Scientists are excited over the latest geological evidence for the existence of life on Mars. But as we have seen many times in the past, the Red Planet guards its secrets closely with the possibility of non-life explanations for the formations.

As geologist Joel Hurowitz said in our recent conversation on Quirks & Quarks, the rock in question was found by the Perseverance rover that has been exploring what was once a river delta that drained into an ancient Martian lake.

Two chemicals found in the delta, vivianite and greigite, are most often formed on Earth by microbial activity in the soil — hence the excitement. But under certain conditions, they can also be formed by chemical reactions. In other words, we have potential evidence for life on Mars, but no conclusive proof.

Mars has been tantalizing us with the possibility of life on its surface since American astronomer Percival Lowell looked through his telescope in 1894 and thought he saw straight lines crisscrossing the planet.

He called them canals, which are artificial, not naturally formed channels. Lowell believed these enormous features were engineered like the Panama or Suez Canals, but on a much larger scale. To him it suggested there was a civilization on Mars with amazing technical skills.

Lowell's claims were fuel for science fiction writers and movie makers, leading to stories such as H.G. Wells' War of the Worlds, where Earth is invaded by benevolent Martians.

It was not until the 1970s that robotic spacecraft sent to Mars debunked Lowell's theory, revealing a landscape pockmarked with craters that looked more like the moon than a habitable planet.

But further robotic exploration did discover dried up river beds, or channels, proving that water once flowed on the surface billions of years ago.

On Earth where there is water, there is life, so perhaps there is or was life on Mars. It is just not in the form of cities and networks of canals.

Watch: Animation of water drying up in the ancient river where Perseverance found signs of life: 

Further compelling evidence came in 1976, when two Viking landers carried out experiments to look for life in the Martian soil. When the soil was warmed up and fed a drop of dilute nutrient solution, it gave off a gas as though microbes were present. But another experiment searching for organics found none, so it was assumed the reaction was chemical, not biological.

Then there was great fanfare in 1996 about a Martian meteorite found in Antarctica that contained what appeared to be a tiny fossilized worm. Finally, real proof of Martian life. But again, the geochemists stepped in and declared the shape could also be formed by non-biological processes. 

That debate continues, especially since NASA's Curiosity rover recently detected the largest organic molecules found on Mars to date, which might be fragments of a common component of life seen here on Earth.

Despite more than a century of exploration, no definitive life has been found on the surface of Mars — at least not yet. But what about the possibility of life within the Martian crust? 

As microbial biogeochemist Karen Lloyd talks about in her new book, Interterrestrials, there is a multitude of life forms that thrive deep underground on Earth. Kilometres beneath the surface, microbial life survives independent of sunlight, living in water that seeps through cracks in the rock. 

Mars too has a permafrost layer that raises the possibility of liquid water deeper down where microbial life could thrive. That could be a promising place to look.

Looking beyond Mars to the more than 6,000 verified planets orbiting other stars, and thousands more identified as candidates, many appear to have hostile surfaces. But that doesn't mean there couldn't be life. The universe might be teaming with life mostly hidden from view underground.

NASA recently introduced a new class of astronaut candidates who will be among those to return to the moon and possibly to venture on to Mars. As they learn to fly spaceships and survive on other worlds, perhaps they should also get lessons on how to dig deep holes, so when they land on Mars they will have not just a flag to plant in the ground, but a drill.