A History of Astrobiology

The connection between space exploration and astrobiology (then called exobiology) was highlighted and given early legitimacy by molecular biologist-turned-exobiologist Joshua Lederberg. Even before NASA was formally established, he was reaching out to colleagues about the possibilities of finding life beyond Earth. He won the Nobel Prize (at age 33, for discoveries about the genetics of bacteria) the same year NASA was founded.

By 1960 he was writing in the journal Science that: “Exobiology is no more fantastic than the realization of space travel itself, and we have a grave responsibility to explore its implications for science and for human welfare with our best scientific insights and knowledge.”

While the 1960s were defined within NASA primarily by the efforts to land humans on the Moon, all during that period the agency was also supporting a robust effort to prepare for a mission to Mars. Its core goal: To search for signatures of life beyond Earth.

That effort required substantial research into and inevitable debate about the nature of the “life” that the Viking landers would be looking for. What’s more, those in the biological fields became properly concerned about what microbial life the Viking landers might bring to Mars from Earth, and projecting further on extraterrestrial life that might some day be returned to our planet.

So while hunting for present or past life on Mars was a very popular idea, it opened a Pandora’s box of extremely difficult questions about the still-mysterious nature and origins of life. Nonetheless, the possibility of actually finding extraterrestrial life reached a fever pitch of excitement during the Viking landing in 1976. Many predicted that life would be found on Mars – including Carl Sagan, who looked forward to encountering, via Viking, visible, perhaps floating creatures.

But those predictions gave way to first images of a bleak and barren Martian landscape, and then to negative but also confusing scientific conclusions about whether signs of life, or even of organic compounds, had been detected.

The experience was sufficiently sobering that the study of Mars took an abrupt backseat, and it would be decades before interest recovered. And while orbiters, landers, and rovers returned to Mars in the 1990s and 2000s, it wasn’t until the 2012 landing of Curiosity that another astrobiology (though not life detection) mission began. Fortunately, a great deal had been learned in the intervening years – and the Perseverance mission now on Mars is collecting samples from the Martian surface that could be brought back to Earth as part of the planned Mars Sample Return mission.

An example of what has been learned over the years is that previously unknown microbial communities have been discovered on Earth that survive – thrive, even – in what were previously considered dead, uninhabitable environments. The first major “extremophile” discovery was made in the blackness of the deep ocean off the Galapagos Islands, alongside the hydrothermal vents that dot the seafloor. Not only were microbes and later tube worms found living in the total dark, but they were living in water made scaldingly hot by the vents.

That 1977 discovery led researchers to extreme environments around the world, where they found microbes living in bitter cold, in highly acidic and salty water, in the rock of goldmines dug miles underground, in the atmosphere high above ground, and in surroundings with high levels of radioactivity.

This explosion of often NASA-sponsored research told scientists a great deal about life on Earth, but it also quite clearly suggested that life can exist beyond Earth in conditions long deemed unsurvivable – such as the frozen-over oceans of Jupiter’s moon Europa.

Researchers have also found all the chemicals needed for life in space, and many of the key building blocks in meteorites and even comets. Amino acids, for instance, were found in samples of the comet Wild 2 after NASA’s Stardust spacecraft passed through the comet’s dusty coma in 2004 and were also found on the asteroid Ryugu by the Japanese Hayabusa2 mission, which returned samples from the asteroid in 2021. Nucleobases have also been discovered by NASA scientists in meteorites and on Ryugu. These results from the field of “astrochemistry” have told scientists that the ingredients presumed to be needed for life are actually falling on planets, moons, and asteroids everywhere.

How those and other organic compounds might organize into self-replicating forms, and ultimately organisms, has been among the most challenging fields in astrobiology. By both digging into the genetic infrastructure of life as well as trying to recreate it in the laboratory, scientists have pushed back the mystery of life’s origins to an early RNA world and even a pre-RNA world. But the process through which non-living substances took on the attributes of life remains elusive.

Earth-based research has been essential to astrobiology and has significantly changed our understanding of Earth and what might be possible on other worlds. But NASA, European, and Japanese robotic missions and space telescopes have most often been the engines that drive the field.

Guided by the mantra “follow the water,” NASA missions in our solar system have discovered a surprising variety of astrobiology targets. First came Jupiter’s moon Europa, with an ocean beneath its icy crust. On-going research suggests that the water is salty, a brine with apparent parallels to our oceans. And most recently plumes of that water may have been detected leaking from the moon – similar in some ways to those spurting out of Saturn’s moon Enceladus.

The water story on Mars has been especially promising, with the identification of deep river channels, valley systems, alluvial fans, and, more recently, lakes and suggestions of a once-grand northern ocean. The dwarf planet Ceres and Jupiter’s moon Ganymede now also appear to hold inner oceans, and the possibilities for finding more water worlds seem endless.

That’s because the past twenty years have witnessed a revolution in our understanding of exoplanets – bodies that orbit distant suns. Scientists have long suspected that other stars produce solar systems, but it wasn’t until 1995 that the first was detected. Since then thousands more have been identified, especially by NASA’s Kepler Space Telescope, but also through ground-based observations.

As the estimated number of exoplanets has grown into the many billions, the possibility that some are home to living organisms has become more plausible and the subject of substantial research. Scientists have determined that some of the planets are rocky and “Earth-like,” and orbiting their sun well within a “habitable zone” – at a distance where water can remain liquid on the surface of the planet for at least some of the time. Far more than a rocky surface and occasionally liquid water is needed to make a planet truly habitable, but it’s an important start.

In retrospect, we can see that a broad range of advances in astrobiology set the stage for what immediately became the biggest news of all — the possible detection of signs of ancient martian life.

Headlines in 1996 told of a NASA research team, led by David McKay, that had found six indicators of past life in a meteorite from Mars. The famous ALH84001 meteorite, uncovered in the Allan Hills region of Antarctica in 1984, was presented as containing clear signs that microbial life once existed on Mars. There were even images of what was interpreted to be the fossil remains of a bacterium-like life form.

As with the Viking results, however, many in the Mars and astrobiology communities were not convinced. While the authors of both the Viking results and the Mars meteorite results stand by their work, the scientific consensus has largely rejected them — concluding that the findings could be explained without the presence of biology.

Nonetheless, the Mars meteorite and the excitement surrounding it gave a jumpstart to NASA’s renewed search for life beyond Earth. The NASA Astrobiology Institute was founded two years after the Mars meteorite paper was released, with Nobel laureate Baruch Blumberg as its director, and the organization has been funding wide-ranging research ever since.

Some of the work involves studying environments on Earth to better understand potentially similar ones beyond Earth (so-called “analogue environments”). Other work goes into technology development for use on other planets and moons, while other research explores the origins and early development of life on our planet.

The 2000s saw a renewed interest in exploring Mars with NASA orbiters, landers, and rovers. None had specifically astrobiological missions, but all contributed to better understanding pathways into the discipline’s goals. The Phoenix lander, for instance, found water ice in the north of Mars, ground-truthing the theory that Mars had substantial ice deposits just under its surface. The MER rovers, Opportunity and Spirit, detected carbonates and other minerals important to understanding the potential for biology in the Martian past.

And then came Curiosity and Perseverance, which have an explicitly astrobiological mission – to determine whether ancient Mars was habitable or potentially inhabited. While Curiosity did not have the capacity to assess whether the planet was actually once inhabited by microbial life, the results it has collected have convinced its science team that portions of the Gale Crater landing site were once perfectly capable of supporting life. It was the first formal identification of a habitable environment beyond Earth. Now the same has been determined about Jezero Crater, where Perseverance is exploring.

As is always the case with astrobiology, it was a combination of results — gathered by way of geology, geochemistry, mineralogy, sedimentology, super-high temperature chemistry and precision photography — that led to the conclusion that Mars was once habitable. Those findings support the theory that Mars was warmer and much wetter during its earliest days, even though climate modelers can’t figure out how an ancient Mars could have been warm enough, and had an atmosphere thick enough, to keep that water liquid for potentially tens of millions of years.

As technologies and scientific understandings have progressed, astrobiology has entered ever more fields. Moving beyond the astronomical detections of a cosmic menagerie of exoplanets, efforts are now underway to analyze the atmospheres, and ultimately the surfaces, of those bodies. The James Webb Space Telescope, launched in 2021, was modified to allow for intensive study of exoplanet atmospheres.

Carbon dioxide, water, and other compounds have already been detected in exoplanet atmospheres, but the ultimate goal is to find concentrations and mixes of oxygen, ozone and perhaps methane – gases which are associated with biology. Because oxygen and ozone quickly bond with other elements, the presence of large reservoirs of elemental oxygen, for instance, would tell scientists that it is constantly being produced. On Earth, the production of oxygen is largely a function of life, although abiotic means also exist and could be more pronounced on another planet.

The goal of searching for life beyond Earth has now also been embraced by the National Academy of Sciences, which in its most recent decadal study recommended a major NASA commitment to a next generation observatory that would have the power to clearly detect signs of life on distant exoplanets. NASA teams have begun the process of planning for such a future Grand Observatory, that will have widest telescope mirror ever sent to space. A NASA-European Space Agency Mars sample return mission is also both underway with rock caching by Perseverance and in development for a planned 2030s delivery to Earth of pieces of Mars.

With so many lines of research underway, NASA leaders are optimistic about finding life beyond Earth. When, where and what kind of life remain uncertain questions, but the quest is clearly well underway.

Updated: August 7, 2022