Forging Planets in a Shoebox: The EXCISS Mission
By Shruthi Venkatesh
Cosmofluencer (Season 03)
Have you ever wondered how planets form? It all starts with tiny building blocks called chondrules. These millimeter-sized spheres are the most common component of primitive meteorites, which are leftover debris from the early solar system.
These tiny balls of rock have a unique composition and structure. Made primarily of silicate minerals like olivine, pyroxene, etc., chondrules often contain a distinctive mix of melted and unmelted textures.
This suggests they have experienced a rapid heating event followed by quick cooling. Scientists believe they played a crucial role in the birth of our solar system’s planets.
Imagine these tiny spheres swirling around in a vast disk of gas and dust called the protoplanetary disk. Over time, through collisions and gravitational attraction, chondrules would have begun to clump together, eventually forming the cores of what would become planets. But how exactly did these individual chondrules form?
That’s a bit of a mystery. This is where the EXCISS experiment comes in.
The EXCISS Experiment
EXCISS is a fascinating experiment that took place aboard the International Space Station (ISS) in 2018. This German-led project, spearheaded by Dr.Daniel Hutmacher from Goethe University Frankfurt, aimed to test a specific theory about chondrule formation (the nebular lightning hypothesis).
The Nebular Lightning Hypothesis
Imagine the early solar system as a swirling disk of gas and dust called the protoplanetary disk. According to the nebular lightning hypotheses, electrical discharges, similar to lightning, ripped through this dusty environment.
These nebular lightning strikes could have rapidly heated dust particles to extremely high temperatures, causing them to melt and stick together, eventually forming chondrules. The unique properties of chondrules, including their distinctive textures and mineral compositions, strongly support this theory.
Why do chondrule properties support the nebular lightning hypotheses?
- Melted and unmelted grains: Many chondrules contain a mixture of melted glass and unmelted mineral grains. This suggests a rapid heating event that melted some of the dust particles, followed by quick cooling that froze the molten material in place. Nebular lightning’s intense, short-lived heat pulse could explain this unique texture.
- Fractional melting: Some chondrules show evidence of only specific minerals within them, while others remain unaffected. This suggests a process with varying degrees of heating, which could be caused by the uneven distribution of energy during a lightning strike.
- Mineral composition: The presence of rare minerals like some iron-sulfide inclusions, within chondrules, suggests extremely high temperatures during their formation. Nebular lightning’s intense heat could explain the formation of these exotic minerals
Comparison with other hypotheses:
- Nebular shock heating: This theory proposes that shockwaves from collisions within the protoplanetary disk could have rapidly heated dust particles. While shockwaves can produce high temperatures, it’s challenging to explain the observed variations in melting within chondrules.
- Nebular condensation: This theory suggests that chondrules are formed through the slow cooling and condensation of hot gas in the protoplanetary disk. However, this process wouldn’t create the melted textures and exotic minerals observed in chondrules
- Overall, the unique textures and mineral composition of chondrules are more consistent with a rapid heating event, like that proposed by the nebular lightning hypotheses. While other theories exist, they struggle to explain all the observed features of chondrules.
EXCISS in action
The EXCISS experiment was a marvel of miniaturization. Designed to fit within the confines of the ISS, the entire apparatus was no bigger than a shoebox. Inside this miniature space laboratory, were specially created dust particles composed of forsterite, a mineral common in chondrules.
These particles (most likely created using a process called laser ablation) were levitated between two electrodes in a chamber filled with argon gas, mimicking the dusty environment of the protoplanetary disk.
Over 30 days, the experiment remotely fired and precisely controlled electrical discharges at the dust particles simulating nebular lightning under the unique microgravity conditions of space.
How were the dust particles specially made?
- Laser ablation: This technique focuses a powerful laser beam on a target material ( in this case, forsterite) to vapourize tiny amounts. As the vapour cools and condenses, it forms microscopic particles with a very specific size and composition. This allows scientists to create dust particles that closely resemble the size and composition of those believed to exist in the protoplanetary disk
Significance of Using Forsterite:
While chondrules can contain a variety of silicate minerals, forsterite was chosen for the EXCISS experiment for several reasons:
- Simplicity: Forsterite is a relatively simple mineral. Composed of just magnesium and silicon. This makes it easier to study the effects of stimulated nebular lightning on the melting and fusion process without the complications of additional elements present in other silicate minerals like olivine or pyroxene.
- Relevance: Though not the most abundant mineral in all chondrules, forsterite is a common component. Using forsterite allowed the researchers to investigate a fundamental building block potentially involved in chondrule formation.
- Mimicking Melting Behavior: Forsterite has a lower melting point compared to other silicate minerals like olivine. This might have been chosen to replicate the rapid melting process believed to occur during chondrule formation caused by nebular lightning.
By using specially made forsterite dust particles, the EXCISS experiment could isolate and investigate the role of rapid heating on dust particles, a key aspect of the nebular lightning hypotheses.
The Verdict
The return of the EXCISS experiment from the ISS marked the beginning of a new phase -analysis.
Researchers at Goethe University Frankfurt meticulously analyzed the dust particles that had been exposed to the simulated nebular lightning.
They found that the electrical discharges indeed cause the forsterite particles to melt and partially fuse, with some particles even exhibiting signs of evaporation.
This provided strong evidence that nebular lightning could be a viable mechanism for chondrule formation. The analysis also showed that the melting process was extremely rapid, occurring in milliseconds, which aligns with the properties observed in natural chondrules.
A Spark of Discovery
While EXCISS did not prove the nebular lightning hypothesis, it proved a significant leap forward in our understanding of chondrules formation.
The experiment validates the nebular lightning hypothesis as a potential mechanism and paves the way for future research, both on Earth and in space, to refine our understanding of this critical step in planetary formation.
Future studies can build upon EXCISS by incorporating different dust compositions, pressure, and discharge parameters to create a more comprehensive picture of chondrule formation under various nebular conditions.
Conclusion
EXCISS peered into the mystery of how tiny chondrules formed planets! Simulating nebular lightning on dust particles abroad the ISS, it revealed this process could be a key ingredient. The experiment observed melting and fusion happening in milliseconds, mirroring natural chondrules, a critical finding.
While EXCISS doesn’t provide the final answer, it paves the way for future research. Understanding chondrules isn’t just about science, it’s about unlocking the secrets of our solar system’s birth.
By deciphering this code, we might just inch closer to understanding our place in the cosmos. So, the next time you look up, remember -the stars whisper a story of chondrules, and continued research will help us all hear it!
References
- SpringerLink | A chondrule formation experiment aboard the ISS – Dominik Spahr, Tamara E. Koch, David Merges, Lkhamsuren Bayarjargal, Philomena-Theresa Genzel, Oliver Christ, Fabian Wilde, Frank E. Brenker & Björn Winkler
- Goethe University Frankfurt | EXCISS – Experimental Chondrule Formation at the ISS