Impact splash chondrule formation during planetesimal recycling

This blog article is a plain text summary of the research efforts soon to be published in Icarus, openly accessible at arXiv:1711.02103.

What are chondrules, and why should I care?

The early Solar system was very likely an uncomfortable place. The young Sun was surrounded by a ring of gas and dust, the protoplanetary disk in which the planets were forming. In the inner part, close to the Sun, it was extremely hot, potentially a few thousand Kelvin, and outside, beyond the current orbit of Jupiter, the temperatures were freezing cold, at only a few dozen Kelvin. In the midplane of the disk, myriads of planetesimals (small asteroid-like rocky bodies in the early solar system) were forming and colliding with each other to build up the solar system planets. However, until to date, there’s lots of uncertainty regarding the environment of that time and the details of the accretion process. We would like to know exactly how it all came to be, because these physical and chemical processes are our prime example of the general principles of planet formation.

Luckily, we are in the reasonably comfortable situation to possess at least a few contemporary witnesses of that time. These are the rocks in the world’s meteorite collection, and by analysing them in laboratories we can measure their ages and chemical compositions. The oldest and most pristine ones of these are called ‘chondrites’, because (nearly) all of them feature tiny spherules inside them that are called ‘chondrules’, which are usually 0.1–2 mm in size. The interior of the most frequent class of chondrites is shown in Figure 1A below, and a zoom-in onto a very specific chondrule in Figure 1B.

Figure 1: Close-in picture of chondrules and metal grains in chondritic meteorites. (A) Ordinary chondrite with chondrules and metal grains (source). (B) Concentric compound chondrule with inner and outer metal ring (Wasson & Rubin, 2010).

These images highlight two things: (1) Chondrules are usually really abundant in chondritic meteorites. Depending on the specific type of meteorite, the relative amount of chondrules to more tiny dust grains (‘matrix’) varies from 20–80%. (2) In many chondrites a third abundant component (in addition to chondrules and matrix) is ‘metals’, like pure iron or iron-nickel alloys.

Why do I tell you all this? Because, still until today, we do not know how chondrules formed in the first place… That has to do with their rather unique and enigmatic physical and chemical features, which do not easily fit into our common view of planet formation. Indeed, they are so weird, that if they would not be so ubiquituous in meteorites, no one would ever have theoretically predicted them! For example, from experimental petrology we know that chondrules were formed during extreme heating events, that reached temperatures above ~1900 K. That alone would not be that surprising, we can think of many mechanisms being able to heat a tiny speck of dust. But, we also know that after these heating events that melted the dust specks, all chondrules cooled down on a time scale of ~hours to days. And this combination is indeed weird. Why? Imagine the conditions in the protoplanetary disk (extremely cold, or extremely hot, many small floating dust grains embedded in a diffuse gaseous medium), you have two options to reach the chondrule-forming temperature: Either, you heat up the entire disk (or large parts of it) to reach ~1900 K, but afterwards you also need to cool down this disk rapidly to achieve the required cooling time. The other option is to localise the energy event, for instance you heat up individual dust grains separately, and not the gas. However after the energetic event is over, the dust grain will cool rapidly in the cold environment, probably on the order of seconds. (Physically speaking, the cooling of a dust grain in space is so efficient, because it is entirely dominated by surface energy. So the total amount of energy that can be stored in grains of ~mm in size is tiny compared to its potential to lose energy via radiation.)

Chondrule formation models to date

Ok, so we must think of a way to satisfy these primary constraints, in addition to many others. (I will not go into further details here about chondrule features and chemical, petrographical, and textural constraints, but refer to the literature for the vast amounts of evidence piled up in the more than 100 years since the first description of chondrules.) Over the years, people came up with all possible and impossible ways to form chondrules, none so far is generally accepted. The two most widely discussed theories include (1) gas shocks in the protoplanetary disk, that heat up the dust grains (‘nebula models’), and (2) planetesimals that collide with each other and launch sheets of tiny magma droplets into space (‘planetary models’, Figure 2). In both cases, the chondrules formed during the heating event must later be accumulated in newly formed bodies and eventually end up in the asteroid belt (because the asteroid belt is the source region of Earth’s meteorites).

Figure 2: Formation of chondrules via impact splash collisions among partially molten planetesimals (Asphaug et al., 2011).

Here, we deal with the second type of models. Especially, we will have a look into another constraint from chondrite meteorites for chondrule formation models: the abundant metal grains (or beads) in and around chondrules, which can be seen in Figure 1. But before doing so, let’s shortly recap the theory behind these types of models. The general idea of the collision hypothesis is, that planetesimals were (somehow) formed shortly after the birth of the Sun, and then underwent large numbers of collisions among each other to grow into (terrestrial) planets. The impact energy during these collisions may have been sufficient to melt the rock of the bodies participating in the collision and the collisions would launch a myriad of magma droplets (= chondrules) into space that later reaccreted onto planetesimals/asteroids to finally end up in the asteroid belt.

Recent collision models now utilise the fact that these early planetesimals were efficiently heated from the decay of aluminum-26. Aluminum-26 is a short-lived radioactive isotope that was present in the early solar system (but is not active anymore, since its half-life is short, ~717,000 years). The decay energy from aluminum-26 was sufficient to melt the planetesimals from the inside. So instead of colliding two rocky bodies with each other, you can imagine it as two balls of magma splashing each other (therefore, impact splashes). In this case, the impacts can be much less energetic to reach sufficient temperatures for chondrule formation (~1900 K). That is shown in Figure 2, where the expanding sheets after a collision between two pre-heated planetesimals are shown. That the planetesimal collision velocities can be much slower than in the case of a collision of two cold rocks is much appreciated by advocates of collision models. It turns out, that it wouldn’t even be possible to smash two ‘cold’ planetesimals into each other with the required energies, because the ambient gas (from the disk) dampens the relative velocities of planetesimals and makes it really hard to achieve sufficiently high collision velocities (higher than a few km/s).

Overcoming a major inconsistency of ‘planetary’ models

However, one major criticism of these models deals with the abundance of metal grains in and around chondrules in chondritic meteorites (see again Figure 1). If planetesimal collisions are supposed to form chondrules, then they must be able to preserve the metals from the accretion of the planetesimals and not lose them during the radiogenic heating of the planetesimal. Let me expand a bit on this argument first. When a planetesimal heats up, and the rock turns into magma, the metal grains which are all in and around the silicate rocks are lost to the center of the planetesimal, because they are denser than the rock. They ‘rain out’ and form an iron core in the center of the planetesimal. This is not a theoretical idea, we have actual evidence for this process in the form of iron meteorites. So we know that some planetesimals formed a core, and presumably they did so when the planetesimals heated up and differentiated into a structure of core, mantle and crust. That’s a serious problem for collision models. Heating is good, because you can make chondrules from low-velocity collisions, but on the other hand it is bad because you lose all your precious metals to the core! So, what to do?

Figure 3: Two end-member pathways for planetesimals, depending on their formation time and radius/size. (Top) Low-energy bodies heat up, and form a small core from percolation, then subsequently cool down. (Bottom) High-energy bodies heat up and form a magma ocean during their heat climax (that can last millions of years). They form a large core from metal rainout, which depletes their mantles from the majority of metals. Models highlighted in red are not suitable for chondrule formation, because they are either too cold (and would require too high collision velocities) or are metal-depleted. Green models, however, are eligible for chondrule formation from collisions. (Lichtenberg et al., 2017)

In fact, the story is not so simple. Aluminum-26 indeed was a strong heating mechanism in the early solar system. However because of its short-half life time, the actual degree of heating in the planetesimals is a strong function of both their size and formation time. If a planetesimal is small, it can efficiently cool via its surface, if it is big, the internal heating surpasses the degree of cooling and melt can readily accumulate in the interior. Early-formed planetesimals (when a lot of aluminum-26 was present) therefore received a lot of heating. When the planetesimal formed later, most aluminum-26 was already decayed and could not heat the planetesimal anymore. The resulting interior evolution pathways of planetesimals depending on these two parameters are shown and explained in Figure 3. The evolution of the silicate rock during the high-energy evolution pathway is illustrated in the video below.

Video 1: Heated interior of planetesimal due to decay of aluminum-26 in a planetesimal with 110 km radius that formed 0.1 Myr after CAIs. Shown is solid-state buoyancy-driven convection in a three-dimensional finite-differences fluid dynamics simulation (Lichtenberg et al., 2016).

What does this mean for chondrule formation via planetesimal collisions? You can do it, but only bodies that received a relatively balanced amount of heating can serve as potential colliding bodies. That, in fact, means a lot for the standard theory of terrestrial planet accretion. Standard models of planet formation start out with a protoplanetary disk seeded with planetesimals, which then go through several phases (runaway and oligarchic growth, and subsequently the giant impact phase that produced the Moon in case of the Earth) to form the giant planets and terrestrial planets. Now, if the planetesimals formed early, they would receive a massive amount of heating and all end up in the high-energy regime from Figure 3. The countless collisions among these planetesimals inevitably would produce large amounts of collision ejecta in form of droplets. These droplets would then flood the entire solar system and reaccumulate to form new generations of planetesimals and also inevitably end up in the asteroid belt. However, these droplets would look intrinsically different than the chondrules we observe in chondritic meteorites!

Figure 4: Syrup in a water glass. (source)

Why? First of all, they would be metal-depleted, because the impacts cannot be sufficiently energetic to remix all the planetesimal cores with the planetesimal mantle material. And there’s a second issue, which I did not mention so far. These ominous spherules produced in impact splash collisions between completely molten planetesimals would be isotopically indistinguishable! To visualise this, imagine a glass of water with some viscous and colored syrup mixed into (like cranberry syrup). Without any further effort, the syrup barely mixes with the water and mostly sinks to the bottom of the glass or hangs out in its own dense cloud within the water. Only once you stir the glass with a spoon both liquids mix in a satisfying way and you can drink the tasty beverage. The same goes with any primordial isotopic differences in chondrites and chondrules. In recent years cosmochemists were able to analyse chondrites with new laboratory techniques to an unprecedented level of detail. It turns out, no piece of chondritic meteorite is like the other! The smallest building blocks (including the chondrules) can all be distinguished from one another on extremely fine scales. This brings up another problem with collisions among fully molten planetesimal that are supposed to generate lots of debris and flood the inner solar system: all the debris would be isotopically similar, because it would be mixed in the magma ocean inside the planetesimals. In this case, there is of course no spoon around that could mix the isotope-syrup. However, if a planetesimal would be largely molten inside, the magma is mixed via turbulent convection, which acts similarly to a spoon stirring our glass of water. Thus, the isotopic homogeneity (water-syrup mixing degree) in the magma would be too high to be consistent with the vastly different isotopic signatures in chondritic meteorites and thus the asteroid belt objects!

What does this mean for chondrite parent bodies….

Ok, let’s come back to our planetesimal collisions. Planetesimals were abundant. They collided. They were the seeds of the planets. So where is all the debris and basaltic spherules that must look intrinsically different than chondrules? For that to answer, let’s look at how these considerations still allow to make a chondrite from collisionally reprocessed material. This is shown in Figure 5.

Figure 5: How to form a chondrite parent body from collisions of (somewhat) pre-heated planetesimals. After planetesimal formation, the body collides with a similar-sized object and ejects a cloud of chondrules, that further reaccumulate or accrete onto a different object. Before the final parent bodies are formed, the material could go through multiple cycles of liberation and reaccumulation with varying degree of injected energy, accumulation times scales and chemistry. (Lichtenberg et al., 2017)

… and planet formation in the solar system

These schematics of chondrite parent body accretion have very direct consequences for planet formation models and our understanding of how the asteroid main belt came into being. Apparently, the global accretion dynamics prohibited that the asteroid belt was enriched in large amounts of material ejected from planetesimals with deep and convecting magma oceans (the high-energy bodies from Figure 3). From a dynamical perspective, that may not be easy to achieve. In general, accretion models predict efficient throughput of planetesimals during accretion, and thus much debris from largely molten planetesimals would be mixed into subsequent generations of planetesimals.

So, for now we see three methods to generate chondritic signatures from collisionally reprocessed planetary materials: (1) planetesimals in the asteroid belt region (or better, the planetesimals, that eventually ended up in the asteroid belt) formed late, and so avoided strong radiogenic heating; (2) planetesimals were formed small (< 50 km in radius) on average, with the same result as in (1); or (3) collisional reprocessing was so efficient that planetesimals did not even have time to heat up. In the latter case, the collisional throughput of planetesimals must have been so high that planetesimals only heated to just below (or around) the solidus of the silicate material (when rock starts to melt) and thus never experienced magma ocean phases.

How to form the iron meteorites then? In this case, these bodies must come from much larger bodies, for instance several hundred km in size. Such bodies would be massive enough to not be disrupted from the constant impactor flux to be able to differentiate into the typical core, mantle, crust signature. And what about chondrules? If the collisional processing of planetary materials is indeed so efficient, then chondrules are more likely to form by collisions than ever, because any potentially earlier formed chondrules would be erased during the constant impact events and reaccumulation cycles.

Where to go from here?

To summarise, we can learn a lot from meteoritic samples about the environment and accretion dynamics of the planets and early solar system. It turns out that the chemical and isotopic signatures of chondrules are at odds with our standard theory of terrestrial planet formation. These signatures suggest that the majority of source material that fed the asteroid belt cannot originate from strongly heated planetesimals that underwent repeated collision events, which would be predicted by disk-wide and rapid planetesimal formation.

These implications spawn new constraints for planet formation models: how can we form planets from agglomeration of smaller planetesimals on the disk time scale (~5 Myr) and at the same time avoid to flood the asteroid belt with lots of differentiated material? Questions like these will in the mid-term future enable a clearer view of how our solar system came into being. To do so, we must further synchronise the available evidence from our meteorite collections, potential future space missions (like NASA’s Psyche mission), and new types of planet formation models, that are powerful enough to simultaneously solve for the global disk dynamics and account for the local-scale collisional remixing of solids.


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