Can you give us a brief summary of your work on the Allan Hills meteorite?
We published a paper in the journal Science1 presenting evidence for fossil life on Mars. Those lines of evidence were the presence of carbonate globules or pancakes found in the Allan Hills meteorite, ALH84001, from Mars. In the same meteorite we also found the presence of organic material, PAHs, polyaromatic hydrocarbons. The third line of evidence was the presence of very tiny crystals of magnetite and sulfide. We proposed the magnetite, 10- to 100-nm sized crystals, was formed by bacteria. And the last line of evidence was a number of morphological features that we interpreted as possible microfossils. So, taken all together, we thought a reasonable interpretation was that they were all related to primordial life.
Figure 1. This high-resolution scanning electron microscope image shows an unusual tube-like structural form that is less than 1/100th the width of a human hair. It was found in meteorite ALH84001, a meteorite believed to be of Martian origin. This structure will be the subject of future investigations that could confirm whether or not it is fossil evidence of primitive life on Mars 3.6 billion years ago.
How do you know it's extraterrestrial and not earth contamination?
We know that meteorite and 12 others are from Mars for several reasons. One, they all have fusion crusts on them, which can only form when something goes through the atmosphere at high speed. So, we know they're not from earth. Why do we think they're from Mars? Because when we sent Viking there in 1976, we got enough information on the composition of the Mars atmosphere that when we analyzed these meteorites carefully for their gas content, it turned out that they were extremely similar to what we analyzed on Viking.
Furthermore, when their oxygen isotopes (18O, 17O, and 16O) are analyzed and plotted, they form a tight little family of oxygen isotopes that is significantly different from oxygen isotopes of earth, other meteorites, or the moon.
This tells us that these 13 meteorites were formed from the same planetary body and that planetary body was different from anything else we know about. Based on the rare gas content found in four or five of them, which matches Viking so well, we can conclude that these 4 or 5 meteorites all came from Mars. And based on the oxygen isotope data and the unique family defined by these isotopes, we reason that if any of the meteorites came from Mars, they all came from Mars.
Other supporting data includes a wide range of crystallization ages for the igneous rocks (4.0 billion years to 160 million years). Such a range could only be formed on a planetary body that has been volcanically active over most of its lifetime, and this rules out most small bodies in the solar system. (Io is an exception.)
As you opened up the meteorite, the farther you went in, the more evidence there was of the extraterrestrial organisms, correct?
That's correct. For example, as we did profiles from the outside to the inside of the meteorite, the number of PAHs increased as we went farther into the interior. Their number is lowest right at the fusion crust. Even a millimeter or so in from the fusion crust, the numbers are low because that area was heated, though not melted, when the meteor shot through the atmosphere. That plus a number of other lines of evidence showed that the PAHs cannot be Antarctic contamination.
We have carefully analyzed clean ice from the same ice field and found no detectable PAHs. In fact, ALH84001 shows abundances of PAHs several orders of magnitude above the detection limit of the technique, so these values were at least several orders of magnitude above the abundance of PAHs in the ice. Careful laboratory experiments showed that carbonates do not preferentially concentrate or collect PAHs, as has been asserted by others. So if the ALH84001 PAHs are Antarctic contaminants, how did they become so concentrated in the meteorite? We know of no plausible mechanism. Furthermore some of the PAHs we found in ALH84001 are so insoluble in water that they are very unlikely to move around, even if they were present in the ice.
We also analyzed a number of porous micrometeorites collected in the same Antarctic ice. Each contained a unique set of PAHs easily distinguishable in type and relative abundance from the other micrometeorites collected within less than a meter's distance. If the PAHs came from Antarctic contamination, the abundance patterns in these micrometeorites should have been similar; they are not. So the obvious interpretation is they are indigenous to the micrometeorites. These micrometeorites are much more porous than ALH84001, so if they showed no evidence of Antarctic PAHs, one would expect ALH84001 to also show no evidence.
There is some evidence in the meteorite, though, of earth contamination in the form of amino acids, probably from Antarctica, and carbon 14 from our atmosphere. But, based on the reasoning outlined above, we conclude that some of the organic compounds (PAHs and possibly others that we did not measure) in this rock absolutely come from Mars. There's just no question in our minds that they're martian.
Furthermore, the magnetite, which we now consider our strongest evidence, is embedded in the carbonate pancakes or globules. These carbonates are a mixture of Ca, Mn, Mg, and Fe carbonate, some of which are concentrated in concentric bands. Most of the magnetite is in the iron-rich bands near the outer boundary of carbonates in the rock. Those carbonates, based on isotopic composition, are quite different from any terrestrial carbonates based on their relatively heavy oxygen isotopes.
It's the oxygen isotope ratios in the -CO3. Oxygen normally has 16 nucleons, 8 protons, and 8 neutrons; the isotope of interest here has 18 nucleons. It's a very heavy oxygen isotope. And the carbonates are enriched by about 18-30 parts 18O per thousand compared to a terrestrial standard of ocean water. Although it is still a small percentage of the total oxygen, you just don't get natural carbonates on earth with anything close to that.
Based solely on their oxygen isotopes it was realized early that those carbonates were formed on Mars. Recent careful Rb-Sr and Pb-Pb age dating has shown that the carbonates formed about 4.0 billion years ago; much older than any known earth carbonate, but still not as old as the silicate minerals in the Allan Hills rock, which are 4.5 billion years old.
Those magnetites meet all six criteria proposed by Joseph Kirschvink at California Institute of Technology for biogenic magnetites (that is, magnetites made by bacteria). These criteria include single-domain grain size, very narrow range of size distribution, and high chemical purity. No natural or laboratory terrestrial magnetite from earth has been found to exhibit these six criteria unless it is associated with bacterial cells.
In ALH84001, these magnetites are a subpopulation of the total number of magnetites in the carbonates. But about 25 percent of the total magnetites in the carbonates of this rock meet Kirschvink's criteria. While it is difficult to prove a negative, we know of simply no other way these magnetites can form other than by bacteria.
What are magnetites used for?
On earth, magnetites are formed by bacteria to help them sense the magnetic force lines of the earth. This makes them more efficient in finding their energy supply. They look for certain layered zones in lakes and oceans where the oxidation-reduction reactions are most easily used for their energy production. By following the magnetic lines of force, which normally have a vertical component, these bacteria can reduce a 3-dimensional random search to a 1-dimensional random search. This strategy greatly improves their search efficiency and has given them an evolutionary edge over bacteria that do not sense magnetic fields and therefore may waste their time (or lifetime) searching for the best layer where the energy-producing iron reduction or oxidation reactions can take place.
The Mars orbiting spacecraft now mapping Mars has detected paleomagnetic fields generated from magnetized rock units that formed when Mars had a core dynamo. These bacteria on Mars would have used that magnetic field to help them find their optimum environment for energy.
The magnetites in the Allan Hills sample are firmly embedded in these carbonates and we know the carbonates are made on Mars because of their nonterrestrial oxygen isotope ratios. We don't see any way that these magnetites can be made by earth bacteria. They had to be made by Mars bacteria.
I recently saw you on TV talking about new microorganisms found in other meteorites. Can you elaborate?
We've looked at two other Mars meteorites. These are not from Antarctica. One of them fell on Egypt in 1911. It's called Nakhla. It was seen falling and was picked up the next day or so. There were several fragments. Some were put in an Egyptian museum. One of those fragments was almost completely sealed in a fusion crust and has been sitting in a British museum for the past 87 years. That was brought here last April to Houston where it was cracked open in the NASA Astro materials clean lab under a nitrogen atmosphere. It was then distributed to 100 or so scientists around the world for analysis. It is within that rock, Nakhla, that we see a number of forms that are almost identical in appearance to known terrestrial microfossils of bacteria. Their morphology is the same. And their chemical composition -- iron oxide -- leads us to believe they are fossil bacteria because that is how many bacteria get fossilized. So, that's what we think we're seeing in Nakhla. Some also appear to be the locus for the crystallization of a clay mineral (smectite).
We have not proved beyond a shadow of a doubt that these are fossilized bacteria, that they are from Mars and not earth, although they are deeply embedded in the Mars meteorite. Those are two things that we plan to do in the future with detailed analysis, mainly isotopic analysis of oxygen and carbon and also transmission electron microscope analysis of the internal structures and the internal composition.
Now, the big difference between these possible fossils and Allan Hills' is that these are much bigger than anything we saw in the Allan Hills meteorite that may have been fossils. The Nakhla objects are a half micron to 2 microns in diameter and that's right in the size range of most common earth bacteria. So we won't get into this argument about the nanobacteria here.
There's another meteorite from India called Shergotty, which fell in 1865. It was seen to fall, it was picked up immediately, and it's been in museums there ever since. Some of it's now been distributed to the science community. In looking at Shergotty, we saw some very similar things we saw in Nakhla. We've also made comparisons now with dozens of known fossil bacteria samples from earth, including some we mineralized in experiments using bacteria extracted from deep within the Columbia River basalt in cooperation with scientists from Pacific Northwest Laboratories, Battelle. There's a pretty good match.
It's interesting that the carbonates in the Allan Hills meteorite have been dated at 4.0 billion years by two different laser fusion techniques. That would make these magnetites the oldest known fossils, far older than the oldest earth fossil, which is roughly 3 - 3.6 billion years.
The second thing that is really interesting is the Nakhla from Egypt was formed about 1.3 billion years ago and the Shergotty from India was formed about 165 million years. So, the Shergotty is relatively quite young. If there really is fossilized life in that youngest meteorite, that means that over much of Mars' history, we have evidence for life and furthermore nothing has happened in the last 165 million years that we know of that would kill off life on Mars. If it was there 165 million years ago, it's probably still there. That means when we go there for samples, there's a serious chance that living bacteria could be in the Mars sample that we bring back.
That leads me to my next question. What future Mars missions are planned?
The Mars Surveyor '98 mission, which is on its way right now, will land near the south pole of Mars on December 3, 1999. This mission also includes the Mars Climate Orbiter, which will begin orbiting Mars in September 1999. These missions will look for water and ice in the southern polar cap and will collect new data on the atmosphere and climate of Mars. There is no specific experiment to look for life on it.
Figure 2. These are possible Martian fossilized microbial cells attached to a mineral in the Egyptian meteorite Nakhla. They range from about 1 to 2 µm in size and each one is firmly attached to the crystal by clay minerals, which are known to commonly form on cells as part of the mineralization or fossilization process. The scale bar is 5 µm.
In 2001, the Mars Surveyor '01 mission will launch another lander and orbiter. The lander will not specifically look for life but will do a lot of characterization of the rocks and soils and may reveal geologic environments favorable to life.
In 2003 and 2005, more Mars Surveyor missions are planned. The current concept is for another lander and orbiter to launch each year. The lander may collect samples to bring back and may put them into orbit awaiting recovery by the 2005 mission. The samples will actually be brought back by a small return vehicle, which will collect the samples from Mars orbit. If all works out, we'll have roughly half a kilogram from one location, collected in 2003, and another half a kilogram from another location, collected in 2005.
The returned samples will undergo a very rigorous quarantine and search for life. They'll be treated as very dangerous biohazard material and will be carefully contained. If there is life, and it's determined to be not dangerous, only then will we take the life forms out of quarantine and look at them in detail.
How do you know if they're dangerous or not?
That's a good question. Those protocols have to be worked out. The medical biologists are thinking about that right now. How do you test whether something is dangerous to life? In the days when we brought back lunar samples, we fed them to rats and we did various other kinds of experiments to directly test whether they would be harmful to living things. They were shown to be not harmful to anything. I don't know what the procedures will be this time. One protocol may be to challenge different kinds of cells to see if they cause problems with cell reproduction and growth, cause DNA mutations, inhibit chemical transmission, and so forth.
Figure 3. This shows two possible fossilized Martian cells and the fragments of others. The cell in the center has the remains of a fossilized biofilm partly covering its surface. The cell to the right is partly embedded in the clay mineral that fills veins or cracks in the Nakhla meteorite. This clay mineral is now known to have formed on Mars about 700 million years ago. If these bumps are truly fossilized Martian microbes, they are then about 700 million years old.
Is there a possibility of getting DNA from fossilized bacteria?
That's a good question and one that we're looking at carefully. DNA is known to deteriorate over time, and the time it takes to deteriorate is a function of the temperature, whether it is kept moist or dried out, and how much radiation dose it receives. It's conceivable that there might be some DNA left in these bacteria. The ones in Shergotty and Nakhla could be as young as the time the meteorite left Mars, which is on the order of 10 to 30 million years. We think the DNA can probably last that long if it's favorably preserved. The evidence so far indicates that these fossil-like forms that we describe have been chemically altered by the mineralization process. If so, that means the DNA, if any, is gone. It's been replaced by minerals.
Does earth atmosphere permeate the rock?
Andrew Steele, who is part of my team, has found fungal filaments penetrating the fusion crust and growing into the interior of Nakhla. If they can do that, then certainly terrestrial gases can get in there. We know Nakhla is contaminated. We know Allan Hills was contaminated. Both of them contain evidence for earth life. So we have to understand that terrestrial contamination and be able to sort it out from anything that might be true Mars biology. It's a challenging job, but it's not impossible because the isotopes are likely to be different. Therefore, that's what we're looking at right now.
So, the defining technique is the isotopes of oxygen.
I think so. Also, some other isotopes may have a role. The best evidence for fossils will likely be based on transmission electron microscope data. If indeed they are fossils, the defining technique as to whether they are martian will be based on the composition of isotopes of oxygen, carbon, deuterium, maybe sulfur. If they have common terrestrial signatures for those isotopes, then they are not martian. We hope to have a definitive answer, one way or the other, within a year.
If you find extraterrestrial life, what does that mean to you personally as a scientist?
I would be just incredibly excited about it. I think it means, from a science point of view, hey, life may develop anywhere where the conditions are favorable. Life may be a spontaneous process that simply starts wherever you have water. That seems to be the one defining criterion.
It may also mean -- and this would have to develop after we have a lot more data on Mars life, such as DNA genome sequencing -- that earth life started on Mars and came here in a meteorite 4 billion years ago. In one of those ironies of life, we'd all be related. That is, all life throughout the solar system would be related. Of course, that conclusion would have to be reconstructed through genome sequencing.
I think we have an exciting decade ahead of us because we will acquire Mars samples, we will have new insight into the origin of life, and we will probably definitely find out if life began on Mars and might even still be there. If Mars turns out to have diverse cellular life forms, we will have strong incentive to send people there to study them and their geologic environment. Even if life never existed there, or is extinct, we will learn why and we will likely be much more appreciative of life on earth. By studying Mars, we will ultimately learn much about the development and evolution of life on earth.
David S. McKay is the Senior Scientist for Planetary Science and Exploration and Director of the Johnson Space Center for the Study of Biomarkers in Astromaterials. He led the team that discovered possible evidence for life on Mars in Mars meteorites. He has received numerous awards and has been published more than 300 times. He was interviewed by Frederick Su.
1. D.S. McKay, et al., "Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001," Science 273, 924 (1996).