The recent announcement that scientists have spotted what they believe to be evidence suggesting ancient life on Mars seemed like the latest in an endless cycle of tantalizing hints followed by pleas for more funding. What will it take for them to declare something definitive?

This time, the SUV-sized Perseverance Rover detected minerals called vivianite and greigite, which typically form in marshes, lake bottoms, or hydrothermal sediments — environments where microbes thrive. Here on Earth, microbes play a role in their formation by altering the chemical forms of iron and sulfur. A NASA team concluded that life is the most likely explanation for these minerals’ presence on Mars.

Scientists already have several lines of evidence suggesting life may have arisen on Mars. Raising the probability is intriguing but won’t fundamentally alter our understanding of the universe. What would? Actually finding remnants of alien life and using them to understand how it functioned. Martian life might consist of different building blocks and use something other than DNA or RNA to store and copy information.

Because all earthly life shares a common origin, finding a second tree of life would forever change humanity’s understanding of ourselves. It would help guide our search for life elsewhere in the solar system and on the many seemingly habitable planets astronomers have discovered orbiting other stars across the galaxy.

But to actually see what’s inside potential Martian life, scientists would need to bring samples back to Earth to study — a project NASA launched but has struggled to complete.

In the early 2000s, the space agency proposed a relatively inexpensive sample-return mission called Groundbreaker. Christopher McKay, a planetary scientist and astrobiologist at the NASA-Ames Research Center, compared the mission to a VW Beetle. NASA instead chose to pursue a Cadillac version, which began with Perseverance.

Perseverance, which landed on Mars in 2021, has been collecting samples and storing them in sealed titanium containers, which are now awaiting retrieval for a return to Earth. The return was slated for as early as 2033, but the schedule has fallen behind to at least 2040. A recent independent review estimated the mission would cost between $8 billion and $11 billion, a price tag that is untenable in today’s budget reality. So, it’s been put on hold.

Meanwhile, China successfully used a relatively simple technique to return moon rock samples and has devised plans to apply the same technology to bring Martian samples back to Earth within five years.

But how will scientists in the US or China recognize alien life? They’ll have compelling evidence if they find highly complex molecules such as hemoglobin or chlorophyll, which are very unlikely to form outside of living systems. Yet extraterrestrial organisms might rely on molecules we’ve never seen before.

In recent years, scientists have developed a mathematical framework to determine whether any given molecule is too complex to have formed without life. This framework, assembly theory, measures complexity by counting the number of steps needed to build a molecule from basic building blocks. Once that number reaches about 15, the probability of the molecule forming without a biological process guiding its construction becomes highly unlikely.

There is already strong evidence that Mars supported a warm, watery environment early in its history, before it lost its atmosphere and dried out. Life appeared early on Earth in a similar setting — by 3.7 billion years ago at the latest — which is why scientists are so hopeful it may have originated on both planets. Even if all life on Mars went extinct billions of years ago, it could have left behind complex molecules that could tell scientists how it was assembled.

The older Curiosity Rover has detected organic compounds that could have come from dead Martians, but the compounds aren’t complex enough to rule out non-biological origins. Analyzing Mars samples back here on Earth could reveal much smaller traces of past life.

The bad news is that we might return a sample and find only the same kind of life that exists here. In the early solar system, debris was frequently hurled between planets. Scientists have identified several Martian meteorites that landed on Earth, and in 1996, they announced they might have found signs of fossilized life.

The evidence was never considered sufficient, and excitement quickly faded. Later studies, however, showed that rocks traveling between planets can preserve delicate magnetic structures, suggesting that ejection into space wouldn’t necessarily be lethal. It’s not impossible that life originated on Mars and seeded life on Earth.

Finding evidence of interplanetary transfer of life would be exciting — but also a bit of a let-down because we wouldn’t gain a second form of life. That’s part of the gamble of doing science: the universe doesn’t always conform to our highest hopes.

Still, a sample return is a safer bet than sending astronauts to Mars without knowing whether life exists there. Astronauts would take precautions to avoid contaminating Mars, but they might still inhale dangerous spores or bring them back to Earth. Biologists have shown that spores can survive in suspended animation in extremely hostile environments. Discovering life on Mars would be world-changing — but it’s not something we’d want to learn the hard way.

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This column reflects the personal views of the author and does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

F.D. Flam is a Bloomberg Opinion columnist covering science. She is host of the “Follow the Science” podcast.

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