Allende

The Rosetta Stone of the Solar System

January 21, 2019
By John Curchin
It went something like this…

It was February 7th, 1969 and Elbert King was frustrated. A geology professor at the University of Houston, ‘Bert’ was also overseeing the finishing touches on the LRL, the Lunar Receiving Laboratory that NASA had been putting together in anticipation of analyzing the incoming priceless trove of moon rocks. The instruments were all in place: saws and grinders to make wafer thin slices of rock mounted on glass slides – thin sections – from which the interior of the moon rocks could be explored:  their mineral grains, textures, compositions, etc. The slides could also be mounted on microprobes to acquire elemental abundances. Mass spectrometers were ready to count isotope ratios of oxygen, carbon, sulfur…it was the most advanced scientific laboratory of the time, and Bert was just northeast of town, in Crosby, Texas, hunting for a recently reported fallen meteorite. If he could just find a fresh rock from space to give the new labs’ instruments a few months of dry runs, he’d be much more comfortable with the moon rocks once they arrived sometime later this year. But this days’ exploring had come up empty (no meteorites from around Crosby would ever be found).

Early the next morning, just after 1 a.m. local time in the quiet Mexican town of Hidalgo del Parral, residents were abruptly awakened by a series of thunderous booms, bright lights, and a rain of stones. A particularly large one fell not 30 feet from the post office of the nearby Pueblito de Allende, from which this meteorite fall would eventually take its name. Over 2 tons fell that morning, in an elongated elliptical area, or strewn field, 30 miles long by 5 miles wide, one of the largest ever. Later that day, locals easily found the black rocks sitting in small pits on the desert floor, and began to collect them in case they were messengers from heaven. How close that idea was to the truth, they could not have known…

After another fruitless day searching for the ‘Crosby meteorite’ 1000 miles to the east, Bert turned on the radio once again for the drive home. It wasn’t long before his mood took a decided turn for the better – over the airwaves came the announcement of a bright fireball seen over southern New Mexico and northern Mexico that morning. He quickly had his secretary make some calls, and not 12 hours later was rolling into the dusty town of Parral in a rental car, ready to meet with the local newspaper editor and purchase some stones. He was welcomed as a representative of NASA, and with the help of the local mayor and police, acquired many a fine specimen for his lab. Over the next few months, the LRL would initiate the research on what has been called the most studied rock in the universe, the Allende meteorite.

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What makes this meteorite so important?

In addition to being one of the first any serious collector typically purchases (although some can still be found down in the Chihuahua desert), the fact that so much of it arrived at such a propitious time is only the beginning. Allende is classified as a CV3, or carbonaceous chondrite of the Vigarono type, petrologic grade of 3. Carbonaceous chondrites are extremely rare, even for meteorites, as they are very fragile, easily contaminated, and do not survive long in the terrestrial environment…all reasons Bert high-tailed it down to Parral once he heard the news. The name carbonaceous refers to the fact that they have a high carbon content, including many water-bearing minerals and organic compounds. Carbonaceous chondrites had been found before, but most were small, and had been poorly studied. And none were freshly fallen in a desert, where the carbon and water bearing minerals could be definitively identified as non-terrestrial.

The ‘chondrite’ part of Allende’s classification refers to the presence of pin-head to pea-sized chondrules, or tiny spherules of silicate (olivine and pyroxene) glass embedded in a fine-grained matrix. Chondrules appear to be some of the earliest solids produced by the solar system as it condensed out of a vast cloud of gas and dust 4 ½ billion years ago. Theories to their origin are as numerous and diverse as those who study them, particularly because ‘ordinary’ chondrites make up over 85% of all meteorite falls, and may well represent the majority of material that went into making the terrestrial planets. Again, this idea could be dissected and discussed until we actually begin mining the asteroids to better assess the role of chondrules as building blocks of the planets.

What else?

What really makes the Allende stand apart from other carbonaceous chondrites is its low petrologic grade: technically a 3.2. This grading refers to the amount of heating the parent asteroid of any given meteorite has been subjected to, and the lowest possible is 3.0. Therefore, the Allende is about as unmetamorphosed as a meteorite can be. Its components are essentially pristine pieces from the soon-to-be solar system. And the real kicker is the abundance of CAI, or calcium aluminum inclusions, in the matrix. These are irregular-shaped aggregations of minerals containing calcium, aluminum, titanium and many other ‘refractory’ elements – those in compounds that condense at the highest temperatures from a cooling solar nebula of gas and dust. In other words, the age of the CAI gives us Time Zero, the moment that the first condensed solids composed entirely of ‘us’, this particular mix of gas and dust in the galaxy, formed. The given date of the birth of our solar system, 4.5682 billion years ago, is the most recently determined age for the CAI from within the Allende meteorite. The only rocks we’ll ever find older than this will come from planets elsewhere in the galaxy.

Intriguingly, something akin to this kind of material from exosystems may exist within the Allende as well. Tiny diamond crystals have been identified with non-solar system isotope ratios. Micro-diamonds are formed in the expanding envelopes of red giant stars, and perhaps the Allende preserves some of this stardust from a nearby giant in our own stellar nursery prior to 4.5 billion years ago. It is why they are known as presolar grains, and are found in very few meteorites. Even more recently deep within the matrix another form, or allotrope, of carbon has been found: fullerenes. Also known as buckyballs, these are tiny ‘spheres’ of 60 carbon atoms with the exact same geometry of pentagons and hexagons in a stitched soccer ball.

Other complex organic (carbon-bearing) compounds abound. A 70 carbon form of buckminsterfullerene has been identified, as well as a suite of fulleranes – fullerenes with attached H atoms. PAHs, or polycylic aromatic hydrocarbons, have also been found. These are benzene ring-based organic molecules – multiple six carbon rings attached to each other, and with side groups of hydrogen, methane, amines, etc. Two well-known terrestrial ones found in Allende are naphthalene, the odor-producing compound in moth balls, and anthracene, common in coal formations. Even more complex organic compounds such as benzofluoranthene and corennulene have been detected. Although none of these molecules are considered biological, such meteorites as Allende may well have provided the carbon-rich compounds seeding the early earth from which life did eventually emerge.

Mineralogically, Allende has also proven to be a bonanza. Currently, 76 valid mineral species have been identified among the chondrules, CAI, and matrix of the meteorite, 18 of them new to science. Hence, the Allende meteorite is considered the ‘type locality’ for each of those minerals, including, of course, Allendeite. All in all, truly a Rosetta stone of the solar system.