For centuries, astronomers stared at the Red Planet. The better their telescopes, the more tantalizing it seemed. Its proximity to the Sun meant a warm day on Mars might be akin to springtime day in New Orleans, while a cold day would feel a little chillier than Antarctica in winter. It was quite a swing, and its average temperature is 80 below zero, but humans — or Martians — could survive there.

“Mars has long been — rightfully so — the world that has revealed the most extensive history of liquid water on its surface,” says Kevin Hand, an astrobiologist and planetary scientist at NASA’s Jet Propulsion Laboratory. “We can see on the surface evidence of ancient rivers, evidence of ancient seas, and possibly even vast oceans.”

Through umber discs, telescopes pointing towards Mars soon saw shading. There were dark patches — were they mountains or oceans? In fact, they were polar ice caps, just like the ones on Earth. In the mid-1800s, we worked out Mars’ first maps. A couple decades laters, we observed yellow clouds. Then in 1877, Asaph Hall, an American astronomer, found two moons circling Mars. Percival Lowell, who built the observatory that found Pluto, spent the early twentieth century obsessed with what he believed were artificially-carved canals all across the Martian surface.

NASA engineers spent the late 1960s hunched over drafting boards, slide rules in hand, working out how to build a robot that might descend through the Red Planet’s wispy atmosphere and land on its terrain.

In 1969, the agency contracted with building material supplier Martin Marietta to send two robots (in case one crashed), both part of a project called Viking. Nothing was off the table for what we might find. We bet that the planet was most likely Earth-like, though we also feared it might have Barbasol soil, with the consistency of shaving cream.

During planning meetings, Carl Sagan, the celebrity astronomer who helped choose the Viking landing sites, even filibustered meetings, demanding lights be added to the two landers. He reasoned that Martian animals might be attracted to the strobes at night. They might scamper up close for a better look.

To protect the Martians from invasive Earth-born microbes, engineers even baked both landers for 45 hours, 22 hours of which reached temperatures of 233 degrees Fahrenheit. When we found life, we did not want it to be microbes we brought from home. And we did not want to infect a new world with deadly disease.

In 1976, Viking 1 and Viking 2 landed successfully on opposite sides of the planet. Engineers at the mission operations center at NASA Jet Propulsion Laboratory switched on the cameras.

Neither probe touched down into shaving cream soil; instead they discovered rocks, dirt, and later, a sienna sky. They did not, however, discover cacti, bushes, tumbleweeds, or scorpions. And nothing crawled up to get a better look at either alien visitor.

Mars, as far as complex life forms went, was likely barren. Even the chemistry sets installed on the landers to determine if microbial life wiggled its way through the upper inches of Martian soil showed inconclusive results, at best.

“The Viking missions — the orbiters and landers — were incredibly successful,” says Hand. “The problem is that Mars was not, and continues not, to be very generous. If Mars does, in fact, have any signs of life that existed in the past, those signs of life, those chemicals, those compounds, those minerals are somewhat elusive.”

And then NASA pivoted.

An underappreciated aspect of space exploration is that the study of other worlds invariably redounds upon our own. Before a veritable flotilla of spacecraft began probing Venus in the sixties, little was known about the greenhouse effect.

Venus is about the same size as Earth, with the same density, gravity, and composition, and its orbital path isn’t that far from our own. Unlike Earth, however, its surface is 900 degrees — hot enough to melt lead. Measurements made around a planet beset by a rampant greenhouse effect gave context to Earth’s atmosphere, increased the scientific understanding of how greenhouse gasses could affect global temperatures here, and added urgency to the climate discussion.

It was the same with Mars. “Because of Viking,” explains Hand, “NASA made a major pivot in the search for life beyond Earth: from the macro to the micro.”

Mars, says Hand, was once awash with oceans, rivers, ponds, and creeks. The liquid water on its surface vanished, though no one knows where to. Across millions of years, any microorganisms there might have adapted to the changing planet, and could still be  thriving in now-arid lakes, or at the very least, have left fossilized remains.

According to Hand, to learn more about possible life on Mars, NASA in the 1990s began investing heavily in the study of extremophiles on Earth — microbes that thrive in harsh environments such as volcanoes, Antarctica, the ocean floor, or scorching deserts. By understanding the habitability of our own ecosystem, the agency hoped to learn how to better search for life elsewhere in the solar system — a new mission which it called “follow the water.”

Though Mars is the most Earth-like planet in the solar system — and thus thought to once be the only place circling our Sun that might harbor alien life — scientists have since started to also look for life in new, and even more distant, territories.

“The discovery of vast quantities of deep, salty oceans of liquid water beyond the asteroid belt have led to yet another pivot by NASA in the search for life,” says Hand, who is also the author of Alien Oceans: The Search for Life in the Depths of Space.

He explains that these water oceans beyond Earth are now known to be a common planetary phenomenon. And just as Mars led to serious investment in Earth’s deep biome, so too has the discovery of alien, subsurface oceans led to further curiosity about our own.

Including the Viking orbiters and landers, NASA has successfully sent fifteen spacecraft to Mars in the last fifty years, nine of which operate on or operated on the surface, each designed to chip away at the “life question,” one piece at a time.

Back on Earth, the field of biology has experienced its own revolution. In the late 1970s and early 1980s, geneticists slowly came to better understand DNA. An international effort to sequence the human genome started in 1990 and the advancement of computers enabled scientists to complete the project in 2003. They have since begun doing the same for bacterial genomes.

Spaceflight is also entering a second golden age, with spacecraft able to make astoundingly precise measurements once thought impossible, around worlds once considered inaccessible. Planetary science, meanwhile, has fundamentally transformed as a field of study. What began as solely astronomy has now come to incorporate geology, chemistry, and biology. This has enabled an interdisciplinary understanding of the origin and evolution of planetary bodies. After all, if you want to know where life came from, you need to know where the habitats supporting life did as well.

Shannon MacKenzie, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory, says the action on Mars today is beneath its surface.

“Maybe half a meter and further down into Mars's rocky crust might be a habitable environment, very similar to the way that down deep in Earth’s rocky crust are microbes hanging out, doing their thing, and they might be super old.”

This thriving region is called the “deep biome,” and even on Earth, it is a challenging environment to study. Doing so, however, yields new insights into how life can emerge and even thrive. Some of these creatures live for a thousand years before reproducing. “The deep biome in the Earth is one of the biggest biospheres on our own planet that we don't really understand,” says MacKenzie. Scientists who hope to study the Martian deep biome with robots will encounter not only the terrestrial problems of accessing extreme environments, but also the difficulties of operating anything in space. Fortunately, NASA is getting better. The Perseverance rover that the agency landed on Mars in 2021 can drill into Martian rock and soil and collect sample vials of pristine soil. A future rover will collect the vials and launch them back to Earth to study in labs.

Next year, NASA will launch the spacecraft Europa Clipper to determine the habitability of Europa. But Europa is not the only moon in possession of a subsurface ocean or the potential for life. Two of Saturn’s moons, Enceladus and Titan, are also now known to be ocean worlds, and NASA has plans to explore each of them in turn. The agency will send a quadcopter called Dragonfly to Titan in 2027. NASA is also developing a concept called Enceladus Orbilander, which will fly through the plumes of water being blasted from Titan’s interior, land on its surface, and also search for evidence of life.

While the Perseverance and Curiosity rovers are currently rolling across Mars looking for signs of ancient, fossilized life, the search for life on ocean worlds will involve looking for extant life: things that swim. For now, NASA can only get at the surfaces of these water bodies, but scientists are still hopeful.

“If we find signs of life on the surface on the ice of these ocean worlds, then those signs might actually be indicative of life that is alive today in the oceans below. That's exciting because if we do discover extraterrestrial life, we can study its biochemistry and figure out how it may or may not relate to life on Earth — and the origin of life itself,” says Hand.

The discovery of life on another world, alive or extinct, would not only transform the fields of planetary science, biology, or chemistry, but would also have profound implications for philosophy and religion. Accordingly, the search for it, and any announcement of its discovery, must be made only with sober and serious intention. As Carl Sagan once wrote, “Life is the hypothesis of last resort.”

MacKenzie endorses this sentiment. “I like to joke that the only slam dunk is if we drop a camera in one of these oceans and a whale swims by,” she says. Even on Earth, after all, the definition of life is up for debate. Biologists do not know if viruses are alive or not, for example. “It’s that same level of ambiguity, except we're going to go out in space and explore,” says MacKenzie.

And she says that it falls to the public to make sure scientists don’t get too carried away:

“It's on all of us to be a little skeptical. It's fun to think about, but the notion of life elsewhere in the solar system must be taken seriously and given the consideration that it deserves. It is a worthwhile question to ask, and must be answered in the right way.”