Eighteen years after a NASA-funded research team offered contested, yet startling claims of possible traces of fossilized bacteria in a meteorite of Martian origin,a second science team with significant participation from the first is noting evidence for water movement through a considerably younger meteorite from the red planet laced with the carbon building blocks for life.
The latest developments involving efforts by scientists from NASA's Jet Propulsion Laboratory and Johnson Space Center -- combined with 2012 findings from NASA's Mars Curiosity rover -- are re-fueling debate over life there, while lending urgency for the collection of Martian soil and rock samples that can be returned to Earth for analysis.
The latest developments, outlined in the February edition of the journal Astrobiology, address tiny features observed in Yamato 000593, a 30-pound Martian meteorite recovered from an Antarctic glacier by the Japanese Antarctic Research Expedition in 2000.
Lead author Lauren White, of JPL, and her colleagues describe tunnel and micro-tunnel structures threaded throughout Y000593.The undulations are similar to tiny structures found in volcanic glasses on Earth and reported by researchers who study terrestrial interactions between bacteria and similar lava mineral formations. The research team also describes tiny spherules enriched in carbon sandwiched within the rock layers.
White and her co-authors -- among them the late David McKay, co-leader of the 1996 study involving the possible fossilized structures within the Martian meteorite Alan Hills 84001 -- do not exclude non biological mechanisms as the source of the carbon found in Y000593. But in a Feb. 27 statement from JPL, they note textural and compositional similarities to features in Earth samples that have been interpreted as biogenic.
"This is no smoking gun," said White. "We can never eliminate the possibility of contamination in any meteorite. But these features are nonetheless interesting and show that further studies of these meteorites should continue."
Suggestions the Yamato features were contaminated on Earth are tempered by the discovery of similar structures within Naklah, a signature Martian meteorite that was collected shortly after it fell to Earth in Egypt in 1911 and presumably unexposed.
Y000593 formed about 1.3 billion years ago in a Martian lava flow, according to analysis.
Analyses found that the meteorite was formed about 1.3 billion years ago from a lava flow on Mars. Around 12 million years ago, Mars was struck by an impactor with enough force to eject Y000593 from the surface. It circled the sun until it fell into Antarctica about 50,000 years ago.
The ALH 84001 story is similar, though its origins date to 4.6 billion years ago, when the solar system was forming. It remained below ground and was exposed to water during a warmer, wetter era on Mars between 3.6 and 4 billion years ago, according to analysis. It, too, was liberated from Mars by the impact of a large comet or asteroid about 16 million years ago. After wandering around the sun, the 4.2 pound specimen entered the Earth's atmosphere about 13,000 years ago and fell into Antarctica, where it was discovered in 1984.
McKay and his colleagues at NASA and Stanford University based their claims on several lines of discovery within the Alan Hills meteorite, including carbon compounds, mineral phases typical in primitive microscopic organisms found on Earth as well as possible microfossils. All were found within close proximity.
Critics, however, have argued the NASA/Stanford researchers failed to establish a clear biogenic source for the curious Alan Hills 84001 features or that they actually formed while the rock was on Mars.
Within months of landing in Gale Crater on Mars in August 2012, the nuclear-powered, roving Curiosity/Mars Science Laboratory found mineral evidence at Yellowknife Bay that a neutral form of water -- not too acidic and not too alkaline -- flowed through the rocks and soil before Mars turned cold and dry. The conditions may have been suitable for life to emerge.
While nature has conveniently provided experts stuck on Earth with scientifically rich chunks of Mars in the form of meteorites, there may be no way of clearly establishing whether life emerged there without collecting samples of rock and soil, then preserving and returning them to Earth for analysis.
"The nature and distribution of Martian carbon is one of the major goals of the Mars exploration program. Since we have found indigenous carbon in several Mars meteorites, we cannot overstate the importance of having Martian samples available to study in Earth-based laboratories," said Everette Gibson, a lead researcher in both the Allan Hills and Yamato studies. “Furthermore, the small sizes of the carbonaceous features within the Y000593 meteorite present major challenges to any analyses attempted by remote techniques on Mars."
NASA's latest attempt at sample return calls for the 2020 launching of a Curiosity-based rover that would collect and cache Mars samples until a follow-on robotic or human mission could retrieve them. The European and Russian space agencies, meanwhile, are collaborating on ExoMars 2018, a rover designed to drill for and analyze samples of the Martian surface for signs of life in situ.