Mention the meteorite Allan Hills 84001 in a room full of planetary scientists, and they'll know exactly what you're taking about.

Martian meteorite press conference

David McKay (lower left) and his colleagues at their August 1996 press conference announcing possible signs of ancient microbes in the Martian meteorite Allan Hills 84001.

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.

Mars meteorite ALH 814001

Weighing in at 4¼ pounds (1.9 kg), Allan Hills 84001 is a Martian meteorite with an age of 4.1 billion years &mdash: and interesting stories to tell about the Red Planet's ancient past.


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).

ALH 84001 thin section

Yellow globules of carbonate mineral are rimmed with black-and-white "oreo cookie" deposits in this magnified section of the Martian meteorite ALH 84001. The black rims contain some of the controversial magnetite crystals. The view is 0.5 mmwide.

Courtesy John Valley and Allan Treiman.

"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.


Image of Simon Hanmer

Simon Hanmer

October 23, 2011 at 7:05 am

Kelly ...

18 degrees C is intriguing, but we have to beware. It's a nice surface temperature from our anthropocentric perspective ... but to my knowledge there's no unequivocal evidence that these carbonate "nodules" formed at or close to the martian surface. While hydrothermal temperatures are thought of as being in the 100s degrees C, the temperature gradient must drop to ambient somewhere ... possibly passing thru 18 degrees. Nonetheless, your words of caution regarding the duration of the measured temperature regime are well taken -----


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Kelly Beatty

October 23, 2011 at 11:22 am

Simon... thanks for your comments -- always glad to get feedback from the pros. modeling by J. Melosh and others shows that any material ejected from an impact at or above escape velocity had to be in a spall layer very close to the surface. not sure what that means for ALH 84001, but it should become clear once we find its "ground zero".

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October 25, 2011 at 11:30 pm

This story on Allan Hills 84001 was interesting but what I want to know is how scientists can be certain where this meteor came from out of all the innumerable possibilities. It seem unlikely that a meteor would come from a planet. What evidence could point to it's origin as being Mars?


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November 3, 2011 at 12:42 pm

Oddly, more meteorites have been identified on Earth as coming from Mars than from our own moon. They presumably are launched by meteorite impacts on the originating body, in this case Mars. A large enough strike can accelerate impact debris to escape velocity and, behind the original impactor, open a low-pressure tunnel in the atmosphere above the impact, enabling ejecta to escape.

Why has Mars been identified as the origin for ALH 84001 and similar meteorites? A summary can be found here:
In short, Martian meteorite ages are younger than those of meteorites originating from asteroids; they contain trapped gasses matching those measured for the Martian atmosphere; and elemental and oxidation characteristics match Mars alone.

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Roger Venable

November 14, 2011 at 9:42 am

Our perspective on this matter may be somewhat colored by our perception that Mars is a cold place. However, it is widely thought that the polar regions on the Red Planet undergo extremes of heating and cooling over millions of years, associated with the slow changes in obliquity that result from the lack of a massive moon.

Though the obliquity at present is 25 degrees, it varies over a cycle of about 120,000 years from 13 degrees to 35 degrees, and over a long, chaotic "cycle" of a few million years it is calculated to get as high as 47 degrees. Under such a condition, the polar caps would have no residuae in the summers, and the polar and subpolar regions would be quite warm near the summer solstices. And, in the winters, they would be much colder than at present.

It seems to me likely that the temperature requirements that Halevy, Eiler, and Fischer state for the formation of the carbonates with their measured isotopic composition could be met by the changes in obliquity. This would not require any proposition of a grossly warmer Mars 4 billion years ago than in the last few million years.

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