Meteorite Yields Evidence of Primitive Life on Early Mars
"Extraordinary claims require extraordinary evidence" - Carl Sagan
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 17 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 meteorties. All, except the original Nakhlites and Chassignites are considered of the "shergottite" family.
All SNCs, with the single and very notable exception of ALH84001, these rocks are basaltic (volcanic) and have radiometric ages of 150 million years to 1.3 billion years. They 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 give some fragments of 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 occured, but the rock knocked off of Mars subsequently broke into smaller pieces 2.7 and 0.5 million years ago as the result of subsequent collisions in space.
"The" Martian Meteorite: Allan Hills 84001
What is ALH84001?
(the above web site has excellent details)
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 prestigous 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:
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 glueing 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 clearly associated with the carbonate globules which 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 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 inthe 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.
Sadly, I am now ending this Internet web page, where I have tried to present technical papers about the martian meteorite ALH84001 for the non-specialist. With time, I have formed fairly strong opinions about the hypothesis of McKay et al. (1996), and have found it harder and harder to be insightful and remain objective. I have enjoyed writing for this web page, and I hope that my efforts have been helpful. Thank you for your support.There is now evidence for the widespread infestation of bacteria and fungi in all meteorites, including the very carefully preserved ALH84001. Further, carbonate globules have now been made inorganically, including with magnetites like those in ALH84001. Magnetites can be made via shocks. The magnetites in ALH84001 have at least as many differences from terrestrial bacterially produced magnetites as they have similarities. Bacteria-shaped objects can form without biology. The sulfites do not show the kind of isotopic signatures in sulfur typically found in terrestrial, bacterially produced sulfites. The wormlike things are clearly coated by the materials used to use the microscope, leaving no room left for the "worm" inside, given the minimum possible thickness for a cell wall. Claims for the presence of martian amino acids in the Nakhla meteorite have been dismissed: these amino acids are identical in type and amounts to those found in the Nile Valley and show equal chirality (left and righthandedness) and are most certainly terrestrial contaminants.
According to one scientist at the 31st meeting of the Lunar and Planetary Science Conference in 2000, "The community is moving on."