First Meteorite of Egypt, The Crown Jewel of Mars' Meteorites
By Kevin Kichinka
Nakhla. The "N" of the SNC meteorite triumvirate.
El Nakhla El Baharia. The original name for a piece of Mars fallen from the heavens impacting the ancient dust of the Pharaohs, an object now probed by scholars seeking signs of life lived on that world.
El Nakhla El Baharia Markaz Abu Hommos. Not just a rock, a talisman, coveted by those intrigued by its legend.
This is about Nakhla - the Holy Grail of meteorites.
The circumstances of the Nakhla fall were first reported by W.F.Hume, Doctor of Science and Director of the Geological Survey of Egypt, in an article entitled "The First Meteorite Record in Egypt" published in The Cairo Scientific Journal #59, volume V, August, 1911. The copy provided to me by Dr. Tim McCoy of the Smithsonian is annotated in turn-of-the-century penmanship, "With the author's compliments" and "Nakhla".
Only a few days after the fall, Hume visited the five sites where Nakhla was collected, and carefully recorded the circumstances of the specimens recovered. One particular stone focused his attention. "The specimen from Ezbet Saber is an eight-sided block approximately ten centimeters long, nine centimeters broad, and nine centimeters high, each side being covered with a black glistening varnish except where broken at the edges where the interior is seen to be made up of light green-colored crystals and grains. Only one side is smooth, the remainder being pitted with depressions such as characterize the greater number of meteorites. The interior of the rock, on inspection with a lens, shows prismatic crystals which are probably the mineral enstatite and shining yellow grains, similar to olivine or peridot, both of these being usual constituents of the stony meteorites."
"The specimen has a total weight of 1,320 gms, and a specific gravity of 3.4 as determined in the Survey Chemical Laboratory….The examination made in the Chemical Laboratory has shown that the black varnish is rich in iron, but contains no manganese."
Departing from his review of the Nakhla fall, Hume described the existing meteorite collection of the Egyptian Geological Museum in Cairo. "The meteorites hitherto obtained fall into three groups, the Holosiderites mainly composed of native iron, the Syssiderites, in which minerals such as olivine and enstatite are partly mingled with the native iron, and the Sporadosiderites or Lithosiderites, in which the olivines, etc. are in great excess."
He concluded his report by hinting at the origin of the Nakhla meteorite." It is of interest to note that these extraneous objects have close resemblance to certain rocks on the earth's surface, which are, however, amongst the rarest occurrences, and are present under conditions indicating a deep-seated origin. It has consequently been suggested that they may represent materials thrown out from ancient volcanoes on the earth or from the now extinct lunar volcanoes, but the question is too theoretical for consideration in a preliminary note of this nature."
In a paper dated November 25, 1912, authored by "John Ball, Ph.D. D.Sc. F.G.S. Survey Dept.", he writes: "In form, the stones are mostly sub-angular, that is, they are of shapes such as one would obtain by breaking up a mass of homogenous rock with a hammer and then very slightly rounding off sharp edges...Many of the stones are entirely, and all the others are partially covered with a glossy black skin, as if they had been varnished with pitch. The surfaces in places show shallow pittings such as would be caused by pressing the thumb into a mass of putty." After listing the museums to which the Egyptian Government would be presenting various smaller specimens "where they are highly prized as additions to collections already embracing large numbers of specimens from various parts of the globe", Ball concludes that "the value of the Nakhla specimens to such collections is enhanced, not only by the circumstances that they represent the first authenticated fall of a meteorite in Egypt, but also by the fact that... they belong to a new type of meteoric stone."
Ball, F. Berwith, S. Meunir of the Academy of Science in Paris, and G.T. Prior, "Keeper of the Minerals in the British Museum" all published their findings on "El Nakhla El Baharia" in 1911-12. Prior also prepared thin sections for microscopic inspection. He described one specimen as having "thin, black, varnish-like, fused crust, as glossy as that of the Sherghotty and Juvinas meteorites. No metallic iron or chondrules are visible, and the few grains attractable by a magnet from the powdered stone consist only of magnetite. The material is very friable and can be reduced to a course powder simply by rubbing between the finger and thumb." Examining the thin sections under the microscope he saw "well-defined prismatic crystals of diopside" and "olivine in larger irregular grains showing no sharp outlines of crystal-faces." Looking closer he detected "interstitial material which...resolves itself into a crystalline aggregate of feldspar-laths with grains of augite and magnetite, like the matrix of a fine-grained basalt."
Prior crushed over 9.2 grams of material to perform an overall analysis and to calculate the amounts of alkalis, ferrous iron, sulfur and water. He also tested to find the proportion of soluble to insoluble silicates. Prior concluded that "the meteorite of El Nakhla approaches most closely to the angrite group as represented by the meteorite of Angra dos Reis, Brazil."
In the ensuing years, as Nakhla remained a curiosity outside the boundaries of classification, the world's greatest meteoriticists relentlessly probed between the atoms hoping to glean its origins and geologic chronology. This section will include their findings, with relevant discussions of the associated nakhlites, Lafayette and Governor Valderes, and the other SNC's.
What is the "stuff" that Nakhla is made of? In 1975, Robert Hutchison, N.H.Gale and J.W.Arden wrote the seminal study of Nakhla up to that time, "The Chronology of the Nakhla Achondritic Meteorite". The reasons they chose to study this meteorite are made clear in the opening sentence - "We undertook this study because Nakhla is a rare unbrecciated achondrite."
In their introduction, Nakhla is described as "the type meteorite of the nakhlite (diopside-olivine) class of calcium-rich achondrites, (it) is the most calcium-rich achondrite after Angra dos Reis and has the most iron-rich olivine, FO31, of any equilibrated meteorite. A modern interpretation of its texture indicates that Nakhla is a cumulate; it formed by settling of pyroxene and olivine crystals in a slowly cooling melt, followed by further precipitation as overgrowths to pre-existing, cumulus crystals and by the crystallization of interstitial liquid. The intercumulus material comprises an intergrowth of plagioclase and clinopyroxene crystals about 0.1 mm in length, with some smaller magnetite and sulfide grains. In addition, a reddish-brown material occurs intimately associated with olivine, along cracks in pyroxene or in interstitial patches...it may also include the material thought by Bunch and Reid to be an alteration product after olivine, possibly iddingsite. G.T. Prior found 0.17% H2O in Nakhla, which could be consistent with this interpretation of the reddish-brown material. In agreement with Bunch and Reid we coincide that there is apparently no textural evidence for shock or metamorphism in Nakhla."
Following a many-faceted investigation that reduced 43 grams of Nakhla to dust and chips, they made several still-standing observations regarding the age, chemical makeup and formation of this meteorite. However, no mention of a Martian origin was indicated. "There are geochemical similarities between Nakhla and the Earth, but not between Nakhla and the moon, other achondrites, or ordinary chondrites."
So where did Nakhla come from? In 1979, H. McSween, D. Walker (et al.), and J.T. Wasson and G.W. Wetherill, wrote separately of the relatively recent crystallization ages (1.3 billion- 180 million years) of the meteorites Shergotty, Nakhla, and Chassigny, and the lack of an asteroidal or planetary "smoking gun" (or volcano as it were) so late into the development of the solar system. In 1981, C.A.Wood and L.D.Ashwal asked, "SNC meteorites - Igneous Rocks from Mars?" Some believe this was also the first use of the term "SNC".
In 1982 R.E.Grimm and McSween ran a computer simulation of crystal settling in shergottite magmas to estimate the size of the gravitational field needed to match the pattern observed in certain shergottite meteorites. A parent body at least as large as the moon was indicated. But that proof was still suspect as a similar pattern of crystal settling could also have been created by flow differentiation and convection currents within the parent magma.
The strongest clue emerged for a red planet origin when an isotopic measurement of trapped Argon gas found in the shock melt of the EET79001 shergottite in 1983 by Bogard and Johnson matched the composition of the Martian atmosphere as measured by the Viking lander in 1976. Several subsequent studies detected molecular abundances and isotopic compositions for a variety of gases that matched the Martian atmosphere. Ironically, a 1994 paper by Drake, Swindle, et al., reveals that the measurements of the atmosphere performed by the Viking provided data of such low quality, that the "high precision measurements performed on "lithology C" of EETA79001, rather than the low precision Viking measurement, represents our best estimate of the isotopic composition of the Martian atmosphere."
Harry McSween Jr. published a landmark paper in 1985 that suggested that the SNC meteorites "might be used as probes of Martian geologic processes and history." He sited the differences they had from other achondrites and a mineralogy indicating rapid cooling in a magma containing water and exposure to a high degree of oxidation. He suggested a planetary formation, and even matched the rocks to certain Martian volcanoes in the Tharsis region of a similar age. McSween agreed that the trapped gases in the shock melt matched data collected by the Viking Lander, but was skeptical that a meteoric collision could cause the launch of the SNC's into orbit.
Obviously, Nakhla's not a shergottite. Looking for its origins in trapped little bubbles was a non-starter, as this meteorite shows virtually no evidence of shock, and consequently, no shock melt. A "back door" in a search for trapped atmospheric evidence could be opened by leaching out iddingsite, (a mixture of clay and iron-oxides produced when liquid water interacts with olivine) from a sample and isolating and measuring for Xenon29/Xenon32 v. Krypton84/Xenon32. By comparing that ratio with known values for Chassigny, most shergottites and lithology C of Antarctic meteorite EET79001, Drake, Swindle, Owen and Musselwhite (1994) concluded that they'd found proof of fractionated Martian atmosphere in Nakhla.
Interestingly, iddingsite makes up a little less than 2.5% by volume of the nakhlites and is classified as a sedimentary rock. SNC's contain 0.04 - 0.4% water by weight (Karlson, et al. 1992), and formed in a low sodium/potassium tholeiitic magma containing water (McSween, 1985). According to Drake and Swindle et al. (1994), the nakhlites show signs of aqueous alteration - they got wet.
By 1994, after a decade of studies had built a mountain of coincidental evidence supporting the red planet as the origin of these meteorites, and with the discovery of ALH84001, a possible fourth type of Mars rock different than the SNC's, Mittlefehldt proposed dropping the "SNC" nomenclature for simply "Martian".
When did the nakhlites form? In 1962 Stauffer found a K-Ar (Potassium-Argon) age of 1.3 Ga (billion years). Podesek found corroborating evidence checking Argon40/Argon39 in 1973. Papanastassiou and Wasserburg studied the Rb-Sr (Rubidium-Strontium) ratio in 1974 and deduced a range of 1.31 - 1.37 Ga. Scientists have since looked at other isotopic relationships (with varying degrees of agreement) but 1.3 Ga is the accepted standard for the age of Nakhla.
What were the geological circumstances of their formation? According to Berkley, et al. in 1980 and prolific researcher Allan Treiman in 1986, the compositions of Fe-Ti (iron-titanium) oxides showed that crystallization had occurred under oxidizing conditions, and the augite crystals were oriented in a way that suggested that the rock was a cumulate.
For a 1987 comparative study of the Earth and Mars' mantles, as sampled by nakhlites and a rare augite-rich basalt from Canada, Treiman (et al.) showed that "clinopyroxenites of Theo's Flow (an Archaen-era basalt flow in Ontario) are nearly identical to the nakhlite's in mineralogy, mineral proportions, grain sizes, and textural relations." According to Treiman, Earth and Mars shared a period of common petrogenesis as represented by these two rocks. From its "LREE-enriched-with-minimal-Europium-anomaly" profile to it's MREE abundances, Treiman's group declared that "If a Theo's flow pyroxenite fell from space, it would be classified as a nakhlite." Other test results were inconclusive, causing Treiman to conclude his paper with a question, "Could the source mantle for the nakhlites (and the other SNC's) be as unrepresentative of the Martian mantle as the theolite source mantle is of the Earth's mantle?"
I add this informational note to meteorite collectors. Although a Theo's Flow rock has a physical resemblance to Nakhla, Treiman has written me that: "Theo's Flow rocks are both metamorphosed (very slightly) and weathered a bit, so that a good hand sample petrologist, or a beginning microscopist could tell them apart immediately. In Theo's flow, augite is still augite. The original olivine is now serpentine, and the original plagioclase and some pyroxenes are replaced by metamorphic minerals like pumpellyite. Weathering is obvious as red staining."
Concurrently, a series of papers were presented attempting to explain the nakhlites complex crystallization. Struggling with this perplexing problem, researchers introduced new elements into the mix. A theory for an intermediate cumulus phase of augite with olivine grains as a xenocryst (foreign crystal) was proposed (Treiman, 1986). The team of Longhi and Pan, (1989) suggested that the interstitial melt reacting with the iron and magnesium in the olivine caused their diffusive re-equilibration. In a 1992 paper, Ralph Harvey of Case Western Reserve University in Cleveland, and Harry McSween elegantly tied together the loose ends, writing that all three nakhlites (including Governador Valadares and Lafayette) "appear to be a series of relatively simple cumulate rocks which have undergone various amounts of late-magmatic and subsolidus diffusion, possibly reflecting their relative positions in a cooling cumulate pile... Cooling was most rapid in Nakhla and Governador Valadares, and the presence of Fe-rich pigeonite in these meteorites indicates that they were quenched from temperatures above 860*C...Lafayette cooled more slowly, which allowed complete re-equilibration of the olivine, more highly developed re-equilibration of the augite, and substantial reaction to form orthopyroxene."
Were they from the same parent magma, the same Martian volcano? In an e-mail to me dated February 16, 1998 McSween wrote, "It is almost a certainty that all three meteorites were ejected from the same body of rock on Mars at the same time. Whether they were ejected as three different pieces or on one piece, we can't say. The fact that Gov Val has experienced a different annealing history suggests to me that this requires different depths of burial, and it is hard to get a really large chunk off Mars."
And the other Mars meteorites? Studies by Jones (1989) and Longhi (1991) suggest the possibility that all of the SNC meteorites were formed from the same parent magma melting over and over....
How deep into the Martian ground were the nakhlites and other SNC's prior to their ejection into space? In the same February correspondence, McSween wrote "Either this (Nakhla) was a plutonic rock or a volcanic flow that was quickly buried under other hot flows, so that heat could not escape easily."
There are ways to determine how long ago Nakhla was blasted off the Martian crust. When an object is in space, or on another object devoid of a shielding atmosphere, it is exposed to cosmic radiation at a rate as steady as an atomic clock. The "exposure age" is calculated by measuring the amounts of 3Helium, 21Neon and 38Argon present in the meteorite. But these easily obtained results can be tainted by other mitigating factors, and call for researchers to accommodate certain variables. Cosmic radiation can only penetrate about a meter into an object in space. The interior of a larger object is shielded from exposure. If additional collisions take place, a new "clock" starts ticking on freshly exposed surfaces. The possibility that cosmic radiation exposure began when a rock was laying on the surface of an asteroid, moon or planet before its journey to Earth began must be considered.
Ott (1988) built models showing exposure ages of 3 Ma for the shergottites (for an object of around two meters in diameter) and 10-12 Ma for the nakhlites and Chassigny (for objects of around 1 meter in diameter). Collisional disruption in space may account for the difference. Significant differences in 3Helium yields caused Ott to question if Chassigny was ejected in the same event as the nakhlites. Ott interpreted cosmogenic nuclide data to preclude the exposure of the SNC's to cosmic radiation while still on Mars, implying that these were quickly buried under subsequent lava flows.
Assuming that the shergottites, nakhlites and Chassigny were ejected in the same impact (a theory still under scrutiny) and were in close proximity, why aren't Chassigny and the shergottites altered by water like the nakhlites? Where did the water come from? The nakhlites and Chassigny apparently have the same crystallization ages and cosmic ray exposure ages, yet Chassigny doesn't contain iddingsite. Drake, Swindle, et al. (1994) wondered if it was possible to "distinguish between the existence of a stable aqueous reservoir vs. a transient aqueous fluid produced by interaction between cooling magma and water ice in a permafrost layer."
Ultimately, and most difficult to explain, was how the SNC's were ejected into space. The consensus for a Martian origin of Nakhla and the other SNC's sparked a revolution in the field of impact physics. Until a few years ago, the accepted conclusion was that any meteoric (or cometary) impact capable of launching material past the 5 km/sec velocity required to escape Mars gravitational field would vaporize or melt everything on contact. Imagine the additional difficulty in building a model of an ejection mechanism for the unshocked nakhlites! "Necessity is the mother of invention" never rang truer as researchers struggled to come up with a plausible explanation for the intact ejection of these Mars rocks into space. One of the first papers on this was L.E. Nyquist's (1983), "Do oblique impacts produce Martian meteorites?"
The defining variables of a model allowing for the ejection of intact fragments from Mars were many. Most researchers assumed a 10km/sec. velocity for an impactor. Obviously, smaller pieces are easier to launch into space than larger ones. Measured cosmic ray exposure age and the possibility of collisional disruption in space of fragments (Bogard, et al. 1984) first defined the parameters of the fragment size able to exit Mars intact. The calculated ablation loss from entering earth's atmosphere (Vickery, 1986) refined those size limits. Even the effect on ejecta acceleration caused by vaporizing Martian permafrost was considered (Wasson and Wetherill, 1979).
To date, although researchers agree that a way exists to propel material into space from Mars, no single model fully explains this phenomenon. A study by A.M. Vickery and H.J. Melosh (1987) used a "spallatial model" to predict the size of a crater required to eject material consistent with the known collective weight of the SNC's. They concluded that "the SNC meteorites were probably ejected from a very large crater (greater than 100 kilometers in diameter) about 200 Ga., and that cosmic ray exposure of the recovered meteorites was initiated after collisional fragmentation of the original ejecta in space at much later times (0.5 to 10 Ga.).
Drake and Swindle et al. (1994) suggested that "since Nakhla shows no evidence of shock it must have been ejected from Mars by impact as a near surface "Grady-Kipp" fragment (Melosh, 1988). An exact definition of "near surface" is not possible, as the thickness of Grady-Kipp fragments scales with the magnitude of the impact event, which ejected the nakhlites from Mars. Nevertheless, the nakhlites must have come from no deeper than a few kilometers below the Martian surface...". (Note - A "Grady-Kipp" object is defined as broken rock at the impactor's target area.)
Wetherill (1984) calculated that 35% of material ejected from Mars into a heliocentric orbit would reach Earth in about 10 Ma (and for those of us owning or interested in E-chondrite meteorites, he also calculated that any material originating on Mercury was about one hundred times less likely to make it to Earth).
In a follow-up paper in 1994 McSween writes, "Planetary ejection of these rocks has promoted an advance in the understanding of impact physics, which was accomplished by a model involving spallation during large cratering events. Ejection of all the SNC meteorites (except ALH84001) in one or two events may provide a plausible solution to most constraints imposed by chronology, geochemistry, and cosmic ray exposure, although problems remain with this scenario; ALH84001 may represent older Martian crust (>3.5 Ga.) sampled during a separate impact."
In Part Two, we'll discover a major discrepancy with the total known weight of Nakhla, then journey to the strewnfield with Mr. Meteorite himself, Bob Haag. We'll close out this magnum opus by resurrecting a dead dog.