Happy May 4th to all of my fellow Star Wars geeks! We might not be able to jump into hyperspace just yet, but we do have a pretty great space program. The Cassini spacecraft has been orbiting Saturn since 2004 and is now maneuvering its months-long grand finale of high-risk dives around Saturn’s rings, before plummeting into the gas giant’s atmosphere1. With Cassini’s long voyage coming to an end I thought that it would be worth a look back at one of its many discoveries.
Space is not empty: The ‘void’ between solar systems has much more going on than a massive expanse of nothingness. Dark matter aside, this part of space is still quite complicated; the interstellar medium contains cosmic rays, gases (mostly hydrogen and helium), and dust particles, to name a few of its components. This area of space is particularly interesting because we have such a limited picture of it. What exactly is out there? Are those dust particles the same presolar dust that created our solar system? Ignoring our human compulsion to just want to know, these are practical questions. Having just some of this information will give insight to the active conditions and physical realities in this region of space. We can understand what makes up of our solar neighborhood, and vastly improve interstellar modeling.
Cassini did not venture into the local interstellar space. Instead, it identified and tested particles that were blown into our solar system through an interstellar dust stream. First identified by the Ulysses spacecraft, this flow of interstellar particles is carried into the region around Saturn during certain parts of its orbit. Based on the speed and direction of the dust particle, Cassini was able to separate the interstellar dust from the local dust around Saturn. Over ten years, 36 interstellar dust particles were collected. Vaporized upon collision, Cassini’s Cosmic Dust Analyzer could detect the chemical composition of each particle.
Analysis of all 36 particles revealed some interesting things. First this dust is mainly silica based, containing high levels of magnesium, calcium, and iron inclusions. This composition means that the dust particles are tiny building blocks of rock. Secondly, these interstellar dust particles are unexpectedly similar to one another: Presolar dust grains, particles around at the formation of our solar system, are made up of highly diverse heavy elements, so interstellar dust is very clearly different. It would seem that this interstellar dust is highly processed, cycling through destruction and reconstitution. The similarity of these particles indicates that they have been mixing and homogenizing, breaking apart and joining back together. The authors suggest that this process occurs when the particles shift between various temperature and density zones within the interstellar medium, colliding and devolatilizing in warm zones while condensing in cold regions. Until we can venture into the far reaches of interstellar space, these types of experiments are giving us some of the most meaningful insight into the nature and cosmic processes of our galaxy.
- NASA: Jet Propulsion Laboratory. “Cassini: The grand finale: Timeline”. NASA. Accessed: 05/03/2017. https://saturn.jpl.nasa.gov/the-journey/timeline/#launch-from-cape-canaveral
- Altobelli, N., et al. “Flux and composition of interstellar dust at Saturn from Cassini’s Cosmic Dust Analyzer.” Science 352.6283 (2016): 312-318.