On February 18th, NASA's Perseverance rover successfully landed on the surface of Mars. For the next two years of its primary mission, it will search the Jezero Crater (an ancient lakebed with a preserved delta fan) for possible biosignatures. This will include the first sample-return mission from Mars, where Perseverance will collect soil and drill samples and place them in a cache.
This will be picked up in a few years by a joint NASA-ESA mission, which will consist of a lander, rover, launch vehicle, and orbiter. These robotic elements will retrieve the sample cache and fly them back to Earth for analysis. In this, Perseverance and the sample-return mission are the latest in a long line of efforts to determine if Mars once supported life.
While the search for life on Mars has only been taking place for a few decades, our preoccupation with Martian life is centuries-old. Ever since scientists became aware that Mars was a planet much like Earth, the concept of Martians has fired our imaginations (and haunted our dreams!)
While much of the mythology of civilizations and "little green men" has been dispelled, there is still the possibility that life once existed on Mars (and perhaps still does). As our knowledge of the Red Planet has evolved, so too have our notions of what life on Mars could look like.
With the potential discovery of life just a few years away, perhaps it is time for a retrospective on what we expected to find. David Bowie, a little mood music, if you please!
As a matter of scientific speculation, the idea of life on Mars began gaining traction by the 18th century, thanks to the invention of modern telescopes. In 1610, Galileo became the first astronomer to view Mars through a telescope that he built himself. However, it was not until the 1700s that telescopes had the resolving power to spot features on the surface.
From 1704 - 1719, Italian astronomer Giacomo Miraldi observed Mars from the Paris Observatory (using the Campani Telescope). After spotting what he described as "white spots," he began speculating (correctly) that he was looking at ice caps. Between 1777 and 1783, British Astronomer Royal Sir William Herschel studied Mars using telescopes of his own creation (like Galileo).
In 1784, Herschel shared his observations in a paper titled, "On the remarkable appearances at the polar regions of the planet Mars, and its spherical figure; with a few hints relating to its real diameter and atmosphere." In it, he identified the many similarities that existed between Earth and Mars.
These included the way Mars' polar ice caps appeared to advance and recede, but also the length of a single day (diurnal motion), its axial tilt, and its distance from the Sun. From this, Herschel concluded that Mars experienced seasonal change and was rather "Earth-like":
"The analogy between mars and the Earth is, perhaps, by far the greatest in the whole Solar System. Their diurnal motion is nearly the same; the obliquity of their respective ecliptics, on which the seasons depend, not very different; of all the superior planets the distance of Mars from the Sun is by far the nearest alike to that of the Earth: nor will the length of the Martian year appear very different from that which we enjoy..."
Herschel also noted many dark and bright albedo features on the surface, which he mistakenly thought were oceans and landmasses. He further theorized that Mars "has a considerable but moderate atmosphere, so that its inhabitants probably enjoy a situation in many respects similar to ours."
These conclusions were echoed almost a century later by William Whewell (a fellow of Trinity College, Cambridge). In 1854, he released a study where he argued that Mars has green seas and red land, and maybe even life forms. These observations led to an overall increase in speculation by the mid-19th century that Mars could be habitable.
From canals to little green men
The greatest stimulus for the idea of Martian life came in the late-19th century by Italian astronomer Giovanni Schiaparelli. In 1877, he began a campaign of Martian observation to create a detailed map of its surface. This led to the development of a nomenclature system for Martian geological features that is still used today.
In particular, Schiaparelli noted many long dark streaks that he named "canali" ("channels" or "grooves") and which he named after rivers on Earth. Later observations showed that these were optical illusions. However, when the map was published, "canali" was misinterpreted to mean "canals," which further fueled speculation that there was such a thing as a Martian civilization.
Inspired by Schiaparelli, American astronomer Percival Lowell founded an observatory in Flagstaff, Arizona, where he began making his own observations of Mars. Between 1895 and 1906, he published a series of books in which he proposed that the canals were built by a now-extinct civilization.
This, in turn, inspired British writer H. G. Wells to write The War of the Worlds in 1897, which told the story of invading Martians fleeing their rapidly desiccating planet. In chapter four, the narrator describes seeing Martians for the first time:
"A big greyish rounded bulk, the size, perhaps, of a bear, was rising slowly and painfully out of the cylinder. As it bulged up and caught the light, it glistened like wet leather. Two large dark-colored eyes were regarding me steadfastly. The mass that framed them, the head of the thing, was rounded, and had, one might say, a face. There was a mouth under the eyes, the lipless brim of which quivered and panted, and dropped saliva. The whole creature heaved and pulsated convulsively. A lank tentacular appendage gripped the edge of the cylinder, another swayed in the air.
"Those who have never seen a living Martian can scarcely imagine the strange horror of its appearance. The peculiar V-shaped mouth with its pointed upper lip, the absence of brow ridges, the absence of a chin beneath the wedgelike lower lip, the incessant quivering of this mouth, the Gorgon groups of tentacles, the tumultuous breathing of the lungs in a strange atmosphere, the evident heaviness and painfulness of movement due to the greater gravitational energy of the earth—above all, the extraordinary intensity of the immense eyes—were at once vital, intense, inhuman, crippled and monstrous.
"There was something fungoid in the oily brown skin, something in the clumsy deliberation of the tedious movements unspeakably nasty. Even at this first encounter, this first glimpse, I was overcome with disgust and dread."
The popular trope of Martian invaders and "greys" (aka. "little green men") was born! Even though observations performed just a few years later disproved the existence of Martian canals and cast doubt on the habitability of the planet, the tropes would endure until the latter half of the 20th century.
Lowell's books would also inspire Edgar Rice Burroughs, the famed science fiction writer who wrote the Barsoom series (released between 1912 and 1943). In the first installment (A Princess of Mars), an American Civil War veteran (John Carter) is transported to Mars and finds multiple species of humanoid embroiled in war.
In 1938, Orson Welles directed and narrated the most famous production of War of the Worlds, which was presented as a radio drama (part of The Mercury Theatre on the Air series) on the night before Halloween. The broadcast is renowned for allegedly causing panic among listeners who did not realize it was a dramatization.
Olaf Stapledon's Last and First Men (1930) tells the story of humanity's future and includes a description of a war between Earth and Mars that lasts for tens of thousands of years. The trope of invading Martians remained popular throughout the "golden age of science fiction."
Examples include "Marvin the Martian" (who appeared in a series of Warner Bros. cartoons from 1948-1963), Mars Attacks The World (1938), Flying Disc Man from Mars (1950), Flight to Mars (1951), Red Planet Mars (1952), Invaders from Mars (1953), The War of the Worlds (1953), It! The Terror from Beyond Space (1958), The Angry Red Planet (1959), The Day Mars Invaded Earth (1963).
The Space Age turns its focus to Mars!
With the dawning of the space age in 1958, scientists and the general public were about to get their first up-close and personal look at Mars. What they saw would forever dispel the notion of a Martian civilization but would not deter the search for life there one bit! The first mission to reach Mars and send back information on its atmosphere was NASA's Mariner 4 mission.
This robotic orbiter flew past Mars on July 14th, 1965, and provided the first close-up photographs of another planet. These revealed that atmospheric pressure on Mars is about 1% that of Earth's, and daytime temperatures of −148 °F (−100 °C). It also found no evidence of a planetary magnetic field or Martian radiation belts, indicating that life would have a hard time surviving there.
By 1971, NASA's Mariner 9 probe and the Soviet Union's Mars 2 and 3 missions reached Mars and revealed more about its landscape. This included a planet-wide dust storm that was raging at the time and surface features that indicated how water once flowed on Mars.
These efforts culminated in the Viking 1 and 2 missions in 1975, which consisted of an orbiter and lander element. The landers gathered meteorologic, seismic, and magnetic readings on Mars from the surface and conducted the first search for biosignatures. The results of these experiments were inconclusive, which further fueled doubts about finding life there.
The exploration of Mars stalled somewhat for the next twenty years, with only two missions sent during the 1980s. These were the Soviet Phobos 1 and 2 missions sent to explore Mars' two satellites (Phobos and Deimos) but failed before completing their missions.
But by 1996, a new era of Martian exploration began with the deployment of the Mars Pathfinder (renamed the Carl Sagan Memorial Station) lander and the Sojourner rover - the first rover to operate another planet. By 2004, it was joined by the Spirit and Opportunity rovers.
In 2008, the Pheonix Lander set down in Mars' northern polar region and spent the next five months assessing the habitability and history of the area. The Curiosity rover arrived by 2012 and was followed by NASA's InSight lander (2018) and the Perseverance rover (2021).
During this same stretch of time, NASA and the ESA deployed several missions that explored Mars' atmosphere from orbit. NASA's Mars Global Surveyor was first in 1996, followed by Mars Odyssey in 2001, the ESA's Mars Express in 2003, and NASA's Mars Reconnaissance Orbiter (MRO) in 2006.
In 2014, the Mars Orbiter Mission (Mangalyaan) became India's first mission to Mars, arriving just two days after NASA's MAVEN orbiter. And in February of 2021 (about the same time Perseverance arrived), Tianwen-1 and the Emirates Mars Mission (aka. Hope) - China's first Martian orbiter, and the first mission launched to space by an Arab nation (respectively).
Warmer, wetter past
These missions gathered volumes of data on Mars' atmosphere and surface, which allowed scientists back on Earth to further characterize the Martian environment and the planet's geological history. This revealed that Mars once had a magnetic field, a warmer and thicker atmosphere, and flowing water on its surface.
Roughly 4.2 billion years ago, Mars lost this magnetic field when its interior cooled, arresting geological activity and action in the core. With the magnetic field gone, the Martian atmosphere was slowly stripped away by solar wind, between 4.2 and 3.7 billion years ago. All of the surface water not currently frozen at the poles was either lost to space or retreated underground.
Interestingly, the large-scale disappearance of Mars' atmosphere is also the reason that evidence of its warmer and wetter past has been so perfectly preserved. Without erosion caused by weather or wind erosion, the ancient river basins, lakebeds, and the Northern Lowlands (which were once the site of an ocean) are still there.
As for life, microbial organisms may have emerged on Mars billions of years ago when conditions were still favorable. Finding evidence of this past life is the primary objective of the Perseverance rover and the sample-return mission that will follow.
However, it is merely the latest in a long line of missions whose purpose was to find out if there was ever such a thing as Martian life. For decades, space agencies have been sending robotic missions of every kind to answer the age-old question: "Is there life on Mars?"
Now that we are closer than we've ever been to answer that question, just what is it scientists are hoping to find?
At this juncture, scientists anticipate that if Mars had life in the past, the best places to look for evidence of it would be the Gale Crater, the Jezero Crater, and other locations where standing lakes or rivers once existed. The evidence would likely take the form of fossilized bacteria, much like those that have been uncovered here on Earth.
Here on Earth, the oldest evidence of life consists of fossilized microbes found around hydrothermal vents on the ocean floor. These environments were conducive to life because volcanic vents distributed essential elements from Earth's interior, and these then mix with cold, oxygen-rich seawater.
Also, these vents provided the necessary energy (heat) to fuel metabolism. As such, Earth's oldest microorganisms would have found the space around these vents to be hospitable environments. Evidence of these lifeforms is indicated by the presence of a certain type of organic carbon molecule they leave behind.
Other evidence includes stromatolites, solid sedimentary formations created by photosynthetic cyanobacteria dated to ca. 3.5 billion years ago. These formations result from bacteria producing adhesive compounds, which causes sand and minerals to stick together and grow over time.
In short, scientists anticipate that if there once was life on Mars, it will have left behind similar "biosignatures" - i.e., carbon compounds and features that form in the presence of bacteria and water. Since the conditions on the surface today are very harsh, scientists anticipate that if life still exists on Mars, it is likely located in briny patches of water beneath the surface.
In this respect, past bodies of water and past life on Mars may have found their way to the same place (underground) and were able to survive. While it's not exactly as glorious as the idea of little green men, flying saucers, and an ancient civilization that might want to invade us, finding evidence of Martian life (past or present) will be groundbreaking!
What will it mean?
Finding fossilized bacteria or organic carbon that confirms the existence of microbes on Mars (ca. 3.7 to 3.5 billion years ago) will confirm that life emerged on both of our planets in a similar time frame. They might even point the way towards a common origin, like the possibility that microbes were distributed throughout the Solar System by meteorites and asteroids ("panspermia theory").
But first, we need to find compelling evidence that life once existed there first. To quote the late and great science communicator, Carl Sagan:
"If we see a hedgehog staring in the camera, we would know there’s current and certainly ancient life on Mars, but based on our past experiences, such an event is extremely unlikely. Extraordinary claims require extraordinary evidence, and the discovery that life existed elsewhere in the universe would certainly be extraordinary."
The search for life on Mars could take generations, even millennia. Much like the search for extraterrestrial intelligence (SETI), it is a mystery that may never be solved. Or it could be solved with a single, groundbreaking, mind-blowing discovery.
We can't be sure. All we know is to keep looking!