BEIJING, Dec. 15 -- Did Mars ever harbor life? Scientists have found new evidence for possible life on the Red Planet in a piece of Martian meteorite that landed on Earth after about 700,000 years of space travel.

According to research carried out by teams of Chinese, German, Swiss, and Japanese scientists, more than 10 pieces of coal-like carbon particles, thinner than one-tenth of the width of a strand of hair, were found in a thumb-sized piece of the meteorite.

"We used advanced equipment to determine the carbon particles are organic matter, and to rule out the possibility of graphite, which is inorganic," said Lin Yangting, a lead scientist of the research team from the Institute of Geology and Geophysics, Chinese Academy of Sciences.

"Furthermore, we found an enrichment of the light carbon isotope in the organic matter," said Lin. "It's so exciting! This could be a promising indicator of life on Mars."

The lighter the carbon isotope, the greater the possibility of biological activity, while a heavy carbon isotope indicates the opposite, Lin said. Paleontologists analyze ancient rocks' carbon isotope ratio to determine the date of Earth's earliest life forms.

He explained that organic matter like coal and petroleum on Earth are formed as a result of biological activity. But not all organic matter is related to biological activity. Organic compounds have been synthesized in labs. Carbon isotopes are a key indicator in judging whether organic matter resulted from life.

Lin's research team published their findings this month in the professional journal "Meteoritics & Planetary Science."

They used the NanoSIMS, an ion microprobe that can analyze particles smaller than one-millionth of a meter, to analyze their elemental and isotopic composition. "No one has ever seen the organic carbon components in the stone with such clarity," Lin said.

But could the signs of life have come from Earth, rather than Mars? What if the stone was contaminated immediately when it landed on Earth?

Lin's team ruled out this possibility by analyzing the hydrogen isotope in the organic compounds.

"It has a Martian fingerprint, different from the one on Earth. So we say the organic compounds come from Mars," Lin said.

Previously, scientists had claimed to find organic compounds or signs of life in Mars meteorites, but were met with doubts. Lin says his team's findings need further testing.

More than 120 pieces of Martian meteorite have fallen to Earth, according to the meteorite database of the Meteoritical Society. They were blasted off the planet when asteroids hit Mars.

"Most of them were recovered after staying for a long time in Atlantic ice or hot deserts," Lin said. "People have no idea when they came to earth and, year by year, they may have been contaminated by substances on earth."

But the piece Lin's team looked at is quite special. The meteorite, officially named Tissint, is new to Earth, with witnesses who saw it fall.

At about 2 a.m. local time on July 18, 2011, a bright fireball was observed by several people in the region of the Oued Draa Valley, east of Tata, Morocco, according to the Meteoritical Society, an organization that records all known meteorites.

It was first yellow in color, and then turned green, illuminating the entire area before it appeared to split into two parts, said eyewitness Aznid Lhou.

Three months later, nomads began to find fresh, fusion-crusted stones near Tissint village.

Chinese scientists bought some of them from meteorite collectors for research. The stones are mostly coated in a glistening black fusion crust, and in some places the crust has broken, revealing a pale gray interior.

Though there have been four other meteorites with witnesses before Tissint, the most recent was more than 51 years ago, Lin said. "Tissint is a new Martian meteorite that can supply us with fresh samples."

"At first we were looking for traces of water in it, and accidentally found carbon particles. That's a rare case," Lin said. Though scientists have confirmed that Tissint formed six hundred million years ago on Mars, it's still unclear when the organic carbon components came into being.

Scientists say the surface of Mars has not been suitable for life for the past three billion years. "If life existed after that, it might have been living underground," Lin said.

The red planet resembles Earth in many ways. It is made of rock, and it has an atmosphere and weather systems.

In recent years, Mars orbital and rover missions have brought abundant evidence of water or methane on the planet -- potential signs of primitive life.

The Mars Odyssey probe, launched by the United States in 2001, discovered a vast amount of ice beneath the Martian surface.

In 2003, NASA launched two rovers, Spirit and Opportunity, and both found signs that water once flowed on the planet's surface.

In 2004, the ESA's Mars Express spacecraft detected plumes of methane gas on the red planet, while the Mars rover Curiosity, which landed on the planet in 2012, has not found direct evidence of life.

Aside from sending spacecraft to Mars, the other approach is to analyze fallen martian meteorites, the only available rocks from Mars, Lin said.

"Lin's team's new finding in Tissint is so far the most inspiring evidence for life on Mars," said Ouyang Ziyuan, chief scientist of China's lunar exploration project.

Despite the inspiring findings, Lin admits that they cannot draw a final conclusion about life on Mars until they analyze samples collected directly from Mars.

But the 52-year-old scientist seems confident. "If I were to make a bet," he said, "I would wager that there was once life on that planet."