Asterismos

Category: Physics

  • The Wonders of the Universe: The Importance of Saturn’s Rings as an Astrophysical Laboratory

    The Wonders of the Universe: The Importance of Saturn’s Rings as an Astrophysical Laboratory

    Keywords

    Physics, Astrophysics, Astronomy, Planets, Exoplanets, Deep Space, Space Telescopes

    Introduction

    Of all the wonders that can be observed within our solar system, there are few that are more iconic than the rings of Saturn. As a result of their outstanding beauty, scientists for centuries have been drawn to Saturn’s ring system, studying this system extensively. This knowledge of planetary rings and the mechanics behind them now serves an important purpose. As we, the human race, begin to look further out into the cosmos, we can put to use the lessons learnt from our stellar neighborhood to identify objects further out in space.  

    Image of Saturn and It’s Ring System.

    Saturn’s Ring System

    From observations alone, you would be forgiven for thinking that Saturn’s rings are solid objects which orbit the planet. However, upon closer inspection, it is revealed that the structure of the planetary rings consist of millions and millions of tiny ‘dust’ like particles. These particles vary quite significantly in size, from being just a few microns in diameter to over 30 meters wide.  

    Another feature of interest when considering Saturn’s rings, would be the presence of gaps within the rings such as, the Cassini division. Since, we know that the ring structure is made up of millions of particles, why do gaps exist within the ring system, and what causes them to form? 

    Close up of Saturn’s Rings, showing the structure including gaps.

    At present, there are two known mechanisms responsible for the formation of gaps within planetary rings. The first of which is known as embedded moons. These are similar to traditional moons that would be found orbiting a planet however, these are found to be orbiting within the ring system itself. As a result of the moon being physically located within the ring system, a gap is cleared within the system where the embedded moon is gravitationally dominant. The gap that has now been created by this moon will now be maintained for as long as the moon exists, furthermore, the width of the gap actually relates to the mass and therefore the size of the moon.  

    The second mechanism is a touch more complicated; this gap formation method occurs due to orbital resonance. Orbital resonance occurs when two or more orbiting objects have orbital periods (the time taken to complete one full orbit) that can be related by a ratio of small integers (e.g. 3:1). As a result of this relationship between orbital periods, the two bodies exert a regular, periodic gravitational influence on one another.  

    Consider a moon orbiting outside of a ring system and a particle within the ring system. The particle in the ring system might orbit the planet twice for each orbit of the moon. These two objects are considered to be in an orbital resonance. At each point the orbits align the moon and the particle exert a gravitational ‘tug’ on each other, over time the so called gravitational ‘tug’ can build up to have significant impact.  

     When the particles within the ring system are found to be in orbital resonance with a moon outside of the ring, the gravitation influence of the moon is found to change the orbit of the particle in the ring. As a result, there is a region created within the ring system that is cleared of particles simply due to the influence of a moon located outside of the ring system. Since, Saturn has lots of moons, there are also lots of gaps within the ring system that oftentimes relate to the presence of a moon.  

    A short animation ofJupiter’s Moons which display a 4:2:1 orbital resonance.

    Understanding the Strange Case of J1407B

    So how does this information help the modern astronomer, looking out deeper into space? As astronomy continues to evolve as a discipline the way we use telescopes is changing, among others, a new role for telescopes now exists, survey telescopes. Survey telescopes are designed to map an entire area of the sky, quickly and efficiently, collecting lots of data that can then be studied by scientist at a later date. In particular, telescopes like the Kepler telescope and the TESS telescope are designed to survey the light coming from stars. 

    Artists rendition of the Kepler space telescope. Credit: JPL Laboratory

    With proper analysis, the light of a distant star can reveal all kinds of information about the star such as, the mass of the star, the diameter of the star, and even about any objects that might be orbiting a star. An example of this would be what astronomers call the Transit method, used to identify exoplanets.  

    The transit method is a simple analysis of the light of stars. When a celestial object such as a planet passes between the star and the Earth, from the Earth we observe a significant dip in light intensity. From the duration of the dip and the amount of light that is restricted, an astronomer can discover important details about the nature of the object.  

    In one particular case, when examining the light curve for a star named V1400 Centauri (Catchy, I know) some very unusual features were noticed. First of all, the period of transit was extremely long, the transit lasted for more than 56 days, which is way to long for a planet to be orbiting the star. Secondly, the distinctive u-shaped dip of the transit, was not present, instead the light curve was chaotic. 

    An example of a light curve containing a transiting planet, data taken from the Kepler Mission.

    The object, designated J1407b initially stumped scientist and posed somewhat of a question mark. However, scientists were able to figure out, from studying Saturn’s rings, that this object was in fact a massive ring system. There remains some margin for error, as we cannot directly observe this object, but it is generally accepted that J1407b is a circumplanetary disk, or in other words a massive cloud of dust and gas that will one day may form into moons, exomoons and satellites.

    The chaotic structure of the light curve stems from the design of the ring system, and scientists have been applying what we know about the formation of Saturn’s rings to attempt to figure out how this giant ring system came to be. This is important area of research, as this is a window into the very beginnings of a new stellar system, with potential to learn about the mechanisms that underlie star formation as well as planet formation, areas of astrophysics that are at present not completely understood. 

    Artists rendition of the J1407b system. Credit Ron Miller

    Conclusion

    This specific case is just one example of how the lessons we have learnt, studying our own galactic neighborhood, can help to develop our understanding of objects that are found within deep space. By studying the rings of Saturn, we have gained knowledge of much more complex systems such as nebula, spiral galaxies and accretion disks.  

    This specific case highlights the significance of understanding our local stellar systems, these objects act as astronomical observatories, regions where our ideas and theories can be rigorously tested before our understanding can be applied to more distant objects, or perhaps even the universe at large.  

    Further Reading

    ​P. Sutton, “Mean Motion Resonances with Nearby Moons: An Unlikley Origin for the Gaps Observed in the Ring Around the Exoplanet J1407b,” Monthly Notices of the Royal Astronomical Society, vol. 486, pp. 1681-1689, 2019.  

    M. M. Hedman, P. D. Nicholson, K. H. Baines, B. J. Buratti, C. Sotin, R. N. Clark, R. H. Brown, R. G. French and E. A. Marouf, “The Architecture of the Cassini Division,” The Astronomical Journal, vol. 139, pp. 228-251, 2010.  

  • The Magic Dance in the Night Sky

    The Magic Dance in the Night Sky

    The Science Behind the Northern Lights

    Photo by stein egil liland on Pexels.com

    Introduction

    Of all of the wonders of the natural world, the Northern lights are perhaps the most striking. The hues of greens, purples and reds that light up the otherwise dimly lit night sky are adored by many around the world, with a great number of people travelling hundreds of miles just for the chance to catch a glimpse of the aurora.  

    “The northern lights rise like a delicate veil of green fire, dancing across the night sky, whispering secrets of the cosmos to those who dare to look up.”

    — Unknown

    This article aims to take a dive into the physics behind the phenomena, with the objective of describing what otherwise, seems to be a magic show, to the lay audience. Join us on this educational journey to learn everything about why these simply spectacular lights can be seen in our night sky.  

    It all Starts with the Sun

    Looking up at the night sky, you would be forgiven for thinking that the universe is filled with only stars, planets and moons. In actuality, the “empty” space that we observe between these celestial objects is filled with streams of particles, radiation and the reflection of that radiation off of the planets and the moons.  

    Many of these particles begin their journey within stars, much like our Sun. Stars are comprised of many different layers, and the relative motion between said layers means that each star generates a massive magnetic field. However, the full picture is not this straightforward. Overall, these layers do move relative to each other, but in smaller sections we can see that the motion is somewhat random. This random motion gives rise to local magnetic fields within the overall magnetic field. These local magnetic fields manifest themselves as solar activities, an example of which would be sunspots.  

    Photo by Pixabay on Pexels.com

    One consequence of the solar activities is the production of high velocity charged particles, Protons and electrons, small sub-atomic particles. These particles are given a velocity greater than the escape velocity of the Sun, such that, the particles are able to escape the Sun’s gravity. The Sun emits these particles in all directions and as a result some of them are directed towards the Earth. The amount of these particles that reach the Earth varies with the level of solar activity at the time.

    It’s Almost Show Time

    Upon their ejection from the surface of the Sun or it’s corona, these charged particles begin on a journey of around ninety-three million miles, on the path between the Earth and the Sun. This journey can take between two and four days to complete, with the particles reaching speeds of over one million miles per hour.  

    The charged particles now arrive at the Earth. Although they don’t make it the Earth’s surface, they do reach the Earth’s magnetic field or magnetosphere if you will. Much like the Sun, the Earth is comprised of multiple layers. A solid core, a liquid magma mantle, and once again solid crust. These layers also move relative to one another. Hence, the Earth has its own magnetic field although, on a much smaller scale than that of the Sun.

    It is here that the charged particles of the solar wind are affected by the electromagnetic force imposed upon them. The charged particles follow the field lines of the magnetic field towards the polar regions. As the particles are accelerated along the magnetic field lines, they enter a region of the Earth’s atmosphere where the magnetosphere and the ionosphere overlap.  

    The Ionosphere is a region of the Earth’s atmosphere where we can find even more of these charged particles, in this case rather than the mix of electrons and protons that we find in the solar wind, we find ions of oxygen, nitrogen and other elements found within the Earth’s atmosphere. Current understanding suggests that when the charged particles of the solar wind reach the ionosphere, collisions occur between the ions and the charged particles.  

    A result of the collision, the electrons within the ions are excited to a higher energy level. When these excited electrons relax back to their ground state, the energy gained from the collision is released in the form of a photon, or particle of light. This light is otherwise known as the Aurora. 

    The different colors that we can observe in the aurora, occur from the different collisions that occur in between the charged particles and the ions. For example, the green light that we often see when observing the aurora, is characteristic of Oxygen molecules whilst, the blues and the purples are indicative of the different excitations that can occur within Nitrogen.  

    Final Thoughts

    In summary the brilliant display of lights that can be seen above our heads at night stem from a stream of particles that travel over ninety-three million miles to reach our planet. These electrically charged particles are driven towards the polar regions by Earth’s magnetic field. It is here that these particles interact with ions in the upper atmosphere to produce the stunning array of colorful lights that we can observe.  

    Hence the Northern lights are not a ‘magic’ as I have claimed in the title of this article. Rather they arise from a number of physical phenomena from multiple disciplines, ranging from the astrophysics of stars, to quantum particle interactions. Whilst the aurora may not be ‘magic’ understanding the physics does not take away from the natural beauty of the lights.

    Further Reading

    Qian, W. (2023) ‘A physical explanation for the formation of Auroras’, Journal of Modern Physics, 14(03), pp. 271–286. doi:10.4236/jmp.2023.143018.


    Rehnberg, M. (2021), Understanding aurora formation with ESA’s Cluster mission, Eos, 102, https://doi.org/10.1029/2021EO162945. Published on 07 September 2021.

    Rezhenov, B. V. and Vardavas, I. M.: A possible mechanism for <theta> aurora formation, Ann. Geophys., 13, 698–703, https://doi.org/10.1007/s00585-995-0698-3, 1995.