ANSMET Anniversaries: 40 years ago, the 1979-80 season.

Field Number 1043, also known as EET(A) 79001.

Before they were known to be from Mars,  the Martian meteorites were affectionately called the “SNC” meteorites, often pronounced “Snicks”.  The name comes from three specimens-  Shergotty, which fell in India in 1865; Nakhla, which fell in Egypt in 1911, and Chassigny, which fell in France in 1815. They were thought to be related to each other primarily because they shared two features that were unusual among the meteorites known before Antarctic recoveries. First, they were relatively coarse-grained mafic-to-ultramafic igneous rocks, and second, they contained the pseudo-mineral Maskelynite,  basically feldspar that retains its characteristic tabular shape but loses its crystallinity, transformed into glass by shock.

It was an ANSMET meteorite recovered in 1979 that led to the discovery that the SNC’s were of Martian origin.  By the mid-seventies,  there were 6 SNC’s known;  the three listed above, two additional nakhlites (Governador Valadares and Lafayette) and one additional shergottite (Zagami).  The discovery of the first Antarctic SNC,  ALH 77005*,  led to significant new interest in the group.  It turned out to be a “new” type, petrographically speaking.  Highly shocked (with areas of whole-rock melt),  it had regions of two distinct textures. Some areas look basaltic like Shergotty and Zagami; a subequal mix of pyroxene and maskelytinized feldspar, with a few xenocrysts (distinct inclusions of ultramafic minerals such as olivine and clinopyroxene) floating around, while other regions were much coarser-grained, with large orthopyroxene and clinopyroxene poikilitically enclosing olivine.  As a result it was called a “lherzolitic shergottite”, the first new class of SNC’s. In the post-Apollo, “let’s point our fancy instruments at all these Antarctic meteorites!” excitement of the late seventies, ALH 77o05 was widely studied.

Jump forward a few years, and we still didn’t know where the SNC’s were from, but speculation was rampant. The literature from that era proposes a wide range of origins for the SNC’s as well as an equally diverse number of studies shooting them all down.  ANSMET was gaining its footing and recoveries were ramping up, leading to the recovery of another SNC sample-  EET 79001*.  The largest sample recovered by ANSMET in the 1979-1980 season (almost 8 kilograms), it turned out to be another shergottite; but it had several very unique features.  First, it contained two distinct lithologies meeting at a planar contact. Lithology A lookes a lot like Shergotty, though the ultramafic xenocrysts are more prominent; lithology B is finer grained and is for the most part xenocryst-free. There was also a third lithology (logically called “lithology C”) which consisted of large black glassy pockets and veins of impact melt.

As is so often the case in the natural sciences,  it was the messy, difficult-to-unravel stuff  that provided the key.  One of the first things we want to know about any novel igneous rock that falls into our hands is its crystallization age;  how old is this rock, so we can try to put it into chronological context.  But that proved difficult for EET 7900 given its mix of lithologies.  In an effort to deconstruct the problem,  Don Bogard and P. Johnson (and a number of other prominent research groups) sought to measure the rare-gas and isotopic composition lithology C and figure out when that impact melt formed,  presumably returning an ejection age from the SNC parent body, if nothing else.

What they found (and would be confirmed by other researchers) would be a paradigm shift in the study of planetary materials.  While lithology C didn’t immediately provide a definitive ejection age, it did reveal a high concentration of rare gases occuring in distinctive relative abundances. The pattern of abundances were wildly different than those seen in other meteoritic, terrestrial and lunar samples;  but there was one sample they did match, almost 1-for-1.  That sample?  The atmosphere of Mars as sniffed out by the Viking landers a few years previously.  This was the long-sought-after “smoking gun” for the origin of the SNC’s;  given that the ratios of noble and other gases serve as virtually unique fingerprints for planetary atmospheres,  there was no way that EET 79001 DIDN’T come from Mars.  Multiple replications and extensions of this study, by Bogard and others, quickly led to the growing belief that EET 79001 and (by extension) the other SNC’s had to be Martian samples.

There is so much more can be said here-  literally tens of thousands of research studies grew out of early research on this singular specimen.  From a historical perspective the story isn’t complete either;  it would take the discovery of ALH 81005 (the first lunar meteorite) for the Martian origin of the SNC’s to be widely accepted.  But while I certainly can’t do justice to the scientific significance of EET 79001 in one post, my goal is simpler- put a few significant moments in Antarctic meteorite history into context for you. There are few more scientifically noteworthy moments than the recovery of that one specimen during ANSMET’s fourth field season in 1979-1980.

-Posted by RPH from Novelty, OH.    


Additional notes:

* Through most of this post I used the modern formal name for the US Antarctic meteorites, consisting of three letters that designate a field area, two digits representing the year, followed by 3 or 4 digits making up the rest of the name. Thus “EET 79001”.  In ANSMET’s early seasons, however, a four-letter prefix was used;  the additional letter was to designate specific field teams should multiple teams be operating independently at a single site. That convention was dropped by the mid-eighties when it became clear the “two teams at one site” thing was unlikely to happen.   Thus many early references refer to the formal names ALHA 77005 or EETA 79001, but more modern references often drop the superfluous “A”.

For those seeking a comprehensive description of the Martian meteorites, both Antarctic and non-Antarctic,  the absolute best reference (in my humble opinion)  is the Martian Meteorite Compendium  prepared and hosted by the Antarctic Meteorite Curator at NASA’s Johnson Space Center. If you can’t find the information there, you didn’t need it anyway.

And if you want to see the original rare gas work noted in this post, look up Bogard D. and Johnson P. (1983) Martian Gases in an Antarctic Meteorite? Science 221, 651-654.   Don would go on to be JSC’s Antarctic Meteorite Curator for many years, helping bring the modern system to life.