New Study of Meteorite Provides More Evidence for Ancient Life on Mars

New Study of Meteorite Provides More Evidence for Ancient Life on Mars
This image of the meteorite, seen through a scanning electron microscope, shows bumps that resemble a fossilized colony of microbacteria. Some of the rounded bumps are preserved at the top of the surface and resemble individual spherical and ovoid-shaped microbes. Image credit: NASA.

( -- In 1996, when scientists examined a meteorite from Mars previously uncovered in Antarctica, they were intrigued by what looked like microscopic fossils of ancient Martian life forms. Now, using new technology that wasn't available 13 years ago, NASA scientists have found further evidence that the materials and structures in the meteorite are likely signs of ancient life, rather than the results of inorganic processes.

ALH84001 History

Scientists estimate that the , called Allan Hills 84001 (ALH84001), formed on about 4.5 billion years ago, making it one of the oldest known objects in the solar system. Because the meteorite contains microscopic carbonate disks that are about 4 billion years old, scientists have previously hypothesized that the meteorite interacted with water that may have existed on Mars at this time.

Much later, about 15 million years ago, a larger meteorite likely struck Mars and ejected ALH84001 into space. After spending most of that time traveling throughout the , the meteorite landed on Earth about 13,000 years ago. Then, in 1984, a team of US scientists discovered it in Antarctica. The meteorite finally made news headlines in 1996, when NASA scientist David McKay and others peered at the rock under a and saw what appeared to be nanoscale fossils of bacteria-like life forms.

Bacterial or Thermal Origin?

Now, McKay, along with Kathie Thomas-Keprta, Everett Gibson, Simon Clemett, and Susan Wentworth, all of NASA's Johnson Space Center, have revisited the original hypothesis with new observations of the meteorite. The study is published in a recent issue of the journal Geochimica et Cosmochimica Acta.

In the new study, the scientists used advanced microscopy techniques to investigate the carbonate disks and, more importantly, the magnetite within the disks. These embedded magnetites are the apparent fossils that exhibit features similar to contemporary magnetotactic bacteria.

During the past 13 years, different groups of scientists have proposed competing hypotheses to explain the origins of these magnetites. Some of the leading hypotheses are non-biological, suggesting that the magnetites were formed via thermal decomposition of the carbonates in which ALH84001 was struck by other meteorites. Such impacts may have increased the temperature of ALH84001 and caused the carbonates to decompose into magnetites via bond redistribution. In some models, ALH84001 may have experienced this shock by a random meteorite impact while still on Mars, while in other models, thermal decomposition may have occurred due to the impact event that ejected ALH84001 from its home planet.

But whatever event might have triggered a thermal decomposition process, the scientists argue in the current study that very few - if any - of the magnetites embedded in ALH84001 carbonates are a product of thermal decomposition. By analyzing details such as the percentage of magnetite volume in the carbonate disks, the trace amounts of impurities observed in some of the magnetites, and the lack of siderite which some previous models suggested may have decomposed to form magnetite, the scientists concluded that these new observations were inconsistent with the previous inorganic-based thermal decomposition hypotheses.

By showing that it’s very unlikely that the magnetite originated from the decomposition of ALH84001’s carbonate, the scientists argue that possible biological origins of the magnetite need to be considered more seriously than before.

“For the past 10 years, the leading (and only) viable non-biologic hypothesis for the origin of the nanophase magnetites concentrated in ALH84001 has been thermal or shock decomposition of iron-bearing carbonates, a process known to produce small magnetite crystals,” Thomas-Keprta told “Our paper has falsified this non-biologic hypothesis by showing, based on thermodynamics and minor element chemistry, that this non-biologic hypothesis simply cannot produce the ultrapure magnetites actually present in ALH84001 as a significant population of all magnetites. By falsifying this non-biologic hypothesis, we are left with only the biologic hypothesis to explain the detailed properties of the magnetites in this martian meteorite.”

Magnetite Biosignature

Although they have not yet developed a model for the origin of the magnetite in ALH84001, the researchers’ new observations are consistent with the possibility that the magnetite has an “allochthonous origin,” in which it was exposed to aqueous solutions such as water.

As Thomas-Keprta explained, the magnetite in ALH84001 could have been one of several ferromagnetic minerals produced by magnetotactic bacteria that live in aquatic environments. When these bacteria die and their shells degrade, a chain of magnetite is released into the environment. Without its confining shell, the magnetite chain configuration cannot be maintained, so individual magnetite crystals begin to mix with inorganic particles in the water.

On Earth, magnetotactic bacteria are quite common in aqueous environments, and scientists often find magnetites in surface and subsurface sediments.

“For many years, the presence of the specific kind of nanomagnetite formed by magnetotactic bacteria on Earth have been completely accepted as a biosignature when found in any Earth sediment or rock,” Thomas-Keprta said, noting that these magnetite have very specific properties.

“When we first documented these specific properties in the ALH84001 carbonates, the only alternate non-biologic hypothesis that was commonly accepted as viable was the thermal decomposition of iron-bearing carbonate,” she said. “Now that we have completely falsified this hypothesis with this latest paper, we are still left with the specific properties of the ALH84001 magnetite that, if found on Earth, would be a robust biosignature indicating production by bacteria.

“We also point to the many discoveries since our original paper showing supporting evidence such as an early strong magnetic field on Mars (necessary for the development of magnetotactic bacteria); the presence of near surface water at many locations on current-day Mars; the presence of possible oceans, major drainage channels, and other features associated with an early wet Mars; and the recent evidence for variable releases of methane into the Martian atmosphere. . . . We do not believe it is too incautious to restate our original hypothesis that such magnetites constitute strong evidence of early life on Mars.”

Explore further

Relic of life in that Martian meteorite? A fresh look

More information: K.L. Thomas-Keprta, S.J. Clemett, D.S. McKay, E.K. Gibson, and S.J. Wentworth. “Origins of magnetite nanocrystals in Martian meteorite ALH84001.” Geochimica et Cosmochimica Acta, 73 (2009) 6631-6677.

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Dec 17, 2009
Are there any experiments that the Mars Rovers can perform that would detect (the remains of) such magnetotactic bacteria on the surface of Mars today ?

Dec 17, 2009
It makes sense, I suppose, and Mars WAS more clement earlier on...but even so, life as an explanation for any non-terrestrial phenomenon seems like it should be the explanation of last resort. And maybe there's some explanation that's been overlooked, that no one's thought of yet? I suppose, if the...blobs in the rock are fossilized Martians, then they would have had to show up in it quite some time after the rock itself formed--either that, or life would have had to have arisen on Mars in the narrow window of time between the point when the planet formed and the time when (if life was there) it was sterilized by the late heavy bombardment, which doesn't seem like it leaves much time. I don't know; I'm hopeful, but...skeptical.

Dec 17, 2009
@Ronan: the article suggests that although the rock was formed billions of years ago, it likely interacted with water about 15 million years ago and then was ejected from the planet. The life forms could be from as early as 15 million years presumably.

This is exciting news.

Dec 17, 2009
Actually, according to the article, the microscopic carbonate disks are about 4 billion years old. They were ejected from Mars about 15 million years ago and eventually ended up in Antarctica about 13,000 years ago. So if life created these disks, it was life from a very long time ago. Another interesting point the article doesn't bring up - what is the likelihood of life on Mars and life on Earth independently evolving to be so similar? Might it be possible that bacterial life started on Mars, and Earth was "seeded" with that primitive life, leading to all the life we see on Earth today? Technically, we'd all be Martians then - a profound possibility. But that's putting the cart way ahead of the horse. The acceptance of the existence of life on Mars will require clear, unequivocal evidence from multiple locations; this rock alone won't answer the question.

Dec 17, 2009
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Dec 17, 2009
Great work by Thomas-Keprta et al! Linking NASA w/ the likely hood of early life on Mars is a breakthrough admission by the agency. If there was life on Mars 4 billion yrs ago there is a high probability that Mars still harbors life. In fact Viking detected what appeared to be life in the early '70s, i.e. diurnal emission of CO2, but no other missions to Mars since then has included life detecting instrumentation (as far as we know). Why is this? Never A Straight Answer!

Dec 17, 2009
Well Fabian, like wise if its similiar life, why cant Mars have been "seeded" by Earth?? If Mars can eject a water related rock to Earth..why not vice versa?

It would seem, since we have an abundance of life and water here on Earth, its more likely that the meteorite is from Earth originally.

Dec 17, 2009
Yellowdart - Keep in mind, Mars is farther from the sun than Earth is. It's much easier for a rock originating on Mars to get pulled closer to the sun and into Earth's orbit than vice versa. I suppose it's remotely possible that a really large asteroid strike on Earth might be energetic enough to send rocks from Earth on a highly elliptical trajectory out to Mars. But Earth's higher gravity, the friction created by its much thicker atmosphere and its closer proximity to the sun make that much less likely. Without doing the calculations, it would seem that the size of asteroid strike to Earth that could do the trick would be so catastrophically large that it would have obliterated life on Earth in the process. Again, I haven't crunched the numbers, but I hope you see my point. Gravity makes it much easier for rockes to go from Mars to Earth than the other way around. Therefore, the Mars-to-Earth seeding idea is more likely.

Dec 17, 2009
Either there is evidence or there is not. If there is evidence then its not speculation. Saying there is MORE evidence implies that the question has been settled which they tiptoe around and never deal with.

Dec 17, 2009
Either there is evidence or there is not. If there is evidence then its not speculation. Saying there is MORE evidence implies that the question has been settled which they tiptoe around and never deal with.

You're confusing evidence with proof.

Dec 17, 2009
@Yellowdart. Besides Fabian's ballistic argument, consider that Mars being farther from the Sun and smaller than Earth, surface temperature from earlier crust and mantle cooling should have made Mars biocompatible earlier than Earth. This seeding hypothesis is indeed very interesting, IMHO.

Dec 17, 2009
It will probably never be possible to prove that life started on one specific planet before the other, at least not while we are alive. While it is easier to launch rocks from Mars than from Earth, the intense bombardment during the solar system's youth makes it possible -even likely- that big rocks on more than one occasion have been thrown from Earth into space and eventually crashed on Mars. Thus, the inner planets can have a common biosphere (in terms of heritage), in theory they could even have seeded Europa with microbes before the ocean froze over.

Dec 18, 2009
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Dec 18, 2009
Yes the Earth is closer to the sun than Mars. But there are two major reasons expelling from Earth would have no problem reaching Mars. 1. Earth's gravity is 1600 times stronger than the sun's is being 93,000,000 miles away. If you can overcome the Earth's gravity, the sun's effect is a relatively small resistance. 2. Potential for the Radiometer Effect. Being closer to the sun, than Mars, and expelling away from Earth and sun, would help here.

Dec 18, 2009
Oh and at its closer points, Mars can be only about 36 million miles away...

Dec 19, 2009
Interesting that it shows even more signs of fossilization. I have just have never really been sold on the validity that it absolutely came from Mars... I'm not sure what evidence they have that shows it couldn't have come from somewhere way out of our solar system, from a body that has some similarities to mars, at least elementally. What proves that it could have only come from Mars and no where else? Sounds a little too convenient, I dunno.

Don't get me wrong, evidence of life in any form from ANYWHERE in our Galaxy is a big deal! I wouldn't say that if it turns out that it wasn't from mars after all that that would make the meteorite any less important... Mars on the other hand... well, it would hurt our hopes of find evidence of Martian life, I suppose.

Dec 21, 2009
Maybe an elementary question from a non-scientist - but how is it known that the meteorite came from Mars? That seems to have been established even before various robots were landed on Mars(1984).

Dec 26, 2009
I dont see how the whole "seeding" theory is possible in the first place. How can bacteria survive floating through a rock in space for years while being constantly rained on by deadly radiation from the sun?

Dec 28, 2009

Thanks. I agree you need a mechanism for propelling out a rock. I'm just pointing out that twinkies come from a twinkie to speak.

I think they are responding to Kerr's article in 2003 in Science journal are they not, that opposed the bacterial origin of the magnetite?

The main problem I have with the research here, is that it is still compared to 1976 Voyager atmospheric composition, as opposed to actual Mars rock composition, which we know from Taylor and Mittlefehldt (Science Oct 2000) is not representative of atmospheric composition.

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