Embedded in the meteors that orbit our star are the oldest objects formed in the solar system. As the first solids to condense out of the gaseous nebula that surrounded the young Sun, their appearance is key to understanding the formation of terrestrial planets such as the Earth.
Unfortunately for those wishing to unlock our origins, their formation has previously been an unresolved mystery.
Identified by their glassy texture, these ancient wanderers are named ‘chondrules’ and make up around half the mass of the most common types of meteorites. By directly measuring the quantity of radioactive isotopes in the rock, scientists have confirmed that their age puts them as contemporaries to the oldest known solids in the solar system. At the point of their formation some 4500 million years ago, the Sun was a young star and orbited by a disc of gas and dust that would one day birth the planets.
The problems with chondrule formation begin with the surrounding rock. Based on the vaporisation point of its constituent compounds, this matrix material can never have been exposed to temperatures greater than 600–700 K. Yet, to form a chondrule, the temperature must have soared to a staggering 1800 K.
After formation, the molten rock needs to cool. If the chondrule was exposed straight to the vacuum after a flash heating event, it would cool radiatively in a matter of seconds and produce crystals. However, chondrules do not have a crystalline structure, but are smooth and glassy, suggesting a cooling time of several hours.
One way out of this situation is to suggest that the chondrules formed separately to their surrounding matrix, joining only after their massive temperature rise. However, this can easily be proved false by measuring the fraction of heavy elements present in the meteorites.
The heavy element content of an object depends on the time and location of its formation, making it a reliable measure of where an object originated. In the case of a chondrule-containing meteorite, the total heavy element fraction matches that of the Sun, regardless of the percentage of chondrules within it. This means that the chondrules and their surrounding rock formed from the same mixture of dust.
“If you take two meteorites,” Professor Mordecai-Mark Mac Low at the American Museum of Natural History in New York explains, “and one consists of 20% chondrules and the other 70% chondrules, both will contain the same fraction of heavy elements. If the chondrules formed separately, the remaining meteor material would have the Sun’s heavy element fraction, and the chondrules would add more heavy elements on top of that. The heavy element fractions for these two meteorites would then be very different.”
Working with the idea that the formation mechanism had to be a common phenomenon, Mac Low began his explorations on chondrules by asking a simple question: What basic fact do we know about the gas disc in which the chondrules were forming?
The answer was that it was an accretion disc: a rotating reservoir from which the young Sun was pulling gas to increase its own mass. In order for this gas to break its orbit and spiral towards the disc centre, there must have been a force to destabilise the circular motion. This force came from the magnetic field.
In a differentially rotating disc (where the rate of rotation changes with radius), the force from even a weak magnetic field can become highly disruptive. It was this ruction that drove the gas in the disc to become turbulent, allowing it to transfer its rotational energy outwards so it could spiral in and accrete onto the Sun.
Moreover, accretion wasn’t the sole result of the magnetic turbulence. As the gas was stirred up, it caused the magnetic field to bend. These field bends induced a current, forming curved surfaces in the turbulent disc along which charged particles flowed – a formation known as a current sheet.
While it sounds exotic, the existence of current sheets has been long established as an unavoidable consequence of any kind of magnetised turbulence. However, Mac Low thought that this common yet energetic process might hold the key to chondrule formation.
At first, Mac Low’s discoveries were not promising. As with your toaster oven, a running current will meet with resistance and produce heat. How much heat is produced depends on the size of the current and a property of the object called resistivity. However, since the disc was largely neutral, the number of charged particles available to produce a current sheet was small. This meant very little heating.
Then, Alexander Hubbard, a postdoctoral researcher working with Mac Low, began to look at the problem. Hubbard discovered that if small temperature perturbations in the current sheet could bump the temperature up to just below 1000 K, alkali metals such as potassium and sodium could become ionised. The burst in the number of charged particles would boost the current, increasing the heating and thereby ionising even more elements.
This runaway process could create significantly hotter patches within the current sheet wherever it approached 1000 K, but it still wasn’t enough to hit the temperatures needed to create a molten chondrule. Then Hubbard and Mac Low realised a crucial fact: while most applications in astrophysics assume the resistivity is constant throughout a material, in the case where a temperature rise significantly increases the number of charged particles, it can become strongly temperature dependent. This means that within the hotter patches in the current sheet, the resistivity would plummet and the current would soar. “It was like a short circuit,” Mac Low says.
The increased current would then drag on the magnetic field lines to create a very thin layer, with the majority of the current channelled through the area of low resistivity. The result was a sheet with a hot spot capable of melting rock. Small clumps of dust could pass through these creations in a few hours, allowing them to melt into a chondrule before cooling slowly as the rock exited the sheet.
“The key is to get the disc temperature up to the maximum the matrix rock can stand and then form a hot patch inside the current sheet to ionise the alkali metals,” Mac Low concludes. “The resistivity then drops to create spots hot enough to melt a chondrule.”
Once the chondrules form, the first solid objects in the solar system have been made, and, from there, our own planet’s story can begin.
Image: Denton Ebel, meteoriticist and associate curator in the Museum’s Department of Earth and Planetary Sciences, uses an electron microprobe to excite the atoms on the surfaces of meteorite samples, revealing their mineral composition. © AMNH/D. Ebel. Reproduced with permission