Eighteen years after a Martian meteorite sparked a debate over alien-looking "nanofossils," researchers are reporting that different structures inside an even bigger space rock suggest biological processes might have been at work on the Red Planet hundreds of millions of years ago.

"We're convinced that this is another one of the important data points that is going toward answering the big question: Was there life on Mars?" Everett Gibson, a researcher at NASA's Johnson Space Center who was involved in both studies, told NBC News.

He made clear, however, that even the latest study won't settle the big question. "We don't come out and say we have found life on Mars," Gibson said.

Microscopic structures

Gibson and his colleagues focused on microscopic structures deep within a 30-pound (13.5 kilogram) meteorite known as Yamato 000593, which was found in Antarctica by a Japanese team in 2000. An analysis of the rock's composition showed that it was formed on Mars about 1.3 billion years ago and altered by interaction with water on Mars. Scientists say the rock was blasted into space by a cosmic impact and fell to Earth within the past 10,000 years.

In February's issue of the journal Astrobiology, the research team — led by Lauren White of NASA's Jet Propulsion Laboratory — describes microscopic tunnels that thread their way through the meteorite's interior, as well as tiny blobs of carbon-rich minerals that are embedded within layers of rock.

This photomicrograph shows bands of minerals inside a meteorite from Mars, including "micro-tunnels" that researchers say are suggestive of microbial weathering. White et al. / Astrobiology

The red circle in this photomicrograph highlights spherules in a meteorite from Mars that are enriched in carbon, compared with the background material indicated within the blue circle. White et al. / Astrobiology

The team says such structures are suggestive of ancient weathering through biological processes. If the meteorite had come from the bottom of Earth's oceans, "we'd say, 'Gee, this rock contains evidence that there was microbial activity that was eating away at the rock,'" Gibson said.

The researchers emphasize that they "cannot exclude the possibility that the carbon-rich regions in both sets of features" are the product of non-biological processes. However, they say the "textural and compositional similarities to features in terrestrial samples, which have been interpreted as biogenetic, imply the intriguing possibility that the Martian features were formed by biotic activity."

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White told NBC News that she didn't want to make any "life on Mars" claim prematurely. "I definitely don't think this is a 'smoking gun' paper," she said. "I want the reader to decide in the context of everything that we show."

Revisiting controversy

A similar tale was told in 1996 about a Martian meteorite known as ALH84001. Back then, researchers said that chemical analysis as well as wormlike features they called nanofossils supported their view that life was once present. The report caused a sensation — but other experts insisted that the features weren't biological in origin. As a result, the "life on Mars" claims faded into scientific limbo.

Since then, Gibson and other researchers who were involved in the original ALH84001 study have been trying to gather more evidence for their case. White joined the team as a summer-session researcher in 2007 and was asked to take a close look at Yamato 000593. "Who's going to turn that down, right?" she said.

White was intrigued by the micro-tunnels and the spherules, and worked with other researchers to determine they were similar to geological features on Earth created through biological processes. The key challenge was to show that the features were the result of activity on Mars rather than earthly contamination. "Whatever these features are, the data suggests they likely originated from Mars," White said.

One of the paper's authors was NASA scientist David McKay, who took center stage during the controversy over ALH84001. McKay died a year ago after a long struggle with heart problems. His health difficulties complicated the years-long publication process.

"He was actually working on this paper the day he passed away," White said. "I promised his wife that I would publish this paper, because it meant so much to him to keep this work going."

Skepticism persists

Most planetary scientists accept the view that Mars was once warmer, wetter and more Earthlike — and thus more hospitable to life — than it is today. It's also plausible to suggest that if life did exist on ancient Mars, it should have left characteristic traces in rocks from the Red Planet. But the evidence laid out in the Astrobiology paper isn't likely to settle the controversy.

"I don't think the science community will find 'textural and compositional similarities' compelling enough to be proof of a biological origin," Chris McKay, an astrobiologist at NASA's Ames Research Center and no relation to David McKay, told NBC News in an email.

"We have our critics, and that's what science is all about."

Gibson acknowledged that the case for life on Mars is far from closed. "We have our critics, and that's what science is all about," he said.

He and his colleagues are following up on their findings with more detailed chemical analysis. "We have to go to the next step of going in there and tearing these carbon molecules apart," Gibson said.

NASA's Curiosity rover is searching for evidence of organic carbon on the Red Planet. But to settle the big question conclusively will probably require bringing fresh rock samples back from Mars and analyzing them on Earth with high-precision scientific instruments. That could take a decade or more.

"Until that time, we need to use the best thing we have," Gibson said, "and that's the meteorites from Mars."

In addition to White, Gibson and McKay, the authors of "Putative Indigenous Carbon-Bearing Alteration Features in Martian Meteorite Yamato 000593" include Kathie Thomas-Keprta and Simon Clemett.

Tip o' the Log to NBC News space analyst James Oberg.

NBC News' Alan Boyle will discuss the latest from Mars as well as developments in commercial spaceflight from 10 to 11:30 p.m. ET Tuesday on "The Space Show," a live streaming-audio program hosted by Dr. David M. Livingston.