundance of liquid water. The 1975 Vikings lander measured the atmospheric and soil composition of Mars which proved vital in tracing the origin of meteorites to Mars.
The later Mars Global Surveyor and Mars Odyssey orbiters provided data that pointed towards possible origins of ALH84001 to the Valles Marines, a canyon near the equator. Such findings inform landing locations of future missions since the possibility of finding biomarkers would be higher here. Spirit and Opportunity rovers contributed immensely by revealing past water activity on Mars. Spirit’s wheel exposed a silica rich ground, the Earth analogs of which sustain thriving microbes. The Phoenix lander also exposed water ice beneath the surface of Mars. The Mars Express orbiter provided, along with stunning pictures of Mars, a detailed chemical composition of the Martian atmosphere. Most importantly, it revealed the presence of methane, which could be a by-product of biochemical processes. Curiosity rover found complex organic molecules which could be of biological origin. All these and other missions have helped further our search for life on Mars, directly or indirectly.
What is next in this quest? Future missions to Mars will all get us one step closer to the answers. Perseverance rover is on a mission to find biomarkers in Martian soil. A sample return mission from Mars is being planned that will get us a pristine piece of Mars to examine in Earth laboratories. A human mission to Mars will be the biggest undertaking towards the search for life as humans will be more dexterous, insightful and will have larger reach than robots. And human intuition simply cannot be fully substituted with planned experiments via a robot.
Although McKay and team could not convince the scientific society that they had found proof of past life on Mars, the debate that ensued has benefited science greatly. Implications of finding life outside Earth will be as profound as Copernicus saying that Earth is not the center of the Universe or Darwin saying that apes and humans are distant cousins. As Carl Sagan said, and has been quoted many times in this journey, “Extraordinary claims require extraordinary evidence”. And to get the extraordinary evidence, equally extraordinary space faring missions are required.