Mars wasn't always the dry, dusty husk of a planet we know today.
In fact, more than 3 billion years ago, it may have been a place of chequered plateaus, deep valleys and, most vitally, flowing water. At least, that's the picture painted by scientists analyzing data from the Mars rover.
But so far, we've seen nary a sign that those conditions actually led to life on Mars.
Or did we? A former NASA scientist is convinced that we actually did find proof of life there 40 years ago — but the results were dismissed as an anomaly.
The scientist, Gilbert V. Levin, published an opinion piece in Scientific American this month claiming the 1976 mission that sent Viking landers to Mars netted a tantalizing find: soil that contained organic matter.
Martian soil had been widely considered bereft of microbial life. But an experiment conducted by the Viking probes — and dubbed Labeled Release (LR) — begged to differ.
For the test, the probes inserted nutrients into the seemingly dead soil. If there was any kind of life in that dirt, it would eat those nutrients and leave an echo of the act — a faint gas that would be captured by radioactive monitors.
Levin, who was the principal investigator on the NASA experiment, called it "a very simple and fail-proof indicator of living microorganisms."
First, the test was conducted on the untouched soil. And then the test was repeated on soil that had been heated to the point that all life in it would be dead. If the soil consumed the nutrients in the first test, but not the second, then it would seem that a biological reaction had really occurred. In other words, it would be a telltale sign that the soil hosted the essence of life.
The result of those experiments, according to Levin, were conclusive. The raw Martian soil gobbled up the nutrients, while the cooked soil did not.
"As the experiment progressed, a total of four positive results, supported by five varied controls, streamed down from the twin Viking spacecraft landed some 4,000 miles apart," Levin wrote.
"It seemed we had answered that ultimate question."
Or did they?
The reaction disappeared from follow-up experiments. NASA eventually dismissed that early result as a false positive. It wasn't a sign of life, but rather a chemical reaction that scientists couldn't quite understand.
Levin left little doubt where he stands on the issue, titling his article, "I'm Convinced We Found Evidence of Life on Mars in the 1970s."
But how to explain the failure to replicate the early results of the LR experiment? Was life on Mars so incredibly shy it retreated from subsequent investigations?
NASA's position, Levin notes, was that they had discovered "a substance mimicking life, but not life."
And over the next 43 years, most scientists had to stick to that conclusion, as none of the Mars landers that followed Viking were outfitted with life detection equipment.
But that's changing. Over the years, Mars has left a kind of breadcrumb trail for life-hunting scientists. Last year, the Curiosity rover found organic compounds and molecules in soil samples taken from the planet's Gale crater, a 3-billion year-old mudstone crevice. Although organic matter isn't in itself life, it can be considered a food source, or "chemical clue" for life.
And in 2020, more breadcrumbs may be collected by the new lander that's scheduled to depart for the Jezero Crater, a region that may have once boasted a river delta that flowed into an ancient lake.
While the new rover won't include life detection equipment, it will have an instrument capable of looking for past signs of life.
For his part, Levin is hoping NASA will revive the decades-old LR experiment, revising its parameters for the new rover. By analyzing that new data, a panel of experts may draw the same conclusion he did so many years ago.
"Such an objective jury might conclude, as I did, that the Viking LR did find life."