Mention the meteorite Allan Hills 84001 in a room full of planetary scientists, and they'll know exactly what you're taking about.
Scooped from atop the icy wastes of Antarctica in 1984, ALH 84001 became a prized specimen several years later when geochemists realized that it had been blasted off the surface of Mars. But it absolutely exploded onto the public stage 15 years ago when a team of NASA researchers announced that they'd found evidence for fossilized microbes inside it.
The hoopla has long since died down — most, but not all, of the purported biogenic evidence has been negated. Yet ALH 84001 remains unique among the several dozen known Martian meteorites because it's the only one that is truly ancient (4.1 billion years old) and because it's riddled with tiny round nodules of carbonate with a striking layered composition.
Over the years, inside and outside the "fossil" debate, researchers have offering competing theories for how the carbonate beads formed. Some scenarios required high-temperature conditions (impacts, volcanoes), while others would have taken place in standing water at much lower temperatures (perhaps below water's freezing point). Consensus proved elusive because, well, no one really knew how hot or cold Mars was a few eons ago.
But now we do know — at least for the spot on Mars where ALH 84001 called home (wherever that was). In a recent issue of the Proceedings of the National Academy of Sciences, three researchers explain how they've been able to take the temperature of ancient Mars by subjecting the meteorite to something called clumped-isotope thermometry. Developed by coauthor John Eiler (Caltech), this technique precisely measured the isotopes of carbon and oxygen present in the ALH 84001 carbonates. The relative abundances of the rare isotopes oxygen-18 and carbon-13 is very sensitive to formation temperature, and in this case they imply a formation temperature of about 64°F (18°C).
"The thing that's really cool is that 18° is not particularly cold nor particularly hot," says co-author Woody Fischer in a Caltech press release. "It's kind of a remarkable result."
Eiler and Fischer, together with lead author Itay Halevy (Weizmann Institute of Science, Israel), are not saying that ancient Mars basked in the kind of warm, wet climate that would have welcomed the development of life. The carbonates could have been deposited in ALH 84001 over just a few hours, and such a brief warm-up could have followed a regional impact or a burst of warmth from a hydrothermal source near the planet's surface.
Not long afterward the planet went into a global deep-freeze that continues today. But the three researchers have shown that the conditions were once far more clement — at least in spots. So it's not surprising that geologists are eager to find ALH 84001's launch site.
A few years ago researcher Vicky Hamilton (then at the University of Hawaii) pinpointed Eos Chasma in Valles Marineris as a strong candidate, but that suggested target might change pending full analysis of super-detailed surface imagery and spectra from NASA's Mars Reconnaissance Orbiter.