‘Life on Mars?’ Is More Than a David Bowie Song—It’s a Question Humans Keep Asking

Following the 1990s discovery that Mars had once been a watery oasis in the solar system’s habitable zone, NASA devised a strategy to search for evidence of H2O. “Follow the water” became the watchwords of every Mars mission thereafter.

The hopes of finding life on Mars took a new turn in 1996. In August, a team of scientists from NASA and Stanford University announced that a Mars meteorite found in the Allan Hills of Antarctica contained evidence of ancient Martian life. Scientists hypothesized that the 4.2-pound (1.9-kilogram) potato-sized rock, to which they gave the less-than-catchy name ALH84001, formed as an igneous rock about 4.5 billion years ago, when Mars was a much warmer place. Then about 15 million years ago, a large asteroid hit the Red Planet and jettisoned the rock into space, where it remained until crashing into Antarctica in around 11,000 BCE. According to some experts, ALH84001 appeared to contain fossil-like remains of Martian microorganisms 3.6 billion years old.

In the summer of 1996, scientists announced evidence of organic molecules in this Martian meteorite, which suggested that primitive life may have existed on early Mars. This discovery stimulated enthusiasm for a new exploration mission. Prior to the meteorite study, NASA had already initiated a program in which it planned to send two spacecraft, an orbiter and a lander, to Mars roughly every two years over the course of a decade. Making the search for life a central goal for future Mars exploration efforts, NASA formed a multidisciplinary group to develop strategies leading to the discovery of signs of life.

As it turned out, when other scientists examined the findings of the NASA-funded team most rejected its conclusion that ALH84001 provided evidence of past life. Even without a scientific consensus on the significance of these findings, they nevertheless led to a renewed interest in Mars exploration.

After the Mars Pathfinder mission in 1997, NASA shifted the focus of its Mars exploration efforts toward seeking evidence of water. In 1998 the Mars Global Surveyor (MGS) entered Martian orbit with the primary goal of mapping the planet’s surface. This was completed in January 2001, with more than 98 percent of the surface mapped, and so the MGS turned to pursuing “follow the water” efforts to reimage in greater detail areas that showed evidence of ancient flooding. Using its images, planetary scientists identified more than 150 geographic features that were created by fast-flowing water.

Launched in August 2005, with a more sophisticated instrument package than its predecessor, the Mars Reconnaissance Orbiter (MRO) started operations on Mars on March 10, 2006. It joined several other probes exploring the planet, either in orbit or on the surface. These included MGS, the European Space Agency’s Mars Express, 2001 Mars Odyssey, and two Mars Exploration rovers, Spirit and Opportunity. At the time, this set a record for the most operational spacecraft on Mars. Images from MRO, alongside additional images from MGS, showed further evidence of dry riverbeds, floodplains, and gullies on Martian cliffs and crater walls that suggested the presence of water flowing on the surface at some point in the history of Mars. MRO remains operational, continuing to “follow the water.”

But the most intriguing possibility came with the realization that life-forms might still be living on Mars today, beneath polar caps or in subterranean hot springs warmed by vents from the core. Discoveries from many spacecraft and the evidence signifying that water once freely flowed suggested that Martian equivalents of single-celled microbes that dwell in Earth’s bedrock might be found in underground caverns. Scientists were quick to add, however, that these theories were unproven.

Of the fifty-one missions to Mars over the course of the Space Age, only slightly more than half—twenty-six in total—can be considered fully successful. While this statistic has improved with time— missions to Mars have fared better than this 50 percent success rate since the 1990s—even in the years since 1992, eight of twenty-six missions have failed or partially failed.

So what makes Mars exploration so difficult? It is a complex question, but the distance from Earth and the state of human technology have much to do with this issue. Mars remains hard to reach, and difficult to navigate and investigate on arrival.

The history of missions to Mars offers a reality check for anyone who believes that we might easily send humans to the planet in the near term. Thus far, only nation state actors have undertaken missions to Mars. That may change in the future if sufficient profit motive for undertaking these missions is found. For most of the Space Age, Mars missions were undertaken only by the United States and the Soviet Union. That has changed in the last few decades. Six of the twenty-six missions since the end of the Cold War are by entities other than the USA or Russia. A broadening of actors will continue into the future.

The various types of robotic missions thus far undertaken to Mars range from flybys to orbiters to landers to rovers to, most recently, atmospheric flyers. We will see an increasing number of landers, rovers, and flyers in the future.

Despite NASA’s multiple failures to obtain approval for sending humans to Mars in the twentieth century, it remains an important goal for many as this new century unfolds. There is good evidence that sometime during the twenty-first century, humans will reach Mars.

A sustained effort to learn more about Mars has been underway since the dawn of the century, and robotic explorers have unveiled a rich and enticing world. A search for life on Mars— even in the form of microorganisms lying beneath its polar caps or in subterranean hot springs warmed by vents from the Martian core—fuels this quest. These endeavors will not abate as the century progresses.

Sending humans to Mars presents a significant challenge, even though it remains an inviting goal. But there are proposals that could work if astronauts were able to “live off the land.” The first humans to Mars may well extract fuel and consumables from the Martian environment. Such a mission would require a twoyear-plus timetable to fly to Mars, work on the surface, and then return to Earth. It would also require a vehicle for getting there, a lander with a scientific laboratory and habitat module, a power plant, rovers, a way to grow food, and, most critically, an ascent vehicle for leaving.

Fuel could be manufactured on Mars from the local atmosphere, which mostly consists of carbon dioxide. This gas could be split into its component elements in a reaction chamber. The process, first discovered by French chemist Paul Sabatier (1854–1941), produces methane and water. The methane would be pumped through a cryogenic cooler, which reduces it to a liquid state, and stored as rocket fuel. The water could be turned into hydrogen and oxygen for use by astronauts.

Upon arrival, humans would need to deploy a greenhouse, most likely inflatable, to grow food. Using rovers, the crew could begin exploration of the surrounding terrain. They would collect samples for analysis and drill into the Martian substrata in search of water and any subterranean life that may exist. They could also search for fossils and seek to confirm the existence of any other natural resources on Mars. Once their time on the planet ended, the crew would undertake a hundred-day trip back to Earth.

The technical problems of such a mission are considerable, but they can be overcome with sufficient time and resources. Engineers would need to develop low-cost, high-reliability technologies to make this a reality.

During such a trip, as well as on the surface, the crew would be exposed to diverse types of radiation. A fast transit time is the best protection against radiation, but solar flares could also be lethal, especially in the unprotected vacuum of space. Engineers would have to develop protective systems for the mission to be successful. It may also be necessary to maintain some artificial gravity on the spacecraft carrying the crew to and from Mars to help minimize biomedical problems associated with prolonged exposure to low-gravity environments. This could be accomplished by rotating areas of the vessel.

If humans do go to Mars in this century, it will be because those on Earth are willing to expend enough resources to overcome these obstacles. At present, there is only modest public support for such expenditures.

Of course, we could send human expeditions to Mars. There is nothing magical about it, and a multinational mobilization to do so could be successful. But a human Mars landing would require a decision to accept considerable risk and to expend substantial funds over an extended period. Using Apollo as a model, anyone seeking a decision to mount a human expedition to Mars must ask a critical question: What political, military, social, or economic reason, cultural challenge, scenario, or emergency can they envision to which the best response would be a major commitment to sending humans to Mars?

In addition, with a record of accomplishment that includes numerous failures of robotic probes to Mars, are we willing to accept significant risk to humans on such a mission? Absent a major surprise that would change this equation, it is doubtful humans will land on Mars before the latter half of this century.

Read more in Smithsonian Atlas of Space, which is available from Smithsonian Books. Visit Smithsonian Books’ website to learn more about its publications and a full list of titles. 

Excerpt from Smithsonian Atlas of Space by Roger D. Launius © 2024 Quarto Publishing Plc.

Roger D. Launius | READ MORE

ROGER D. LAUNIUS served as chief historian of NASA between 1990 and 2002 and is a recipient of the NASA Exceptional Service Medal and the Exceptional Achievement Medal. From NASA, he moved to the Smithsonian's National Air and Space Museum in Washington, DC, where he served most recently as Associate Director for Collections and Curatorial Affairs. He has written or edited more than 30 books on aerospace history, including The Smithsonian History of Space Exploration, Apollo's Legacy, and NASA Space Shuttle: 40th Anniversary.