Martian Meteorites

Meteorite Yields Evidence of Primitive Life on Early Mars
 

"Extraordinary claims require extraordinary evidence" - Carl Sagan



 On the Question of the Mars Meteorite
 Mars Meteorites (JPL)
 Meteorites from Mars (Johnson Space Center): brief and in-depth

What are the Martian meteorites?
How do we know they are from Mars?
How did they get off of Mars and to Earth?

A total of  28+ meteorites are now identified as of Martian origin.  They are collectively referred to as SNCs ("snicks"), as an acronym for the Shergottites (several stones that fell  in Shergotty, India in 1865), the Nakhlites (40 stones that fell in El Nakhla, Egypt in 1911), and the Chassignites (one meteorite that fell in Chassigny, France, in 1815).  Additional SNCs have been collected from Nigeria (1962), Indiana (1931), Brazil (1958), Libya (1998), Los Angeles (1999), Oman (2000), and Morocco (2001) and several from Antarctica, most importantly ALH84001 (found in 1984).  As many of these were observed falls (rather than finds), we know they are meteorites; in addition, chemically and isotopically, they form a very distinct class, different from terrestrial and lunar rocks and from other meteorites.  All, except the original Nakhlites and Chassignites are considered of the "shergottite" family.

All SNCs, with the single and very notable exception of ALH84001, are basaltic (volcanic) and have radiometric ages of 150 million years to 1.3 billion yearsThey therefore had to form on some parent body (a planet) that was volcanically active within the last 1.3 billion years.  This eliminates all asteroids, Mercury, the Moon, and all other moons in the solar system except Io.  Given the incredible energies that would be required to loft a meteorite off of Io and give it escape velocity from Jupiter, an Io origin seems impossible.  And given the enormously thick atmosphere of Venus and the difficulties of lofting a meteorite off of that planet; in addition, the SNCs contain hydrous minerals (water bearing clays) that almost certainly could not have formed on Venus. Given all of this evidence plus the recent origin of these volcanic rocks, their oxygen isotopic pattern that is clearly non-terrestrial, Mars appears to be the only likely site of origin for the SNCs.

The proof of their Martian origin appears to be almost absolutely conclusive, based on the chemical signatures of gases (132Xe, 84Kr, 20Ne, 36Ar, 40Ar, N2, CO2) trapped inside the meteorite EETA79001.  Among other things, these gases show the same enrichment in deuterium to hydrogen and 15N to 14N as is seen on Mars.  These gases appear almost identical to those in the Martian atmosphere, as measured by Viking in 1976, which in turn is unique in the solar system.  See graph.

How did they get from Mars to Earth? Probably, one or more large impacts ejected them from Mars.  This impact was sufficiently violent to eject some fragments with eject escape velocity from Mars, but not sufficient to have remelted the rocks.  They have cosmic ray exposure ages of either 0.5, 2.6 or 11 million years (three groupings).  These ages indicate how long the surfaces of these rocks were exposed to cosmic rays in space.  The three different ages could reflect three different impact+ejection events on and from Mars. Alternatively, only one impact+ ejection event might have occurred 11 MY ago, but the rock knocked off of Mars subsequently broke into smaller pieces 2.6 and again 0.5 million years ago as the result of subsequent collisions in space.

"The" Martian Meteorite: Allan Hills 84001
ALH 84001

What is ALH84001?

(the above web site has excellent details)

ages: Is it from Mars? Yes.
 


What's the big deal?

On August 7, 1996, Dr. David McKay and his co-workers released evidence (in a NASA sponsored press conference and in an article in the prestigious journal Science) suggesting that martian bacteria had once lived in ALH84001.  If they are correct, this is an absolutely phenomenal discovery.  If McKay et al. are right, life may have arisen independently on Mars and Earth or life may have arisen on Mars and been transferred via a meteorite to Earth.

What evidence did McKay et al. present? See Fossil Life in ALH 84001 for an excellent and detailed discussion, briefly summarized here:

What is the evidence for ancient life?
  1. Shapes that resemble bacteria
  2. microscopic mineral grains (magnetite, pyrrhotite, greigite, iron oxide, iron sulfide) that appear to be of bacterial origin.  All can be found on Earth as products of biological activity.
  3. PAHs (polycyclic aromatic hydrocarbons): organic (carbon-bearing) molecules claimed to be of bacterial origin.
How good is this evidence? (summarized from Fossil Life in ALH 84001):

Shapes that resemble bacteria: the evidence
The evidence is the presence of elliptical, rope-like and tubular structures that look bacteria-like.  Although these were smaller than any known bacteria, c. 1997, since that time nanobacteria have been found on Earth.  But similar shapes can be produced in many non-biological ways, including from laboratory processes such as cleaning and gluing and coating samples.  It is not absolutely clear that the shapes are not contaminants.  And if they are truly bacterial, could they be terrestrial contaminants? Again, McKay et al. made a good faith attempt to demonstrate this but the evidence is not unequivocal.

Microscopic Mineral Grains
McKay et al. show that these grains are associated with the carbonate globules that clearly are of Martian origin.  Thus, terrestrial contamination is not an issue.  Magnetite is made by some magnetotactic bacteria on Earth.  Pyrrhotite and greigite are produced as waste products by some bacteria.  Iron oxide and iron sulfide are not usually found in the same biological environments, but some bacteria can produce both.    All these mineral grains do look similar to the sizes and shapes of such grains produced by terrestrial bacteria.  But non-biological processes can also produce all these minerals of similar shapes and sizes.  McKay et al. suggest that the grouping of these minerals is highly unusual in non-biogenic environments, but this is not certain.

PAHs
PAHs can form biologically (decomposition of bacteria, for example) but also can form in many other ways.  Many different kinds of PAHs exist, and many carbonaceous chondrites as well as interplanetary space is full of PAHs; however, these PAHs appear different from those in other meteorites and those known elsewhere in the galaxy.  McKay et al. show convincingly that the PAHs in ALH84001 are not contaminated by laboratory handling and analysis.  They also show that the number of PAHs increases toward the inside of the meteorite, strongly suggesting but not proving that the PAHs are not a terrestrial contaminant.    Most likely, the PAHs are of uniquely Martian origin, although not necessarily of biological origin.

What have we learned since 1996?

After the original announcement by McKay et al., NASA and NSF jointly designated funds for extensive studies of the claim for ancient life on Mars.  A tremendous amount of work has been done in this regard over the last few years.  Allan Treiman, of the Lunar and Planetary Institute, in Houston, has maintained a web page attempting to explain and synthesize all of the papers on the Martian meteorite (also, less recent papers) in a very impartial, non-biased way.   As of December 12, 2000, he posted this note, which I interpret as an incredibly strong statement by an expert but impartial follower of this debate that the debate is over.

According to one scientist at the 31st meeting of the Lunar and Planetary Science Conference in 2000, "The community is moving on."

added note, May 22, 1997: Shocked Carbonates May Spell  in Martian Meteorite  ALH84001

 http://www.psrd.hawaii.edu/May97/ShockedCarb.html

In an electrifying paper published in August, 1996 in the journal Science, David McKay (NASA Johnson Space Center) and his colleagues suggested there were fossils of martian organisms associated with carbonate minerals in martian meteorite ALH84001. How these carbonate minerals formed (biologic origin or not) and the temperature at which they formed (low or high) are hotly debated questions. We have proposed an entirely different origin: the carbonates in ALH84001 formed in seconds at high temperatures (>1000oC) from melts produced during a large impact on Mars 4.0 billion years ago (Scott and others, 1997). We infer that it is unlikely that the carbonates or any minerals in them contain mineralogical evidence for ancient martian life.

added note, 3/22/2002:  No Knockouts in Martian Meteorite Showdown
By David L. Chandler
Sky & Telescopel
March 17, 2002

All last week, attendees at the 33rd annual Lunar and Planetary Science Conference in Houston, Texas, looked forward to one of the meeting's final sessions, whose main attraction was the controversial 4½-billion-year-old Martian meteorite known as ALH 84001. For years David S. McKay (NASA/Johnson Space Center) and his coauthors have maintained that this celebrated stone contains strong evidence - but not proof - of fossilized microbial life.

Friday's debate focused on tiny, uniform, and chemically pure crystals of magnetite embedded in carbonate globules within the meteorite, crystals that look remarkably similar to those produced by certain strains of terrestrial bacteria. Dadigamuwa C. Golden (Hernandez Engineering) and Douglas W. Ming (NASA/Johnson Space Center) reported that the perfectly formed, chemically pure magnetite crystals they've created in their laboratory also share the distinctive size and shape of those in ALH 84001.

But members of the McKay team countered that 3-D views shown by Ming did not unambiguously reveal the "truncated hexa-octahedrals," or THOs, that would signify a unique biological signature. Kathie Thomas-Keprta (Lockheed Space Systems) argued that while the synthesized crystals might be THOs, they were more likely cubo-octahedrons - the most common shape of magnetite formed by artificial means. Golden, in return, conceded that the images he presented might not provide proof but claimed he had other images that would. To complicate matters further, all parties agree that most of the meteorite's magnetite grains were formed by some inorganic process.

And so the debate remains about where it has been for the last few years: a standoff. But some tantalizing new research hints that the issue might indeed be resolved after additional work. A second team, Andrea M. Koziol (University of Dayton, Ohio) and Adrian J. Brearley (University of New Mexico), has also synthesized meteorite-mimicking crystals, and it may be only a matter of time before a few convincing images clinch the case for nonbiological origin. However, as McKay stressed during Friday's presentations, the laboratory conditions used to synthesize the crystals differ significantly from those encountered by the meteorite itself.

Meanwhile, a team led by Joseph L. Kirschvink (Caltech) introduced some brand-new techniques for studying these contentious crystals, which are less than 100 nanometers (2 millionths of an inch) long. The results presented are ambiguous, because so far only bulk material from ALH 84001 has been tested. But Kirschvink's group has used three different methods to analyze magnetite from a wide variety of sources. One of these outcomes did indeed show the Martian magnetites to be much closer to those produced by certain terrestrial bacteria than to the synthesized versions. However, results from the other two methods, though only preliminary and less clear, suggest that the Martian crystals share characteristics with both the synthesized versions and those from fossilized bacteria (but not those produced by living bacteria).

These techniques hold great promise to help resolve the question, McKay said after the session. His group is also pursuing various lines of further research, including close scrutiny of some additional formations in the meteorite that may turn out to be microscopic fossils. But, clearly stung by the intense and often bitter controversy that has surrounded their work since the initial publication, he said the group will not attempt to publish such findings until they have conducted sufficient tests to make the results "bulletproof."

added note, May 14, 2002:  Resolution of a Big Argument About Tiny Magnetic Minerals in Martian Meteorite

http://www.psrd.hawaii.edu/May02/ALH84001magnetite.html
 

     --- Magnetic minerals in Martian meteorite ALH 84001 formed as a result of impact heating and decomposition of carbonate; they were never used as compasses by Martian microorganisms.

Written by Edward R. D. Scott (Hawai'i Institute of Geophysics and Planetology) and David J. Barber (Cranfield University and University of Greenwich)

Planetary Science Research Discoveries
May 13, 2002

Tiny grains of magnetite, an iron oxide mineral, from a Martian meteorite are markedly similar in size, shape, and composition to the little oxide magnets used by bacteria on Earth and different from other naturally formed magnetites. Is this good evidence for life on Mars? Or did the Martian magnetite grains form by another process? Our studies reveal that the planes of atoms in the Martian magnetites are aligned with atomic planes in the carbonate in which the magnetites are embedded. This shows that the magnetites formed in the rock and not inside microorganisms.

     References:

     Barber, D. J. and Scott, E. R. D. (2002) Origin of supposedly biogenic magnetite in the Martian meteorite Allan Hills 84001. Proceedings of the National Academy of Sciences 99, 6556-6561.

     Thomas-Keprta, K. L., Clement, S. J., Bazylinski, D. A., Kirschvink, J. L., McKay, D. S., Wentworth, S. J., Vali, H., Gibson, E. K., Jr.,  McKay, M. F., and Romanek, C. S. (2001). Truncated hexa-octahedral magnetite crystals in ALH 84001: presumptive biosignatures. Proceedings of the National Academy of Sciences 98, 2164-2169.