SNC Group - Martian Meteorites

The SNC group is named for the historic meteorite falls of Shergotty, Nakhla, and Chassigny, each representing a distinct rock type of a common parent body. Shergotty is a basaltic rock that is considerably evolved, whereas the clinopyroxenite Nakhla and the dunite Chassigny represent equally evolved, plutonic cumulates. These meteorites were initially classified with the HED group, but were later reclassified when their young crystallization ages were ascertained, and their unique oxygen isotope fractionation line was determined. Obviously, the SNC meteorites represent a distinct group of highly evolved achondrites having some most unusual features.

With crystallization ages between 1.35 and 0.15 billion years, most SNC members are very young compared to other achondrites. The HED members show absolute ages of about 4.5 billion years, and even the youngest lunar mare basalts exhibit relatively ancient crystallization ages of about 2.8 billion years. Obviously, the SNC members formed on a parent body that has retained its igneous activity until very recent times, suggesting that the source of these strange rocks is a planet. From what we know, only Venus, Mars, and Earth are geologically active worlds, and we can easily rule out our own planet as a possible source of the SNC meteorites because terrestrial rocks plot on a different oxygen isotope fractionation line. The question of whether the SNC members originate on Venus or Mars was finally answered in 1985 after the discovery of trapped gas inclusions inside the shergottite EET 79001. Based on data obtained by the Viking probes, which landed on Mars in 1976, the composition of this trapped gas is identical to the Martian atmosphere. Subsequently, this comparison has been repeated for several other SNC members, proving the correlation, and convincing most scientists that the members of this group actually are genuine samples of our red neighbor, the planet Mars. Consequently, the SNC members are also known as Martian meteorites.

Martian meteorites are of major scientific importance because they represent the only known samples of another planet available for research. From a geological standpoint, they are most intriguing, but more importantly, they provide tantalizing clues to the question of the possibility of life on other planets. Not only do most Martian meteorites contain minerals that have been altered by the presence of water, but some also contain traces of amino acids, the fundamental building blocks of life. Moreover, some even contain microscopic structures that have been interpreted as microfossils by some researchers. However, it is hard to rule out the possibility of terrestrial contamination in these samples, and therefore the evidence of "microfossils" has been vigorously challenged by other scientists who claim that an inorganic formation process could be responsible for these odd structures. The debate about whether there is, or has ever been, life on Mars probably won't end until the current planned missions to Mars return their pristine samples of rocks and soil. Only then will we be able to make comparisons, under appropriate sterile conditions, to the Martian meteorites in our collections.

If we exclude all probable pairings, 24 different Martian meteorites have been recovered to date, comprising a total known weight of about 83 kg (have a look at our list of Martian meteorites). Based on their mineral compositions, the SNC members are further divided into four subgroups: shergottites, nakhlites, chassignites, and orthopyroxenites, the latter two subgroups consisting of just one single member. We will discuss each subgroup of Martian meteorites below. >> top...

Shergottites

The shergottites are named for their type specimen, an achondrite that fell in Shergotty, India, in 1865. Comprising 17 distinct members, they represent the most abundant type of Martian meteorite. Shergottites are igneous rocks of volcanic or plutonic origin, and they resemble terrestrial rocks more closely than do any other achondrite group. They all show exceptionally young crystallization ages of about 150 to 200 million years, and they usually exhibit signs of severe shock-metamorphism. Typically, the plagioclase in shergottites has been converted to maskelynite, a glass that is produced when plagioclase is subjected to shock pressures of at least 30 GPa. It is likely that the maskelynite was formed by the impact forces that blasted the shergottites from the Martian surface and into space. Calculations show that it requires a major impact event to accelerate any material to a speed high enough to escape the planet's gravity - one of the reasons why the SNC meteorites are such a rare class. Based upon their mineral compositions, the shergottites are further subdivided into two distinct subgroups: the basaltic subgroup, and the lherzolitic subgroup.

Basaltic shergottites: The members of this subgroup are basaltic rocks of volcanic origin, consisting primarily of the clinopyroxenes pigeonite and augite, minor plagioclase converted to maskelynite, and accessory phases such as fine-grained mesostasis, impact-melt, olivine, orthopyroxene, and various oxides, sulfides and phosphates. Their igneous textures range from fine- to coarse-grained, and some members display multiple lithologies. Several basaltic shergottites contain evidence of interaction with Martian water, either in the form of hydrated silicates, or in the form of carbonate and sulfate salts. Typical basaltic members are Shergotty, Zagami, Los Angeles, and two of our recent finds from Morocco - NWA 480, a unique coarse-grained Martian meteorite with a low total known weight of just 28 grams, and NWA 856, also known as "Djel Ibone", a fairly fresh basaltic shergottite exhibiting well-developed and oriented pyroxene crystals. Both texturally and chemically, NWA 856 appears to be a true intermediate between Shergotty and Zagami.

Lherzolitic shergottites: The members of this subgroup are cumulate, ultramafic rocks of plutonic origin, resembling terrestrial lherzolites and harzburgites. They consist primarily of medium-grained olivine and chromite that are poikilitically enclosed by large orthopyroxene crystals. Minor interstitial areas of plagioclase glass (maskelynite) occur, and clinopyroxenes with accessory oxides and phosphates are present. The lherzolitic shergottites probably crystallized as cumulates from residual melts in magma chambers, and they share many mineralogical and chemical features with the basaltic shergottites. The  lherzolitic members are five finds from the ice fields of Antarctica - ALH 77005, Yamato 793605, LEW 88516, Yamato 1075, and GRV 9927. The unique Lherzolite ouside Antarctica is NWA 1950, the freshest of all the group.

Transitional members: Considering the strong mineralogical and chemical links that exist between the basaltic and lherzolitic groups, it isn't too surprising that some shergottites in our collections represent intermediate forms between these two groups. Specifically, the olivine-rich shergottites from Dar al Gani, Libya, (DaG 476 and pairings), the shergottites from Sayh al Uhaymir, (SaU 005 and pairings), and the shergottite from Dhofar 019, Oman, all exhibit transitional features. Although usually grouped with the basaltic shergottites, these samples all represent different transitional stages between the basaltic and lherzolitic groups. This suggests a formation through the partial melting of lherzolitic and other source rocks, resulting in a magma that crystallized within an extruded lava flow near the Martian surface. In April 2001, our team recovered another olivine-rich shergottite from Morocco, NWA 1068, also known as "Louise Michel". This unique shergottite also shows transitional features, but it seems to represent a new, previously unsampled type of shergottite, an olivine-rich picrite. Ongoing research will have to show the further implications of this sensational new find.  >> top...

Nakhlites

The Martian meteorites of this group are named for Nakhla, a fall that occurred in El Nakhla, Egypt, in 1911. The nakhlites are clinopyroxenites, displaying a sugary, fine-grained texture. They consist primarily of green cumulate augite crystals with minor olivine in a very fine-grained mesostasis. This mesostasis is composed of plagioclase, alkali feldspar, pyroxenes, iron-titanium oxides, sulfides and phosphates. Traces of pre-terrestrial aqueous alteration products are present in the form of hydrated minerals, e.g. amphibole, clay minerals similar to iddingsite or smectite, and carbonate and sulfate salts. Some researchers have proposed that the presence of these hydrated minerals in the nakhlites, in addition to concentrations of water-soluble ions such as Cl, K, Na, and Ca, suggests that they resided in an environment in which liquid seawater was present for some time, perhaps inside of an ancient Martian ocean. Furthermore, scientists have identified microscopic structures embedded within the iddingsite of Nakhla, possible evidence of fossilized Martian microbes similar to terrestrial iron-reducing bacteria. If this finding should prove to be true, the nakhlites tell a most intriguing story about their parent body, Mars.

Nakhlites crystallized only 1.3 to 1.4 billion years ago, and two competing scenarios have been proposed to explain this very recent formation. According to the older theory, the nakhlites are cumulates that formed in magma plutons deep inside the Martian crust. A more recent theory places their origin in a lava flow near the Martian surface similar to Theo's flow, an extrusive Archean lava flow in Canada that formed pyroxenites similar to the nakhlites. However, subsequent aqueous alteration of the resulting rocks took place much later as indicated by the formation ages of the clay minerals in nakhlites; these show absolute ages of about 700 million years. Perhaps in the not-too-distant past Mars wasn't the dry and hostile place that we know today.

For decades, only three nakhlites were known - Nakhla, Lafayette, and Governador Valadares. For a time, the latter two finds were even suspected of being dislocated or mislabeled individuals of Nakhla due to their virtually identical mineralogical and textural features. However, in late 2000, fate intervened with the recovery of two new nakhlites. A team of Japanese researchers recovered the first antarctic nakhlite, Yamato 000593, and its pairing Yamato 000749. A fifth nakhlite was recovered simultaneously from the Moroccan desert by our team, a stone that would prove to be quite different from the other nakhlites. It has been named NWA 817, and is a fusion-crusted individual of just 104 grams. Not only does NWA 817 have a higher modal proportion of mesostasis than the other nakhlites, but it also contains more evidence of pre-terrestrial aqueous alteration. This alteration is manifest through the high abundance of clay minerals carrying high water content. Further research will reveal more of the interesting story that our most important find has to tell. We will keep you informed in our news section. >> top...

Chassignites

This group is named for its only member, Chassigny, a meteorite that fell in France in 1815. As a witnessed fall that was recovered in the early days of meteoritics, it was one of the first meteorites to be recognized as a genuine rock from space, and it subsequently proved to be one of the most intriguing ones. Chassigny is a cumulate rock, resembling a terrestrial dunite. It consists of about 91% iron-rich olivine, 5% clinopyroxene, 1.7% plagioclase, 1.4% chromite, 0.3% melt inclusions, and other accessory minerals and phases. The melt inclusions contain rare amphiboles, and cracks within Chassigny are filled with carbonate and sulfate salts, indicating a mild pre-terrestrial aqueous alteration.

Chassigny displays a rather typical oxygen isotopic composition, placing it clearly within the SNC field. Its crystallization age of 1.36 billion years and its compositional and elemental trends, indicate a close relationship exists between Chassigny and the nakhlites, and suggests an origin from the same parent magma on Mars. However, Chassigny contains noble gas values that are entirely different from those found in other SNC members or in the Martian atmosphere. It is suspected that these gases might originate from the Martian mantle, suggesting a formation for Chassigny within a magma pluton deep inside the Martian crust. >> top...

Orthopyroxenites

In late 1984, the Antarctic meteorite Allan Hills 84001 was found. It remains the only member of this unique group of Martian meteorites. It is a cumulate rock consisting of 97% coarse-grained, magnesium-rich orthopyroxene, with minor plagioclase (maskelynite), chromite, and carbonate. It was initially classified as a member of the HED group, specifically a diogenite, because these achondrites are also primarily composed of orthopyroxene. However, the presence of oxidized iron in the chromite of ALH 84001 led to its reclassification as a Martian meteorite; a fact that has been subsequently confirmed by its oxygen isotope composition.

ALH 84001 is a most unusual member of the SNC group. With a crystallization age of about 4.4 billion years, it is by far the oldest Martian meteorite yet discovered. It most likely represents a sample of the early Martian crust, providing evidence for the earliest geologic history of Mars. However, public attention has been focused on a minor aspect of this unique rock during the past years - the presence of small orange-colored carbonate spherules that probably formed 3.9 billion years ago. These "orangettes" are barely visible to the naked eye, ranging up to 200 microns in size, and they seem to have formed in the presence of liquid water within fractures inside ALH 84001. In 1996, McKay and his co-workers published a paper announcing the discovery of traces of fossil Martian life within the orangettes, consisting of organic molecules, several biominerals, and microfossils that resemble terrestrial nanobacteria. Ever since then, his discovery has been vigorously debated, splitting the scientific community into advocates and prosecutors of the existence of (former) primitive life on Mars. Future missions to Mars will confirm whether we are alone in a hostile universe, or if there are unsuspected neighbors in our own solar system, thus implying a biophile nature for our universe where life is not the exception but the rule. >> top...

Complete List of Martian Meteorites: