Chemicals in a Martian meteorite that were once held up as possible evidence of life on ancient Mars were more likely the product of heat, water and chemistry, according to a new study. Researchers from the Carnegie Institution of Washington and the University of Oslo in Norway reached that conclusion after comparing the four-pound (two-kilogram) extraterrestrial rock, ALH84001, with samples of earthly volcanic material—and discovering a matching pattern of minerals consistent with a chemical process that yields carbon compounds after rapid heating and cooling.
Although the study does not support the existence of life on Mars, researchers say it shows that some of the chemical precursors of life—at least as we know it—were kicking around on the Red Planet some 4.5 billion years ago.
Discovered in 1984 in Allan Hills, Antarctica, researchers believe that meteorite ALH84001 struck Earth some 3.5 to four billion years ago after being kicked up from the Martian surface and into space by the impact of another meteorite. NASA researchers triggered international headlines in 1996 when they discovered, among other possible indicators of life, traces of polycyclic aromatic hydrocarbons (multiringed carbon molecules found in living cells) along surface fractures in ALH84001.
Initial excitement that such compounds might represent traces of Martian microbes faded within a year or two as researchers came up with other possible explanations for the meteorite's unique features.
"What was missing was whether Mars could actually undertake organic chemistry itself," says Carnegie geophysicist Andrew Steele, who led the new study. To find out, he and his colleagues examined the chemical makeup of 0.1-millimeter carbon globules from samples taken at several depths from ALH84001. They identified rings of magnetite (iron oxide) arranged around the sooty spheres in the same pattern as in frocks from a volcano on the island of Svalbard, Norway.
The researchers, whose results appear in the September 2007 issue of Meteoritics & Planetary Science, attribute the pattern to known chemical reactions in which strong heating followed by rapid cooling causes carbon dioxide to rearrange itself into more complex molecules in water, with iron oxide serving as a catalyst. Additional carbon molecules could have been cooked up when ALH84001 was ejected from Mars, they note.
"This process may be making a lot of the stuff of life, without any help from things that are alive," says John Rummel, senior scientist for astrobiology for NASA's Planetary Science Division, who was not involved in the study. And that, he adds, means there could still be life there waiting to be discovered.
Steele says the results lay the groundwork for interpreting future chemical studies of the Martian surface, such as experiments to be carried out by the Mars Science Laboratory rover mission, scheduled to launch in fall 2009. "If you find complex organic species on Mars, it's not necessarily life."