Nasa has found what it calls “strong evidence” that life once existed on Mars after re-examining a meteorite¹.

近日，美国宇航局发现火星曾存在生物体的“有力证据”，这一结论是在该机构对一枚陨星进行重新检测后得出的。

An image of meteorite No.ALH84001. Nasa has found what it calls “strong evidence” that life once existed on Mars after re-examining a meteorite.

A research team at Johnson Space Centre in Houston has been re-examining a meteorite（陨石） that hit Antarctica 13,000 years ago, and found the most compelling（强迫性的，令人注目的） evidence yet that the planet once harboured bacterial² life.

The team says that microscopic³ crystals（晶体） found in the rock are almost certainly fossilised bacteria that have many characteristics in common with bacteria found on Earth.

“The evidence supporting the possibility of past life on Mars has been slowly building up during the past decade,” said David McKay, Nasa chief scientist for exploration and astrobiology（太空生物学）.

“This evidence includes signs of past surface water including remains⁴ of rivers, lakes and possibly oceans and signs of current water near or at the surface.”

The debate centres on magnetite（磁铁矿） crystals found in the meteorite. A study carried out in 1996 suggested that they were biogenic（由生物活动产生的） - produced by living organisms.

Critics, however, suggested that the magnetite was formed by a chemical process called thermal⁵ decomposition⁶（热分解）.

The new paper uses more advanced instruments to refute（驳斥） the critics’ theory, arguing that ancient life remains the most plausible⁷ explanation（似有道理的解释）.

“We believe that the biogenic hypothesis（假设） is stronger now than when we first proposed it 13 years ago,” said Everett Gibson, a Nasa scientist.