What do we know about the Mars meteorite ALH84001?

This meteorite, weighing 1.9 kilograms, is one of a small list of recovered Antarctic meteorites whose trapped gas isotopic abundances match very closely those seen by the Viking Lander in 1976:

Name...............................Type................
The SNC or Sherogotty-Nakhla-Chassigny meteorites:

EETA-79001A                  Basaltic shergottite
Shergotty                    Basaltic shergottite
Zagama-1                     Basaltic shergottite
Zagama-2                     Basaltic shergottite

ALH77005                     Lherzolitic shergottite
LEW88516                     Lherzolitic shergottite

Nakhla                       Clinopyroxenite/Wehrlite
Lafayette                    Clinopyroxenite/Wehrlite
Governador Valadares         Clinopyroxenite/Wehrlite

Chassigny                    Dunite
......................................................
AHL84001                     Orthopyroxenite
......................................................

With the exception of ALH84001, these martian meteorites have late crystallization ages from about 1.3 billion to 180 million years ago. Trapped argon gas in the EET79001 stone has the same isotopic signature as detected by the Viking lander in the atmosphere of Mars, especially the abundances of Zenon-132, Krypton-84, Neon-20, Argon-36, Argon-40 and CO2 which match the martian atmospheric values almost exactly. So, there is little dispute that the stones were at one time on Mars, and that their young crystallization ages indicate ejection by some event within the last 1.3 billion years.

In a paper by Haraldur Karlsson, Robert Clayton, Everett Gibson Jr and Toshiko Mayeda published in the March 12, 1992 issue of the journal Science ( vol. 255, p. 1409) they examine the 'SNC' meteorites and discover they contain 0.04 to 0.4 percent water by mass, locked up in the crystalline and mineralogical compounds. Most of this is terrestrial contamination, however, they find evidence from oxygen-17 isotopic analysis that a significant portion is extraterrestrial, and presumably attesting to evidence of water on Mars at some time in its past.

Attempts at finding likely locations on Mars where they could have been ejected have been inconclusive. ALH84001 has a ( disputed) age of about 4.5 - 4.6 billion years corresponding to what is called the martian 'Noachian Period', but volcanic rocks of this age are only thought to make up about 1.5 percent of the planet's surface. Areas volcanically resurfaced within the last 1.3 billion years correspond to the 'Amazonian period' and constitute only about 156 percent of the surface. Late Amazonian volcanic basalts dating from 150 million years are only 1.5 percent of the surface. One possibility proposed in McSween's review article is that one impact sampled material at several different ages from 1.3 billion to 150 million years ago. Ejection in one event may explain why the Chassigny and Zagami meteorites fell on the same day, but 174 years apart which " might imply residence in a meteoroid stream" produced from a single martian ejection. A consensus of opinion seems to be that the most likely region on Mars that may be young enough to have significant 180 million year-old volcanic flows is the Tharsis region.

The cosmic ray track density in these meteorites suggest that there was little shielding, so that the parent bodies were not much bigger than perhaps a half-meter across. They would have ablated only about 50 percent of their mass upon entering the atmosphere. Dynamical models of impact ejecta show that for impact velocities of 10 kilometers per second, the parent body that collided with Mars could have excavated a 10-kilometer crater and ejected 0.5 meter rocks. A 30 kilometer crater would indicate an impactor capable of ejecting meter-sized stones. Specific craters have been searched for as possible impact sites. 25 craters have been identified that are big enough ( more than 12 km across) to have been produced by an impacting body capable of launching meter-sized rocks from the surface at the escape velocity of 5 kilometers/sec or more, and without vaporizing them. None, however, would have been capable of sampling material 1.3 billion years old, at least not if these rocks are near the surface.

ALH84001, being the oldest meteorite, but with the youngest age in terms of its cosmic ray age ( some 10 - 15 million years) would have been ejected from a very recent impact on Mars, in a region of Mars with older rock present. It is widely recognized has containing samples of older martian crust, still its crystallization age is not known with any certainty.

There are two important papers on ALH84001. The first is the 'discovery' paper by david W. Mittlefehldt at Lockheed, published in 1994 in the journal Meteoritics, vol. 29, p.214-221 and titled " ALH84001, a cumulate orthopyroxenite member of the martian meteorite clan". The second paper is by Allan H. Treiman at the Lunar and Planetary Institute in Huston Texas. It is published in the same journal in 1995, vol. 30, p.294-302 and is titled, " A petrographic history of martian meteorite ALH84001: Two shocks and an ancient age". Herewith is an overview of what these articles have to say about this meteorite.

1. ALH84001 has an oxygen isotopic composition indistinguishable from the 'Nakhlite' martian meteorite family, so it is not a radically different type of material than the other martian stones.
2. It is a course-grained, cataclastic orthopyroxenite with much of the original magmatic/metamorphic texture preserved.
3. It contains 100 micron sized grains of maskelynite; interstitial grains of 10 micron Augite; 300 micron grains of Apatite; Pyrite grains 10 micron sized; large interstitial carbonate grains 100 microns in size; small interstitial carbonate grains 10 microns in size.
4. Mittlefehldt states that "fine-scale zoning is present in these [large] carbonates...indicating that they were formed before the last shock event." and that the small carbonate grains are frequently found in "crushed zones or in fractures in orthopyroxene, indicating that they were formed after a shock event, or were remobilized by that event"
5. The 'zoning' of the large 'early' carbonate grains consists of light and dark bands of iron and calcium-rich materials and magnesite, indicating "early carbonates were formed from multiple influxes of fluids". They were formed under varying conditions of the fluid influxes, and even at different times relative to the shock since some can be found cleaved by fractures in the rock, and others along fractures in the rock.
6. Zoning was not found in the small 'late' carbonate grains suggesting that they were formed under different environmental circumstances, consistent with having been produced in a shock event.
7. The compositions of the large, 'early' carbonate grains, according to Mittlefehldt, contains euhedral pyrite which forms at maximum temperatures bellow 742 C and that the large carbonate grains may have been formed at relatively high temperatures above 500 C. "The early carbonates, therefore, are products of hydrothermal alteration, rather than low temperature weathering products. The inferred high temperature origin for both the early and late carbonates is incompatible with genesis by Antarctic weathering..."
8. Tremian proposes that evidence for two shocks, not the single one proposed by Mittlefehldt, is present in the structure of ALH84001. None of the other martian samples shows evidence for multiple shocks. This could imply that the original parent rock on Mars had already been involved in an impact event, and might correspond to an older' highland' rock type perhaps more than 3 billion years old.
9. It is difficult to radiometrically date ALH84001 because the common nuclear chronometer isotopes are either absent, or their abundances may have been contaminated by entrained martian atmosphere.
10. ALH84001 consists of 95 percent pyroxene, 2 percent chromite, 1 percent maskelynite, 1 percent carbonate and traces of other minerals such as apatite, olvine, augite and pyrite.
11. The carbonate 'globules' have hemispherical or ellipsoidal shapes with complex sub-micron concentric zoning "...that is sharp on a sub-micron scale". Extensive deformation features called crush zones by Mittlefehldt, are also present indicating a shock event following the formation of the globules.
12. Treiman proposes a history for this sample that begins with a crystallization from a magma, metamorphism in a 'first shock' event; thermal metamorphism; a low temperature chemical alteration process, and finally a 'second shock' event possible the one which launched the rock into space off of the martian surface.
13. The first shock event produced the 'granular bands' and pervasive cracking of the pyroxene material. This could have been produced by a tectonic event, but Mars is known to be inactive for this activity, while meteoritic/asteroidal impacts are a far more likely mechanism. In this first impact, the parent material remained on Mars as part of an old impact crater bedrock.
14. After the first shock event, the parent rock was heated so that the granular bands could anneal to form the existing 'granoblastic-polygonal texture", and to explain the intergranular textures within the granular bands. The maximum temperature was probably near 850 C.
15. A subsequent period of alteration is suggested by the presence of the carbonate grains, "...The carbonate must have been deposited after the granular bands had annealed, because the concentric stratigraphy of the carbonate globules is superimposed on the equilibrated texture of the bands."
16. Some of the globules are deformed, so there must have been a second shock event that happened after the carbonate globules formed.
17. Unlike Mittlefehldt's assertion that the carbonate globules formed under high temperature conditions near 700 C, Treiman argues that, "...the stratigraphy and composition of the carbonate globules suggest that they were formed at low temperature, not high temperature.." he sites experiments that show that carbonate grains homogenize in about one month at 350 C so that any 'zones' would be washed away at high temperatures. The ALH84001 globules are strongly zoned and could therefore not have been subjected to high temperatures. Temperatures between 0 to 350 C lead to carbonate formation, so this is a low-temperature process, not a high-temperature process.
18. The repetitive light-dark banding at the rims of the globules suggests cyclic changes in their formation conditions or fluid compositions.
19. The second shock produced micro-faults cutting the carbonate globules, and radial cracks around other components of the meteorite. This could have been either the impact shock that launched the rock off of Mars, or an impact in space with another interplanetary body. Further analysis of ALH84001 is expected to distinguish between these possibilities on the basis of refined abundance analysis.
20. Samarium and neodynium dating of ALH84001 suggest an age near 4.6 billion years, but it is not certain what event this date refers to.

So, prior to the NASA announcement of the discovery of possible martian micro-fossil 'nano-bacteria' in the carbonate globules, there was a significant recent debate over the history of ALH84001. The key issues are 1) the age of the sample; and 2) the temperature at which the carbonate globules formed. If the globules had formed at temperatures above 400 C, then there could be no fossils present. If the globules formed as a by-product of the shocks to which the sample had been subjected, then any discussion of 'micro-fossils' would be washed away as simply a peculiar by product of the shock process. However, Treiman's argument that sustained temperatures above 300 C are enough to homogenize the carbonate globules is significant. The advocates for micro-fossils propose that the globules condensed from a carbonate-rich martian ocean at low temperature, forming the zones much like tree rings. If it can be demonstrated that these globules are only consistent with a low-temperature history, that strengthens the argument micro-fossil argument.

ALH84001 clearly is an exciting meteorite. Only a few dozen grams of its 1.9 kilograms have been meticulously studied, with the prospect of more to be learned from it a very real possibility.