Our ancient Solar System was a violent place, where primordial objects crashed into one another, blasting each other into a multitude of fragments. This chaotic, turbulent mess of primordial crashes, occurring between rampaging objects, has inspired some planetary scientists to refer to our ancient, still-forming Solar System as a “cosmic shooting gallery”. Indeed, some of these invading objects wreaked havoc when they crashed into the newborn Earth, often contributing more and more of their material to our still-forming planet. Planetary formation models show that the solid, terrestrial inner planets of our Sun’s familiar family–Mercury, Venus, Earth and Mars–were born as a result of the accretion of tens of Moon-to-Mars-sized planetary embryos through raging, energetic giant impacts. In April 2018, a team of astronomers published their new findings suggesting that a space rock that fell to Earth may have come from a long-lost proto-planet from the early Solar System–and that tiny bits of iron and sulfur embedded in diamonds within this meteorite likely were created under high pressures found only deep within planets the size of Mercury or Mars.
Alas, tattle-tale relics of these large, lost proto-planets have been difficult to find. Ureilites are one of the major families of achondritic meteorites and their parent body is thought to have been catastrophically blasted to pieces by an impact during the first 10 million years of our 4.56 billion-year-old Solar System. Achondrites lack chondrules and originate in differentiated bodies–such as planets. A chondrule is a spheroidal mineral grain, that is present in large numbers, within some stony meteorites. In the April 17, 2018 issue of Nature Communications, a team of planetary scientists published their report announcing that they had studied a chunk of the Almahata Sitta ureilite using transmission electron microscopy. The scientists found, scattered within this chunk, large diamonds that could only have formed at high pressure deep within a parent body. The team of researchers detected chromite, phosphate, and (Fe,Ni)-sulfide (iron, nickel, sulfide) inclusions embedded in diamond, and they reported that the composition and morphology of the inclusions can only be explained if the formation pressure was greater than 20 GPa. These pressures indicate that the ureilite parent-body was a Mercury-to-Mars-sized planetary embryo.
Sulfide inclusions in diamonds are the most common of all inclusions, and they contain important information about the timing and physical/chemical conditions prevailing during diamond formation.
The story of the Almahata Sitta ureilite began when an asteroid, designated as Asteroid 2008 TC3, crashed down in the Nubian desert in Sudan in 2008, and its recovered batch of meteorites, called Almahata Sitta, are mostly composed of ureilites with a variety of chondrites. Ureilite fragments are coarse-grained rocks that are primarily made up of olivine and pyroxene that originated from the mantle of the ureilite parent body (UPB), that has experienced a disruption resulting from a catastrophic impact that occurred early in our Solar System’s existence. High concentrations of carbon distinguishes ureilites from all other achondrite meteorites, with graphite and diamond nestled between grains of silicate.
Cosmic Shooting Gallery
When our Solar System was first forming, strange things were occurring. Primordial planetary building blocks, called planetesimals, traveled away from where they had been born, and violently crashed into one another as a result. Sometimes these wandering planetsimals merged, but quite frequently they collided, leaving only the wreckage of both bodies behind to tell the tragic story of their ancient, deadly collision.
The history of our Solar System is one of turmoil, and this is also the case with distant planetary systems around other stars beyond our Sun. Stars are born surrounded by a whirling disk made up of gas and dust, termed a protoplanetary accretion disk. These swirling disks form at about the same time that the baby star, called a protostar, is born within its blanketing, obscuring natal cloud.
Protoplanetary accretion disks contain large quantities of gas and dust that feed growing, voracious protoplanets. Our own Solar System, as well as other planetary systems, form when a relatively small and very dense blob, embedded within the billowing, undulating swirls of a cold, dark, giant molecular cloud, collapses under the merciless influence of its own gravity. Floating throughout our Milky Way Galaxy in huge numbers, these phantom-like, beautiful clouds, serve as the bizarre nurseries of fiery baby stars. These gigantic, frigid clouds are mostly composed of gas, but they also harbor smaller quantities of very fine dust. Although it seems counterintuitive, things have to get cold in order for a hot baby star to be born.
Most of the collapsing blob collects at the center, and ultimately ignites with a brilliant stellar fire as a result of the process of nuclear fusion–thus forming a new stellar infant (protostar). The remaining gas and dust eventually evolves into the protoplanetary accretion disk from which planets, their moons, and other smaller objects are born. In its earliest stages, a protoplanetary accretion disk is both very massive and searing-hot–and it can circle its host star for as long as ten million years.
By the time a brilliant star, that is about the same mass as our Sun, reaches what is termed the T Tauri phase of its development, the very hot, massive surrounding accretion disk has become considerably cooler–and thinner. A T Tauri is a stellar tot–a very young, variable star that is similar to our Sun, and is quite active at the age of a mere 10 million years. These fiery stellar toddlers sport large diameters that are several times greater than that of our own Star at present. However, T Tauris are still in the process of shrinking. Unlike human babies, Sun-like stellar tots shrink as they grow up. By the time the stellar toddler has reached this stage, less volatile materials have started to condense close to the center of the surrounding accretion disk, thus creating very fine and extremely sticky motes of dust. The delicate dust particles carry within them crystalline silicates.
The dust motes collide and then merge within the very dense environment of the accretion disk. As a result, they continue to grow in size, from dust-particle size, to boulder size, to mountain size, to moon size, to planet size. These growing bodies become the primordial planetesimals, and they can reach impressive sizes of 1 kilometer across–or greater. These planetary building blocks represent an abundant population within the ancient accretion disk, and they can linger around their star long enough for some of them to still be around billions of years after a mature planetary system has emerged. In our own Solar System, comets are the frozen, dusty, icy relics of the planetesimals that contributed to the formation of the quartet of outer gaseous giant planets–Jupiter, Saturn, Uranus, and Neptune. On the other hand, the asteroids are the leftover rocky and metallic planetesimals that served as the “seeds” of the inner solid planets–Mercury, Venus, Earth, and Mars.
A Tattle-Tale Chunk From A Vanished Ancient World
The paper published in the April 17, 2018 issue of Nature Communications suggests that the chunk of the Almahata Sitta ureilite being studied is probably a piece of a Mars-sized protoplanet–one of the first planets to exist in our Solar System. Alas, this ancient planet has long since disappeared. The authors of the paper, who are of the Ecole Polytechnique Federale de Lausanne (EPFL) in Lausanne, Switzerland, analyzed very tiny pieces of the Almahata Sitta meteorites. These particular meteorites are famous because they came from the first-ever asteroid to be tracked from its orbit to the ground–as it was in the process of crashing down to the Nubian desert. These ureilites possess compositions that are different from those of the known solid, inner planets of our Sun’s family, and contain 100-micrometer diamonds. This means that the diamonds are too large to have formed in the shock of two asteroids blasting into one another. Diamonds this large, however, could form within asteroids that are at least 1,000 kilometers in diameter, because pressures within these bodies would be sufficient to compress carbon.
During their study, the researchers–that include Dr. Phillippe Gillet, Dr. Farhang Nabiel, and their colleagues from the EPFL–discovered something strange that made them doubt that these tiny diamonds formed within any asteroid at all. This is because the gems had grown around even tinier crystals of iron and sulfur, which normally repel each other–and will not mix in a way that has been likened to that of water and oil. Those crystals could only remain stable at extreme pressures of 20 gigapascals–equivalent to almost 200,000 times the atmospheric pressure at sea level on our own planet. This means that they could only have formed in the center of a major planet, approximately the same size as Mercury, about 4,900 kilometers wide, or in the core-mantle boundary of a world as large as the planet Mars. Mars is approximately 6,800 kilometers wide.
Such long-lost planets likely dwelled in the primordial Solar System about 4 billion years ago. However, there are only a handful of survivors left from that violent, turbulent time–the quartet of solid inner planets that currently circle close to the warmth and light of our Sun. Supercomputer simulations indicate that most of these ancient planets blasted into one another and were destroyed. This ancient planetary smash-up probably occurred during the first 100 million years of our Solar System’s existence.
Basically, there are three mechanisms that could explain diamond formation in ureilites: (i) growth under static high-pressure within the UPB; (ii) a shock-driven metamorphosis of graphite into diamond as a result of a high-energy impact; (iii) growth by chemical vapor deposition (CVD) of a heavily carbon-laden gas floating around within the solar nebula.
Recent studies of the chunk of the Almahata Sitta ureilite show clusters of diamond single crystals that have almost identical crystallographic orientation, and are separated by bands of graphite. This means that individual, large diamond single crystals are present in the sample, and that these have later segmented through graphitization. The formation of such large single-crystal diamond grains along with the zoning seen in diamond segments cannot occur during a dynamic event. This is because of its brief duration (up to only a few seconds) and–even more importantly–by CVD mechanisms. This means that static high-pressure growth is the only possible origin of the single-crystal diamonds.
Diamonds frequently encapsulate and imprison minerals and melts that are present in their original environment in the form of inclusions. This is because of the gem’s stability, melting temperature, and mechanical strength. In the case of the diamonds found on Earth, this feature has enabled scientists to estimate the depth of diamond formation, as well as to identify the composition and petrology of phases sampled at that depth. This indicates that diamonds formed inside the ureilite parent body can potentially solve the mystery surrounding the size and composition of the long-since-vanished ancient world.
This new study confirms the existence of lost primordial Solar System planets. However, in itself, the probability that these vanished worlds once existed long ago isn’t especially surprising. The new findings are important because, for the first time, it has provided direct meteoritic evidence for the existence of a large, vanished protoplanet inhabiting our ancient Solar System.